The Color Purple – Why Does It Appear on Brown Concrete?

These two pieces of concrete are from a three-day-old slab integrally
colored a medium brown. The piece on the right has a purplish cast
on the surface, while the piece on the left shows the true brown color
on the inside.

Question:
A day after pouring a concrete slab with a medium-brown integral color, the color looks more purple than brown. Why is this, and how do I get the brown color back?

Answer:
As discussed in previous entries of Sullivan’s Corner, the true color of concrete lies within (see photo). The purple color phenomenon you’ve observed is actually a surface effect resulting from one or a combination of factors. Concrete integrally colored a shade of brown (from light tan to dark chocolate) can take on a purplish cast due to efflorescence, a high water-cement ratio, or a rough surface finish (broom or sweat).

Luckily, this issue is easily remedied. A very mild acid wash (40 parts water to one part muriatic acid) or cleaning with a chemical product specifically for removing efflorescence often removes the purple cast and brings back the true brown color. Once the slab is dry and the brown color restored, apply a good sealer for long-term maintenance. To minimize the purple color phenomenon on your next project, I recommend using a concrete mix with a low water-cement ratio (consider adding water reducers or plasticizers) and apply a good-quality cure and seal or curing compound.