Rub it before you guess
Run a dry finger across it first. If it comes off as a fine, pale powder, you are looking at efflorescence, mineral salt left on the surface after water carried it up and evaporated. If instead the surface is flaking, pitting, or coming away in small chips, that is a different problem, surface scaling, not efflorescence, and it does not wipe off with a dry hand.
Efflorescence itself is not eating the concrete. It is a byproduct sitting on top of it, calcium hydroxide the mix released during curing, carried to the surface by moisture and left behind once the water evaporates.
- A fine white powder that wipes or brushes off
- Usually heaviest in the first year while the slab finishes curing
- Can also show up on any older slab that stays damp underneath
Find out where the moisture is coming from
Efflorescence needs water moving through the concrete to happen at all. On a new slab, that is often just the curing process itself, water working its way out as the mix hardens, and it settles down as the slab dries through its first year.
On an older slab, moisture moving up from underneath is the more common story, ground water wicking through a subgrade that never had a vapor barrier under it, a downspout draining onto the surface, or a low spot that holds water after every rain. Figuring out which one you have decides whether this fades on its own or keeps coming back.
Clear what is already there
Start plain. A stiff broom and water lifts most efflorescence without doing anything to the slab. Let it dry and check again in a day, since some deposits build back within hours if the moisture behind them has not slowed down.
Stubborn buildup sometimes needs a masonry cleaner made for mineral deposits rather than a general purpose one, and on new concrete it is worth waiting until the slab has had time to cure before you scrub or pressure wash it hard. Pushing water into concrete that is still green works against the same drying process that is causing the residue in the first place.
Whether sealing helps or just hides it
Sealing does not stop the moisture that is moving through the slab, it changes what happens once it gets to the surface. A penetrating sealer still lets vapor pass through while cutting down on new mineral deposits forming on top, which is usually the better fit here than a film forming sealer that traps moisture underneath it.
Sealing too early is its own mistake. If the slab is still actively curing or the moisture source underneath has not been dealt with, sealing over efflorescence just locks the problem under a coating instead of solving it.
If it keeps coming back within days
Efflorescence that clears with a wash and stays gone is nothing to worry about. Efflorescence that reappears within a few days, especially in the same spot every time, is telling you water is still actively moving through that section of slab, and it is worth figuring out where that water is coming from before you clean it again.
Bring us the details, when it shows up, whether it is spreading, and what is near that section of slab, a downspout, a low spot, a garden bed running close to the edge. That is usually enough to point at the source.