Characterisation of Nigerian Mining Waste Clays as Sustainable Precursors for Production of Low Carbon Geopolymer Binders

Characterisation of Nigerian Mining Waste Clays as Sustainable Precursors for Production of Low Carbon Geopolymer Binders | 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 Research Article Characterisation of Nigerian Mining Waste Clays as Sustainable Precursors for Production of Low Carbon Geopolymer Binders Adebayo Matthew Adeleye, G. M. Ayininuola, Mary B. Ogundiran This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7214593/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract The replacement of Portland limestone cement, with geopolymer as a binder is gaining global attention, due to reduced carbon-footprint, in compliance with United Nations agenda on sustainable development goals. Geopolymer binders (GBs) are usually produced from aluminosilicate precursors. Precursor’s availability at cheap price is essential for sustainability. Therefore, the possibility of utilising mining waste clays (MWCs) as precursors in the production of GBs was investigated. The MWCs were collected from mining sites located in four different states in Nigeria [(Awo, Osun state (OS), Ibeshe, Lagos state (LA), Ijero, Ekiti state (EK) and Owode, Ogun state (OG)] and were characterised for thermal properties, oxide composition, functional groups, morphology/elemental composition, and mineralogical phases using Thermogravimetry/differential thermal analysis (TGA/DTA), X-ray fluorescence, Fourier-transform infra-red (FTIR), Scanning electron microscopy/Energy dispersive spectrometery (SEM/EDS), X-ray diffraction (XRD), spectroscopic techniques. They were calcined at different temperatures and time and their geopolymer binders were produced with different NaOH/Na 2 SiO 3 activator ratio. Carbon emissions for GBs production was estimated using combustion energy equation. Thermal dehydroxylation of MWCs into reactive amorphous phase occurred within the range of 420–740ºC. The sum of SiO 2 and Al 2 O 3 composition were 70.5–82.2%, suggesting aluminosilicate material, confirmed by FTIR peaks at 3691, 1114, 1032 and 3620cm − 1 indicating O-H, Si-O, Si-O-Si and Al-OH bonds, respectively characteristic of kaolinite aluminosilicate materials. The SEM’s irregular non crystalline microstructure and XRD’s hallow peaks or humps at 2Ɵ (20º − 30º) of thermally treated clays suggested amorphisation. Optimised production conditions for GBs were 600–740ºC calcining temperature, 6hrs calcining time, and 1:1.5 NaOH/Na 2 SiO 3 ratio. The compressive strength values of the GBs ranged from 26.1 to 33.7 MPa which were above ASTM standards, with Carbon emission reduction of 92.9% compared to cement. The Nigerian MWCs could be applied for sustainable geopolymer binders’ production. Mining waste clays calcining temperature calcining time carbon emission reduction geopolymer binders 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 1. Introduction Recent findings have shown that the world cement production has increased by about 20% in the past decade and has reached over 4.1 billion tonnes in 2019, largely driven by demand from developing countries (Zhang et al. 2021 ). The world Portland limestone cement (PLC) production has attained an increment over the 2010 level by 24% (Zhang et al., 2021 ; Zhou et al., 2017 ). It was further speculated that this figure was expected to increase astronomically in the years to come. This is because many developing societies of the world are experiencing increased urbanisation and population growth, thus impacting the demand for housing and infrastructure development. High and excessive energy requirement has been found to be a flaw associated with PLC manufacturing. According to Temuujin and Van Riessen (2009), the energy requirement for the manufacturing of PLC is just next to that of steel and aluminium. In addition, the high release of dust into the environment, during the manufacturing process of PLC is of major health concern. Coupled with these problems, a major adverse impact from cement manufacture is the high quantity of carbon (iv) oxide (CO 2 ) being released into the atmosphere during production. It has been reported that the manufacture of PLC generates about 700 to 900 kg of CO 2 emission per tonne of PLC (Zhou et al., 2017 ; Mahasenan et al .,2003). Mahasenan et al. ( 2003 ) informed that the process of CO 2 release during cement manufacture derives majorly from the heating of limestone (calcium carbonate) at very high temperatures, to produce lime and carbon dioxide and also indirectly through the use of energy if that involves the emission of CO 2 . Efforts are being put in place at finding greener alternatives to PLC, particularly to align with the United Nations agenda on sustainable development goals (SDG’s 2030) on the environment and reducing global warming, which impacts on climate change. The replacement of PLC by less carbon-intensive materials is widely accepted as an effort towards reducing the overall CO 2 emission associated with cement manufacturing. Geopolymer has been identified as one of such materials that have potentials to replace PLC in building and construction materials. This acceptance was premised on the advantages of geopolymer which include faster setting time, higher mechanical strength, higher water resistance, resistant to acid attacks and environmental weathering making them more applicable for external construction materials. However, cost of production seems to be a major shortcoming of geopolymer, which is still being investigated. Geopolymer is a term coined by a French scientist, Professor Joseph Davidovits, in the 1970s (Davidovits, 1991 ). The materials used for the production of geopolymer are essentially aluminosilicate materials in the presence of alkali activators (Ogundiran and Kumar, 2015 ). Such materials include Ground Granulated Blast Furnace Slag (GGBS), kaolinite clay, coal fly ash, Palm Oil Fuel Ash (POFA) among others and alkali activators which include either or a combination of some of the following: Na 2 SiO 3 , NaOH, and Ca(OH) 2 . The reaction of aluminosilicate containing source materials in an alkali medium results in geopolymer formation (Zarina et al., 2013 ). This involves the dissolution of aluminosilicate materials in an alkaline medium leading to oligomers which are precursors for geopolymer formation. Studies are on-going globally on developing geopolymer binder materials, using source materials from waste sources for sustainability. The utilisation of waste materials as precursors for geopolymer production is expected to reduce its disadvantage of cost and unsustainability (Neupane et al. 2018 ). Therefore, the aim of this study was to assess Nigerian mining waste clays for their potentials as precursors for geopolymer binders. The need to evaluate the potentials of mining waste clays, to produce geopolymer binders as a greener, sustainable, durable and carbon saving alternative to Portland limestone cement motivated this research. Mining waste clays from four mining sites located in the South-western part of Nigeria were characterised for their physical, chemical, mineralogical and thermal properties, in the raw, calcined and geopolymerised forms. The results revealed chemical, phase and mineralogical transformations arising from calcining and geopolymerisation processes. Optimal process conditions and material mix ratios were experimentally determined, leading to the formulation of process pathway to produce geopolymer binders and were tested for their mechanical capacity (Compressive strength). Carbon emission evaluation study was done for the production of geopolymer binder from Nigerian mining waste clays and compared with existing value of carbon emission for PLC manufacturing. 2. Materials and methods 2.1 Materials Samples of raw mining waste clays (MWCs) were collected from four mining locations in South West Nigeria, namely; Awo, Osun state (OS), Ibeshe, Lagos state (LA), Ijero, Ekiti state (EK) and Owode, Ogun state (OG), mining sites. The Global Positioning System (GPS) for the locations of the mining sites were as follows; Latitude 7 0 50'29"N, Longitude 4 0 2'57"E, (Ijero), Latitude 7 0 46'14''N, Longitude 4 0 24'16''E (Awo), Latitude. 7 0 8'30''N, Longitude. 3 0 26'36''E (Owode), Latitude. 6 0 34'56.1''N, Longitude. 3 0 29'15.8''E (Ibeshe). The mapping of these mining sites in South West Nigerian States is represented in Fig. 1 . The Samples were air dried and sieved into 212 microns and labeled OSR, LAR, EKR and OGR respectively. 2.2 Physical and structural characteristics of the mining waste clays 2.2.1 Loss on ignition Loss on ignition of the mining waste clays was determined by subjecting 1 g each of oven-dried (105 0 C) samples to heat treatment at 500 0 C, for about 4 hours in a muffle furnace, cooled in a desiccator and weighed. 2.2.2 Specific gravity. Mining waste clays samples of about 50g each were dried in the oven at 105 0 C to constant weight. Each sample was poured into a pre-weighed bottle of a suitable volume, to the brim. This was weighed. The bottle was emptied and cleaned. Water was poured into the same bottle to the brim and weighed. The weight of the bottle was recorded as M 1 , the weight of the bottle + sample as M 2, and the weight of the bottle + water as M 3 . Specific gravity was calculated using the ratio of the weight of sample to that of water of equal volume. 2.2.3 Bulk density. The clay samples of about 50g each were dried in the oven at 105 0 C, to constant weight. The final weight was taken and the sample was poured into a measuring cylinder. The sample was gently compacted and the volume was recorded. The bulk density was estimated using Eq. 1 (Atanda et al ., 2012). Bulk density (g/cm 3 ) = weight of dried sample (g) / Volume (cm 3 ) …………………….. (1) 2.2.4 Thermal properties Thermal properties data on occurrence of specific mineralogical and phase transformation leading to the determination of the dehydroxylation temperatures of the clays, was obtained by using thermogravimetry/differential thermal analysis method (TGA/DTA), NETZSCH STA 449 F3 Jupiter thermal analyser, set at 40–900°C with a heating rate of 10°C per minute under a nitrogen atmosphere. 2.2.5 Oxide composition The calcined clay samples were prepared into pellets. and the oxide composition of the samples was determined using an X-ray fluorescence spectrometer (XRF), by Sky Ray Instruments; EDX3600B X-ray fluorescence spectrometer. The system detects elements between Magnesium (Mg, Z = 12) and Uranium (U, Z = 92). Calibration of the equipment was achieved using a pure silver sample and a working curve was selected for the sample. Output data revealing the elemental composition of the samples were read on Excel software. 2.2.6 Functional groups Raw samples of the mining waste clays were analysed with a state-of-the-art FTIR instrument (Agilent Technologies brand of scanning frequency range of 4000 to 400 cm − 1 and resolution of 4cm − 1 ). 2.2.7 Mineralogical phases The mineral phases of the mining waste clay samples were obtained using XRD analysis by an automated powder diffractometer, NASENI-LASER instrument, with CuKα radiation of 30KV accelerating voltage, filament current of 10mA, 2 Theta range from 10 0 to 70 0 , and scan speed of 0.02 0 /5s. 2.2.8 Morphology and elemental composition One of the raw mining waste clays (OGr) was selected for this analysis and oven dried at 105 0 C. The particle morphology and microstructure was obtained by using scanning electron microscopy instrument equipped with an energy dispersive spectrometer (SEM/EDS), model JEOL-JSM 7600F, 20KV/127micronm. 2.3 Optimisation of geopolymer binder production conditions Appropriate conditions and material mixing ratios were optimised to achieve the best possible geopolymer products. These conditions included calcination temperature, calcination time, and mixing ratios (NaOH/Na 2 SiO 3 ). Each of the optimisation experiments was done by producing replicate samples of geopolymer, and the resulting geopolymers were subjected to compressive strength tests at 7, 14, 21, and 28 days of curing time. 2.3.1 Optimisation of clays’ calcination temperature Three temperature points around the DTA dehydroxylation peak, for each clay were marked T1, T2 and T3 representing pre-peak, peak point and post-peak respectively. Geopolymer binders were produced from clays calcined at these three different temperature points for 6 hours as observed in previous works (Adeniyi et al ., 2020) and compressive strength measured at 7, 14, 21 and 28 days. The calcination temperature that produced the geopolymer binder with the highest compressive strength at 28 days was considered the optimal calcination temperature for the particular clay source. Clays were therefore calcined at their respective optimal calcination temperatures and labeled; Ogun Calcined (OGC), Lagos Calcined (LAC), Osun Calcined (OSC) and Ekiti Calcined (EKC) clays respectively, for subsequent experiments and instrumental analysis. The production of geopolymer binders followed established procedure (Ogundiran et al., 2013 ; Ogundiran and Ikotun, 2014 ). Further to compressive strength tests, these clays calcined at the varying temperature points were also subjected to FTIR tests to observe structural changes based on varying calcining temperatures (T1, T2, T3). 