In concrete production, cement is primarily used, which consumes a lot of energy & resources. Numerous greenhouse gases are being released to our atmosphere during cement manufacture. Cement production emits around 2.2 billion tons of CO2 per year worldwide accounting for about 8% of total global CO2 emissions (Wallah et al., 2010; Turner et al., 2013; Duan et al., 2015). The use of alumino-silicate compounds as a substitute for cement has become increasingly significant. When compared to OPC-based concrete, geopolymers might reduce total unfavorable emissions by 44 to 64 percent (Alhawat et al., 2022).
Geopolymer (GP) is an inorganic alumino-silicate polymer formed by a chemical reaction (geopolymerisation) involving alumino-silicate materials (such as FA, GGBS, metakaolin, steel slag etc.) and alkali activator. In the last couple of decades, use of GPC as a replacement for ordinary cement concrete has gained traction as a mean of addressing expanding environmental concerns as well as providing a better substitute as a building material with high compressive strength (CS), low creep and shrinkage and high resistance to aggressive chemical surroundings (Li et al., 2004; Duxson et al., 2007; Komnitsas et al., 2007). Geopolymer is more resistant to acidic environments and operates better in high saline situations than OPC systems (Adjei et al., 2022). Heavy metals, dyes, and other radioactive contamination can be successfully absorbed by GPC (Cong and Cheng, 2021).
Previous study discovered that FA and GGBS-based mixes produced 45 percent less CO2 than a typical OPC mixture (Gartner, 2004). For improving the mechanical strength of GPC, the replacement amount of GGBS with FA would typically vary by 30 to 50% (Shreyas, 2017). The strategy of creating high-quality geopolymers under ambient circumstances will aid in the growth of fly ash use and the industrialization of Class F fly ash-based geopolymers (Ge et al., 2022). Geopolymer matrix’s compressive strength is remarkably governed by physical and chemical properties of the precursors like FA and GGBS. The particle size of binder component plays a major effect in the mechanical characteristics of the GP matrix (Assi et al.; 2018; Sharma et al., 2019). A finer particle distribution of the components results in a larger specific surface area and a lower porosity of the activated material, which boosts a component's pace of leaching, resulting in quicker gelation, setting, and, in most circumstances, increased CS of the emerging geopolymer.
The addition of calcium oxide (CaO) speeds up the setting process and enhances the mechanical characteristics of GP mixes. Several mechanisms have been proposed to explicate the contribution of CaO in GPC, including the creation of calcium silicate hydrate (CـSـH) or calcium aluminosilicate hydrate gel (C-A-S-H) that helps in densifying paste (Dombrowski et al., 2007). The role of calcium compounds is to provide a better environment for the precipitation of the GP compounds. The charge regulating capability of calcium in the alumino silicate matrix is also a significant factor (Fernández-Jiménez et al., 2010; Garcia-Lodeiro et al., 2011). One of the most essential aspects governing the CS of geopolymers is the alkali content of the activator solution. In general, it is documented that increasing the alkali content resulted in an surge in the mechanical characteristics of geopolymers (Hardjito et al., 2004; Lee et al., 2002). However, it has been noted that an excess of alkali in the activator solution reduces its strength (Komljenovic' et al., 2010). The bond strength of GPC is significantly higher than PCC and therefore its compressive strength (Abdulrahman et al., 2022).
Another factor affecting the CS of the GPC is the presence of soluble silica, in addition to the alkali content, primarily in the form of NS solution in the alkali activator solution. Numerous investigations have reported better compressive strength of GP mixtures using NaOH and Na2SiO3 solution as compared to using only NaOH (Chindaprasirt et al., 2007). Increasing the value of NS/NH value from 0.4 to 2.5 resulted in a greater CS for both 8 M and 14 M conc. of NaOH (Hardjito et al., 2004).
It has been discovered that adding sodium silicate solution beyond a specific concentration i.e. 14–16 M to mixes reduces the compressive strength of GPC (Baščarević et al., 2015). Based on the specific alkali ratios incorporated in the study, several studies have indicated an ideal range of NS/NH ratio of the activator solution to be 1.5–2.5 (Chindaprasirt et al., 2007; Baščarević et al., 2015). At temperatures below 20°C, the process of geopolymerisation retards greatly (Puertas et al., 2000), thus lowering its mechanical strength.
The manufacture of GPC utilizing FA and GGBS cured at elevated temperatures is a major issue for the large scale uses of GPC in structural sector. The present research will also look at the effect of NS/SH ratio, NH molarity and S/B ratio on the workability, mechanical, and microstructural characteristics of FA and GGBS based GPC cured at room temperature (see Fig. 1). This discovery might be useful for the construction of high-strength sustainable residential structures.