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Do Admixtures Inhibit Stain Color?

Question:

We seem to be experiencing more and more concrete slabs that are not taking acid stains, even when we grind them first with a #150-grit diamond disc. The acid stain is not even producing full color development inside a ¼-inch sawcut. The problem is only happening on slabs less than two years old. We were told by stain and sealer manufacturers that the problem is due to the addition of plasticizers to the concrete or that the portland cement in the concrete was replaced with as much as 20% fly ash. Do you have any test data to substantiate this, and could the use of sodium silicates and lithium hardeners also have negative effects on the development of stain color?

Answer:

I have no hard evidence to support that stains don't take due to the addition of plasticizers to the concrete, but evidence does exist to support that a high fly ash content will limit stains from developing their full color. If you consider that admixtures such as plasticizers and water reducers have been used for decades in concrete, I doubt a strong case can be made that these are causing the problems. Plasticizers are used in such small amounts in comparison to the overall volume of concrete that it would take a mix design with massive amounts of these chemicals to cause an issue. Chemical admixtures used in excess may cause a reduction in color development, but the chemistry and historical track record do not support outright color rejection.

In terms of fly ash, this can cause a problem, but only if it is used in high doses—in excess of 20%. Fly ash will increase durability and concrete density and consume free lime, thus "stealing" the raw materials that stains need to produce color. (Acid stains need to be able to penetrate into the concrete to get at the free lime for a reaction to occur.) I always recommend pure cement mix designs or the use of less than 10% fly ash if staining is to be performed.

The other factor to consider is that in the last two years, the green movement has come into its own, promoting the use of all kinds of eco-friendly methods and materials. Using fly ash is considered to be an eco-friendly manufacturing method and increases the LEED (Leadership in Energy and Environmental Design) value of concrete. It also reduces material costs at the ready-mix facility. Those forces have made it politically correct to use more fly ash, without much consideration for the effects down steam.

Keep in mind that it is your responsibility, as the stain applicator, to make sure the slab is ready to accept the stain, whether you poured the concrete or not. Take the time to look at the batch tickets to see what's in the concrete mix, and always do a sample prior to stain installation to ensure that the color is correct.

We also need to look at why and how stains work to understand why problems might be occurring. I will throw out a few ideas and opinions based on what I have observed in regard to acid stains over the last few years.

• Advancements in placement and finishing techniques create a tighter, denser concrete, which makes it harder for acid stains to take. Increased surface preparation is needed.
• "Closing" the surface vs. "opening" the surface by grinding. I am seeing more and more applicators using diamonds to try and open up the concrete, but in actuality they are tightening the surface even more.
• Total lack of surface preparation—just apply the stain and hope for the best. There are not many situations in which concrete does not need some type of surface preparation prior to staining.

To answer the second part of your question regarding the use of chemical hardeners and densifiers, they will cause color reduction if applied prior to the stain. No matter whether the hardener is a sodium-, lithium- or potassium-based product, these materials create a harder and denser surface through chemical means. Anything that will retard stains from getting into the concrete will have an effect on stain color. That said, the type of concrete and surface profile will play a big part as well. The tighter the surface, the more effect a chemical hardener will have in terms of retarding stain color. The only way to determine the effect these chemicals will have on color development is to prepare a prejob sample.

As a side note, the use of concrete dyes eliminates most of the issues discussed above. Dyes are significantly smaller in particle size than acid stains, penetrate into much harder and denser concrete than stains, and don't need any free lime to develop their color. The trend in the industry is moving toward dyes and away from stains based on ease of use, time and labor savings, and elimination of potential color problems.

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