2.3.2 Optimisation of calcination time The clays were calcined at the optimised temperatures, activator ratio of 1:1 (8MNaOH/Na 2 SiO 3 ) but at varying time 1, 3, 6, 10, 14 and 20 hours. Geopolymer binders were produced using these clays. Other production conditions remain fixed as in previous section. Compressive strength test was conducted on the geopolymers. The calcination time that gave the geopolymer with the highest strength at 28 days, was considered optimal calcination time. 2.3.3 Optimisation of mix ratio of activator (NaOH/Na 2 SiO 3 ) Geopolymer binders were produced from varying ratios of 8M NaOH/Na 2 SiO 3 . The mixing ratios were 1:1, 1:1.5, 1:2, and 1:2.5. The products were subjected to compressive strength tests and the mix ratio that produced the geopolymer with the highest strength was considered the optimal mix ratio. Samples of geopolymers produced at optimised conditions of temperature, time and activator specifications are displayed in Fig. 2 . 2.4 Characterisation of calcined clays and geopolymer binders The calcined clays and geopolymer binders produced at the optimal conditions for each of the clays crushed and sieved to 212 microns, were characterised. The powder samples were treated with isopropyl alcohol to terminate the geopolymerisation reactions (Ogundiran and Kumar, 2015 ). These samples characterised by FTIR, XRD and SEM/EDX. 3. Results and discussion 3.1 Physical properties of mining waste clays The results of loss on ignition (LOI), specific gravity (SG) and bulk density (BD) of the clays are presented in Fig. 3. The LOI of samples define the loss of sample mass due to ignition or combustion, which usually serves as an indicator of the amount of organic matter, moisture, volatile substances, and plant and animal fossils present in the sample originally. From the results, the order of the values observed for the loss on ignition of the clays is EK ˃ OS ˃ LA ˃ OG. This was correlated with the respective compressive strengths of the geopolymer binders. The specific gravity of a material is defined as the ratio of the mass of a given volume of that material to the mass of an equal volume of water. This parameter is generally used to characterise soils and similar materials. As observed from Fig. 3, the order of the specific gravity which is OG ˃ LA ˃ OS ˃ EK, seems to be opposite in trend with the LOI values. Bulk density is the dry weight of a unit volume of soil expressed in g/cm 3 . It is inversely related to pore space. It has an important influence on the reactivity of clay materials because bulk density can be related directly to the clay permeability and flow of liquid activator during the process of material mixing. The result of the respective bulk densities of the mining waste clays had the order of magnitude as follows: OG ˃ LA ˃ OS ˃ EK. These properties have correlations with the compressive strength development of the respective clays. 3.2 Thermal properties of the mining waste clays The DTA/TGA curves of the mining waste clays OSR, EKR, OGR, and LAR are presented in Figs. 4a, b, c and d respectively. The dehydroxylation process for clay minerals when thermally treated, is accompanied by mass loss at different temperature ranges. According to Zhou et al. (2017), dehydroxylation has been understood to depend essentially on the clays’ layered structures and the amount of hydroxyl groups contained. The respective dehydroxylation temperature ranges labeled Pre-Peak (T1), Peak point (T2) and Post-Peak (T3) for each of the four clay samples are presented in Table 1. Table 1: Dehydroxylation temperatures of Nigerian mining waste clays based on DTA curves CLAY Dehydroxylation Peak into metakaolinite ( 0 C) T1 T2 T3 EKr 440 550 702 LAr 420 575 740 OGr 455 580 704 OSr 446 517 603 The thermogrammes revealed percentage mass loss for the samples of Nigerian mining waste clays. For OSR dehydration/onset temperature was 88 0 C to 133 0 C with insignificant mass loss of 0.31% attributable to loss of adsorbed and interlayer water, indicative of dehydration, characteristic of when 2:1 clay minerals lose adsorbed, pore, and interlayer water (Kenne et al ., 2015, Adeniyi et al ., 2020). Major mass loss of 3.08% was observed, attributable to dehydroxylation at temperature range 449 0 C to 602 0 C. This is the occurrence of loss of structural OH that leads to the transformation of crystalline kaolin into the more reactive amorphous metakaolin (Longhi, 2015). For EKR, dehydration/onset temperature, was95 0 C to 131 0 C with insignificant mass loss of 0.61% attributable to loss of adsorbed and interlayer water. Major mass loss of7.88% attributable to dehydroxylation observed at temperature range 430 0 C to 701 0 C. However, for OGR dehydration/onset temperature was at 58 0 C to 130 0 C with insignificant mass loss of 0.18% attributable to loss of adsorbed and interlayer water. Major mass loss of 7.41% was observed at temperature range 458 0 C to 703 0 C, attributable to dehydroxylation. Whereas, LAR sample’s dehydration/onset temperature was at 58 0 C to 131 0 C with insignificant mass loss of 0.57% attributable to loss of adsorbed and interlayer water. Major mass loss of; (97.41 – 88.39) 9.02% attributable to dehydroxylation at temperature range 466 0 C to 741 0 C. The sample LAR had the highest mass loss due to dehydroxylation, implying largest quantity of decomposed minerals into reactive amorphous clay. The maximum temperature of dehydroxylation on the DTA/TGA curve also was the highest, providing more temperature range to allow for more minerals to get decomposed. This could be responsible for the compressive strength of the geopolymer product which was only lesser than OGR, among all the samples studied. The sample EKR had the highest mass loss due to dehydration of adsorbed and interlayer water. This may be due to the presence of much hydrated 2:1 clay minerals in this sample (Adeniyi et al ., 2020). This clay also turned out as the sample with the geopolymer having lowest compressive strength. It could be suspected that the sample EKR has the least of kaolinite composition compared to others, which decomposed to the amorphous metakaolinite during calcination. The other decomposed minerals within the calcining temperature range may not have impacted positively on the compressive strength. The temperature values corresponding to T1, T2 and T3 on the DTA curves provided basis for choice of the set of temperature values tested for the optimisation of calcination temperature of the mining waste clays. Generally, for all the clays, the temperature range for the dehydroxylation of kaolinite which occurred between 430 0 C and 740 0 C, had the most intense mass loss among the three peaks in the respective thermograms, suggesting a relatively large presence of kaolinite alongside the composition of the 2:1 clay types suggested by the XRF results. This assumption is further ascertained by the XRD results, revealing intense kaolinite peaks for the clays. At a temperature range above 800 0 C, the occurrence here was the dehydroxylation of some 2:1 clay minerals; illite, montmorillonite, and mullite (Selmani et al ., 2015). 3.3 Chemical composition of the clays The chemical compositions of Nigerian mining waste clays, as revealed by the XRF analysis are presented in Table 2. They are expressed as oxides of constituent metals. The results show that the dominant components across all the samples are SiO 2 and Al 2 O 3 , which suggests the presence of clay minerals such as kaolinites, quartz, feldspar, and others, consistent with composition of clay minerals suitable for the geopolymerisation process (Ferone et al ., 2015). It could be directly derived from the results that, the sum percentages of the Si and Al oxides in each clay sample were, 71%, 72%, 82%, and 79% for EK, OS, OG, and LA clays respectively. The high content of these Si and Al oxides, at least above 70%, implied that they were all kaolinite aluminosilicates and are suitable as pozzolanic materials (Ogundiran and Kumar, 2015; Ayininuola and Adekitan, 2016). Table 2 : Chemical composition of clay samples by XRF analysis (Oxides % weight) EKC OSC OGC LAC Al 2 O 3 25.5 23.7 27.7 28.3 SiO 2 45.1 48.6 54.5 50.8 CaO 0.03 0.03 0.03 0.28 TiO 2 Nil Nil 0.73 1.67 Fe 2 O 3 0.01 0.41 3.92 5.08 NiO 0.43 0.07 0.11 0.09 The presence of iron oxide in a considerable proportion, particularly in OG and LA clays, suggests not only iron minerals such as pyrite or goethite but is also indicative of clay minerals such as chlorite, montmorillonite, and illite (Zhou et al ., 2017). The absence of TiO 2 in clays EK and OS implied the absence of minerals like anatase and rutile, while it is present in clays OG and LA (Ferone et al ., 2015). In all the clays, CaO is very low, implying that they are less calcite clays. Calcite clays having an excess of 20% CaO content are known to set too fast and are usually not recommended for the geopolymer source material (Srinivasula et al ., 2016). The silica-to-alumina ratio of the clays, according to the XRF data; EKC (1.77), OSC (2.1), OGC (1.97) and LAC (1.79) generally tends toward 2. This is suggestive of the 2:1 clay type. 3.4. Functional group of the waste clays The FTIR spectra that show the basic functional groups of raw OSR, EKR, OGR and LAR clays are presented in Figs. 5a, b, c and d respectively. The FTIR spectra of the raw clays generally showed peaks at major regions of 4000 to 3000 and 1300 to 500 cm -1 . Peaks at 3691, 1114, 1032 and 3620cm -1 in the raw clays indicating the presence of functional groups O-H, Si-O, Si-O-Si and Al-OH respectively. For all the clays, the O-H stretching vibrations typical of the presence of kaolinite occurred at absorption bands 3698 to 3694 cm -1 (Adeniyi et al ., 2020). Also Al-OH peaks typical of 2:1 clay minerals (montmorillonite and illite) which were also observed by Adeniyi et al ., (2020) occurred at absorption bands 3626 to 3620 cm -1 . These OH vibrations are due to hydroxyl groups that are attached to aluminium octahedron sheet of the clay minerals (Ogundiran and Kumar, 2015). Only EKr had a peak at 1636cm -1 representing the carboxylate group. Peaks at 1114 and 1120 cm -1 indicated the vibrations of Si-O-Si. Bands at 1004 and 911 represent Al-OH bending vibrations which identified these clays as mainly kaolinite (Ogundiran and Kumar, 2015). The stretching vibrations of OH bonds at 3691, 3690, 3688, and 3621cm -1 generally indicate the presence of crystalline kaolinite mineral (Selmani et al ., 2015; Kenne et al ., 2015). Peaks observed in all the clays around 1109, 1112, 1114, and 1032 cm −1 are associated with Si-O and Si-O-Si stretching vibrations (Ogundiran and Kumar, 2015). 3.5. Mineralogical phases of the clays The results of the XRD analysis of the clays are presented in diffractogrammes of 2θ at 10°2θ to 70°2θ range in Fig. 6 for OSR, EKR, OGR and LAR. The respective diffractogrammes show that the clays in their raw forms contained predominantly crystalline kaolinite as shown in similar reports (Ogundiran and Sanjay, 2015’ Adeniyi et al., 2020). Other minerals present in the clays were illite, anatase and quartz. 3.6. Microstructure of the mining waste clays The SEM image of one of the mining waste clays sample was done to provide a representative result of the raw clay samples studied. Fig. 13a presents the SEM image of Ogun raw clay (OGR). Observations from the SEM image revealed a type of plate-like particle morphology consistent with crystalline phase kaolinite clay. The Si (45%) and Al (20%) composition as revealed by the EDS coupled with the SEM instrument further confirmed the large percentage composition of silica and alumina in the clay, confirming it as an aluminosilicate source. 3.7. Optimisation of process conditions and material mixing 3.7.1. Optimisation of calcining temperature of the clays Compressive strength values were used to determine the optimised parameters for geopolymer binders’ production. The results of the compressive strength of the geopolymers produced from clays calcined at varying temperatures are presented in Fig. 7. The DTA’s dehydroxylation peak provided guiding data to predict the calcination temperature at which metakaolin was formed (Fig. 4). This was complemented with compressive strength values of geopolymers produced from clays calcined at the pre-peak (T1), peak point (T2), and post-peak (T3) temperature points and at an extreme temperature 850 0 C (T4), for each of the clays. The optimal calcination temperatures for the mining clays at the highest values of the compressive strengths were 700, 740, 600, and 700 ( 0 C) for OG, LA OS, and EK respectively. From the compressive strength test results, it was observed that the calcination of clays at the T3 dehydroxylation (post-peak) temperatures gave the best geopolymer performance. This suggests that the thermal process of clay dehydroxylation into amorphous metakaolin continued after the spectra peak and had only come to completion at the post-peak point (T3). The temperature corresponding to the highest compressive strength, represents the optimal calcining temperature. 3.7.2 Optimisation of calcination time of the mining waste clays The results of the compressive strength of the geopolymer binders produced from OG, LA, OS, and EK calcined at their respective optimised temperatures over varying calcination time are presented in Fig. 8. The compressive strength values range from 6.10 to 29.5 MPa for calcining time range from 1 to 20 hours. The highest compressive strength values (OG; 29.5, LA; 27.1, OS; 25.4 and EK; 23.1 MPa) for all the clays was observed at 6hours calcining time. The results suggest that the optimal calcination time for these mining waste clays was 6 hours. The decline in the compressive strength at higher calcining time further suggest that at a calcination time longer than 6 hours, there is an increase in the quantity of non-amorphous materials which were unreactive, resulting in reduced reactivity of the clays. 