Blue and green chemical stains, while artistically beautiful, can darken a concrete surface, even turning it black. Chemists hypothesize that the color change could be caused by such things as concrete's moist environment, the pH level in the concrete, and the effect of copper in acid stain. There are jobsite practices that can be used to alleviate this problem: careful grading, preparing a well-drained subgrade, using a moisture barrier, and assuring that the concrete is well cured. Still, Larry Good, Specialty Concrete Products, Columbia, S.C., cautions, “When using blue or green chemical stains, tell the customer to expect some darkening and consider putting it in the contract.”
Acid stains work by reacting with the calcium hydroxide in concrete. The acid, however, is not what is reacting, but rather the metallic salts in a water-based acidic solution. The reaction achieves a mottled look and becomes a permanent part of the concrete surface. Most blue and green acid stains are copper-based compounds which end up creating copper hydroxide in the surface layer of the concrete. Iron-based compounds yield rusty colors, manganese compounds produce blacks and dark browns, and chromium is used to modify the affect of several colors.
The acid in the stain works to open the concrete surface and allows the metallic salts to react with the calcium hydroxide. In concrete pH is measured on a scale of 1 (very acidic) to 14 (very alkaline) with 7 being neutral. Concrete is on the upper end of the pH scale at 13 or 14, making it a very alkaline material. To preserve green and blue colors, to keep the copper in the hydroxide form, you need to maintain that alkaline environment. Other variables that affect staining include cement properties, the weather that day, and the admixtures in the mix design.
Why color changes
Opinions vary about the cause of the color change of blue and green acid stains. It may be that the copper hydroxide that creates the green and blue colors can further oxidize to form dark copper oxides—changing form from a hydroxide state to an oxide. Another theory is that the darkening may be caused by a fungus. Good doesn't subscribe to that idea saying that copper is toxic to most molds and is used as an anti-fungal ingredient to kill mold, thus it could not encourage mold growth in concrete. He sees the problem as free moisture in concrete and hypothesizes that controlling free moisture will control the darkening.
“A high water-cement ratio can leave free water in the slab even after the slab is fully hydrated. These slabs are more porous and susceptible to water intrusion from the ground, rainwater, or even high humidity causing the blue or green acid stains to darken. The reaction sometimes becomes more apparent once the slab is sealed,” says Good, who reached these conclusions through field observation and lab experimentation. One test began by casting and curing small concrete samples and then applying blue acid stain to the surface. Good sealed one sample only on the top and another was sealed on all its surfaces. The two pieces were left standing, but not submerged in water. The sample sealed on all surfaces lasted a year before it darkened, while the sample sealed only on the top darkened in four months “Make the intrusion of water difficult; address the drainage for slabs on grade by placing a layer of gravel, a layer of 6 mil plastic, and then another layer of 3/8-inch stone. That system allows a place directly under the slab for bleed water to exit and provides a moisture barrier to prevent ground-water from moving upward through the slab,” says Good.
Mix design considerations
Another way to prevent darkening is to avoid calcium chloride. “The chloride ion is the problem,” says Gabriel Ojeda, president, Fritz-Pak, Dallas. “The chloride ion in salty seawater rusts boats.” Using nonchloride accelerators in a mix design avoids the chloride ion and helps to diminish color change.
You may also consider a lower water-cement ratio to help prevent the darkening. However, that presents a problem because the acid stain needs the canals left after initial set to penetrate the concrete for the reaction to occur.
Bob Harris, Decorative Concrete Institute, Temple, Ga., says that warmer temperatures and dampness promote the reactive phenomena. Harris says that a correction can be made, assuming there is no sealer on the slab, by stripping away the darkened color with an acid wash and taking care not to remove too much material. “But, the phenomenon is not entirely predictable. I installed a green acid-stained sidewalk 10 feet from the edge of water and expected a color change. It hasn't occurred,” says Harris. The green was lightly applied and completely neutralized, scrubbing the slab more than normal before it was sealed with a solvent-based acrylic. Also make sure to use a breathable sealer. Water trapped in the slab is clearly deleterious to acid stain.
Is a polymer-modified overlay a safer base for blue and green acid stains? Overlays are cementitious breathable products and placed onto concrete on grade would be prone to similar water intrusion. Placing an overlay over a nonconcrete surface such as wood has a greater measure of safety. Both concrete and overlay material indoors are usually subject to less weather and may be a safer place to use blue and green stains.
“Be cautious with customers, contractors, and business owners,” says Wes Vollmer, Alternative Finishes, San Antonio. Vollmer conducts a calcium-chloride test on all commercial jobs to determine how much moisture is left in the concrete before staining. If he doesn't conduct a formal test, he tapes a 12-inch square of clear plastic onto the concrete. If the plastic shows dampness, he doesn't apply stain. Vollmer recommends the standard minimum of 28 days before application of an acid stain to a slab.
Contractors turn to topical stains for a blue or green color. “If a job calls for an acid stain and a portion is green or blue, I discuss the use of water-based stains with the owner,” says Gerald Taylor, Images in Concrete, El Dorado, Ark.
Stain manufacturer Kemiko has customers who stain their own floors. “All Kemiko's printed material advises homeowners to stain just before they affix the base boards-advocating months for the concrete to cure,” says Kemiko's Barbara Sargeant.
The copper hydroxide state is considered metastable by chemists. If you take copper hydroxide into the laboratory and warm it up, it decomposes immediately into black copper oxide. The exact reaction mechanism of changing from blue to black is unknown. We invite you to write to us with your thoughts on this subject.