Concrete from Cement

Despite its deceptively complex nature, the process of making concrete is actually very simple. That is, it is very simple if you pick things up from the point where a regular contractor or DIY enthusiast comes in. For them, concrete is made by mixing cement, water, sand, and potentially other aggregates together to create a mixture of the desired consistency. 

It would be easy to stop there and say that there is not much involved in the making of concrete, but you only have to take a look one step further down the line to see that that is not the case.

Concrete may only require those few meager ingredients, but getting those ingredients in the first place is no small task.

Granted, “how is concrete made” isn’t the kind of question most people ask themselves, but you’re here, so there’s a good chance you’re interested in all the gory details. Keep reading, because that’s precisely what we’ve got for you.

How Concrete is Made

Let’s start at the top and work our way back. As we mentioned at the top, concrete is made by mixing cement, water, sand, and other aggregates together. The crucial part of this process is the interaction between the water and cement. Cement—that is, Portland Cement, which the most commonly used today—is a hydraulic compound that reacts with water. There are a number of reasons we include sand and other aggregates in our concrete, which we’ll get into later, but it’s worth mentioning that you could make concrete with just cement and water.

This reaction between water and cement also makes the mixing process a time-sensitive one. Once you have started mixing your concrete, you have a limited amount of time to pour it before the concrete starts to cure and becomes unviable. Interestingly, though concrete starts to cure relatively quickly, it can take a very long time to reach full strength.

The exact proportions of the structure play a large role in determining how long it takes concrete to cure fully, but it will often take decades for a concrete structure to reach its full strength without any help from the engineers designing the structure. Case in point; the Hoover Dam was laced with pipes that were filled with water to cool the concrete as it cured. Had this not been done, it would have taken over a century for the concrete to cure fully.

It is also worth noting that the full strength of concrete is not always a requirement. For example, if you pour a concrete driveway, it may take a year or two before the concrete is technically cured to its full strength, but that does not mean it is not strong enough to hold the weight of your car after the first few weeks. On the other hand, it may feel strong after the first day or two, but that doesn’t mean it can take the weight of your car that soon.

What are Aggregates For?

Knowing that you could simply mix water and cement together to get a concrete mix of sorts, it is understandable to wonder what aggregates like sand are actually for. The truth is that the role of cement in a typical concrete mix is more of a binding agent than anything else. The bulk of the strength comes from the aggregates, such as sand and gravel.

The cement binds these aggregates together, forming a cohesive structure that incredibly strong. Take the example of crushed stone, for instance. If concrete is made using a crushed stone aggregate, compressing that concrete to breaking point would mean compressing it with enough force to crush the stones that make up the concrete, a task made all the more challenging by the reinforcement that the cement provides.

Were concrete to be poured without aggregates, it would not only be weaker, but it would also be prohibitively expensive. Cement is by far the most expensive ingredient in concrete and makes up a relatively small part of the construction material by volume. The exact ratios of the ingredients will vary depending on the use, but a concrete mix that didn’t include sand and aggregate would require around seven times as much cement, which would represent an extremely large price hike over concrete that used aggregates. This would make large construction projects entirely unworkable.

What Are Some Examples of Aggregates and What are the Differences?

We have already mentioned sand, of course, but aggregates can come in a variety of sizes. For the most part, the size of the individual pieces of aggregate is the main thing that distinguishes each type, as most aggregates are some variant of stone. It may be gravel, crushed stone—even sand is largely made up of tiny grains of stone.

The difference in size affects the final concrete mix in both an economical and an aesthetic sense. The economic factor comes in because of the cost of cement that we mentioned above. Having a larger size of aggregate means that you will be able to make more concrete mix with less cement since more of the volume of the poured concrete will be taken up by the aggregate.

On the aesthetic side of things, a larger aggregate means you are limited by the finish you can achieve. With a small aggregate like sand, you can get a very smooth finish, even glass-like, with some polishing after the concrete has cured. With a larger aggregate, the surface will be much rougher. For this reason, the aggregate used for any given concrete mix is often made as large as it can be without affecting the desired look and feel of the concrete since it is more cost-effective to use a larger aggregate.

That being said, sand is typically included in any concrete mix, regardless of what other aggregates are used, or even if there are no other aggregate being used. Sand serves the same purpose as the larger aggregates like gravel and crushed stone, but on a much smaller scale.

Aggregates also need to be non-reactive, as any chemical reactions that take place during the curing process could interfere with the concrete, resulting in a weaker product.

Aggregates: Shape Matters

Looking from the outside in, it would be understandable to think that aggregates can be any kind of crushed stone or sand-like substance, but this is not the case. The internal structure of concrete needs to be a certain way in order to have the kind of strength that is needed. The individual pieces of aggregate need to be angular so that they interlock with each other. The idea here is that the stresses that are placed on the concrete are as evenly distributed among the aggregate as possible, and that would not be the case with, for example, a rounded aggregate.

This factor is prominent in one area of concern for future concrete production—sand mining.

Sand is the second most exploited natural resource after water—an interesting parallel with concrete, which is the second-most in-demand substance after water—and we are rapidly running out of it. Some estimates have certain regions running out of sand within a few years, while more global estimates throw up the year 2050 as the likely period we will run out of sand at our current rate of use.

