From Ancient Practice to Modern Innovation: Solving the Clay Mineral Puzzle in Brickmaking

From Ancient Practice to Modern Innovation: Solving the Clay Mineral Puzzle in Brickmaking | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article From Ancient Practice to Modern Innovation: Solving the Clay Mineral Puzzle in Brickmaking Yunfei Xi, Sen Wang, Lloyd Gainey, Lihui Liu, Zijun Zeng, Shusheng Xiao, and 3 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-6743297/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted You are reading this latest preprint version Abstract Clay minerals are the most crucial constituents in brickmaking clays, yet the specific role of different clay minerals remain poorly understood due to their natural coexistence and formidable challenges of separation. Here, we design a novel kaolinite-illite-quartz-feldspar system through a multiple-stage purification strategy, enabling direct evaluation of two most critical clay minerals - kaolinite and illite. Precise tuning the initial clay mineral assemblage yields brick strength up to 10 times the ASTM C62 Grade MW requirement, without upgrading existing production method. Mineralogical mechanisms responsible for substantial strength enhancement are revealed. Extensive analyses of 60 brick formulations uncovered: a breakdown of conventional linear relationships among different brick properties due to the temperature-dependent response of kaolinite and illite; specific formation pathways of mullite and amorphous phases; over-firing defects above 1200°C due to K + -induced fluxing from illite. Notably, a previously unrecognized negative correlation between mullite and brick strength is identified, suggesting a need to reconsider the widely held assumption of mullite as a strengthening phase. This work not only presents valuable insights for optimizing high-strength brick formulations but also opens new avenues for investigating the intricate contributions of distinct starting components to key brick performance. Physical sciences/Materials science/Structural materials/Ceramics Earth and environmental sciences/Solid Earth sciences/Mineralogy clay minerals fired bricks mechanical strength kaolinite illite firing temperatures Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 1. Introduction Fired bricks have been manufactured for over 6000 years, 1 yet low mechanization in traditional brickmaking significantly limited production efficiency and consistency, and the complexity of brickmaking clays, which comprises clay minerals, quartz, feldspars, carbonates, organic matter, Fe and/or Al oxyhydroxide, etc., 2 , 3 made it difficult to understand underlying material science of brick formation. Modern factories have adopted high-tech machinery - crusher, mixers, extruders, and temperature control systems - to ensure efficient and consistent production. Additionally, the commercialization of analytical tools like X-ray diffraction (XRD) and X-ray fluorescence (XRF) enabled detailed analysis of the composition and thermal behavior of clays. Tools like the SiO 2 -Al 2 O 3 -MgO and SiO 2 -Al 2 O 3 -CaO phase diagrams now allow researchers to map high-temperature mineral transformations more accurately. 4 Thus, a comprehensive understanding of the relationship between raw materials and final brick properties has only recently emerged. 1 , 5 However, despite these advances, defect rates in current brick production remain stubbornly high - approximately 3–5% even in top-performing production lines. 6 , 7 These defects, including cracks, spalling, chips, efflorescence, etc. translate into substantial economic losses, given the annual production of over 1,500 billion bricks. 8 More critically, defective bricks contribute to significant energy waste and environmental pollution due to the increased CO 2 emissions associated with carbonate decomposition. 8 , 9 A major source of these defects is the inherent variability in raw clays, driven by weathering processes that differ across regions and even within a single mining site. 10 , 11 This variability is further amplified by the gap between scientific research and industrial practice. Most existing studies are based on region-specific clays subjected to varied firing regimes, with limited use of controlled experimental designs and systematic compositional gradients. 12 , 13 This narrow focus makes manufacturers struggle to extract practical, locally adaptable guidelines from the current body of scientific knowledge. Consequently, brick research and production still echo ancient trial-and-error methods without innovative breakthroughs. To date, the most pressing challenge in brick industry is establishing a universal understanding of how different components in clay influence the final properties of bricks. However, precisely tuning these components remains challenging due to their inherent coexistence in natural clay. In addition, clay minerals warrant special attention owing to their diversity and pivotal roles like imparting essential plasticity and vitrification upon firing to densify the brick body. 14 In 1992, Dunham 5 systematically investigated brick-making clays in the UK, classifying clay minerals into six categories and identifying two primary variations: the kaolinite/illite (Kln/Ilt) ratio and the quartz/total clay mineral ratio. These ratios are particularly significant because Kln and Ilt are considered two most common clay minerals in ceramic clays, 15 and their proportional variations have been widely documented from brickmaking in Spain, 16 Italy, 17 Australia, 18 Czechia, 19 and Brazil. 20 Despite the recognized importance and wide compositional variability Kln and Ilt, there remains a surprising lack of studies directly linking them to critical brick properties. Limited research has explored their influence on mullite and cristobalite formation, 21 and the contributions of Ilt-rich clay to brick bulk density and Young’s modulus. 15 To date, no definitive conclusions have been reached regarding how Kln and Ilt affect the thermal behavior of clay mixes and key technological performance of final bricks. This lack of fundamental insights not only significantly limits innovation in the brick industry but also increases the risk of producing defective bricks due to mismatched raw materials and processing conditions. This study aims to complete a critical piece of the brick manufacturing puzzle by systematically revealing the roles of clay minerals in brick production. We designed a Kln-Ilt-Quartz-Feldspar (KIQF) system that for the first time, allows flexible tailoring in types, proportions, and characteristics of clay mineral in brick raw materials. Gradient variations of Kln and Ilt were obtained using KIQF, and a total of 60 different brick formulations were prepared across a wide firing temperature range. The impacts of Kln and Ilt on phase composition, microstructure, color, and the physical and mechanical properties of fired bricks were comprehensively assessed. Specifically, optimal clay mix formulations at different firing temperatures were proposed and the underlying mineralogical mechanisms were elucidated. Quantitative correlation analyses presented new insights into the relationships among phase compositions, porosity, and strength of fired bricks. These findings offer practical guidance for improving product consistency and mechanical performance in industrial brick manufacturing. 2. Results 2.1. Design of KIQF System Precisely tuning clay minerals is a critical first step in understanding their roles in brickmaking but this remains a major challenge and has rarely been reported. 18 This difficulty stems from the commonly coexistence of clay minerals, which exhibit similar particle sizes and overlapping structural characteristics, making separation and independent manipulation extremely difficult. 10 , 22 , 23 In addition, as clay minerals are usually formed through the weathering of different rocks, naturally occurring clay minerals with exceptionally high purity are rare. 22 , 24 To address this challenge, we designed a KIQF system composed of three adjustable ingredients: heavy particles (Hp), Kln, and Ilt, through a multi-stage purification strategy. KIQF enables precise control of different clay minerals, providing a fundamentally new design concept that enables systematic differentiation of their individual contributions. Hp fraction was derived from industrial brick-making clay in Queensland, Australia, with its clay mineral components removed through purification (details provided in Supplementary Note 1 ). Hp represents the non-clay components typically present in brick raw materials. In addition, Kln and Ilt are high-purity natural minerals, further refined in the laboratory to enhance their purity, as described in Supplementary Note 2 . XRD analysis on the powder and clay sides from fine fraction of powder samples are shown in Figure S1 and S2 . Rietveld refinement demonstrates purity of Kln and Ilt at 93.5% and 93.9%, respectively, while Hp consists of quartz (65.8%), feldspar (2.57%), hematite, goethite, and minor clay minerals ( Figure S3 , Supporting Information). To further characterize the raw materials, morphological analyses and particle size distribution were performed. SEM images reveal that both Kln and Ilt exhibit layered stacking, with Ilt showing an irregular plate-like morphology and Kln presenting a pseudohexagonal shape ( Figure S4 , Supporting Information). According to laser particle size analysis ( Figure S5 , Supporting Information), the volume-weighted mean particle size (D [4,3] ) of Hp is 460.00 µm, whereas Kln and Ilt display closely comparable values of 5.75 µm and 5.28 µm, respectively, supporting good particle size interchangeability. To comprehensively capture the variability of Kln and Ilt contents encountered in practical brick production and fully elucidate their roles, a broad compositional and temperature space was systematically established. Three independent variables were set: 1) total clay mineral content, calculated as (Kln + Ilt)/(Kln + Ilt + Hp), at 20%, 35%, and 50%; 2) Kln-to-clay mineral ratio, calculated as Kln/(Kln + Ilt), at 0%, 33.3%, 50%, 66.6%, and 100%; and 3) firing temperatures at 900°C, 1050°C, 1200°C, and 1350°C. It is important to note that the variability within and between geological formations of brick clays primarily arises from differences in Kln/Ilt and quartz/total clay mineral ratios, making their study particularly relevant. 5 In addition, the wide temperature range was selected to span both low-temperature energy efficiency and high-temperature strength optimization typical of industrial brick firing. Table 1 summarizes the 15 clay mixes prepared. With each composition subjected to four firing temperatures, a total of 60 brick formulations were generated. To the best of our knowledge, such a comprehensive and systematic exploration of raw material variations has not yet been reported in existing literature. Table 1 Formulations of clay mixes. Clay mix ID (Kln + Ilt)/(Kln + Ilt + Hp) (%) Kln/(Kln + Ilt) (%) 20/0 20 0 20/33 20 33 20/50 20 50 20/66 20 66 20/100 20 100 35/0 35 0 35/33 35 33 35/50 35 50 35/66 35 66 35/100 35 100 50/0 50 0 50/33 50 33 50/50 50 50 50/66 50 66 50/100 50 100 2.2. Thermal Expansion and Shrinkage of Clay Mixes Prior to examining the properties of fired bricks, thermal shrinkage/expansion of clay mixtures were investigated as these processes dictate porosity evolution and sintering - both significantly affecting the final physical and mechanical performance of bricks. Across 25-1350°C, thermal shrinkage/expansion can be divided into five distinct stages (Fig. 1 a, Figure S6 , Supporting Information). 14 Notably, at a fixed total clay mineral content, variations in the Kln/(Kln + Ilt) ratio only affect the final three stages (above 500°C). In stage 3, brick expansion decreases from 0.64–0.15% as the Kln/(Kln + Ilt) ratio rises from 0 to 100% (Fig. 1 a). This decrease is due to the structural collapse of Kln after dehydroxylation, counteracting the abrupt expansion caused by the α - to β -quartz inversion. 4 , 25 This indicates that incorporating Kln can reduce cracking risks in clay bodies between 500°C and 650°C. However, Ilt does not exhibit this effect because its crystal structure remains after dehydroxylation. It is reported that Ilt continuously expand upon firing below 1000℃, 26 aligning with the expansion observed in the 35/0 sample below 900℃ (Fig. 1 a). In stage 4 (650–900℃), replacing Ilt with Kln shifts the clay mix from expansion (0.21% for 35/0) to shrinkage (-0.12% for 35/100) (Fig. 1 a), attributed to the structural collapse caused by reorganization of Al coordination in metakaolin in preparation for spinel/mullite formation. 27 In stage 5 (900–1350℃), the firing shrinkage decreases from 10.98% (35/0) to 6.27% (35/50) and 3.24% (35/100) with increasing Kln/(Kln + Ilt) ratio. This indicates that compared to Kln, Ilt in the clay mix leads to higher degree of sintering, likely due to the fluxing action of K + ions in its interlayer space. 28 , 29 X-ray fluorescence test in Table S1 (Supporting Information) confirmed that Ilt, with a combined K, Na, Mg, and Ca oxide content of 10.79%, is a high-fluxing material, while Kln and Hp exhibit much lower levels of fluxing agents, at 0.64% and 2.49%, respectively. In addition, under scanning electron microscopy observation, pure Ilt melts at 1050℃, while Kln partially reserves its morphology even after 1200℃ firing ( Figure S7 , Supporting Information, Fig. 1 b), further demonstrating the higher vitrification capacity of Ilt than Kln. Apart from the thermal dilatometry, the dimensional change of brick buttons after firing were also examined. Green bodies show an expanding trend after 900°C (Fig. 1 c), with the expansion value decreasing as the Kln/(Kln + Ilt) ratio increases. At 1050°C and 1200°C, higher Kln/(Kln + Ilt) ratios result in reduced firing shrinkage due to the lower vitrification capacity of Kln compared to Ilt (Fig. 1 d and 1 e). Interestingly, bricks made from Ilt and Hp exhibit approximately 10% expansion at 1350°C instead of shrinkage (Fig. 1 f), attributed to the so-called “overfiring”. 30 During firing, open pores typically close as the brick densifies. However, beyond maximum densification, rising gas pressure in fully closed pores can lead to expansion as the temperature continues to rise. Introducing Kln to the clay mix can mitigate this effect, due to the superior refractory properties of Kln compared to Ilt (Fig. 1 b). In addition, higher total clay mineral content leads to greater firing shrinkage across most formulations (Fig. 1 c- 1 f), consistent with the thermal dilatometry results in Figure S6 (Supporting Information). An exception occurs when Kln/(Kln + Ilt) = 0 and the firing temperature is 900°C, where increased Ilt content results in expansion rather than shrinkage (Fig. 1 a). This is likely due to irreversible lattice expansion of Ilt following dehydroxylation, 31 as evidenced by the increased d (001) spacing after firing (inset in Fig. 1 c). 