Influence of phosphorus impurities on the performances of calcium sulfoaluminate cement

When phosphogypsum (PG) is used as the added gypsum of calcium sulfoaluminate (CSA) cement, the phosphorus impurities in PG negatively affect the performances. To select an appropriate PG pretreatment method to render it suitable to serve as the added gypsum of CSA cement, it is necessary to ascertain the influence of different types of phosphorous impurities on the performances of CSA cement. Analytical reagents such as H3PO4, Ca(H2PO4)2·H2O, CaHPO4·2H2O, and Ca3(PO4)2 were used to investigate the influence of phosphorus impurities on CSA cement performances. The results demonstrate that the presence of phosphate and phosphoric acid leads to an increase in the water demand for normal consistency and prolongs the setting time of CSA cement. Within 6 h, the phosphates and phosphoric acid inhibit the hydration of C4A3S¯ in CSA cement, leading to a decrease in the compressive strength. In terms of inhibitory action, the following sequence is observed: H3PO4 > Ca(H2PO4)2·H2O > Ca3(PO4)2 > CaHPO4·2H2O. After 12 h, CaHPO4·2H2O boosts the hydration of C4A3S¯ in CSA cement. At 1 d and 28 d, the compressive strengths of the CaHPO4·2H2O-added and Ca(H2PO4)2·H2O-added samples exceed that of the blank sample, separately. For the Ca3(PO4)2-added sample, the compressive strength is equivalent to that of the blank sample at 28 d, while the H3PO4-added sample exhibits a much lower compressive strength over 28 d. Therefore, removing H3PO4 from PG or transforming it into a phosphate is an effective approach to render PG suitable to serve as the added gypsum of CSA cement.