Integrated meta analytical and multi criteria evaluation of walnut shell as a sustainable aggregate in concrete with emphasis on durability performance

This study investigates the feasibility of using walnut shells (WS), an agricultural by-product, as a partial replacement for natural aggregates in concrete production. The effects of walnut shells on the microstructure, physical properties, and durability performance of concrete were evaluated in the research. In this context, the aim was to convert agricultural waste into sustainable construction materials. The interaction between the cement matrix and walnut shell particles was examined using scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) analyses. The findings showed that at low substitution rates (5–10%), mechanical performance improved or decreased only slightly due to better interfacial adhesion. In contrast, at high substitution rates (30% and above), porosity and microcrack formation increased, which reduced strength. Physical experiments have shown that the use of walnut shells significantly reduces concrete density, with a density reduction of approximately 33% at a 40% replacement rate, bringing the concrete closer to the lightweight concrete class. Ultrasonic pulse velocity (UPV) values decreased steadily with increasing substitution ratio. In terms of durability, significant increases in water absorption values were observed as the walnut shell ratio increased; however, thermal conductivity decreased significantly (by more than 50%), improving the insulation performance of the concrete. The tests conducted against sulfate, chloride, and acid effects showed that medium replacement ratios (around 20%) yielded results similar to or better than the control samples, but durability weakened at high ratios. To synthesize these findings, a log response ratio (lnRR)-based meta-analysis was used to capture the overall trends in performance, and an anchored polynomial meta-regression with bootstrap-based uncertainty estimation was then applied to model the changes in different properties with WS content. Finally, a durability-constrained multi-criteria decision analysis was conducted based on thermal performance, durability metrics, ΔGWP, and ΔCost, and both a screening-level life cycle assessment and techno-economic analysis suggested that moderate levels of WS replacement provide the best trade-off between mechanical performance, environmental benefits, and cost, therefore, the integrated methodology proposed in this study provides a quantitative decision-support framework for determining sustainable levels of walnut shell substitution in concrete aggregates.