2.3. Physical and Mechanical Properties of Fired Bricks 2.3.1. Microstructure and Appearance BSE-SEM on fine polished bricks effectively reveals pore distribution and phase interaction, providing critical insights into the physical and mechanical properties. At 35% clay minerals and 1350°C, decreasing the Kln/(Kln + Ilt) ratio results in 1) reduced pores and their connectivity, 2) transition from elongated to rounded-shape pores, and 3) more discrete pore size distribution ( Figure S8 , Supporting Information). These changes are primarily attributed to the superior vitrification behavior of Ilt compared to Kln, which enhances pore coalescence and elimination, as well as grain consolidation. Moreover, a distinct evolution of pore characteristics is observed with increasing temperature (Fig. 2 a- 2 d) − 900°C pores from loose particle packing; 1050°C and 1200°C elongated pores (> 100 µm) from differential shrinkage of Kln, Ilt, and quartz particles; 1350°C closed and rounded pores from substantial liquid-phase formation - reflecting the progressive sintering and densification of the brick structure. 32 , 33 In addition to pore evolution, EDS mapping reveals the contact relationships between different phases ( Figure S9 , Supporting Information). The evolution of clay mineral matrix was also observed with firing temperature. At 900°C and 1050°C, the matrix comprises loosely packed Kln and Ilt with clearly defined phase boundaries (Fig. 2 e- 2 f and 2 i- 2 j). EDS analysis confirms that Kln is Al-rich, while Ilt contains higher concentrations of K, Fe, and Mg ( Figure S10 , Supporting Information). Both phases retain their layered morphology, indicating limited vitrification. At 1200°C, a partially vitrified matrix forms (Fig. 2 c), incorporating fine, poorly crystallized mullite (< 2 µm) (Fig. 2 g and 2 k). Some residual lamellar structures remain, with an Al: Si ratio of ~ 1: 1, suggesting they are unreacted Kln ( Figure S11 , Supporting Information). Upon firing to 1350°C, the microstructure transitions to a highly vitrified and homogeneous glassy matrix (Fig. 2 d), accompanied by pore closure and reduction in size (Fig. 2 h). Well-developed needle-shaped mullite crystals emerge, some exceeding 10 µm (Fig. 2 l). EDS analysis shows an Al: Si ratio of ~ 3:2 in these areas ( Figure S12 , Supporting Information), indicative of secondary mullite formation promoted by the presence of alkali fluxes. 34 In terms of appearance, dried bricks shift their colors from oyster to neutral grey, becoming lighter at higher clay mineral contents ( Figure S13 , Supporting Information). For fired bricks, color deepens with temperature, ranging from white at 900°C to mushroom pink at 1050°C, cloud grey at 1200°C, and dark brown at 1350°C (Fig. 2 m). This darkening is due to the increased crystal size of hematite with temperature ( Figure S14 , Supporting Information). 18 In addition, higher clay mineral content results in lighter colors, attributed to the lower Fe content in Kln and Ilt compared to Hp. Decreasing the Kln/(Kln + Ilt) ratio also deepens the brick color, possibly because the higher fluidity of the Ilt-based glass matrix facilitates the growth of larger hematite crystals. Interestingly, surface defects such as black spots and cracks emerge on Hp-Ilt bricks fired at 1200°C or higher (Fig. 2 n and 2 o). XRD analysis reveals that the black spot consists of hercynite (FeAl 2 O 4 ), hematite (Fe 2 O 3 ) and quartz. According to Laita, Bauluz 35 , hercynite can evolve from Fe particles at high temperatures, partially replacing hematite crystals. In this experiment, the hercynite-rich exudate increases with both Ilt and Hp content. Their presence is due to the Fe-Al interaction in the low vitreous glassy matrix resulting from intensive melt of Ilt. In addition, cracks gradually develop with increasing Ilt content in the starting clay (Fig. 2 o), likely due to enhanced liquid-phase sintering, which produces large, closed pores that expand during heating and generate internal stress within the brick (Fig. 2 p). 2.3.2. Phase Composition and In-situ High-temperature XRD As a critical factor in determining brick performance, a quantitative investigation of phase composition is essential. In the absence of Ca and Mg, the primary phases in bricks are quartz, mullite, and amorphous phases. 36 , 37 Quantitative XRD analyses of these phases were performed through Rietveld refinement ( Figure S15-S24 , Supporting Information), with results presented in Fig. 3 a– 3 i. In addition, in-situ high-temperature XRD was employed to reveal the real-time phase transitions of clay mix fired up to 1200°C (Fig. 3 j and 3 k). Mullite Formation : Mullite forms through the decomposition of clay minerals, with a formation temperature at 975°C (Fig. 3 k). Previous study suggest that fluxing agents like K and Na enhance the fluidity of Si-rich amorphous phases, promoting greater mullite crystal growth. 38 However, our experiment reveals that mullite formation is a complex process influenced by fluxing agents, the Al/Si ratio of clay minerals, and firing temperature: At low firing temperatures (900°C): Mullite is primarily observed in Ilt-rich samples (Fig. 3 a). The presence of Kln in the starting material promotes the formation of Al-spinel instead of mullite at this temperature, as confirmed by previous TEM observations. 38 At 1050°C: Maximum mullite content (10.19%) occurs when Kln and Ilt coexist (Fig. 3 a). Compared to Ilt with an Al: Si of 1: 2, the Al: Si ratio of Kln (1: 1) is closer to that of mullite (3: 2 or 2: 1), promoting mullite crystallization. However, the highly viscose Si-rich amorphous phase from Kln decomposition simultaneously limits mullite growth. As for Ilt, although the highly fluid amorphous phase favors mullite formation, its lower Al: Si ratio (2: 1) hinders it. Therefore, a combination of Kln and Ilt yields the highest mullite content (10.19%) at 1050℃. Moreover, the effect of fluxing agents appears more significant than the Al: Si ratio, as evidenced by the lower mullite in the 35/100/1050 brick (1.92%) compared to the 35/0/1050 brick (7.22%) (Fig. 3 a). At 1200°C and 1350°C: Both Kln and Ilt melt extensively (Fig. 1 b and Figure S6, Supporting Information), making the Al: Si ratio more critical than the fluxing agent for mullite formation. This explains why higher Kln increases mullite, reaching a maximum at 38.77% for 50/100/1350 (Fig. 3 a). Additionally, while mullite forms from the decomposition of clay minerals, a higher total clay mineral content in the starting clay does not always guarantee more mullite in the final bricks: when Kln is the dominant clay mineral, mullite content increases with clay mineral content above 1200°C (Fig. 3 b); in contrast, when Ilt is the primary clay mineral, this increase occurs at a lower temperature of 900°C (Fig. 3 c). Amorphous Phase : Amorphous phase evolution inversely correlates with mullite because both phases derive from decomposed clay minerals (Fig. 3 d) - amorphous phase comes from Si-rich components, while mullite forms from Al-rich components. Thus, the changes in amorphous content with both Kln/(Kln + Ilt) and clay mineral content are opposite to those of mullite (Fig. 3 d- 3 f). Note that the decrease in amorphous content above 1200°C may also result from cristobalite formation from amorphous Si. In summary, both the formation of mullite and amorphous are significantly affected by initial Kln/(Kln + Ilt) and clay mineral content, highly dependent on firing temperature. Quartz : Unlike mullite and amorphous phase, quartz content remains relatively stable across varying Kln/(Kln + Ilt) ratios (Fig. 3 g- 3 i). A specific interpretation on quartz variation among different formulations can be seen in Supplementary Note 3 . 2.3.3. Water Absorption and Bulk Density Water absorption (WA) and bulk density are critical parameters that significantly influence the structural integrity, durability, and overall performance of obtained brick. 39 The effect of Kln/(Kln + Ilt) on both WA and bulk density is strongly temperature dependent. At 900°C, the increase of Kln/(Kln + Ilt) slightly reduces WA (Fig. 4 a), primarily due to the higher firing shrinkage of Kln compared to Ilt (Fig. 1 a), though the overall difference in WA is relatively minor. At 1050°C and 1200°C, a higher Kln proportion leads to increased WA (Fig. 4 b and 4 c), as the weaker vitrification of Kln relative to Ilt limits pore closure and retains more interconnected porosity. At 1350°C, the 50/50 formulation achieves the lowest WA value (1.30%) (Fig. 4 d), indicating superior sintering performance. In addition, detailed variation of WA with starting clay mineral content is discussed in Supplementary Note 4 . Regarding the bulk density, the obtained value varies from 1.62 g/cm³ to 2.23 g/cm³ (Fig. 4 e- 4 h) across 60 formulations. At 900°C, bulk density decreases as the Kln/(Kln + Ilt) ratio increases, ascribed to the higher thermal mass loss of Kln compared to Ilt (Fig. 4 i). The reduction in bulk density at 1050°C and 1200°C is attributed to both a lower mass and an increased dimension of brick (see the reduced firing shrinkage with Kln/(Kln + Ilt) in Fig. 1 d and 1 e). At 1350°C, bulk density first increases with Kln/(Kln + Ilt) ratio due to reduced amount of large, closed pores (Fig. 4 j-l), then declines after Kln/(Kln + Ilt) exceeds 50% as brick mass decreases and firing shrinkage remains stable (Fig. 1 f). In addition, the bulk density of bricks correlates closely with total clay mineral content. An increase in the clay mineral content leads to lower bulk density at 900°C regardless of the Kln-to-Ilt ratio (Fig. 4 e). This is due to the higher thermal mass loss of both Kln and Ilt compared to Hp as shown in the TGA analysis (Fig. 4 i). At 1050°C and 1200°C, Ilt-rich clays tend to increase bulk density, resulting from greater firing shrinkage, while Kln-rich clays lead to a lower density with clay mineral content because of higher mass loss and reduced firing shrinkage (Fig. 4 f and 4 g). At 1350℃, the considerably low bulk density for formulations with Kln/(Kln + Ilt) = 0 is attributed to the expansion of the brick body by the formation of large, closed pores (Fig. 4 h). 2.3.4. Compressive Strength Compressive strength (CS) is crucial as fired bricks are primarily designed to withstand significant compressive loads. 4 The relationship between CS and Kln/(Kln + Ilt) ratio varies across 900–1350℃. At 900℃, there is a positive correlation between CS and Kln/(Kln + Ilt) (Fig. 5 a). Higher Kln content results in greater firing shrinkage due to the structural collapse and reorganization of Kln after dehydroxylation (Fig. 1 a and Fig. 4 a), increasing the obtained CS. In contrast, Ilt expands after dehydroxylation, producing porosity and reducing CS. 4 At 1050℃ and 1200℃, a negative correlation between CS and Kln/(Kln + Ilt) is observed (Fig. 5 b and 5 c). This is due to the higher degree of vitrification of Ilt than Kln, which enhances brick densification, reducing porosity and increasing CS. At 1350℃, the relationship between CS and Kln/(Kln + Ilt) becomes more complex (Fig. 5 d). A combination of Kln and Ilt is required for optimal CS. Generally, Ilt promotes liquid sintering, reducing porosity and enhancing particle-bonding, but excessive Ilt leads to cracks and the formation of large pores (Fig. 2 o and Fig. 4 j). As for Kln, while it increases mullite formation, its low level of vitrification weakens the liquid sintering and elevates obtained porosity (Fig. 1 b). Consequently, the highest CS at 1350℃ is achieved in formulations where Ilt and Kln coexist, i.e. 20/50 (163.5 Mpa), 35/33 (172.4 Mpa), and 55/33 (166.8 Mpa). Regarding the effect of total clay mineral content: Higher clay mineral content leads to lower CS at 900℃. This is due to the looser packing of fine clay mineral particles, which increases the initial porosity (Fig. 2 i, Figure S10, Supporting Information). At 1050°C and 1200°C, the vitrification of clay minerals densifies the bricks, so the higher clay mineral contents, the higher CS values (Fig. 5 b and 5 c). However, at 1350°C, formulations with Kln/(Kln + Ilt) ratios ≥ 50% exhibit reduced CS when clay mineral content increases (Fig. 5 d). This is attributed to the higher number of initial pores introduced by the addition of clay minerals, which fail to completely close due to the high Kln content. Additionally, at 900–1200°C, CS is more sensitive to the Ilt content than to the Kln content (Fig. 5 a- 5 c). A significant reduction in CS is observed with increasing Ilt content at 900°C (from 20/0 to 50/0), while a notable increase occurs at 1050°C and 1200°C. In contrast, CS remains consistent with total clay mineral content when Kln dominates the clay mineral. Furthermore, most formulations show increased CS at higher firing temperatures (Fig. 5 e- 5 g), in line with existing literature. 40 , 41 Two exceptions are: 1) the 50/0 formulation shows reduced CS at 1350°C compared to 1200°C, likely due to the formation of cracks from excessive liquid sintering (Fig. 2 o); 2) for bricks with 20% total clay mineral content, lower CS is observed when increasing the temperature from 1050°C to 1200°C (Fig. 5 e). This is attributed to the high quartz content in the starting clays. Quartz typically transforms into cristobalite above 1200°C. Upon cooling, the formed cristobalite undergoes transition from β - to α -phase between 280 − 190°C, resulting in ~ 5% volumetric shrinkage. This transformation may induce microcracks within the matrix, thereby compromising its structural integrity. 42 The considerable increase in cristobalite is confirmed by XRD in Fig. 5 h and 5 i. However, no further CS reduction is observed from 1200°C to 1350°C, likely due to enhanced sintering at 1350°C, which imparts a more robust matrix capable of resisting cristobalite phase inversion. To better visualize the CS value under different clay mineral assemblages, contour maps for 60 formulations are presented in Fig. 5 j- 5 m. Specifically, achieving a high CS (53.6 MPa) at 900°C requires a low total clay mineral content and a high Kln/(Kln + Ilt) ratio. However, the clay mineral content should also be sufficient to ensure proper extrusion in practical brickmaking. At 1050°C and 1200°C, the trend reverses: a high clay mineral content and a low Kln/(Kln + Ilt) ratio improve CS, reaching peaks of 126.2 MPa and 142.4 MPa, respectively. For bricks fired at 1350°C, achieving a high CS (172.4 MPa) requires avoiding extreme values for both clay mineral content and the Kln/(Kln + Ilt) ratio. These results demonstrate that by precisely adjusting the initial clay minerals, the brick CS can be significantly enhanced - reaching 3, 7, 8, and 10 times the minimum required compressive strength (17.2 MPa) for Grade Moderate Weathering (MW) bricks, as specified by ASTM C62. 43 The recommended optimal clay formulations for enhanced CS at different firing temperatures are illustrated in Fig. 6 . 