Optimising LC3 composites with alternative calcined clays: hydration, strength, and CO₂ reduction

This study evaluates the impact of different calcined clays and limestone substitutes on the fresh, hydration, mechanical, microstructural, and environmental performance of LC3-70 and LC3-80 blends for structural applications. The different calcined clays tested include metakaolin (MK), calcined Na-bentonite (BE(C)), calcined local mixed clay (LC(C)), and over-calcined wastes, such as brick waste (BW) and terracotta waste (TW), with limestone substitution investigated using garden lime (GL), BW and TW. Isothermal calorimetry revealed that BE(C) accelerated early hydration, while BW and TW, as limestone substitutes, enhanced early hydration due to a filler effect. MK and BE(C) reduced workability, while BW and TW had minimal effect. At 28 and 56 days, LC3 blends with BW- and TW-based limestone substitutes achieved higher compressive strength than GL in combination with MK and BE(C). The strength development and hydration heat results were supported by the portlandite consumption, as evidenced by comparisons of diffraction patterns and thermogravimetric mass loss. An environmental assessment of LC3-70 and LC3-80 blends showed that blends containing BW and TW as calcined clays reduced Global Warming Potential (GWP) from 19.24% to 28.93% by avoiding the additional energy required for clay calcination. LC3 blends with MK, BE(C), and LC(C) achieved reductions of 13.38% to 26%. These findings demonstrate that waste-derived clay sources can enhance both performance robustness and sustainability in LC3-70 and LC3-80 cementitious systems, supporting their viability as practical low-carbon alternatives to OPC.