Question: I am patching and repairing some street curbs. The quick-mix products tend to be dark gray, which does not match the lighter sandy color of my curbs and sidewalks. The new concrete highways tend to be almost white. What do they add to get an almost white color?

Answer: This kind of patching isn't as easy as you might think. The white pavement that you refer to might be the curing agent used on the pavement, which is typically colored white to help workers achieve an even application. But within a couple months the surface usually turns lighter due to carbonation. The other difficulty for color-matching patches is that the concrete isn't just one color, there is variability that becomes obvious when you add a one-color patch.

Here are some mixes you can try for a better match:

  • Purchase a commercial polymer patching product made with white portland. Add a little regular colored cement to it to deepen the color. Patching cements look darker than the color of the cement because the polymer bonding agents in them make the concrete denser.
  • Tint a patch mix with metallic oxide colors. You can get these at paint stores (they must be water-based pigments) or at construction supply houses that sell integral color for concrete. You can buy a full range of colors, including white. Liquid dispensed colors are particularly good for this.
  • Get a small amount of the same portland cement and sand used on your project by your ready-mix supplier. Add a polymer bonding agent and color to offset the deeper coloration resulting from the bonding agent.
  • Add a pozzolan called metakaolin to your patch mix. It is very light in color.

Patching requires trial-and-error attempts until you find the right recipe for a given situation.