Question. We are installing colored textured concrete in an enclosed mall being constructed in a winter climate. The general contractor is using temporary unvented construction heaters that put both the heat and the exhaust into the enclosure. When we came back the following day I noticed that some of the color was “dusting” off the surface of our work. Why is this happening and what should we do?

Answer. The dusting is caused by a reaction between freshly placed concrete and the high amount of carbon dioxide from the heaters. The carbon dioxide reacts with calcium hydroxide in the concrete to produce calcium carbonate on the surface. It's the surface dust that you are referring to (plus some color particles). There is still some calcium hydroxide in the surface regions also.

This isn't a perfect solution, but applications of magnesium or zinc floro silicates will react with the remaining calcium hydroxides to harden the surface. This can pose problems with colored concrete because efflorescence can also result. Apply three applications. Dilute the first one 75% with water, the second one with a 50% water dilution, and then apply the third one at full strength. If you notice any signs of efflorescence developing, wash the surface immediately to remove it. When the process is complete the floor surface will never be as hard as it would be if it was cured properly and the carbonation hadn't occurred but it could be 50% to 75% of what it would be.

It's always best to place concrete in spaces where construction heaters are vented outside the working area. There is much less risk that your concrete will dust and it's healthier for workers. If you are placing plain concrete where the surface appearance isn't important, cover it with poly plastic after it's finished.