Transforming cement chemistry for net-zero: role of metakaolin in low-carbon composites

The global demand for sustainable construction materials has accelerated interest in alternative binders to ordinary Portland cement due to the significant environmental and energy impacts of conventional cement production. Metakaolin (MK) and other supplementary cementitious materials (SCMs) have emerged as promising candidates for enhancing the mechanical performance, durability, and environmental sustainability of geopolymer and alkali-activated concretes. This review comprehensively examines the influence of MK on alkali-activated and conventional concrete systems, focusing on fresh and hardened properties, curing conditions, hydration mechanisms, and interactions with SCMs such as fly ash, ground granulated blast furnace slag, silica fume, rice husk ash, and calcium hydroxide. The synergistic effects of MK with these SCMs show notable improvements, including compressive strengths up to 70 MPa and strength increases of about 20% compared to conventional mixes. In repair applications, MK-based geopolymers demonstrate bond strength up to 1.5 times higher than epoxy systems, while impact resistance improves significantly, with increases up to 63% and impact values reaching 658/712 under drop-weight testing. These materials also exhibit enhanced structural compatibility and durability. Advanced characterisation methods, including X-ray diffraction, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and nuclear magnetic resonance, are discussed to elucidate the structural evolution and geopolymerisation processes influenced by MK and MgO additions.