Concrete VS Cement

Concrete and cement often get confused by people who do not work with them on a regular basis, and with good reason. Despite their differences, cement and concrete are often used interchangeably, even by people who know the difference. 

As it turns out, cement is actually an ingredient of concrete, rather than a different substance, but cement can be used by itself also. 

We’ll get into the use cases for each, and why it is necessary to have both rather than just use one, but first let’s take a closer look at what each of these construction materials is made of.

What is Cement?

Cement is a substance that sets and hardens and is commonly used in construction. There are hydraulic and non-hydraulic forms of cement, though hydraulic is the most commonly used of the two. Hydraulic cement hardens through a process called hydration, which, contrary to popular belief, requires water rather than “dries out.” This gives it the useful property of being able to set underwater.

The most commonly used type of cement today is called Portland Cement, though cement in various forms has been used for thousands of years.

What is Concrete?

Concrete is a mixture of cement and an aggregate, such as gravel, sand, and other similar substances. The exact ingredients may vary, but cement typically makes up around ten to fifteen percent of the materials in concrete, which is preferred for larger construction projects. Concrete may also be reinforced with things like steel bars in order to add additional strength.

Though concrete is made to be strong enough for the purpose, adding aggregate to the cement does weaken the final result. Fortunately, the strength of the product can be made to suit by adding more cement to the mix as needed.

Why is there Concrete and Cement?

The main reason we have both concrete and cement rather than just the latter is cost. Cement is relatively expensive to make, which makes it economically unviable to use in the kinds of quantities that are needed for large constructions, such as buildings.

Fortunately, cement is a binding agent and can create a perfectly viable construction material when mixed with an aggregate. The exact aggregate user for any given job will depend on the type of construction that is being carried out, but as a general rule, a fine aggregate like sand is used for situations when a smooth finish is required, whereas very large constructions will use a larger aggregate.

The aggregate fills out the mixture, making it much cheaper to manufacture a larger amount of concrete. The most important trait of the aggregate is that it does not react with the chemical processes that happen in the cement, since that could affect the strength of the final product. After that, it just needs to be strong, which is why stone in various forms is often used.

How is Portland Cement Made?

We mentioned above that Portland Cement is the most commonly used kind of cement these days, so it makes sense to wonder how this material is made.

Quite a few ingredients go into Portland Cement, including limestone, shale, clay, and iron ore. All of this is crushed up placed in a kiln where it is heated to very high temperatures. This process produces ball-like objects called “clinkers.” These clinkers are then ground into a fine powder to produce the cement you will find in your local DIY store.

Interestingly, there is a variant of Portland Cement that is white (imaginatively called White Portland Cement) that uses many different ingredients, though the properties of the finished product are the same.

What are the Different Aggregates?

Aggregates can generally be broken into two groups, fine and coarse. Fine aggregates are often made from crushed stone, and are around 9.55mm and under. Coarse aggregates, on the other hand, can be as much as 37.5mm across. Here are the most common materials used to form these aggregates;

Crushed Stone and Manufactured Sand

Stone that is quarried can be ground down to produce an aggregate of any desired size, from coarse stone to fine sand. This is one of the most commonly used aggregates for large-scale construction because the size of the aggregate can be determined artificially.

Gravel

One way to think of gravel is as naturally ground stone. It is formed from unconnected rocks fragments, often worn down by water erosion, to some degree. Technically, this description covers anything from a granule-sized piece of stone to an entire boulder. However, in practice, gravel is often at the smaller end of that scale.

Sand

Essentially tiny pieces of stone, sand is naturally formed over years of erosion. It is much finer than gravel and commonly used in situations where the finish of the concrete needs to be smooth. Unlike manufactured sand, this sand also contains a number of other things, such as minerals and fragments of seashells.

Recycled Concrete

Concrete is an increasingly controversial material in times of growing concern for our climate. This is due to the fact that the process of making it is not particularly good for the environment, in no small part due to the amount of carbon dioxide it releases. When you couple that with the fact that concrete is the second most-consumed substance in the world behind water and you can see why this could be a problem.

To that end, using recycled concrete is one way in which the impact of the manufacturing process on the environment can be lessened, even if only by a small amount.

Recycled concrete comes from crushing existing concrete to the desired size, usually after demolishing a structure that originally used concrete in its construction.

Final Thoughts

Cement is undoubtedly a remarkable construction material. It is strong and versatile, and its hydraulic nature means that it actually gets stronger over time. In fact, depending on the size of the structure, concrete can may still be gaining strength for years after it is laid. The need for aggregates, turning cement into concrete, is an unavoidable reality of economics. It is simply not feasible to make the grand structures we have built as a species using pure cement.