Where the extra water ends up on a fresh pour
Dusting comes from the same place crazing does: too much water working its way to the top of a fresh pour, and finishing happening before that surface has firmed up the way it should. Extra water at the surface dilutes the cement paste right where the wear happens, so the top few thousandths of an inch never gains the strength the rest of the slab does. Finishing over that watery layer, or troweling a slab that has bled more water to the top than usual, works loose cement into the surface instead of solid, bonded paste. Once the slab cures, that thin weak layer stays weak. It never bonds the way the concrete underneath it did, and it breaks down into powder under traffic instead of holding together.
Where dusting turns up and why it keeps coming back
Garage floors and basements are the usual complaint, since both see steady foot and tire traffic on a slab that often was not sealed, and neither one gets much air movement to help the surface cure evenly. A few conditions make it more likely on a given pour:
A little light dust the first time you sweep brand new concrete is normal and is not what this term describes. Dusting as a defect means the powder keeps coming back sweep after sweep, months or years after the pour, because the top layer itself never gained real strength.
- Finishing done while bleed water was still standing on the surface
- A slab that cured too fast, losing moisture before the surface fully hardened
- Cold weather finishing without the surface protected from an early freeze
- A slab with no sealer, left to take direct tire and foot traffic