With our experience building precast concrete fences and walls in California, we've become pretty comfortable with a good understanding of the concrete and cement business. Generally "concrete" and "cement" are not the same thing. Sidewalks are made from concrete, not cement, although cement is a vital ingredient of concrete. There are other ingredients which may include gravel or crushed stone , sand, water and, other performance-enhancing additives. The trucks you see with the swirling container that most people refer to as cement mixers are really concrete mixers; cement, like talcum powder, is transported mainly in tank trucks.

The cement in concrete is known as Portland cement, because Joseph Aspdin, an English bricklayer who invented the earliest version, felt that its color was almost the same as limestone quarried on the Isle of Portland. Aspdin got a patent for cement in 1824. He used to heat limestone and clay in a kiln until parts of the mixture fused, then he ground the burned and desiccated result into a fine powder. Adding water to the powder yielded a workable paste and started a chemical process, hydration, in which the water bonded with compounds of calcium, silicon, aluminum and iron, and caused the whole thing to combine into a rigid mass. Wet Portland cement doesn't merely "dry," hydration transforms it into a chemically distinct material, which continues to gain strength over time.

Though concrete is very hard to crush, it's pretty easy to actually pull apart. A way to compensate for this tensile weakness (that means it's easy to break apart) is to add steel reinforcing rods, known as rebar, which hold the concrete in place overall when it cracks. Concrete reinforced with rebar must crack, Meyer explained. "That may sound funny to the layperson," he said, "but the reason is that if it doesn't crack, you wouldn't need the steel. It is the challenge of the engineer to keep cracks small, so that rather than having a few big cracks, we have many little cracks."

Concrete is essentially fireproof, but it can fall apart in very high temperatures as free water trapped inside turns to steam, expands, and blows it apart from within. So if you want to reinforce cement even more, you can add lengths of threadlike fibers made of steel, polypropylene, polyolefin, and other materials-samples. Such concrete can provide extra protection in structures that may be exposed to any of a variety of increasingly ordinary-seeming perils of modern existence, among them fires, explosions, and bomb blasts. By adding polypropylene fibers to the mix it can reduce the risk of such failures, because in high heat the fibers melt, leaving voids that act like relief valves for steam.