Improvement of the Mechanical Properties of Beta-Hemihydrate Phosphogypsum by Incorporating Wet-Ground Low-Calcium Fly Ash Slurries

Beta-hemihydrate phosphogypsum (beta-HPG) is a low-carbon cementitious material used in construction. Nevertheless, the poor water resistance and low mechanical strength of beta-HPG limit its utilization. In this study, two low-calcium fly ash (FA) slurries with median particle sizes of 1.83 mu m and 1.14 mu m were prepared using a wetgrinding process for the improvement of beta-HPG, and the mechanical strength and softening coefficient were tested to assess the effect. The results show that wet-ground low-calcium FA can significantly enhance the mechanical strength and softening coefficient of beta-HPG. Compared with specimens without wet-ground low-calcium FA, the dry compressive strength and softening coefficient of specimens with 30% wet-ground low-calcium FA (D50=1.14 mu m) increased by 104.9% and 71.8%, respectively. It is worth noting that the 1 d compressive strength also increased by 11.9%. Specifically, wet-ground low-calcium can increase the pH value and conductivity of the FA slurry, further accelerate the hydration process of FA in beta-HPG, forming more ettringite and gels to fill in the pores between dihydrate gypsum (DH) crystal, and reducing the larger pores in the hardened paste. This study can develop a high-performance cementitious material that will enable the effective use of beta-HPG.