[Note that this article is a transcript of the video embedded above.]

If you have to know the answer right away, it’s no; or at least, my goal with this video is to convince you that the world is not running out of sand. But if it were that simple, I wouldn’t be here (right?) and you probably wouldn’t be either. In fact, I was really surprised by some of the things I didn’t know as I dug deeper into the topic further, and how some of the most widely spread sand “facts” are dead wrong.

The wide world of sand is complicated, and not in a boring, pedantic kind of way. This simple material touches nearly every part of our lives, and the science and engineering behind it is rich and deep, and to me at least, hard not to become obsessed with. There’s a good chance you’ve seen articles or videos over the past few years with essentially the same story about sand. The “Sand Wars” documentary kind of kicked off the modern discussion, and then Vince Beiser wrote an excellent book on the topic, “The World in a Grain.” Of course, a lot of the book’s best reviews focus on the fact that sand is kind of a topic that’s “taken for granted” or “neglected.” But, at least in civil engineering, it is one of the most glected materials out there. And I’d like to give you a peek behind the curtain and show you how we think about this seemingly unlimited resource and why it’s worth knowing a little more about it. But first, I need to head out to the garage and put some sand in a rock tumbler, because I want to do a material property shootout, and this is going to take a little while. I have something really cool in the other barrel, and I’ll show it to you later on in the video. I’m Grady, and this is Practical Engineering.

What the heck is sand anyway? It’s kind of a “know it when you see it”-type material. If we use the US Department of Agriculture’s soil textural triangle, sand is any granular material that is at least 85% sand… So, what the heck is sand? For a better answer, we have the Unified Soil Classification System. Geotechnical engineers sometimes say that “dirt” is a four-letter word. Maybe because it undermines the importance of soil (which is also a four-letter word, by the way), but I like to think it’s because we have better names for all the dirts around the world, and here they are. In fact, there are four specific kinds of sand, but they all fit this one criterion, and it’s all about the size of the particles. At least half of those particles have to make it through a Number 4 sieve (about 5 millimeters), but no more than half can go through a Number 200 sieve (less than a tenth of a millimeter, or about 75 microns). That is a pretty wide range of materials, but I think when you picture sand in your mind, you probably imagine what the USCS would call “clean sand” where less than 12% pass the 200 sieve. So, just to make it simple, let’s say that sand is a material where the particles fit through this, but they don’t fit through this. But still, that encompasses a huge range of different dirts. And I hope, at this point, you’re asking, “Who Cares?” because I would love to answer that question.

In his book, Beiser calls sand “the most important solid substance on earth…the literal foundation of modern civilization…” We use it to make glass, semiconductors, fiber optics, filters, and abrasives, use it to texture surfaces, to play in, for beauty, and more. But, probably more than anything else, sand is an essential ingredient in concrete. And, you know, I’m a civil engineer; this is a channel about the built environment; so I wanna talk about concrete. And, in fact, if this video sparks your curiosity about one of my favorite materials, I have a whole playlist of topics I’ve covered in the past so you can learn more after this. You can’t really overstate how important concrete is and how much of it we use. There’s a bigger conversation to be had about its environmental impacts, but when compared to alternative building materials, it just has so much going for it. It is an extremely low-cost, durable substance that can be made into just about any shape you can imagine. Concrete has enabled us to build structures that last for generations from some very simple ingredients that are (mostly) available across the world: water, cement, gravel, and sand.

Most of those ingredients are mined and used directly as raw materials. And they’re usually mined close by. Transportation makes up a big part of the cost associated with sands used for construction, so the distance between where they’re found and where they need to go is highly correlated with how economical they can be. And that often leads to environmental impacts, some worse than others, depending on local regulations. It turns out that the best sand for concrete often comes from rivers, and mining in rivers can be particularly destructive because the impacts can spread upstream and downstream through changes in the nature of the channel. (I have a series of videos on that topic, too, by the way). Sand isn’t spread evenly throughout the world, and it’s a non-renewable resource. Geologic processes produce it a lot slower than we can use it. So, it makes some intuitive sense to say that we could eventually run out. But here’s a fact that is often overlooked in the discussion: we can make sand.

And it’s not that complicated, either. I talked about the definition of sand a little earlier, but here’s another one: it’s just very small rocks. And we have engineered machines that can transform big rocks into small ones. In fact, I have such a machine in my garage. It’s called a hammer. Some might argue that this isn’t the best use of my time, but I spent about an hour to artisanally manufacture a batch of sand just to hammer this point home. First, you crush the rocks. Then you put them through the sieves to remove the stuff that’s too big or too small. It takes a little extra processing, but this is not grain surgery. And this has a lot of benefits compared to sourcing natural sand. Hard rock quarries and crushing operations are already out there producing coarse aggregates like gravel, so sometimes the smaller stuff is a waste product anyway. It opens up possibilities when natural deposits aren’t available and can move mining operations upland, away from rivers, where the environmental impacts are less severe. And, it can make the concrete stronger. Let me show you what I mean.