3.7.3. Optimisation of activator mixing ratio NaOH/Na 2 SiO 3 The results of the optimisation of mixing ratio of activator consisting of NaOH and Na 2 SiO 3 , is presented in Fig. 10. It was observed from the results that the NaOH/Na 2 SiO 3 ratio of 1: 1.5 generally achieved the production of geopolymers with the highest compressive strength. It can be suggested that the optimal ratio is therefore 1:1.5. On the final analysis the geopolymer binders produced at optimal conditions had compressive strengthvalues of 33.7, 29.8, 28.3 and 26.1 at 28 days for OG, LA, OS and EK respectively. 3.8. Characterisation of calcined clays and geopolymer binders 3.8.1. Fourier transform infrared spectroscopy (FTIR) The results of the FTIR characterisation of the calcined clays at different and geopolymers of the four mining waste clays are presented in Fig 11 a, b, c and d. The results revealed the thermally induced dehydroxylation of the clays, as initially indicated by TGA/DTA results. Where T1 refers to the temperature at the onset of dehydroxylation peak, T2 describes the temperature at the peak, and T3 is for the temperature after the peak. Also, the produced geopolymer binders’ spectra labelled GPOG, GPLA, GPOS, and GPEK respectively, are presented. It was observed that all spectra corresponding to clays calcined at temperatures; T1, T2, and T3 had no peak at the fingerprint region of the spectra, implying loss of adsorbed water, whereas the OGGP spectrum for the geopolymer had peaks at 3000, 3170 and 3440 (cm -1 ) similar to such as existed in raw clays, which was ascribed to as the presence of adsorbed atmospheric water in them. The explanation for the presence of adsorbed water in geopolymer is the water used in the production of the geopolymer. An interesting trend in the three calcination temperatures was that, though they had similar spectra patterns but different bands at the functional group region; for T3 are usually broader and occur much earlier than T2, while the same trend occurs between T2 and T1. This trend suggests the extent of dehydroxylation and metakaolin formation being in the order of T3 ˃ T2 ˃ T1, thus validating the compressive strength tests which had the geopolymers of T3 calcination temperature with the highest strength. This was observed to be consistent across all four clays investigated. The strong band at 1086 cm-1 in most of the calcined clays shifted towards the lower wavenumber after the geopolymerisation reaction. The spectrum shift was approximately 78 cm -1 . This indicated that there had been a change in the microstructure during the hydration process, which produced a new product having a different microstructure not similar to that of metakaolin. This spectra transformation is similar to what was observed in the XRD patterns of the calcined and geopolymer samples respectively. According to Zhang and Li (2009), a 1086 cm -1 peak resulted from symmetrical vibration of the Si-O bond. It can be suggested that the shift to a lower wave number in geopolymer could be attributed to the partial replacement of SiO 4 tetrahedron by AlO 4 tetrahedron, which gives rise to a change in the chemical atmosphere of the Si-O bond. 3.8.2. X-ray diffraction analysis (XRD) Results of the XRD analysis of the mining clays in raw, calcined and geopolymer forms are presented in Fig. 12 for OG, LA, OS and EK. This result enhanced the understanding of the impact of thermal treatment on the clays and the subsequent geopolymerisation reaction to achieving the geopolymer products. Observing the diffractograms of the calcined clays and geopolymers, they confirm transformation into amorphous form, by the presence of hallow peaks or humps at 2Ɵ (20º - 30º). The disappearance kaolinite characteristic peaks at 2Ɵ (12º) revealed the absence kaolinite and suggests conversion to metakaolinite. Most of the diffractograms peaks of the calcined clays were found closely related or similar in some features when compared with those of the corresponding raw clays. This can be attributed to the fact that the clays are naturally occurring complex mixtures of various clay minerals and even non-clay minerals, especially as they are mine wastes. Hence, some minerals could survive thermal treatment without being decomposed within the temperature range utilised. All of the four clays, from the XRF data, had high silica content and this was justified by the strong peaks for quartz, a silica-based mineral, in all of their XRD diffractograms. Also, they all contained potassium oxide from XRF result and this is also evident in their XRD peaks representing the presence of illite, potassium-containing mineral. 3.8.3. Scanning electron microscopy (SEM/EDS) The result of SEM/EDS for samples of raw clay, calcined clay and geopolymer binder of OG clay is presented if Fig. 13. The SEM image observed in the calcined clay revealed a more dispersed irregular morphology, consistent with an amorphous phase of clay material. This microstructure differs from that of the raw clay, where the morphology was orderly and layered, being crystalline. It can be suggested that the change in morphology, as seen, was due to thermal treatment (Selmani et al ., 2015). A careful observation of the geopolymer’s SEM image presented here suggests denser particle morphology and the spongy appearance of the geopolymeric structure. This also suggests the presence of interlayer water as well as a well-solidified system with some unreacted activator molecules (Selmani et al ., 2015). The SEM image showed that the geopolymer produced contained some un-dissolved calcined clay and an amount of newly formed sponge-like aluminosilicate species. The gels of the geopolymer is coarse-textured, sponge-like and contained round ball-like materials with bridging in between. This observation is very similar to what was reported by Ogundiran and Kumar (2015). An attempt was made in this study to compute an estimated value of carbon dioxide emission reduction through the use of Nigerian mining clay wastes to produce geopolymer binder and compared with CO 2 emission associated with cement production. Assumption: it was assumed that the specific heat capacity of Nigerian mining waste clays is the same as that of normal clays. The value of the specific heat capacity of clay is given as 900 J/KGK (Zhou et al ., 2017). Recall the energy formular: The energy required to raise 1 tonne of Nigerian clay to the optimised calcining temperature. E = M x C x ΔT …………………… …………………………3.18 Where E = energy in KWh, M = mass of Nigerian clay in Kg, C = specific heat capacity of Nigerian clay in J/KgK, ΔT = temperature change in 0 C. Energy E in KWh (Conversion factor of KWh = MJ x 0.27777) Mass M = considering 1 tonne = 1000kg Specific heat capacity C = 900 J/KgK Temperature change ΔT = optimised calcining temperature – initial temperature of clay before heating in the furnace. The average calcination temperature utilised for Nigerian clays can be arrived at thus: (700 + 740 + 700 + 600) / 4 = 2740/4 = 685 0 C. ΔT = (685 – 32) 0 C = 653 0 C For one tonne of Nigerian clay (1000 Kg); E = M x C x ΔT E = 1000 x 900 x 653 = 587.7 MJ = 163.4 KWh The carbon emission factor of biomass combustion is given as 0.35 Kg CO 2 /KWh (Zhou et al., 2017). The implication is that the calcination of 1 tonne of Nigerian clay would generate an amount of CO 2 calculated as; 163.4 KWh x 0.35 KgCO 2 /KWh = 57.19Kg CO 2 . per tonne of clay. It can be inferred from the above calculation that: Calcination activities to prepare Nigerian clay for utilisation as a geopolymer binder, for making construction materials generally, will contribute only an estimated value of 57.19Kg CO 2 /tonne to the carbon emissions. The measurement of this same parameter for the manufacture of Portland cement was estimated at 800 Kg CO 2 per tonne (Zhou et al ., 2017). In comparison, Carbon emission difference: 800 – 57.19 = 742.81 Percentage carbon emission reduction: 742.81/800 x 100 = 92.9%. The implication of this is that the utilisation of calcined Nigerian clays as geopolymer binders in the production of construction materials has the potential of reducing contribution to carbon emission by 92.9%. 4. Conclusion The need to evaluate the potentials of Nigerian mining waste clays (NMWCs), for the production of geopolymer binders, as a sustainable, durable and carbon saving alternative to Portland limestone cement motivated this research. Screened and sieved samples of mining wastes from four states in Nigeria were characterised by their physical, thermal, chemical, structural, mineralogical and microstructural properties. The impact of their physical properties; loss on ignition, specific gravity and bulk density on strength development of geopolymer binders produced was observed. the values of bulk density and specific gravity were directly proportional to the compressive strength values of the clays, while loss on ignition was inversely proportional to compressive strength. Characterisation using DTA/TGA, XRF, FTIR, XRD and SEM/EDS methods of instrumental analysis of NMWCs suggested that the studied Nigerian clays, decomposed on thermal treatment at temperature within 420 to 740 0 C. Though mainly kaolinite, NMWCs were essentially a complex mixture of clay and non-clay minerals which included 1:1 layered structure (kaolinite), 2:1 layered structure (illite, montmorillonite) clay types and quartz (non-clay). Optimised process conditions for NMWCs were obtained. The optimised calcining temperature was within the range of 600 to 740 ( 0 C), for 6 hours optimal time. Also, the appropriate mix ratio for the activator constituents (NaOH/Na 2 SiO 3 ) were experimentally determined as 1:1:5. Geopolymer binders were produced and compressive strength at 28 days were in the range of 33.7, 29.8, 28.3 and 26.1 at 28 days, with Ogun clay having highest Compressive strength and Ekiti the lowest. Carbon emission reduction for using NMWCs was 92.9% in comparison with that of conventional Portland cement. Declarations Acknowledges Acknowledgement to Professor Mary B. Ogundiran 9professor of Analytical Chemistry) for providing the setting out design for the research and to Professor (Engineer) G.M. Ayininuola for his technical guidance on Civil Engineering details of the research. Also due acknowledgement to Dr. Abiodun Ogbesejana for his proof-reading assistance towards the quality of writing and presentation of the article. Funding No organizational funding was received for this research. Author Information: Authors and Affiliations 1. Adebayo Matthew Adeleye Department of Chemistry, University of Ibadan, Nigeria and Department of Chemical Sciences, Joseph Ayo Babalola University, Nigeria. 2. G.M Ayininuola Department of Civil Engineering, University of Ibadan, Nigeria 3. Mary B. Ogundiran Department of Chemistry, University of Ibadan, Nigeria. Contributions 1. A.M.A: Research results and data generation, provision of samples, methodology, writing. 2. G.M.A and M.B.O: Research design and Supervision. Corresponding author Adebayo Matthew, Adeleye at [email protected] Consent to participate declaration Not applicable. Ethics declarations All ethical standards have been duly observed during the research. Consent for publication Not applicable. Competing interests The authors declare no competing interests. References Adeniyi, F. I., Ogundiran, M. B. Hemalatha T., Hanumantrai, B. B. 2020. Characterization of raw and thermally treated Nigerian kaolinite‑containing clays using instrumental techniques. Springer Nature Applied Sciences Journal. 2:821 Ayininuola, G. M., Adekitan, O. A. 2016. Characterization of Ajebo Kaolinite Clay for Production of Natural Pozzolan. World Academy of Science, Engineering and Technology International Journal of Civil, Environmental, Structural, Construction and Architectural Engineering. 10:9. Davidovits, J., 1991. Geopolymers: Inorganic Polymeric New Materials, Journal of Thermal Analysis, 37, 1633–1656 Ferone, C., Liguori, B., Capasso, I., Colangelo, F., Cioffi, R., Cappelletto, E. and DiMaggioc, R. 2015. Thermally treated clay sediments as geopolymer source material. Elsevier. Applied Clay Science 107: 195-204 Kenne, B.B., Elimbia A., Cyrb M., Dika, M. J., Tchakoute, K. H. 2015. Effect of the rate of calcination of kaolin on the properties of metakaolin-based geopolymers. Journal of Asian Ceramic Societies . 3 (1): 130-138 Longhi, M.A., 2015. Valorisation of a kaolin mining waste for the production of geopolymers. Journal of Cleaner Production. 10.1016.12.011 Mahasenan, N., Steve,S., Kenneth, H. and Kaya, Y. 2003. The Cement Industry and Global Climate Change: Current and Potential Future Cement Industry CO 2 Emissions. Greenhouse Gas Control Technologies– 6th International Conference. Oxford: Pergamon. 995–1000 Neupane, K., Chalmers, D., Kidd, P. 2018. High strength geopolymer concrete properties, advantages and challenges. Advances in materials. 7 (2) : 15-25. Ogundiran, M. B., Nugteren, H. W., Witkamp, G. J. 2013. Immobilisation of lead smelting slag within spent aluminate-fly ash based geopolymers. Journal of Hazardous Materials 248-249: 29-36. Ogundiran, M. B., Ikotun, O. J. 2014. Investigating the Suitability of Nigerian Calcined Kaolins as Raw Materials for Geopolymer Binders. Trans. industrial ceramic society. 73(2):138-142. Ogundiran, M. B., Kumar S., 2015. Synthesis and characterisation of geopolymer from Nigerian clay; Elsevier, Journal of Applied clay science. 