Effect of borax-modified activator on mechanical properties and drying shrinkage of alkali-activated slag/metakaolin mortar

Effect of borax-modified activator on mechanical properties and drying shrinkage of alkali-activated slag/metakaolin mortar | 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 Effect of borax-modified activator on mechanical properties and drying shrinkage of alkali-activated slag/metakaolin mortar Haiming Chen, Ziguang Qin, Jie Chen, Yadong Zhang, Peng Wu This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-3837552/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 08 Apr, 2024 Read the published version in Scientific Reports → Version 1 posted 9 You are reading this latest preprint version Abstract Alkali-activated materials (AAMs) possess several advantages, such as high strengths and low carbon emissions. However, their application is hindered due to their significant shrinkage. This study explored the effect of borax-modified sodium silicate activator and metakaolin (MK) on the mechanical properties and drying shrinkage (DS) of alkali-activated slag (AAS) and AAS/MK (AASM) mortars. X-ray diffraction, scanning electron microscopy, and Fourier-transform infrared spectroscopy were used to characterize the hydration products. The results show that the DS reduction of the AAS mortar is related to decreased Na 2 O content, a reduction in the proportion of mesopores, and the formation of moisture-retaining borate compounds. The DS reduction of the AASM mortar is attributed to the ultra-fine differential effect induced by MK, reducing the connected pores. The modified activator combined with MK increased the chemically bound water content in the matrix. Additionally, the B-O bond and highly active MK improved compactness of the AASM mortar. Physical sciences/Engineering/Civil engineering Physical sciences/Materials science/Structural materials/Composites Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Figure 11 Figure 12 Introduction The cement industry emits approximately 1.45 Gt of CO 2 per year, accounting for approximately 4% of global fossil fuel emissions 1,2 . As infrastructure construction continues to expand in developing countries, greater use of ordinary Portland cement (OPC) is expected, resulting in more pronounced environmental pollution from cement production. Alkali-activated materials (AAMs) offer an environmentally friendly solution for reducing carbon emissions in the cement industry. These materials primarily use industrial solid waste (e.g., slag 3–5 , lithium slag 6,7 , and fly ash 8,9 ) as a low-carbon binder for precursors 10 . This approach not only provides cost-effective benefits but also promotes solid waste reuse, leading to a reduction in CO 2 emissions of approximately 40% 11 . Previous studies have demonstrated that AAMs exhibit high strengths, low permeabilities, and excellent corrosion resistances, making them promising alternatives to cement-based materials 12,13 . However, the issue of drying shrinkage (DS) poses a significant obstacle to the application of AAMs. Cracks resulting from DS accelerate the erosion of harmful substances such as carbon dioxide, acid, and base ions, thereby reducing the materials' durability. Consequently, reducing the shrinkage is a theoretically effective approach to enhance the erosion resistance of AAMs. AAMs condense quickly, and DS primarily occurs in the initial stage of hydration and the subsequent gradual evaporation of free water during curing. The widely recognized mechanism of DS is that when the water gradually evaporates, surface tension is generated when the menisci are formed in the gel pores and capillary pores, thereby causing an equal compressive stress in the skeleton and causing volume shrinkage 14,15 . Current research primarily focuses on reducing the DS of AAMs by incorporating fibers 16–18 , chemical admixtures 4,19 , and mineral admixtures 20,21 , as well as changing the curing system 22–24 . Changing the curing system, while effectively reducing shrinkage, may also lead to a decrease in mechanical properties and issues of unstable internal curing humidity 24 . Adding fibers can enhance the toughness of AAMs and reduce shrinkage, but it also introduces uncertain factors such as uneven fiber distribution and lower fluidity than cement mortar 25 . The activator brings greater alkalinity to AAMs than OPC, which is one of the key factors affecting DS. As a commonly used activator in AAMs, liquid sodium silicate accelerates the polycondensation reaction of the system due to the presence of [SiO 2 (OH) 2 ] 2− , which has a better excitation effect but also results in rapid setting and greater shrinkage to AAMs 11,26 . Conversely, NaOH and Na 2 CO 3 as activators exhibit lower shrinkage than Na 2 SiO 3 but compromise the mechanical properties of AAMs 27,28 . The choice of activators significantly affects the performances of AAMs. Therefore, by modifying the types of ions present in the activators, there is a possibility of improving the performances of AAMs. Borate ions have been shown to act as retarders in cement-based materials 29,30 . Compounds containing borate ions, such as borax and boric acid, have been successfully used as retarders in various cement types, such as Portland cement and magnesium phosphate cement 31,32 . The retarding mechanism involves the reaction between calcium ions and borate ions to form hydrated calcium borate. This compound covers the surfaces of clinker particles, partially or fully, thereby decelerating clinker dissolution and postponing hydration 30 . Some studies 8,33,34 have investigated the curing of borate ions in AAMs and found that they can enhance the workability of AAMs. Rakhimova et al. 34 prepared activators containing borate ions with pH values of 8.5 and 10.5, simulating radioactive solutions containing borates in pressurized water reactors of nuclear power plants. The results showed that the two solutions provided acceptable setting times and 28-d compressive strengths of 49.7–56.1 MPa for alkali-activated slag (AAS) paste with a 7% alkali equivalent. Boron (B) shares similar coordination characteristics with silicon (Si) and aluminum (Al). Theoretically, [BO 4 ] will likely share oxygen atoms with [AlO 4 ] and [SiO 4 ] to change the gel structure, thus influencing the macro- and micro-properties of AAMs. Recently, some studies have attempted to partially replace commonly used activators with borates in AAMs and geopolymers, obtaining satisfactory outcomes in both mechanical performance and workability 35–37 . Revathi et al. 35 investigated the performance changes of fly-ash-based geopolymers by partially substituting sodium silicate with borax. Substituting 10–30% of the sodium silicate resulted in a final setting time of over 200 min for the geopolymer without affecting the mechanical properties at 28 d. In addition, tetrahedral boron absorption bands with wavenumbers of 1380–1310 cm − 1 and 1134 cm − 1 were observed via Fourier-transform infrared spectroscopy (FTIR). Attenuated total reflectance (ATR) FTIR showed that B partially replaced the Al in the gel network, indicating that Si-O-T(B, Al) was compatible with the gel structure. Bagheri et al. 37 compared the environmentally friendly substitution of borax in AAMs and geopolymers and found that a 10–30% borax content improved the mechanical properties of the geopolymers more significantly than AAMs. While these studies provided detailed micro-level analyses, research on the shrinkage performances and durability of AAMs in this new excitation environment is still limited. If the addition of borax leads to excessive shrinkage, the application value of AAMs may be diminished. In general, in calcium-rich AAS systems and calcium-poor geopolymers, [SiO 4 ] 4− and [AlO 4 ] 5− produced by different activators dissolve the precursor materials and balance with alkali metal cations (Ca 2+ , Na + ) to form C-S-H and C(N)-A-S-H 38 . However, under different excitation environments, the polycondensation reaction in AAMs may change, resulting in variations in the reaction speed or the resulting gel network and subsequent performance changes. For instance, the presence of certain concentrations of [BO 3 ] 3− , [BO 4 ] 5− , and [PO 4 ] 3− in the activator has the potential to alter the alkalinity of the mortar's pore solution and the internal humidity of AAMs 37,39 . Based on environmental sustainability and durability considerations, this study explored the influence of a boron environment on the DS and mechanical properties of different AAMs using a modified sodium silicate activator with borax. Experimental investigations were conducted to analyze the factors of the borax content on the fluidity, DS, mechanical properties, and microscopic characteristics of the AAS mortar to determine the optimal borax content. With the optimal borax content, the impact of various metakaolin (MK) contents (5%, 10%, 15%, and 20%) as an alternative to ground granulated blast furnace slag (GGBS) in the AAS mortar was studied. The results demonstrated that the boron environment reduced the DS and improved the mechanical properties of AAMs. Additionally, substituting MK for slag further decreased the DS while enhancing the mechanical properties of AAMs. Therefore, borax-modified activators offer a method to improve the performances of AAMs, contributing to carbon emissions reductions and environmental protection. Materials and methods Materials. S95 GGBS and MK were sourced from Gongyi Wanying Environmental Protection Materials Co., Ltd. (China), and their chemical compositions and X-ray diffraction (XRD) patterns are presented in Table 1 and Fig. 1 , respectively. The particle size distributions of the GGBS and MK were measured using a laser particle size analyzer, as shown in Fig. 2 , indicating median particle sizes of approximately 17.38 and 5.01 µm for GGBS and MK, respectively. Liquid sodium silicate was obtained from Guangzhou Suixin Chemical Co., Ltd. (China), with an initial modulus (Ms) = 3.57, where Ms is the ratio of the amount of SiO 2 to the amount of Na 2 O. The main indicators are provided in Table 2 The content of liquid sodium silicate with different Ms was adjusted with an appropriate proportion of sodium hydroxide and deionized water to obtain different Ms Values. The sand used in this study was sourced from the Huaihe River in China, with a fineness modulus of 2.36 (Type II) and an apparent density of 2550 kg/m 3 . Anhydrous borax (Na 2 B 4 O 7 ), which was obtained from Tianjin Guangfu Development Co., Ltd. (China), is a white powder, and its aqueous solutions are weakly alkaline. Table 1 Chemical composition (%) of GGBS and MK measured by XRF. Composition SiO 2 Al 2 O 3 CaO MgO Fe 2 O 3 TiO 2 SO 3 Na 2 O MnO K 2 O Others GGBS 31.81 18.44 34.59 9.93 0.29 1.01 2.45 0.62 0.30 0.40 0.16 MK 48.91 49.55 - 0.06 0.76 0.13 0.02 0.04 0.01 0.48 0.04 Table 2 Physical and chemical indicators of liquid sodium silicate. Modulus Baume degree (°Bé) Na 2 O (wt%) SiO 2 (wt%) Density (g/cm³, 20℃) 3.57 39.7 8.32 28.77 1.38 Mix proportions In this study, nine experimental groups were established to analyze the effects of modified activators on the performances of different AAMs. According to previous studies 20,34 , the water–binder ratios for all groups were maintained at 0.41, the cement–sand ratio was 0.5, the adjusted Ms was set at 1.2, and the water in the activator was calculated as part of the mixing water. The alkali equivalent levels tested were 4.9%, 5.6%, 6.3%, and 7%. The activator substitution levels examined were 10%, 20%, and 30%. Additionally, the GGBS replacement levels investigated were 5%, 10%, 15%, and 20%. Table 3 shows the mixed design of AAS and AASM mortars. The control groups, C-N7 and C-N5.6, did not contain borax and MK. C-N5.6 represents the AAS mortar with an alkali equivalent of 5.6%. The B-M groups were slag/MK systems, such as B20-M10, where B20 indicates a replacement of 20% of the sodium silicate with borax, and M10 represents a replacement of 10% of the slag with MK. Na 2 O content (alkali equivalent) indicates the percentage of Na 2 O mass in the activator to the total mass of the cementitious material. Table 3 Mix proportions of mortars (g/kg). Mix ID Na 2 O content (%) Borax to Na 2 SiO 3 (wt%) Materials consumption(g/kg) GGBS MK Na 2 SiO 3 Borax C-N7 7% 0 685 0 315 0 C-N5.6 5.6% 0 732 0 268 0 B10-M0 6.3% 10% 685 0 283.5 31.5 B20-M0 