Environmental Durability and Microstructural Evolution of Industrial-Waste-Modified Pisha Sandstone Cementitious Composites

The water-induced swelling and disintegration of pisha sandstone (PS) are modified through gel-mediated cementation, thereby providing a technical pathway for the resource utilization of PS. This study investigated the durability of PS geopolymer cementitious materials (PSCM) incorporating four industrial solid wastes, including blast furnace slag (BFS), fly ash (FA), silica fume (SF), and steel slag (SS), under dry–wet cycles, freeze–thaw cycles, and water immersion. X-ray diffraction, thermogravimetric analysis, and scanning electron microscopy were employed to characterize the evolution of the mineral phase composition and gel structure of PSCM. The results show that BFS can effectively improve the durability of PSCM. With 40% BFS, the strength of PSCM reaches 25.82 MPa after 10 dry–wet cycles; the strength loss rate drops to 27% after 10 freeze–thaw cycles; and the softening coefficient reaches 0.89 after 28 d immersion. With 20% BFS, the strength loss of PSCM specimens is only 10.3% and 9.3% after 10 dry–wet and freeze–thaw cycles followed by 7 d immersion. BFS promotes the formation of calcium-rich C-A-S-H gels, enhancing interparticle bonding and matrix integrity, thus enhancing the durability of PSCM. This study provides a theoretical basis for understanding the microstructure evolution and durability differences in PSCM modified by different solid wastes.