Effect of Synergistic Modification of Building Materials Based on α-Hemihydrate Phosphogypsum by Portland Cement/H-PDMS on Water Resistance

Alpha-hemihydrate phosphogypsum (α-HPG) is a cementitious material obtained by dehydration of phosphogypsum (PG), a byproduct of phosphoric acid production. Poor water resistance of α-HPG has usually restricted its application in construction materials. In this study, hydroxy-terminated polydimethylsiloxane (H-PDMS) and Portland cement (PC) were used for the hydrophobic modification of α-HPG. The fluidity, setting times, compressive strength, flexural strength, ratio of compressive to flexural strength, water absorption rate, softening coefficient, pore structure, chemical information, and microstructure of the samples were measured to evaluate the modification effect of H-PDMS and PC. The results showed that H-PDMS and PC significantly improved waterproof properties of α-HPG and reduced its porosity, total pore area, and pore diameter. Specifically, PC provided the reactive group −OH that reacted with H-PDMS. Also, due to the coverage of hydrophobic −CH3 groups, PG was given an overall hydrophobicity with a contact angle of 134° (1.5% H-PDMS). H-HPG (H-PDMS- and PC-modified α-HPG) hydrophobic material can be used in building materials with waterproof requirements and achieve the comprehensive utilization of solid waste PG.