Eco-friendly treatment of recycled concrete fines as supplementary cementitious materials

Demolished or waste concretes can be crushed into particles with appropriate sizes to be used as coarse or fine aggregates in new concretes. This recycling process also produces 5% to 20% small particles with size less than 0.15 mm. which can be recycled as the recycled concrete fines (RCFs). Besides low-end applications and land-filling, few appropriate applications have been found for RCF. This study develops an eco-friendly treatment to turn these underutilized RCFs into supplementary cementitious materials (SEM) for new concretes. Without any treatment, RCFs can significantly reduce the strength and durability of the produced concrete due to their low pozzolanic activity and high porosity. This study exploits a plant-based compound, tannic acid (TA) to treat RCFs so they can be used in new concrete to partially replace Portland cement without reducing the performance of the concrete. This is done by a simple two-step mixing process. In the first step, RCF particles are mixed with a low-concentrated TA solution for about one hour to allow for potential reaction between the TA and the RCF particles. In step II, the slurry produced by step I is mixed with cement and aggregate to make concrete. Reaction between the TA and the RCF particles can produce submicron particles deposited on the surface of the RCF particles. Those particles not only fill the pores of the RCF particles, but also provide the nucleation sites for the hydration of the cement. This promotes the hydration of the cement and densifies the microstructure of the hardened paste. As a result, multiple benefits can be induced by the proposed TA treatment. Experimental results show that up to 26% increment in compressive strength of the mortar has been achieved by the proposed treatment. More importantly, the risk of cracking in the early age of the mortar is reduced and the corrosion life of the reinforcing bar is greatly enhanced.