2.4. The Importance of Firing Temperature According to the above analyses, key properties of fired bricks are strongly influenced by starting clay minerals, with these effects varying significantly with firing temperature. This variability is primarily due to the differing responses of Kln and Ilt to heating, governed by their crystal structures (1: 1 or 2: 1) and chemical compositions (Al: Si ratio and fluxing agent concentration). Since in most brick manufacturing scenarios, where Kln coexists with Ilt, and firing temperatures are around 1050°C or 1200°C, 44 the lower vitrification degree of Kln compared to Ilt results in lighter brick colors. This difference in vitrification also necessitates the coexistence of Kln and Ilt to achieve a high mullite content at 1050°C, as discussed in Section 2.3.2 . However, vitrification no longer limits mullite formation from metakaolin at 1200°C. For applications requiring lower bulk density (e.g., for easier transport), at 1050°C and 1200°C, increasing the Kln content can be beneficial, although this compromises the durability of bricks by increasing porosity. Conversely, if higher bulk densities are required (enhancing durability and strength), a higher Ilt content in the starting clay is recommended. Additionally, to prevent the melting of hercynite and hematite on the brick surface, particularly when Hp contains ≥ 6.5% of Fe 2 O 3 , the clay mineral may need to exceed 35%. Specific considerations also apply at the two less commonly used firing temperatures of 900°C and 1350°C. At 900°C, Ilt fails to vitrify, weakening brick strength and making it unsuitable as a primary starting clay. At 1350°C, optimal strength is achieved with an Ilt-to-Kln ratio of either 2: 1 or 1: 1, depending on the total clay mineral content. 2.5. Correlations among Diverse Brick Properties Firing temperature impacts not only thermal evolutions of raw clays and related brick properties but also the interrelationships among these properties. While prior studies have frequently reported strong linear correlations (R 2 ≥ 0.90) - such as a negative relationship between WA and firing shrinkage, 45 , 46 a positive correlation between CS and bulk density, 12 , 47 a negative correlation between CS and WA, 12 , 47 , 48 and a negative correlation between WA and bulk density 49 - the present study did not observe similarly strong relationships after systematically examined 60 different formulations. The highest coefficient of determination (R 2 ) found in this study is only 0.59 for the CS-WA relationship, followed by 0.34 (CS-amorphous content), 0.10 (CS-mullite content), and 0.08 (CS-bulk density) (Fig. 7 a and 7 b). These values suggest that the strong linear correlations reported may only be applicable within narrow temperature or compositional ranges as adopted in previous studies. In contrast, the broader firing range examined here (900–1350°C) revealed temperature-dependent variations that weaken these correlations, which was usually overlooked before. For instance, CS and bulk density are nearly uncorrelated at 900°C, likely due to the unique behavior of Kln, which undergoes both significant shrinkage and water loss. This contrasts with the positive correlation between these two properties above 1050°C (Fig. 7 c). In addition, at 1350°C, the excessive melting of Ilt results in cracks and formation of closed pores, leading to irregular relationships among CS, bulk density, and WA. According to quantitative analysis, the importance of factors influencing CS ranks as follows: WA (-0.78) > amorphous content (0.51) > bulk density (0.40) > mullite content (0.29) (Fig. 7 d). These correlations strongly depend on temperature (Fig. 7 c), peaking mostly at 1200°C, except for mullite content. The unexpectedly low impact of bulk density compared to literature is mainly due to its weak correlation with CS at 900°C. Interestingly, although mullite is widely acknowledged to strengthen ceramics by forming interlocking acicular crystal networks, refining Griffith flaws, and mismatched thermal expansion coefficient with glass phase matrix, 50 , 51 , 52 , 53 we have identified a negative correlation between mullite and CS at 900°C, 1200°C and 1350°C (the correlation coefficient at 1050°C is also quite low at 0.3, Fig. 7 c). This may be attributed to that mullite typically forms at the expense of the amorphous/glass phase, which contributes more significantly to CS development by facilitating sintering and particle bonding (see the positive correlation coefficient values in Fig. 7 c). It can be reasonably inferred that glass matrix is more important to fired bricks than other ceramic products as the clayey materials used in brickmaking are generally coarser in particle size, creating larger initial voids and thus requiring more glass to achieve effective bonding. In fact, phase-related factors (amorphous and mullite contents) only play pronounced role at low temperature of 900°C, while above 1050°C, porosity-related factors (WA and bulk density) are more important (Fig. 7 c). Nevertheless, brick CS is governed by a combination of factors, including porosity (open and closed), phase composition and contact patterns, the degree of particle bonding (sintering), and the presence of cracks. 3. Discussion This study introduces a groundbreaking Kaolinite-Illite-Quartz-Feldspar (KIQF) system for precisely adjusting the inherent phase composition of natural clays, thus allowing universal understanding of the roles of different components in brickmaking. Specific focus was put on kaolinite and illite - two most common clay minerals in brick clay. We systematically varied total clay mineral content (20%, 35%, and 50%), kaolinite-to-total clay mineral ratios (0%, 33%, 50%, 66%, and 100%), and firing temperatures (900°C, 1050°C, 1200°C, and 1350°C) and created 60 distinct brick formulations. Comprehensive material characterization revealed that optimal clay mineral assemblages are highly dependent on firing temperature, fundamentally governed by the unique crystalline structures of kaolinite and illite and their temperature-dependent thermal behaviors. At 900°C, formulations with 20% clay minerals composed entirely of kaolinite exhibited enhanced compressive strength (53.6 MPa) and reduced water absorption (11.38%), attributed to the shrinkage from dehydroxylation and subsequent structural reorganization of kaolinite. In contrast, at 1050°C and 1200°C, bricks with 50% clay minerals predominantly consisting of illite demonstrated superior strength (126.2 Mpa and 142.4 Mpa), likely due to the abundant K⁺ ions in illite promoting sintering and densification. The highest compressive strength (172.4 MPa) and lowest water absorption (1.3%) were achieved at 1350°C using a formulation with 35% clay minerals, where one-third of the clay minerals were kaolinite, effectively mitigating over-firing issues associated with illite vitrification. Beyond these findings, an unexpected negative correlation between mullite content and compressive strength is revealed, with Pearson correlation coefficients of -0.89, -0.37, and − 0.37 at 900°C, 1200°C, and 1350°C, respectively. This suggests that mullite formation may consume excessive glass phases that are more crucial to brick strength due to enhanced sintering and particle bonding. The formation of mullite and amorphous/glass phases is influenced by clay mineral content, Al: Si ratios, fluxing agents, and firing temperature. Additionally, the widely reported linear relationships among various technological properties of bricks persist only within narrow temperature ranges and deteriorate over the broader 900–1350°C interval, due to the temperature-sensitive transformations of kaolinite and illite. Quantitative correlation analyses indicated that phase-related factors (amorphous and mullite contents) primarily influence compressive strength at lower temperatures (900°C), while porosity-related factors (water absorption and bulk density) become dominant above 1050°C. Other notable findings include that increased clay mineral content and higher kaolinite-to-illite ratios result in lighter brick colors because higher fluidity of the illite-based glass facilitates the growth of larger hematite crystals. However, excessive vitrification of illite from the interlayer K⁺ leads to hercynite exudation above 1200°C and the formation of closed, rounded pores at 1350°C, causing defects such as approximately 10% expansion and large cracks. Through strategic tuning of clayey raw materials, our study demonstrates that the compressive strength of bricks can achieve 3-, 7-, 8-, and 10-fold over the ASTM Moderate Weather strength requirement at 900°C, 1050°C, 1200°C and 1350°C firing. These insights offer valuable guidance for optimizing complex raw materials in industrial brick manufacturing, and more importantly, provide a framework to ultimately explain the role of different components in determining key brick performance. 4. Methods Materials Three raw materials - Hp, Kln, and Ilt, were used. Hp derived from a commercial clay mix supplied by Austral Bricks Pty. Ltd. in Queensland, Australia. Kln and Ilt were supplied by Austral Bricks Pty. Ltd. and Guzhang Shan Lin Shi Yu Mineral Co., Ltd., respectively. Sodium hexametaphosphate (powder, purity of 96%) was purchased from Merck Pty. Ltd. Fabrication Process of Fired Bricks The preparation of clay bricks could be primarily divided into three steps: molding, drying, and firing. First, the required amounts of Hp, Kln, and Ilt (totally 30 grams) were homogenized in an electrical blender (KitchenAid Classic) for 30 mins (Table 1 ). After adding 12% of water to the mix, the green bodies were obtained through hydraulic compression and then dried and fired following procedure in Figure S25 . Raw Materials Characteristics The chemical compositions of Hp, Kln, and Ilt were analyzed using a Bruker S8 TIGER XRF spectrometer with a 4 kW Rh X-ray tube. Before the test, an aliquot of powder sample (0.74g) was fused into a glass disc using a lithium metaborate/lithium tetraborate flux (50: 50 LiT: LiM, 0.5 wt% LiI) in a platinum crucible using an electric fusion furnace (Katanax, model X600). The particle size distributions of Hp, Kln, and Ilt were analyzed using a Mastersizer 3000 laser particle size analyzer with deionized water as the dispersion medium. The volume-weighted mean diameter was calculated using the following equation: \(\:{D}_{\left[\text{4,3}\right]}=\frac{\sum\:{n}_{i}{d}_{i}^{4}}{\sum\:{n}_{i}{d}_{i}^{3}}\) (Eq. 1) where \(\:{D}_{\left[\text{4,3}\right]}\) , \(\:{n}_{i}\) , and \(\:{d}_{i}\) are volume-weighted mean diameter, number diameter of particles in size class, and diameter of particles in size class, respectively. Thermogravimetry/derivative thermogravimetry analyses of Hp, Ilt, and Kln were performed on a Netzsch STA4493F from 25℃ to 1400℃ at a heating rate of 10℃/min in a 50 mL/min airflow. Expansion/Shrinkage of Clay Mix The expansion/shrinkage of clay mixes during firing were investigated using two methods. Cylindrical specimens (Φ 6 mm × 25 mm) were prepared from green bodies and their in-situ dimensional changes were measured using a Netzsch 402C thermal dilatometer from 25°C to 1350°C at a ramping rate of 5°C/min under static air. Additionally, firing shrinkage of brick buttons was calculated using Eq. 2 provided below: \(\:Firing\:shrinkage\:=\frac{\text{D}-{D}_{0}}{{D}_{0}}\times\:100\%\) (Eq. 2) where \(\:{D}_{0}\) and \(\:D\) represent the diameter of brick buttons before and after firing, respectively. In-situ Phase Evolution and Rietveld Refinement For in-situ phase evolution, high-temperature XRD measurement was conducted on a Rigaku Smartlab X-ray diffractometer (Cu Kα, 40 kV, 40 mA) equipped with an Anton Paar HTK-1200 N chamber. XRD patterns were collected from 7° to 45° 2θ at a step size of 0.02°. The temperature increased from 25°C to 1200°C with a ramp rate of 5°C/min under 100 mL/min dry airflow. Data was collected with an interval of 25°C. The sample height was aligned at each temperature to prevent expanded sample holder height compromising XRD reflections. As for the quantitative phase analysis, XRD patterns were acquired using a Bruker D8 Advance diffractometer (Co Kα, 35 kV, 40 mA) from 2° to 90° 2θ at a step size of 0.015° and a scan speed of 1.5° 2θ/min. A 10 wt.% internal standard (corundum) was used for quantification of amorphous content with details shown in Supplementary Note 5 . Phase identification of the XRD pattern was conducted in DIFFRAC.EVA v7 in ICDD PDF-5 + 2025 database. Quantitative phase analysis was conducted through Rietveld refinement in DIFFRAC.TOPAS v7. In addition, to identify the swelling clay minerals in raw materials, sedimentation experiments were applied to prepare clay slides for XRD measurement with details shown in Supplementary Note 6 . Microstructural Characterizations The micromorphology of fired bricks was imaged using a field emission scanning electron microscope (FESEM, JEOL 7001F) using backscattered electron imaging at 20 kV and 12 mA. An X-Max 80 (Oxford, UK) energy-dispersive X-ray spectroscopy system was applied to determine elemental compositions. Sample preparation procedures are shown in Supplementary Note 7 . Physical and Mechanical Properties of Fired Bricks The 24-h cold water absorption of fired bricks were measured as per the ASTMC67/C67M standard. 54 The bulk density of fired bricks were calculated using the following equation: \(\:\text{B}ulk\:density=\frac{\text{m}}{{\pi\:}{\text{r}}^{2}l}\) (Eq. 3) where \(\:m\) , \(\:r\) , and \(\:l\) stand for the mass, radius and thickness of fired bricks, respectively. Measurements were taken at five positions for each dimension, and the average value was used. Compressive strength tests were conducted on fired brick buttons (Φ 40 mm × 12 mm) using the Instron 6800 universal testing machine, with the maximum bearing capacity of 300 kN and the loading rate 1 mm/min. Declarations Funding: Australian Research Council Mid-Career Industry Fellowship (IM230100132), and The Clay Minerals Society (USA) 2023 Student Research Grant Acknowledgements This work is supported by an Australian Research Council Mid-Career Industry Fellowship (IM230100132) and the Clay Minerals Society (USA) 2023 Student Research Grant. The authors would like to acknowledge Brickworks Ltd., and the Central Analytical Research Facility (CARF), QUT for sample characterizations. Data Availability Statement The authors declare that the data supporting the findings of this study are available within the paper and its supplementary information files. Ethics Declarations Competing interests The authors declare no competing interests. References Zhang LY. 