Assessment of a microwave-assisted recycling process for the recovery of high-quality aggregates from concrete waste

• A microwave pretreatment was studied for producing high-quality aggregates.
• Weakening of concrete samples was observed even at low energy microwave heating.
• Decrease of the fracture energy and increase of the aggregate liberation.
• Assessment of the robustness of the process for treating concrete waste.

This study presents an innovative method for concrete waste up-cycling based on concrete weakening through microwave heating before impact crushing. Two series of tests were conducted in order to assess the influence of the aggregate properties (size distribution, mineralogical nature) and the influence of the operating conditions of the microwave heating pretreatment on concrete fragmentation; and thus to evaluate the feasibility and the robustness of this process. Experiments were carried out on lab-made, cylindrical concrete specimens and on no-slump concrete waste with a multimode cavity microwave equipment (2.45 GHz, 6 kW) and an impact crusher. Results showed that microwave heating always induced an embrittlement of concrete samples which resulted in lower fracture energy, higher fragmentation of samples and higher liberation of aggregates (i.e. free of cement paste). A microwave-assisted comminution process is therefore an effective recycling technique for the recovery of high-quality aggregates from concrete waste.