Some time after moving into a new building, the occupant many find that foot or wheel traffic is creating a lot of dust on the concrete floors. Investigation shows that the floor surface is soft and easily scratched. This is another example of an old problem- the carbonation of fresh concrete that causes dusting of concrete surfaces. The problem was identified and discussed 70 years ago, and the construction industry has been warned about it many times since then. However, the lesson has not yet been learned by everyone involved. Each new generation of craftsmen must be made aware of the phenomenon of carbonation and how to avoid the problems it can cause. Combustion heaters- those that burn wood, oil, gas, coal or coke-are the principal cause of soft, dusty concrete floor surfaces. Used during winter to protect fresh concrete from freezing, these heaters emit exhaust gases that contain, among other things, carbon dioxide, which is harmful to fresh concrete. If the air is still, the carbon dioxide will settle and concentrate near the floor, rather than distributing itself evenly throughout the air, and do more damage than otherwise. Once the carbon dioxide enters the work area, it reacts with the moisture on the fresh concrete surface to form carbonic acid. This acid then combines with the hydrated lime created during the hydration of cement to form weak calcium carbonate rather than the calcium silicate hydrate that normally would be formed as a product of hydration. In normal cement paste, calcium silicate hydrate forms dense bonded aggregations between the other crystalline phases and the remaining unhydrated cement grains. It is thus chiefly responsible for the setting, hardening and strength development of cement paste. When it is replaced by weak calcium carbonate, the concrete loses strength and wear resistance. To prevent the formation of destructive carbonic acid, there are two alternatives: immediately sealing the fresh concrete with liquid curing membrane or using vented heaters or other sources of heat.