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Supplementary Files SupportingInformation25May2025.docx From Ancient Practice to Modern Innovation: Solving the Clay Mineral Puzzle in Brickmaking GraphicAbstract.docx Cite Share Download PDF Status: Under Review Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-6743297","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":475108689,"identity":"62e855ee-2249-4826-8770-12c99199432e","order_by":0,"name":"Yunfei Xi","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAAsklEQVRIiWNgGAWjYJCCAx8gtAHxWg7OYGCQIE0LMw9JWgwO8Bgetik7XMfA3rxNgqHmMDFa2BIO55w7LMHAc6xMguEYUVqYDxzObQNqkcgxk2BgI0oLY8NhS5AW+TdALf+ItYURbAuPmQSQQViL5GG2hIM959Il23jSii0S+9IJa+E73mP84UeZNT8/++GNNz58syasRQHsEjYwYmBIIKyBgUG+AaplFIyCUTAKRgFOAACJGzVOmGrdiwAAAABJRU5ErkJggg==","orcid":"https://orcid.org/0000-0003-2924-9494","institution":"School of Chemistry and Physics, Faculty of Science, Queensland University of Technology","correspondingAuthor":true,"prefix":"","firstName":"Yunfei","middleName":"","lastName":"Xi","suffix":""},{"id":475108690,"identity":"810de68e-8373-4311-8fd9-6fd3e6017439","order_by":1,"name":"Sen Wang","email":"","orcid":"","institution":"School of Chemistry and Physics, Faculty of Science, Queensland University of Technology","correspondingAuthor":false,"prefix":"","firstName":"Sen","middleName":"","lastName":"Wang","suffix":""},{"id":475108691,"identity":"61afc0f1-58a7-474b-9878-faa24a1c8166","order_by":2,"name":"Lloyd Gainey","email":"","orcid":"","institution":"Brickworks Building Products Ltd","correspondingAuthor":false,"prefix":"","firstName":"Lloyd","middleName":"","lastName":"Gainey","suffix":""},{"id":475108692,"identity":"8b1d3486-f7a3-4021-9bdf-1b60bf012f9d","order_by":3,"name":"Lihui Liu","email":"","orcid":"","institution":"chool of Environmental and Municipal Engineering, Lanzhou Jiaotong University","correspondingAuthor":false,"prefix":"","firstName":"Lihui","middleName":"","lastName":"Liu","suffix":""},{"id":475108693,"identity":"fe6c73c9-d0fb-404e-ad37-2e971d4fbc64","order_by":4,"name":"Zijun Zeng","email":"","orcid":"","institution":"School of Chemistry and Physics, Faculty of Science, Queensland University of Technology","correspondingAuthor":false,"prefix":"","firstName":"Zijun","middleName":"","lastName":"Zeng","suffix":""},{"id":475108694,"identity":"cea722e0-5881-49fd-a25c-7fdcd06c50e3","order_by":5,"name":"Shusheng Xiao","email":"","orcid":"","institution":"School of Chemistry and Physics, Faculty of Science, Queensland University of Technology","correspondingAuthor":false,"prefix":"","firstName":"Shusheng","middleName":"","lastName":"Xiao","suffix":""},{"id":475108695,"identity":"b4d00a69-9cb2-45a2-ac45-d922458db495","order_by":6,"name":"Runliang Zhu","email":"","orcid":"","institution":"Guangzhou Institute of Geochemistry","correspondingAuthor":false,"prefix":"","firstName":"Runliang","middleName":"","lastName":"Zhu","suffix":""},{"id":475108696,"identity":"5b391c88-92bd-4200-91a7-9e6f528d9fc5","order_by":7,"name":"Sara Couperthwaite","email":"","orcid":"","institution":"School of Mechanical, Medical and Process Engineering, Faculty of Engineering, Queensland University of Technology","correspondingAuthor":false,"prefix":"","firstName":"Sara","middleName":"","lastName":"Couperthwaite","suffix":""},{"id":475108697,"identity":"3b70846d-2b70-4db5-a511-3e60cb407c57","order_by":8,"name":"Yuantong Gu","email":"","orcid":"https://orcid.org/0000-0002-2770-5014","institution":"Queensland University of Technology","correspondingAuthor":false,"prefix":"","firstName":"Yuantong","middleName":"","lastName":"Gu","suffix":""}],"badges":[],"createdAt":"2025-05-25 11:05:17","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-6743297/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-6743297/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":85327634,"identity":"85f7640c-dffc-4a22-aa94-f2561e63c0ad","added_by":"auto","created_at":"2025-06-24 16:58:23","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":368571,"visible":true,"origin":"","legend":"\u003cp\u003eThermal dilatometry (TD) and the first derivative (dL/dt) curves (DTD) of 35/0, 35/50, and 35/100 clay mixes (a); Schematic illustration showing the morphological evolution of Kln and Ilt fired at different firing temperatures (b); Firing shrinkage of clay mixtures as a function of the Kln/(Kln+Ilt) ratio at 900°C (c), 1050°C (d), 1200°C (e), and 1350°C (f). Inset in (c): Comparison of the (001) XRD reflection of Ilt before and after firing at 900°C.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-6743297/v1/12fdd5a4474e3115f8e0d109.png"},{"id":85327633,"identity":"8d4a107f-344f-4e52-ba32-b023f6d8c41f","added_by":"auto","created_at":"2025-06-24 16:58:23","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":1085386,"visible":true,"origin":"","legend":"\u003cp\u003ePore evolution in 35/50 formulation fired at 900°C (a), 1050°C (b), 1200°C (c), and 1350°C (d); Clay mineral matrix in 35/50 fired at 900°C (e and i), 1050°C (f and j), 1200°C (g and k), and 1350°C (h and l); Appearance of 72 fired brick buttons (m); black spot on 1200°C fired 35/0 brick and corresponding XRD pattern (n); cracks evolution on 1350°C fired 20/0, 35/0 and 50/0 bricks (o); closed, rounded-shape pores in 50/0/1350 sample (p). Legends: Qz (quartz), Hem (hematite), and Hc (hercynite).\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-6743297/v1/7f861c87c38e950c522e66a7.png"},{"id":85327632,"identity":"b3d301e8-b0ec-48c5-af95-acca49954f5b","added_by":"auto","created_at":"2025-06-24 16:58:23","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":464413,"visible":true,"origin":"","legend":"\u003cp\u003eQuantitative analysis of mullite, amorphous phase, and quartz in relation to the Kln/(Kln+Ilt) ratio and total clay mineral content: (a, d, g) vs. Kln/(Kln+Ilt) ratio; (b, e, h) vs. total clay content when Kln is the sole clay mineral; (c, f, i) vs. total clay content when Ilt is the sole clay mineral. (j) \u003cem\u003eIn-situ\u003c/em\u003e XRD setup; (k) 2D \u003cem\u003ein-situ\u003c/em\u003e XRD patterns of 35/50 formulation heated between 25-1200°C. Notable phase changes include the dehydroxylation of kaolinite by 550°C, and of illite between 600-700°C, with crystal structure of illite fully destroyed above 1000°C. Hematite and mullite begin to form above 650°C and 975°C, respectively. Plagioclase and anatase phases disappear above 925°C and 1125°C, respectively.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-6743297/v1/fd3c3ef60678282159e6a8db.png"},{"id":85327631,"identity":"40382396-290e-4f10-8574-b1e0d390aa05","added_by":"auto","created_at":"2025-06-24 16:58:23","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":639308,"visible":true,"origin":"","legend":"\u003cp\u003eWater absorption of bricks fired at 900℃ (a), 1050℃ (b), 1200℃ (c), and 1350℃ (d) with respect to Kln/(Kln+Ilt) ratio and and total clay mineral content; Bulk density of bricks fired at 900℃ (e), 1050℃ (f), 1200℃ (g), and 1350℃ (h) with respect to the Kln/(Kln+Ilt) ratio. Cyan, blue, and green represent 20%, 35%, and 50% total clay mineral contents, respectively; Thermogravimetric analyses of Hp, Ilt, and Kln (i); Reduction of large, closed pores from 35/0/1350 (j) to 35/0/1350 (k) and 35/0/1350 (l).\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-6743297/v1/a30378a80d409679506937dc.png"},{"id":85327951,"identity":"fdce889c-09ea-4afb-9c20-4a9716d10c68","added_by":"auto","created_at":"2025-06-24 17:06:23","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":590336,"visible":true,"origin":"","legend":"\u003cp\u003eCS of bricks fired at 900℃ (a), 1050℃ (b), 1200℃ (c), and 1350℃ (d) with respect to Kln/(Kln+Ilt) ratio; Evolution of CS with firing temperature at 20% (e), 35% (f), and 50% (g) clay mineral contents; XRD patterns of cristobalite in 1050℃ and 1200℃ fired 20/0 (h) and 20/100 bricks (i). Contour map of CS versus total clay mineral content and Kln/(Kln+Ilt) ratio at 900℃ (j), 1050℃ (k), 1200℃ (l), and 1350℃ (m).\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-6743297/v1/5061c01b0fa846932f4f9b3f.png"},{"id":85328662,"identity":"2c23c08b-64f9-4415-bf35-35a2b6cb9216","added_by":"auto","created_at":"2025-06-24 17:14:23","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":576562,"visible":true,"origin":"","legend":"\u003cp\u003eProposed optimal clay formulations for enhanced CS of bricks fired across 900-1350°C, with underlying mechanistic insights.\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-6743297/v1/ee949935241582836960b1b9.png"},{"id":85327635,"identity":"4b9c21cb-afaa-4d4a-a2a5-9ce6b85f89d9","added_by":"auto","created_at":"2025-06-24 16:58:23","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":440311,"visible":true,"origin":"","legend":"\u003cp\u003e3D distribution and corresponding 2D projections of bulk density, WA, and CS (a); amorphous content, mullite content, and CS (b). Dashed lines indicate linear fitting; Pearson’s correlation coefficients between CS and other properties at independent firing temperature (c); Pearson’s correlation coefficients among various technical properties over the whole range of 900-1350°C (d).\u003c/p\u003e","description":"","filename":"7.png","url":"https://assets-eu.researchsquare.com/files/rs-6743297/v1/579f8b719768649ad5531666.png"},{"id":85328836,"identity":"fd1d3b83-7165-42c1-906d-c8ba4ee3939d","added_by":"auto","created_at":"2025-06-24 17:22:26","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":4694813,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-6743297/v1/8ab5212d-9f1b-4400-875d-30f87ae86703.pdf"},{"id":85327640,"identity":"b1db1d68-13a2-4741-89c0-47a8c141b554","added_by":"auto","created_at":"2025-06-24 16:58:23","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":3512231,"visible":true,"origin":"","legend":"From Ancient Practice to Modern Innovation: Solving the Clay Mineral Puzzle in Brickmaking","description":"","filename":"SupportingInformation25May2025.docx","url":"https://assets-eu.researchsquare.com/files/rs-6743297/v1/f4b98b90fd34d6d3a5c70980.docx"},{"id":85327630,"identity":"f6ee2953-2a84-4958-8816-f93c4006e872","added_by":"auto","created_at":"2025-06-24 16:58:23","extension":"docx","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":120754,"visible":true,"origin":"","legend":"","description":"","filename":"GraphicAbstract.docx","url":"https://assets-eu.researchsquare.com/files/rs-6743297/v1/072a6b3697aa203063025d41.docx"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"From Ancient Practice to Modern Innovation: Solving the Clay Mineral Puzzle in Brickmaking","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eFired bricks have been manufactured for over 6000 years,\u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e yet low mechanization in traditional brickmaking significantly limited production efficiency and consistency, and the complexity of brickmaking clays, which comprises clay minerals, quartz, feldspars, carbonates, organic matter, Fe and/or Al oxyhydroxide, etc.,\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e, \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u003c/sup\u003e made it difficult to understand underlying material science of brick formation. Modern factories have adopted high-tech machinery - crusher, mixers, extruders, and temperature control systems - to ensure efficient and consistent production. Additionally, the commercialization of analytical tools like X-ray diffraction (XRD) and X-ray fluorescence (XRF) enabled detailed analysis of the composition and thermal behavior of clays. Tools like the SiO\u003csub\u003e2\u003c/sub\u003e-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-MgO and SiO\u003csub\u003e2\u003c/sub\u003e-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-CaO phase diagrams now allow researchers to map high-temperature mineral transformations more accurately.\u003csup\u003e\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e\u003c/sup\u003e Thus, a comprehensive understanding of the relationship between raw materials and final brick properties has only recently emerged.\u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e, \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u003c/sup\u003e However, despite these advances, defect rates in current brick production remain stubbornly high - approximately 3\u0026ndash;5% even in top-performing production lines.\u003csup\u003e\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e, \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e\u003c/sup\u003e These defects, including cracks, spalling, chips, efflorescence, etc. translate into substantial economic losses, given the annual production of over 1,500\u0026nbsp;billion bricks.\u003csup\u003e\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u003c/sup\u003e More critically, defective bricks contribute to significant energy waste and environmental pollution due to the increased CO\u003csub\u003e2\u003c/sub\u003e emissions associated with carbonate decomposition.\u003csup\u003e\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e, \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003cp\u003eA major source of these defects is the inherent variability in raw clays, driven by weathering processes that differ across regions and even within a single mining site.\u003csup\u003e\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e, \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e\u003c/sup\u003e This variability is further amplified by the gap between scientific research and industrial practice. Most existing studies are based on region-specific clays subjected to varied firing regimes, with limited use of controlled experimental designs and systematic compositional gradients.\u003csup\u003e\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e, \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003e This narrow focus makes manufacturers struggle to extract practical, locally adaptable guidelines from the current body of scientific knowledge. Consequently, brick research and production still echo ancient trial-and-error methods without innovative breakthroughs.\u003c/p\u003e \u003cp\u003eTo date, the most pressing challenge in brick industry is establishing a universal understanding of how different components in clay influence the final properties of bricks. However, precisely tuning these components remains challenging due to their inherent coexistence in natural clay. In addition, clay minerals warrant special attention owing to their diversity and pivotal roles like imparting essential plasticity and vitrification upon firing to densify the brick body.\u003csup\u003e\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e\u003c/sup\u003e In 1992, Dunham \u003csup\u003e\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u003c/sup\u003e systematically investigated brick-making clays in the UK, classifying clay minerals into six categories and identifying two primary variations: the kaolinite/illite (Kln/Ilt) ratio and the quartz/total clay mineral ratio. These ratios are particularly significant because Kln and Ilt are considered two most common clay minerals in ceramic clays,\u003csup\u003e\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e\u003c/sup\u003e and their proportional variations have been widely documented from brickmaking in Spain,\u003csup\u003e\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e\u003c/sup\u003e Italy,\u003csup\u003e\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e\u003c/sup\u003e Australia,\u003csup\u003e\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003e Czechia,\u003csup\u003e\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e\u003c/sup\u003e and Brazil.\u003csup\u003e\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003cp\u003eDespite the recognized importance and wide compositional variability Kln and Ilt, there remains a surprising lack of studies directly linking them to critical brick properties. Limited research has explored their influence on mullite and cristobalite formation,\u003csup\u003e\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e\u003c/sup\u003e and the contributions of Ilt-rich clay to brick bulk density and Young\u0026rsquo;s modulus.\u003csup\u003e\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e\u003c/sup\u003e To date, no definitive conclusions have been reached regarding how Kln and Ilt affect the thermal behavior of clay mixes and key technological performance of final bricks. This lack of fundamental insights not only significantly limits innovation in the brick industry but also increases the risk of producing defective bricks due to mismatched raw materials and processing conditions.