I'm a perfectionist when it comes to concrete finishing. I want to know why discoloration occurs in some instances on recently placed slabs. The discolorations are usually light and dark patches. I know that concrete slabs eventually bleach out with exposure to the sun, but sometimes the building shell is built before bleaching occurs, leaving a permanently discolored slab. I've asked many finishers why this discoloration occurs. Their answers have included spraying water on the surface to make finishing easier, excessive finishing, and improper placement of thermal blankets. Are there other possible causes?
We asked our readers this same question in June 1991 and published many suggested causes in October 1991. These included steel troweling, variable subgrade composition, and variations in the concrete's water-cement ratio. One reader cited a detailed study conducted by the Portland Cement Association (PCA) in the mid-1960s. This study showed that discoloration could be produced by using calcium chloride in the concrete, applying a hard-trowel finish, and failing to moist cure for the first day after concrete placement. Discoloration was caused primarily by calcium chloride retarding the reaction of tetracalcium aluminoferrites in the cement. When hydration of this compound is retarded, it retains its dark color; hydration lightens it markedly. Some discoloration that's reported occurs when there's no calcium chloride in the concrete and no hard troweling. This form of discoloration is believed by some to be related to water that's trapped just beneath the surface during finishing. It seems to occur most often in coastal areas with high ambient relative humidities. For further discussion of the problem and some solutions, see "Pinto Concrete: Is There a Cure?" in Concrete Technology Today, Vol. 17, No. 1, March 1996, p. 4. You can purchase a copy by calling PCA at 800-868-6733.