Tea stain-inspired treatment for fine recycled concrete aggregates

Abstract

A massive fraction of recycled concrete aggregates (RCAs) is fine RCA. Once recycled in new concrete, fine RCA can significantly reduce the mechanical strength and durability of the concrete since fine RCA has high water absorption and high porosity. To address this issue, this study proposes to treat the fine RCA using a tea-stain inspired compound, tannic acid (TA). To this end, fine RCA is first immersed in dilute solution of TA. In this process, TA can react with calcium hydroxide (CH) and calcite of RCA to produce nanoparticles on the surface of the RCA particles. These nanoparticles can fill the capillary nanopores in the mortar, significantly reduce the porosity of the produced mortar. This can be confirmed by the scanning electron microscopy (SEM) and mercury intrusion porosimetry (MIP) analysis. Experimental studies have been carried out to examine the effect of this treatment on the hydration and mechanical behavior of the mortars made with the treated fine RCA. Testing results confirm that the strength and durability of the mortar can be significant enhanced by the proposed method, which can be mainly attributed to two reasons. First, TA coated on the surface of the fine RCA can capture calcium ions to induce local mineralization and facilitate the hydration of cement in the interface transition zone (ITZ), densifying the ITZ according to nanoindentation test. Second, the cohesion of hardened cementitious material can be enhanced by the unique binding capacity of TA through covalent and non-covalent interactions.