Because the disposal of phosphogypsum (PG) can lead to serious contamination of the air, soil, and water, recycling of PG has attracted wide attention. This study investigated the effect and solidification of phosphorus in the production of calcium sulfoaluminate (CSA) cement using PG as the sole CaO source. The effects of three phosphorus impurities (Ca3(PO4)2, CaHPO4, Ca(H2PO4)2) on the decomposition of CaSO4, formation of minerals, microstructure of the clinker, and the hydration and mechanical properties of the cement were studied. Experimental results show that Ca3(PO4)2 and Ca(H2PO4)2 promoted the decomposition of CaSO4 and the formation of clinker minerals with the increase in P2O5 content, whereas CaHPO4 showed a promoting effect only when the P2O5 content was more than 1.5 wt%. The increase in phosphorus incorporation in Ca2SiO4 leads to the transformation of β-Ca2SiO4 to α'-Ca2SiO4 and then to Ca7Si2P2O16. The presence of three phosphates in the clinker enhanced the growth of crystal grains and the generation of a liquid phase. Compared with Ca4Al6SO16 without phosphorus, the hydration reaction of phosphorus-bearing Ca4Al6SO16 started later and ended earlier, and the reaction time was shorter. The presence of phosphorus impurities reduces the 1-day strength of CSA cement but does not affect the development of the 3-day and 28-day strengths. Considering environmental aspects, the solidification of phosphorus in the production of CSA clinker were quantified by measuring the distribution of elements. The results indicated that phosphorus is solidified by Ca4Al6SO16, Ca2SiO4, and Ca4Al2Fe2O10, and Ca2SiO4 has a stronger ability to solidify phosphorus than the other two minerals. Ca3(PO4)2 is more difficult to solidify than CaHPO4 and Ca(H2PO4)2. This study is of great significant to guide the large-scale clean utilization of PG in the production of CSA cement.
Keywords: Calcium sulfoaluminate cement; Mineral formation; Phosphogypsum; Phosphorus; Solidification.
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