A concrete explanation
A gentleman named Hugh Andrew has written about a common error that irritates him, and since it irritates me as well, I quote him in full. If you aspire to be known as a careful and precise writer, pay attention.
One current word usage continues to bother me. It is the use of the word “Cement” when the writer or speaker means “Concrete.” I learned the difference between the two terms more than fifty years ago in the School of Engineering at Johns Hopkins. I know that the language evolves, but this particular evolution tends to be confusing.
Cement or Portland cement, to use the technical name, is a greenish-gray finely ground powder that is one of the ingredients in concrete. The Portland Cement Association, a trade group representing manufacturers of this material, maintains an informative web site at www.cement.org/basics. When this manufactured chemical is mixed with gradated aggregates—usually coarse sand and either gravel or crushed stone—and water is added the resulting mixture is concrete. A chemical reaction will then take place that will cause the mixture to harden within a few hours. If the ingredients have been properly proportioned, and if the product has not been subjected to climate extremes, in approximately 28 days, this man-made conglomerate will become strong enough to withstand a compressive force of 3,000 pounds per square inch or more. Concrete is an ideal construction material. Because it is put in place in a semi-liquid state, it can be molded into shapes that are structural or decorative or both.
However, it is concrete, and cement is just one ingredient. Therefore, to speak of a cement sidewalk or a cement mixer does not make sense.
Technically, Portland cement concrete is not the only kind of concrete. When a stone aggregate is mixed with a bitumen—usually petroleum-based asphalt—the resulting material can be called bituminous concrete. It is much simpler and more descriptive to call this product by its common name. We properly call it blacktop, unless it is a paving material used in an airport, when it mysteriously becomes tarmac. Much of the paved surface at airports these days is concrete and not blacktop. I suspect that news people use the word tarmac to describe all of these air terminal surfaces. At least, they do not call them cement runways!