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Laboratory studies of mottling discoloration of horizontal concrete slabs are described, showing that discoloration is increased by hard trowling, by use of calcium chloride admixtures, or by poor curing of the slabs. Local discoloration can also be caused by non-uniform curing conditions. Avoiding the use of calcium chloride in hard-troweled flatwork would eliminate much flatwork discoloration. If calcium chloride is necessary, curing procedures recommended to minimize local discoloration are water ponding or use of sprayed membrane curing compounds. The effect of calcium chloride is dependent on the alkali content of the cement. Immediate and thorough washing of the concrete surface with water seems the easiest way of "erasing" discoloration. Special chemical treatments to erase discoloration are of additional benefit. This report is also found on DVD021.
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