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Powdering at the surface of a concrete slab is called dusting. It is characterised by a light-powdery surface that may weather to expose the aggregate. Dusting surfaces powder under any kind of traffic and can be easily scratched with a nail or even by sweeping.
Bleed water carries cement and finer particles to form a layer of laitance on the slab surface. Excessive bleed will create a relatively porous ‘skin’ that has a tendency to dust.
In cold weather concrete sets slowly. If the humidity is relatively high, water will condense on the freshly placed concrete. If trowelled into the surface dusting may occur. On formed surfaces, over application of release agents can cause the surface to retard, the resultant concrete being dusty.
Very rapid drying of the surface from wind may reduce the water available at the surface for cement to hydrate fully, leaving dusty surface.
To prevent dusting, do not carry out finishing operations with excess water on the surface and provide effective curing (producing a stronger surface, more resistant to abrasion, chemical spillage etc) by using a curing compound or covering the surface with wet hessian or plastic sheet. Immature concrete must be protected from the prevailing environments.
Dusting can be rectified by the application of a chemical floor hardener or, in severe cases, wet-grinding the surface and laying a bonded proprietary screed. Alternatively, the surface can be covered with an overlay of flooring such as carpet or vinyl.
Acknowledgement: The Concrete Society
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