108: 171-181 Selmani S., Essaidi N., Gouny F., Bouaziz S., Joussein E., Driss A., Sdiri A. and Rossignol S. 2015. Physical–chemical characterization of Tunisian clays for the synthesis of geopolymers materials. Journal of African Earth Sciences , Volume 103, 113-120 Srinivasula, R. M., Dinakar, P. and Hanumantha, R. B. 2016. A review of the influence of Source material’s oxide composition on the compressive strength of geopolymer concrete. Elsevier Journal; Microporous and Mesoporous materials . 234: 12-23. Temuujin, J., VanRiessen, A. 2009. Effect of fly ash preliminary calcination on theproperties Of geopolymer. Journal of Hazardous Materials . 164:634–9. Zarina, Y., Mohd, M. A., Kamarudin, H., Khairul, N. I., Andre, V. S. and Vizureanu, P., 2013. Chemical and Physical Characterization of Boiler Ash from Palm Oil Industry Waste for Geopolymer Composite. 1408-1412 Zhang, Z., Nielsen, M. K., Horsholt, S., Muralidharan, G., Jorgensen, J. B. 2021. Digitalization, control and optimisation for cement plants.computer aided Engineering. 50: 1319-1324. Zhou D., Wang R., Tyrer M., Wong H., Cheeseman C. 2017. Sustainable infrastructure development through use of calcined excavated waste clay as a supplementary cementitious material. Journal of Cleaner Production 168:-1180-1192. Additional Declarations No competing interests reported. Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. 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18:57:30","extension":"html","order_by":48,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":107695,"visible":true,"origin":"","legend":"","description":"","filename":"earlyproof.html","url":"https://assets-eu.researchsquare.com/files/rs-7214593/v1/a58f48a02dddc9ae7215910d.html"},{"id":92979634,"identity":"c4546504-fd5d-4a6b-9eb0-1e639f894996","added_by":"auto","created_at":"2025-10-07 19:05:28","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":170283,"visible":true,"origin":"","legend":"\u003cp\u003eMap showing selected mining locations in the southwest region of Nigeria.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-7214593/v1/ed515d1aced66e567267baf7.png"},{"id":92979233,"identity":"ebbc30c6-f01b-49c6-861b-5e5921e61b9f","added_by":"auto","created_at":"2025-10-07 18:57:28","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":196127,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eSamples of geopolymers produced at optimised conditions of the mining waste clays\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-7214593/v1/19703634ca77c334345b9967.png"},{"id":92979635,"identity":"51868608-c83c-456a-b955-2a033689c010","added_by":"auto","created_at":"2025-10-07 19:05:28","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":19804,"visible":true,"origin":"","legend":"\u003cp\u003eSome\u003cstrong\u003e \u003c/strong\u003ephysical properties of the mining waste clays\u003cstrong\u003e \u003c/strong\u003e(a) Loss on ignition, (b) Specific gravity, (c) Bulk density\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-7214593/v1/29d54b47acf2b707206528d4.png"},{"id":92980104,"identity":"479cdb3a-bef0-4c6f-98d0-4bc2000ccd58","added_by":"auto","created_at":"2025-10-07 19:13:28","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":77970,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eTGA/DTA curves of a) OSR, b) EKR, c) OGR and d) LAR\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-7214593/v1/8aba7711847d858833a29323.png"},{"id":92980363,"identity":"2adec3a3-2977-4450-aef2-16199b3bb4bd","added_by":"auto","created_at":"2025-10-07 19:21:28","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":83783,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eFTIR spectra of a)OSR, b) EKR, c) OGR and d) LAR raw clays\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-7214593/v1/8461225a56185b6be44a8256.png"},{"id":92979238,"identity":"840d2086-7c99-4a5b-9eed-b118628bf389","added_by":"auto","created_at":"2025-10-07 18:57:28","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":107532,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eXRD diffractograms of the raw waste mining clays\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-7214593/v1/a2c3fb7226c1393fcd2ae273.png"},{"id":92980362,"identity":"ee1d56aa-b259-4f05-9125-329b99ee9661","added_by":"auto","created_at":"2025-10-07 19:21:28","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":11463,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eCompressive strength of geopolymers of different clays at varying calcining temperatures\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"7.png","url":"https://assets-eu.researchsquare.com/files/rs-7214593/v1/8c2f43523bf18c7e1092d4dd.png"},{"id":92979242,"identity":"fb3dc2a7-a4bf-4fda-b3c9-c19c56e50bcc","added_by":"auto","created_at":"2025-10-07 18:57:28","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":31117,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eCompressive strength of geopolymers of different clays at varying calcination time\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"8.png","url":"https://assets-eu.researchsquare.com/files/rs-7214593/v1/b827635272dc481ec7ad0f0e.png"},{"id":92979640,"identity":"4e82a1e3-2032-4832-8bc1-1f7d21767a0d","added_by":"auto","created_at":"2025-10-07 19:05:29","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":18707,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eFig. 10.\u003c/strong\u003e \u003cstrong\u003eCompressive strength of geopolymers with varying Na\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e2\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003eSiO\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e3\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e/NaOH ratio\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"10.png","url":"https://assets-eu.researchsquare.com/files/rs-7214593/v1/3bf145b450d11c1706a68527.png"},{"id":92979244,"identity":"62bf7447-cecb-408f-bc16-541692b9ecff","added_by":"auto","created_at":"2025-10-07 18:57:29","extension":"png","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":49742,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eFig.11.\u003c/strong\u003e FTIR of a) LA, b) EK, c) OG and d) OS clays for raw, calcined at varying temperatures and their geopolymers.\u003c/p\u003e","description":"","filename":"11.png","url":"https://assets-eu.researchsquare.com/files/rs-7214593/v1/49d174d9999119f871da8319.png"},{"id":92979246,"identity":"048d437e-8f47-40d9-891c-ea9dbaa29bba","added_by":"auto","created_at":"2025-10-07 18:57:29","extension":"png","order_by":11,"title":"Figure 11","display":"","copyAsset":false,"role":"figure","size":98254,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eFig. 12.\u003c/strong\u003e XRD diffractograms for raw and calcined clays and geopolymer samples\u003c/p\u003e","description":"","filename":"12.png","url":"https://assets-eu.researchsquare.com/files/rs-7214593/v1/e8d2c636550ea302fb5cbb33.png"},{"id":92979641,"identity":"2ca07818-e373-43eb-979e-8d754f8fed2b","added_by":"auto","created_at":"2025-10-07 19:05:29","extension":"png","order_by":12,"title":"Figure 12","display":"","copyAsset":false,"role":"figure","size":250169,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eFig. 13.\u003c/strong\u003e SEM image and EDS of raw and calcined clay and geopolymer product\u003c/p\u003e","description":"","filename":"13.png","url":"https://assets-eu.researchsquare.com/files/rs-7214593/v1/594485cae98a4154b10b898f.png"},{"id":97136252,"identity":"c04e3c20-ccb5-4955-8b3b-aa80c775a2bd","added_by":"auto","created_at":"2025-12-01 09:56:14","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":2384023,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7214593/v1/7f3e7045-31f3-447b-a4df-413eda78e356.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Characterisation of Nigerian Mining Waste Clays as Sustainable Precursors for Production of Low Carbon Geopolymer Binders","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eRecent findings have shown that the world cement production has increased by about 20% in the past decade and has reached over 4.1\u0026nbsp;billion tonnes in 2019, largely driven by demand from developing countries (Zhang et al. \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). The world Portland limestone cement (PLC) production has attained an increment over the 2010 level by 24% (Zhang et al., \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e2021\u003c/span\u003e; Zhou et al., \u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e2017\u003c/span\u003e). It was further speculated that this figure was expected to increase astronomically in the years to come. This is because many developing societies of the world are experiencing increased urbanisation and population growth, thus impacting the demand for housing and infrastructure development.\u003c/p\u003e\u003cp\u003eHigh and excessive energy requirement has been found to be a flaw associated with PLC manufacturing. According to Temuujin and Van Riessen (2009), the energy requirement for the manufacturing of PLC is just next to that of steel and aluminium. In addition, the high release of dust into the environment, during the manufacturing process of PLC is of major health concern. Coupled with these problems, a major adverse impact from cement manufacture is the high quantity of carbon (iv) oxide (CO\u003csub\u003e2\u003c/sub\u003e) being released into the atmosphere during production. It has been reported that the manufacture of PLC generates about 700 to 900 kg of CO\u003csub\u003e2\u003c/sub\u003e emission per tonne of PLC (Zhou et al., \u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e2017\u003c/span\u003e; Mahasenan \u003cem\u003eet al\u003c/em\u003e.,2003). Mahasenan et al. (\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2003\u003c/span\u003e) informed that the process of CO\u003csub\u003e2\u003c/sub\u003e release during cement manufacture derives majorly from the heating of limestone (calcium carbonate) at very high temperatures, to produce lime and carbon dioxide and also indirectly through the use of energy if that involves the emission of CO\u003csub\u003e2\u003c/sub\u003e. Efforts are being put in place at finding greener alternatives to PLC, particularly to align with the United Nations agenda on sustainable development goals (SDG\u0026rsquo;s 2030) on the environment and reducing global warming, which impacts on climate change. The replacement of PLC by less carbon-intensive materials is widely accepted as an effort towards reducing the overall CO\u003csub\u003e2\u003c/sub\u003e emission associated with cement manufacturing. Geopolymer has been identified as one of such materials that have potentials to replace PLC in building and construction materials. This acceptance was premised on the advantages of geopolymer which include faster setting time, higher mechanical strength, higher water resistance, resistant to acid attacks and environmental weathering making them more applicable for external construction materials. However, cost of production seems to be a major shortcoming of geopolymer, which is still being investigated.\u003c/p\u003e\u003cp\u003eGeopolymer is a term coined by a French scientist, Professor Joseph Davidovits, in the 1970s (Davidovits, \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e1991\u003c/span\u003e). The materials used for the production of geopolymer are essentially aluminosilicate materials in the presence of alkali activators (Ogundiran and Kumar, \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e2015\u003c/span\u003e). Such materials include Ground Granulated Blast Furnace Slag (GGBS), kaolinite clay, coal fly ash, Palm Oil Fuel Ash (POFA) among others and alkali activators which include either or a combination of some of the following: Na\u003csub\u003e2\u003c/sub\u003eSiO\u003csub\u003e3\u003c/sub\u003e, NaOH, and Ca(OH)\u003csub\u003e2\u003c/sub\u003e. The reaction of aluminosilicate containing source materials in an alkali medium results in geopolymer formation (Zarina et al., \u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e2013\u003c/span\u003e). This involves the dissolution of aluminosilicate materials in an alkaline medium leading to oligomers which are precursors for geopolymer formation.\u003c/p\u003e\u003cp\u003eStudies are on-going globally on developing geopolymer binder materials, using source materials from waste sources for sustainability. The utilisation of waste materials as precursors for geopolymer production is expected to reduce its disadvantage of cost and unsustainability (Neupane et al. \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). Therefore, the aim of this study was to assess Nigerian mining waste clays for their potentials as precursors for geopolymer binders.\u003c/p\u003e\u003cp\u003eThe need to evaluate the potentials of mining waste clays, to produce geopolymer binders as a greener, sustainable, durable and carbon saving alternative to Portland limestone cement motivated this research. Mining waste clays from four mining sites located in the South-western part of Nigeria were characterised for their physical, chemical, mineralogical and thermal properties, in the raw, calcined and geopolymerised forms. The results revealed chemical, phase and mineralogical transformations arising from calcining and geopolymerisation processes. Optimal process conditions and material mix ratios were experimentally determined, leading to the formulation of process pathway to produce geopolymer binders and were tested for their mechanical capacity (Compressive strength). Carbon emission evaluation study was done for the production of geopolymer binder from Nigerian mining waste clays and compared with existing value of carbon emission for PLC manufacturing.