This study investigates the influence of rice husk ash (RHA) characteristics and replacement level on the fresh and hardened performance of mortars incorporating Portland limestone cement (CEM II/A-LL 32.5R). Four RHA types (RHA-A, RHA-B, RHA-C, and RHA-D) with different physicochemical characteristics were incorporated at replacement levels of 5–60% at a constant water-to-binder ratio of 0.50. The effects of silica structure and content, loss on ignition, and particle fineness on mortar flowability, compressive strength, and tensile strength were evaluated at 7, 28, and 91 days. The results demonstrate that RHA performance is strongly dependent on its physicochemical characteristics and replacement level. RHA-A and RHA-B exhibited a progressive reduction in mechanical performance at high replacement levels, whereas RHA-C and RHA-D maintained or enhanced strength over a substantially wider replacement range. Notably, RHA-C and RHA-D achieved substantial strength enhancement at replacement levels of up to 40%, with RHA-D exhibiting the highest later-age performance. Based on the combined assessment of compressive strength and microhardness, the optimum replacement levels were identified as 30% for RHA-A, 20% for RHA-B, 50% for RHA-C, and 60% for RHA-D. At these levels, the 28-day compressive strength was comparable to the reference cement mortar for RHA-A and increased by 2.18%, 10.45%, and 7.43% for RHA-B, RHA-C, and RHA-D, respectively. The findings demonstrate that the suitability of RHA as a high-volume supplementary cementitious material is governed not only by its replacement level but also by its physicochemical characteristics, highlighting the importance of RHA-specific characterization when designing sustainable cementitious systems.

rice husk ash, portland limestone cement, supplementary cementitious materials, physicochemical characteristics, compressive and tensile strengths, sustainable cementitious materials