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The shape of the sand grains from our planet's deserts is too round to be used in concrete (1, 2, 3). This is why we are drawing the last sand resources from natural beaches for making concrete for the ever-increasing human demands.

Would it not make sense to crush the round desert sand grains in a mill to produce the sand required for concrete? If we break each sphere into two fragments, the resulting grains are sharp-edged and asymmetric, with "grip".

Is it a matter of cost?

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    $\begingroup$ Beach sand might be used where you live, but not where I live. One of the issues with using beach sand is that it needs to be washed, to remove salt, prior to it being used in concrete. Geologically, sand is a loose material with a specific range of grain sizes. Breaking existing grains of sand is expensive and it would create a smaller sized material, some of it the size of silt, which could be unsuitable for use in concrete. $\endgroup$ – Fred Nov 19 '17 at 23:59
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Yes, cost seems to be the limiting factor. Specifically costs to haul the sand where it is needed. Where I live there are huge piles of sand being removed from the local river within a few miles of the concrete plants. In the New Yorker article, reference #3 in your question, one of the engineers they interviewed said after about 60 miles it becomes too costly to transport the sand for construction activities. If cost isn't a limiting factor, and if the milled sand could be sorted to an appropriate size for the cement, other sources further from where the sand is needed could be utilized.

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Cost is a factor, but gradation is probably more important. Round aggregate is very nice for making concrete: the same mix proportions with rounded aggregate will produce a higher slump (more workable) concrete than will crushed aggregate. Now it does turn out that crushed aggregate may produce a higher ultimate strength, because the crushed aggregate tends to interlock, whereas rounded aggregate tends to lose connection to the main mass.

ASTM C-33 specifies a gradation for sand which minimizes void space - the approved gradation doesn't need so much paste to bind everything together. Many unusable sands, like some beach sands, may be overrepresented in one size range, so to get the desirable overall gradation, most of the sand must be discarded (or used elsewhere). On the other hand, when the cement is the item to be tested, rounded sand of a narrow size range is used (Ottawa Sand, from Illinois, windblown). Paste cubes made from Ottawa Sand have a failure mechanism which is highly dependent on cement properties (strength, adhesion) rather than on aggregate (adhesion, interlocking).

But ultimately, cost does determine whether there is enough value in the sand, or does it need to be processed to make it usable.

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  • $\begingroup$ Round aggregate? I thought round aggregate was not used precisely because it is round (i.e. desert sand formed from wind erosion) and doesn't have sharp edges or surface area to hang onto the binder. $\endgroup$ – DKNguyen Oct 29 '20 at 22:17
  • $\begingroup$ Concrete with rounded aggregate (yeah, spheres seem to be too rounded!) flows better when fresh (unset) than with crushed aggregate, therefore needs less water for a given slump and strength. Surface composition is just as important as irregular structure. Silica is the usual rounded material, but limestone bonds better to cement, so rounded is easier to work with (screed, push), and usually gives enough strength for sidewalks, etc. And 5000 psi isn't really any better than 3000 psi if it looks bad after a few freeze-thaw cycles peel thin layers of paste off the flat rock. $\endgroup$ – Jim Gaidis Oct 30 '20 at 14:09

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