Advancing sustainable concrete through lime sludge and metakaolin for reduced energy consumption and waste valorisation

Nearly 8% of carbon dioxide is emitted by the cement industry, which highlights the demand for alternatives which supports sustainable development. this study focuses on developing a sustainable concrete by the partial replacement of cement with calcium rich lime sludge (LS) and aluminosilicate material from calcinated kaolinite clay popularly known as Metakaolin (MK). The main aim is to formulate eco-friendly concrete composites that advance the 3R principles (Reduce, Reuse, Recycle) and align with sustainable development goals (SDGs) 9 and 11. 16 concrete mixes of M25 grade are prepared, varying LS and MK content from 0 to 15%, to evaluate the combined effect on workability, strength, and durability. Mechanical and durability tests were supported by microstructural (SEM) and elemental (EDS) analyses to assess matrix refinement and hydration properties. The mix with 10% LS and 10% MK delivered the better performance, attaining a 22.5% and 25.8% hike in 28- and 90-day compressive strength, respectively, compared to the control mix. The tensile and flexural strengths improved by 12.5 and 13.2%, respectively. M10L10 also exhibited a greater than 10% hike in density, water absorption (reduction from 8% to 6.4%), and lesser acid-induced weight loss (from 10 to 8%), reflecting chemical resistance and enhanced impermeability. The SEM results showed a compact microstructure dominated by CSH gel, while EDS indicated balanced Ca/Si (1.8) and Si/Al (26) ratios, denoting improved stability of gel. The synergistic reaction between MK and LS not only improved mechanical and durability parameters but also cut cement demand and reduced carbon dioxide emissions. The regression analysis effectively predicts behaviour of concrete and validates the efficiency of MK-LS blends as a sustainable, high-performance alternative to conventional cementitious materials (CCMs). Therefore, this dual-waste approach offers a 20-25% energy saving in clinker production and promotes industrial waste valorisation, contributing to a circular, low-carbon, and scalable pathway for sustainable concrete development.