Desert sand is too smooth to bind to make concrete, so sand is taken from river beds such as the Mekong
That's really interesting, but makes sense. The sand in most deserts has been blowing around for millennia (or longer), and so would be extremely smoothed-off. Sand in the Mekong is relatively young, and not that far removed from when it was a rough chip off a rock upstream. The difference here is called sediment maturity [] IIRC from first-year geology.
But can anyone answer: why is mature, rounded, fine sand not suitable for binding concrete or for glassmaking? And how fine can the sand be -- wouldn't extremely fine grains classify as clays instead?
Not to argue with the BBC, but this statement is just factually incorrect. Desert sand is not suitable for concrete because it is poorly graded and is all essentially the same size. Concrete mixes call for sands of various grain sizes, and so to use desert sand you would need to mix in these other sizes of sand.
I have seen this rumor repeated often and I think it is journalists trying to explain something they don't understand in a way that seems intuitive.
"But can anyone answer: why is mature, rounded, fine sand not suitable for binding concrete or for glassmaking?"
It's the jagged edges on 'fresh' sand that gives the mix strength. Smooth sand is fine for glassmaking (something I do along with metal and jewelry work) but horrible for construction, as it doesn't grip into the binding agent as well, and any serious load makes it turn straight into dust.
The sand can be as fine as possible, as long as it isn't rounded, and still be useful for construction work.
I'm not a concrete mix design expert by any means, but I disagree with this post on a few levels. Concrete strength is derived from multiple sources, and grain interlocking is just one of them. I think recent research has shown that grain interlocking does not play a large role in concrete strength. It's unclear to me if your post is talking about grain interlocking or cement/aggregate bonding. I'm not up to speed on research in this area, but I do not believe that you can have a rounded sand that is so smooth that bonding doesn't occur and the idea that the concrete "turns to dust" under any serious load is something that I personally have never heard of.
In fact, the more angular the aggregate is the less workable the concrete becomes. There is a sweet spot of angularity that is desired. I believe the main advantage to increased angularity is in durability (which is due to interlocking, I think, as well as increased void ratio leading to better air entrainment).
Also - the size of the grains is more important than angularity, so I also disagree with the statement that sand "as fine as possible" can be useful for construction work in the context of concrete. While tautologically true because anything too fine (passing #200) would be classified as a silt instead of a sand, the way that it's written is deceptive.
"Also - the size of the grains is more important than angularity, so I also disagree with the statement that sand "as fine as possible" can be useful for construction work in the context of concrete."
Disagree all you want but that doesn't change the fact I've done building construction, including laying foundation, and finer sand is far superior to coarser sand. In fact, coarser sand is rather horrible for construction.
I still don't understand what exactly you're talking about. Are you talking about sand used for concrete or sand in general (e.g. - for subgrade soils).
Sand used in concrete needs to meet a gradation requirement. The gradation requirement uses all sizes of sand as aggregate.
And if we are comparing credentials (which I assume is what your building construction story and youtube link is for?), then I've done nothing but put holes in the ground and fill them up with concrete for almost a decade. Here's what I was doing in 2014: https://www.youtube.com/watch?v=E4Ixw69woSA
I did the construction globally, from USA to UK to Saudi Arabia.
I'm talking about both sand in construction and in general (like for glassblowing, etc.) Portland cement uses the finest sands possible for holding concrete aggregate, which you only really tend to find in desert areas (if you're lucky enough to find a spot that hasn't been above water for more than a million years, that's one reason construction-grade stuff is so hard to find.)
Now days, I do very similar to your link, except it's through mountains, and terminates inside the mountain. Minerals extraction is fun!
I suppose then there's something else about desert sand (all deserts??) that the article claims makes it unsuitable for glassmaking. Maybe compositional.
The composition doesn't matter once you melt and everything undergoes specific gravity separation. Melt a bucket of sand, swing it around in a circle for three minutes at high speed, you have a cullet of purified glass with contaminated exterior layers.
Sand is just a classification of grain size of soil. We make sand all the time at quarries by crushing rocks. Most of the aggregate used in construction is made this way or taken from a pit/dredged.
Because crushing rocks costs more money than dredging rivers. Especially when there are weak or no environmental protection laws or regulations for dredging, like on the Mekong.
If you're willing to do that, it's probably easier to just start with gravel/rocks (also abundant) and crush them. I'm pretty sure this is where most of the construction material near me comes from.
Desert sand can be used in concrete. Concrete can be up to 40% stronger, after 90 days, with addition of up to 20% of dune sand instead of Portland cement.
See "Compressive strength and hydration with age of cement pastes containing dune sand powder" Salim Guettala, Bouzidi Mezghiche, Algeria.
That's really interesting, but makes sense. The sand in most deserts has been blowing around for millennia (or longer), and so would be extremely smoothed-off. Sand in the Mekong is relatively young, and not that far removed from when it was a rough chip off a rock upstream. The difference here is called sediment maturity [] IIRC from first-year geology.
But can anyone answer: why is mature, rounded, fine sand not suitable for binding concrete or for glassmaking? And how fine can the sand be -- wouldn't extremely fine grains classify as clays instead?
[] https://en.wikipedia.org/wiki/Maturity_(sedimentology)