Influence of crystal modifier on the preparation of α-hemihydrate gypsum from phosphogypsum

Crystal modifier is an important factor in preparation of α-hemihydrate gypsum (α-CaSO4·0.5H2O) from phosphogypsum (PG). Here, α-CaSO4·0.5H2O was prepared from PG using a semi-dry method. SEM, XRD, XPS were used to explore the effects of one or more crystal modifiers on crystal growth and mechanical properties after hydration and hardening of α-CaSO4·0.5H2O. The results showed that addition of RCOO− can delay the hydration process of α-CaSO4·0.5H2O and improve productivity. Al3+ can be adsorbed selectively on the gypsum crystal surface, inhibiting the crystal growth along axis c, which could alter the speed in all directions and turn the crystal into a short hexahedron pole. But when RCOO− and Al3+ were composited, a superposition effect occurred, resulting in production of a crystal with a smaller length to diameter ratio and more perfect crystal form. When RCOO− and Al3+ were both added in 0.06% level, the flexural and compressive strength of the hardened α-CaSO4·0.5H2O could reach to 6.7 and 25.65 MPa, respectively.