\u003c/p\u003e"},{"header":"2. Materials and methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\u003ch2\u003e2.1 Materials\u003c/h2\u003e\u003cp\u003eSamples of raw mining waste clays (MWCs) were collected from four mining locations in South West Nigeria, namely; Awo, Osun state (OS), Ibeshe, Lagos state (LA), Ijero, Ekiti state (EK) and Owode, Ogun state (OG), mining sites.\u003c/p\u003e\u003cp\u003eThe Global Positioning System (GPS) for the locations of the mining sites were as follows; Latitude 7\u003csup\u003e0\u003c/sup\u003e50'29\"N, Longitude 4\u003csup\u003e0\u003c/sup\u003e2'57\"E, (Ijero), Latitude 7\u003csup\u003e0\u003c/sup\u003e46'14''N, Longitude 4\u003csup\u003e0\u003c/sup\u003e24'16''E (Awo), Latitude. 7\u003csup\u003e0\u003c/sup\u003e8'30''N, Longitude. 3\u003csup\u003e0\u003c/sup\u003e26'36''E (Owode), Latitude. 6\u003csup\u003e0\u003c/sup\u003e34'56.1''N, Longitude. 3\u003csup\u003e0\u003c/sup\u003e29'15.8''E (Ibeshe). The mapping of these mining sites in South West Nigerian States is represented in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. The Samples were air dried and sieved into 212 microns and labeled OSR, LAR, EKR and OGR respectively.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e\u003ch2\u003e2.2 Physical and structural characteristics of the mining waste clays\u003c/h2\u003e\u003cdiv id=\"Sec5\" class=\"Section3\"\u003e\u003ch2\u003e2.2.1 Loss on ignition\u003c/h2\u003e\u003cp\u003eLoss on ignition of the mining waste clays was determined by subjecting 1 g each of oven-dried (105\u003csup\u003e0\u003c/sup\u003eC) samples to heat treatment at 500\u003csup\u003e0\u003c/sup\u003eC, for about 4 hours in a muffle furnace, cooled in a desiccator and weighed.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec6\" class=\"Section3\"\u003e\u003ch2\u003e2.2.2 Specific gravity.\u003c/h2\u003e\u003cp\u003eMining waste clays samples of about 50g each were dried in the oven at 105\u003csup\u003e0\u003c/sup\u003eC to constant weight. Each sample was poured into a pre-weighed bottle of a suitable volume, to the brim. This was weighed. The bottle was emptied and cleaned. Water was poured into the same bottle to the brim and weighed. The weight of the bottle was recorded as M\u003csub\u003e1\u003c/sub\u003e, the weight of the bottle\u0026thinsp;+\u0026thinsp;sample as M\u003csub\u003e2,\u003c/sub\u003e and the weight of the bottle\u0026thinsp;+\u0026thinsp;water as M\u003csub\u003e3\u003c/sub\u003e. Specific gravity was calculated using the ratio of the weight of sample to that of water of equal volume.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec7\" class=\"Section3\"\u003e\u003ch2\u003e2.2.3 Bulk density.\u003c/h2\u003e\u003cp\u003eThe clay samples of about 50g each were dried in the oven at 105\u003csup\u003e0\u003c/sup\u003eC, to constant weight. The final weight was taken and the sample was poured into a measuring cylinder. The sample was gently compacted and the volume was recorded. The bulk density was estimated using Eq.\u0026nbsp;1 (Atanda \u003cem\u003eet al\u003c/em\u003e., 2012).\u003c/p\u003e\u003cp\u003eBulk density (g/cm\u003csup\u003e3\u003c/sup\u003e)\u0026thinsp;=\u0026thinsp;weight of dried sample (g) / Volume (cm\u003csup\u003e3\u003c/sup\u003e) \u0026hellip;\u0026hellip;\u0026hellip;\u0026hellip;\u0026hellip;\u0026hellip;\u0026hellip;\u0026hellip;.. (1)\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec8\" class=\"Section3\"\u003e\u003ch2\u003e2.2.4 Thermal properties\u003c/h2\u003e\u003cp\u003eThermal properties data on occurrence of specific mineralogical and phase transformation leading to the determination of the dehydroxylation temperatures of the clays, was obtained by using thermogravimetry/differential thermal analysis method (TGA/DTA), NETZSCH STA 449 F3 Jupiter thermal analyser, set at 40\u0026ndash;900\u0026deg;C with a heating rate of 10\u0026deg;C per minute under a nitrogen atmosphere.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec9\" class=\"Section3\"\u003e\u003ch2\u003e2.2.5 Oxide composition\u003c/h2\u003e\u003cp\u003eThe calcined clay samples were prepared into pellets. and the oxide composition of the samples was determined using an X-ray fluorescence spectrometer (XRF), by Sky Ray Instruments; EDX3600B X-ray fluorescence spectrometer. The system detects elements between Magnesium (Mg, Z\u0026thinsp;=\u0026thinsp;12) and Uranium (U, Z\u0026thinsp;=\u0026thinsp;92). Calibration of the equipment was achieved using a pure silver sample and a working curve was selected for the sample. Output data revealing the elemental composition of the samples were read on Excel software.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec10\" class=\"Section3\"\u003e\u003ch2\u003e2.2.6 Functional groups\u003c/h2\u003e\u003cp\u003eRaw samples of the mining waste clays were analysed with a state-of-the-art FTIR instrument (Agilent Technologies brand of scanning frequency range of 4000 to 400 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and resolution of 4cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e).\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec11\" class=\"Section3\"\u003e\u003ch2\u003e2.2.7 Mineralogical phases\u003c/h2\u003e\u003cp\u003eThe mineral phases of the mining waste clay samples were obtained using XRD analysis by an automated powder diffractometer, NASENI-LASER instrument, with CuKα radiation of 30KV accelerating voltage, filament current of 10mA, 2 Theta range from 10\u003csup\u003e0\u003c/sup\u003e to 70\u003csup\u003e0\u003c/sup\u003e, and scan speed of 0.02\u003csup\u003e0\u003c/sup\u003e/5s.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec12\" class=\"Section3\"\u003e\u003ch2\u003e2.2.8 Morphology and elemental composition\u003c/h2\u003e\u003cp\u003eOne of the raw mining waste clays (OGr) was selected for this analysis and oven dried at 105 \u003csup\u003e0\u003c/sup\u003eC. The particle morphology and microstructure was obtained by using scanning electron microscopy instrument equipped with an energy dispersive spectrometer (SEM/EDS), model JEOL-JSM 7600F, 20KV/127micronm.\u003c/p\u003e\u003c/div\u003e\u003c/div\u003e\u003cdiv id=\"Sec13\" class=\"Section2\"\u003e\u003ch2\u003e2.3 Optimisation of geopolymer binder production conditions\u003c/h2\u003e\u003cp\u003eAppropriate conditions and material mixing ratios were optimised to achieve the best possible geopolymer products. These conditions included calcination temperature, calcination time, and mixing ratios (NaOH/Na\u003csub\u003e2\u003c/sub\u003eSiO\u003csub\u003e3\u003c/sub\u003e). Each of the optimisation experiments was done by producing replicate samples of geopolymer, and the resulting geopolymers were subjected to compressive strength tests at 7, 14, 21, and 28 days of curing time.\u003c/p\u003e\u003cdiv id=\"Sec14\" class=\"Section3\"\u003e\u003ch2\u003e2.3.1 Optimisation of clays\u0026rsquo; calcination temperature\u003c/h2\u003e\u003cp\u003eThree temperature points around the DTA dehydroxylation peak, for each clay were marked T1, T2 and T3 representing pre-peak, peak point and post-peak respectively. Geopolymer binders were produced from clays calcined at these three different temperature points for 6 hours as observed in previous works (Adeniyi \u003cem\u003eet al\u003c/em\u003e., 2020) and compressive strength measured at 7, 14, 21 and 28 days. The calcination temperature that produced the geopolymer binder with the highest compressive strength at 28 days was considered the optimal calcination temperature for the particular clay source. Clays were therefore calcined at their respective optimal calcination temperatures and labeled; Ogun Calcined (OGC), Lagos Calcined (LAC), Osun Calcined (OSC) and Ekiti Calcined (EKC) clays respectively, for subsequent experiments and instrumental analysis. The production of geopolymer binders followed established procedure (Ogundiran et al., \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e2013\u003c/span\u003e; Ogundiran and Ikotun, \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e2014\u003c/span\u003e). Further to compressive strength tests, these clays calcined at the varying temperature points were also subjected to FTIR tests to observe structural changes based on varying calcining temperatures (T1, T2, T3).\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec15\" class=\"Section3\"\u003e\u003ch2\u003e2.3.2 Optimisation of calcination time\u003c/h2\u003e\u003cp\u003eThe clays were calcined at the optimised temperatures, activator ratio of 1:1 (8MNaOH/Na\u003csub\u003e2\u003c/sub\u003eSiO\u003csub\u003e3\u003c/sub\u003e) but at varying time 1, 3, 6, 10, 14 and 20 hours. Geopolymer binders were produced using these clays. Other production conditions remain fixed as in previous section. Compressive strength test was conducted on the geopolymers. The calcination time that gave the geopolymer with the highest strength at 28 days, was considered optimal calcination time.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec16\" class=\"Section3\"\u003e\u003ch2\u003e2.3.3 Optimisation of mix ratio of activator (NaOH/Na\u003csub\u003e2\u003c/sub\u003eSiO\u003csub\u003e3\u003c/sub\u003e)\u003c/h2\u003e\u003cp\u003eGeopolymer binders were produced from varying ratios of 8M NaOH/Na\u003csub\u003e2\u003c/sub\u003eSiO\u003csub\u003e3\u003c/sub\u003e. The mixing ratios were 1:1, 1:1.5, 1:2, and 1:2.5. The products were subjected to compressive strength tests and the mix ratio that produced the geopolymer with the highest strength was considered the optimal mix ratio. Samples of geopolymers produced at optimised conditions of temperature, time and activator specifications are displayed in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003c/div\u003e\u003cdiv id=\"Sec17\" class=\"Section2\"\u003e\u003ch2\u003e2.4 Characterisation of calcined clays and geopolymer binders\u003c/h2\u003e\u003cp\u003eThe calcined clays and geopolymer binders produced at the optimal conditions for each of the clays crushed and sieved to 212 microns, were characterised. The powder samples were treated with isopropyl alcohol to terminate the geopolymerisation reactions (Ogundiran and Kumar, \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e2015\u003c/span\u003e). These samples characterised by FTIR, XRD and SEM/EDX.\u003c/p\u003e\u003c/div\u003e"},{"header":"3. Results and discussion","content":"\u003cp\u003e\u003cstrong\u003e3.1 Physical properties of mining waste clays\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe results of loss on ignition (LOI), specific gravity (SG) and bulk density (BD) of the clays are presented in Fig. 3. The LOI of samples define the loss of sample mass due to ignition or combustion, which usually serves as an indicator of the amount of organic matter, moisture, volatile substances, and plant and animal fossils present in the sample originally. From the results, the order of the values observed for the loss on ignition of the clays is EK ˃ OS ˃ LA ˃ OG. This was correlated with the respective compressive strengths of the geopolymer binders.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe specific gravity of a material is defined as the ratio of the mass of a given volume of that material to the mass of an equal volume of water. This parameter is generally used to characterise soils and similar materials. As observed from Fig. 3, the order of the specific gravity which is OG ˃ LA ˃ OS ˃ EK, seems to be opposite in trend with the LOI values.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eBulk density is the dry weight of a unit volume of soil expressed in g/cm\u003csup\u003e3\u003c/sup\u003e. It is inversely related to pore space. It has an important influence on the reactivity of clay materials because bulk density can be related directly to the clay permeability and flow of liquid activator during the process of material mixing. The result of the respective bulk densities of the mining waste clays had the order of magnitude as follows: \u0026nbsp;OG ˃ LA ˃ OS \u0026nbsp; ˃ EK.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThese properties have correlations with the compressive strength development of the respective clays.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e3.2 Thermal properties of the mining waste clays\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe DTA/TGA curves of the mining waste clays OSR, EKR, OGR, and LAR are presented in Figs. 4a, b, c and d respectively.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe dehydroxylation process for clay minerals when thermally treated, is accompanied by mass loss at different temperature ranges. According to Zhou \u003cem\u003eet al.\u003c/em\u003e (2017), dehydroxylation has been understood to depend essentially on the clays\u0026rsquo; layered structures and the amount of hydroxyl groups contained.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe respective dehydroxylation temperature ranges labeled Pre-Peak (T1), Peak point (T2) and Post-Peak (T3) for each of the four clay samples are presented in Table 1.