The air classification of manufactured sand (i.e., fine aggregates from crushed rock with a particle size smaller than about 4 mm) is utilised to separate crushed rock material into a fine particle fraction (filler, <100 mu m) and a coarse particle fraction (>100 mu m). Reducing the filler content of the manufactured sand is especially important for the concrete industry to ensure the rheology of the cement paste. There is an on-going search for production methods that can be applied to produce suitable manufactured sand that fulfils the requirements set by ready-mix concrete producers. A gravitational air classifier was investigated using computer fluid dynamics (CFD) to increase the understanding of the parameters controlling the flow field and how they affect the behaviour and efficiency of the classification process. The influence of the design and the geometrical parameters of the classifier on the flow field and air classification performance were also simulated and evaluated using the CFD technique. (C) 2012 Elsevier Ltd. All rights reserved.