Influence of Metakaolin and Lime Sludge on Rheology, Mechanical Properties of Concrete and Its Sustainability Analysis: A Review

Researchers are exploring alternate cementitious materials to augment or partially replace Portland cement due to the increased need for sustainable infrastructure development and the push to minimise construction sector carbon emissions. Metakaolin, a highly reactive pozzolanic derivative of kaolin clay, and industrial lime sludge, a calcium-rich waste byproduct from paper, sugar, and water treatment industries, seem promising. This review study critically analyses and synthesises literature on metakaolin and lime sludge's impacts on concrete's rheology, mechanical performance, and sustainability. Metakaolin, with its high silica and alumina content, improves early strength gain, reduces permeability, and refines concrete microstructure through secondary pozzolanic reactions, while lime sludge, when used wisely, fills the concrete and helps develop long-term strength while reducing waste and cost. These compounds improve concrete's workability, viscosity, and flowability as well as its compressive, flexural, and split tensile strengths under conventional and aggressive exposure circumstances. Due to their pore-refining properties, these materials reduce chloride penetration, increase acid and sulphate resistance, and reduce shrinkage and cracking in concrete. Microstructural investigations like SEM, XRD, and TGA reveal that lime sludge and metakaolin interact with hydration products like CSH and CAH gels to densify the matrix and improve performance. On the sustainability front, lime sludge diverts a lot of industrial waste from landfills, while metakaolin lowers cement clinker, reducing carbon dioxide emissions by 25% or more depending on the replacement ratio. Cost-efficiency is crucial, especially in poor countries where lime sludge is abundant and cheap. Thus, improved concrete mixes are technically superior, economically viable, and environmentally friendly for large-scale infrastructure projects.