We see it all around us; concrete is the most used building material in the world. But what is it?
Traditional concrete is a composite material made from cement, sand, stones, and often a reinforcing material such as steel. It is a strong material, that even when made poorly can handle quite a heavy load. For example, a 30 day old domestic concrete foundation can handle the weight of an average automobile on every square inch of its upper surface. But for all its compressive strength, unreinforced concrete lacks an ability to hold a similar tensile strength.
The early users of concrete knew this, and also lacked a readily available material that could serve this function in their concrete mixes. This lead them to build most of their structures to channel compression. Rome’s Pantheon, 1900 years old, and for 1400 years mankind’s largest dome is built as if it were a semispherical arch. From the oculus atop the dome, to the pillars that hold its weight at ground level, all is in compression.
With the addition of steel, the construction of Roman buildings might have been very different. Steel has made the modern era’s concrete capable of defining our time. Steel expands and contracts at a rate quite similar to concrete, and it allows bridges to span large chasms, and buildings to reach into the sky.
Cement is the binder that holds concrete together. In a foundation it represents 10-14% of the total weight. In countertops, and other home furnishings, the cement content can be 20% for a traditional mix, and up to 45% for more esoteric mixes. Think of it a a mineral glue. Typically it is made by grinding limestone and a high silica clay together, heating them to 1450º in a rotary kiln. The heating causes the limestone and clay to lose hydrogen and oxygen, and combines the once separate materials into glass nodules called clinkers. These are then crushed into a fine powder, and gypsum is added to help stabilize the mix for storage, and also help smooth the mixture’s reaction with water.