Recovery of Homogeneous Materials from Composite Masonry Structures by Crushing and Mechanical Separation -- Limitations of Prevalent Processes.

One means of bringing recycled construction materials back into the materials cycle is to reuse them as feedstock in production. To establish a closed-loop cycle in the masonry industry it is necessary to ensure a sufficient purity of crushed and sorted recycled material. Mixed rubble resulting from the demolition of masonry structures has to be processed mechanically by conventional crushing devices to obtain bulk materials with a defined particle size distribution. Particle size reduction is the most common means used to liberate homogeneous particles from composite masonry materials, without any attached plasters or mortars. Fully liberated particles of the main components (clay, calcium-silicate, lightweight concrete bricks, and autoclaved aerated concrete blocks) are a necessary prerequisite for the success of downstream sorting processes. Investigations of composite masonry materials have shown that the effectiveness of homogeneous particle liberation processes is practically independent of the type of stress exerted by the crushing devices. Rather, homogeneous particle liberation depends mainly on the particle size of the resulting bulk material. Particle sizes can be further reduced in order to achieve a greater purity at the expense of increasing waste in the form of fines which cannot be sorted. Therefore, the formation of composite particles cannot practically be avoided by prevalent crushing processes if coarser aggregates are required. Composite particles in recycling mixtures may cause problems for subsequent sorting processes. Downstream mechanical processes can be used to increase the degree of liberation and to remove attached plasters and mortars mechanically from the particles. It has been experimentally demonstrated that attached mortars, plasters, or other impurities with a lower mechanical comminution resistance could be separated selectively by friction or shearing. Based on these results, the first draft of a sophisticated processing system has been developed. [ABSTRACT FROM AUTHOR]