Effects of Fineness Modulus Variation in Preheated Sand on Geopolymer Material Properties

The fineness modulus (FM) denotes the average particle size of fine aggregates. The FM of the sand, and other constituent qualities, have a major effect on the strength and durability of geopolymer mortar. Finer particle size increases the surface area, influencing heat absorption and emission, impacting the mechanical strength of geopolymer concrete. Heat energy notably accelerates reaction kinetics, accelerate geopolymerization processes. The purpose of this study is to ascertain how the compressive strength (CS) of fly ash (FA) and Ground granulated blast-furnace slag ($S_{\mathrm{lag}}$) based geopolymer material (GPM) is affected by the fineness modulus (FM) of sand that has been heated to $100°\mathrm{C}\pm 5°\mathrm{C}$. To make the paste for the geopolymer material, natural river sands with FM of 1.85, 2.41, 2.79, and 3.32 were used. For the investigation, 36 samples of a $50\times 50\times 50{\mathrm{mm}}^3$ cube were hot cured at 50°C for 3 h to determine the CS of GPM paste after 3 h, 1 day, and 7 days. The study’s conclusions show that sand FM has a large effect on the flow rate (%) of GPM. In particular, a greater flow rate (%) of GPM paste was connected to a higher sand FM. Notably, the most pronounced improvements in CS growth rate (%) were seen in GPM paste made with heated sand that had FM values of 1.85 (fine sand) and 2.79 (medium sand). Furthermore, when heated sand was utilized, the FM had a substantial impact on the CS of GPM matrix. The mineral composition, including unreacted FA and $S_{\mathrm{lag}}$ components, underwent assessment using energy dispersive X-Ray analysis (EDX) and scanning electron microscopy (SEM). These analyses revealed that fine aggregates, with increased surface area content, contributed to higher concrete strength compared with those with a higher concentration of coarse aggregates. It follows that when attempting to create geopolymer material with strong early-strength qualities, fine sand, well graded sand ideal match to the ASTM upper and lower bound limits are the best options for fine aggregates.