Concrete Evidence

Anyone can use a mixture of sand and water to form a shape that will stick together temporarily, but without cement, it weakens as it dries. Add cement to the mix and you have concrete, which actually strengthens as it cures — in some cases enough to last thousands of years.

Before we get into the history of concrete, here's a quick a vocabulary lesson: Cement and concrete are not interchangeable terms. Cement is one of the ingredients that make up concrete (which also contains water and aggregates, such as sand, gravel and often ash). Cement is the binder that holds everything together (like the eggs in cookie dough).

Although Egyptians used a mortar similar to concrete (probably a blend of lime and gypsum) to construct stone pyramids in 3000 B.C., Ancient Romans are considered the experts who first made and used actual concrete to build structures (such as the Pantheon and Coliseum) and infrastructure (viaducts and bridges). Roman concrete was made with pozzolana, a volcanic ash. Today we know that the science of concrete requires great understanding of chemistry, physics, geology and engineering. Either the Romans got lucky or they knew more than we might imagine, because many of their 2,000-year-old monuments remain standing, despite the fall of the empire in the fifth century A.D. However, the technology the Romans had developed — including the recipe for concrete — –was lost. Over the next 14 centuries, people struggled to make a comparable building material, eventually combining lime and clay to create a natural, though fairly inconsistent, type of cement.

Modern concrete mixtures contain fly ash, a byproduct of burning coal. Its chemistry is similar to volcanic ash, an ingredient the Romans used to build the Pantheon and other enduring structures more than 2,000 years ago.

It wasn't until 1824 that British stonemason Joseph Aspdin literally cooked up and patented a formula for reliably strong cement using a mixture of ground limestone and clay. Aspdin named his creation Portland cement because it reminded him of the limestone found on the Isle of Portland. The concoction required heating and mixing and more heating and grinding. Portland cement endures – in formula and in formation – to this day.

Advancements in the application of concrete quickly took form all over the world. In 1867, French gardener Joseph Monier earned the first of several patents for using iron mesh to give concrete added strength. As a result of his discoveries, the first reinforced-concrete bridge, the Alvord Lake Bridge, was constructed in San Francisco, California, in 1889 and still stands today.

As demand for concrete expanded, cooking up the cement used to make it required bigger and better kilns. In 1885 a British inventor designed a rotating horizontal kiln that could operate constantly — a vast improvement over previous stationary ovens that cooled between firings, wasting energy. Because the rotating kiln was slanted, the materials slid through, making cement production a much more efficient process. Thomas Edison improved on the idea with his 1909 patent for a 150-ft. kiln — 70 ft. longer than previous models. (Modern cement plants use kilns that are 500 ft. or longer.)

By the 20th century, concrete was on its way to becoming the most-used material for construction. The Hoover Dam, completed in 1936, is just one of concrete's many monumental applications – and it would have been impossible to build without this super-strong material. The proven durability of concrete gives us hope that, like the Roman baths and aqueducts, the Coliseum and the Pantheon, our modern concrete structures will still be standing 2,000 years from now — and possibly beyond.

More Masonry Semantics
In addition to cement, another often misused masonry term is cinder block. An actual cinder block contains ash; a block that does not contain ash is officially and more accurately called a concrete masonry unit (or CMU). Furthermore, a block is different from a brick, which is made of clay and fired in a kiln to harden. Bricks were made and used long before concrete was developed.