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eTable 1:\u0026nbsp;\u003c/strong\u003eDehydroxylation temperatures of Nigerian mining waste clays based on DTA curves\u003c/p\u003e\n\u003ctable border=\"1\" cellspacing=\"0\" cellpadding=\"0\"\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 99px;\"\u003e\n \u003cp\u003eCLAY\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd colspan=\"3\" valign=\"top\" style=\"width: 314px;\"\u003e\n \u003cp\u003eDehydroxylation Peak into metakaolinite \u0026nbsp;(\u003csup\u003e0\u003c/sup\u003eC)\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 99px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 118px;\"\u003e\n \u003cp\u003eT1\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 98px;\"\u003e\n \u003cp\u003eT2\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 98px;\"\u003e\n \u003cp\u003eT3\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 99px;\"\u003e\n \u003cp\u003eEKr\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 118px;\"\u003e\n \u003cp\u003e440\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 98px;\"\u003e\n \u003cp\u003e550\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 98px;\"\u003e\n \u003cp\u003e702\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 99px;\"\u003e\n \u003cp\u003eLAr\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 118px;\"\u003e\n \u003cp\u003e420\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 98px;\"\u003e\n \u003cp\u003e575\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 98px;\"\u003e\n \u003cp\u003e740\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 99px;\"\u003e\n \u003cp\u003eOGr\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 118px;\"\u003e\n \u003cp\u003e455\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 98px;\"\u003e\n \u003cp\u003e580\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 98px;\"\u003e\n \u003cp\u003e704\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 99px;\"\u003e\n \u003cp\u003eOSr\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 118px;\"\u003e\n \u003cp\u003e446\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 98px;\"\u003e\n \u003cp\u003e517\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 98px;\"\u003e\n \u003cp\u003e603\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n\u003c/table\u003e\n\u003cp\u003eThe thermogrammes revealed percentage mass loss for the samples of Nigerian mining waste clays. For OSR \u0026nbsp; dehydration/onset temperature was 88\u003csup\u003e0\u003c/sup\u003eC to 133\u003csup\u003e0\u003c/sup\u003eC with insignificant mass loss of \u0026nbsp;0.31% attributable to loss of adsorbed and interlayer water, \u0026nbsp;indicative of \u0026nbsp; dehydration, characteristic of when 2:1 clay minerals lose adsorbed, pore, and interlayer water (Kenne \u003cem\u003eet al\u003c/em\u003e., 2015, Adeniyi \u003cem\u003eet al\u003c/em\u003e., 2020). Major mass loss of 3.08% was observed, attributable to dehydroxylation at temperature range 449\u003csup\u003e0\u003c/sup\u003eC to 602\u003csup\u003e0\u003c/sup\u003eC. \u0026nbsp; This is the occurrence of loss of structural OH that leads to the transformation of crystalline kaolin into the more reactive amorphous metakaolin (Longhi, 2015). For EKR, dehydration/onset temperature, was95\u003csup\u003e0\u003c/sup\u003eC to 131\u003csup\u003e0\u003c/sup\u003eC with insignificant mass loss of 0.61% attributable to loss of adsorbed and interlayer water. Major mass loss of7.88% attributable to dehydroxylation observed at temperature range 430\u003csup\u003e0\u003c/sup\u003eC to 701\u003csup\u003e0\u003c/sup\u003eC. However, for OGR dehydration/onset temperature was at 58\u003csup\u003e0\u003c/sup\u003eC to 130\u003csup\u003e0\u003c/sup\u003eC with insignificant mass loss of 0.18% attributable to loss of adsorbed and interlayer water. Major mass loss of 7.41% was observed at temperature range 458\u003csup\u003e0\u003c/sup\u003eC to 703\u003csup\u003e0\u003c/sup\u003eC, attributable to dehydroxylation. Whereas, LAR sample\u0026rsquo;s dehydration/onset temperature was at 58\u003csup\u003e0\u003c/sup\u003eC to 131\u003csup\u003e0\u003c/sup\u003eC with insignificant mass loss of 0.57% attributable to loss of adsorbed and interlayer water. Major mass loss of; (97.41 \u0026ndash; 88.39) 9.02% attributable to dehydroxylation at temperature range 466\u003csup\u003e0\u003c/sup\u003eC to 741\u003csup\u003e0\u003c/sup\u003eC. The sample LAR had the highest mass loss due to dehydroxylation, implying largest quantity of decomposed minerals into reactive amorphous clay. The maximum temperature of dehydroxylation on the DTA/TGA curve also was the highest, providing more temperature range to allow for more minerals to get decomposed. This could be responsible for the compressive strength of the geopolymer product which was only lesser than OGR, among all the samples studied. The sample EKR had the highest mass loss due to dehydration of adsorbed and interlayer water. This may be due to the presence of much hydrated 2:1 clay minerals in this sample (Adeniyi \u003cem\u003eet al\u003c/em\u003e., 2020). This clay also turned out as the sample with the geopolymer having lowest compressive strength. It could be suspected that the sample EKR has the least of kaolinite composition compared to others, which decomposed to the amorphous metakaolinite during calcination. The other decomposed minerals within the calcining temperature range may not have impacted positively on the compressive strength. The temperature values corresponding to T1, T2 and T3 on the DTA curves provided basis for choice of the set of temperature values tested for the optimisation of calcination temperature of the mining waste clays. Generally, for all the clays, the temperature range for the dehydroxylation of kaolinite which occurred between 430\u003csup\u003e0\u003c/sup\u003eC and 740\u003csup\u003e0\u003c/sup\u003eC, had the most intense mass loss among the three peaks in the respective thermograms, suggesting a relatively large presence of kaolinite alongside the composition of the 2:1 clay types suggested by the XRF results. This assumption is further ascertained by the XRD results, revealing intense kaolinite peaks for the clays. At a temperature range above 800\u003csup\u003e0\u003c/sup\u003eC, the occurrence here was the dehydroxylation of some 2:1 clay minerals; illite, montmorillonite, and mullite (Selmani \u003cem\u003eet al\u003c/em\u003e., 2015).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e3.3 Chemical composition of the clays\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe chemical compositions of Nigerian mining waste clays, as revealed by the XRF analysis are presented in Table 2. They are expressed as oxides of constituent metals. The results show that the dominant components across all the samples are SiO\u003csub\u003e2\u003c/sub\u003e and Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e, which suggests the presence of clay minerals such as kaolinites, quartz, feldspar, and others, consistent with composition of clay minerals suitable for the geopolymerisation process (Ferone \u003cem\u003eet al\u003c/em\u003e., 2015). It could be directly derived from the results that, the sum percentages of the Si and Al oxides in each clay sample were, 71%, 72%, 82%, and 79% for EK, OS, OG, and LA clays respectively. The high content of these Si and Al oxides, at least above 70%, implied that they were all kaolinite aluminosilicates and are suitable as pozzolanic materials (Ogundiran and Kumar, 2015; Ayininuola and Adekitan, 2016).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eTable 2 :\u003c/strong\u003eChemical composition of clay samples by XRF analysis (Oxides % weight)\u003c/p\u003e\n\u003ctable border=\"0\" cellspacing=\"0\" cellpadding=\"0\"\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd colspan=\"2\" valign=\"top\" style=\"width: 40.6386%;\"\u003e\n \u003cp\u003e\u003cstrong\u003e\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; EKC\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 17.9487%;\"\u003e\n \u003cp\u003e\u003cstrong\u003eOSC\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 17.9487%;\"\u003e\n \u003cp\u003e\u003cstrong\u003eOGC\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 17.9487%;\"\u003e\n \u003cp\u003e\u003cstrong\u003eLAC\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 22.3512%;\"\u003e\n \u003cp\u003e\u003cstrong\u003eAl\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 17.9487%;\"\u003e\n \u003cp\u003e25.5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 17.9487%;\"\u003e\n \u003cp\u003e23.7\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 17.9487%;\"\u003e\n \u003cp\u003e27.7\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 17.9487%;\"\u003e\n \u003cp\u003e28.3\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 22.3512%;\"\u003e\n \u003cp\u003e\u003cstrong\u003eSiO\u003csub\u003e2\u003c/sub\u003e\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 17.9487%;\"\u003e\n \u003cp\u003e45.1\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 17.9487%;\"\u003e\n \u003cp\u003e48.6\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 17.9487%;\"\u003e\n \u003cp\u003e54.5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 17.9487%;\"\u003e\n \u003cp\u003e50.8\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 22.3512%;\"\u003e\n \u003cp\u003e\u003cstrong\u003eCaO\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 17.9487%;\"\u003e\n \u003cp\u003e0.03\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 17.9487%;\"\u003e\n \u003cp\u003e0.03\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 17.9487%;\"\u003e\n \u003cp\u003e0.03\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 17.9487%;\"\u003e\n \u003cp\u003e0.28\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 22.3512%;\"\u003e\n \u003cp\u003e\u003cstrong\u003eTiO\u003csub\u003e2\u003c/sub\u003e\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 17.9487%;\"\u003e\n \u003cp\u003eNil\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 17.9487%;\"\u003e\n \u003cp\u003eNil\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 17.9487%;\"\u003e\n \u003cp\u003e0.73\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 17.9487%;\"\u003e\n \u003cp\u003e1.67\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 22.3512%;\"\u003e\n \u003cp\u003e\u003cstrong\u003eFe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 17.9487%;\"\u003e\n \u003cp\u003e0.01\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 17.9487%;\"\u003e\n \u003cp\u003e0.41\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 17.9487%;\"\u003e\n \u003cp\u003e3.92\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 17.9487%;\"\u003e\n \u003cp\u003e5.08\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 22.3512%;\"\u003e\n \u003cp\u003e\u003cstrong\u003eNiO\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 17.9487%;\"\u003e\n \u003cp\u003e0.43\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 17.9487%;\"\u003e\n \u003cp\u003e0.07\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 17.9487%;\"\u003e\n \u003cp\u003e0.11\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 17.9487%;\"\u003e\n \u003cp\u003e0.09\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n\u003c/table\u003e\n\u003cp\u003eThe presence of iron oxide in a considerable proportion, particularly in OG and LA clays, suggests not only iron minerals such as pyrite or goethite but is also indicative of clay minerals such as chlorite, montmorillonite, and illite (Zhou \u003cem\u003eet al\u003c/em\u003e., 2017). \u0026nbsp;The absence of TiO\u003csub\u003e2\u003c/sub\u003e in clays EK and OS implied the absence of minerals like anatase and rutile, while it is present in clays OG and LA (Ferone \u003cem\u003eet al\u003c/em\u003e., 2015). In all the clays, CaO is very low, implying that they are less calcite clays. Calcite clays having an excess of 20% CaO content are known to set too fast and are usually not recommended for the geopolymer source material (Srinivasula \u003cem\u003eet al\u003c/em\u003e., 2016). The silica-to-alumina ratio of the clays, according to the XRF data; EKC (1.77), OSC (2.1), OGC (1.97) and LAC (1.79) generally tends toward 2. This is suggestive of the 2:1 clay type. \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e3.4. Functional group of the waste clays\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe FTIR spectra that show the basic functional groups of raw OSR, EKR, OGR and LAR clays are presented in Figs. 5a, b, c and d respectively. The FTIR spectra of the raw clays generally showed peaks at major regions of 4000 to 3000 and 1300 to 500 cm\u003csup\u003e-1\u003c/sup\u003e. Peaks at 3691, 1114, 1032 and 3620cm\u003csup\u003e-1\u003c/sup\u003e in the raw clays indicating the presence of functional groups O-H, Si-O, Si-O-Si and Al-OH respectively. For all the clays, the O-H stretching vibrations typical of the presence of kaolinite occurred at absorption bands 3698 to 3694 cm\u003csup\u003e-1\u003c/sup\u003e(Adeniyi \u003cem\u003eet al\u003c/em\u003e., 2020). Also Al-OH peaks typical of 2:1 clay minerals (montmorillonite and illite) which were also observed by Adeniyi \u003cem\u003eet al\u003c/em\u003e., (2020) occurred at absorption bands 3626 to 3620 cm\u003csup\u003e-1\u003c/sup\u003e. These OH vibrations are due to hydroxyl groups that are attached to aluminium octahedron sheet of the clay minerals (Ogundiran and Kumar, 2015). Only EKr had a peak at 1636cm\u003csup\u003e-1\u003c/sup\u003e representing the carboxylate group. Peaks at 1114 and 1120 cm\u003csup\u003e-1\u003c/sup\u003e indicated the vibrations of Si-O-Si. Bands at 1004 and 911 represent Al-OH bending vibrations which identified these clays as mainly kaolinite (Ogundiran and Kumar, 2015). \u0026nbsp; The stretching vibrations of OH bonds at 3691, 3690, 3688, and 3621cm\u003csup\u003e-1\u003c/sup\u003e generally indicate the presence of crystalline kaolinite mineral (Selmani \u003cem\u003eet al\u003c/em\u003e., 2015; Kenne \u003cem\u003eet al\u003c/em\u003e., 2015). Peaks observed in all the clays around 1109, 1112, 1114, and 1032 cm\u003csup\u003e\u0026minus;1\u0026nbsp;\u003c/sup\u003eare associated with Si-O and Si-O-Si stretching vibrations (Ogundiran and Kumar, 2015). \u0026nbsp; \u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e3.5. Mineralogical phases of the clays\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe results of the XRD analysis of the clays are presented in diffractogrammes of 2\u0026theta; at 10\u0026deg;2\u0026theta; to 70\u0026deg;2\u0026theta; range in Fig. 6 for OSR, EKR, OGR and LAR. The respective diffractogrammes show that the clays in their raw forms contained predominantly crystalline kaolinite as shown in similar reports (Ogundiran and Sanjay, 2015\u0026rsquo; Adeniyi et al., 2020). Other minerals present in the clays were illite, anatase and quartz.