Unveiling the Effects of Quicklime on the Properties of Sulfoaluminate Cement–Ordinary Portland Cement–Mineral Admixture Repairing Composites and Their Sulphate Resistance

Aiming at the problem of repeated repair of concrete caused by the damage of concrete structure repair system in a sulphate environment, the quicklime modified sulphoaluminate cement (CSA)–ordinary Portland cement (OPC) –mineral admixture composite repair material was utilised to obtain the law and mechanism of quicklime, improving the mechanical properties and sulphate resistance of composite repair materials. In this paper, the effects of quicklime on the mechanical properties, as well as sulphate resistance of CSA–OPC–ground granulated blast furnace slag (SPB) and CSA–OPC–silica fume (SPF) composites, were studied. The findings reveal that the addition of quicklime improves the stability of ettringite in SPB and SPF composite systems, promotes the pozzolanic reaction of mineral admixtures in composite systems, and significantly increases the compressive strength of both SPB and SPF systems. The 8 h compressive strength of SPB and SPF composite systems increased by 154% and 107%, and the 28 d compressive strength enhanced by 32% and 40%. After the quicklime was added, the formation of C-S-H gel and calcium carbonate in SPB and SPF composite systems was promoted, the porosity was reduced, and the pore structure was refined. The porosity was reduced by 2.68% and 0.48%, respectively. The mass change rate of various composite systems under sulphate attack was reduced, and the mass change rate of the SPCB30 and SPCF9 composite systems decreased to 0.11% and −0.76% after 150 dry–wet cycles. Additionally, the mechanical strength of different composite systems under sulphate attack was improved, so that the sulphate resistance of different ground granulated blast furnace slag and silica fume composite systems was improved.