Recycling phosphogypsum as the sole calcium oxide source in calcium sulfoaluminate cement production and solidification of phosphorus.

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 (Ca(PO), CaHPO, Ca(HPO)) on the decomposition of CaSO, formation of minerals, microstructure of the clinker, and the hydration and mechanical properties of the cement were studied. Experimental results show that Ca(PO) and Ca(HPO) promoted the decomposition of CaSO and the formation of clinker minerals with the increase in PO content, whereas CaHPO showed a promoting effect only when the PO content was more than 1.5 wt%. The increase in phosphorus incorporation in CaSiO leads to the transformation of β-CaSiO to α'-CaSiO and then to CaSiPO. The presence of three phosphates in the clinker enhanced the growth of crystal grains and the generation of a liquid phase. Compared with CaAlSO without phosphorus, the hydration reaction of phosphorus-bearing CaAlSO 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 CaAlSO, CaSiO, and CaAlFeO, and CaSiO has a stronger ability to solidify phosphorus than the other two minerals. Ca(PO) is more difficult to solidify than CaHPO and Ca(HPO). This study is of great significant to guide the large-scale clean utilization of PG in the production of CSA cement.