Preparing sulfoaluminate cement using phosphogypsum as the only calcium oxide source: Effects of fluoride impurities

Phosphogypsum (PG) is a solid waste generated during the wet process of phosphoric acid, the utilization of which is hindered by impurities. Herein, natural gypsum and limestone were replaced by PG as the only source of CaO for the production of sulfoaluminate cement (SAC). The effects of fluoride impurities such as NaF, CaF2, and Na2SiF6 on the mineral formation and hydration of SAC were investigated. Results show that with increasing fluoride content, the decomposition of CaSO4 initially decreased and then increased. An increase in the NaF content facilitated mineral formation in the clinkers, whereas an increase of over 1.0 wt% in the contents of CaF2 and Na2SiF6 each resulted in a positive impact. The fluoride impurities helped convert o-C(4)A(3)S(-) to c-C(4)A(3)S(-) and alpha'-C2S to beta-C2S as well as influenced the hydration behavior of SAC. They reduced the 1-day compressive strength of SAC while enhancing its 3- and 28- day strengths. Moreover, a quantitative analysis of the distribution of F in the clinkers revealed that F from NaF and Na2SiF6 was uniformly distributed in all clinker minerals, whereas F from CaF2 was more concentrated in C(4)A(3)S(-) and C2S. Na mainly existed as Na2SO4, which coexisted with CaSO4. Furthermore, F was immobilized by Ca in the clinker minerals to form CaF2. This study will promote the use of PG in SAC preparation.