Influence of ash content and curing age on the workability and residual strength of rice husk ash blended cement mortar

Cement production contributes approximately 7–8% of global carbon dioxide (CO 2 ) emissions, necessitating the development of sustainable clinker reduction strategies. Rice husk ash (RHA), a silica-rich agro-industrial by-product, offers strong pozzolanic potential as a partial clinker substitute. This study evaluates the influence of RHA content (10–50%) and curing age (28 and 180 days) on workability, mechanical strength development, and residual performance after exposure to 950 °C for 60 min. Six cement blends were produced by replacing clinker with RHA at fixed 5% gypsum content including the control (M0) with 95% clinker. Mortars were prepared in accordance with EN 196-1, and strength values represent the mean of three specimens. Water demand increased progressively with RHA content, rising from 27.0% in the control mix to 40.6% at 50% replacement. While 28-day compressive strength decreased with increasing RHA content, significant long-term strength enhancement was observed at moderate replacement levels. At 180 days, the 20–30% RHA mixes achieved comparable or superior compressive strength relative to the control. Residual strength results indicated improved thermal resistance at 20–30% replacement, with the 30% blend demonstrating the highest compressive strength retention after high-temperature exposure. The observed long-term strength gain and enhanced residual performance are consistent with the documented pozzolanic reactivity of silica-rich systems reported in blended and alternative low-carbon binders. Within the scope of this study, 20–30% clinker replacement with RHA provides an optimal balance between mechanical performance, thermal resilience, and sustainability objectives, positioning RHA-blended cement as a practical transitional pathway toward reduced-carbon cementitious materials.