Combined effect of lime sludge (LS) and metakaolin (M) on the strength, physical parameters and microstructural properties of sustainable concrete

The cement industry accounts for nearly 8% of global CO2 emissions, underscoring the urgent need for eco-efficient construction materials that align with the Sustainable Development Goals (SDGs). This study explores the partial replacement of Ordinary Portland Cement with lime sludge (LS), a calcium-rich industrial by-product from paper manufacturing and metakaolin (MK), a reactive pozzolana derived from calcined clay, to develop sustainable concrete composites supporting the 3R principles (Reduce, Reuse, Recycle) SDGs 9 and 11. The research aimed to determine the optimal proportions of LS and MK for enhancing concrete performance while reducing cement consumption and overall energy demand. A total of 16 concrete mixes (M25 grade) were designed with varying LS (0%–15%) and MK (0%–15%) replacement levels. The mix containing 10% LS and 10% MK (L10M10) demonstrated superior performance, showing a 22.8% increase in 28-day compressive strength and a 25.8% improvement at 90 days compared with the control (L0M0). Flexural and tensile strengths improved by 13.2% and 12.5%, respectively. The density of L10M10 increased by over 10%, while water absorption decreased from 8% to 6.4% and acid-induced mass loss reduced from 10% to 8% after 12 weeks, indicating enhanced durability. Scanning electron microscopy analysis revealed a denser microstructure with refined C–S–H gel formation and minimal porosity, whereas energy dispersive spectroscopy confirmed favorable Ca/Si (1.8) and Si/Al (26) ratios promoting stability and strength. This approach demonstrates a 20%–25% potential energy saving during clinker production and encourages valorization of industrial by-products, reducing landfill waste.