Optimisation of alkaline solution for fly ash-sugarcane bagasse ash geopolymer mortars: effects on strength, durability, and microstructure

Geopolymer is an eco-friendly binder due to its low emissions. This innovative research employs fly ash and sugarcane bagasse ash (SCBA) in geopolymer mortars, focusing on the combined effects of NaOH molarity and the Na2SiO3/NaOH mass ratio. Seventy-five mixtures incorporating 0, 10, and 20% SCBA were prepared at a fixed liquid-to-solid ratio of 0.485 and cured at 90 °C for 24 h. Fresh properties and 28-day hardened properties-including compressive and flexural strength, bulk density, apparent porosity, water absorption, and sulphate resistance in a mixed MgSO4–Na2SO4 solution, were evaluated. These findings suggest that increasing NaOH molarity and Na2SiO3/NaOH ratio generally improves mechanical performance and sulphate resistance, particularly in mixtures with moderate SCBA content. The optimum combination achieved 68.7 and 10.1 MPa, respectively, for compressive and flexural strengths. Under sulphate exposure, this mixture showed an initial mass gain (+1.18% at cycle 5), followed by a moderate loss (−2.22% at cycle 30). SEM and FTIR analyses confirmed a dense microstructure with well-developed aluminosilicate gels. Regression models for flow, compressive, and flexural strength showed high predictive accuracy (R2 = 0.72–0.97). The results indicate the potential of using SCBA together with an optimised activator composition to enhance low-carbon geopolymer mortars.