Mechanism of calcination modification of phosphogypsum and its effect on the hydration properties of phosphogypsum-based supersulfated cement

Abstract

Supersulfated cement (SSC) is a kind of environment-friendly and energy-saving cement, which is almost entirely made of industrial waste and could consume a lot of phosphogypsum (PG), which has great potential for industrial application. The calcination treatment at the temperature of 150 °C, 350 °C, 600 °C, 800 °C with calcination time of 0.5 h, 1 h, 1.5 h, 2 h is used to improve the performance of PG. The phase transformation of PG, the removal of soluble phosphorus impurities, and the hydration properties of SSC containing PG with or without modification are investigated, including the water requirement of normal consistency and setting time, the compressive strength, the pH value of pore solution and the microstructure performance of pastes. The relationship between these properties and calcination pretreatment processes are established as well. The results show that the calcination pretreatment of PG reduced the adverse effects of impurities in PG and changed the gypsum phase to prepare SSCs with higher performance in all aspects. While the modification effect varied with calcination temperature and calcination time, 90% of the setting time was shortened after calcination temperature of 350 °C for 2 h, which makes the PG-incorporated SSC meet the requirements of existing standard. And high temperature treatment showed the best modification effect on strength, especially after calcination at 600 °C for 2 h, both early and later strength was significantly increased, which was almost twice of the reference specimen. Besides, more formed hydration products and the denser microstructure were observed in the specimen with high temperature treated PG. Moreover, the slight decreases of 90-day compressive strength in some specimens were observed, which could be due to the carbonation of the specimen.