What is the cause of 'dusting' on the surface of hardened concrete?

Dusting on the surface of hardened concrete is caused by a weak, non-durable surface layer that easily crumbles and produces a fine, powdery residue. This weak surface layer is typically the result of improper finishing or curing practices that prevent proper cement hydration at the surface. Several factors can contribute to dusting: premature finishing, which is working the concrete surface before the bleed water has completely evaporated, diluting the cement paste at the surface and weakening it; inadequate curing, which deprives the surface of the moisture needed for cement hydration; carbonation of the surface layer, where carbon dioxide from the air reacts with calcium hydroxide to form calcium carbonate, which is less durable than calcium silicate hydrate; and the use of excessively wet mixes, which can lead to segregation and bleeding, resulting in a weak surface layer with a high water-cement ratio. Dusting is most common on interior slabs and floors. For example, if a concrete floor is finished too early and then not properly cured, the surface will likely exhibit dusting. To prevent dusting, it is essential to follow proper finishing and curing procedures, avoid over-watering the mix, and protect the concrete surface from rapid drying.