Metakaolin in ultra-high-performance concrete: A critical review of its effectiveness as a green and sustainable admixture

The current research aims to analyze metakaolin's effectiveness as a green and sustainable admixture in ultra-high-performance concrete (UHPC). This study also focuses on addressing the knowledge gap in the previous research by comprehensively analyzing the physical and chemical characteristics of metakaolin and its influence on the hardened properties of UHPC, such as compressive and flexural strength. In addition, this review will examine the durability of UHPC containing metakaolin, considering its resistance to various environmental conditions and its ability to withstand long-term loading. The present paper intends to advance knowledge on the use of metakaolin in UHPC and provide recommendations for future research in this area. It highlights potential areas for further investigation to better understand the properties and behaviour of metakaolin in sustainable UHPC. The production of UHPC has significant environmental impacts, making the development of novel eco-UHPC with lower costs and reduced energy consumption crucial. Metakaolin, produced by calcining kaolin, has high SiO2 and Al2O3 content and is used in concrete to enhance its properties. Based on microstructural tests, mineral admixtures with smaller particle sizes and lower water/binder ratios (0.16-0.24) helped create high-density C-S-H with less ettringite production and alkali-silica reactions in UHPC matrices. The optimal level of metakaolin to enhance compressive strength ranges from 5% to 15% by weight of cementitious materials, while flexural strength usually varies from 5% to 10%. Proper curing is also essential for developing strength in UHPC with metakaolin. This study recommends using metakaolin as a green and sustainable admixture in UHPC to improve its durability significantly and contribute to sustainable construction practices.