Improving the volume stability of the β-hemihydrate phosphogypsum – Cement system by incorporating GGBS

•The coexistence of GGBS and cement improves the mechanical properties of β-HPG.•The presence of GGBS is beneficial for volume stability.•Morphology and location of ettringite altered by GGBS. Using Portland cement to improve the properties of β-hemihydrate gypsum (β-HPG) is a convenient and economical measure. However, the volume stability of the β-HPG-cement system is worrying because the hydration product ettringite is likely to cause expansion damage. This study introduced ground granulated blast furnace slag (GGBS) to enhance the β-HPG-cement system's volume stability. The influence of GGBS on the performance of composite systems was investigated, and the mechanism behind it was discussed. The results showed that when 5 % cement and 15 % GBBS coexisted, the compressive strength of the composite system reached 41.0 MPa, and the softening coefficient was 0.73. As for the volume stability, the 28 d volume change of the specimen containing 5 % cement and 5 % GGBS was consistent with that of the blank group. XRD, TG-DSC, and SEM results showed that the introduction of GGBS increased the content of ettringite but affected its crystal morphology and formation position. Most of the ettringites were thick rods and randomly distributed in the pores, which was speculated to be one reason for the composite system's enhanced volume stability. The altered crystallization driving force of ettringite due to the consumption of OH– by hydration of GGBS was considered responsible for its altered morphology and position. These research results showed that the morphology and location of ettringite had more influence on the expansion properties in the β-HPG -cement-GGBS system, which could provide theoretical guidance for designing composite materials with high-volume stability.