Influence of jaw angle in smart crushing on selective recovery of recycled concrete fines

Recycling concrete is essential for reducing the environmental impact of the construction sector, yet the reuse of recycled cement fines remains limited due to challenges in separation. The Smart Crusher applies to a selective mechanical process designed to separate cement paste from natural aggregates during processing. Its performance depends strongly on the mechanical configuration, particularly the jaw angle. This study investigates two configurations of the moving jaw (90° and 85°) and their influence on the amount and composition of fines obtained from laboratory-made CEM I and CEM III concrete. Results show that the 85° jaw angle produced more fines due to stronger size reduction, but these fines presented higher SiO2, indicating increased quartz contamination and reduced selectivity. Conversely, the 90° configuration generated fewer fines, reflected by lower available SiO2 and higher CaO content, suggesting improved preservation of cement paste and reduced aggregate crushing. This result was confirmed through X-ray diffraction analysis, showing lower quartz and higher portlandite peaks for the 90° angle. Overall, the study highlights a clear trade-off: a narrower jaw angle increases yield but decreases selectivity, while the wider angle improves cement paste recovery at the expense of quantity.