Freeze–Thaw Durability Enhancement of Cement Mortar Incorporating Milled RHA: Hydration Kinetics, Microstructural Refinement, and Strength Evolution

Large quantities of agricultural waste, particularly rice husk ash (RHA), are generated worldwide each year, and the lack of rational, value-added disposal pathways poses both environmental and resource-utilization challenges. To address this practical problem while improving the freeze–thaw (F–T) durability of cement-based materials in cold regions, this study investigates the effects of replacing silica fume (SF) with finely milled RHA on the hydration behavior, mechanical performance, and durability of cement mortar. From a scientific perspective, the freeze–thaw behavior of RHA-modified cementitious materials and the underlying relationships among hydration kinetics, microstructural evolution, and durability remain insufficiently understood. Mortars with different RHA–SF blending ratios were prepared at a constant water-to-binder ratio. Compressive strength was measured before and after F–T cycling, and the underlying mechanisms were investigated using isothermal calorimetry, water absorption tests, and scanning electron microscopy. Results show that SF significantly enhances pre-F–T compressive strength, with the SF-only mixture reaching 56.8 MPa at 28 d, approximately 28.7% higher than the control. With increasing RHA replacement, pre-F–T strength decreased with a non-monotonic variation (40.1–51.5 MPa). F–T cycling caused severe degradation in the reference mortar, with a strength loss rate of 31.75%, whereas RHA- or SF-modified mortars exhibited substantially lower loss rates (6.30–21.54%). Notably, high-RHA mixtures retained residual strengths of 36.0–38.3 MPa after F–T cycling. Although RHA delayed early hydration and increased water absorption, freeze–thaw resistance was not proportionally reduced. These results demonstrate that freeze–thaw durability is governed primarily by long-term microstructural stability rather than early-age strength, and they provide mechanistic evidence supporting the rational utilization of finely milled RHA as a low-carbon supplementary cementitious material for cold-region applications.