When you pour a mixture to make a new sidewalk or driveway for your house, are you using cement or concrete? What’s the difference between the two?
Although the terms cement and concrete often are used interchangeably, they’re not the same thing. Concrete actually includes cement and other things, like aggregates and paste. The aggregates include small materials such as sand, gravel or crushed stone, while the paste—the thing that holds the aggregates together—is water and Portland cement.
Portland cement is not a brand name. Instead, it’s a generic term for the most prevalent type of cement—just like stainless is a type of steel, not a trademarked name. A Briton, Joseph Aspdin, developed the building material in the eighteenth century. He discovered that adding clay to limestone and superheating the mixture allowed the resulting mixture to set anywhere.
Cement makes up from 10% to 15% of the total mass of concrete; though the exact proportions are different from mixture to mixture, depending on which type of concrete is being made. Portland cement is a type of hydraulic cement, which means that when water is added through a process called hydration, it starts the chemical reaction that causes the cement to harden and set, holding the aggregates together in a rocklike mixture—concrete. Before the concrete is allowed to harden, however, the concrete mix must be poured into a mold so that it will harden into the desired shape.
The amount of time it takes for concrete to harden depends on factors like the amount of gypsum added to the mixture. This time can also be sped up by adding accelerating admixtures or slowed down by adding a set retarding admixture . When concrete sets, tiny crystals in its structure harden. Accelerating and set retarding admixtures affect the development of these crystals.
In climates where concrete is exposed to freezing and thawing conditions, an air entraining admixture should be added. This admixture entrains millions of microscopic bubbles that assist in protecting the concrete from the damaging effects of freezing and thawing. Concrete gets stronger as it gets older because the hardening process continues over time. With the right mix, concrete even hardens underwater and remains strong in wet conditions.
Therefore, you should be sure to get your terminology correct when referring to the distinction between concrete and cement. You don’t walk on a cement sidewalk; you walk on a concrete sidewalk. Similarly, you won’t find a cement mixer—instead, you’ll find a concrete mixer.