Sand production with VSI crushing and air classification: Optimising fines grading for concrete production with micro-proportioning

Crushed sand fines have a pronounced effect on fresh concrete rheological properties, which can be controlled through a concrete micro-proportioning approach, i.e. optimisation of the particle size distribution (PSD) in the very fine range of the grading (<= 250 mu m). The paper describes a study where possibilities of producing crushed sand by combining high-speed (70 m/s) vertical shaft impact (VSI) crushing and static air classification are explored to enable the micro-proportioning approach in full-scale aggregate and concrete production. In addition, the effect of rock resistance to fragmentation (crushability) on the shape improvement and fines generation during high-speed VSI crushing is experimentally studied. The VSI crushing experiment results show that an acceptable level of crushed sand particle (1.25-8 mm) equi-dimensionality (flakiness index lower than 5-8%) can be achieved for all processed feeds, regardless of the parent rock crushability or initial particle shape. The amount of fines smaller than 63 mu m and 125 mu m generated during high-speed VSI crushing is very strongly governed by the resistance to fragmentation of the processed rock materials. Analysis of the air classification results show possibilities of modelling a variety of different crushed sand fine particle (<= 250 mu m) grading curves independently of the rock type, amount of fines or grading of the crushed sand product after the VSI crushing. (C) 2015 Elsevier Ltd. All rights reserved.