Q. We made a 40-cubic-yard concrete pour for a floor on ground that was to receive a machine-troweled finish. The pour started at 7 a.m., and by 10 a.m. finishers who were edging the slab noticed a fine, white dust coming to the surface. This was occurring only along the edges; no dust appeared on the machine-troweled part of the floor. Small white dots on the surface appeared to be the source of the dust. By 5 p.m. the dust almost completely covered the area within 6 inches of the form. We've never seen this before and wonder whether it's efflorescence or has some other cause. The concrete contained 8 ounces of a mid-range water reducer per 100 pounds of cement, and no calcium chloride was added. I first saw the concrete at 10 a.m. and don't know how wet it was when placed.
A. We've neither seen nor heard of this phenomenon. If the concrete was very wet when placed, small channels could have been formed by excessive bleeding. Cement fines and alkalies brought to the surface through these channels may then have dried to form the deposit you describe. If power troweling sealed off the channels in the rest of the slab, fines would have appeared only at the edges. A chemical analysis of the powder would help solve the mystery. Have any other readers experienced this? Do you know what caused it? Send replies to the Problem Clinic editor at the above address.