Separability of cement stone and aggregates in concrete recycling

Abstract

Based on a growing demand for mineral aggregates and high CO2‐emissions of the cement industry, an entire separation of cement stone and original aggregates from concrete waste is desirable. As described in literature, aggregates can be reused e.g. in ready‐mixed concrete and ground cement stone can substitute limestone for clinker production. Thus, extensive test series were carried out to determine specific energies needed to crush concrete waste aggregates. Concrete mixtures with different compressive strengths and maximum grain sizes were thermally damaged using different temperature levels and numbers of thermal cycles. To evaluate the separability quantitatively, different properties from impact‐tests and static tests were derived and compared. An increasing number of thermal cycles, increasing temperature level, low compressive strength and small max. grain size enhance separability. It is found, that parameters from impact‐tests and static tests can be used equally good, even if the both tests include different deterioration mechanisms. From the knowledge of the specific crushing energy a model to estimate ball‐sizes for mills is proposed. Finally, grinding/milling of small grain fractions (e.g. 2/4 or 8/16) improves the quality of recovered aggregates and cement stone.