What causes color differences in a cast-in-place concrete wall?
Cement is usually the main contributor to the color of concrete. Cements from different plants almost always show color differences. Increasing the cement content or decreasing the water-cement ratio usually darkens the color. Water absorption into the form face while the concrete is hardening may also cause darkening. Concrete that has been allowed to dry quickly after hardening is lighter in color initially than concrete that has been kept moist. As a result, removing formwork at significantly different ages after placement can produce color differences in a wall. However, the differences become less marked with time. To minimize color variations, purchase all cement for the job from a single plant. Don't vary mix proportions, especially cement and water content. Seal form faces to prevent absorption and tape joints to prevent leakage. Then strip all of the forms at about the same time after placement. The American Concrete Institute's Guide to the Selection and Use of Hydraulic Cements (ACI 225R-85) lists several other factors that may cause variations in concrete color.