Q.: These photographs show the final texture of a slab on grade. The spots shown are distributed throughout the building area. We wonder if the spots are a clue to the trouble we have had with spalling and flaking of the surface in some locations and with several control joints that have also required repairs. The slab sustains the traffic of a 4000-pound-capacity forklift truck. Could an extra heavy hose used in water curing, or excessive use of curing liquid cause something as indicated?
A.: We don't believe the spotty discoloration is directly related to the spalling, flaking and breakdown of joints--or that it was caused by a heavy curing hose or excessive amounts of curing agent. The discoloration is most likely a separate problem in its own right. We can't be sure what caused these particular spots but they look very much like spots caused by incompletely hydrated ferrites in the cement. N. R. Greening and R. Landgren have reported their study of ferrite discoloration in "Surface Discoloration of Concrete Flatwork," Portland Cement Association, Research Department Bulletin 203, published in 1966. (The article "Discolored Concrete Flatwork" published in our November 1967 issue, page 417, is a summary of that bulletin.)
According to Greening and Landgren, the ferrites in unhydrated portland cement are dark in color. When the ferrites become hydrated they become lighter. Normally this happens, and there is no problem.
Very hard troweling, the authors believe, can lower the water-cement ratio enough in parts of the surface of the concrete to seriously inhibit the hydration of the ferrites and cause spots or smudges of dark discoloration.
The use of calcium chloride, or an admixture that contains calcium chloride, in the mix can also cause dark discoloration. Although calcium chloride accelerates hydration of most cement compounds, it inhibits hydration of the ferrites, leaving them dark in color. If the cement happens to have a high alkali content, the alkali tends to overcome this darkening by calcium chloride--sometimes completely.
Methods that may help prevent dark discoloration are: avoiding the use of calcium chloride in the mix; avoiding hard troweling late in the hardening process; and curing the concrete promptly and thoroughly. If the slab does become discolored, it may be possible to reduce the discoloration by flushing with hot water and drying, perhaps more than once. If a curing compound has been applied to the surface, flushing and drying will be ineffective until the curing compound has been removed or has had time to dissipate.