Effect of sodium sulphate on the rheology, reaction kinetics and early-age strength of limestone calcined clay cements (LC3)

This research has investigated how sodium sulphate (Na 2 SO 4 , ∼0–5 wt%) influences the rheology, reactivity, and strength of limestone calcined clay cement (LC 3 ) pastes with varying metakaolin contents. A comprehensive experimental programme was conducted, including rheometry, calorimetry, TGA, XRD, SEM/EDS and strength tests. LC 3 derived from low-grade (∼ 36% kaolinite) clays has rheological properties comparable to CEM I, CEM II/S and CEM II/V binders. Na 2 SO 4 enhances the dispersion of calcined clay, producing lower initial yield stress and plastic viscosity. Furthermore, Na 2 SO 4 addition enhances hydration and promotes metakaolin reactivity, increasing C-(A-)S-H formation and portlandite consumption. However, LC 3 derived from high-grade (∼84% kaolinite) clays with large Na 2 SO 4 dosages result in U-phase and delayed ettringite formation, suppressing AFm. LC 3 with 1% Na 2 SO 4 can achieve both early and late strengths comparable to CEM II/V and CEM II/S, despite containing only ∼50% clinker. The work demonstrates the beneficial influence of Na 2 SO 4 in optimising low-carbon LC 3 formulations.