It is the glue, not the mix
People say cement when they mean concrete, and the two are not the same thing. Cement is the fine gray powder. Concrete is cement mixed with water, sand and stone, plus whatever admixtures the job calls for. Once water hits the powder, a chemical reaction starts that locks everything else together into one solid mass. That reaction is what sets and cures concrete, not evaporation. A slab that dries out too fast before that reaction finishes never reaches the strength it was ordered at, which is the whole reason curing gets talked about separately.
The amount of cement in a load, relative to everything else, is called the paste content. More paste generally means more strength and a smoother finish, and it also means more cost, since cement is the priciest ingredient by weight in the truck.
Where the amount of it gets decided
How much portland cement goes into your load is not something you pick off a shelf. It comes out of the mix design the plant builds against the strength you ordered and the conditions the slab will face. A footing calls for a different paste content than a driveway that sits outside through a Virginia winter, because the driveway also needs air entrainment worked into that same paste to survive freeze and thaw.
On our own pours, that conversation with the plant happens before the truck is ever loaded. We tell them what the slab is, they hand back a mix design built for it, and the cement content is baked into that design rather than left to guesswork on the day.
What it looks like on a pour
You never see straight cement on a job site. By the time a load arrives, it has already been proportioned with water, sand and stone at the plant and is on its way to being concrete. What you can see is the paste doing its work at the surface, the smooth gray film that a finisher works up with a float or a trowel. That film is cement paste rising to the top, and it is what gives a finished slab its texture.
Watch a crew finish a driveway and most of what they are managing is that thin layer of paste. Too little of it at the surface and the finish comes out rough and hard to close. Too much worked up too soon and it can trap water underneath, which is part of why timing matters as much as the mix itself.
Why curing still matters after the paste sets
Hydration, the reaction between cement and water, does not finish the moment concrete looks hard enough to walk on. It keeps building strength for weeks, and it needs moisture in the slab to keep going. That is the whole reason curing gets managed as its own step on a pour rather than left to chance. A slab that dries out too fast on a hot day can lose that reaction early, and no amount of cement in the original mix makes up for hydration that stopped before it should have.