Physical properties of raw materials
Sieve analysis tests of the OB shown that particle passing 4.75 mm IS sieve and retained on 0.075 mm IS sieve was coarse sand to fine sand. The fine generated was presented in Table 1. Particle size also plays a very vital role in bricks manufacturing because it acts as a moulding capability of the materials. Texture helps the movement of air and water holding capacity rate, which can enter and move through soil (FAO 2020). According to the VSBK program (2008), soil helped in the moulding of soil into bricks therefore this study has coarse sand to fine sand recorded in heavy amount whereas silt and clay in very low amount then its help to manufacturing good or high-quality bricks. According to Rathore et al. (2020) a high proportion of clay results in cracking of bricks due to excessive shrinkage.
The values for SG, BD (g/cm3), porosity (%), FM and MC (%) were 2.22, 2.81, 25, 2.79, and 7.01 respectively (Table 2). According to Rathore et al. (2020), the role of the SG was the mass decider of the bricks made from it and it totally depends on the particle size, texture void ratios, moisture content, and mineral compositions (Adyel et al. 2012.). The typical soil has usually 2.65 to 2.85 of SG, and he was also stated that the iron-rich soil has a larger SG than silica (Bowels 2012). Therefore, the bricks making through clay having higher SG than their weight was high, due to lower SG clay was preferred for clay bricks manufacturing. In this study, the value of SG is lower than the ranged prescribed by the Bureau of Indian standard, hence this OB material is suitable for bricks making.
Mineralogical composition raw materials
The X-Ray Diffraction (XRD) patterns of the coal mine OB shown in Fig. 3. It’s clearly reveals that the quartz (60.897 %), microcline intermediate 2 (18.441 %) and kaolinite (17.024%) are the major phase while muscovite 2M1 (1.883%) and calcite (1.755%) are the minor minerals phase.
Chemical properties of raw materials
The chemical composition of raw materials was shown in Table 3. The coal mine OB contains the oxides form of SiO2 58.86%, Al2O3 29.90%, Fe2O3 4.17%, CaO 1.21%, MgO 0.81 %, TiO2 1.76 %, K2O 3.44%, Na2O 0.11%, P2O5 0.08, and SO3 0.47% (Table 3) whereas OPC were used for study and its chemical composition with the higher composition of CaO 61.24%, SiO2 24.98%, Al2O3 4.84%, Fe2O3 3.02%, MgO 1.42 %, TiO2 0.48 %, K2O 0.39%, Na2O 0.29%, P2O5 0.8, and SO3 2.5% (Table 3). The chemical constituent (%) of sodium silicate (SS) was silica (SiO2), sodium oxide (Na2O), total solid and water content were 34.78, 16.22, 51 and 49 respectively whereas Na2O: SiO2, was 1: 2.14.
The constituent of the raw material when compared to the FCB, the content of the oxides form of Al2O3 was high but the content of Fe2O3 and CaO were low. The content of SiO2, Al2O3 in soil samples ranged between 50 to 60%, 20 to 30% respectively, whereas CaO and Fe2O3 were not exceeded than 10% (Velasco et al. 2014). In this study SiO2, Al2O3 were 58.86 % and 29.30 % respectively and CaO and Fe2O3 were 1.21 and 4.17 respectively then it represents partially similar properties for brick manufacturing. According to Pinheiro et al. (2008) and White et al. (2015) silicon oxide (SiO2) content was higher than 50 % was a strong indicator of the presence of free silica and clay minerals such as kaolinite (Al2O3.2SiO2.2H2O), similarly in this study silicate proportion in OB was 58.86 % then it indicates kaolinite presence. The large amount of SiO2 in coal mine OB may impart greater strength (Behra et al. 2020).
