Microstructural and phase characteristics of phosphogypsum cement mixtures

The effects of the tricalcium aluminate (C(3)A) content of the stabilizing cement [Type II (1.3% C(3)A) cement and Type III cement (9.3% C(3)A)], curing time (seven to 90 days), proportion of PG (5-60%), grain size, and impurities in phosphogypsum (PG)(PG compared to gypsum) on the microstructural (by scanning electron microscopy) and phase properties of gypsum/PG and cement mixtures, a potential road-base material, were studied (by derivative thermogravimetry). Morphologically, two types of ettringite (well-defined crystals, >2 mu m in lengths and ill-defined crystals, below 1 mu m in length) were present in the cement and gypsum/PG mixtures. The impurities in PG retarded the initial hydration of cement in direct proportion to its amount in the mixtures. However, gypsum/PG increased the degree of hydration of cement in the mixtures in the long term. The reacted gypsum/PG produced a large amount of ettringite (within two months) compared to cement only. The amount of carbonation was uniformly low.