• Moign, Alain Le
• Bonnaudin, Fabrice
• Touzé, Solène
• Bru, Kathy
• Menard, Yannick
• Poirier, Jean Eric
• Weid, Frédéric Von Der
• Ruffié, Gilles

Abstract

Hardened concrete is a composite material that contains two main phases: the matrix (hardened cement paste, 20 %) and aggregates (gravels and sand, 80 %). The liberation and the recycling of these constituents can provide an answer to i) the exploration of new aggregates supply sources imposed by the depletion of natural deposit and the faced difficulties when trying to open new quarries and ii) the reduction of CO2 emissions in the clinker manufacturing process through the substitution of part of the limestone by a non-carbonated lime source (cement paste) in the raw mix. Traditional recycling methods are based on crushing and grinding but these methods have two main shortcomings: they are not selective regarding the concrete composition and they induce an extensive destruction of the matrix and of the aggregates. Therefore, most of the recycling products are used as low-quality materials. Moreover, none of the current processes allows recovering and recycling the hardened cement paste. In order to overcome these drawbacks, it is necessary to develop innovative technologies that mark a break with the current recycling practices. This study, supported by the French national research agency (ANR) through the COFRAGE project, aims at assessing two technologies of embrittlement and selective fragmentation of concretes which could allow the production of two high-quality materials (aggregates and cement paste) while minimizing the associated energy consumption. These technologies are: - A selective fragmentation technique that applies high voltage pulses, (a few kV per cm), through blocks immersed in water. The basic principle is the flow of an electrical power between two electrodes in a water-filled discharge vessel that causes an explosive expansion along the discharge plasma channel (electrodynamic fragmentation) and that induces the propagation of a shock-wave front in the water (electrohydraulic fragmentation); - An embrittlement technology that uses electromagnetic energy of microwaves and their selective effects of internal heating on the different mineral phases. Some mineral phases belonging to the group of dielectric materials with strong absorption capacity of microwaves warm up more quickly than other minerals belonging to the groups of insulating materials when they are exposed a few seconds to microwaves. This induces different thermal expansion and then generates mechanical constraints which weaken the material. Experiments were performed in lab-scale equipment firstly on concrete waste collected on a French demolition site and secondly on lab-made concrete samples with a defined composition. Influence of the operating parameters was investigated and the two technologies were compared. It was shown that both of these technologies induces cracks along the grain boundaries of the material and therefore could allow a selective liberation of the aggregates and cement paste during grinding. The impact of these new processes on the comminution energy was also evaluated.

Topics

• impedance spectroscopy
• mineral
• grain
• phase
• experiment
• grinding
• crack
• composite
• cement
• thermal expansion
• lime