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e3.6. Microstructure of the mining waste clays\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe SEM image of one of the mining waste clays sample was done to provide a representative result of the raw clay samples studied. Fig. 13a presents the SEM image of Ogun raw clay (OGR). Observations from the SEM image revealed a type of plate-like particle morphology consistent with crystalline phase kaolinite clay. \u0026nbsp;The Si (45%) and Al (20%) composition as revealed by the EDS coupled with the SEM instrument further confirmed the large percentage composition of silica and alumina in the clay, confirming it as an aluminosilicate source.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e3.7. Optimisation of process conditions and material mixing\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e3.7.1. Optimisation of calcining temperature of the clays\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eCompressive strength values were used to determine the optimised parameters for geopolymer binders\u0026rsquo; production. The results of the compressive strength of the geopolymers produced from clays calcined at varying temperatures are presented in Fig. 7. \u0026nbsp;The DTA\u0026rsquo;s dehydroxylation peak provided guiding data to predict the calcination temperature at which metakaolin was formed (Fig. 4). This was complemented with compressive strength values of geopolymers produced from clays calcined at the pre-peak (T1), peak point (T2), and post-peak (T3) temperature points and at an extreme temperature 850\u003csup\u003e0\u003c/sup\u003eC (T4), for each of the clays. The optimal calcination temperatures for the mining clays at the highest values of the compressive strengths were 700, 740, 600, and 700 (\u003csup\u003e0\u003c/sup\u003eC) for OG, LA OS, and EK respectively. From the compressive strength test results, it was observed that the calcination of clays at the T3 dehydroxylation (post-peak) temperatures gave the best geopolymer performance. This suggests that the thermal process of clay dehydroxylation into amorphous metakaolin continued after the spectra peak and had only come to completion at the post-peak point (T3). The temperature corresponding to the highest compressive strength, represents the optimal calcining temperature.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e3.7.2 Optimisation of calcination time of the mining waste clays\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe results of the compressive strength of the geopolymer binders produced from OG, LA, OS, and EK calcined at their respective optimised temperatures over varying calcination time are presented in Fig. 8. The compressive strength values range from 6.10 to 29.5 MPa for calcining time range from 1 to 20 hours. The highest compressive strength values (OG; 29.5, LA; 27.1, OS; 25.4 and EK; 23.1 MPa) for all the clays was observed at 6hours calcining time. The results suggest that the optimal calcination time for these mining waste clays was 6 hours. The decline in the compressive strength at higher calcining time further suggest that at a calcination time longer than 6 hours, there is an increase in the quantity of non-amorphous materials which were unreactive, resulting in reduced reactivity of the clays.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e3.7.3. Optimisation of activator mixing ratio NaOH/Na\u003csub\u003e2\u003c/sub\u003eSiO\u003csub\u003e3\u0026nbsp;\u003c/sub\u003e\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe results of the optimisation of mixing ratio of activator consisting of NaOH and Na\u003csub\u003e2\u003c/sub\u003eSiO\u003csub\u003e3\u003c/sub\u003e, is presented in Fig. 10. It was observed from the results that the NaOH/Na\u003csub\u003e2\u003c/sub\u003eSiO\u003csub\u003e3\u0026nbsp;\u003c/sub\u003eratio of 1: 1.5 generally achieved the production of geopolymers with the highest compressive strength. It can be suggested that the optimal ratio is therefore 1:1.5.\u003c/p\u003e\n\u003cp\u003eOn the final analysis the geopolymer binders produced at optimal conditions had compressive strengthvalues of 33.7, 29.8, 28.3 and 26.1 at 28 days for OG, LA, OS and EK respectively.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e3.8. Characterisation of calcined clays and geopolymer binders\u003c/strong\u003e\u003cstrong\u003e\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e3.8.1. Fourier transform infrared spectroscopy (FTIR)\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe results of the FTIR characterisation of the calcined clays at different and geopolymers of the four mining waste clays are presented in Fig 11 a, b, c and d. The results revealed the thermally induced dehydroxylation of the clays, as initially indicated by TGA/DTA results. Where T1 refers to the temperature at the onset of dehydroxylation peak, T2 describes the temperature at the peak, and T3 is for the temperature after the peak. Also, the produced geopolymer binders\u0026rsquo; spectra labelled GPOG, GPLA, GPOS, and GPEK respectively, are presented. It was observed that all spectra corresponding to clays calcined at temperatures; T1, T2, and T3 had no peak at the fingerprint region of the spectra, implying loss of adsorbed water, whereas the OGGP spectrum for the geopolymer had peaks at 3000, 3170 and 3440 (cm\u003csup\u003e-1\u003c/sup\u003e) similar to such as existed in raw clays, which was ascribed to as the presence of adsorbed atmospheric water in them. The explanation for the presence of adsorbed water in geopolymer is the water used in the production of the geopolymer. An interesting trend in the three calcination temperatures was that, though they had similar spectra patterns but different bands at the functional group region; for T3 are usually broader and occur much earlier than T2, while the same trend occurs between T2 and T1.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThis trend suggests the extent of dehydroxylation and metakaolin formation being in the order of T3 ˃ T2 ˃ T1, thus validating the compressive strength tests which had the geopolymers of T3 calcination temperature with the highest strength. This was observed to be consistent across all four clays investigated. The strong band at 1086 cm-1 in most of the calcined clays shifted towards the lower wavenumber after the geopolymerisation reaction. The spectrum shift was approximately 78 cm\u003csup\u003e-1\u003c/sup\u003e. This indicated that there had been a change in the microstructure during the hydration process, which produced a new product having a different microstructure not similar to that of metakaolin. This spectra transformation is similar to what was observed in the XRD patterns of the calcined and geopolymer samples respectively. According to Zhang and Li (2009), a 1086 cm\u003csup\u003e-1\u003c/sup\u003e peak resulted from symmetrical vibration of the Si-O bond. It can be suggested that the shift to a lower wave number in geopolymer could be attributed to the partial replacement of SiO\u003csub\u003e4\u003c/sub\u003e tetrahedron by AlO\u003csub\u003e4\u003c/sub\u003e tetrahedron, which gives rise to a change in the chemical atmosphere of the Si-O bond.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e3.8.2. X-ray diffraction analysis (XRD)\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eResults of the XRD analysis of the mining clays in raw, calcined and geopolymer forms are presented in Fig. 12 for OG, LA, OS and EK. This result enhanced the understanding of the impact of thermal treatment on the clays and the subsequent geopolymerisation reaction to achieving the geopolymer products. Observing the diffractograms of the calcined clays and geopolymers, they confirm transformation into amorphous form, by the presence of hallow peaks or humps at 2Ɵ (20\u0026ordm; - 30\u0026ordm;). The disappearance kaolinite characteristic peaks at 2Ɵ (12\u0026ordm;) revealed the absence kaolinite and suggests conversion to metakaolinite. Most of the diffractograms peaks of the calcined clays were found closely related or similar in some features when compared with those of the corresponding raw clays. This can be attributed to the fact that the clays are naturally occurring complex mixtures of various clay minerals and even non-clay minerals, especially as they are mine wastes. Hence, some minerals could survive thermal treatment without being decomposed within the temperature range utilised. All of the four clays, from the XRF data, had high silica content and this was justified by the strong peaks for quartz, a silica-based mineral, in all of their XRD diffractograms. Also, they all contained potassium oxide from XRF result and this is also evident in their XRD peaks representing the presence of illite, potassium-containing mineral.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e3.8.3. Scanning electron microscopy (SEM/EDS)\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe result of SEM/EDS for samples of raw clay, calcined clay and geopolymer binder of OG clay is presented if Fig. 13. The SEM image observed in the calcined clay revealed a more dispersed irregular morphology, consistent with an amorphous phase of clay material. This microstructure differs from that of the raw clay, where the morphology was orderly and layered, being crystalline. It can be suggested that the change in morphology, as seen, was due to thermal treatment (Selmani \u003cem\u003eet al\u003c/em\u003e., 2015). A careful observation of the geopolymer\u0026rsquo;s SEM image presented here suggests denser particle morphology and the spongy appearance of the geopolymeric structure. This also suggests the presence of interlayer water as well as a well-solidified system with some unreacted activator molecules (Selmani \u003cem\u003eet al\u003c/em\u003e., 2015). The SEM image showed that the geopolymer produced contained some un-dissolved calcined clay and an amount of newly formed sponge-like aluminosilicate species. The gels of the geopolymer is coarse-textured, sponge-like and contained round ball-like materials with bridging in between. This observation is very similar to what was reported by Ogundiran and Kumar (2015).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eAn attempt was made in this study to compute an estimated value of carbon dioxide emission reduction through the use of Nigerian mining clay wastes to produce geopolymer binder and compared with CO\u003csub\u003e2\u003c/sub\u003e emission associated with cement production. \u0026nbsp;\u003c/p\u003e\n\u003cp\u003eAssumption: it was assumed that the specific heat capacity of Nigerian mining waste clays is the same as that of normal clays. The value of the specific heat capacity of clay is given as 900 J/KGK (Zhou \u003cem\u003eet al\u003c/em\u003e., 2017).\u003c/p\u003e\n\u003cp\u003eRecall the energy formular:\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe energy required to raise 1 tonne of Nigerian clay to the optimised calcining temperature.\u003c/p\u003e\n\u003cp\u003eE = M x C x \u0026Delta;T \u0026hellip;\u0026hellip;\u0026hellip;\u0026hellip;\u0026hellip;\u0026hellip;\u0026hellip;\u0026hellip; \u0026hellip;\u0026hellip;\u0026hellip;\u0026hellip;\u0026hellip;\u0026hellip;\u0026hellip;\u0026hellip;\u0026hellip;\u0026hellip;3.18\u003c/p\u003e\n\u003cp\u003eWhere E = energy in KWh, M = mass of Nigerian clay in Kg, C = specific heat capacity of Nigerian clay in J/KgK, \u0026Delta;T = temperature change in \u003csup\u003e0\u003c/sup\u003eC.\u003c/p\u003e\n\u003cp\u003eEnergy E in KWh (Conversion factor of KWh = MJ x 0.27777)\u003c/p\u003e\n\u003cp\u003eMass M = considering 1 tonne = 1000kg\u003c/p\u003e\n\u003cp\u003eSpecific heat capacity C = 900 J/KgK\u003c/p\u003e\n\u003cp\u003eTemperature change \u0026Delta;T = optimised calcining temperature \u0026ndash; initial temperature of clay before heating in the furnace.\u003c/p\u003e\n\u003cp\u003eThe average calcination temperature utilised for Nigerian clays can be arrived at thus:\u003c/p\u003e\n\u003cp\u003e(700 + 740 + 700 + 600) / 4 \u0026nbsp; \u0026nbsp;= 2740/4 = 685\u003csup\u003e0\u003c/sup\u003eC.\u003c/p\u003e\n\u003cp\u003e\u0026Delta;T = (685 \u0026ndash; 32) \u003csup\u003e0\u003c/sup\u003eC = 653 \u003csup\u003e0\u003c/sup\u003eC\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eFor one tonne of Nigerian clay (1000 Kg);\u003c/p\u003e\n\u003cp\u003eE = M x C x \u0026Delta;T\u003c/p\u003e\n\u003cp\u003eE = 1000 \u0026nbsp; x \u0026nbsp;900 \u0026nbsp;x \u0026nbsp;653 = 587.7 MJ \u0026nbsp;= \u0026nbsp;163.4 KWh\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe carbon emission factor of biomass combustion is given as 0.35 Kg CO\u003csub\u003e2\u003c/sub\u003e/KWh (Zhou et al., 2017).\u003c/p\u003e\n\u003cp\u003eThe implication is that the calcination of 1 tonne of Nigerian clay would generate an amount of CO\u003csub\u003e2\u003c/sub\u003e\u0026nbsp; \u0026nbsp;calculated as;\u003c/p\u003e\n\u003cp\u003e163.4 KWh x 0.35 KgCO\u003csub\u003e2\u003c/sub\u003e/KWh = 57.19Kg CO\u003csub\u003e2\u003c/sub\u003e. per tonne of clay.