I took some sand out of my kids’ sandbox and put it in a rock tumbler for a week to try and simulate the erosion it might see over years in windswept dunes of a desert. Obviously, this erosion reduced the overall size of the particles. So, I classified both materials with the sieves to make them a closer match for a fair comparison. And both batches are within the spec for concrete sand in the US. Looking in the microscope, you can clearly see the differences. The tumbled sand is rounded with roughly spherical, smooth grains. The manufactured sand is jagged with sharp, angular corners. And watch what happens when I pile them up. I filled up two pieces of pipe with the same amount of sand, and then pulled the pipe away. It’s not a huge difference, but you can see the rounded sand spreads out a little further because the particles have less friction. It makes intuitive sense that concrete made from this sand would be weaker than with this sand. Let’s see if that’s true.

I mixed up a simple batch of concrete from the crushed sand, and the tumbled sand keeping the weights of all the ingredients equal. Here’s my recipe if you want to try this experiment yourself. Then I molded some concrete cylinders and let them cure for a week. Most concrete mixes are meant to reach their design strength after 28 days, but concrete strength gain is fairly predictable, so the relative difference between the samples should be consistent in time. Most importantly, my little benchtop hydraulic press would not be able to break these samples if I waited that long. And this load cell is not calibrated, so I’m doing this test in arbitrary Practical Engineering units of force. The tumbled sand cylinder broke at around 2500 units. And the manufactured sand concrete broke at 7500 units. You can easily see the difference in the results. “Goodnight!” It was 3 times as strong. Of course, this is my garage, not a testing lab, and I only did one sample because my arm got tired of hammering rocks. Luckily, people much smarter than me have tested this out, and the results are pretty conclusive that, if you keep everything the same, the angularity of fine aggregate increases the strength of concrete. And that’s the story you probably know if you’ve read anything on this topic. It’s the common explanation for why we don’t use dune sand, the most visible of earth’s sand resources, in concrete. It’s intuitive. Rounded grains don’t lock together. Beiser makes the claim not once but three times in his book. But it turns out it’s not that simple, because strength isn’t the only property of concrete that we care about.

Before concrete has to be strong, it has to be placed. Ask anyone who’s done this kind of work, and they’ll tell you it’s hard. Well, it’s liquid at first, but it’s hard to work with. Concrete is about two-and-a-half times the density of water. It’s heavy stuff, and to get it into the forms often can require a lot of different tools: wheelbarrows, buggies, chutes, pumps, hoses, and more. The better the concrete flows, the easier it is to do a good job of placing it. And that matters. If a mix is too stiff, it can clog up hoses, trap air bubbles, and ultimately lead to poor quality in the installed product. This property of concrete is usually called workability. It’s often measured using a slump test. Fill up a cone of concrete, pull the cone away, and see how far the concrete slumps. But the problem with workability is that, in one big way, it works against strength. And it all has to do with water.

What happens when the concrete truck shows up to your job site and the mix is too stiff? Depends on if the engineer is there or not, but in a lot of cases, you just tell the driver to add a few gallons of water to the mix. More water; better flow; easier to place. It’s pretty straightforward, but there’s a reason you don’t want the engineer to know: water decreases concrete strength. I’ve done a whole video about this with some garage demos, so again, check that out if you want to learn more. The gist is that the ratio of water to cement is one of the most important factors determining concrete's strength when it cures. Cement isn’t like some types of glue that harden as the water or solvent evaporates. It goes through a chemical reaction, incorporating the water into the final product. That’s why we say concrete “cures” instead of “dries”. But, cement can only react with around 35 percent of its weight in water, so any more than that is just taking up volume in the mix that could be used by the stronger ingredients. More water; less strong. And here’s where the shape of the sand grains comes into play.

I did a little garage slump test to gauge the workability of those two mixes I made for the earlier demonstration. Here’s the rounded sand mix… and here’s the manufactured sand. “Haha, no slump at all.” Honestly I expected this to be a subtle difference, but it was like night and day. They weren’t even close. So I wondered, what would happen if, instead of holding ingredient ratios constant, I used the workability as the controlled variable? Let’s find out. First I used the manufactured sand with enough water to get it to a workable level. 100 milliliters got it to here, which is a bit better than the first one. Then I did a second mix with the tumbled sand, slowly adding water and running the slump test until it was pretty close. It took only 70 ml of water to make them match, 30% less than the first batch. After a week, I tested the samples. The tumbled sand sample broke at 4,800 units. And the manufactured sand broke at only 4,300 units. The tumbled sand with the rounder grains was stronger this time (by about ten percent), and it’s all due to the lower water content in the mix.