5.6% 20% 685 0 252 63 B20-M5 5.6% 20% 650.75 34.25 252 63 B20-M10 5.6% 20% 616.5 68.5 252 63 B20-M15 5.6% 20% 582.25 102.75 252 63 B20-M20 5.6% 20% 548 137 252 63 B30-M0 4.9% 30% 685 0 220.5 94.5 Methods Fluidity tests. The fluidity of the fresh mortar was assessed following ASTM C1437-20 40 . The mortar was poured into a truncated cone mold and compacted uniformly by ramming the rod from the periphery toward the center. Excess mortar above the mold's top surface was removed using a trowel. Subsequently, the mold was gently lifted, and the flow table test was immediately initiated. The table was vibrated once per second, completing 25 vibrations in approximately 25 ± 1 s. After vibration, the maximum spread diameter and orthogonal length of the mortar on the flow table were measured using a steel ruler, and the average value was calculated. Flexural and compressive strengths tests . The flexural and compressive strengths of the specimens were determined according to ASTM C348-21 41 and ASTM C349-18 42 respectively. The raw materials were mixed intensively for 3 min using a mortar mixer to ensure uniformity. Subsequently, water and activator were added and stirred for 2 min. Fresh mortar was poured into a polyethylene mold (40 mm × 40 mm × 160 mm) and vibrated on a vibration table for 5–10 s to eliminate air bubbles. The specimens were placed in a curing room maintained at a temperature of 23 ± 2°C and a humidity of above 95%. After 1 d of curing, the specimens were placed in a curing box under the same conditions after demolding and cured to 3, 7, and 28 d. The flexural and compressive strengths were calculated using Formulas (1) and (2) respectively 43 : σ f = 2.8 P (1) σ c = 0.62 P (2) where σ f is the flexural strength, MPa; σ c is the compressive strength, MPa; and P is the maximum load in the strength test, kN. Drying shrinkage tests. DS amounts of all the specimens were tested following ASTM C596-18 44 . Three columnar specimens (25 mm × 25 mm × 280 mm) were prepared for each group, and the molds were removed after 24 h of curing in the curing room. The specimens were submerged in water at a temperature of 20 ± 1°C for 48 h and then removed. The initial length was measured using a comparator after wiping the samples with a wet cloth. The measuring accuracy of the comparator is 0.001mm. Subsequently, the samples were stored indoors at a temperature of 20 ± 1°C and a humidity of 50% ± 3%. The length changes will be measured during the curing process until a specified age period. The microstrain was calculated according to Formula (3): $${\mu }_{\epsilon }=\frac{{L}_{0-}{L}_{T}}{280}\times {10}^{6}$$ 3 Where µ Ɛ is the microstrain; L 0 is the initial length of the specimen, mm; L T is the test length of the specimen at the age to be tested, mm. Mercury Intrusion Porosimetry tests. The pore structure characteristics of the mortar samples were tested using mercury intrusion porosimeter (MIP). The mercury pressure range was 0.1 to 61000 psia, and the contact angle was set to 130°. Microscopic tests. The specified aged paste and mortar samples were immersed in anhydrous ethanol for 7 d and then dried in a vacuum chamber for 48 h. The microstructures of the mortar samples were analyzed using scanning electron microscopy (SEM, Flex 1000). Before the analysis, metal particles were sputtered onto the test block using an MSP-2S magnetron ion diffractometer for 90 s to enhance the conductivity. Characterization of hydration products using X-ray diffractometer (XRD, Smartlab SE) manufactured in Japan and Fourier-transform infrared spectroscopy (FTIR, Nicolet IS50). The XRD scanning speed was set to 5°/min, with a sampling interval of 0.01° ± 2θ, and the scanning range was from 5° to 60°. FTIR testing scanned 32 times from 4000 to 400 cm − 1 at 4 cm − 1 resolution. Results and discussion Fluidity. Figure 3 shows the effect of the borax-modified activator on the fluidity of the AAS mortar: with an increasing borax content in the sodium silicate activator, the mortar's fluidity initially increased and then decreased. Among the samples, B10-M0 exhibited the highest fluidity (183 mm), which was 5% higher than that of C-N7. However, excessive borax reduced the mortar's fluidity, and the minimum fluidity was exhibited by B30-M0 (161.5 mm). The fluidity change of the AAS mortar was primarily influenced by two factors: (1) Alkali equivalent. Borax was used to partially replace the activator, resulting in a reduction in the Na 2 O content of the activator. The positive impact of the Na 2 O content on the fluidity of the AAS mortar was that the additional [SiO 4 ] 4− provided by the activator enhanced the electrostatic repulsion between the particles, thereby increasing the dispersion of free water among the particles. Consequently, the apparent viscosity of the AAS mortar decreased, leading to an increase in its fluidity 45 . As shown in Fig. 3 , the lower the substitution rate was, the greater the alkali equivalent and fluidity of the mortar were. However, too high a content of Na 2 O accelerated the dissolution of gel particles and the polycondensation reaction of [SiO 4 ] 4− groups with Ca 2 + 46 . The content of alkali silicate gel in the mortar increased, and the particles adsorbed by the higher-density gel increased, leading to a decrease in the fluidity. Therefore, the fluidity of C-N7 was lower than that of C-N5.6. (2) Free water content. During borax dissolution, the consumption of free water gradually increased. Compared to the electrostatic repulsion effect between ions, at this point, the available free water for dispersing slag particles was limited, leading to higher sensitivity of fluidity to the loss of free water 47,48 . The hydrolysis of borax produces boric acid, generating borate ions under alkaline conditions 49 . The hydrolysis reaction equations of borax are shown in (4) and (5). $$({B}_{4}{O}_{7}{)}^{2-}+7{H}_{2}O\to 2O{H}^{-}+4{H}_{3}B{O}_{3}$$ 4 $${H}_{3}B{O}_{3}+O{H}^{-}\to B(OH{)}_{4}^{-}$$ 5 Figure 4 shows the fluidity of the fresh AAS/MK (AASM) mortar. Incorporating MK decreased the fluidity of the mortar, and the greater the amount of MK added was, the lower the fluidity was. This phenomenon could be ascribed to MK's water absorption and its irregular morphology on the fluidity 50 . Flexural and compressive strengths. An appropriate proportion of the borax-modified activator proved beneficial in optimizing the strength of the AAS mortar. As shown in Fig. 5 a, with the increase in the borax substitution rate, the flexural strength (FS) of the AAS mortar exhibited an initial increase followed by a subsequent decrease. Figure 5 b presents a consistent trend in the compressive strength (CS) of the AAS mortar, similar to FS. The FS of the AAS mortar at both 7d and 28d exceeded that of the control group. The optimal substitution level was found to be 20%, and the CS and FS at 28d of B20-M0 were respectively 9.35% and 29.29% higher than those of C-N7 in the control group. The 7-d CS of C-N5.6 was 11.46% lower than that of C-N7, while the FS and CS were similar at 28 d. Additionally, the addition of borax decreased the strength of the AAS mortar at 3 d. However, when borax was introduced under the same alkali equivalent conditions, the optimized modified AAS mortar (B20-M0) exhibited higher FS and CS at both 7 and 28 d compared to C-N5.6 and C-N7. This clearly illustrated the positive impact of incorporating borax on the development of strength in the AAS mortar. When utilizing only sodium silicate as activator, the alkali equivalent proved to be the key factor influencing the strength of the AAS mortar. While a higher alkali equivalent displayed better dissolution ability for precursor materials like GGBS compared to a lower alkali equivalent activator, it did not exhibit an absolute growth trend 51–53 . The study indicated that when the alkali equivalent exceeded a certain range, the high alkali equivalent activator would rapidly dissolve GGBS in the early stage, accelerate the polymerization reaction of [SiO 4 ], [AlO 4 ] and Ca 2+ , and the gel produced would wrap the unreacted particles. The excessive gelation of the AAS mortar in the early stage hindered the hydration reaction in the later stage, ultimately reducing the mechanical properties of the AAS mortar. The application of the borax-modified sodium silicate activator enhanced the mechanical properties of the AAS mortar, which was due to the fact that the use of composite activator reduced the rapid condensation reaction caused by high alkali equivalent, and at the same time, in the early stage of hydration, borate ions formed calcium borate complexes with Ca 2+ and adhered to the surface of unreacted GGBS, thus slowing down the condensation rate. However, unlike the excessive gelatinization caused by higher alkali equivalent, this complex reaction only affected the development of the AAS mortar strength at the 3d. This was because the complex on the slag surface had semi-permeable membrane properties. With the progression of the hydration reaction, the complex was constantly permeated and destroyed, facilitating the hydration reaction of the AAS mortar tend to be normal 49 . The complex reaction was shown in formula (6). In addition, due to [BO 4 ] has the same coordination characteristics as [SiO 4 ] and [AlO 4 ], the [BO 4 ] generated from borax hydrolysis condensed with [SiO 4 ] and [AlO 4 ], increasing the degree of gel polymerization, thus improving the strength of the AAS mortar 35 . However, maintaining an appropriate proportion of borax in the activator is crucial, as a significant reduction in [SiO4] supplied by sodium silicate would markedly diminish the activator's alkalinity, thereby weakening its ability to dissolve GGBS and affecting the hydration reaction. $$2B(OH{)}_{4}^{-}+C{a}^{2+}\to Ca(B(OH{)}_{4}{)}_{2}$$ 6 Figure 5 c shows that the FS of the AASM mortar increased initially and then decreased with the increase in the MK content. The optimal substitution rate was determined to be 15%. The FS of B20-M15 increased by 15.54% at 28 d compared with that of B20-M0. As shown in Fig. 5 d, with the increase in the MK content, the CS of the AASM mortar showed a trend of decreasing first, then increasing, and then decreasing. The 28-d CS of B20-M15 increased by 8.92% compared with that of B20-M0, which was 12.97% higher than that of C-N5.6. The primary factors that contribute to improving the mechanical properties of the AASM mortar with MK can be summarized as follows. With the increase in MK substitution rate, the levels of alumina (Al 2 O 3 ) and silica (SiO 2 ) in the mortar also rose. The increased levels of Al and Si enhanced geopolymerization reaction and the formation of N-A-S-H and C-A-S-H gels 20 , ultimately leading to the improvement in the mechanical properties of the AASM mortar. Additionally, MK displayed a finer particle size and a larger specific surface area compared to GGBS. Its high reactivity facilitated the hydration reactions and improved the interfacial bonding performance and compactness of the AASM mortar. However, when the MK content exceeded 20%, a decrease in the calcium oxide supplied by GGBS resulted in a reduction in the proportion of C-(A)-S-H gel, subsequently leading to a decrease in mortar strength. Porosity. According to the International Union of Pure and Applied Chemistry (IUPAC) definition, porous materials are categorized into three classes based on pore size: micropores ( 50 nm) 21 . In order to analyze the impact of borax-modified sodium silicate activator and MK on the porosity of the AAS and AASM mortars, a comparison of the pore size distribution was conducted among the initial control group (C-N7), the optimal group B20-M0, and B20-M15. Figure 6 shows the cumulative pore volume curve of the mortars, with porosity at 28 d recorded as 14.73%, 12.94%, and 11.48% for the three mortar samples, respectively. The characteristics of the pore diameter distribution of the mortars were shown in Fig. 7 , with all three curves showing a peak value, corresponding to the most probable pore diameter, representing the pore size with the highest frequency within different pores. The most probable pore diameter for the control group and B20-M0 are around 50 nm, while for B20-M15, it was around 20 nm. Overall, the proportion of pores in the modified mortars smaller than 50 nm was lower. The reduction in porosity of the modified mortars was primarily