Optimising silane-siloxane treatment for enhanced performance of mixed recycled aggregate: rsm- central composite design approach

Abstract This study explores the optimisation of silane-siloxane (SS) treatment conditions for Mixed Recycled Aggregate (MRA) using Central Composite Design (CCD) under Response Surface Methodology (RSM). Three key independent variables—SS concentration, water-to-aggregate ratio, and soaking duration—were systematically analysed to minimise water absorption and LA abrasion, two critical durability parameters of MRA. The optimised treatment conditions (13.29% SS concentration, 0.59 water-to-aggregate ratio, and 12-hour soaking duration) resulted in a 53.55% reduction in water absorption and a 7.57% improvement in (Los Angeles) LA abrasion resistance, enhancing aggregate performance. The accuracy of the predictive models was validated through statistical analysis, with R 2 values of 0.99 for water absorption and 0.98 for LA abrasion, confirming a strong correlation between experimental and predicted results. The treated MRA was then incorporated into concrete at replacement levels of 0%, 25%, 50%, 75%, and 100%, and its influence on density, workability, and mechanical properties was evaluated. SS treatment significantly improved fresh and hardened density, slump, compressive, tensile, flexural strengths, water absorption and volume of permeable voids compared to untreated MRA, mitigating strength loss and enhancing durability. The findings demonstrate that SS treatment effectively modifies MRA, making higher replacement levels viable for structural applications due to its coating and filling mechanism, which fills the pores and voids of the MRA surface. This study contributes to sustainable construction by promoting use of MRA, thereby reducing dependency on natural aggregates and advancing the principles of a circular economy within the construction industry.