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

•Recycled concrete fines can be used as supplementary cementitious materials after treating with tannic acid.•Tannic acid can react with RCF to produce to produce submicron particles.•Tannic acid treatment drastically reduces the porosity and enhances the elastic modulus and packing density of the hydration products.•Compressive strength at 28d of the mortar made with RCF as SCM can be improved over 26% by the proposed treatment.•The early-age cracking of the mortar is significantly reduced by the tannic acid treatment.•The corrosion time of the reinforcing bar is enhanced by the proposed method over 55%. 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 landfilling, 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 t