associated with the [BO 4 ] produced by the hydrolysis of borax. In comparison to a single sodium silicate activator, the additional [BO 4 ] participates in the polymerization reaction between [SiO 4 ] and [AlO 4 ], enhancing the polymerization of C-(A)-S-H gel and thus favoring the refinement of mortar pores. The incorporation of MK further decreased the porosity of the AASM mortar. On one hand, MK facilitated geopolymerization reactions, leading to the additional formation of N-A-S-H gel, resulting in a denser matrix. On the other hand, due to its finer particle size relative to GGBS, MK also contributes to a filling effect. The change in porosity correlated with the strength of the mortar, indicating that as the porosity decreased, the strength of the mortar increased. Drying shrinkage. When AAMs are exposed to acid, base ions, and CO 2 , the cracks resulting from DS can exacerbate the extent of the damage. The DS value serves as a measure of the volume change in an AAM caused by water evaporation, allowing for the evaluation of the susceptibility of AAMs to damage. As shown in Fig. 8 , an appropriate amount of borax in the activator had a positive influence on reducing the DS of the AAS mortar. The optimal group was B20-M0, with its DS value at 7 d being 34% and 25% lower than those of C-N7 and C-N5.6, respectively, while the DS at 28 d was 12% and 8% lower, respectively. C-N7 showed greater DS than C-N5.6, indicating that as the alkali equivalent increased, the DS of the AAS mortar increased. According to the theory of capillary tension in pores, when the cementitious material gradually loses free water in an unsaturated air environment, surface tension is generated due to the formation of meniscus inside the gel pores and capillaries. Consequently, this induces isotropic compressive stresses within the matrix, leading to volume shrinkage. Collins et al. 14 pointed out that significant shrinkage stresses occur only when mesopores with diameters smaller than 50 nm lose water, while macropores larger than 50 nm and micropores smaller than 2.5 nm do not generate shrinkage stresses. MIP results indicated a decrease in the proportion of mesopores for B20-M0 and B20-M15 compared to the control group C-N7, suggesting that the composite activator improved the pore size distribution of the AAS mortar. This contributed to alleviating the mortar's DS. When using only the sodium silicate activator, the hydration reaction accelerated with the increase in alkali equivalent. The proportion of gel pores also increased with the formation of more gel, and the consumption of free water within the pores led to greater shrinkage stress, ultimately resulting in an increase in the DS 54 . Therefore, the drying shrinkage of B20-N5.6 was less than that of C-N7. This was consistent with previous studies, where a higher alkali equivalent leads to a faster condensation rate, which is detrimental to the volume stability of the mortar 21 . When using a borax-modified activator, the reduction in the dosage of the sodium silicate activator was beneficial for reducing the DS. Additionally, borate ions participated in the formation of borate compounds. Studies have indicated that the high moisture retention properties of these compounds helped to stabilize the internal humidity of the AAS mortar and further reduce its DS 47 . However, excessive substitution of borax for the sodium silicate activator led to a reduction in alkali equivalent to 4.9%. In this context, the alkalinity of the solution was unfavorable for the development of the later gel network, leading to a weakening of the gel matrix's ability to resist shrinkage stresses. Considering all factors, it is suggested that the borax content be maintained at an optimal substitution level of 20% to achieve an excellent performance in the AAS mortar. Replacing 10–20% of GGBS with MK could effectively reduce the DS of AAMs. The development of the DS in the AASM mortar with MK contents of 5%, 10%, 15%, and 20% in a boron environment is shown in Fig. 9 . The DS of mortar in each group increased rapidly from 1 to 7 d, and the DS of the AASM mortar was lower than those of C-N5.6 and C-N7. As the age progressed, the DS in each group gradually slowed down. This trend became more pronounced with higher MK contents, and the DS of the AASM mortar decreased with increasing MK contents. The DS amounts of C-N5.6 and C-N7 were 23% and 27% lower, respectively, than that of the optimal group, B20-M20. As shown in Fig. 6 , the DS of each mortar group exhibited a significant growth trend from 7 to 28 d. However, as shown in Fig. 9 , the DS tended to stabilize within the 7-d to 28-d range. These two different trends indicated that the incorporation of MK was beneficial for reducing the DS of AAS mortar modified with borax, and the DS reduction effect became more significant after 7 d. As the MK content increased, it effectively reduced the DS of the AASM mortar for the following reasons. The small particle size and high specific surface area of MK induced an ultrafine differentiation effect, resulting in a reduction of interconnected pores within the AASM system 55 . Consequently, this decrement in pore count mitigated the shrinkage forces caused by capillary effects, ultimately leading to a decrease in the DS of the AASM mortar. Additionally, The addition of MK to GGBS introduced Al, thereby enhancing the crack resistance of the C(N)-A-S-H gel. Moreover, an increased Al/Si ratio effectively reduced the sensitivity of AAMs to moisture loss during the drying process, thus contributing to a reduction in DS 20,56 . SEM analysis. Figure 10 (a) and 10(c) shows the SEM images of C-N7 and C-N5.6 at 28 d, respectively. The gel structure shown in Fig. 11 a possessed a blocky and needle-like structure with significant local defects, which may be caused by a high alkali equivalent. The gel in Fig. 11 c appeared more intact and was attached to some low-crystallinity hydrated calcium silicate gel, but numerous continuous microcracks were evident. Figure 10 (b) and10(d) shows the SEM images of the AAS (B20-M0) and AASM (B20-M15) mortars under the influence of the modified activator, respectively. As shown in Fig. 10 (b), the gel formed by hydration after the incorporation of borax was smoother and denser 48 , without noticeable micro-cracks, which microscopically explained that B20-M0 excited by the modified activator had better mechanical properties. The gel reaction degree shown in Fig. 10 (d) was better than other three groups, and the surface was aggregated with flaky calcium silicate aggregates, which corresponded to the high reactivity of the MK. Additionally, the MK had a smaller particle size than GGBS, and the filling effect made the mortar structure denser. XRD analysis. Figure 11 (a) and 11(b) presents the XRD patterns of the AAS and AASM pastes at 28 d, respectively. The main peak represented the calcite phase, and the phases on both sides were mainly calcium silicate hydrate. Ulexite (NaCaB 5 O 6 (OH) 6 (H 2 O) 5 ) was present in the B10-M0, B20-M0, and B30-M0 samples. With the increase in the proportion of borax in the activator, the content of calcite and C-S-H gel remained relatively stable. The decreased intensity of diffraction peaks for akermanite and gehlenite, along with the emergence of ulexite diffraction peaks, suggested the involvement of borate ions in the formation of the gel network 35 . However, the excessive addition of borax reduced the amount of calcite generated, primarily due to the decrease in the proportion of sodium silicate in the activator. The reduction in alkali equivalent resulted in a decrease in the release of Ca 2+ and Al 3+ ions from GGBS, thereby impacting the strength of the mortar. This validated the analysis of the mortar's mechanical properties. With the increase in the substitution ratio of MK for GGBS, there was a higher content of Al 2 O 3 and SiO 2 , leading to the formation of C(N)-A-S-H gel in the matrix 20,50,57 . However, no distinct diffraction peaks were observed, possibly due to potential overlap with the diffraction peaks of calcite 58 . Additionally, the formation of hydrotalcite is advantageous for the corrosion resistance of AAMs 59 . FTIR analysis. To elucidate the effects of borax and MK on the phase transition of AAMs samples, FTIR spectra analysis was performed on the bands of Si-O-T (T represents Si, Al or B), O-C-O, and H-O-H, which correspond to gels, calcium carbonate, and chemically bound water, respectively 60 . Figure 12 (a) exhibits the effects of different borax levels. The spectral band near 1650 cm − 1 represents the bending vibration of H-O-H 61 . With the increase of borax content, the peak of H-O-H band increases. This may be caused by the crystallization of borate compounds. The spectral band near 1415 cm − 1 represents the symmetric tensile vibration of O-C-O 62 , which is associated with the carbonization of the sample. In all samples, there was a major absorption peak near 1109 cm − 1 , corresponding to the asymmetric tensile vibration of Si-O-T bonds caused by the dissolution of the silicate phase. The absorption peaks at 671 cm − 1 and 536 cm − 1 represent the symmetric tensile vibration of Si-O-Si, the bending vibration of B-O-B, respectively 35,63 . These peaks are associated with the formation of gel. With the increase of borax, the peak increases. That is, the internal polymerization reaction of the AAS mortar increases after using the borax-modified activator, which increases the amount of the gel phase. This may be related to the involvement of B-O bonds in the composition of the gel network, and the resulting C-S(B)-H increases the degree of ploymerization of the gel 64 . Figure 12 (b) exhibits the effects of different MK levels, and the peak of Si-O-T band increases with the increase in MK contents. This is due to increased levels of Si and Al in the mortar leading to additional C(N)-A-S-H gel formation, which is accordance with the mechanical properties. The increase of the peak of H-O-H and –OH bands indicates that the AASM mortar has a higher bound water than the AAS mortar, which is conducive to further alleviating shrinkage stress and reducing the DS of the AASM mortar. Conclusion The effects of the borax-modified sodium silicate activator on the properties of the AAS mortar were investigated, and the optimal substitution level of borax was determined. Under the condition of an optimal borax replacement rate, the performance of the AASM mortar with different MK contents to replace GGBS were studied. The conclusions drawn from the test results are as follows. • The fluidity of the AAS mortar initially increased and then decreased with higher anhydrous borax content in the activator. This behavior could be attributed primarily to the changes in the Na2O content and the consumption of free water. The increase in MK content led to a decrease in the fluidity of the AASM mortar, attributed to MK's water absorption and irregular morphology. • The incorporation of an appropriate amount of borate enhanced the mechanical properties of the AAS mortar. This was attributed to the presence of [BO 4 ] resulting from the dissolution of borax, which actively participated in the gel network composition alongside [SiO 4 ] and [AlO 4 ]. Incorporating an appropriate amount of MK markedly enhanced the strength of the AASM mortar. This improvement was attributed to MK's smaller specific surface area, which facilitated and promoted geopolymerization reaction. • The reduction in the DS of the AAS mortar, resulting from the modified activator, could be attributed to the decreased alkali equivalent of the mortar system, a reduction in the proportion of mesopores, and the formation of borate compounds with high moisture retention properties. • The incorporation of MK further reduced the DS of the AASM mortar. This was primarily associated with the finer particle size and larger specific surface area of MK compared to those of GGBS. This not only enhanced the interfacial bonding and compactness of the AASM mortar but also exhibited an ultrafine differentiation effect, reducing the number of interconnected pores and mitigating shrinkage stress caused by water loss. • The borax-modified activator reduced the DS and improved sulfate resistance of AAMs, while also reducing the dosage of sodium silicate activator. In order to expand the application range of AAMs and promote environmental protection, it is recommended to conduct further research on the effects of the composite activators containing boron-containing waste solutions on the shrinkage and durability of AAMs. Declarations Acknowledgments We thank LetPub (www.letpub.com) for its linguistic assistance during the preparation of this manuscript. Funding This work was supported by the National Natural Science Foundation of China (Project nos. 41440018, 41672278). CRediT authorship contribution statement Haiming Chen: Methodology, Investigation, Conceptualization, Resources, Supervision, Writing – review & editing, Project administration. Ziguang Qin: Methodology, Investigation, Writing – original draft. Jie Chen: Investigation. Yadong Zhang: Investigation. Peng Wu: Investigation. 