Upcycling of construction and demolition waste by oxalic acid surface activation of recycled aggregates: a novel processing approach.

The construction sector generates over 3.57 billion tons of construction and demolition waste (CDW) globally each year. However, the large- scale use of recycled aggregates (RA) in concrete remains constrained by adhered mortar and inferior physico- mechanical performance. The literature indicates that inorganic acids can deteriorate aggregate integrity and weaken concrete durability. Therefore, this study explores the use of organic oxalic acid (C2H2O4) as a surface- modification strategy. Advanced characterisation of the processed RA using XRD, FESEM- EDS, FTIR, TGA and TEM- EDS confirmed the formation of surface calcium oxalate, calcium redistribution and pozzolanic crystalline deposits, leading to improved interfacial densification. The results demonstrate that organic acid treatment reduced the impact value by 4.6% and 5.9% compared to H2SO4- and HCl- treated aggregates, respectively. The water absorption of RA decreased from 3.68% (untreated) to 2.80% (H2SO4- treated), 2.75% (HCl- treated), and further to 2.45% with oxalic acid treatment, indicating improved surface densification and reduced accessible porosity. This indicates surface pore refinement and reduced porosity. The compressive and splitting tensile strengths increased by 29.5% and 35.2%, respectively, at 28 days, compared to untreated RA concrete. The superior performance of oxalic acid was associated with the combined effects of adhered mortar modification and calcium oxalate-rich surface formation rather than mortar removal alone. The use of organic oxalic acid offers practical advantages over hazardous inorganic acids, including comparatively reduced handling and storage hazards. A preliminary screening-level carbon and life- cycle assessment (LCA) further indicated its potential environmental advantage, thereby minimising safety concerns and operational costs for large- scale construction applications. The findings establish oxalic acid treatment as a sustainable route to produce high- performance RA for circular concrete applications.