The limit of CaO in clay content ranged up to 10% which was low in OB samples. Consequently, to overcome this cement is added while the preparation of bricks (Velasco et al. 2014). OPC can better hydrate and stabilize when in contact with water as well as maintaining and developing its strength (Lv et al. 2015; Du et al. 2019). SiO2 and Al2O3 contributed more than 50% by weight which helps in making good bricks, as particles react with cement and form cementitious gels of silicate and aluminate (Zhou et al. 2012) in this study contribution of SiO2 and Al2O3 were more than 50% then its indication of making good bricks. Al2O3 was high it leads to higher mechanical strength in the brick but the presence of Fe2O3 can cause the problem of effloresces therefore it ranges was recommended 10% (Velasco et al. 2014). The loss on ignition of coal mine OB was which was due to the removal of hydrate presence in OB and the un-burnt form of carbon in it. The limit for loss on ignition was 12% as a greater amount of organic matter will interfere in bricks (Webb 1994).
Mineralogical composition manufactured brick
The X-Ray Diffraction (XRD) patterns of the manufactured bricks of coal mine OB with cement shown in Fig. 4. It’s reveals that kaolinite (32.046 %), quartz (27.108 %), microcline intermediate (13.651 %) calcite (9.569 %) and vaterite (7.916 %), were major mineral composition and whereas minor mineral composition was muscovite 2M1 (4.400 %), hematite (2.376 %), anatase (2.060 %), and periclase (0.874 %).
Mechanical properties of manufactured brick
Compressive strength (CS)
The CS of the brick specimen was shown in Table 4 and it plays a vital role in the application of bricks in construction or building materials. According to the FCB standard (IS: 3495-(Part1), (1992), the minimum CS required is 3.5 N/mm2.
This study indicated that the maximum CS was 9.5 N/mm2 in specimen B3 in ratios (OB: C, 08:02,) and minimum in 4.2 N/mm2 in A1 (OB: SS, 09:01) (Table 4). The CS increased with an increase in OPC and sodium silicate content in Table 4. The highest CS was observed at 20 % addition of OPC with OB. According to Islam et al. (2014), cement is responsible for the development of strength. The cement helps for a long-term build-up of strength (Nagaraj et al. 2014). It is postulated that if the silica percent is more than 17-25% then the strength increases further. The maximum amount of silica and alumina extends the setting time of the OPC during the formation of brick (Raj 2017).
Water absorption (WA)
The WA of the bricks as shown in Table 4, and it is one of the essential mechanical properties of bricks. It depicted the porousness of bricks and it’s designated the degree of the reaction of bricks. It was also observing the exact in geo-polymerization bricks temporarily higher degree of geo-polymerization upshots in a less porous and permeable matrix (Ahmari and Zhang 2012).
Outcomes of this study showed that the maximum value (6.56 %) WA recorded in the group and in specimen A1 ratios (09:01, OB: SS) and minimum value (4.32 %) in group B, the specimen B3 (08:02, OB: C). these all of the ratios were under the prescribed Indian standard for the FCB (IS 3495 (Part 2), 1992). The WA of a common FCB value was not more than 20% (IS 3495 (Part 2), 1992). According to Raheem et al. (2010), the cement steadied interconnecting bricks/blocks were of good quality. Numerous of reports also designated that the WA of all the manufacturing bricks at unrelatedly of the firing temperature was less than 20% (Lamani et al. 2015). A similar study was done by Jamal and Sidharth (2008), discussed in bricks manufacture from OB, the WA (%) was not more than 10%. In this study, WA varied from 4.32 to 6.56 % and in this regard, it’s also shown the quality of manufacturing bricks from OB was comparatively better than others.
Relation between water absorption and compressive strength
According to the results of the current study in Figure 3, indicated that the CS increased when WA was decreased in the same way the number of authors also reported the same results (Morchhale et al. 2006). When sodium silicate, as well as cement content, increased than its CS increased and WA decreased (Figure 4a, b) and then a first decision was made that there was a correlation between the CS and WA in both the ratios. The outcomes and fitting equations were shown in Figures 4a, 4b, and equations 1 and 2 respectively.
When the coal mine OB with SS ratio is (09:01 (A1), 17:03 (A2) and 08:02 (A3)),
Y= -2.2131 X + 18.316 (R2= 0.9358) (1)
When the coal mine OB with C ratio is (09:01 (B1), 17:03 (B2) and 08:02 (B3)).
