Numerical analysis of mortar-aggregate separation induced by microwave heating

Microwave heating (MWH) is more and more popular for its high selectivity in mortar-aggregate separation that has been traditionally dominated by mechanical method. In this work, COMSOL Multiphysics software was used to predict the distribution of multi-field inside mortar and aggregate, and the reliability of the numerical model was verified by microwave heating experiment. The simulation results show that there are hot spots and cold spots in the sample, and the temperature at the edge is high, which is consistent with the experimental results. The separation is induced by the high temperature gradient at the interface due to thermal mismatch. The failure of the interface is mainly incurred by normal stress perpendicular to the interface. Microcracks are induced when it reaches the strength limit, which appear first at the edge of the interface. With the increase of time, the generation and interconnection of cracks lead to the increase of interface separation rate continuously. The separation rate of the interface and aggregate damage rate are also changing at varying positions. The separation efficiency has no direct relationship with the energy utilization rate. The aggregate damage rate is basically zero at the appropriate position, while the interface separation rate reaches 95%. In addition, the dehydration of calcium silicate hydrate and ettringite and steam migration are accompanied by the heating process. The model developed can also be helpful to predict the optimal location of effective separation of mortar-aggregate.