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Thermo-mechanical behaviour of fly ash-ladle furnace slag blended geopolymer with incorporation of decahydrate borax. Constr. Build. Mater. 331 , 127337 (2022). Additional Declarations No competing interests reported. Cite Share Download PDF Status: Published Journal Publication published 08 Apr, 2024 Read the published version in Scientific Reports → Version 1 posted Editorial decision: Revision requested 11 Feb, 2024 Reviewers agreed at journal 07 Feb, 2024 Reviews received at journal 31 Jan, 2024 Reviewers agreed at journal 18 Jan, 2024 Reviewers invited by journal 16 Jan, 2024 Editor assigned by journal 16 Jan, 2024 Editor invited by journal 10 Jan, 2024 Submission checks completed at journal 10 Jan, 2024 First submitted to journal 05 Jan, 2024 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. 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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-3837552","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":266307736,"identity":"cd87a7ce-e0ae-4e56-8446-ea5cbc2d547e","order_by":0,"name":"Haiming 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mortars.\u003c/p\u003e","description":"","filename":"7.jpg","url":"https://assets-eu.researchsquare.com/files/rs-3837552/v1/5ddb95e9a3b3cec9c96f0595.jpg"},{"id":49541552,"identity":"2edcc159-37d1-4255-adc1-5e5e824414c3","added_by":"auto","created_at":"2024-01-12 17:28:47","extension":"jpg","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":52269,"visible":true,"origin":"","legend":"\u003cp\u003eDS of mortars with different borax content in the modified activator.\u003c/p\u003e","description":"","filename":"8.jpg","url":"https://assets-eu.researchsquare.com/files/rs-3837552/v1/8989fd823a436afdf70e7ab7.jpg"},{"id":49543065,"identity":"2a101313-1dee-4679-8489-ac560ed1945a","added_by":"auto","created_at":"2024-01-12 17:36:47","extension":"jpg","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":59424,"visible":true,"origin":"","legend":"\u003cp\u003eDS of mortars with various ratios of MK to GGBS.\u003c/p\u003e","description":"","filename":"9.jpg","url":"https://assets-eu.researchsquare.com/files/rs-3837552/v1/eaf2023367225466497052e1.jpg"},{"id":49540774,"identity":"1e360c05-36bc-4c13-954b-bda52f195bd6","added_by":"auto","created_at":"2024-01-12 17:20:47","extension":"jpg","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":143209,"visible":true,"origin":"","legend":"\u003cp\u003eSEM micrographs of mortars at 28 d.\u003c/p\u003e","description":"","filename":"10.jpg","url":"https://assets-eu.researchsquare.com/files/rs-3837552/v1/02a383857c01879f24527d91.jpg"},{"id":49543063,"identity":"ada6f02a-b751-4144-b85e-39931267c61e","added_by":"auto","created_at":"2024-01-12 17:36:47","extension":"jpg","order_by":11,"title":"Figure 11","display":"","copyAsset":false,"role":"figure","size":49019,"visible":true,"origin":"","legend":"\u003cp\u003eXRD patterns of hydration products of AAS and AASM pastes at 28 d: (a) with different borax contents and (b) with different MK contents.\u003c/p\u003e","description":"","filename":"11.jpg","url":"https://assets-eu.researchsquare.com/files/rs-3837552/v1/ea3ea82bdb3d6590d8a88507.jpg"},{"id":49541548,"identity":"c54d546e-c3a7-4c7c-b3f0-ce132f15d858","added_by":"auto","created_at":"2024-01-12 17:28:47","extension":"jpg","order_by":12,"title":"Figure 12","display":"","copyAsset":false,"role":"figure","size":59562,"visible":true,"origin":"","legend":"\u003cp\u003eFTIR spectra of AAS and AASM pastes:(a) with different borax contents and (b) with different MK contents.\u003c/p\u003e","description":"","filename":"12.jpg","url":"https://assets-eu.researchsquare.com/files/rs-3837552/v1/ba0b57b49af47f6c30b66c73.jpg"},{"id":54712975,"identity":"d7f1aac3-45ea-4091-817d-ce464f1c2855","added_by":"auto","created_at":"2024-04-15 15:14:04","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":976352,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-3837552/v1/66336d94-b818-47b9-acd6-06eadb8040e1.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Effect of borax-modified activator on mechanical properties and drying shrinkage of alkali-activated slag/metakaolin mortar","fulltext":[{"header":"Introduction","content":"\u003cp\u003eThe cement industry emits approximately 1.45 Gt of CO\u003csub\u003e2\u003c/sub\u003e per year, accounting for approximately 4% of global fossil fuel emissions\u003csup\u003e1,2\u003c/sup\u003e. As infrastructure construction continues to expand in developing countries, greater use of ordinary Portland cement (OPC) is expected, resulting in more pronounced environmental pollution from cement production. Alkali-activated materials (AAMs) offer an environmentally friendly solution for reducing carbon emissions in the cement industry. These materials primarily use industrial solid waste (e.g., slag\u003csup\u003e3\u0026ndash;5\u003c/sup\u003e, lithium slag\u003csup\u003e6,7\u003c/sup\u003e, and fly ash\u003csup\u003e8,9\u003c/sup\u003e) as a low-carbon binder for precursors\u003csup\u003e10\u003c/sup\u003e. This approach not only provides cost-effective benefits but also promotes solid waste reuse, leading to a reduction in CO\u003csub\u003e2\u003c/sub\u003e emissions of approximately 40%\u003csup\u003e11\u003c/sup\u003e. Previous studies have demonstrated that AAMs exhibit high strengths, low permeabilities, and excellent corrosion resistances, making them promising alternatives to cement-based materials\u003csup\u003e12,13\u003c/sup\u003e. However, the issue of drying shrinkage (DS) poses a significant obstacle to the application of AAMs. Cracks resulting from DS accelerate the erosion of harmful substances such as carbon dioxide, acid, and base ions, thereby reducing the materials' durability. Consequently, reducing the shrinkage is a theoretically effective approach to enhance the erosion resistance of AAMs.\u003c/p\u003e \u003cp\u003eAAMs condense quickly, and DS primarily occurs in the initial stage of hydration and the subsequent gradual evaporation of free water during curing. The widely recognized mechanism of DS is that when the water gradually evaporates, surface tension is generated when the menisci are formed in the gel pores and capillary pores, thereby causing an equal compressive stress in the skeleton and causing volume shrinkage\u003csup\u003e14,15\u003c/sup\u003e. Current research primarily focuses on reducing the DS of AAMs by incorporating fibers\u003csup\u003e16\u0026ndash;18\u003c/sup\u003e, chemical admixtures\u003csup\u003e4,19\u003c/sup\u003e, and mineral admixtures\u003csup\u003e20,21\u003c/sup\u003e, as well as changing the curing system\u003csup\u003e22\u0026ndash;24\u003c/sup\u003e. Changing the curing system, while effectively reducing shrinkage, may also lead to a decrease in mechanical properties and issues of unstable internal curing humidity\u003csup\u003e24\u003c/sup\u003e. Adding fibers can enhance the toughness of AAMs and reduce shrinkage, but it also introduces uncertain factors such as uneven fiber distribution and lower fluidity than cement mortar\u003csup\u003e25\u003c/sup\u003e. The activator brings greater alkalinity to AAMs than OPC, which is one of the key factors affecting DS. As a commonly used activator in AAMs, liquid sodium silicate accelerates the polycondensation reaction of the system due to the presence of [SiO\u003csub\u003e2\u003c/sub\u003e(OH)\u003csub\u003e2\u003c/sub\u003e]\u003csup\u003e2\u0026minus;\u003c/sup\u003e, which has a better excitation effect but also results in rapid setting and greater shrinkage to AAMs\u003csup\u003e11,26\u003c/sup\u003e. Conversely, NaOH and Na\u003csub\u003e2\u003c/sub\u003eCO\u003csub\u003e3\u003c/sub\u003e as activators exhibit lower shrinkage than Na\u003csub\u003e2\u003c/sub\u003eSiO\u003csub\u003e3\u003c/sub\u003e but compromise the mechanical properties of AAMs\u003csup\u003e27,28\u003c/sup\u003e. The choice of activators significantly affects the performances of AAMs. Therefore, by modifying the types of ions present in the activators, there is a possibility of improving the performances of AAMs.\u003c/p\u003e \u003cp\u003eBorate ions have been shown to act as retarders in cement-based materials\u003csup\u003e29,30\u003c/sup\u003e. Compounds containing borate ions, such as borax and boric acid, have been successfully used as retarders in various cement types, such as Portland cement and magnesium phosphate cement\u003csup\u003e31,32\u003c/sup\u003e. The retarding mechanism involves the reaction between calcium ions and borate ions to form hydrated calcium borate. This compound covers the surfaces of clinker particles, partially or fully, thereby decelerating clinker dissolution and postponing hydration\u003csup\u003e30\u003c/sup\u003e. Some studies\u003csup\u003e8,33,34\u003c/sup\u003e have investigated the curing of borate ions in AAMs and found that they can enhance the workability of AAMs. Rakhimova et al.\u003csup\u003e34\u003c/sup\u003e prepared activators containing borate ions with pH values of 8.5 and 10.5, simulating radioactive solutions containing borates in pressurized water reactors of nuclear power plants. The results showed that the two solutions provided acceptable setting times and 28-d compressive strengths of 49.7\u0026ndash;56.1 MPa for alkali-activated slag (AAS) paste with a 7% alkali equivalent. Boron (B) shares similar coordination characteristics with silicon (Si) and aluminum (Al). Theoretically, [BO\u003csub\u003e4\u003c/sub\u003e] will likely share oxygen atoms with [AlO\u003csub\u003e4\u003c/sub\u003e] and [SiO\u003csub\u003e4\u003c/sub\u003e] to change the gel structure, thus influencing the macro- and micro-properties of AAMs.\u003c/p\u003e \u003cp\u003eRecently, some studies have attempted to partially replace commonly used activators with borates in AAMs and geopolymers, obtaining satisfactory outcomes in both mechanical performance and workability\u003csup\u003e35\u0026ndash;37\u003c/sup\u003e. Revathi et al.\u003csup\u003e35\u003c/sup\u003e investigated the performance changes of fly-ash-based geopolymers by partially substituting sodium silicate with borax. Substituting 10\u0026ndash;30% of the sodium silicate resulted in a final setting time of over 200 min for the geopolymer without affecting the mechanical properties at 28 d. In addition, tetrahedral boron absorption bands with wavenumbers of 1380\u0026ndash;1310 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and 1134 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e were observed via Fourier-transform infrared spectroscopy (FTIR). Attenuated total reflectance (ATR) FTIR showed that B partially replaced the Al in the gel network, indicating that Si-O-T(B, Al) was compatible with the gel structure. Bagheri et al.\u003csup\u003e37\u003c/sup\u003e compared the environmentally friendly substitution of borax in AAMs and geopolymers and found that a 10\u0026ndash;30% borax content improved the mechanical properties of the geopolymers more significantly than AAMs.