The precast concrete industry makes a decisive contribution to cover the growing need for residential and non-residential buildings. This task is associated with a substantial consumption of mineral aggregates. As a new approach to increasing resource efficiency, the creation of material cycles has been started. An example is the use of recycled aggregates for the manufacture of R (resource-conserving) concrete, resorting to „end-of-life concrete“ as anthropogenic raw material deposit.
A resource that has been hardly used so far is masonry rubble – a mix consisting of different wall-building materials and also gypsum, in the most unfavorable case. These mixes – used in downcycling such as in earthworks projects at best so far – are ideal as raw material for the manufacture of lightweight aggregates. Regarding their properties they are equal to those of lightweight aggregates manufactured of expanded clays. Demolished pumice concretes are a possible source of raw material, too. Lightweight granules made on the basis of this rubble can replace traditional pumice building materials for which the end of availability is clearly foreseeable.
For many years now, IAB Weimar has been developing practical solutions for an implementation of the circular economy. Since the beginning of the 2000s, many R & D activities have been focusing on the life cycle approach „production – utilization – recycling“. In this regard, the recycling of masonry rubble as raw material for the manufacture of lightweight aggregates plays an increasing role. It was proven that at a firing temperature of 1200 °C the release of expanded gases required for the pore formation takes place simultaneously with the plasticizing of the surface as well as the chemical transformations, resulting in a stable material structure.
Meanwhile a standard of knowledge has been obtained that allows us to transfer the positive results achieved in the laboratory to an industrial scale. A pilot plant is available for this purpose equipped with a gas-fired rotary kiln as a highlight. In the first test the research team of IAB managed to transfer this process into practice. At the same time, they were able to prove an effective sulfate reduction by means of the above-mentioned thermal treatment. The gypsum contained in the construction rubble could be thermally decomposed and totally transferred to the gas phase. This makes the homogenous recycling of gypsum possible, which is an additional advantage of the procedure.
The development described exploits alternative sources of raw materials for the manufacture of lightweight R concrete, thus being of great relevance to the conservation of natural resources.