Q&A / 

Calcium Chloride Concrete Slab Discoloration

Quick Column Summary:

  • What causes concrete discoloration
  • Were all slabs poured on same day
  • What additives were used in the concrete
  • Possible solutions for overall concrete color

Paula Butterworth, who had some concrete work  done at her home in Kennewick, WA, is not happy with the color of the finished slab. Her story appeared in the opens in a new windowSeptember 28, 2014 Newsletter.

Here's what she told me:

"Based on the attached photos can you tell me what went wrong to cause the discoloration of my garage floor? The concrete finisher added 1 percent calcium - poured April 23, 2014, as well as two other slabs - all with 1 percent calcium and the other slabs look great. Temps were 48 - 53 degrees. Redi Mix plant is about 20 minutes away. Is it due to delay in troweling, too much water added?  Thank you for any information you can provide."

Here's the photo that Paula sent me. You can see the uneven coloration in the slab. Photo credit: Paula Butterworth

I asked some followup questions to Paula:

  • Which slab was finished LAST?
  • This one?  Did it rain after they left or the next day?
Paula then responded:
"The job tickets confirm the other slab was poured two days before the spotted one! There were two loads on two different trucks with travel time to the job site averaging 25 minutes. However, the delivery tickets indicate 2 percent calcium chloride was added, not 1 percent as previously thought. It didn't rain at all that week."
 Here's my advice to Paula:
Paula, adding calcium chloride to ready-mix concrete can cause all sorts of issues. It's an additive that's used to speed up the hydration chemical reaction in the concrete so the concrete transform from a plastic material to a solid faster in colder weather. Without the calcium chloride, the concrete can stay very wet and plastic for hours and hours delaying the finishing process.
If the calcium chloride is not mixed thoroughly in the concrete it can lead to discoloration. There are also other complicated chemical reactions that can happen with the Portland cement and other additives (fly ash, slag, etc.?) that *might* be in the concrete.
You can get trowel burns, which produces a darkening, when you use calcium chloride.
The addition of water to the concrete during the finishing process can create discoloration too. But if the weather was cool / cold so as to require the calcium chloride, I doubt the finishers would be sprinkling water on the slab. But you never know.
At the end of the day, if you want a monolithic coloration you may have a few choices.
One thing you can try in an out-of-the-way location that would be covered by a garbage can or ???? is to etch the slab with a solution of muriatic acid. This solution will dissolve the upper ultra-thin layer of cement paste uncovering some of the sand in the finish. It MAY even out the look of the slab. You would mix 1 part acid to 10 parts water. Just try it in an area about 1 foot by 1 foot. Let the solution work for about 20 minutes then LIGHTLY scrub. Rinse well and inspect it after it dries.
Take great before and after photos of this spot for comparison sake.
If this fails, then you can resort to one of the epoxy coatings for concrete. I don't like these too much because they turn a maintenance-free surface into one that requires periodic maintenance.

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