Influence of alkaline activator concentration on the mechanical, microstructural properties of geopolymer mortar using Class C fly ash and M-sand

This study investigates the mechanical performance and microstructural characteristics of geopolymer mortar produced using Class C fly ash as the sole precursor under ambient curing conditions. Sodium hydroxide (NaOH) and sodium silicate (Na 2 SiO 3 ) were employed as alkaline activators, using NaOH molarities of 8 M, 10 M, 12 M, and 14 M alongside silicate-to-hydroxide ratios ranging from 1:1 to 2.5:1. The effects of activator molarity, silicate-to-hydroxide ratio, curing regime, and binder-to-aggregate ratio on early-age mechanical properties were investigated. Compressive, flexural, and direct tensile strengths were evaluated up to 28 days. Results show that a NaOH molarity of 12 M combined with a silicate-to-hydroxide ratio of 2.5:1 provides superior performance, achieving 28-day compressive, flexural, and tensile strengths of 40.50 MPa, 4.77 MPa, and 3.70 MPa, respectively. 8 M resulted in insufficient aluminosilicate dissolution; 10 M and 12 M molarities enhance dissolution, gel formation, and mechanical performance, whereas higher molarity 14 M promoted crystalline phase formation without significant strength improvement. Microstructural analyses confirmed dominant geopolymer gel formation, with XRD revealing a strong amorphous phase and increased baseline shift at 12 M. FTIR spectra confirmed Si–O–T (T = Si or Al) bond formation, while SEM images indicated a denser matrix. EDAX elemental ratios at 12 M indicate enhanced aluminosilicate network formation and effective charge balancing. Microstructural analyses (XRD, FTIR, SEM, and EDAX) confirmed that a Ca/Si ratio of 0.18, Si/Al ratio of 4.46 and Na/Al ratio of 1.18 at 12 M signify an optimally cross-linked aluminosilicate network and denser matrix. The 12 M sodium hydroxide with a 2.5:1(silicate: hydroxide) ratio, cured under ambient conditions, produced a compressive strength that was 15%, 5%, and 9.5% higher than the 8 M,10 M,14 M mixes, respectively. The increase in flexural strength was found to be 8.4%, 2.6%, and 6%, while the direct tensile strength showed an increase of 13.9%, 5.7%, and 8.8% compared to the same molarities. The 12 M NaOH concentration improved the dissolution of silica and alumina, producing a denser matrix with a compact microstructure, improved bond between the binder and fine aggregate, providing the alkaline environment for geopolymerisation, resulting in a higher strength.