Consecutive pozzolanic layerings to depress internal-sulfate-attack corrosion of OPC by phosphogypsum-based cold bonded aggregates

Phosphogypsum-based cold bonded aggregates (PCBAs), as incorporates excessive soluble sulfate, potentially induce Internal Sulfate Attack (ISA) corrosion to OPC, which severely constrains its utilization and advancement. This work applies consecutive pelletization using pozzolanic layerings (fly ash (FA) and phosphorus slag (PS) basis) for surface modification. The basic, exterior hydration and microstructure properties of PCBAs, as well as the interfacial corrosions of PCBAs-OPC, are investigated via XRD, TG-DTG, X-CT, SEM-EDS, μ-XRD, and nanoindentation technologies. Results indicate the FA and PS layerings block sulfate leaching by 15–30%. When blended with OPC, these layerings capture releasable sulfate within PCBAs exterior via interfacial hydration that also depletes portlandite to generate tighter ITZ. Therefore, the reactive layerings effectively alleviate ISA corrosion via in-front blockage and back-end reaction, as supported by reduced sulfate ingress within 30–40 μm, less degradation products (ettringite and gypsum) formation, and improved hardness of vicinity OPC. The FA-based layering performs slightly poorer due to its lower density causing inefficient consolidation of PCBAs. Consequently in macroscopic, the ISA-inducing fissures of concrete surface are diminished, followed by improved 90-d compressive strength from 36.43 MPa to optimally 48.38 MPa. This study elucidates the rich-gypsum aggregates-type ISA corrosion mechanisms to OPC and proposes an instructive solution using pozzolanic layerings.