What Is the Mix for Concrete? The 4 Ingredients Explained

What Is the Mix for Concrete?

If you have never ordered concrete before, the words on a quote can look like code. C20, GEN 3, slump, ratio. It sounds complicated, but the material itself is not.

Concrete is only four things: cement, sand, stone and water.

That is it. Everything else, every grade and every number, is just a different balance of those same four ingredients. Once you know what each one does, the rest of the language starts to make sense.

Here is the short version before we go through them one by one.

Those figures shift depending on the job. A strong structural mix uses more cement. A cheap filling mix uses less. But the four ingredients never change.

1. Cement: The Glue

Cement is the grey powder. On its own, it does almost nothing useful. Add water, and it starts a chemical reaction called hydration, which slowly turns it into a hard, stone-like paste.

Think of cement as the glue in a bag of mixed nuts. The nuts do not stick together on their own. The glue coats every piece and locks them in place.

This matters more than people expect. Cement is the most expensive ingredient in the bag, and it is the one that decides how strong the finished concrete will be. More cement, all else being equal, means a stronger slab. That is the main reason a structural mix costs more than a garden path mix.

Cement is also the reason concrete gets warm as it sets. Hydration gives off heat, so a fresh pour on a cold morning can steam slightly. That is normal, and it is a good sign.

2. Sand: The Gap Filler

Sand is called fine aggregate. Its job is simple: fill the small spaces between the bigger stones.

Picture a bucket filled with tennis balls. Even when the bucket looks full, there are gaps everywhere. Pour in dry sand, and it runs down into all those spaces. Concrete works the same way. Without sand, you would end up with a slab full of holes, which would be weak and would let water straight through.

Sand also makes concrete easier to work with. A mix with the right amount of sand moves smoothly off a chute and levels off nicely with a float. Too little sand and the mix feels harsh and stony. Too much and it goes sticky and loses strength, because sand needs cement to coat it and there is only so much cement to go round.

The sand has to be clean. Building sand from a reputable supplier is washed and graded. Sand dug out of the garden usually carries clay, soil and organic bits that stop the cement bonding properly.

3. Coarse Aggregate: The Backbone

Coarse aggregate is the stone. Usually crushed rock or gravel between 10mm and 20mm across, depending on the job.

This is the part that actually takes the weight. Cement paste is strong, but stone is stronger and far cheaper. So concrete is designed to be mostly stone, held together by just enough cement paste to lock it in place. When a lorry drives over a slab, the load travels through the network of stones, not through the paste.

Aggregate size is chosen to suit the pour. Smaller stone, around 10mm, is used where concrete has to flow into narrow gaps or around dense steel reinforcement. Larger stone, around 20mm, is the standard choice for slabs and foundations because it is cheaper and needs less cement.

If you look at a broken piece of good concrete, the stones themselves will usually be split clean through rather than pulled out whole. That tells you the bond did its job.

4. Water: The Ingredient Most People Get Wrong

Water does two separate jobs, and this is where first-time buyers go wrong.

Job one is chemical. Water reacts with cement to make it harden. Without water, cement stays a powder forever. Concrete does not dry out to get hard, it reacts. That is why a slab can cure underwater.

Job two is practical. Water makes the mix loose enough to pour, spread and level. Builders call this workability, and it is measured with a slump test, where a cone of fresh concrete is turned out, and the amount it sags is recorded in millimetres. A low slump is stiff. A high slump is runny.

Here is the important part.

The chemical reaction only needs a small amount of water. Everything you add beyond that makes the concrete permanently weaker.

The ratio of water to cement is the single biggest factor in how strong your concrete ends up. Add extra water on site, and the mix flows beautifully, spreads with almost no effort and saves you an hour of hard work. Then that surplus water has nowhere to go. It sits in the mix, the cement reacts around it, and when it eventually evaporates, it leaves behind millions of tiny voids running through the slab.

