Aggregates' innovation: friend or foe? February 27 2015
Extracted from Aggregates Business Magazine : September/October 2014
“The aggregates’ industry typically has branded any crushed product that is less than 4 mesh (4.75mm) as ‘manufactured sand’. This of course is not the case. Manufactured sand should be a product that you have intentionally produced, not merely the waste fraction of a process that is targeting larger aggregate sizes.” 1
The produce sands from crushed rock, whether to replace ever-dwindling supplies of naturally occurring sands, or simply to reduce the huge amounts of unsaleable crushed rock fines generated in many quarries, there are several key characteristics with which we should be concerned. Those related to the suitability of the source rock, perhaps best loosely described as chemical properties, may be addressed by using admixtures in the concrete-making process, but, apart from the elimination of certain specific size ranges in which deleterious material may be concentrated, there is little the crushing and sizing process can do. Other factors, however, are well within the influence of existing process technology, and chief among these are particle size distribution, shape and surface texture.
The particle size distribution of manufactured sand is controlled by screening or sometimes by other methods of classification, the latter usually involving water, which brings its own problems. Until recently, this grading was usually governed to a great extent by the output of the crusher(s), and the breaking characteristics of the rock, but in the last few years, more interest has been shown in means of modifying the natural pattern of breakage, in particular to increase the sub-1mm particle population at the expense of coarser sizes, in order to improve packing density in concrete mixes.
The classification process has no effect upon either particle shape (or indeed on surface texture, as this is a property of the parent rock), so it is necessary to impose improvement with the use of suitable processing. Traditional rock crushers, whether compression (cone) type, or impact (blow-bar or hammer) type, can be made to produce adequate shape in coarse aggregates, but typically product below about 6mm (¼ inch) contains a high proportion of flaky or elongated particles, resulting in many of the problems inherent in using such material as a sand substitute. Grain shape is a function of the crushing process (and, of course, the natural cleavage and fracture characteristics of the parent rock). Suitable shape is generally described, unhelpfully, as cubical, the ultimate expression of which would be a shape enclosed by six identical square faces. In contrast, we should try to produce a shape that has good volume to surface area ratio, and an easily-packed form, to minimise the amount of binder we need to stick it together in the form of concrete. Given these attributes, it will automatically tend to assist workability, unlike our cube! The ideal form can perhaps be described as equally dimensional, and without sharp corners or edges.
Manufactured sand need not mimic the natural product: in fact manufactured sand can have some advantageous properties entirely due to its dissimilarity to natural sands. For instance, natural sands must have nearly all of the microfines removed, because they might contain deleterious material, whereas crushed rock from a uniform homogeneous deposit without incursions may generally retain its microfines without detriment. Much successful research has been carried out on the inclusion of high levels of microfines in manufactured sand.
1 Hudson B P. “Crushers affect Product Quality”, Quarry magazine, April 1999