The problem is not so much a lack of sand in general, but a lack of the right kind of sand. The best kind of natural sand for concrete is riverbed sand, but there are only so many rivers in the world and only so much easily accessible sand in those rivers, and we are using a lot of it, and have been for a long time. There are deserts, of course, but you may recall we mentioned above that rounded aggregates would not make for a very good concrete mix, and that is precisely the problem with desert sand. Thousands of years of being blown around the desert by wind and friction with other grains of sand have turned desert sand smooth, making it not suitable as a concrete aggregate.

There is good news on this front, however. We have the technology to make manufactured sand. While manufactured sand is not as cheap as scooping natural sand off of the ground, the fact that it is manufactured means the general shape of the aggregate can be managed, making for an optimal aggregate product. It is also the case that the makeup of that aggregate can be more refined. Natural sand is great for concrete, but it contains many things we don’t necessarily want, such as organic components that can interfere with the chemical processes in curing concrete. And the minerals that make the grains of natural sand can be from several different kinds of stone. With manufactured sand, the aggregate is “pure,” and knowing exactly what materials were used to make the sand makes it possible to better judge the strength of the final product.

The Magic Ingredient: Cement

The process of mixing concrete is relatively simple, but making concrete involves more than throwing some ingredients in a mixer. Sure, the process of getting gravel or crushed stone isn’t necessarily a walk in the park—and we’ve touched on the struggles that are looming with regards to getting sand—but all of those things are relatively simple compared to the key ingredient in concrete; cement.

Cement is the binder, the thing that holds it all together. It is in cement where the magic happens, so to speak, so it should come as no surprise that there is a little more work involved in making it than there is in acquiring the other ingredients.

What are the Ingredients in Portland Cement?

Cement can have a wide variety of ingredients, including shale, iron ore, silica sand, blast furnace slag, and even seashells. But there are two main ingredients that are essential to Portland Cement; lime and clay. The union of these two elements is the driving force in the way concrete reacts to water, which is essential to the way concrete cures.

It’s worth noting that both lime and clay have been used as a construction material in their own rights for thousands of years. Clay, of course, is still used today for smaller-scale items like pots, and lime can still be found enjoying widespread use in things like Venetian Plaster and Tadelakt.

How is Cement Made?

So you know what goes into cement, but how is it made? Unlike concrete, there is more to it than merely throwing the ingredients into a container and giving them a good stir. Far from it, in fact.

The ingredients for cement are fed into a rotating kiln where they are heated to an impressive 2,640 degrees Fahrenheit. This heating causes the ingredients to form into marble-sized nuggets that are called clinkers. From there, the clinkers are ground down into a very fine powder, and it is that powder that makes up cement. After that, there is little more to do other than bag it all up and ship it out to construction sites across the world.

The process of making cement is, as you might have guessed from that high temperature we mentioned, where the increased expense of cement comes in. While aggregate and sand can be pulled directly from the ground, cement has to undergo this high-energy process that involves multiple ingredients, each of which needs to be obtained in a similar manner to the aggregates. This is why aggregate is as essential from an economic standpoint as it is from a structural one.

Concrete’s Carbon Footprint

Concrete doesn’t have the best of reputations when it comes to the health of the climate. The energetic process required to make cement means that the carbon footprint for cement is quite high. Couple that with the fact that cement is as widely used as it is—it is thought that around four billion tonnes of the stuff is used every year—and it is easy to see why it might not look good from an environmental standpoint.

In fact, if you were to take the estimated carbon output from concrete manufacturing across the globe and compare it to the total carbon output of entire countries, concrete would be the third-highest producer of carbon behind the United States and China. When you consider that carbon is the main driving force behind global warming, that’s not a good look for concrete.

Further exacerbating this problem is the fact that the enormous demand for concrete is often down to new developments required to accommodate our continually growing population. That often means areas of nature are being built over since there is only so much space in already developed environments to build on. Presently, there is no greater mechanism for removing carbon dioxide from our atmosphere than nature itself. Plants breathe in carbon dioxide and breathe out oxygen, and concreting over areas of plantlife removes some of our planet’s capacity to deal with the extra carbon dioxide we are pumping into it.

There are ways to mitigate the impact of concrete, such as green roofs, which involve covering the roof of a structure with plantlife so that we do not entirely lose that footprint of nature, but there is little denying that concrete does not have a great record when it comes to keeping carbon out of our atmosphere.

The Future of Concrete

It is hard to say what the future of concrete may look like due to the many opposing factors. On the one hand, concrete is providing to be very bad for the environment, and the growing demand for it only makes that impact worse. On the other hand, there is growing demand, mostly because of our growing population, and we do not presently have a more practical alternative to concrete that is as strong and cost-effective.

Similarly, we look to be on course to run out of natural sand of the sort that can be used in concrete, but at the same time, we have the technology to manufacture sand, though this process is more expensive and will add an additional toll on the environment.

There are also attempts to make concrete more environmentally friendly, such as is the case with “grasscrete,” which is essentially regular concrete with a regular pattern of small holes that are seeded with grass. Of course, halfway measures like these often end up leaving nobody satisfied with the end result, but the point is people are trying to find a solution.

Given that we are expected to reach a global population of nine billion over the next thirty or forty years, there certainly doesn’t look to be a slowing of demand for concrete on the horizon. And with a higher percentage of that growing population expected to be city-dwellers, concrete will be playing an even larger role in our lives if we do not come up with an alternative.

Final Thoughts

The future of concrete may be somewhat uncertain, but one thing that is certain is that this construction material has played an enormous role in the development of our various civilizations, and its remarkable properties make it very hard to replace.