\u003c/p\u003e \u003cp\u003eWhile these studies provided detailed micro-level analyses, research on the shrinkage performances and durability of AAMs in this new excitation environment is still limited. If the addition of borax leads to excessive shrinkage, the application value of AAMs may be diminished. In general, in calcium-rich AAS systems and calcium-poor geopolymers, [SiO\u003csub\u003e4\u003c/sub\u003e]\u003csup\u003e4\u0026minus;\u003c/sup\u003e and [AlO\u003csub\u003e4\u003c/sub\u003e]\u003csup\u003e5\u0026minus;\u003c/sup\u003e produced by different activators dissolve the precursor materials and balance with alkali metal cations (Ca\u003csup\u003e2+\u003c/sup\u003e, Na\u003csup\u003e+\u003c/sup\u003e) to form C-S-H and C(N)-A-S-H\u003csup\u003e38\u003c/sup\u003e. However, under different excitation environments, the polycondensation reaction in AAMs may change, resulting in variations in the reaction speed or the resulting gel network and subsequent performance changes. For instance, the presence of certain concentrations of [BO\u003csub\u003e3\u003c/sub\u003e]\u003csup\u003e3\u0026minus;\u003c/sup\u003e, [BO\u003csub\u003e4\u003c/sub\u003e]\u003csup\u003e5\u0026minus;\u003c/sup\u003e, and [PO\u003csub\u003e4\u003c/sub\u003e]\u003csup\u003e3\u0026minus;\u003c/sup\u003e in the activator has the potential to alter the alkalinity of the mortar's pore solution and the internal humidity of AAMs\u003csup\u003e37,39\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eBased on environmental sustainability and durability considerations, this study explored the influence of a boron environment on the DS and mechanical properties of different AAMs using a modified sodium silicate activator with borax. Experimental investigations were conducted to analyze the factors of the borax content on the fluidity, DS, mechanical properties, and microscopic characteristics of the AAS mortar to determine the optimal borax content. With the optimal borax content, the impact of various metakaolin (MK) contents (5%, 10%, 15%, and 20%) as an alternative to ground granulated blast furnace slag (GGBS) in the AAS mortar was studied. The results demonstrated that the boron environment reduced the DS and improved the mechanical properties of AAMs. Additionally, substituting MK for slag further decreased the DS while enhancing the mechanical properties of AAMs. Therefore, borax-modified activators offer a method to improve the performances of AAMs, contributing to carbon emissions reductions and environmental protection.\u003c/p\u003e"},{"header":"Materials and methods","content":"\u003cp\u003e \u003cb\u003eMaterials.\u003c/b\u003e S95 GGBS and MK were sourced from Gongyi Wanying Environmental Protection Materials Co., Ltd. (China), and their chemical compositions and X-ray diffraction (XRD) patterns are presented in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e and Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e, respectively. The particle size distributions of the GGBS and MK were measured using a laser particle size analyzer, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e, indicating median particle sizes of approximately 17.38 and 5.01 \u0026micro;m for GGBS and MK, respectively. Liquid sodium silicate was obtained from Guangzhou Suixin Chemical Co., Ltd. (China), with an initial modulus (Ms)\u0026thinsp;=\u0026thinsp;3.57, where Ms is the ratio of the amount of SiO\u003csub\u003e2\u003c/sub\u003e to the amount of Na\u003csub\u003e2\u003c/sub\u003eO. The main indicators are provided in Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e The content of liquid sodium silicate with different Ms was adjusted with an appropriate proportion of sodium hydroxide and deionized water to obtain different Ms Values. The sand used in this study was sourced from the Huaihe River in China, with a fineness modulus of 2.36 (Type II) and an apparent density of 2550 kg/m\u003csup\u003e3\u003c/sup\u003e. Anhydrous borax (Na\u003csub\u003e2\u003c/sub\u003eB\u003csub\u003e4\u003c/sub\u003eO\u003csub\u003e7\u003c/sub\u003e), which was obtained from Tianjin Guangfu Development Co., Ltd. (China), is a white powder, and its aqueous solutions are weakly alkaline.\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\u003eChemical composition (%) of GGBS and MK measured by XRF.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"12\"\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 \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c8\" colnum=\"8\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c9\" colnum=\"9\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c10\" colnum=\"10\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c11\" colnum=\"11\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c12\" colnum=\"12\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eComposition\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSiO\u003csub\u003e2\u003c/sub\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eAl\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eCaO\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eMgO\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eFe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c7\"\u003e \u003cp\u003eTiO\u003csub\u003e2\u003c/sub\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c8\"\u003e \u003cp\u003eSO\u003csub\u003e3\u003c/sub\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c9\"\u003e \u003cp\u003eNa\u003csub\u003e2\u003c/sub\u003eO\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c10\"\u003e \u003cp\u003eMnO\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c11\"\u003e \u003cp\u003eK\u003csub\u003e2\u003c/sub\u003eO\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c12\"\u003e \u003cp\u003eOthers\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eGGBS\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e31.81\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e18.44\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e34.59\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e9.93\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e0.29\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e1.01\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e \u003cp\u003e2.45\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c9\"\u003e \u003cp\u003e0.62\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c10\"\u003e \u003cp\u003e0.30\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c11\"\u003e \u003cp\u003e0.40\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c12\"\u003e \u003cp\u003e0.16\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eMK\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e48.91\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e49.55\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.06\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e0.76\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e0.13\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e \u003cp\u003e0.02\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c9\"\u003e \u003cp\u003e0.04\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c10\"\u003e \u003cp\u003e0.01\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c11\"\u003e \u003cp\u003e0.48\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c12\"\u003e \u003cp\u003e0.04\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003ePhysical and chemical indicators of liquid sodium silicate.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"5\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eModulus\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eBaume degree (\u0026deg;B\u0026eacute;)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eNa\u003csub\u003e2\u003c/sub\u003eO (wt%)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eSiO\u003csub\u003e2\u003c/sub\u003e (wt%)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eDensity (g/cm\u0026sup3;, 20℃)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e3.57\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e39.7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e8.32\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e28.77\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e1.38\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003eMix proportions\u003c/h2\u003e \u003cp\u003eIn this study, nine experimental groups were established to analyze the effects of modified activators on the performances of different AAMs. According to previous studies\u003csup\u003e20,34\u003c/sup\u003e, the water\u0026ndash;binder ratios for all groups were maintained at 0.41, the cement\u0026ndash;sand ratio was 0.5, the adjusted Ms was set at 1.2, and the water in the activator was calculated as part of the mixing water. The alkali equivalent levels tested were 4.9%, 5.6%, 6.3%, and 7%. The activator substitution levels examined were 10%, 20%, and 30%. Additionally, the GGBS replacement levels investigated were 5%, 10%, 15%, and 20%. Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e shows the mixed design of AAS and AASM mortars. The control groups, C-N7 and C-N5.6, did not contain borax and MK. C-N5.6 represents the AAS mortar with an alkali equivalent of 5.6%. The B-M groups were slag/MK systems, such as B20-M10, where B20 indicates a replacement of 20% of the sodium silicate with borax, and M10 represents a replacement of 10% of the slag with MK. Na\u003csub\u003e2\u003c/sub\u003eO content (alkali equivalent) indicates the percentage of Na\u003csub\u003e2\u003c/sub\u003eO mass in the activator to the total mass of the cementitious material.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab3\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 3\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eMix proportions of mortars (g/kg).\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"7\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eMix ID\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eNa\u003csub\u003e2\u003c/sub\u003eO content (%)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eBorax to Na\u003csub\u003e2\u003c/sub\u003eSiO\u003csub\u003e3\u003c/sub\u003e (wt%)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colspan=\"4\" nameend=\"c7\" namest=\"c4\"\u003e \u003cp\u003eMaterials consumption(g/kg)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eGGBS\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eMK\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eNa\u003csub\u003e2\u003c/sub\u003eSiO\u003csub\u003e3\u003c/sub\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c7\"\u003e \u003cp\u003eBorax\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eC-N7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e7%\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e685\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e315\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eC-N5.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e5.6%\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e732\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e268\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eB10-M0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e6.3%\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e10%\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e685\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e283.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e31.5\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eB20-M0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e5.6%\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e20%\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e685\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e252\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e63\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eB20-M5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e5.6%\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e20%\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e650.75\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e34.25\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e252\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e63\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eB20-M10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e5.6%\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e20%\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e616.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e68.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e252\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e63\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eB20-M15\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e5.6%\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e20%\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e582.25\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e102.75\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e252\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e63\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eB20-M20\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e5.6%\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e20%\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e548\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e137\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e252\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e63\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eB30-M0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e4.9%\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e30%\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e685\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e220.