Recycling-Oriented Investigation of Local Porosity Changes in Microwave Heated-Concrete

Abstract

Large quantities of concrete waste are being produced continuously throughout the world, of which only a fraction are downcycled as construction backfill or as road-base. Seeking total concrete recyclability, this work concerns the development of microwave-based solutions for the separation of individual constituents of concrete. By focusing on the interaction between microwaves and concrete at the microscopic level, the paper makes important connections between local changes in the microwave-heated concrete texture and macroscopic changes in mechanical properties. Through analysis of the concrete texture using SEM imaging, it is found that the microwave heating of concrete causes fracture porosity. The size and shape of fracture porosity can be correlated with recycling performance indicators; namely aggregate liberation, concrete strength and product fineness. In particular, the work finds that only a short exposure to microwaves promotes the formation of a primary fracture network responsible for selective liberation of aggregates. Longer exposure to microwave heating creates a secondary network of smaller fractures that spreads throughout the cement phase, which is directly associated with the changes in mechanical strength of concrete and product fineness.The work introduces the concept of textural versus physical liberation, and shows that while microwave heating creates a high selective textural liberation of aggregate particles, the comminution of microwave-heated concrete may not necessarily yield high physical liberation. The work concludes that the key to designing a microwave-based process for concrete recycling resides in finding comminution and separation technologies that can best harvest the benefits of the textural and mechanical changes produced by microwave heating.