What Causes Dusting and How We Can Fix It
Concrete is attractive and durable when it is installed correctly, but mistakes during the installation process can lead to problems. One issue that we see frequently is called ‘dusting’.
If a fine powdery material easily flakes off the surface of hardened concrete it is probably caused by ‘dusting’. It can be problematic either indoors or outdoors, but it tends to be more of an issue indoors because dust can be tracked all over a home or other building.
In fresh concrete, aggregates, cement, and water are evenly distributed. It takes a period of time for the cement and water to react and form hardened concrete. The cement and aggregates are partly suspended in the water before they harden and tend to sink over time. As they move down, displaced water rises and bleed water appears at the surface. Laitance, a thin layer of water, cement, and fine particles, forms at the top. The laitance is the weakest, most permeable, and least wear-resistant layer of concrete.
Dusting can be caused by floating and troweling concrete with bleed water on top because it mixes excess water back into the surface of the concrete and weakens it. Dusting can also occur if water is applied during finishing, if concrete is exposed to rain during finishing, if dry cement is spread over the surface to speed up the finishing process, if there is a low cement content, if the mix is too wet, if the cement does not cure properly (especially if the surface dries rapidly), if the surface freezes, or if the aggregate is dirty.
Heaters are often used to protect concrete if it is installed at temperatures near or below freezing. Oil- and propane-fueled blowers and coke-burning salamanders produce dry heat. It is necessary to prevent rapid drying of the concrete near the heater.
Burning of fuel also produces carbon dioxide, which reacts with calcium hydroxide in fresh concrete and creates a weak surface layer of calcium carbonate. If this happens, traffic can cause dusting. A heater that produces carbon dioxide should not be used when placing and finishing concrete or for the first 24 to 36 hours in the curing process, unless the enclosure is properly vented.
We can correct a dusting problem by grinding off the top 1/8 to ¼ inch to get down to solid concrete. We then apply a lithium silicate densifier to harden the surface followed by the application of an epoxy flake style coating to create a durable and attractive concrete floor.