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.

Eutectic phase change material (EPCM), compared with single phase change material (PCM), is widely studied by many scholars due to its flexibility in practical engineering applications by the characteristic of changing phase change temperatures. However, there is still a lack of theoretical prediction research on the thermal performance parameters of EPCM. The existing theories about EPCM are unable to accurately predict its thermal performance parameters, which increases the difficulty of selecting the composition ratio of EPCM and affects its practical application. Based on a background of the accurate prediction of EPCM thermal parameters, 12 binary EPCMs, and 7 ternary EPCMs are prepared by mixing capric acid, n-octanoic acid and tetradecane with different proportions, respectively. By using the cooling curves and DSC test, the variation patterns in thermal performance parameters of EPCMs are studied. After comparing various thermodynamic models, the Schrader model was selected and combined with experimental results to compare and calculate the experimental and theoretical values of thermal performance parameters. The results show that the binary PCM reaches the minimum eutectic point of 1.42 °C at a mass ratio of 0.33:0.67 for capric acid to n-octanoic acid, while the ternary PCM reaches the minimum eutectic point of 0.34 °C at a mass ratio of 0.231:0.469:0.3 for capric acid, n-octanoic acid, and n-tetradecane. The latent heat of a EPCM is between the lowest and highest values of the latent heat of the constituent components, and its value increases with the enhancement of the overall thermal storage density of the material. The modified Schrader equations can effectively predict the phase change temperatures and latent heats of EPCMs at different composition ratios. The equation has a fitting accuracy of over 0.986 and a bias error of less than 6%, demonstrating excellent accuracy and providing a reliable theoretical basis for the proportion design and thermal parameter prediction of EPCM in actual engineering applications.

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.

Stepwise phase change material (PCM) aggregate concrete has advantages in controlling temperature and resisting frost heave, but its freeze–thaw resistance performance is still unclear. This paper explored the impact of replacing ordinary coarse aggregate with stepwise aggregate on the freeze–thaw resistance characteristics of concrete. Firstly, the compressive strength, splitting tensile strength, and their relationship were evaluated. Then, the freeze–thaw resistance properties of PCM aggregate concrete were investigated, including macroscopic changes, mass loss, relative dynamic elasticity modulus loss, and compressive strength loss. Subsequently, the pore changes before and after freeze–thaw cycles were tested through non-destructive testing and nuclear magnetic resonance (NMR) testing, and the evolution of pores under freeze–thaw cycles was explored. The results show that adding 100% PCM aggregate reduces the strength of concrete by 32%. However, due to the high porosity in the 100% PCM aggregate concrete, it would have an adverse impact on corrosion resistance. The corrosion resistance of concrete increases firstly and then decreases with the addition of PCM aggregate, which can be attributed to PCM aggregate having a limiting effect on pore development. Overall, a substitution rate of 60% is acceptable for compressive strength and corrosion resistance.