\u003c/p\u003e \u003cp\u003eThis study aims to complete a critical piece of the brick manufacturing puzzle by systematically revealing the roles of clay minerals in brick production. We designed a Kln-Ilt-Quartz-Feldspar (KIQF) system that for the first time, allows flexible tailoring in types, proportions, and characteristics of clay mineral in brick raw materials. Gradient variations of Kln and Ilt were obtained using KIQF, and a total of 60 different brick formulations were prepared across a wide firing temperature range. The impacts of Kln and Ilt on phase composition, microstructure, color, and the physical and mechanical properties of fired bricks were comprehensively assessed. Specifically, optimal clay mix formulations at different firing temperatures were proposed and the underlying mineralogical mechanisms were elucidated. Quantitative correlation analyses presented new insights into the relationships among phase compositions, porosity, and strength of fired bricks. These findings offer practical guidance for improving product consistency and mechanical performance in industrial brick manufacturing.\u003c/p\u003e"},{"header":"2. Results","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1. Design of KIQF System\u003c/h2\u003e \u003cp\u003ePrecisely tuning clay minerals is a critical first step in understanding their roles in brickmaking but this remains a major challenge and has rarely been reported.\u003csup\u003e\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003e This difficulty stems from the commonly coexistence of clay minerals, which exhibit similar particle sizes and overlapping structural characteristics, making separation and independent manipulation extremely difficult.\u003csup\u003e\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e, \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e, \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e\u003c/sup\u003e In addition, as clay minerals are usually formed through the weathering of different rocks, naturally occurring clay minerals with exceptionally high purity are rare.\u003csup\u003e\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e, \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003cp\u003eTo address this challenge, we designed a KIQF system composed of three adjustable ingredients: heavy particles (Hp), Kln, and Ilt, through a multi-stage purification strategy. KIQF enables precise control of different clay minerals, providing a fundamentally new design concept that enables systematic differentiation of their individual contributions. Hp fraction was derived from industrial brick-making clay in Queensland, Australia, with its clay mineral components removed through purification (details provided in \u003cb\u003eSupplementary Note 1\u003c/b\u003e). Hp represents the non-clay components typically present in brick raw materials. In addition, Kln and Ilt are high-purity natural minerals, further refined in the laboratory to enhance their purity, as described in \u003cb\u003eSupplementary Note 2\u003c/b\u003e. XRD analysis on the powder and clay sides from fine fraction of powder samples are shown in \u003cb\u003eFigure \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e\u003c/b\u003e and \u003cb\u003eS2\u003c/b\u003e. Rietveld refinement demonstrates purity of Kln and Ilt at 93.5% and 93.9%, respectively, while Hp consists of quartz (65.8%), feldspar (2.57%), hematite, goethite, and minor clay minerals (\u003cb\u003eFigure S3\u003c/b\u003e, Supporting Information).\u003c/p\u003e \u003cp\u003eTo further characterize the raw materials, morphological analyses and particle size distribution were performed. SEM images reveal that both Kln and Ilt exhibit layered stacking, with Ilt showing an irregular plate-like morphology and Kln presenting a pseudohexagonal shape (\u003cb\u003eFigure S4\u003c/b\u003e, Supporting Information). According to laser particle size analysis (\u003cb\u003eFigure S5\u003c/b\u003e, Supporting Information), the volume-weighted mean particle size (D\u003csub\u003e[4,3]\u003c/sub\u003e) of Hp is 460.00 \u0026micro;m, whereas Kln and Ilt display closely comparable values of 5.75 \u0026micro;m and 5.28 \u0026micro;m, respectively, supporting good particle size interchangeability.\u003c/p\u003e \u003cp\u003eTo comprehensively capture the variability of Kln and Ilt contents encountered in practical brick production and fully elucidate their roles, a broad compositional and temperature space was systematically established. Three independent variables were set: 1) total clay mineral content, calculated as (Kln\u0026thinsp;+\u0026thinsp;Ilt)/(Kln\u0026thinsp;+\u0026thinsp;Ilt\u0026thinsp;+\u0026thinsp;Hp), at 20%, 35%, and 50%; 2) Kln-to-clay mineral ratio, calculated as Kln/(Kln\u0026thinsp;+\u0026thinsp;Ilt), at 0%, 33.3%, 50%, 66.6%, and 100%; and 3) firing temperatures at 900\u0026deg;C, 1050\u0026deg;C, 1200\u0026deg;C, and 1350\u0026deg;C. It is important to note that the variability within and between geological formations of brick clays primarily arises from differences in Kln/Ilt and quartz/total clay mineral ratios, making their study particularly relevant.\u003csup\u003e\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u003c/sup\u003e In addition, the wide temperature range was selected to span both low-temperature energy efficiency and high-temperature strength optimization typical of industrial brick firing. Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e summarizes the 15 clay mixes prepared. With each composition subjected to four firing temperatures, a total of 60 brick formulations were generated. To the best of our knowledge, such a comprehensive and systematic exploration of raw material variations has not yet been reported in existing literature.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eFormulations of clay mixes.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"3\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eClay mix ID\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003e(Kln\u0026thinsp;+\u0026thinsp;Ilt)/(Kln\u0026thinsp;+\u0026thinsp;Ilt\u0026thinsp;+\u0026thinsp;Hp) (%)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eKln/(Kln\u0026thinsp;+\u0026thinsp;Ilt) (%)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e20/0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e20\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e20/33\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e20\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e33\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e20/50\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e20\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e50\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e20/66\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e20\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e66\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e20/100\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e20\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e100\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e35/0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e35\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e35/33\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e35\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e33\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e35/50\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e35\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e50\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e35/66\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e35\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e66\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e35/100\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e35\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e100\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e50/0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e50\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e50/33\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e50\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e33\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e50/50\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e50\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e50\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e50/66\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e50\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e66\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e50/100\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e50\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e100\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2. Thermal Expansion and Shrinkage of Clay Mixes\u003c/h2\u003e \u003cp\u003ePrior to examining the properties of fired bricks, thermal shrinkage/expansion of clay mixtures were investigated as these processes dictate porosity evolution and sintering - both significantly affecting the final physical and mechanical performance of bricks. Across 25-1350\u0026deg;C, thermal shrinkage/expansion can be divided into five distinct stages (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea, \u003cb\u003eFigure S6\u003c/b\u003e, Supporting Information).\u003csup\u003e\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e\u003c/sup\u003e Notably, at a fixed total clay mineral content, variations in the Kln/(Kln\u0026thinsp;+\u0026thinsp;Ilt) ratio only affect the final three stages (above 500\u0026deg;C).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eIn stage 3, brick expansion decreases from 0.64\u0026ndash;0.15% as the Kln/(Kln\u0026thinsp;+\u0026thinsp;Ilt) ratio rises from 0 to 100% (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea). This decrease is due to the structural collapse of Kln after dehydroxylation, counteracting the abrupt expansion caused by the \u003cem\u003eα\u003c/em\u003e- to \u003cem\u003eβ\u003c/em\u003e-quartz inversion.\u003csup\u003e\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e, \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e\u003c/sup\u003e This indicates that incorporating Kln can reduce cracking risks in clay bodies between 500\u0026deg;C and 650\u0026deg;C. However, Ilt does not exhibit this effect because its crystal structure remains after dehydroxylation. It is reported that Ilt continuously expand upon firing below 1000℃,\u003csup\u003e\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e\u003c/sup\u003e aligning with the expansion observed in the 35/0 sample below 900℃ (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea).\u003c/p\u003e \u003cp\u003eIn stage 4 (650\u0026ndash;900℃), replacing Ilt with Kln shifts the clay mix from expansion (0.21% for 35/0) to shrinkage (-0.12% for 35/100) (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea), attributed to the structural collapse caused by reorganization of Al coordination in metakaolin in preparation for spinel/mullite formation.\u003csup\u003e\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003cp\u003eIn stage 5 (900\u0026ndash;1350℃), the firing shrinkage decreases from 10.98% (35/0) to 6.27% (35/50) and 3.24% (35/100) with increasing Kln/(Kln\u0026thinsp;+\u0026thinsp;Ilt) ratio. This indicates that compared to Kln, Ilt in the clay mix leads to higher degree of sintering, likely due to the fluxing action of K\u003csup\u003e+\u003c/sup\u003e ions in its interlayer space.\u003csup\u003e\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e, \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e\u003c/sup\u003e X-ray fluorescence test in \u003cb\u003eTable \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e\u003c/b\u003e (Supporting Information) confirmed that Ilt, with a combined K, Na, Mg, and Ca oxide content of 10.79%, is a high-fluxing material, while Kln and Hp exhibit much lower levels of fluxing agents, at 0.64% and 2.49%, respectively. In addition, under scanning electron microscopy observation, pure Ilt melts at 1050℃, while Kln partially reserves its morphology even after 1200℃ firing (\u003cb\u003eFigure S7\u003c/b\u003e, Supporting Information, Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb), further demonstrating the higher vitrification capacity of Ilt than Kln.\u003c/p\u003e \u003cp\u003eApart from the thermal dilatometry, the dimensional change of brick buttons after firing were also examined. Green bodies show an expanding trend after 900\u0026deg;C (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ec), with the expansion value decreasing as the Kln/(Kln\u0026thinsp;+\u0026thinsp;Ilt) ratio increases. At 1050\u0026deg;C and 1200\u0026deg;C, higher Kln/(Kln\u0026thinsp;+\u0026thinsp;Ilt) ratios result in reduced firing shrinkage due to the lower vitrification capacity of Kln compared to Ilt (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ed and \u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ee). Interestingly, bricks made from Ilt and Hp exhibit approximately 10% expansion at 1350\u0026deg;C instead of shrinkage (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ef), attributed to the so-called \u0026ldquo;overfiring\u0026rdquo;.\u003csup\u003e\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e\u003c/sup\u003e During firing, open pores typically close as the brick densifies. However, beyond maximum densification, rising gas pressure in fully closed pores can lead to expansion as the temperature continues to rise. Introducing Kln to the clay mix can mitigate this effect, due to the superior refractory properties of Kln compared to Ilt (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb).\u003c/p\u003e \u003cp\u003eIn addition, higher total clay mineral content leads to greater firing shrinkage across most formulations (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ec-\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ef), consistent with the thermal dilatometry results in Figure S6 (Supporting Information). An exception occurs when Kln/(Kln\u0026thinsp;+\u0026thinsp;Ilt)\u0026thinsp;=\u0026thinsp;0 and the firing temperature is 900\u0026deg;C, where increased Ilt content results in expansion rather than shrinkage (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea). This is likely due to irreversible lattice expansion of Ilt following dehydroxylation,\u003csup\u003e\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e\u003c/sup\u003e as evidenced by the increased d\u003csub\u003e(001)\u003c/sub\u003e spacing after firing (inset in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ec).