Effect of FeSO₄·7H₂O on self-healing performance of pre-cracked WGP-UHPC: chemical regulation and self-healing promotion

The reuse of the chemical byproduct ferrous sulfate heptahydrate (FeSO₄·7H₂O) presents a promising pathway for advancing the circular economy of solid waste. This study investigates the effects of FeSO₄·7H₂O content and re-curing ages on the self-healing performance of pre-cracked ultra-high performance concrete containing waste glass powder (WGP-UHPC). Microstructural analyses are employed to elucidate the underlying mechanisms. The results indicate that an appropriate content of FeSO₄·7H₂O substantially enhances the self-healing performance of WGP-UHPC, with an optimal content identified as 1%. At moderate contents of FeSO₄·7H₂O (≤1%), Fe²⁺ released undergoes oxidation to form oxyhydroxide (α-FeOOH), consuming OH⁻ in a controlled manner and creating a favorable local pH within cracks. This environment promotes the later pozzolanic reaction of WGP, facilitating the thorough consumption of portlandite (CH), as well as enhancing carbonate reactions that lead to substantial calcite (CaCO₃), ultimately achieving effective crack filling. In contrast, at FeSO₄·7H₂O contents ≥2%, excessive Fe²⁺ oxidation overconsumes OH⁻, causing a marked decrease in the internal pH of the matrix. This reduced pH environment not only inhibits the CH dissolution and the precipitation of calcite but also induces a Ca/Si imbalance due to insufficient Ca²⁺ supply. The system favors the formation of abundant porous silica gel characterized by a porous structure and low Ca/Si, resulting in the deterioration of the self-healing performance. This study elucidates the role of FeSO₄·7H₂O in modulating the self-healing microenvironment, offering a theoretical basis for exploiting industrial byproducts to enhance the self-healing performance of WGP-UHPC.