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e94.5\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\u003eMethods\u003c/h2\u003e \u003cp\u003e \u003cb\u003eFluidity tests.\u003c/b\u003e The fluidity of the fresh mortar was assessed following ASTM C1437-20\u003csup\u003e40\u003c/sup\u003e. The mortar was poured into a truncated cone mold and compacted uniformly by ramming the rod from the periphery toward the center. Excess mortar above the mold's top surface was removed using a trowel. Subsequently, the mold was gently lifted, and the flow table test was immediately initiated. The table was vibrated once per second, completing 25 vibrations in approximately 25\u0026thinsp;\u0026plusmn;\u0026thinsp;1 s. After vibration, the maximum spread diameter and orthogonal length of the mortar on the flow table were measured using a steel ruler, and the average value was calculated.\u003c/p\u003e \u003cp\u003e \u003cb\u003eFlexural and compressive strengths tests\u003c/b\u003e. The flexural and compressive strengths of the specimens were determined according to ASTM C348-21\u003csup\u003e41\u003c/sup\u003e and ASTM C349-18\u003csup\u003e42\u003c/sup\u003e respectively. The raw materials were mixed intensively for 3 min using a mortar mixer to ensure uniformity. Subsequently, water and activator were added and stirred for 2 min. Fresh mortar was poured into a polyethylene mold (40 mm \u0026times; 40 mm \u0026times; 160 mm) and vibrated on a vibration table for 5\u0026ndash;10 s to eliminate air bubbles. The specimens were placed in a curing room maintained at a temperature of 23\u0026thinsp;\u0026plusmn;\u0026thinsp;2\u0026deg;C and a humidity of above 95%. After 1 d of curing, the specimens were placed in a curing box under the same conditions after demolding and cured to 3, 7, and 28 d. The flexural and compressive strengths were calculated using Formulas (1) and (2) respectively\u003csup\u003e43\u003c/sup\u003e:\u003c/p\u003e \u003cp\u003e \u003cem\u003eσ\u003c/em\u003e \u003csub\u003e \u003cem\u003ef\u003c/em\u003e \u003c/sub\u003e\u0026thinsp;=\u0026thinsp;2.8\u003cem\u003eP\u003c/em\u003e (1)\u003c/p\u003e \u003cp\u003e \u003cem\u003eσ\u003c/em\u003e \u003csub\u003e \u003cem\u003ec\u003c/em\u003e \u003c/sub\u003e\u0026thinsp;=\u0026thinsp;0.62\u003cem\u003eP\u003c/em\u003e (2)\u003c/p\u003e \u003cp\u003ewhere \u003cem\u003eσ\u003c/em\u003e\u003csub\u003e\u003cem\u003ef\u003c/em\u003e\u003c/sub\u003e is the flexural strength, MPa; \u003cem\u003eσ\u003c/em\u003e\u003csub\u003e\u003cem\u003ec\u003c/em\u003e\u003c/sub\u003e is the compressive strength, MPa; and \u003cem\u003eP\u003c/em\u003e is the maximum load in the strength test, kN.\u003c/p\u003e \u003cp\u003e \u003cb\u003eDrying shrinkage tests.\u003c/b\u003e DS amounts of all the specimens were tested following ASTM C596-18\u003csup\u003e44\u003c/sup\u003e. Three columnar specimens (25 mm \u0026times; 25 mm \u0026times; 280 mm) were prepared for each group, and the molds were removed after 24 h of curing in the curing room. The specimens were submerged in water at a temperature of 20\u0026thinsp;\u0026plusmn;\u0026thinsp;1\u0026deg;C for 48 h and then removed. The initial length was measured using a comparator after wiping the samples with a wet cloth. The measuring accuracy of the comparator is 0.001mm. Subsequently, the samples were stored indoors at a temperature of 20\u0026thinsp;\u0026plusmn;\u0026thinsp;1\u0026deg;C and a humidity of 50% \u0026plusmn; 3%. The length changes will be measured during the curing process until a specified age period. The microstrain was calculated according to Formula (3):\u003cdiv id=\"Equ1\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ1\" name=\"EquationSource\"\u003e\n$${\\mu }_{\\epsilon }=\\frac{{L}_{0-}{L}_{T}}{280}\\times {10}^{6}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e3\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eWhere \u003cem\u003e\u0026micro;\u003c/em\u003e\u003csub\u003e\u003cem\u003eƐ\u003c/em\u003e\u003c/sub\u003e is the microstrain; \u003cem\u003eL\u003c/em\u003e\u003csub\u003e0\u003c/sub\u003e is the initial length of the specimen, mm; \u003cem\u003eL\u003c/em\u003e\u003csub\u003e\u003cem\u003eT\u003c/em\u003e\u003c/sub\u003e is the test length of the specimen at the age to be tested, mm.\u003c/p\u003e \u003cp\u003e \u003cb\u003eMercury Intrusion Porosimetry tests.\u003c/b\u003e The pore structure characteristics of the mortar samples were tested using mercury intrusion porosimeter (MIP). The mercury pressure range was 0.1 to 61000 psia, and the contact angle was set to 130\u0026deg;.\u003c/p\u003e \u003cp\u003e \u003cb\u003eMicroscopic tests.\u003c/b\u003e The specified aged paste and mortar samples were immersed in anhydrous ethanol for 7 d and then dried in a vacuum chamber for 48 h. The microstructures of the mortar samples were analyzed using scanning electron microscopy (SEM, Flex 1000). Before the analysis, metal particles were sputtered onto the test block using an MSP-2S magnetron ion diffractometer for 90 s to enhance the conductivity. Characterization of hydration products using X-ray diffractometer (XRD, Smartlab SE) manufactured in Japan and Fourier-transform infrared spectroscopy (FTIR, Nicolet IS50). The XRD scanning speed was set to 5\u0026deg;/min, with a sampling interval of 0.01\u0026deg; \u0026plusmn; 2θ, and the scanning range was from 5\u0026deg; to 60\u0026deg;. FTIR testing scanned 32 times from 4000 to 400 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e at 4 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e resolution.\u003c/p\u003e \u003c/div\u003e"},{"header":"Results and discussion","content":"\u003cp\u003e \u003cb\u003eFluidity.\u003c/b\u003e Figure\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e shows the effect of the borax-modified activator on the fluidity of the AAS mortar: with an increasing borax content in the sodium silicate activator, the mortar's fluidity initially increased and then decreased. Among the samples, B10-M0 exhibited the highest fluidity (183 mm), which was 5% higher than that of C-N7. However, excessive borax reduced the mortar's fluidity, and the minimum fluidity was exhibited by B30-M0 (161.5 mm). The fluidity change of the AAS mortar was primarily influenced by two factors: (1) Alkali equivalent. Borax was used to partially replace the activator, resulting in a reduction in the Na\u003csub\u003e2\u003c/sub\u003eO content of the activator. The positive impact of the Na\u003csub\u003e2\u003c/sub\u003eO content on the fluidity of the AAS mortar was that the additional [SiO\u003csub\u003e4\u003c/sub\u003e]\u003csup\u003e4\u0026minus;\u003c/sup\u003e provided by the activator enhanced the electrostatic repulsion between the particles, thereby increasing the dispersion of free water among the particles. Consequently, the apparent viscosity of the AAS mortar decreased, leading to an increase in its fluidity\u003csup\u003e45\u003c/sup\u003e. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e, the lower the substitution rate was, the greater the alkali equivalent and fluidity of the mortar were. However, too high a content of Na\u003csub\u003e2\u003c/sub\u003eO accelerated the dissolution of gel particles and the polycondensation reaction of [SiO\u003csub\u003e4\u003c/sub\u003e]\u003csup\u003e4\u0026minus;\u003c/sup\u003e groups with Ca\u003csup\u003e2\u0026thinsp;+\u0026thinsp;46\u003c/sup\u003e. The content of alkali silicate gel in the mortar increased, and the particles adsorbed by the higher-density gel increased, leading to a decrease in the fluidity. Therefore, the fluidity of C-N7 was lower than that of C-N5.6. (2) Free water content. During borax dissolution, the consumption of free water gradually increased. Compared to the electrostatic repulsion effect between ions, at this point, the available free water for dispersing slag particles was limited, leading to higher sensitivity of fluidity to the loss of free water\u003csup\u003e47,48\u003c/sup\u003e. The hydrolysis of borax produces boric acid, generating borate ions under alkaline conditions\u003csup\u003e49\u003c/sup\u003e. The hydrolysis reaction equations of borax are shown in (4) and (5).\u003cdiv id=\"Equ2\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ2\" name=\"EquationSource\"\u003e\n$$({B}_{4}{O}_{7}{)}^{2-}+7{H}_{2}O\\to 2O{H}^{-}+4{H}_{3}B{O}_{3}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e4\u003c/div\u003e\u003c/div\u003e\u003cdiv id=\"Equ3\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ3\" name=\"EquationSource\"\u003e\n$${H}_{3}B{O}_{3}+O{H}^{-}\\to B(OH{)}_{4}^{-}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e5\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e shows the fluidity of the fresh AAS/MK (AASM) mortar. Incorporating MK decreased the fluidity of the mortar, and the greater the amount of MK added was, the lower the fluidity was. This phenomenon could be ascribed to MK's water absorption and its irregular morphology on the fluidity\u003csup\u003e50\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003cb\u003eFlexural and compressive strengths.\u003c/b\u003e An appropriate proportion of the borax-modified activator proved beneficial in optimizing the strength of the AAS mortar. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ea, with the increase in the borax substitution rate, the flexural strength (FS) of the AAS mortar exhibited an initial increase followed by a subsequent decrease. Figure\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eb presents a consistent trend in the compressive strength (CS) of the AAS mortar, similar to FS. The FS of the AAS mortar at both 7d and 28d exceeded that of the control group. The optimal substitution level was found to be 20%, and the CS and FS at 28d of B20-M0 were respectively 9.35% and 29.29% higher than those of C-N7 in the control group. The 7-d CS of C-N5.6 was 11.46% lower than that of C-N7, while the FS and CS were similar at 28 d. Additionally, the addition of borax decreased the strength of the AAS mortar at 3 d. However, when borax was introduced under the same alkali equivalent conditions, the optimized modified AAS mortar (B20-M0) exhibited higher FS and CS at both 7 and 28 d compared to C-N5.6 and C-N7. This clearly illustrated the positive impact of incorporating borax on the development of strength in the AAS mortar.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eWhen utilizing only sodium silicate as activator, the alkali equivalent proved to be the key factor influencing the strength of the AAS mortar. While a higher alkali equivalent displayed better dissolution ability for precursor materials like GGBS compared to a lower alkali equivalent activator, it did not exhibit an absolute growth trend\u003csup\u003e51\u0026ndash;53\u003c/sup\u003e. The study indicated that when the alkali equivalent exceeded a certain range, the high alkali equivalent activator would rapidly dissolve GGBS in the early stage, accelerate the polymerization reaction of [SiO\u003csub\u003e4\u003c/sub\u003e], [AlO\u003csub\u003e4\u003c/sub\u003e] and Ca\u003csup\u003e2+\u003c/sup\u003e, and the gel produced would wrap the unreacted particles. The excessive gelation of the AAS mortar in the early stage hindered the hydration reaction in the later stage, ultimately reducing the mechanical properties of the AAS mortar. The application of the borax-modified sodium silicate activator enhanced the mechanical properties of the AAS mortar, which was due to the fact that the use of composite activator reduced the rapid condensation reaction caused by high alkali equivalent, and at the same time, in the early stage of hydration, borate ions formed calcium borate complexes with Ca\u003csup\u003e2+\u003c/sup\u003e and adhered to the surface of unreacted GGBS, thus slowing down the condensation rate. However, unlike the excessive gelatinization caused by higher alkali equivalent, this complex reaction only affected the development of the AAS mortar strength at the 3d. This was because the complex on the slag surface had semi-permeable membrane properties. With the progression of the hydration reaction, the complex was constantly permeated and destroyed, facilitating the hydration reaction of the AAS mortar tend to be normal\u003csup\u003e49\u003c/sup\u003e. The complex reaction was shown in formula (6). In addition, due to [BO\u003csub\u003e4\u003c/sub\u003e] has the same coordination characteristics as [SiO\u003csub\u003e4\u003c/sub\u003e] and [AlO\u003csub\u003e4\u003c/sub\u003e], the [BO\u003csub\u003e4\u003c/sub\u003e] generated from borax hydrolysis condensed with [SiO\u003csub\u003e4\u003c/sub\u003e] and [AlO\u003csub\u003e4\u003c/sub\u003e], increasing the degree of gel polymerization, thus improving the strength of the AAS mortar\u003csup\u003e35\u003c/sup\u003e. However, maintaining an appropriate proportion of borax in the activator is crucial, as a significant reduction in [SiO4] supplied by sodium silicate would markedly diminish the activator's alkalinity, thereby weakening its ability to dissolve GGBS and affecting the hydration reaction.