Standing there with a hose, wondering if you should spray it down
The slab is a day old, the surface looks dry, and somewhere you heard that concrete needs to be kept wet while it cures. So you are standing in the yard with a hose, wondering if you are supposed to be spraying it down right now.
The instinct is half right. Concrete does need water to cure. What it does not need is you adding more of it with a hose in those first couple of days, and doing so can actually work against the finish you just paid for.
That confusion is common, because keeping a slab wet is genuinely part of good practice on some jobs, just not the part homeowners usually picture. The wet part is something the crew manages on a schedule, not something added at random from a garden hose whenever it looks dry.
What the water in the mix is already doing
Fresh concrete is mixed with more water than the cement chemically needs, and that reserve is what drives the hydration reaction for weeks after the pour. Some of it works its way to the surface on its own in the first hours, what finishers call bleed water, and the crew reads that before the final finish goes on.
That reserve inside the slab is what the reaction runs on. Water sprayed onto the surface from a hose does not soak down and join it in any meaningful way. What it can do is disturb a finish that has not fully set, wash out fine paste from the top layer, or leave marks that were never going to happen otherwise.
The mix design itself accounts for this. More water goes into a batch than the cement will ever use up, specifically so the reaction has enough to draw on for the full multi-week process instead of running dry a few hours after the pour. Adding water from outside does not top off that reserve, since it never gets past the top few millimeters of a surface that is already closing itself off as it cures.
That is also why a slab that looks bone dry on top can still be well into its reaction underneath, drawing on water it already has rather than anything happening on the surface. The visible dryness and the actual cure are two different clocks running side by side.
Why the guidance is keep off, not hose down
The actual guidance is the opposite of instinct: keep hoses, sprinklers and heavy foot traffic off a new slab for the first couple of days, keep hard rain off if you can reasonably manage it, and lean on the cure guidance given for your specific pour and the weather it went into. None of that is about denying the slab water. It is about not disturbing the surface while the reaction already underway does its work.
Depending on the weather and the finish, the crew may also put down a curing compound, a sprayed-on layer that seals the mix's own moisture in so nothing has to be added from the outside at all. On other pours the plan is a wet cure managed on our own schedule. Either way, it is not something to guess at from the driveway with a garden hose.
This is not a rule specific to hot weather or cold weather either. It applies to a pour at any time of year, because the risk is not the slab drying out from lack of water, it is the surface losing what it already has too fast, or being physically disturbed while it is still soft.
If you are ever unsure whether a slab is far enough along to handle water from a hose for an unrelated reason, rinsing off dust after a dry spell or washing a spill, ask before you do it. The safe window depends on the pour and the weather it went down in, the same as every other timing question on a new slab.