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Manufactured sand is not simply crushed rock screened finer. Gradation, particle shape and fines content all have to be managed
20 Feb 2024
Manufactured sand differs from natural sand because it is produced by breaking rock rather than by weathering it. That single difference explains most of the problems buyers report. Crushed particles are angular, and an angular particle packs differently from a rounded one, so the same sieve analysis can behave differently in concrete. Shape is produced by the crushing method: an impact or VSI stage tends to give a more equant particle, while a cone stage, worked too hard, tends to produce flatter and more elongated pieces.
Gradation is controlled at two points. The first is the crushing stage, which decides how much of the output falls into the sand range at all. The second is screening, which decides how that material is split and how much of the coarse fraction is returned. Screening that is pushed beyond its capacity does not only lose efficiency, it distorts the split: the fine product ends up carrying material that should have stayed coarse, which is the most common cause of an out-of-specification sand.
Fines content is the third variable and the one most often mishandled. A certain proportion of very fine material is normal and can be useful, but too much makes the sand demand more water, and too little leaves a harsh mix. Washing or classifying is how the excess is removed, and the decision of how far to wash is a decision about the concrete the customer is making, not about the crusher. The practical lesson is that a sand line is a system: changing the crusher, the screen or the washer alone moves the problem rather than solving it, and the gradation target has to be stated before any of the three is chosen.
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