Those voids never fill back in. The slab is now full of microscopic holes it was never designed to have. The result is concrete that is weaker, more likely to crack, and far quicker to let frost and water in. As a rough guide, a bucket of extra water in a small load can knock a meaningful slice off the final strength, and no amount of waiting will bring it back.

The Concrete Centre, the UK body that publishes guidance on concrete design and specification, sets out the same point clearly in its guidance on water in concrete.

So if a mix feels too stiff on site, the answer is not the hosepipe. Talk to the driver. Workability is something that should be set in the mix design before the pour, not fixed with a bucket afterwards.

What About Admixtures?

You may see a fifth item on a spec sheet. Admixtures are small doses of liquid added to the mix to change how it behaves.

Common ones include:

• Plasticisers, which make concrete flow more easily without adding water. This is the proper way to get a wetter-feeling mix while keeping the strength.
• Retarders, which slow the set down. Useful in hot weather or on a long pour.
• Accelerators, which speed the set up. Useful in cold weather or when you need to get back on the slab quickly.
• Air entrainers, which create tiny controlled bubbles that help concrete survive repeated freezing and thawing. Often used on external slabs and driveways.

Admixtures are not a fifth ingredient in the way the other four are. They are adjustments. The core recipe is still cement, sand, stone and water.

Curing: The Bit After the Pour

Getting the ingredients right is only half the job. Concrete carries on gaining strength for weeks after it is laid, and it can only do that while there is still moisture inside it for the cement to react with.

Concrete reaches most of its rated strength at 28 days, which is why grades are quoted at 28 days. It keeps gaining slowly after that, but 28 days is the standard benchmark.

If a fresh slab dries out too fast, usually in wind or strong sun, the reaction stops early, and the concrete never reaches the strength you paid for. Covering the slab with plastic sheeting, or keeping the surface damp for the first few days, protects the work. It costs almost nothing, and it makes a real difference.

How the Balance Changes for Different Jobs

The four ingredients stay the same. What changes is the proportions, and that is what a grade actually describes. A grade such as C20 means the concrete reaches 20 newtons per square millimetre of compressive strength at 28 days. Higher number, more cement, stronger slab.

Here is how that plays out on two very different jobs.

A shed base or garden slab

This is light-duty work. It holds a timber building, a few tools and maybe a bike. Nothing heavy ever rolls across it.

A general-purpose mix is right here. Something like C20 concrete has a moderate amount of cement, plenty of 20mm aggregate to keep the cost down, and a slump chosen so it spreads easily by hand across a small area. Spending more on a stronger mix would not make the shed any safer. It would just cost more.

A structural slab or reinforced foundation

Now the concrete is carrying real load, possibly the weight of a building, and it may be working alongside steel reinforcement.

The balance shifts. More cement, so the paste is stronger. Often a smaller aggregate, so the mix flows properly around the steel rather than getting caught on it. A tighter water-to-cement ratio, so the finished concrete is dense and durable. This is where mixes such as GEN 3 concrete come in for domestic foundations and floor slabs, and where the grade is usually specified by an engineer or on the building control drawings rather than chosen by eye.

The difference between those two pours is not a different material. It is the same four ingredients, weighed out differently.

Quick Comparison

Where to Go Next

Now that you know what the four ingredients do, there are two natural next steps.

If you want the actual numbers, how much cement to how much sand to how much stone, read our guide to the ideal concrete mix ratio for your project. It takes this article one level deeper and shows you the proportions behind each common grade.

If you already know roughly what you are building and just need to pick a grade, our guide to C25, C30 and C40 concrete strength walks through which number suits which job, and what happens if you go too low or too high.

Get the Right Mix Delivered

Knowing the four ingredients is useful. Getting them measured accurately on your site is what actually gives you a strong slab.

Because we mix on site in volumetric mixers, your cement, sand, aggregate and water are batched at the point of pour, to the grade you need, in the volume you need. Nothing is mixed at a depot an hour before it arrives, and you are not paying for concrete you do not use.

If you are not sure which mix suits your job, tell us what you are building, and we will point you to the right one. Find out more about our concrete delivery in Cambridgeshire, or get in touch for a quote.