\u003c/p\u003e\n\u003cp\u003eIt can be inferred from the above calculation that:\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eCalcination activities to prepare Nigerian clay for utilisation as a geopolymer binder, for making construction materials generally, will contribute only an estimated value of 57.19Kg CO\u003csub\u003e2\u003c/sub\u003e/tonne to the carbon emissions. The measurement of this same parameter for the manufacture of Portland cement was estimated at 800 Kg CO\u003csub\u003e2\u003c/sub\u003e per tonne (Zhou \u003cem\u003eet al\u003c/em\u003e., 2017).\u003c/p\u003e\n\u003cp\u003eIn comparison,\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eCarbon emission difference: \u0026nbsp; 800 \u0026ndash; 57.19 = 742.81\u003c/p\u003e\n\u003cp\u003ePercentage carbon emission reduction: \u0026nbsp;742.81/800 x 100 = 92.9%.\u003c/p\u003e\n\u003cp\u003eThe implication of this is that the utilisation of calcined Nigerian clays as geopolymer binders in the production of construction materials has the potential of reducing contribution to carbon emission by 92.9%.\u003c/p\u003e"},{"header":"4. Conclusion","content":"\u003cp\u003eThe need to evaluate the potentials of Nigerian mining waste clays (NMWCs), for the production of geopolymer binders, as a sustainable, durable and carbon saving alternative to Portland limestone cement motivated this research. Screened and sieved samples of mining wastes from four states in Nigeria were characterised by their physical, thermal, chemical, structural, mineralogical and microstructural properties. The impact of their physical properties; loss on ignition, specific gravity and bulk density on strength development of geopolymer binders produced was observed. the values of bulk density and specific gravity were directly proportional to the compressive strength values of the clays, while loss on ignition was inversely proportional to compressive strength. Characterisation using DTA/TGA, XRF, FTIR, XRD and SEM/EDS methods of instrumental analysis of NMWCs suggested that the studied Nigerian clays, decomposed on thermal treatment at temperature within 420 to 740\u003csup\u003e0\u003c/sup\u003eC. Though mainly kaolinite, NMWCs were essentially a complex mixture of clay and non-clay minerals which included 1:1 layered structure (kaolinite), 2:1 layered structure (illite, montmorillonite) clay types and quartz (non-clay). Optimised process conditions for NMWCs were obtained. The optimised calcining temperature was within the range of 600 to 740 (\u003csup\u003e0\u003c/sup\u003eC), for 6 hours optimal time. Also, the appropriate mix ratio for the activator constituents (NaOH/Na\u003csub\u003e2\u003c/sub\u003eSiO\u003csub\u003e3\u003c/sub\u003e) were experimentally determined as 1:1:5. Geopolymer binders were produced and compressive strength at 28 days were in the range of 33.7, 29.8, 28.3 and 26.1 at 28 days, with Ogun clay having highest Compressive strength and Ekiti the lowest. Carbon emission reduction for using NMWCs was 92.9% in comparison with that of conventional Portland cement.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAcknowledges \u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAcknowledgement to Professor Mary B. Ogundiran 9professor of Analytical Chemistry) for providing the setting out design for the research and to Professor (Engineer) G.M. Ayininuola for his technical guidance on Civil Engineering details of the research. Also due acknowledgement to Dr. Abiodun Ogbesejana for his proof-reading assistance towards the quality of writing and presentation of the article. \u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNo organizational funding was received for this research. \u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor Information:\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cu\u003eAuthors and Affiliations\u003c/u\u003e\u003c/p\u003e\n\u003cp\u003e1. Adebayo Matthew \u003cstrong\u003eAdeleye\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eDepartment of Chemistry, University of Ibadan, Nigeria and \u003c/p\u003e\n\u003cp\u003eDepartment of Chemical Sciences, Joseph Ayo Babalola University, Nigeria.\u003c/p\u003e\n\u003cp\u003e2. G.M \u003cstrong\u003eAyininuola\u003c/strong\u003e \u003c/p\u003e\n\u003cp\u003eDepartment of Civil Engineering, University of Ibadan, Nigeria\u003c/p\u003e\n\u003cp\u003e3. Mary B. \u003cstrong\u003eOgundiran\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eDepartment of Chemistry, University of Ibadan, Nigeria.\u003c/p\u003e\n\u003cp\u003e\u003cu\u003eContributions \u003c/u\u003e\u003c/p\u003e\n\u003cp\u003e1. A.M.A: Research results and data generation, provision of samples, methodology, writing.\u003c/p\u003e\n\u003cp\u003e2. G.M.A and M.B.O: Research design and Supervision.\u003c/p\u003e\n\u003cp\u003e\u003cu\u003eCorresponding author\u003c/u\u003e\u003c/p\u003e\n\u003cp\u003eAdebayo Matthew, Adeleye at [email protected]\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConsent to participate declaration\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNot applicable.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eEthics declarations \u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAll ethical standards have been duly observed during the research.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConsent for publication\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNot applicable.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting interests\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare no competing interests.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n \u003cli\u003eAdeniyi, F. I., Ogundiran, M. B. Hemalatha T., Hanumantrai, B. B. 2020. Characterization of raw and thermally treated Nigerian kaolinite‑containing clays using instrumental techniques. Springer Nature Applied Sciences Journal. 2:821\u003c/li\u003e\n \u003cli\u003eAyininuola, G. M., Adekitan, O. A. 2016. Characterization of Ajebo Kaolinite Clay for Production of Natural Pozzolan. World Academy of Science, Engineering and Technology International Journal of Civil, Environmental, Structural, Construction and Architectural Engineering. 10:9.\u003c/li\u003e\n \u003cli\u003eDavidovits, J., 1991. Geopolymers: Inorganic Polymeric New Materials,\u003cem\u003eJournal of Thermal\u0026nbsp;\u003c/em\u003e\u003cem\u003eAnalysis,\u0026nbsp;\u003c/em\u003e37, 1633\u0026ndash;1656\u003c/li\u003e\n \u003cli\u003eFerone, C., Liguori, B., Capasso, I., Colangelo, F., Cioffi, R., Cappelletto, E. and DiMaggioc, R. 2015. Thermally treated clay sediments as geopolymer source material. Elsevier. Applied Clay Science 107: 195-204\u003c/li\u003e\n \u003cli\u003eKenne, B.B., Elimbia A., Cyrb M., Dika, M. J., Tchakoute, K. H. 2015. Effect of the rate of calcination of kaolin on the properties of metakaolin-based geopolymers. \u003cem\u003eJournal of \u0026nbsp;\u003c/em\u003e\u003cem\u003eAsian Ceramic Societies\u003c/em\u003e. 3 (1): 130-138\u003c/li\u003e\n \u003cli\u003eLonghi, M.A., 2015. Valorisation of a kaolin mining waste for the production of geopolymers. Journal of Cleaner Production. 10.1016.12.011\u003c/li\u003e\n \u003cli\u003eMahasenan, N., Steve,S., Kenneth, H. and Kaya, Y. 2003. The Cement Industry and Global Climate Change: Current and Potential Future Cement Industry CO\u003csub\u003e2\u003c/sub\u003e Emissions. Greenhouse Gas Control Technologies\u0026ndash; 6th International Conference. Oxford: Pergamon. 995\u0026ndash;1000\u003c/li\u003e\n \u003cli\u003eNeupane, K., Chalmers, D., Kidd, P. 2018. High strength geopolymer concrete properties, advantages and challenges. Advances in materials. 7 (2) : 15-25.\u003c/li\u003e\n \u003cli\u003eOgundiran, M. B., Nugteren, H. W., Witkamp, G. J. 2013. Immobilisation of lead smelting slag within spent aluminate-fly ash based geopolymers. Journal of Hazardous Materials 248-249: 29-36.\u003c/li\u003e\n \u003cli\u003eOgundiran, M. B., Ikotun, O. J. 2014. Investigating the Suitability of Nigerian Calcined Kaolins as Raw Materials for Geopolymer Binders. Trans. industrial ceramic society. 73(2):138-142.\u003c/li\u003e\n \u003cli\u003eOgundiran, M. B., Kumar S., 2015. Synthesis and characterisation of geopolymer from Nigerian clay; Elsevier, Journal of Applied clay science. 108: 171-181\u003c/li\u003e\n \u003cli\u003eSelmani S., Essaidi N., Gouny F., Bouaziz S., Joussein E., Driss A., Sdiri A. and Rossignol S. 2015. Physical\u0026ndash;chemical characterization of Tunisian clays for the synthesis of geopolymers materials. \u003cem\u003eJournal of African Earth Sciences\u003c/em\u003e, Volume 103, 113-120\u003c/li\u003e\n \u003cli\u003eSrinivasula, R. M., Dinakar, P. and Hanumantha, R. B. 2016. A review of the influence of Source material\u0026rsquo;s oxide composition on the compressive strength of geopolymer concrete. \u003cem\u003eElsevier Journal; Microporous and Mesoporous materials\u003c/em\u003e. 234: 12-23.\u003c/li\u003e\n \u003cli\u003eTemuujin, J., VanRiessen, A. 2009. Effect of fly ash preliminary calcination on theproperties Of geopolymer. \u003cem\u003eJournal of Hazardous Materials\u003c/em\u003e. 164:634\u0026ndash;9.\u003c/li\u003e\n \u003cli\u003eZarina, Y., Mohd, M. A., Kamarudin, H., Khairul, N. I., Andre, V. S. and Vizureanu, P., 2013. Chemical and Physical Characterization of Boiler Ash from Palm Oil Industry Waste for Geopolymer Composite. 1408-1412\u003c/li\u003e\n \u003cli\u003eZhang, Z., Nielsen, M. K., Horsholt, S., Muralidharan, G., Jorgensen, J. B. 2021. Digitalization, control and optimisation for cement plants.computer aided Engineering. 50: 1319-1324.\u003c/li\u003e\n \u003cli\u003eZhou D., Wang R., Tyrer M., Wong H., Cheeseman C. 2017. Sustainable infrastructure development through use of calcined excavated waste clay as a supplementary cementitious material. Journal of Cleaner Production 168:-1180-1192.\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"Mining waste clays, calcining temperature, calcining time, carbon emission reduction, geopolymer binders","lastPublishedDoi":"10.21203/rs.3.rs-7214593/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7214593/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThe replacement of Portland limestone cement, with geopolymer as a binder is gaining global attention, due to reduced carbon-footprint, in compliance with United Nations agenda on sustainable development goals. Geopolymer binders (GBs) are usually produced from aluminosilicate precursors. Precursor\u0026rsquo;s availability at cheap price is essential for sustainability. Therefore, the possibility of utilising mining waste clays (MWCs) as precursors in the production of GBs was investigated. The MWCs were collected from mining sites located in four different states in Nigeria [(Awo, Osun state (OS), Ibeshe, Lagos state (LA), Ijero, Ekiti state (EK) and Owode, Ogun state (OG)] and were characterised for thermal properties, oxide composition, functional groups, morphology/elemental composition, and mineralogical phases using Thermogravimetry/differential thermal analysis (TGA/DTA), X-ray fluorescence, Fourier-transform infra-red (FTIR), Scanning electron microscopy/Energy dispersive spectrometery (SEM/EDS), X-ray diffraction (XRD), spectroscopic techniques. They were calcined at different temperatures and time and their geopolymer binders were produced with different NaOH/Na\u003csub\u003e2\u003c/sub\u003eSiO\u003csub\u003e3\u003c/sub\u003e activator ratio. Carbon emissions for GBs production was estimated using combustion energy equation. Thermal dehydroxylation of MWCs into reactive amorphous phase occurred within the range of 420\u0026ndash;740\u0026ordm;C. The sum of SiO\u003csub\u003e2\u003c/sub\u003e and Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e composition were 70.5\u0026ndash;82.2%, suggesting aluminosilicate material, confirmed by FTIR peaks at 3691, 1114, 1032 and 3620cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e indicating O-H, Si-O, Si-O-Si and Al-OH bonds, respectively characteristic of kaolinite aluminosilicate materials. The SEM\u0026rsquo;s irregular non crystalline microstructure and XRD\u0026rsquo;s hallow peaks or humps at 2Ɵ (20\u0026ordm; \u0026minus;\u0026thinsp;30\u0026ordm;) of thermally treated clays suggested amorphisation. Optimised production conditions for GBs were 600\u0026ndash;740\u0026ordm;C calcining temperature, 6hrs calcining time, and 1:1.5 NaOH/Na\u003csub\u003e2\u003c/sub\u003eSiO\u003csub\u003e3\u003c/sub\u003e ratio. The compressive strength values of the GBs ranged from 26.1 to 33.7 MPa which were above ASTM standards, with Carbon emission reduction of 92.9% compared to cement. The Nigerian MWCs could be applied for sustainable geopolymer binders\u0026rsquo; production.\u003c/p\u003e","manuscriptTitle":"Characterisation of Nigerian Mining Waste Clays as Sustainable Precursors for Production of Low Carbon Geopolymer Binders","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-10-07 18:57:24","doi":"10.21203/rs.3.rs-7214593/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"671caa93-3356-4bda-9938-679a25a655bd","owner":[],"postedDate":"October 7th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2025-11-25T20:08:18+00:00","versionOfRecord":[],"versionCreatedAt":"2025-10-07 18:57:24","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-7214593","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7214593","identity":"rs-7214593","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}