Investigation on fly ash-based geopolymer mortar: Effect of mix proportions and curing conditions on mechanical properties and study on durability characteristics of optimised mix

Geopolymer ingredients offer a sustainable alternative to conventional cement by using industrial by-products and significantly reducing carbon emissions. This study investigates sustainable construction materials by exploring fly ash-based geopolymer mortar (FGPM) development and optimization. Utilizing a fixed NaOH concentration of 14 M and a recipe of sodium silicate (SS) and sodium hydroxide (SH) with an SS/SH proportion of 2, the research examines the effects of varying liquid-to-binder proportions (0.30, 0.35, 0.40) and binder-to-sand ratios (1:1, 1:1.5, 1:2), with specimens subjected to heat curing at 30°C and 60 °C. Workability was assessed via flow table tests, while mechanical performance was evaluated through compressive, tensile, and flexural strength at 7 and 28 days. Results indicate the optimal mix (F/S = 1:1, AAF = 0.35) cured at 60 °C, achieving compressive strengths up to 35.68 N/mm 2 (28 days) and split tensile/flexural strengths of 4.22/5.55 N/mm 2 at 28 days, outperforming 30°C cured specimens by 25–30 % and achieving moderate workability. Heat curing enhanced geopolymerization, reducing water absorption to < 10 % (vs. >12 % at 30°C) while maintaining stable density (∼2181 kg/m³). FGPM specimens demonstrated strong NaCl resistance (+1.07 % strength gain), moderate NaOH tolerance (-14.71 % strength loss), and high vulnerability to MgSO 4 -driven degradation (-31.1 % strength loss). This work advances the field by methodically optimizing the fluid-to-binder ratio (0.35), binder-to-sand ratio (1:1), and moderate heat curing (60°C) to maximize mechanical strength, and balances flowability concurrently. Moreover, NaCl resistance demonstrates exceptional chloride resilience without additives, a key finding for marine/coastal applications, achieving a balance unattained in prior low-calcium fly ash geopolymer studies. This approach offers valuable insights to guide the design of more efficient and robust geopolymer mortars suitable for precast structural applications.