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003e2.3. Physical and Mechanical Properties of Fired Bricks\u003c/h2\u003e \u003cdiv id=\"Sec6\" class=\"Section3\"\u003e \u003ch2\u003e2.3.1. Microstructure and Appearance\u003c/h2\u003e \u003cp\u003eBSE-SEM on fine polished bricks effectively reveals pore distribution and phase interaction, providing critical insights into the physical and mechanical properties. At 35% clay minerals and 1350\u0026deg;C, decreasing the Kln/(Kln\u0026thinsp;+\u0026thinsp;Ilt) ratio results in 1) reduced pores and their connectivity, 2) transition from elongated to rounded-shape pores, and 3) more discrete pore size distribution (\u003cb\u003eFigure S8\u003c/b\u003e, Supporting Information). These changes are primarily attributed to the superior vitrification behavior of Ilt compared to Kln, which enhances pore coalescence and elimination, as well as grain consolidation. Moreover, a distinct evolution of pore characteristics is observed with increasing temperature (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea-\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ed) \u0026minus;\u0026thinsp;900\u0026deg;C pores from loose particle packing; 1050\u0026deg;C and 1200\u0026deg;C elongated pores (\u0026gt;\u0026thinsp;100 \u0026micro;m) from differential shrinkage of Kln, Ilt, and quartz particles; 1350\u0026deg;C closed and rounded pores from substantial liquid-phase formation - reflecting the progressive sintering and densification of the brick structure.\u003csup\u003e\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e, \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eIn addition to pore evolution, EDS mapping reveals the contact relationships between different phases (\u003cb\u003eFigure S9\u003c/b\u003e, Supporting Information). The evolution of clay mineral matrix was also observed with firing temperature. At 900\u0026deg;C and 1050\u0026deg;C, the matrix comprises loosely packed Kln and Ilt with clearly defined phase boundaries (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ee-\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ef and \u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ei-\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ej). EDS analysis confirms that Kln is Al-rich, while Ilt contains higher concentrations of K, Fe, and Mg (\u003cb\u003eFigure S10\u003c/b\u003e, Supporting Information). Both phases retain their layered morphology, indicating limited vitrification. At 1200\u0026deg;C, a partially vitrified matrix forms (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ec), incorporating fine, poorly crystallized mullite (\u0026lt;\u0026thinsp;2 \u0026micro;m) (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eg and \u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ek). Some residual lamellar structures remain, with an Al: Si ratio of ~\u0026thinsp;1: 1, suggesting they are unreacted Kln (\u003cb\u003eFigure S11\u003c/b\u003e, Supporting Information). Upon firing to 1350\u0026deg;C, the microstructure transitions to a highly vitrified and homogeneous glassy matrix (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ed), accompanied by pore closure and reduction in size (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eh). Well-developed needle-shaped mullite crystals emerge, some exceeding 10 \u0026micro;m (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003el). EDS analysis shows an Al: Si ratio of ~\u0026thinsp;3:2 in these areas (\u003cb\u003eFigure S12\u003c/b\u003e, Supporting Information), indicative of secondary mullite formation promoted by the presence of alkali fluxes.\u003csup\u003e\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003cp\u003eIn terms of appearance, dried bricks shift their colors from oyster to neutral grey, becoming lighter at higher clay mineral contents (\u003cb\u003eFigure S13\u003c/b\u003e, Supporting Information). For fired bricks, color deepens with temperature, ranging from white at 900\u0026deg;C to mushroom pink at 1050\u0026deg;C, cloud grey at 1200\u0026deg;C, and dark brown at 1350\u0026deg;C (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003em). This darkening is due to the increased crystal size of hematite with temperature (\u003cb\u003eFigure S14\u003c/b\u003e, Supporting Information).\u003csup\u003e\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003e In addition, higher clay mineral content results in lighter colors, attributed to the lower Fe content in Kln and Ilt compared to Hp. Decreasing the Kln/(Kln\u0026thinsp;+\u0026thinsp;Ilt) ratio also deepens the brick color, possibly because the higher fluidity of the Ilt-based glass matrix facilitates the growth of larger hematite crystals.\u003c/p\u003e \u003cp\u003eInterestingly, surface defects such as black spots and cracks emerge on Hp-Ilt bricks fired at 1200\u0026deg;C or higher (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003en and \u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eo). XRD analysis reveals that the black spot consists of hercynite (FeAl\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e), hematite (Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e) and quartz. According to Laita, Bauluz \u003csup\u003e\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e\u003c/sup\u003e, hercynite can evolve from Fe particles at high temperatures, partially replacing hematite crystals. In this experiment, the hercynite-rich exudate increases with both Ilt and Hp content. Their presence is due to the Fe-Al interaction in the low vitreous glassy matrix resulting from intensive melt of Ilt. In addition, cracks gradually develop with increasing Ilt content in the starting clay (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eo), likely due to enhanced liquid-phase sintering, which produces large, closed pores that expand during heating and generate internal stress within the brick (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ep).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec7\" class=\"Section3\"\u003e \u003ch2\u003e2.3.2. Phase Composition and In-situ High-temperature XRD\u003c/h2\u003e \u003cp\u003eAs a critical factor in determining brick performance, a quantitative investigation of phase composition is essential. In the absence of Ca and Mg, the primary phases in bricks are quartz, mullite, and amorphous phases.\u003csup\u003e\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e, \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e\u003c/sup\u003e Quantitative XRD analyses of these phases were performed through Rietveld refinement (\u003cb\u003eFigure S15-S24\u003c/b\u003e, Supporting Information), with results presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea\u0026ndash;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ei. In addition, \u003cem\u003ein-situ\u003c/em\u003e high-temperature XRD was employed to reveal the real-time phase transitions of clay mix fired up to 1200\u0026deg;C (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ej and \u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ek).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003cem\u003eMullite Formation\u003c/em\u003e:\u003c/p\u003e \u003cp\u003eMullite forms through the decomposition of clay minerals, with a formation temperature at 975\u0026deg;C (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ek). Previous study suggest that fluxing agents like K and Na enhance the fluidity of Si-rich amorphous phases, promoting greater mullite crystal growth.\u003csup\u003e\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e\u003c/sup\u003e However, our experiment reveals that mullite formation is a complex process influenced by fluxing agents, the Al/Si ratio of clay minerals, and firing temperature:\u003c/p\u003e \u003cp\u003e \u003col\u003e \u003cspan\u003e \u003cli\u003e \u003cp\u003eAt low firing temperatures (900\u0026deg;C): Mullite is primarily observed in Ilt-rich samples (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea). The presence of Kln in the starting material promotes the formation of Al-spinel instead of mullite at this temperature, as confirmed by previous TEM observations.\u003csup\u003e\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003c/li\u003e \u003c/span\u003e \u003cspan\u003e \u003cli\u003e \u003cp\u003eAt 1050\u0026deg;C: Maximum mullite content (10.19%) occurs when Kln and Ilt coexist (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea). Compared to Ilt with an Al: Si of 1: 2, the Al: Si ratio of Kln (1: 1) is closer to that of mullite (3: 2 or 2: 1), promoting mullite crystallization. However, the highly viscose Si-rich amorphous phase from Kln decomposition simultaneously limits mullite growth. As for Ilt, although the highly fluid amorphous phase favors mullite formation, its lower Al: Si ratio (2: 1) hinders it. Therefore, a combination of Kln and Ilt yields the highest mullite content (10.19%) at 1050℃. Moreover, the effect of fluxing agents appears more significant than the Al: Si ratio, as evidenced by the lower mullite in the 35/100/1050 brick (1.92%) compared to the 35/0/1050 brick (7.22%) (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea).\u003c/p\u003e \u003c/li\u003e \u003c/span\u003e \u003cspan\u003e \u003cli\u003e \u003cp\u003eAt 1200\u0026deg;C and 1350\u0026deg;C: Both Kln and Ilt melt extensively (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb and Figure S6, Supporting Information), making the Al: Si ratio more critical than the fluxing agent for mullite formation. This explains why higher Kln increases mullite, reaching a maximum at 38.77% for 50/100/1350 (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea).\u003c/p\u003e \u003c/li\u003e \u003c/span\u003e \u003c/ol\u003e \u003c/p\u003e \u003cp\u003eAdditionally, while mullite forms from the decomposition of clay minerals, a higher total clay mineral content in the starting clay does not always guarantee more mullite in the final bricks: when Kln is the dominant clay mineral, mullite content increases with clay mineral content above 1200\u0026deg;C (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb); in contrast, when Ilt is the primary clay mineral, this increase occurs at a lower temperature of 900\u0026deg;C (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ec).\u003c/p\u003e \u003cp\u003e \u003cem\u003eAmorphous Phase\u003c/em\u003e:\u003c/p\u003e \u003cp\u003eAmorphous phase evolution inversely correlates with mullite because both phases derive from decomposed clay minerals (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ed) - amorphous phase comes from Si-rich components, while mullite forms from Al-rich components. Thus, the changes in amorphous content with both Kln/(Kln\u0026thinsp;+\u0026thinsp;Ilt) and clay mineral content are opposite to those of mullite (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ed-\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ef). Note that the decrease in amorphous content above 1200\u0026deg;C may also result from cristobalite formation from amorphous Si. In summary, both the formation of mullite and amorphous are significantly affected by initial Kln/(Kln\u0026thinsp;+\u0026thinsp;Ilt) and clay mineral content, highly dependent on firing temperature.\u003c/p\u003e \u003cp\u003e \u003cem\u003eQuartz\u003c/em\u003e:\u003c/p\u003e \u003cp\u003eUnlike mullite and amorphous phase, quartz content remains relatively stable across varying Kln/(Kln\u0026thinsp;+\u0026thinsp;Ilt) ratios (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eg-\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ei). A specific interpretation on quartz variation among different formulations can be seen in \u003cb\u003eSupplementary Note 3\u003c/b\u003e.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec8\" class=\"Section3\"\u003e \u003ch2\u003e2.3.3. Water Absorption and Bulk Density\u003c/h2\u003e \u003cp\u003eWater absorption (WA) and bulk density are critical parameters that significantly influence the structural integrity, durability, and overall performance of obtained brick.\u003csup\u003e\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e\u003c/sup\u003e The effect of Kln/(Kln\u0026thinsp;+\u0026thinsp;Ilt) on both WA and bulk density is strongly temperature dependent. At 900\u0026deg;C, the increase of Kln/(Kln\u0026thinsp;+\u0026thinsp;Ilt) slightly reduces WA (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea), primarily due to the higher firing shrinkage of Kln compared to Ilt (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea), though the overall difference in WA is relatively minor. At 1050\u0026deg;C and 1200\u0026deg;C, a higher Kln proportion leads to increased WA (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb and \u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ec), as the weaker vitrification of Kln relative to Ilt limits pore closure and retains more interconnected porosity. At 1350\u0026deg;C, the 50/50 formulation achieves the lowest WA value (1.30%) (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ed), indicating superior sintering performance. In addition, detailed variation of WA with starting clay mineral content is discussed in \u003cb\u003eSupplementary Note 4\u003c/b\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eRegarding the bulk density, the obtained value varies from 1.62 g/cm\u0026sup3; to 2.23 g/cm\u0026sup3; (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ee-\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eh) across 60 formulations. At 900\u0026deg;C, bulk density decreases as the Kln/(Kln\u0026thinsp;+\u0026thinsp;Ilt) ratio increases, ascribed to the higher thermal mass loss of Kln compared to Ilt (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ei). The reduction in bulk density at 1050\u0026deg;C and 1200\u0026deg;C is attributed to both a lower mass and an increased dimension of brick (see the reduced firing shrinkage with Kln/(Kln\u0026thinsp;+\u0026thinsp;Ilt) in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ed and \u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ee). At 1350\u0026deg;C, bulk density first increases with Kln/(Kln\u0026thinsp;+\u0026thinsp;Ilt) ratio due to reduced amount of large, closed pores (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ej-l), then declines after Kln/(Kln\u0026thinsp;+\u0026thinsp;Ilt) exceeds 50% as brick mass decreases and firing shrinkage remains stable (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ef). In addition, the bulk density of bricks correlates closely with total clay mineral content. An increase in the clay mineral content leads to lower bulk density at 900\u0026deg;C regardless of the Kln-to-Ilt ratio (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ee). This is due to the higher thermal mass loss of both Kln and Ilt compared to Hp as shown in the TGA analysis (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ei). At 1050\u0026deg;C and 1200\u0026deg;C, Ilt-rich clays tend to increase bulk density, resulting from greater firing shrinkage, while Kln-rich clays lead to a lower density with clay mineral content because of higher mass loss and reduced firing shrinkage (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ef and \u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eg). At 1350℃, the considerably low bulk density for formulations with Kln/(Kln\u0026thinsp;+\u0026thinsp;Ilt)\u0026thinsp;=\u0026thinsp;0 is attributed to the expansion of the brick body by the formation of large, closed pores (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eh).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec9\" class=\"Section3\"\u003e \u003ch2\u003e2.3.4. Compressive Strength\u003c/h2\u003e \u003cp\u003eCompressive strength (CS) is crucial as fired bricks are primarily designed to withstand significant compressive loads.