Complementary effects of palm oil fuel ash, clay residue, and steel fiber on engineering performance and microstructure of high-calcium fly ash-based alkali-activated mortars

This study investigates steel fibre (SF)-reinforced low-carbon alkali-activated binder (LCAAB) mortars based on high-calcium fly ash (FA), palm oil fuel ash (POFA), and clay residue (CR). Binary and ternary systems containing 0–1.00 vol.% SF were evaluated for engineering, wet-dry durability, microstructural, and environmental performance. The results showed that the POFA-based mixtures exhibited the highest early-age compressive and flexural strengths, reaching a maximum 1-day compressive strength of 5.01 MPa, likely due to the combined effects of SiO2- and K2O-rich POFA and calcium-rich FA on matrix development. In contrast, CR-rich mixtures showed lower strength and higher shrinkage, associated with their more porous microstructure and slower reaction kinetics. SF improved flexural response and residual strength under wet–dry cycling, reduced water absorption, porosity, and drying shrinkage, and increased pull-off bond strength to 0.55 MPa. SEM–EDS indicated coexisting aluminosilicate and calcium-containing reaction products. Although SF increased embodied CO2 emissions by up to 10%, all mixtures exhibited lower emissions (0.655–0.776 ton CO2-eq/m3) than those reported for conventional cement-based mortars. Overall, the results indicate complementary roles of precursor composition and SF reinforcement and show the potential of these mortars for sustainable rendering and other non-structural applications.