Enhancing the recovery of cement-rich powder from recycled fine aggregates through thermal shock

Abstract

This study presents a method for recovering cement-rich powder from recycled fine aggregates by thermal shock, during which particles are fragmented and spalled due to differential thermal stress. When recycled fine aggregates (RFA) are exposed to high temperatures, the cement paste-rich boundary between the aggregates is weakened and spalled, liberating cement rich particles due to thermal shock. To investigate this phenomenon, experiments have been carried out by subjecting fine recycled aggregates to high temperatures ranging from 500 °C to 700 °C at different residence times. The result suggests that the particles split and crackle due to thermo-mechanical changes. Following thermal treatment, gentle milling completes the liberation process of recycled cement-rich powder (RCP). The composition of the recovered powder confirms the feasibility of the recovery method. To understand the thermo-mechanical process better, modelling efforts have been carried out on a spherical concrete particle of known diameter. The model predicts the temperature profile, residence time and radial stress inside the particle. According to the model, a 2 mm particle experiences a radial stress high enough to overcome the tensile strength of the concrete within 35 s, causing cracks due to the thermal gradient created between the inner and outer surfaces of the particle. These predictions have been verified by experimental results in the laboratory. This approach not only enhances recovery of RCP but also promotes sustainable construction practices.