Influence of metakaolin content on the microstructure and strength in hardened LC3 paste

This study systematically investigates the impact of varying metakaolin contents on the compressive strength and microstructure of hardened limestone calcined clay cement (LC3) paste. The findings reveal that increasing metakaolin content intensifies the aluminum sulfate reaction peak and accelerates its onset, while decreasing metakaolin leads to higher total calcium hydroxide (Ca(OH)₂) and calcium carbonate (CaCO₃) levels in the system. A specific threshold of 70 % metakaolin content is identified as optimal for pozzolanic activity; excess metakaolin remains unreacted. The addition of metakaolin refines the pore structure, reduces harmful large pores, and promotes the formation of ettringite and other hydration products, enhancing mechanical properties. Notably, a sample with 70 % metakaolin content exhibits higher compressive strength than one with 100 % metakaolin, indicating that metakaolin containing 30 % impurities (referred to as sand powder) demonstrates superior mechanical performance. These results support the development of LC3 as a commercially viable and eco-friendly alternative to Ordinary Portland Cement (OPC).