Y1= -2.8476 X1 + 21.319 (R2= 0.947) (2)
Where, Y and Y1 are the CS of OB with SS and C ratio of (09:01 (A1), 17:03 (A2) and 08:02 (A3)), and (09:01 (B1), 17:03 (B2) and 08:02 (B3)) respectively.; X and X1 were the WA of coal mine OB with sodium silicate and OPC ratio of (09:01 (A1), 17:03 (A2) and 08:02 (A3)), and (09:01 (B1), 17:03 (B2) and 08:02 (B3)) respectively.
As shown in the above-mentioned equation, the correlation coefficients of the fitting curves of coal mine OB with different sodium silicate and cement ratios were all higher.
The result showed that manufactured bricks with various ratios of binding materials in different ratios observed nil to heavy EFF (Table 5 and Figure 5a). Only in one of the groups (A), OB, SS and A (A1, A2, and A3) EFF was observed moderate (Fig.5a, Table 5). While nil EFF was observed in the rest of the group (B) of OB: C (B1, B2, and B3) shown in Fig.5b and Table 5. According to IS: 3495 (Part 3), 1992, the EFF to be not more than moderate (10-50%) up to class and not more than slight (< 10 %) for higher classes. Veinot et al. (1991) reported that the degree of EFF is greatest for SS and it is also increasing in the humid and CO2 environment (Longhi et al. 2019). EFF also arose due to the reaction between silicate materials and the CO2 in the atmosphere. Some of the scientists tested lithium silicate, potassium silicate, and sodium silicate, and outcome was lithium silicate shown the minimum percentage of EFF comparatively sodium and potassium silicate. Similarly, EFF is too reliant on the silica to metal oxide molar ratio and consequently on the alkalinity of the soluble silicate (Veinot et al. 1991). Zhang et al. (2018) reported the formation of EFF on the surface of the bricks didn’t change their mineralogical features. Though, the CS development and compressive modulus of geopolymers may be affected through processes related to the loss of alkalis, and also to sub florescence. The result of EFF (Table 5) depicted that the EFF is heavy when WA is 6.56 % it means it’s directly correlated with WA.
In group A, (OB: SS) three ratios (09:01, 17:03 and 8:02) were manufactured and analysed. A1 bricks fail to fulfil the limit prescribed by Indian standard IS 3495 (Part 3): 1992 w.r.t. EFF (Tables 4 and 5), whereas A2 and A3 bricks fulfil the prescribed limits. Overall A2 was the best bricks in this group because of its economy compared to A3 were heavy to moderate and fulfil the limit prescribed by Indian standard IS 3495 (Part 3): 1992 w.r.t. CS, WA, and EFF whereas sample A1 (9:1) is the best compare to A2 (17:03) and A3 (8:02) (Table 4). Similarly, in group B, (OB: C) three ratios (09:01, 17:03 and 8:02) were developed and analysed. In this group of all the three ratios of bricks manufactured to fulfil to the limit prescribed by IS3495 (Part 3): 1992 w.r.t. CS, WA, and EFF (Table 4). Overall sample B1 is the best in this category because of its economy as compared to sample B2 and B3 (Table 4). Comparing both of the groups B1 is the best bricks due to its economy and full filament the requirement of fire clay bricks IS 1077: 1992 (Reaffirmed 2002).
SEM analysis of the best brick of both of the group found exciting results and its microstructure analysis reveals that a poor interfacial bonds OB and SS bricks (Figure 5a) and a strong interfacial bonding between OB and C bricks. A large number of capillary pores seen in Figure 5a it means manufactured bricks from OB with SS were not solidified and consolidated completely; its role for higher WA and lower CS (Luo et al. 2020: Mendes et al. 2019) and higher EFF. Furthermore, a large CP observed due to water evaporation and dehydroxylation in the process kilning process in the oven. In fig. 5b showed the manufactured brick with OB and cement at optimum temperature 110± 100C, the degree of melting and consolidation of the bricks was comparatively high in flatted section, its leads to the solid construction. The figure 5b showed very small and dispersed capillary pore as well as the cementing of the liquid phase; its help to enhance the CS whereas reducing WA percentage of the bricks (Luo et al. 2020).