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

• Increasing metakaolin accelerates aluminum sulfate reaction, with higher cumulative heat release in groups with 70% and 85% content due to quartz's filler effect. • Less metakaolin means more Ca(OH) 2 and CaCO 3 , fully depleted by 28 days in samples with 100% and 70% metakaolin. • Metakaolin boosts ettringite, refines pores, and reduces harmful large pores. • LC 3 is stronger with 70% metakaolin than with 100%, showing better performance with 30% impurities (sand powder). This study systematically investigates the impact of varying metakaolin contents on the compressive strength and microstructure of hardened limestone calcined clay cement (LC 3 ) 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 LC 3 as a commercially viable and eco-friendly alternative to Ordinary Portland Cement (OPC).