\u003cdiv id=\"Equ4\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ4\" name=\"EquationSource\"\u003e\n$$2B(OH{)}_{4}^{-}+C{a}^{2+}\\to Ca(B(OH{)}_{4}{)}_{2}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e6\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ec shows that the FS of the AASM mortar increased initially and then decreased with the increase in the MK content. The optimal substitution rate was determined to be 15%. The FS of B20-M15 increased by 15.54% at 28 d compared with that of B20-M0. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ed, with the increase in the MK content, the CS of the AASM mortar showed a trend of decreasing first, then increasing, and then decreasing. The 28-d CS of B20-M15 increased by 8.92% compared with that of B20-M0, which was 12.97% higher than that of C-N5.6.\u003c/p\u003e \u003cp\u003eThe primary factors that contribute to improving the mechanical properties of the AASM mortar with MK can be summarized as follows. With the increase in MK substitution rate, the levels of alumina (Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e) and silica (SiO\u003csub\u003e2\u003c/sub\u003e) in the mortar also rose. The increased levels of Al and Si enhanced geopolymerization reaction and the formation of N-A-S-H and C-A-S-H gels\u003csup\u003e20\u003c/sup\u003e, ultimately leading to the improvement in the mechanical properties of the AASM mortar. Additionally, MK displayed a finer particle size and a larger specific surface area compared to GGBS. Its high reactivity facilitated the hydration reactions and improved the interfacial bonding performance and compactness of the AASM mortar. However, when the MK content exceeded 20%, a decrease in the calcium oxide supplied by GGBS resulted in a reduction in the proportion of C-(A)-S-H gel, subsequently leading to a decrease in mortar strength.\u003c/p\u003e \u003cp\u003e \u003cb\u003ePorosity.\u003c/b\u003e According to the International Union of Pure and Applied Chemistry (IUPAC) definition, porous materials are categorized into three classes based on pore size: micropores (\u0026lt;\u0026thinsp;2 nm), mesopores (2\u0026ndash;50 nm), and macropores (\u0026gt;\u0026thinsp;50 nm)\u003csup\u003e21\u003c/sup\u003e. In order to analyze the impact of borax-modified sodium silicate activator and MK on the porosity of the AAS and AASM mortars, a comparison of the pore size distribution was conducted among the initial control group (C-N7), the optimal group B20-M0, and B20-M15. Figure\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e shows the cumulative pore volume curve of the mortars, with porosity at 28 d recorded as 14.73%, 12.94%, and 11.48% for the three mortar samples, respectively. The characteristics of the pore diameter distribution of the mortars were shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e, with all three curves showing a peak value, corresponding to the most probable pore diameter, representing the pore size with the highest frequency within different pores. The most probable pore diameter for the control group and B20-M0 are around 50 nm, while for B20-M15, it was around 20 nm. Overall, the proportion of pores in the modified mortars smaller than 50 nm was lower. The reduction in porosity of the modified mortars was primarily associated with the [BO\u003csub\u003e4\u003c/sub\u003e] produced by the hydrolysis of borax. In comparison to a single sodium silicate activator, the additional [BO\u003csub\u003e4\u003c/sub\u003e] participates in the polymerization reaction between [SiO\u003csub\u003e4\u003c/sub\u003e] and [AlO\u003csub\u003e4\u003c/sub\u003e], enhancing the polymerization of C-(A)-S-H gel and thus favoring the refinement of mortar pores. The incorporation of MK further decreased the porosity of the AASM mortar. On one hand, MK facilitated geopolymerization reactions, leading to the additional formation of N-A-S-H gel, resulting in a denser matrix. On the other hand, due to its finer particle size relative to GGBS, MK also contributes to a filling effect. The change in porosity correlated with the strength of the mortar, indicating that as the porosity decreased, the strength of the mortar increased.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003cb\u003eDrying shrinkage.\u003c/b\u003e When AAMs are exposed to acid, base ions, and CO\u003csub\u003e2\u003c/sub\u003e, the cracks resulting from DS can exacerbate the extent of the damage. The DS value serves as a measure of the volume change in an AAM caused by water evaporation, allowing for the evaluation of the susceptibility of AAMs to damage. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e, an appropriate amount of borax in the activator had a positive influence on reducing the DS of the AAS mortar. The optimal group was B20-M0, with its DS value at 7 d being 34% and 25% lower than those of C-N7 and C-N5.6, respectively, while the DS at 28 d was 12% and 8% lower, respectively. C-N7 showed greater DS than C-N5.6, indicating that as the alkali equivalent increased, the DS of the AAS mortar increased.\u003c/p\u003e \u003cp\u003eAccording to the theory of capillary tension in pores, when the cementitious material gradually loses free water in an unsaturated air environment, surface tension is generated due to the formation of meniscus inside the gel pores and capillaries. Consequently, this induces isotropic compressive stresses within the matrix, leading to volume shrinkage. Collins et al.\u003csup\u003e14\u003c/sup\u003e pointed out that significant shrinkage stresses occur only when mesopores with diameters smaller than 50 nm lose water, while macropores larger than 50 nm and micropores smaller than 2.5 nm do not generate shrinkage stresses. MIP results indicated a decrease in the proportion of mesopores for B20-M0 and B20-M15 compared to the control group C-N7, suggesting that the composite activator improved the pore size distribution of the AAS mortar. This contributed to alleviating the mortar's DS. When using only the sodium silicate activator, the hydration reaction accelerated with the increase in alkali equivalent. The proportion of gel pores also increased with the formation of more gel, and the consumption of free water within the pores led to greater shrinkage stress, ultimately resulting in an increase in the DS\u003csup\u003e54\u003c/sup\u003e. Therefore, the drying shrinkage of B20-N5.6 was less than that of C-N7. This was consistent with previous studies, where a higher alkali equivalent leads to a faster condensation rate, which is detrimental to the volume stability of the mortar\u003csup\u003e21\u003c/sup\u003e. When using a borax-modified activator, the reduction in the dosage of the sodium silicate activator was beneficial for reducing the DS. Additionally, borate ions participated in the formation of borate compounds. Studies have indicated that the high moisture retention properties of these compounds helped to stabilize the internal humidity of the AAS mortar and further reduce its DS\u003csup\u003e47\u003c/sup\u003e. However, excessive substitution of borax for the sodium silicate activator led to a reduction in alkali equivalent to 4.9%. In this context, the alkalinity of the solution was unfavorable for the development of the later gel network, leading to a weakening of the gel matrix's ability to resist shrinkage stresses. Considering all factors, it is suggested that the borax content be maintained at an optimal substitution level of 20% to achieve an excellent performance in the AAS mortar.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eReplacing 10\u0026ndash;20% of GGBS with MK could effectively reduce the DS of AAMs. The development of the DS in the AASM mortar with MK contents of 5%, 10%, 15%, and 20% in a boron environment is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e. The DS of mortar in each group increased rapidly from 1 to 7 d, and the DS of the AASM mortar was lower than those of C-N5.6 and C-N7. As the age progressed, the DS in each group gradually slowed down. This trend became more pronounced with higher MK contents, and the DS of the AASM mortar decreased with increasing MK contents. The DS amounts of C-N5.6 and C-N7 were 23% and 27% lower, respectively, than that of the optimal group, B20-M20. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e, the DS of each mortar group exhibited a significant growth trend from 7 to 28 d. However, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e, the DS tended to stabilize within the 7-d to 28-d range. These two different trends indicated that the incorporation of MK was beneficial for reducing the DS of AAS mortar modified with borax, and the DS reduction effect became more significant after 7 d.\u003c/p\u003e \u003cp\u003eAs the MK content increased, it effectively reduced the DS of the AASM mortar for the following reasons. The small particle size and high specific surface area of MK induced an ultrafine differentiation effect, resulting in a reduction of interconnected pores within the AASM system\u003csup\u003e55\u003c/sup\u003e. Consequently, this decrement in pore count mitigated the shrinkage forces caused by capillary effects, ultimately leading to a decrease in the DS of the AASM mortar. Additionally, The addition of MK to GGBS introduced Al, thereby enhancing the crack resistance of the C(N)-A-S-H gel. Moreover, an increased Al/Si ratio effectively reduced the sensitivity of AAMs to moisture loss during the drying process, thus contributing to a reduction in DS\u003csup\u003e20,56\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003e \u003cb\u003eSEM analysis.\u003c/b\u003e Figure\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003e(a) and 10(c) shows the SEM images of C-N7 and C-N5.6 at 28 d, respectively. The gel structure shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003ea possessed a blocky and needle-like structure with significant local defects, which may be caused by a high alkali equivalent. The gel in Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003ec appeared more intact and was attached to some low-crystallinity hydrated calcium silicate gel, but numerous continuous microcracks were evident. Figure\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003e(b) and10(d) shows the SEM images of the AAS (B20-M0) and AASM (B20-M15) mortars under the influence of the modified activator, respectively. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003e(b), the gel formed by hydration after the incorporation of borax was smoother and denser\u003csup\u003e48\u003c/sup\u003e, without noticeable micro-cracks, which microscopically explained that B20-M0 excited by the modified activator had better mechanical properties. The gel reaction degree shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003e(d) was better than other three groups, and the surface was aggregated with flaky calcium silicate aggregates, which corresponded to the high reactivity of the MK. Additionally, the MK had a smaller particle size than GGBS, and the filling effect made the mortar structure denser.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003cb\u003eXRD analysis.