Hydration-hardening properties of low-clinker composite cement incorporating carbonated waste sintering red mud and metakaolin

•A new low clinker composite cement without strength loss was developed by using C-SRM and MK.•The strengths at 3d, 7d, 28d and 60d were all better than the Control group, when the admixtures substitution ratio was 1:1.•Additional C-(A)SH gels and Mono-carbonate can be formed in the composite cement paste.•The resistance to chloride migration of composite cement was improved. In our previous work, an in-situ wet carbonation method was proposed to enhance the pozzolanic activity of waste-sintering red mud efficiently, producing large amounts of fine calcium carbonate (Cc) and silica-aluminum gels. In this paper, low-clinker composite cement (50% replacement) was prepared by the synergistic use of carbonated sintering red mud (C-SRM) and metakaolin (MK). Also, its mechanical properties, microstructure, and resistance to chloride migration were investigated. Results showed that compared with pure Portland cement, the compressive strength without a loss was achieved when the C-SRM to MK ratio was 1:1. This high performance was mainly attributed to the synergistic effect of Cc in C-SRM and the aluminum phase in MK, as well as the secondary pozzolanic reaction of the silica-aluminum gels in C-SRM, which densified the microstructure of the hardened composite paste. In addition, the increased density of the paste also enhanced the resistance of the composite cement paste to chloride migration, including the chemical binding to produce Friedel’s salts and the physical adsorption of C-(A)SH gels.