Concrete is the most widely used building material on earth. Humans have been using it for thousands of years and have built impressive structures with it that have stood the test of time. If you’re a homeowner, you’ll likely use concrete for some simple DIY jobs: setting a fence post or a basketball hoop, or laying a concrete slab for a patio.
Despite the ubiquity of concrete, there’s a good chance you don’t know that much about it. Let’s remedy that today by going over a few basics about this building material.
What Is Concrete Made Of?
Concrete is a composite material made of three main ingredients: cement, aggregate, and water.
Cement. People often use the words “concrete” and “cement” interchangeably, but they’re not the same thing. Cement is a component of concrete. It binds together the aggregate (more on that in a bit) and, when combined with water, gives concrete its solid and durable properties.
Most modern cement is Portland cement. To make Portland cement, cement manufacturers combine limestone with a silica source (like slag, fly ash, or clay) in a giant kiln. They then heat up the mixture to temperatures as high as 3,000 degrees Fahrenheit. This drives all the water out of the mix as well as the carbon dioxide from the limestone. The heat chemically mixes all the ingredients together to form a new compound called clinker. The clinkeris then ground to a very fine powder and mixed with gypsum.
Boom. Portland cement.
Aggregate. Cement is rarely used on its own. You typically mix it with some sort of aggregate. Aggregate can be sand and small rocks. Cement mixed with sand gives you mortar to lay a brick wall. Cement mixed with sand and epoxies gives you grout to fill the gap between tiles. Cement mixed with sand and small ground-up stones (up to an inch in diameter) gives you concrete.
The ready-mix bags of concrete that you buy at Home Depot already have cement and aggregate combined. You just need to add . . .
Water. Water is the third and final ingredient of concrete. When you add water to your cement and aggregate mix, a chemical reaction occurs between the components called “hydration” or “curing.”
The hydration process results in calcium silica hydrate (CSH). CSH hardens and interlocks to give concrete its solid and durable nature. In short, water is what makes concrete hard.
Understanding that water is what kickstarts a chemical reaction in cement to give concrete its hardness should clear up a popular (yet erroneous) belief that concrete hardens by drying out. In fact, if your newly laid concrete slab dries out too fast, you’ll end up with a weakened concrete slab. This is why, as we’ll see below, properly laying concrete requires that you keep it moist for the first few days, as this is when most of the curing takes place.
Concrete Mixing Tips
Mixing concrete isn’t difficult. For most DIY projects around the home, you can simply use bags of the just-add-water concrete mix you can buy at Home Depot or Lowes.
It’s easiest to mix concrete in a wheelbarrow. The wheelbarrow’s ample tub makes mixing large amounts of concrete and moving your concrete mix to where you need it much easier. But you can also use a bucket or some other tub if that’s all you have.
First dump the dry concrete mix into your wheelbarrow or tub and push it all the way to one side.
Now comes the tricky part: Knowing how much water to put in: Too little and you won’t get the hydration process going; too much, and you’ll get a soupy mix that will result in weakened concrete.
Your ready-mix bag should tell you how much water you need to add, but you also need to apply your own eyeballs and judgement to this general recommendation. Fill a bucket with the recommended amount of water and slowly pour some of it into your dry concrete mix in the wheelbarrow. Don’t add all of the water right away. After you’ve poured some water in, mix it with the concrete mix using a hoe. Pour some more water in. Mix with the hoe. Pour some more water. Mix. Repeat the process until you’ve got a mix that’s neither too dry nor too wet.
Concrete that’s too dry will look like clumpy balls. Concrete that’s too wet will look really soupy. Just-right concrete is pliable but not runny. It looks like a
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