MECHANICAL AND MICROSTRUCTURAL CHARACTERIZATION OF LIME SLUDGE- METAKAOLIN BLENDED CONCRETE COMPOSITE

Abstract

In this study, two by-products, lime sludge (LS) and Metakaolin (MK), were used as partial replacements for cement to enhance the mechanical, durability and microstructural properties of concrete. MK is a reactive aluminosilicate pozzolan, whereas LS is a calcium-rich by-product of the paper industry. The investigation was carried out in two linked phases. In the first phase, M25 grade concrete was prepared with LS and MK at replacement levels of 0–15% each (a 4 × 4 factorial set of 16 mixes), and the effect of these materials was studied in terms of workability, density, compressive strength, split tensile strength, flexural strength, water absorption, acid resistance and chloride ion penetration. Regression analysis was performed to examine the relationships among strength, density and water absorption, and scanning electron microscopy and energy-dispersive spectroscopy were employed to examine the hydration products and the microstructure. The mix with 10% LS and 10% MK (C80M10L10) performed best among all the LS–MK mixes. Compared with the control mix with 0% LS and 0% MK, it attained about 23% and 26% higher compressive strength at 28 and 90 days, 13–15% higher split tensile strength and 13 15% higher flexural strength. Its water absorption was 20% lower (6.4% versus 8.0%), its mass loss after 12 weeks in 5% HCl was 8% against 10% for the control, and its 56-day RCPT charge was about 64% lower (800 C versus 2,200 C). Regression analysis showed positive but moderate correlations between density and the strength properties (R² = 0.24–0.36) and a stronger inverse correlation between water absorption and density (R² = 0.54). The complete 4 × 4 factorial matrix of MK LS blends corresponds to a Taguchi L16 (4²) orthogonal array, with MK and LS as the two control factors at four levels (0, 5, 10 and 15%). ANOVA of the signal-to noise ratios showed that MK was the dominant factor, contributing about 41–89% of the total variation in compressive strength, water absorption, RCPT charge and acid induced mass loss, while LS contributed about 8–31%; because each mix was tested as a single run, the MK × LS interaction could not be separated from the error term. The cost analysis showed that replacing 20% of the cement with 10% MK and 10% LS increased the material cost of the M25 concrete by about 10.2%, mainly because of the higher price of MK, as lime sludge adds very little cost; this marginal increase is justified by the improvements in strength, durability, microstructure and xvi sustainability. Overall, the mix with 10% MK and 10% LS is technically and economically feasible and suitable for durable and eco-friendly concrete applications. Microstructural analysis indicated a denser matrix and a refined interfacial transition zone (ITZ) for C80M10L10, and the EDS-derived Ca/Si ratios were consistent with the formation of additional C–S–H/C–A–S–H gel, although these phases were inferred rather than directly identified. In the second phase, the performance of M40 grade concrete incorporating MK and calcined lime sludge (CLS) at 0–20% each was investigated. Workability decreased with increasing replacement owing to the higher specific surface areas of MK and CLS, whereas the mechanical properties improved at moderate replacement levels. The mix containing 15% MK and 10% CLS (M15CL10) exhibited the highest compressive strength (55.2 MPa at 90 days against 49.0 MPa for the control), the highest split tensile and flexural strengths and the lowest water absorption, indicating a synergistic effect between the pozzolanic activity of MK and the calcium-rich composition of CLS; beyond this replacement level, strength decreased and water absorption increased. Quadratic response surface models fitted to the CLS–MK data confirmed 15% MK and 10% CLS as the optimum (desirability 0.95). A cradle-to-gate life cycle assessment showed that the optimum mixes reduce the global warming potential by about 18% (C80M10L10) and 21% (M15CL10) relative to their respective controls. Therefore, this study establishes LS–MK and CLS–MK blended concretes as technically feasible and more sustainable alternatives to conventional concrete, contributing to waste valorisation and a reduced carbon footprint.