Synergetic impact of volcanic ash and calcium carbide residue on the properties and microstructure of cementitious composites

The synergetic impact of using volcanic ash (VA) and calcium carbide residue (CCR) on the engineering properties of cementitious mortar was assessed. Cement was replaced with VA at 0–40 % by mass and/or CCR at 0–20 % by mass. Blended mortars were tested for flow, setting time, compressive strength, and ultrasonic pulse velocity (UPV). The flow increased when using VA but decreased with the inclusion of CCR. Compared to the cement-based control mix, ternary mixes had a lesser flow. The impact of VA replacement on the initial and final setting times was marginal until a substitution rate of 40 %, after which the setting times were prolonged. Similarly, the setting time was extended with CCR replacement. The compressive strength and UPV improved with the use of 10–20 % VA and/or 5 % CCR for binary and ternary mortar mixes, with the highest strength of 55.4 MPa and UPV of 4490 m/s being recorded for the mix comprising 20 % VA and 5 % CCR. Mixes were further analyzed through isothermal calorimetry, X-ray diffraction, and scanning electron microscopy. The cumulative heat generation was reduced for binary and ternary mixes except for those containing 20 % VA only. The microstructure analysis highlighted the formation of C-S-H and C-A-S-H as reaction products and the consumption of Ca(OH)₂. This study provides novel insights into optimal combinations of VA and CCR for superior mortar performance, offering practical approaches to reducing cement content and lowering carbon emissions without compromising performance.