Where this starts
A concrete slab gains its strength from cement hydrating, and hydration needs water present for days after the pour. Dusting means the surface layer never got enough — so the cement there never fully reacted and what is left is a friable film of fines and unreacted material sitting on sound concrete.
THE COIN TEST SEPARATES THE TWO CASES. Scratch firmly with a coin or a key. A THIN SKIN that comes away cleanly, with hard concrete immediately underneath, is a finishing failure: water was worked to the surface during floating, or water was added at the surface to make trowelling easier, and a weak layer of laitance was created. A surface that KEEPS GIVING, deeper and deeper, is a curing failure and there is no sound layer waiting a millimetre down.
THE HISTORY USUALLY CONFIRMS IT. Slabs poured in hot or windy weather, poured and left uncovered, finished with water sprinkled onto them, or trowelled while bleed water was still rising, all produce it. So does a slab exposed to frost before it set, and so does one in a garage where de-icing salt arrives on a car every winter.
WHY IT MATTERS BEYOND THE MESS: a dusting surface is a bond breaker. Any coating, adhesive, screed or levelling compound applied over it is bonded to a layer that is already failing, so paint peels, tiles lift and resin flakes — all blamed on the product rather than the substrate. Testing the surface before coating anything is the cheapest part of the whole job.
ONE DISTINCTION WORTH MAKING EARLY: dust from the surface is grey and fine and comes back after sweeping. White powder that keeps returning and wipes away with water is efflorescence, which is a different problem about moisture moving through the slab.
What it could be, and what tells them apart
These are candidates to check against your own building, in the order they most often turn out to be the answer. None of them is a diagnosis of your situation.
Laitance from overworking or adding water at the surface
Most commonA thin weak skin over sound concrete, coming away in the coin test with hard material immediately beneath, often on a power-floated slab.
How to check: The coin test. A skin with hard concrete under it is a finishing failure and is removable.
The slab was never cured
Most commonWeakness continuing as you scratch deeper, with no sound layer close to the surface, on a slab poured in hot, windy or freezing conditions or left uncovered.
How to check: Whether the weakness has a bottom. Material continuing to give means the cement never hydrated properly down into the slab.
Trowelled while bleed water was still present
CommonA surface that looks well finished and dusts anyway, sometimes with a slightly darker, glassy appearance, on a slab finished quickly.
How to check: How soon after the pour it was finished. Working bleed water back in raises the water-cement ratio at exactly the surface.
Carbonation from unvented heaters during curing
CommonA slab poured in cold weather and warmed with gas or paraffin heaters, dusting over the whole area, sometimes with a soft chalky surface.
How to check: Whether unflued heating was used while it cured. Combustion gases react with the fresh surface and weaken it.
De-icing salt or chemical attack
CommonDusting and surface scaling in a garage or an entrance, worst in the wheel tracks and near the door, worsening each winter.
How to check: Where on the slab it is worst. Damage that follows the car is salt arriving on it.
Abrasion on a slab never intended to be trafficked
Less commonDusting in traffic routes only, on a slab that was meant to receive a covering and has been left bare, with the rest of the floor sound.
How to check: Whether the dusting follows where people walk or drive. A slab designed as a base is not a wearing surface.
What to check, in order
The order matters — each step is here because of what it rules out for the ones after it.
- 1
A thin skin over sound concrete is a finishing failure and can be removed; weakness continuing downward is a curing failure and cannot.
- 2
Hot, windy or freezing conditions, or a slab left uncovered, account for most curing failures, and the history is usually easier to get than a test.
- 3
Combustion gases carbonate the fresh surface and produce a soft chalky layer over the whole slab, which looks like bad concrete and was bad conditions.
- 4
Traffic routes mean abrasion, the area near a garage door means salt, and the whole area evenly means the pour rather than the use.
- 5
A dusting surface is a bond breaker, so paint, resin, adhesive and levelling compound will all fail on it and be blamed on the product.
The calculations behind this
Several of the causes above are a quantity that was specified or assumed. These compute them, with the formula and the standard behind each.
When to stop and get somebody in
- Where the weakness continues deeper the more you scratch, since that is a curing failure through the slab rather than a surface skin and the remedies are limited.
- Where a coating, resin or floor covering is planned, because the substrate has to be tested and prepared and a failure there is charged to whoever laid the covering.
- Where the slab is in a commercial or industrial setting with a specified surface regularity or abrasion resistance, which is a compliance question.
- Where surface scaling is progressing as well as dusting, particularly in a garage or an entrance exposed to salt, which is material loss rather than a weak film.
This page lists candidates to check, not a diagnosis — nobody has seen your building. If the symptom involves anything structural, read this first.