Many people talk about cement when they mean concrete. Cement is a fine-grained compound that turns into a solid when mixed with water. Cement is used to bind mixtures of materials into a composite solid. Concrete is a mixture of cement, sand and gravel. That is, cement is the glue of concrete.
Now that that’s clear, let’s talk about cement. Cement begins with lime.
Lime, the First Cement
Lime is a substance used since ancient times to make useful things like plaster and mortar. Lime is made by burning, or calcining, limestone—and that’s how limestone gets its name. Also a greenhouse gas is produced in great quantities by the cement industry.
Lime is also called quicklime or calx (from Latin, where we also get the word calcium). In old murder mysteries, quicklime is sprinkled on victims to dissolve their bodies because it is very caustic.
Concrete made with lime cement is known from archaeological sites in both the New and Old World, some more than 5000 years old. It works extremely well in dry conditions. It has two drawbacks:
- Lime cement takes a long time to cure, and while the ancient world had lots of time, today time is money.
- Lime cement does not harden in water but stays soft, that is, it is not a hydraulic cement. So there are situations where it cannot be used.
Ancient Hydraulic Cement
The Pyramids of Egypt are said to contain a hydraulic cement based on dissolved silica. If that 4500-year-old formula can be confirmed and revived, it would be a great thing. But today’s cement has a different pedigree that is still quite ancient.
Around 1000 BCE, the ancient Greeks were the first to have a lucky accident, mixing lime with fine volcanic ash and getting C-S-H.
C-S-H is still a mysterious substance today, but we know it is an amorphous gel without any set crystalline structure. It hardens fast, even in water. And it is more durable than lime cement.
The ancient Greeks put this new cement to use in new and valuable ways, building concrete cisterns that survive to this day. But Roman engineers mastered the technology and constructed seaports, aqueducts and temples of concrete as well. Some of these structures are as good as ever today, two thousand years later. But the formula for Roman cement was lost with the fall of the Roman empire.
Modern Hydraulic Cement
While lime cement continued in use throughout the Dark and Middle Ages, true hydraulic cement was not rediscovered until the late 1700s. English and French experimenters learned that a calcined mixture of limestone and claystone could be made into hydraulic cement. One English version was dubbed “Portland cement” for its resemblance to the white limestone of the Isle of Portland, and the name soon extended to all cement made by this process.
Shortly thereafter, American makers found clay-bearing limestones that yielded excellent hydraulic cement with little or no processing. This cheap natural cement made up the bulk of American concrete for most of the 1800s, and most of it came from the town of Rosendale in southern New York. Rosendale was practically a generic name for natural cement, although other manufacturers were in Pennsylvania, Indiana and Kentucky. Rosendale cement is in the Brooklyn Bridge, the U.S. Capitol building, most 19th-century military buildings, the base of the Statue of Liberty and many other places. With the rising need to maintain historic structures using historically appropriate materials, Rosendale natural cement is being revived.
True portland cement slowly gained popularity in America as standards advanced and the pace of building quickened. Portland cement is more expensive, but it can be made anywhere. It also cures faster, an advantage when building skyscrapers a floor at a time. Today’s default cement is some version of portland cement.
Filed Under: Technology & Science