Performance Optimization and Mechanistic Analysis of Recycling Waste Phosphogypsum into High-Performance Cement

Abstract

Phosphogypsum (PG) is a bulk solid waste generated during the production of phosphoric acid. This study proposes a novel phosphogypsum-based geopolymer (PBG) cement with a 28-day unconfined compressive strength (UCS) exceeding 70 MPa. The PBG cement, composed of dihydrate phosphogypsum (DPG), slag, fly ash (FA), portland cement clinker (PCC), sulfate alumina cement clinker (SACC), and water reducers, incorporates more than 45% by weight DPG. The primary constituents of the hydration products include dihydrate gypsum (CaSO4·2 H2O), ettringite (Ca6(Al(OH)6)2(SO4)3(H2O)25.7), and polymers (─ Si─ O─ Al─ ). Among these constituents, the polymers account for more than 50% of the total composition and provide most of the strength. Ettringite constitutes 7%–10% of the composition, acting as a skeleton in the early strength development process. Utilizing a weak base as an activator, the PBG cement avoids efflorescence and can be cured at 20°C. Based on the calculations, PBG cement has a higher value-to-cost ratio than ordinary portland cement (OPC), providing a new approach for recycling waste gypsum.