We have noted the formation of a greenish/blue discoloration on some concrete. The occurrence of coloration was quite inconsistent, occurring sometimes after the first placement and not occurring again, while at other times it occurred first on the seventh or eighth casting using the same forms. The forms were high density overlaid plywood. The coloring appears to be present when the form panels are stripped, and has occurred in all weather conditions. The pattern of discoloration is random, with irregular, free-form shapes. The concrete mix on one of the jobs reporting the green discoloration was identified as a slag cement. We have been told that cement people claim the greenish/blue discoloration is a phenomenon and of common occurrence with slag cement. It was reportedly caused by sulphide sulphers in the slag, with the discoloration occurring when no air reaches the surface or interface between concrete and form panel during the pour. Where air reaches the panel surface along form panel edges, this area does not discolor.
The discoloration is caused by iron sulfides and ferrous oxides in slag cement. This discoloration seems to occur more often on concrete cast against large continuous slick form panels such as plastic coated plywood or steel forms which are more airtight than forms made with regular lumber. The occurrence and intensity of the discoloration seems to be related to the length of time before the forms are stripped. Loosening or opening the forms at the earliest possible time after placing the concrete will generally prevent occurrence of this discoloration. Controlled time of removing the forms will also help to prevent or minimize discoloration.If discoloration does occur, it will generally fade and disappear with age providing the concrete is exposed to an essentially drying condition such as interior atmosphere or normal weather (sun, rain, and wind). The application of hydrogen peroxide solution will either remove or diminish the discoloration particularly when applied to the concrete immediately after removing the forms.