If you think of bricks as artificial rocks, cement might be considered artificial lava—a liquid stone that is poured into place where it hardens into solidity.
Cement and Concrete
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. Chemically, lime is calcium oxide (CaO) and is made by roasting calcite (CaCO3) to drive off carbon dioxide (CO2). That CO2, 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.
Mixed with water, lime slowly turns into the mineral portlandite in the reaction CaO + H2O = Ca(OH)2. Lime is generally slaked, that is, mixed with an excess of water so it stays fluid. Slaked lime continues to harden over a period of weeks. Mixed with sand and other ingredients, slaked lime cement can be packed between stones or bricks in a wall (as mortar) or spread over the surface of a wall (as render or plaster). There, over the next several weeks or longer, it reacts with CO2 in the air to form calcite again—artificial limestone!
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. Ash can be thought of as naturally calcined rock, leaving silicon in a chemically active state like the calcium in calcined limestone. When this lime-ash mixture is slaked, a whole new substance is formed: calcium silicate hydrate or what cement chemists call C-S-H (approximately SiCa2O4· xH2O). In 2009, researchers using numerical modeling came up with the exact formula: (CaO)1.65(SiO2)(H2O)1.75.
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 research continues to uncover useful secrets from the ancients, such as the unusual composition of Roman concrete in a breakwater built in 37 BCE, which promises to help us save energy, use less lime and produce less CO2.
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 the ingredients can be assembled instead of relying on a lucky rock formation. It also cures faster, an advantage when building skyscrapers a floor at a time. Today's default cement is some version of portland cement.
Modern Portland Cement
Today limestone and clay-containing rocks are sintered—roasted together at nearly melting temperature—at 1400° to 1500°C. The product is a lumpy mixture of stable compounds called clinker. Clinker contains iron (Fe) and aluminum (Al) as well as silicon and calcium, in four main compounds:
- Alite (Ca3SiO5)
- Belite (Ca2SiO4), known to geologists as larnite
- Aluminate (Ca3Al2O6)
- Ferrite (Ca2AlFeO5)
Clinker is ground to powder and mixed with a small amount of gypsum, which slows down the hardening process. And that is Portland cement.
Cement is mixed with water, sand and gravel to make concrete. Pure cement is useless because it shrinks and cracks; it's also much more expensive than sand and gravel. As the mixture cures, four main substances are produced:
- Ettringite (Ca6Al2(SO4)3(OH)12· 26H2O; includes some Fe)
- Monosulfate ([Ca2(Al,Fe)(OH)6] · (SO4,OH,etc) · xH2O)
The details of all this are an intricate specialty, making concrete as sophisticated a technology as anything in your computer. Yet basic concrete mix is practically stupidproof, simple enough for you and me to use.