Effect of different retarders on setting time and mechanical properties of hemihydrate phosphogypsum-calcium sulfoaluminate cement composite binder

The optimization of mechanical properties and regulation in setting time of beta-hemihydrate phosphogypsum (β-HPG) are the key to its utilization in building materials. Calcium sulfoaluminate cement (SAC) can significantly enhance the mechanical properties of β-HPG to form a gypsum-based-composite binder, but have little effect on regulating the setting time. Retarders can regulate the setting time of β-HPG but decrease its mechanical properties. However, the synergistic modification mechanism of retarders and SAC on β-HPG has not been clarified. In this paper, the effects of protein retarder (SC) and citric acid (CA) on the setting time, hydration heat, mechanical properties and microstructure of the β-HPG-SAC composite binder were studied systematically. The results show that incorporating SC and CA can effectively regulate the setting time, but the effects on the mechanical properties differ. SC has a certain optimization effect on mechanical properties at low content, but has a negative effect when the content is over 0.2%, while CA has a continuous negative effect with its increasing content. Specifically, SC hinders the formation of the crystalline structure to a certain extent, and makes the crystallization of β-HPG and the hydration of SAC synergistic, while CA delays the hydration process of β-HPG and SAC simultaneously. Different from SC, CA can inhibit the growth of dihydrate gypsum (DH) crystals in the C-axis direction, transforming long rod crystals into short columns, eventually forming a looser microstructure and decreasing mechanical properties. This work provides an effective method for the modification of β-HPG and the high value-added resource utilization of PG.