The thermal insulated cement matrix further enhances the thermal insulation of PCM aggregate concrete, consequently reducing energy consumption. In this paper, 0–15% rice husk ash (RHA) was used as a replacement for cement. The effect of the substitution amount of RHA on the workability, mechanical properties, thermal properties, and pore structure of concrete was investigated. The results showed that the density of concrete decreased after replacing cement with RHA. The workability of concrete decreased with the increase in RHA content. The filling effect and pozzolanic effect of RHA resulted in an initial increase and subsequent decrease in the mechanical properties of the concrete. After the cement was replaced by 10% RHA, the concrete exhibited the highest compressive strength and splitting tensile strength. The high porosity of RHA reduced the thermal conductivity of concrete by 12.29%. The temperature response indicated that the temperature difference between 15% RHA concrete and the reference concrete can reach up to 1.2 °C, potentially reducing the energy demand. The NMR results showed that the total pore volume was minimal with a 10% RHA admixture. The capillary pore volume increased slightly with the increase in RHA substitution due to the presence of numerous micron-sized pores within the RHA. The micropore and macropore volumes exhibited a decreasing and then increasing trend.

Damage to buildings and infrastructure caused by freeze–thaw cycles is a common problem in cold regions. To counteract this, multi-stage phase change material (PCM) aggregate concrete has gained attention for its potential in structural protection. PCM concrete is a type of intelligent concrete that regulates and controls the temperature by incorporating PCM. PCM aggregate can efficiently absorb and release significant amounts of heat within a defined temperature range. This study explored the feasibility of using agricultural waste rice husk ash (RHA) and industrial waste fly ash (FA) to produce PCM concrete. The combined use of RHA and FA with multi-stage PCM aggregate concrete allowed the two materials, pozzolanic materials and PCM, which have different approaches to improving the freeze resistance of concrete, to be effectively utilized together and synergistically enhance the durability and energy efficiency of buildings in cold regions. An experimental program was conducted to prepare PCM concrete by replacing cement with 5%, 10% and 15% RHA and 10%, 20% and 30% FA in different mixtures. The results show that when the replacement amounts of RHA and FA are both 10%, the compressive strength of the concrete can be effectively improved, while the thermal conductivity and thermal diffusivity are reduced. The incorporation of RHA and FA improved the thermal regulation of PCM concrete. Strength loss, relative dynamic elastic modulus (RDEM) loss and mass loss were all minimal with RHA at 15% and FA at 10% replacement.