Effect of Environmentally Sustainable Materials on the Strength behaviour of Self-Curing Concrete

The increasing demand for sustainable construction materials has led to growing interest in the use of recycled aggregates (RA) as partial replacements for natural aggregates in concrete. However, the wider application of RA is still limited due to its higher porosity, greater water absorption, and lower mechanical strength, mainly caused by the presence of adhered old mortar. Although acid treatment has been applied to improve the quality of RA, and internal curing agents such as polyethylene glycol-400 (PEG-400) have been used to enhance cement hydration, the combined effect of these two approaches has not been thoroughly investigated. This study evaluates the influence of HCl-treated RA used together with 1% PEG-400 as a self-curing agent on the fresh and mechanical properties of concrete. The RA were incorporated at replacement levels of 0%, 10%, 20%, 30%, 40%, and 50%. The concrete mixes were assessed at 28 days through slump, compression strength, split tensile strength, flexural strength, and water absorption tests. The results show that concrete performance improved up to 20% replacement of treated RA. The slump increased from 82 mm for the control mix to 93 mm at 20% replacement. When compressive strength increased from 23.5 N/mm² to 25.5 N/mm², split tensile strength from 1.7 N/mm² to 2.3 N/mm², and flexural strength from 2.9 N/mm² to 3.5 N/mm² at the same replacement level. Water absorption also showed a slight reduction, decreasing from 3.42% to 3.20%. However, when the replacement level exceeded 30%, mechanical strengths gradually declined and water absorption increased, reaching 19.2 N/mm² compressive strength and 4.05% absorption at 50% RA. Overall, the findings indicate that the combined use of HCl-treated RA and 1% PEG-400 improves fresh and mechanical performance at moderate replacement levels, with 20% identified as the optimum proportion under the conditions investigated in this study.