What's the Difference Between Cement and Concrete?

While commuting to work in a place such as, say, New York City, one might find oneself taking in the landscape’s natural beauty: the scent of hot garbage wafting through the air, the wail of various sirens, the odd substances crusting and pooling on the sidewalk. One might ask oneself, Why didn’t I pursue a career as a ski instructor? Also, is this sidewalk made of cement or concrete? And that’s when a weekly What’s the Difference email, and its focused desire to turn the splitting of hairs into a little bit of delight, makes the commute a little more bearable.

Because it turns out, cement and concrete are two very different things. Cement is a powdery substance made up of limestone, sand, clay, bauxite, and/or iron ore, and sometimes includes materials like shells, chalk, marl, shale, blast furnace slag, fly ash, and/or slate. (Some of these “cementitious materials” are actually similar to the volcanic ashes the ancient Romans used to build the Colosseum!) The process of making cement is actually pretty cool: raw materials like limestone and clay are quarried, then crushed into pieces around 3 inches or smaller. The crushed rock is then combined with iron ore or fly ash; ground into even smaller pieces; and then fed into a cement kiln, which has a diameter of 12 feet and, in many cases, is longer than the height of a 40-story building. The high temperatures in the kiln unite the particles into a new substance called clinker, which comes out red-hot in balls the size of marbles. The clinker is cooled in special coolers, then ground into a powder. The resulting cement is so fine that it can pass through a sieve capable of holding water; one pound of cement contains 150 billion grains.

So if cement is a powder, what’s concrete? Concrete is a mixture of sand, gravel, and/or crushed stone (known as “aggregates”) and a paste made of water and—wait for it—CEMENT. This water-cement paste (also used in mortar for brick-laying) coats the sand, gravel, and stone and binds them all together. Through a chemical process called hydration, the paste hardens and gains strength over time; over 90% of a concrete mix’s strength will be reached within four weeks, with the remaining 10% accumulating over decades. Typically, a mix of concrete will contain 10–15% cement, 60–70% aggregates, and 15–20% water, with air bubbles sometimes making up another 5–8%. That concrete is used to build everything from skyscrapers to streets, houses to highways—and probably the very sidewalk you walked down today.

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