A study of blast furnace slag on the mechanical properties improvement and microstructure of hemihydrate phosphogypsum pretreated by calcium hydroxide

Phosphogypsum (PG) pretreatment with calcium hydroxide (CH) effectively reduces environmental pollution but inversely decreases the mechanical strength of the hardened paste, which leads to its relatively low utilization rate in construction industry. Therefore, in order to enhance the properties of PG pretreated with CH, this study mainly investigate the addition of blast furnace slag (BFS) on the proeperties and microstructure of pretreated hemihydrate phosphogypsum by calcium hydroxide. The results indicated that the incorporation of BFS not only prolonged the setting time of PG, but also significantly enhanced mechanical properties and water resistance. The mechanical strength of samples with 40 % BFS was increased to 15.7 MPa at 28 days, which was 2.7 times compared with the reference sample. The analysis of phase composition change showed that the incorporation of BFS promoted the formation of new phases, such as ettringite (AFt) and calcium alumino silicate hydrate (C-A-S-H) gel, significantly altering the microstructure by affecting the relative content of hydration products. In addition, the incorporation of BFS could alter the crystallization behavior and morphology of gypsum, mainly caused by the dissolution rate of HPG particles and the nucleation and growth of gypsum due to the hydration environment change. Meanwhile, C-A-S-H gel encapsulated AFt and gypsum, imparting a dense microstructure characteristic. This study provides a theoretical foundation for optimizing PG and advances solid waste utilization on the development of sustainable building materials.