Feasibility study on sulfate-alkali co-activated cementitious materials incorporating phosphogypsum and electrolytic manganese residue: Chemical precipitation and phase coexistence under high alkalinity.

The large-scale disposal of phosphogypsum (PG) and electrolytic manganese residue (EMR) remains constrained by the leaching risk of manganese, phosphorus and associated trace elements. This study aims to investigated the feasibility of chemical precipitation and phase coexistence under high alkalinity in a sulfate-alkali co-activated cementitious material (SACAM) by activating granulated blast furnace slag (GBFS) with a two-part NaOH/Na2SiO3 alkaline solution and PEMR, a 2:1 (by mass) mixture of EMR and PG that serves as a composite sulfate activator. The effect of MS, Na2O and PEMR contents on compressive strength were systematically evaluated. microstructural characterization was conducted using XRD, SEM-EDS, and XPS, while hydration kinetics were analyzed via isothermal calorimetry and the Krstulović-Dabić model. Leaching toxicity tests were performed to assess heavy metal immobilization. The results show that appropriate MS and Na2O contents promote GBFS dissolution and accelerate gel formation, whereas excessive silicate or alkali disturbs the balance between C-(A)-S-H/N-A-S-H gels and sulfate-bearing hydrates. The maximum 28-day compressive strength reached 45.43 MPa. In the sulfate-content series, 12.9% PEMR gave the highest 28-day strength (37 MPa), while excessive PEMR reduced strength through dilution, sulfate imbalance and reduced C-(A)-S-H formation. Isothermal calorimetry confirmed that SACAM follows increasing Na2O enhances the reaction degree, while sulfate addition weakens and delays the main exothermic peak by promoting ettringite (AFt) formation and redistributing Ca and Al. The Krstulović-Dabić model revealed that alkali addition enhances the hydration degree throughout the process, while PEMR addition shortens the phase-boundary stage. Microstructural characterization demonstrates the coexistence of AFt/AFm, gypsum and C-(A)-S-H gel under high alkalinity. Heavy metal leaching analysis indicates that SACAM can immobilize Mn2+ and phosphorus from EMR and PG via ion exchange and chemical precipitation, with all leachate concentrations meeting the requirements of the Chinese discharge standard GB8978-1996.