\u003csup\u003e\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e\u003c/sup\u003e The relationship between CS and Kln/(Kln\u0026thinsp;+\u0026thinsp;Ilt) ratio varies across 900\u0026ndash;1350℃. At 900℃, there is a positive correlation between CS and Kln/(Kln\u0026thinsp;+\u0026thinsp;Ilt) (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ea). Higher Kln content results in greater firing shrinkage due to the structural collapse and reorganization of Kln after dehydroxylation (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea and Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea), increasing the obtained CS. In contrast, Ilt expands after dehydroxylation, producing porosity and reducing CS.\u003csup\u003e\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e\u003c/sup\u003e At 1050℃ and 1200℃, a negative correlation between CS and Kln/(Kln\u0026thinsp;+\u0026thinsp;Ilt) is observed (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eb and \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ec). This is due to the higher degree of vitrification of Ilt than Kln, which enhances brick densification, reducing porosity and increasing CS. At 1350℃, the relationship between CS and Kln/(Kln\u0026thinsp;+\u0026thinsp;Ilt) becomes more complex (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ed). A combination of Kln and Ilt is required for optimal CS. Generally, Ilt promotes liquid sintering, reducing porosity and enhancing particle-bonding, but excessive Ilt leads to cracks and the formation of large pores (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eo and Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ej). As for Kln, while it increases mullite formation, its low level of vitrification weakens the liquid sintering and elevates obtained porosity (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb). Consequently, the highest CS at 1350℃ is achieved in formulations where Ilt and Kln coexist, i.e. 20/50 (163.5 Mpa), 35/33 (172.4 Mpa), and 55/33 (166.8 Mpa).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eRegarding the effect of total clay mineral content: Higher clay mineral content leads to lower CS at 900℃. This is due to the looser packing of fine clay mineral particles, which increases the initial porosity (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ei, Figure S10, Supporting Information). At 1050\u0026deg;C and 1200\u0026deg;C, the vitrification of clay minerals densifies the bricks, so the higher clay mineral contents, the higher CS values (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eb and \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ec). However, at 1350\u0026deg;C, formulations with Kln/(Kln\u0026thinsp;+\u0026thinsp;Ilt) ratios\u0026thinsp;\u0026ge;\u0026thinsp;50% exhibit reduced CS when clay mineral content increases (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ed). This is attributed to the higher number of initial pores introduced by the addition of clay minerals, which fail to completely close due to the high Kln content. Additionally, at 900\u0026ndash;1200\u0026deg;C, CS is more sensitive to the Ilt content than to the Kln content (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ea-\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ec). A significant reduction in CS is observed with increasing Ilt content at 900\u0026deg;C (from 20/0 to 50/0), while a notable increase occurs at 1050\u0026deg;C and 1200\u0026deg;C. In contrast, CS remains consistent with total clay mineral content when Kln dominates the clay mineral.\u003c/p\u003e \u003cp\u003eFurthermore, most formulations show increased CS at higher firing temperatures (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ee-\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eg), in line with existing literature.\u003csup\u003e\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e, \u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e\u003c/sup\u003e Two exceptions are: 1) the 50/0 formulation shows reduced CS at 1350\u0026deg;C compared to 1200\u0026deg;C, likely due to the formation of cracks from excessive liquid sintering (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eo); 2) for bricks with 20% total clay mineral content, lower CS is observed when increasing the temperature from 1050\u0026deg;C to 1200\u0026deg;C (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ee). This is attributed to the high quartz content in the starting clays. Quartz typically transforms into cristobalite above 1200\u0026deg;C. Upon cooling, the formed cristobalite undergoes transition from \u003cem\u003eβ\u003c/em\u003e- to \u003cem\u003eα\u003c/em\u003e-phase between 280\u0026thinsp;\u0026minus;\u0026thinsp;190\u0026deg;C, resulting in ~\u0026thinsp;5% volumetric shrinkage. This transformation may induce microcracks within the matrix, thereby compromising its structural integrity.\u003csup\u003e\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e\u003c/sup\u003e The considerable increase in cristobalite is confirmed by XRD in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eh and \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ei. However, no further CS reduction is observed from 1200\u0026deg;C to 1350\u0026deg;C, likely due to enhanced sintering at 1350\u0026deg;C, which imparts a more robust matrix capable of resisting cristobalite phase inversion.\u003c/p\u003e \u003cp\u003eTo better visualize the CS value under different clay mineral assemblages, contour maps for 60 formulations are presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ej-\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003em. Specifically, achieving a high CS (53.6 MPa) at 900\u0026deg;C requires a low total clay mineral content and a high Kln/(Kln\u0026thinsp;+\u0026thinsp;Ilt) ratio. However, the clay mineral content should also be sufficient to ensure proper extrusion in practical brickmaking. At 1050\u0026deg;C and 1200\u0026deg;C, the trend reverses: a high clay mineral content and a low Kln/(Kln\u0026thinsp;+\u0026thinsp;Ilt) ratio improve CS, reaching peaks of 126.2 MPa and 142.4 MPa, respectively. For bricks fired at 1350\u0026deg;C, achieving a high CS (172.4 MPa) requires avoiding extreme values for both clay mineral content and the Kln/(Kln\u0026thinsp;+\u0026thinsp;Ilt) ratio. These results demonstrate that by precisely adjusting the initial clay minerals, the brick CS can be significantly enhanced - reaching 3, 7, 8, and 10 times the minimum required compressive strength (17.2 MPa) for Grade Moderate Weathering (MW) bricks, as specified by ASTM C62.\u003csup\u003e\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e\u003c/sup\u003e The recommended optimal clay formulations for enhanced CS at different firing temperatures are illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv id=\"Sec10\" class=\"Section2\"\u003e \u003ch2\u003e2.4. The Importance of Firing Temperature\u003c/h2\u003e \u003cp\u003eAccording to the above analyses, key properties of fired bricks are strongly influenced by starting clay minerals, with these effects varying significantly with firing temperature. This variability is primarily due to the differing responses of Kln and Ilt to heating, governed by their crystal structures (1: 1 or 2: 1) and chemical compositions (Al: Si ratio and fluxing agent concentration).\u003c/p\u003e \u003cp\u003eSince in most brick manufacturing scenarios, where Kln coexists with Ilt, and firing temperatures are around 1050\u0026deg;C or 1200\u0026deg;C,\u003csup\u003e\u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e\u003c/sup\u003e the lower vitrification degree of Kln compared to Ilt results in lighter brick colors. This difference in vitrification also necessitates the coexistence of Kln and Ilt to achieve a high mullite content at 1050\u0026deg;C, as discussed in Section \u003cspan refid=\"Sec7\" class=\"InternalRef\"\u003e2.3.2\u003c/span\u003e. However, vitrification no longer limits mullite formation from metakaolin at 1200\u0026deg;C.\u003c/p\u003e \u003cp\u003eFor applications requiring lower bulk density (e.g., for easier transport), at 1050\u0026deg;C and 1200\u0026deg;C, increasing the Kln content can be beneficial, although this compromises the durability of bricks by increasing porosity. Conversely, if higher bulk densities are required (enhancing durability and strength), a higher Ilt content in the starting clay is recommended. Additionally, to prevent the melting of hercynite and hematite on the brick surface, particularly when Hp contains\u0026thinsp;\u0026ge;\u0026thinsp;6.5% of Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e, the clay mineral may need to exceed 35%. Specific considerations also apply at the two less commonly used firing temperatures of 900\u0026deg;C and 1350\u0026deg;C. At 900\u0026deg;C, Ilt fails to vitrify, weakening brick strength and making it unsuitable as a primary starting clay. At 1350\u0026deg;C, optimal strength is achieved with an Ilt-to-Kln ratio of either 2: 1 or 1: 1, depending on the total clay mineral content.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003e2.5. Correlations among Diverse Brick Properties\u003c/h2\u003e \u003cp\u003eFiring temperature impacts not only thermal evolutions of raw clays and related brick properties but also the interrelationships among these properties. While prior studies have frequently reported strong linear correlations (R\u003csup\u003e2\u003c/sup\u003e\u0026thinsp;\u0026ge;\u0026thinsp;0.90) - such as a negative relationship between WA and firing shrinkage,\u003csup\u003e\u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e, \u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e\u003c/sup\u003e a positive correlation between CS and bulk density,\u003csup\u003e\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e, \u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e\u003c/sup\u003e a negative correlation between CS and WA,\u003csup\u003e\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e, \u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e, \u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e48\u003c/span\u003e\u003c/sup\u003e and a negative correlation between WA and bulk density\u003csup\u003e\u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e49\u003c/span\u003e\u003c/sup\u003e - the present study did not observe similarly strong relationships after systematically examined 60 different formulations.\u003c/p\u003e \u003cp\u003eThe highest coefficient of determination (R\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e) found in this study is only 0.59 for the CS-WA relationship, followed by 0.34 (CS-amorphous content), 0.10 (CS-mullite content), and 0.08 (CS-bulk density) (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ea and \u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003eb). These values suggest that the strong linear correlations reported may only be applicable within narrow temperature or compositional ranges as adopted in previous studies. In contrast, the broader firing range examined here (900\u0026ndash;1350\u0026deg;C) revealed temperature-dependent variations that weaken these correlations, which was usually overlooked before. For instance, CS and bulk density are nearly uncorrelated at 900\u0026deg;C, likely due to the unique behavior of Kln, which undergoes both significant shrinkage and water loss. This contrasts with the positive correlation between these two properties above 1050\u0026deg;C (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ec). In addition, at 1350\u0026deg;C, the excessive melting of Ilt results in cracks and formation of closed pores, leading to irregular relationships among CS, bulk density, and WA.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eAccording to quantitative analysis, the importance of factors influencing CS ranks as follows: WA (-0.78)\u0026thinsp;\u0026gt;\u0026thinsp;amorphous content (0.51)\u0026thinsp;\u0026gt;\u0026thinsp;bulk density (0.40)\u0026thinsp;\u0026gt;\u0026thinsp;mullite content (0.29) (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ed). These correlations strongly depend on temperature (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ec), peaking mostly at 1200\u0026deg;C, except for mullite content. The unexpectedly low impact of bulk density compared to literature is mainly due to its weak correlation with CS at 900\u0026deg;C.\u003c/p\u003e \u003cp\u003eInterestingly, although mullite is widely acknowledged to strengthen ceramics by forming interlocking acicular crystal networks, refining Griffith flaws, and mismatched thermal expansion coefficient with glass phase matrix,\u003csup\u003e\u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e50\u003c/span\u003e, \u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e51\u003c/span\u003e, \u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e52\u003c/span\u003e, \u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e53\u003c/span\u003e\u003c/sup\u003e we have identified a negative correlation between mullite and CS at 900\u0026deg;C, 1200\u0026deg;C and 1350\u0026deg;C (the correlation coefficient at 1050\u0026deg;C is also quite low at 0.3, Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ec). This may be attributed to that mullite typically forms at the expense of the amorphous/glass phase, which contributes more significantly to CS development by facilitating sintering and particle bonding (see the positive correlation coefficient values in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ec). It can be reasonably inferred that glass matrix is more important to fired bricks than other ceramic products as the clayey materials used in brickmaking are generally coarser in particle size, creating larger initial voids and thus requiring more glass to achieve effective bonding. In fact, phase-related factors (amorphous and mullite contents) only play pronounced role at low temperature of 900\u0026deg;C, while above 1050\u0026deg;C, porosity-related factors (WA and bulk density) are more important (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ec). Nevertheless, brick CS is governed by a combination of factors, including porosity (open and closed), phase composition and contact patterns, the degree of particle bonding (sintering), and the presence of cracks.\u003c/p\u003e \u003c/div\u003e"},{"header":"3. Discussion","content":"\u003cp\u003eThis study introduces a groundbreaking Kaolinite-Illite-Quartz-Feldspar (KIQF) system for precisely adjusting the inherent phase composition of natural clays, thus allowing universal understanding of the roles of different components in brickmaking. Specific focus was put on kaolinite and illite - two most common clay minerals in brick clay. We systematically varied total clay mineral content (20%, 35%, and 50%), kaolinite-to-total clay mineral ratios (0%, 33%, 50%, 66%, and 100%), and firing temperatures (900\u0026deg;C, 1050\u0026deg;C, 1200\u0026deg;C, and 1350\u0026deg;C) and created 60 distinct brick formulations. Comprehensive material characterization revealed that optimal clay mineral assemblages are highly dependent on firing temperature, fundamentally governed by the unique crystalline structures of kaolinite and illite and their temperature-dependent thermal behaviors.