Effect of CaO in Alkali-Activated Fly Ash Mortar Under Different Curing Temperatures

Abstract

This work investigates the influence of CaO as a partial substitute for fly ash in alkali-activated fly ash mortars (AAFM), aiming to reduce reliance on conventional thermal curing. Mortars containing 0%, 2%, and 4% CaO were prepared and subjected to two curing regimes: thermal curing at 70 °C for 24 h and ambient curing at 21 °C for 24 h. The materials were thoroughly characterised by XRD, XRF, TGA/DTA, SEM, and particle size distribution, while compressive and flexural strength, density, and porosity were evaluated at 7, 14, and 28 days. The results demonstrated that CaO addition improved mechanical performance in both curing environments, particularly at a 4% substitution level, where compressive strength increased by up to 13.8% under thermal curing conditions. These improvements were associated with the formation of C-S-H and C-A-S-H gels, especially margarite, which contributed to accelerated setting and earlier demoulding. Nonetheless, while CaO incorporation improved mechanical performance and allowed earlier demoulding, it could not fully replicate the effects of heat curing at the studied percentages. Ambient-cured mortars exhibited higher porosity and less compact microstructures than thermally cured samples, which displayed denser, layered morphologies. The study confirms that CaO can act as a partial substitute or reducer for conventional curing, but is not sufficient to enable in situ applications without heat treatment. Future research should explore higher CaO contents in combination with set retarders, intermediate curing regimes, or alternative strategies to balance mechanical performance with energy efficiency.