Concrete dusting
A fine powder comes off the surface under foot traffic or when wiped with a hand. The slab looks sound but never stops shedding, and a garage floor will leave grey marks on everything.
VerdictCosmetic — does not compromise the slab
What causes it
Dusting is a weak surface layer, and the cause is almost always finishing-related. Troweling while bleed water is still on the surface works that water back into the top few millimetres, raising the local water-cement ratio and leaving paste too weak to hold together. Carbonation from unvented heaters during cold-weather placement does the same thing chemically. Inadequate curing compounds the problem by stopping hydration before the surface gains strength.
How it is repaired
A sodium or lithium silicate densifier reacts with free lime in the surface and hardens it, which resolves light dusting and is the usual first attempt. Heavier dusting needs the weak layer mechanically removed by grinding, then either sealing or an overlay. A film-forming sealer over a dusting surface fails, because it bonds to powder rather than to concrete.
How to prevent it
Wait for bleed water to disappear before floating or troweling — this is the single most common cause. Ventilate any combustion heater used during cold-weather placement. Cure continuously for three to seven days rather than letting the surface dry out.