Q: Some of the concrete in a project we'll be building will contain silica fume (microsilica) as an admixture. I've been told that the silica-fume concrete will be darker in color than normal concrete even though the same cement is used in both. How much difference in color can I expect?
A.: Freshly mixed concrete containing silica fume can be almost black, dark gray, or practically unchanged, depending on the dosage of silica fume and its trace minerals content. We asked Harry Tormey of W. R. Grace & Company, a manufacturer of silica-fume admixtures, to send a hardened concrete sample showing the worst-case color difference that might be expected (see photo). The dark side of the sample contains silica fume at a dosage of 105 pounds per cubic yard. The sawed surface has been given a clear coating to accentuate the difference. Note that the color difference isn't as pronounced on the uncoated edge of the sample. Tormey says that the greatest differences in color occur in concretes made with cements that are light in color. Mix proportions may also have an effect. If the color difference is a problem, you might want to use the darkest brand of cement available in your area and try different mix proportions during the mix design process.
This sample shows a worst-case color difference between normal (left side) and silica-fume (right side) concrete. A clear coating was used on the sawed cylinder surface to accentuate the color contrast. The edge of the cylinder is uncoated.