Permeable Crystalline Agents in Cementitious Composites: A Review of Strength, Durability and Self-Healing

Cementitious composites inevitably develop cracks during service, leading to the ingress of harmful agents and durability deterioration. Permeable crystalline agents (PCAs) have attracted extensive attention due to their combined potential for reinforcement, impermeability, and self-healing. However, the existing studies still lack a unified analysis of the controversies regarding the modification mechanisms of different PCAs, and their mechanical properties, durability, and self-healing capacity have not yet been systematically integrated. To clarify the aforementioned divergences and identify future research directions, this paper adopts a narrative literature review approach, systematically searching relevant publications from the past fifteen years in the CNKI and WOS databases. It comprehensively summarizes the compositional characteristics, action mechanisms of PCAs, and their influence on the evolution of strength, durability, and self-healing performance of cementitious composites. The results indicate that the precipitation–crystallization theory and the complexation–precipitation synergy theory each have distinct emphases in explaining the mechanism, and their combined interpretation can more fully reveal the working essence of PCAs. The performance of PCA-modified cementitious composites is influenced by factors such as PCA composition, PCA dosage, and a curing environment; therefore, engineering parameters should be reasonably determined based on specific working conditions. Finally, recommendations for future research directions on PCA-modified cementitious composites are proposed.