Q: In place driveways, stair stoops and sidewalks during the winter, we cover them with insulating blankets to protect the new concrete from freezing. However, we often find white splotches or stains on the concrete surface when we remove the blankets. This white efflorescence always occurs when the new slab is covered the first night after it's placed. If we cover day-old concrete, very little staining occurs.Does entrained air cause these stains? Are they related to the calcium chloride that's sometimes used to speed up setting time?
And how do we remove the stains? Our customers get angry about this surface defect.
A: Entrained air isn't causing this problem. It actually helps prevent efflorescence because it lowers the amount of water needed to produce a given slump. Using non-air-entrained concrete in a cold climate invites surface scaling, which is a much more serious problem than discoloration.The stains you described may be caused by a chloride accelerator, especially if it's added dry and doesn't get thoroughly mixed into the concrete. A simple solution is to use a nonchloride accelerator. Though these accelerators cost more than calcium-chloride products, their use may eliminate the staining problem and callbacks from angry customers. It's possible, though, that the efflorescence is caused by calcium hydroxide