Strength behavior and autogenous shrinkage of alkali-activated mortar made from low-calcium fly ash and calcium carbide residue mixture

In this study, the effects of sodium hydroxide (NaOH) concentrations, ratios of low-calcium fly ash (FA) to calcium carbide residue (CR), and adding 5% of Portland cement on compressive strength and autogenous shrinkage of alkali-activated mortar made from FA-CR mixtures were investigated and verified by examining the microstructure and morphology of alkali-activated paste. Both FA and CR were improved by grinding a finer powder. The alkali activators were a NaOH solution and a liquid sodium silicate at a constant ratio of 2:1. The results indicated that the utilization of CR partially substituted in FA and activated with a higher NaOH concentration significantly improved the compressive strength of alkali-activated mortars. The autogenous shrinkage of alkali-activated mortars increased with the increased NaOH concentration and decreased significantly when the CR partially replaced in FA. An alkali-activated mortar made from an FA-CR mixture (without cement) achieved compressive strengths up to 60.2 MPa and had an autogenous shrinkage of less than 100 × 10⁻⁶ mm/mm. Moreover, the XRD patterns of alkali-activated pastes showed that the reaction products of C–S–H, C–A–S–H, and N–A–S–H were related to the dense structure and the Si, Al, Ca, and Na detected in SEM/EDS results.