Mike Hooker writes:
I have a problem with people using the word “cement” when they mean “concrete”; they are not interchangeable, yet people write and say it all the time.…To clarify, cement is the powder used to make concrete. The hardened surfaces on which we walk and drive are concrete, not cement. It’s really no big deal, just something that leaps out at me when I read it or hear it.
Although many speakers use the words interchangeably to refer to any hard substance, the distinction matters when it comes to putting these materials to use. Fiction writers especially need to know the difference. For example, it would be embarrassing to have a character who is a construction worker mix up the terms.
Cement is a binder.
Concrete is an aggregate that includes cement.
Here are some examples of cement being used where concrete would be the accurate choice:
How to make a nice cement patio
Building a cement patio is no easy chore…
How to Build a Cement Block Patio
The word cement evolved from a word for “small broken stones” to mean “powdered stones.” It entered English from Old French in the 14th century as ciment. The French word came from Latin caementa, “stone chips for making mortar.”
The Romans made their cement by mixing limestone with volcanic ash. They kept this mixture as dry as possible and then pounded it into an arrangement of rocks already in place. They didn’t use rebar, but many of their bridges, aqueducts, and temples still stand.
The most common cement used in the making of modern concrete is Portland cement.
Portland cement was first produced in 1824 by a British stonemason, Joseph Aspdin. He heated a mixture of finely ground limestone and clay in his kitchen stove and then ground the mixture into a powder that hardened with the addition of water. He called it “Portland” cement because of its resemblance to a stone quarried on the Isle of Portland in the English Channel.
The word concrete came into English as an adjective in the late 14th century, from Latin concretus, “condensed, hardened, thick, hard, stiff, curdled, congealed, clotted.” It began as a term of logic, but expanded in meaning until, in 1834, it was being used as a noun meaning “building material made from cement, etc.”
As an adjective, concrete is used as the opposite of abstract.
This, one of many definitions given by the OED, explains why we talk about “concrete nouns”:
concrete: 4. a. Applied by the early logicians and grammarians to a quality viewed (as it is actually found) concreted or adherent to a substance, and so to the word expressing a quality so considered, viz. the adjective, in contradistinction to the quality as mentally abstracted or withdrawn from substance and expressed by an abstract noun: thus white (paper, hat, horse) is the concrete quality or quality in the concrete, whiteness, the abstract quality or quality in the abstract; seven (men, days, etc.) is a concrete number, as opposed to the number 7 in the abstract.
Oxford English Dictionary
Online Etymology Dictionary
“The Riddle of Ancient Roman Concrete,” by David Moore, P.E.