Dusting is the development of a soft, powdery material that can be easily rubbed off the surface of hardened concrete
Diagnosis: Appearance of a powdery substance on the surface of the concrete that rubs off easily.
Cause: There are multiple potential causes for this condition, such as beginning finishing operations while bleed water is still on the surface, condensation worked into the surface, surface carbonation, inadequate curing practices, and exposure to an unfavorable environment (rain, freezing).
Prevention: Concrete slump should never be more than 5”. Allow bleed to finish completely before working the surface. Do not attempt to re-temper the surface with water or cement powder before, or during finishing. Ensure proper ventilation to evacuate emissions from heaters and power equipment. Protect the surface from unfavorable environmental conditions.
Possible concrete repair: A concrete surface with this condition is rendered only moderately hard, absorptive, and very subject to abrasion. Because of this, it is also very hard to bond a re-surfacing or topping material.
For those reasons the preferable approach to this is to harden the slab using a reactive, often silicate-based penetrating liquid. It is applied to the surface and absorbs into the matrix. There, the silicate reacts with the calcium hydroxide in the concrete to produce calcium silicate hydrate, which crystallizes in place. The same materials and basic procedure is used in polishing concrete surfaces to a brilliant shine. Although most dusting slabs can be greatly improved by this process, it may not be able to recover the most compromised examples.