Improving Mixed Construction and Demolition Waste Used as Fine Aggregates in Concrete Production by Physical Pre-treatment

Finding sustainable substitutes for natural sand in concrete is an urgent task for the construction industry. Sri Lanka must look for environmentally friendly alternatives for river sand. At the same time, the reuse and recycling of construction and demolition (C&D) waste has become a major concern, as these wastes threaten the environment and human health. Here we used recycled fine aggregates (RFA) sourced from mixed C&D waste as a substitute for fine aggregates in concrete. High water absorption of RFA, primarily caused by old cement mortar, reduces concrete workability, while the smaller size fractions result in higher mortar contents. However, the properties of RFA can be improved through simple physical pre-treatment procedures, including reduction of attached mortar content and pre-soaking. The physical properties of treated and untreated RFA were found to compare favorably with natural sand. Subsequently, the sand was replaced with treated or untreated RFA at varying intervals from 10 to 60% by weight, while 0% served as control. The properties of wet and hardened concrete, M30, produced at constant slump, were evaluated. Flexural and splitting tensile strength were not significantly affected. However, the compressive strength decreased with an increase in the proportion of RFA but less so with treated RFA. The maximal viable replacement percentage was about 30% for the untreated samples but could be extended to 50% for treated RFA. We conclude that mixed RFA can be used as supplementary fine aggregate and physical pre-treatment is efficient in improving ready mix and can be employed on-site because it is a very simple procedure.