\u003c/b\u003e Figure\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003e(a) and 11(b) presents the XRD patterns of the AAS and AASM pastes at 28 d, respectively. The main peak represented the calcite phase, and the phases on both sides were mainly calcium silicate hydrate. Ulexite (NaCaB\u003csub\u003e5\u003c/sub\u003eO\u003csub\u003e6\u003c/sub\u003e(OH)\u003csub\u003e6\u003c/sub\u003e(H\u003csub\u003e2\u003c/sub\u003eO)\u003csub\u003e5\u003c/sub\u003e) was present in the B10-M0, B20-M0, and B30-M0 samples. With the increase in the proportion of borax in the activator, the content of calcite and C-S-H gel remained relatively stable. The decreased intensity of diffraction peaks for akermanite and gehlenite, along with the emergence of ulexite diffraction peaks, suggested the involvement of borate ions in the formation of the gel network\u003csup\u003e35\u003c/sup\u003e. However, the excessive addition of borax reduced the amount of calcite generated, primarily due to the decrease in the proportion of sodium silicate in the activator. The reduction in alkali equivalent resulted in a decrease in the release of Ca\u003csup\u003e2+\u003c/sup\u003e and Al\u003csup\u003e3+\u003c/sup\u003e ions from GGBS, thereby impacting the strength of the mortar. This validated the analysis of the mortar's mechanical properties. With the increase in the substitution ratio of MK for GGBS, there was a higher content of Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e and SiO\u003csub\u003e2\u003c/sub\u003e, leading to the formation of C(N)-A-S-H gel in the matrix\u003csup\u003e20,50,57\u003c/sup\u003e. However, no distinct diffraction peaks were observed, possibly due to potential overlap with the diffraction peaks of calcite\u003csup\u003e58\u003c/sup\u003e. Additionally, the formation of hydrotalcite is advantageous for the corrosion resistance of AAMs\u003csup\u003e59\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003cb\u003eFTIR analysis.\u003c/b\u003e To elucidate the effects of borax and MK on the phase transition of AAMs samples, FTIR spectra analysis was performed on the bands of Si-O-T (T represents Si, Al or B), O-C-O, and H-O-H, which correspond to gels, calcium carbonate, and chemically bound water, respectively\u003csup\u003e60\u003c/sup\u003e. Figure\u0026nbsp;\u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e12\u003c/span\u003e(a) exhibits the effects of different borax levels. The spectral band near 1650 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e represents the bending vibration of H-O-H\u003csup\u003e61\u003c/sup\u003e. With the increase of borax content, the peak of H-O-H band increases. This may be caused by the crystallization of borate compounds. The spectral band near 1415 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e represents the symmetric tensile vibration of O-C-O\u003csup\u003e62\u003c/sup\u003e, which is associated with the carbonization of the sample. In all samples, there was a major absorption peak near 1109 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, corresponding to the asymmetric tensile vibration of Si-O-T bonds caused by the dissolution of the silicate phase. The absorption peaks at 671 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and 536 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e represent the symmetric tensile vibration of Si-O-Si, the bending vibration of B-O-B, respectively\u003csup\u003e35,63\u003c/sup\u003e. These peaks are associated with the formation of gel. With the increase of borax, the peak increases. That is, the internal polymerization reaction of the AAS mortar increases after using the borax-modified activator, which increases the amount of the gel phase. This may be related to the involvement of B-O bonds in the composition of the gel network, and the resulting C-S(B)-H increases the degree of ploymerization of the gel\u003csup\u003e64\u003c/sup\u003e. Figure\u0026nbsp;\u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e12\u003c/span\u003e(b) exhibits the effects of different MK levels, and the peak of Si-O-T band increases with the increase in MK contents. This is due to increased levels of Si and Al in the mortar leading to additional C(N)-A-S-H gel formation, which is accordance with the mechanical properties. The increase of the peak of H-O-H and \u0026ndash;OH bands indicates that the AASM mortar has a higher bound water than the AAS mortar, which is conducive to further alleviating shrinkage stress and reducing the DS of the AASM mortar.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e"},{"header":"Conclusion","content":"\u003cp\u003eThe effects of the borax-modified sodium silicate activator on the properties of the AAS mortar were investigated, and the optimal substitution level of borax was determined. Under the condition of an optimal borax replacement rate, the performance of the AASM mortar with different MK contents to replace GGBS were studied. The conclusions drawn from the test results are as follows.\u003c/p\u003e\n\u003cp\u003e\u0026bull; \u0026nbsp;The fluidity of the AAS mortar initially increased and then decreased with higher anhydrous borax content in the activator. This behavior could be attributed primarily to the changes in the Na2O content and the consumption of free water. The increase in MK content led to a decrease in the fluidity of the AASM mortar, attributed to MK\u0026apos;s water absorption and irregular morphology.\u003c/p\u003e\n\u003cp\u003e\u0026bull; \u0026nbsp;The incorporation of an appropriate amount of borate enhanced the mechanical properties of the AAS mortar. This was attributed to the presence of [BO\u003csub\u003e4\u003c/sub\u003e] resulting from the dissolution of borax, which actively participated in the gel network composition alongside [SiO\u003csub\u003e4\u003c/sub\u003e] and [AlO\u003csub\u003e4\u003c/sub\u003e]. Incorporating an appropriate amount of MK markedly enhanced the strength of the AASM mortar. This improvement was attributed to MK\u0026apos;s smaller\u0026nbsp;specific surface area, which facilitated and promoted\u0026nbsp;geopolymerization reaction.\u003c/p\u003e\n\u003cp\u003e\u0026bull; \u0026nbsp;The reduction in the DS of the AAS mortar, resulting from the modified activator, could be attributed to the\u0026nbsp;decreased alkali equivalent of the mortar system,\u0026nbsp;a reduction in the proportion of mesopores, and the formation of borate compounds with high moisture retention properties.\u003c/p\u003e\n\u003cp\u003e\u0026bull; \u0026nbsp;The incorporation of MK further reduced the DS of the AASM mortar. This was primarily associated with the finer particle size and larger specific surface area of MK compared to those of GGBS. This not only enhanced the interfacial bonding and compactness of the AASM mortar but also exhibited an\u0026nbsp;ultrafine differentiation effect, reducing the number of interconnected pores and mitigating shrinkage stress caused by water loss.\u003c/p\u003e\n\u003cp\u003e\u0026bull; \u0026nbsp;The borax-modified activator reduced the DS and improved sulfate resistance of AAMs, while also reducing the dosage of sodium silicate activator. In order to expand the application range of AAMs and promote environmental protection, it is recommended to conduct further research on the effects of the composite activators containing boron-containing waste solutions on the shrinkage and durability of AAMs.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAcknowledgments\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eWe thank LetPub (www.letpub.com) for its linguistic assistance during the preparation of this manuscript.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis work was supported by the National Natural Science Foundation of China (Project nos. 41440018, 41672278).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCRediT authorship contribution statement\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eHaiming Chen:\u0026nbsp;Methodology, Investigation, Conceptualization, Resources, Supervision, Writing \u0026ndash; review \u0026amp; editing, Project administration.\u0026nbsp;Ziguang Qin:\u0026nbsp;Methodology, Investigation, Writing \u0026ndash; original draft. Jie Chen: Investigation. Yadong Zhang: Investigation. Peng Wu: Investigation.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eDeclaration of Competing Interest\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eData Availability\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eData will be made available on request. Data can be obtained from the corresponding author. (E-mail address: [email protected])\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eAndrew, R. M. Global CO\u003csub\u003e2\u003c/sub\u003e emissions from cement production, 1928\u0026ndash;2018. \u003cem\u003eEarth Syst. Sci. Data.\u003c/em\u003e\u003cstrong\u003e11\u003c/strong\u003e, 1675\u0026ndash;1710 (2019).\u003c/li\u003e\n\u003cli\u003eTan, C., Yu, X., Guan, Y. 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However, their application is hindered due to their significant shrinkage. This study explored the effect of borax-modified sodium silicate activator and metakaolin (MK) on the mechanical properties and drying shrinkage (DS) of alkali-activated slag (AAS) and AAS/MK (AASM) mortars. X-ray diffraction, scanning electron microscopy, and Fourier-transform infrared spectroscopy were used to characterize the hydration products. The results show that the DS reduction of the AAS mortar is related to decreased Na\u003csub\u003e2\u003c/sub\u003eO content, a reduction in the proportion of mesopores, and the formation of moisture-retaining borate compounds. The DS reduction of the AASM mortar is attributed to the ultra-fine differential effect induced by MK, reducing the connected pores. The modified activator combined with MK increased the chemically bound water content in the matrix. Additionally, the B-O bond and highly active MK improved compactness of the AASM mortar.\u003c/p\u003e","manuscriptTitle":"Effect of borax-modified activator on mechanical properties and drying shrinkage of alkali-activated slag/metakaolin mortar","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-01-12 17:20:42","doi":"10.21203/rs.3.rs-3837552/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2024-02-11T17:28:52+00:00","index":"","fulltext":""},{"type":"reviewerAgreed","content":"978aed36-d8ab-45b4-9f25-f20ccaa63f6b","date":"2024-02-07T15:50:08+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2024-01-31T12:41:04+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"a693ec09-bf90-4c4f-b47a-cf52ff68e087","date":"2024-01-18T09:40:31+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2024-01-16T08:56:29+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2024-01-16T08:54:12+00:00","index":"","fulltext":""},{"type":"editorInvited","content":"","date":"2024-01-10T16:06:38+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2024-01-10T13:24:45+00:00","index":"","fulltext":""},{"type":"submitted","content":"Scientific Reports","date":"2024-01-05T14:48:32+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"scientific-reports","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"scirep","sideBox":"Learn more about [Scientific Reports](http://www.nature.com/srep/)","snPcode":"","submissionUrl":"","title":"Scientific Reports","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Scientific Reports","inReviewEnabled":true,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"55a4abe3-8c07-4494-9d64-357dd4f3d99e","owner":[],"postedDate":"January 12th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[{"id":28066138,"name":"Physical sciences/Engineering/Civil engineering"},{"id":28066139,"name":"Physical sciences/Materials science/Structural materials/Composites"}],"tags":[],"updatedAt":"2024-04-15T15:10:13+00:00","versionOfRecord":{"articleIdentity":"rs-3837552","link":"https://doi.org/10.1038/s41598-024-58172-x","journal":{"identity":"scientific-reports","isVorOnly":false,"title":"Scientific Reports"},"publishedOn":"2024-04-08 15:01:58","publishedOnDateReadable":"April 8th, 2024"},"versionCreatedAt":"2024-01-12 17:20:42","video":"","vorDoi":"10.1038/s41598-024-58172-x","vorDoiUrl":"https://doi.org/10.1038/s41598-024-58172-x","workflowStages":[]},"version":"v1","identity":"rs-3837552","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-3837552","identity":"rs-3837552","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}