Impact of microwave radiation on recycled concrete powder in cement-based materials: Structure, hydration activity and mechanism

The utilization of recycled concrete powder (RCP) in cement-based materials was greatly limited by its low reactivity. This work aimed to activate the activity of RCP by microwave radiation. The effects of microwave radiation at different power (300-800 W) on the grindability, phase assemblage, microstructure and activity index of RCP were studied. Subsequently, 30% RCP was incorporated into cement paste to prepare RCP-PC pastes. The setting time and compressive strength of blended pastes were tested, and the microstructure and hydration products of hardened pastes were also investigated by SEM and TG. Results show that microwave radiation not only promoted the dissociation of RCP particles, but also induced the amorphous transition of crystalline phases in RCP. As a result, the grindability and activity index of RCP were significantly improved. When subjected to microwave radiation at 800 W, the activity index of RCP was increased from 53% to 75% at 7 d and from 59% to 79% at 28 d, respectively. Furthermore, compared to the cement paste with raw RCP, the initial and final setting time of the microwave activated RCP-PC paste was shortened, and the early compressive strength was enhanced. This work was expected to develop a novel method for the activation of RCP, thus promoting its recycling and utilization in cement-based materials.