\u003c/p\u003e \u003cp\u003eAt 900\u0026deg;C, formulations with 20% clay minerals composed entirely of kaolinite exhibited enhanced compressive strength (53.6 MPa) and reduced water absorption (11.38%), attributed to the shrinkage from dehydroxylation and subsequent structural reorganization of kaolinite. In contrast, at 1050\u0026deg;C and 1200\u0026deg;C, bricks with 50% clay minerals predominantly consisting of illite demonstrated superior strength (126.2 Mpa and 142.4 Mpa), likely due to the abundant K⁺ ions in illite promoting sintering and densification. The highest compressive strength (172.4 MPa) and lowest water absorption (1.3%) were achieved at 1350\u0026deg;C using a formulation with 35% clay minerals, where one-third of the clay minerals were kaolinite, effectively mitigating over-firing issues associated with illite vitrification.\u003c/p\u003e \u003cp\u003eBeyond these findings, an unexpected negative correlation between mullite content and compressive strength is revealed, with Pearson correlation coefficients of -0.89, -0.37, and \u0026minus;\u0026thinsp;0.37 at 900\u0026deg;C, 1200\u0026deg;C, and 1350\u0026deg;C, respectively. This suggests that mullite formation may consume excessive glass phases that are more crucial to brick strength due to enhanced sintering and particle bonding. The formation of mullite and amorphous/glass phases is influenced by clay mineral content, Al: Si ratios, fluxing agents, and firing temperature. Additionally, the widely reported linear relationships among various technological properties of bricks persist only within narrow temperature ranges and deteriorate over the broader 900\u0026ndash;1350\u0026deg;C interval, due to the temperature-sensitive transformations of kaolinite and illite. Quantitative correlation analyses indicated that phase-related factors (amorphous and mullite contents) primarily influence compressive strength at lower temperatures (900\u0026deg;C), while porosity-related factors (water absorption and bulk density) become dominant above 1050\u0026deg;C.\u003c/p\u003e \u003cp\u003eOther notable findings include that increased clay mineral content and higher kaolinite-to-illite ratios result in lighter brick colors because higher fluidity of the illite-based glass facilitates the growth of larger hematite crystals. However, excessive vitrification of illite from the interlayer K⁺ leads to hercynite exudation above 1200\u0026deg;C and the formation of closed, rounded pores at 1350\u0026deg;C, causing defects such as approximately 10% expansion and large cracks.\u003c/p\u003e \u003cp\u003eThrough strategic tuning of clayey raw materials, our study demonstrates that the compressive strength of bricks can achieve 3-, 7-, 8-, and 10-fold over the ASTM Moderate Weather strength requirement at 900\u0026deg;C, 1050\u0026deg;C, 1200\u0026deg;C and 1350\u0026deg;C firing. These insights offer valuable guidance for optimizing complex raw materials in industrial brick manufacturing, and more importantly, provide a framework to ultimately explain the role of different components in determining key brick performance.\u003c/p\u003e"},{"header":"4. Methods","content":"\u003cp\u003e \u003cem\u003eMaterials\u003c/em\u003e \u003c/p\u003e \u003cp\u003eThree raw materials - Hp, Kln, and Ilt, were used. Hp derived from a commercial clay mix supplied by Austral Bricks Pty. Ltd. in Queensland, Australia. Kln and Ilt were supplied by Austral Bricks Pty. Ltd. and Guzhang Shan Lin Shi Yu Mineral Co., Ltd., respectively. Sodium hexametaphosphate (powder, purity of 96%) was purchased from Merck Pty. Ltd.\u003c/p\u003e \u003cp\u003e \u003cem\u003eFabrication Process of Fired Bricks\u003c/em\u003e \u003c/p\u003e \u003cp\u003eThe preparation of clay bricks could be primarily divided into three steps: molding, drying, and firing. First, the required amounts of Hp, Kln, and Ilt (totally 30 grams) were homogenized in an electrical blender (KitchenAid Classic) for 30 mins (Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). After adding 12% of water to the mix, the green bodies were obtained through hydraulic compression and then dried and fired following procedure in \u003cb\u003eFigure S25\u003c/b\u003e.\u003c/p\u003e \u003cp\u003e \u003cem\u003eRaw Materials Characteristics\u003c/em\u003e \u003c/p\u003e \u003cp\u003eThe chemical compositions of Hp, Kln, and Ilt were analyzed using a Bruker S8 TIGER XRF spectrometer with a 4 kW Rh X-ray tube. Before the test, an aliquot of powder sample (0.74g) was fused into a glass disc using a lithium metaborate/lithium tetraborate flux (50: 50 LiT: LiM, 0.5 wt% LiI) in a platinum crucible using an electric fusion furnace (Katanax, model X600). The particle size distributions of Hp, Kln, and Ilt were analyzed using a Mastersizer 3000 laser particle size analyzer with deionized water as the dispersion medium. The volume-weighted mean diameter was calculated using the following equation:\u003c/p\u003e \u003cp\u003e \u003cspan class=\"InlineEquation\"\u003e \u003cspan class=\"mathinline\"\u003e\\(\\:{D}_{\\left[\\text{4,3}\\right]}=\\frac{\\sum\\:{n}_{i}{d}_{i}^{4}}{\\sum\\:{n}_{i}{d}_{i}^{3}}\\)\u003c/span\u003e \u003c/span\u003e (Eq.\u0026nbsp;1)\u003c/p\u003e \u003cp\u003ewhere \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{D}_{\\left[\\text{4,3}\\right]}\\)\u003c/span\u003e\u003c/span\u003e, \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{n}_{i}\\)\u003c/span\u003e\u003c/span\u003e, and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{d}_{i}\\)\u003c/span\u003e\u003c/span\u003e are volume-weighted mean diameter, number diameter of particles in size class, and diameter of particles in size class, respectively. Thermogravimetry/derivative thermogravimetry analyses of Hp, Ilt, and Kln were performed on a Netzsch STA4493F from 25℃ to 1400℃ at a heating rate of 10℃/min in a 50 mL/min airflow.\u003c/p\u003e \u003cp\u003e \u003cem\u003eExpansion/Shrinkage of Clay Mix\u003c/em\u003e \u003c/p\u003e \u003cp\u003eThe expansion/shrinkage of clay mixes during firing were investigated using two methods. Cylindrical specimens (Φ 6 mm \u0026times; 25 mm) were prepared from green bodies and their \u003cem\u003ein-situ\u003c/em\u003e dimensional changes were measured using a Netzsch 402C thermal dilatometer from 25\u0026deg;C to 1350\u0026deg;C at a ramping rate of 5\u0026deg;C/min under static air. Additionally, firing shrinkage of brick buttons was calculated using Eq.\u0026nbsp;2 provided below:\u003c/p\u003e \u003cp\u003e \u003cspan class=\"InlineEquation\"\u003e \u003cspan class=\"mathinline\"\u003e\\(\\:Firing\\:shrinkage\\:=\\frac{\\text{D}-{D}_{0}}{{D}_{0}}\\times\\:100\\%\\)\u003c/span\u003e \u003c/span\u003e (Eq.\u0026nbsp;2)\u003c/p\u003e \u003cp\u003ewhere \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{D}_{0}\\)\u003c/span\u003e\u003c/span\u003e and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:D\\)\u003c/span\u003e\u003c/span\u003e represent the diameter of brick buttons before and after firing, respectively.\u003c/p\u003e \u003cp\u003e \u003cem\u003eIn-situ Phase Evolution and Rietveld Refinement\u003c/em\u003e \u003c/p\u003e \u003cp\u003eFor \u003cem\u003ein-situ\u003c/em\u003e phase evolution, high-temperature XRD measurement was conducted on a Rigaku Smartlab X-ray diffractometer (Cu Kα, 40 kV, 40 mA) equipped with an Anton Paar HTK-1200 N chamber. XRD patterns were collected from 7\u0026deg; to 45\u0026deg; 2θ at a step size of 0.02\u0026deg;. The temperature increased from 25\u0026deg;C to 1200\u0026deg;C with a ramp rate of 5\u0026deg;C/min under 100 mL/min dry airflow. Data was collected with an interval of 25\u0026deg;C. The sample height was aligned at each temperature to prevent expanded sample holder height compromising XRD reflections. As for the quantitative phase analysis, XRD patterns were acquired using a Bruker D8 Advance diffractometer (Co Kα, 35 kV, 40 mA) from 2\u0026deg; to 90\u0026deg; 2θ at a step size of 0.015\u0026deg; and a scan speed of 1.5\u0026deg; 2θ/min. A 10 wt.% internal standard (corundum) was used for quantification of amorphous content with details shown in \u003cb\u003eSupplementary Note 5\u003c/b\u003e. Phase identification of the XRD pattern was conducted in DIFFRAC.EVA v7 in ICDD PDF-5\u0026thinsp;+\u0026thinsp;2025 database. Quantitative phase analysis was conducted through Rietveld refinement in DIFFRAC.TOPAS v7. In addition, to identify the swelling clay minerals in raw materials, sedimentation experiments were applied to prepare clay slides for XRD measurement with details shown in \u003cb\u003eSupplementary Note 6\u003c/b\u003e.\u003c/p\u003e \u003cp\u003e \u003cem\u003eMicrostructural Characterizations\u003c/em\u003e \u003c/p\u003e \u003cp\u003eThe micromorphology of fired bricks was imaged using a field emission scanning electron microscope (FESEM, JEOL 7001F) using backscattered electron imaging at 20 kV and 12 mA. An X-Max 80 (Oxford, UK) energy-dispersive X-ray spectroscopy system was applied to determine elemental compositions. Sample preparation procedures are shown in \u003cb\u003eSupplementary Note 7\u003c/b\u003e.\u003c/p\u003e \u003cp\u003e \u003cem\u003ePhysical and Mechanical Properties of Fired Bricks\u003c/em\u003e \u003c/p\u003e \u003cp\u003eThe 24-h cold water absorption of fired bricks were measured as per the ASTMC67/C67M standard.\u003csup\u003e\u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e54\u003c/span\u003e\u003c/sup\u003e The bulk density of fired bricks were calculated using the following equation:\u003c/p\u003e \u003cp\u003e \u003cspan class=\"InlineEquation\"\u003e \u003cspan class=\"mathinline\"\u003e\\(\\:\\text{B}ulk\\:density=\\frac{\\text{m}}{{\\pi\\:}{\\text{r}}^{2}l}\\)\u003c/span\u003e \u003c/span\u003e (Eq.\u0026nbsp;3)\u003c/p\u003e \u003cp\u003ewhere \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:m\\)\u003c/span\u003e\u003c/span\u003e, \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:r\\)\u003c/span\u003e\u003c/span\u003e, and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:l\\)\u003c/span\u003e\u003c/span\u003e stand for the mass, radius and thickness of fired bricks, respectively. Measurements were taken at five positions for each dimension, and the average value was used. Compressive strength tests were conducted on fired brick buttons (Φ 40 mm \u0026times; 12 mm) using the Instron 6800 universal testing machine, with the maximum bearing capacity of 300 kN and the loading rate 1 mm/min.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eFunding:\u003c/strong\u003e Australian Research Council Mid-Career Industry Fellowship (IM230100132), and The Clay Minerals Society (USA) 2023 Student Research Grant\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAcknowledgements\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis work is supported by an Australian Research Council Mid-Career Industry Fellowship (IM230100132) and the Clay Minerals Society (USA) 2023 Student Research Grant. The authors would like to acknowledge Brickworks Ltd., and\u0026nbsp;the Central Analytical Research Facility (CARF), QUT for sample characterizations.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eData Availability Statement\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare that the data supporting the findings of this study are available within the paper and its supplementary information files.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eEthics Declarations\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting interests\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare no competing interests.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eZhang LY. Production of bricks from waste materials - A review. \u003cem\u003eConstr Build Mater\u003c/em\u003e \u003cstrong\u003e47\u003c/strong\u003e, 643-655 (2013).\u003c/li\u003e\n\u003cli\u003eDondi M, Bertolotti GP. 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(2019).\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"nature-portfolio","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"","title":"Nature Portfolio","twitterHandle":"","acdcEnabled":false,"dfaEnabled":false,"editorialSystem":"ejp","reportingPortfolio":"","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"clay minerals, fired bricks, mechanical strength, kaolinite, illite, firing temperatures","lastPublishedDoi":"10.21203/rs.3.rs-6743297/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-6743297/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eClay minerals are the most crucial constituents in brickmaking clays, yet the specific role of different clay minerals remain poorly understood due to their natural coexistence and formidable challenges of separation. Here, we design a novel kaolinite-illite-quartz-feldspar system through a multiple-stage purification strategy, enabling direct evaluation of two most critical clay minerals - kaolinite and illite. Precise tuning the initial clay mineral assemblage yields brick strength up to 10 times the ASTM C62 Grade MW requirement, without upgrading existing production method. Mineralogical mechanisms responsible for substantial strength enhancement are revealed. Extensive analyses of 60 brick formulations uncovered: a breakdown of conventional linear relationships among different brick properties due to the temperature-dependent response of kaolinite and illite; specific formation pathways of mullite and amorphous phases; over-firing defects above 1200\u0026deg;C due to K\u003csup\u003e+\u003c/sup\u003e-induced fluxing from illite. Notably, a previously unrecognized negative correlation between mullite and brick strength is identified, suggesting a need to reconsider the widely held assumption of mullite as a strengthening phase. This work not only presents valuable insights for optimizing high-strength brick formulations but also opens new avenues for investigating the intricate contributions of distinct starting components to key brick performance.\u003c/p\u003e","manuscriptTitle":"From Ancient Practice to Modern Innovation: Solving the Clay Mineral Puzzle in Brickmaking","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-06-24 16:58:18","doi":"10.21203/rs.3.rs-6743297/v1","editorialEvents":[],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"nature-communications","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"NCOMMS","sideBox":"Learn more about [Nature Communications](http://www.nature.com/ncomms/)","snPcode":"","submissionUrl":"https://mts-ncomms.nature.com/","title":"Nature Communications","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"ejp","reportingPortfolio":"Nature Communications","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"6086a786-3aaa-49ed-aa52-8a959c303da7","owner":[],"postedDate":"June 24th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[{"id":50447442,"name":"Physical sciences/Materials science/Structural materials/Ceramics"},{"id":50447443,"name":"Earth and environmental sciences/Solid Earth sciences/Mineralogy"}],"tags":[],"updatedAt":"2026-01-09T16:20:53+00:00","versionOfRecord":[],"versionCreatedAt":"2025-06-24 16:58:18","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-6743297","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-6743297","identity":"rs-6743297","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}