Mechanistic links between multiscale pore network evolution and chloride transport in PCM-integrated concretes subjected to freeze–thaw cycling

This paper investigates multiscale pore structure evolution in phase change material (PCM)-integrated concrete under freeze–thaw (FT) cycling and its governing role in chloride ion transport. Using high-resolution X-ray micro-tomography (XRT), key microstructural parameters were characterized before and after 0, 150, and 300 FT cycles. Results reveal that FT-induced topological changes in control samples lead to increased pore interconnectivity, thereby facilitating chloride transport. In contrast, PCM inclusion increases pore isolation and disrupts continuous transport pathways, significantly restricting ion mobility. These effects are validated through Rapid Chloride Penetration (RCP) and Non-Steady-State Migration (NSSM) experiments. At 0 FT cycles, PCM20 maintains lower permeability than the control despite higher total porosity, highlighting the governing role of connectivity over total porosity. After 300 FT cycles, the control concrete exhibits a pronounced increase in chloride permeability, consistent with FT-induced pore coarsening and the development of more continuous transport pathways observed from XRT analysis. In contrast, PCM10 shows only a minor, but statistically significant increase in charge passed, while PCM20 exhibits no statistically significant change. Overall, the combined microstructural and transport results suggest a relationship between pore network characteristics and transport behavior, where restrained connectivity evolution in PCM-modified systems is associated with reduced formation of continuous pathways and enhanced resistance to chloride ingress under FT exposure.