Study on the composite compatibility and interfacial properties of excess sulfate phosphogypsum cementing system to OPC and C[formula omitted]A

Abstract

Releasable sulfate contained in the Excess Sulfate Phosphogypsum Cementing System (ESPCS) potentially degrades interfaces when contacted with conventional cements, raising incompatibility concerns. This work evaluated fresh connection, adhesion, and aggregate embedment of ESPCS to ordinary Portland cement (OPC) and Calcium sulfoaluminate cement (CS¯A), with mechanical and interfacial properties studied. Results indicate the discrepancy in hydration properties gives Excess-sulfate Phosphogypsum Slag Cement (EPSC) uncoordinated expansion to OPC, followed by cracks (95 μm) generation. Meanwhile, expansive ettringite forms in the interface due to bleeding sulfate-rich water concentration, thus leaving a porous contact zone (1240 μm, 11.29%) and resultant flexural strength reduction of OPC-EPSC mortar (by > 36%). Conversely, EPSC and CS¯A are close in phase assemblages, resulting in compact interface and higher bond strength. Incorporating the Phosphogypsum-based Cold Bonded Aggregates (PCBAs) into OPC induces the interfacial ettringite formation-sulfate attack by internal curing water, yielding the worst composite performance, i.e., the lowest 360 d compressive strength (38.8 MPa) of concrete OPCC, lower than EPSCC (48.6 MPa) and CS¯AC (50.3 MPa). ESPCS is more compatible to CS¯A than OPC which is advised against exposing directly with ESPCS. This work helps specify composite feasibility and conditions of ESPCS with typical cement composites.