So yes, if you use the same amount of water, more angular sand like you might find from a river or manufactured sand is better, but that’s not what happens in real construction. I say this with many, many caveats, but very generally, you only add as much water as you need for workability. Rounded sand gives you better workability, so you can add less water, and thus get stronger concrete. This idea that we can’t use wind-blown sands in concrete because of their shape is a myth. In fact, the American Concrete Institute has a bulletin that says it better than I can:

“The influence of fine aggregate shape and texture on the strength of hardened concrete is almost entirely related to the resulting water-to-cement ratio of the concrete…”

I tried to track down the original source of this idea that we can’t use rounded grains in concrete, but got nowhere. Beiser cites an article from the UN, which itself cites a 2006 paper about using two types of desert sand from China in concrete. But that paper doesn’t mention the roundness of the particles at all. They didn’t include any measure of the shape of the grains in their study, and they didn’t make any suggestions about how that particular property of the desert sand may have affected the results of their tests. In fact, the conclusion of that paper includes saying that desert sand is a feasible alternative to other types of fine aggregates used in concrete. And the whole reason it was a subject of scientific study at all has to do with size, not shape. This is a widely used specification for the distribution of particle sizes of fine aggregate for concrete. Any sand in this area meets the spec. And here’s the soil used in that paper. Even if the conclusion was that it doesn’t work, I think this would have a lot more to do with the results than the shape of the particles.

And that really gets to the heart of this whole discussion. Fine aggregates are found throughout the world. We can even make our own. And concrete is like baking; different ingredients can change the end results. But just like regional bread recipes evolved based on the availability of local ingredients, the construction industry has developed a lot of ways to use different local materials to achieve good structural properties. The real challenge, like many things in engineering, is cost.

It can be more expensive to manufacture sand compared to mining raw materials that can be put directly in a mix, especially when you factor in the other ingredients, like chemical admixtures, that might be required to make it more workable without adding too much water. It’s expensive to transport better quality sand from far away, rather than finding it close to a job site or batch plant. It’s expensive to mine sand in adherence to environmental regulations that are becoming stricter worldwide. It’s catchy to say there’s a scarcity of fine aggregates on earth, but I think it’s misleading. “Sand is getting a lot more expensive than it used to be” just doesn’t make as nice of a headline. And the tricky part is that, in many ways, those costs have always been there; we’ve just externalized them onto the environment and our future.

All the ingredients in concrete are mined or harvested just like other natural resources. It’s just that concrete is made on a scale that blows most other materials out of the water. It’s a huge business, and there’s lots of money flowing, which means a lot of potential environmental harm and social conflict as a result. That’s especially true in places that don’t have robust oversight and enforcement of how sand is extracted. And I think it’s important to point out that the low-cost of sand, because of its simplicity as a material and its abundance, is a big part of why we use so much concrete in the first place, even in situations where it’s not necessarily the best material choice in other respects. Everything in engineering is a tradeoff, and if the economics around sand change, the engineering and construction industries can change with them. Look at other examples of this.

Diamond used to be exclusively a mined material, but now we can make it in a lab. Synthetic diamond gemstones used in jewelry are now less expensive than mined diamonds, but, admittedly, there’s a lot more to that economy than the costs to make them. What I think is more interesting is that 99 percent of diamond used in the world for industrial purposes is synthetic. It used to be a rare mineral, but now you can pick up a diamond drill bit or saw blade from the hardware store for a fairly small premium.

Timber is another example. Natural forests used to be the only source, but now plantations, trees planted specifically for harvest, now make up more than a third of the wood we use globally. And engineered lumber like plywood, OSB, and structural composites can make more efficient use of raw materials. I’m pointing out these examples, not to say they’re good or bad - there are pros and cons in both cases - but just to illustrate how our demand for materials in the construction industry changes with the supply, and how technology can have a huge impact on that. And there’s another parallel between timber and sand: they both can be renewable.

I had another barrel in the rock tumbler not going to use, so I broke up some chunks of concrete and threw them in. I ran these through the grits, just like you would with any other rock in a tumbler. Concrete is pretty soft compared to most natural rocks, so it didn’t polish up that nicely, but the result is still pretty cool. You can really see the constituent materials of the concrete after it spent so long rolling around in there: the small and large aggregates and the cement paste locking them together. But the point of this demo is that concrete is pretty much just rock, that’s mostly what it’s made of in the first place. And just like the rocks I crushed to create manufactured sand, concrete can be recycled into aggregates that get reused in the construction industry, either in new concrete or other materials, reducing demand for virgin sources.

There’s a lot changing in the construction industry, and a lot of growth in the need for materials like sand and gravel. But I don’t think it’s fair to say the world is running out of those materials. We’re just more aware of all the costs involved in procuring them, and hopefully taking more account for how they affect our future and the environment.