Impact of Rice Husk Ash on the Mechanical Characteristics and Freeze–Thaw Resistance of Recycled Aggregate Concrete

With the accelerating growth of infrastructure construction, carbon emission and environmental pollution problems have become increasingly severe. In order to promote the sustainable development of the construction industry, using rice husk ash (RHA) in recycled aggregate concrete has aroused extensive interest. This study aims to investigate the impact of the partial replacement (0%, 10%, 20%, 30% of binder) of ordinary Portland cement (OPC) with RHA by equal mass on recycled concrete’s mechanical characteristics and freeze–thaw resistance. The workability, compressive strength, mass loss and dynamic elastic modulus of recycled concrete were tested, and the hydration products and microstructure were analyzed using scanning electron microscope (SEM) tests. The mechanism of the freeze–thaw damage deterioration of RHA recycled aggregate concrete was revealed. The results indicate that the incorporation of RHA has an adverse effect on the workability of fresh concrete. Its high specific surface area will provide a large number of nucleation sites for the hydration reaction, refining the pore structure in the paste and improving the weak bonding of the interfacial transition zone (ITZ) by enhancing the matrix’s pozzolanic reaction effect and filling effect, thus improving the compressive strength of concrete. Furthermore, the porous structure of the recycled aggregate attached mortar and mesoporous RHA will absorb a lot of water during the freeze–thaw cycles. With the continuous accumulation of expansion pressure, the interior pores and cracks will gradually expand and extend, leading to more severe damage to the concrete, and the degree of freeze–thaw damage deterioration grows as the RHA replacement ratios increase.