Acid stain technology is at least 120 years old, exceeded only by integral color as a concrete coloring medium. They were formulated originally by architectural precast companies who submerged their precast items in stain for as long as two weeks. The desirability of acid stains stems from the infinite variability of color possibilities and the fact that results are unpredictable. The chemistry of concrete and the conditions surrounding its placement have much to do with the outcome.
Acid stains are only one way to color hardened concrete, but it is the most permanent coloring system, surviving abrasive wear and resisting UV light damage. Other coloring methods include water-based stains, which leave mineral oxide color deposits in the pores of a slab without chemical reactions, and staining with dyes that penetrate the surface, yielding translucent colored finishes.
How stains work
Little has changed in the chemistry of acid stain formulations. Composed primarily of metallic salt compounds, water, and muriatic acid, they react with concrete in a complicated process that depends on precise conditions:
- The product you buy contains metallic salt compounds held in water solution by the addition of just the correct amount of acid to provide the required low pH environment.
- When the solution is placed on the concrete, the acid reacts with calcium carbonate and calcium hydroxide (CH) near the surface of the slab. This reaction causes the stain solution to move toward a neutral pH. The strength of the acid must be just right: If it's too strong the reaction won't go forward and if it's too weak the metallic hydroxides or oxides will settle to the bottom of the container.
- While the acid is reacting with calcium compounds in the concrete, the water and metallic salt solution penetrates into the surface of the concrete.
- With the acid neutralized, the pH of the stain solution rises, causing the metallic hydroxide or oxides to precipitate out of the solution and become a solid.
- At the same time, the metallic oxides react with the CH in the concrete to bring out the finished color.
As noted, conditions for acid staining must be precise. Concrete must have enough CH near the surface to neutralize acid and react with the metallic salts, and the surface finish must allow the stains to penetrate.
Determining mix ingredients
When specifying concrete for a stain application, there are several issues worthy of consideration. Concrete mixes best suited for acid staining have a high pH, a good supply of CH, and abundant, large capillaries to allow stain to penetrate through the surface. Any ingredient that significantly reduces these conditions should be avoided.
The type of cement used matters because different cements develop different concentrations of CH. Stan Stratton, technical director for L.M. Scofield, Douglasville, Ga., says that generally Type II and Type V cements with higher levels of calcium silicates produce more CH. Higher levels of CH increase pH levels in the concrete.
Today, good concrete mixes include fly ash, sometimes in high percentages. The type of fly ash, and the amount replacing portland cement, is an important consideration. Kevin MacDonald, the vice president of engineering services for Cemstone, Mendota Heights, Minn., says that some fly ash includes calcium silicate hydrates that elevate pH. But fly ash also reacts with some CH, reducing the amount available for the reaction with acid stains.
The amount of water in a mix is also important because water creates capillaries where CH grows and develops. MacDonald recommends water-cement ratios in the range of 0.45 to 0.55, which equates to 250 to 260 pounds of water in mixes that contain 520 to 600 pounds of cementitious material.
The best slab accepts stain uniformly, which requires an even distribution of capillaries throughout the concrete. MacDonald says that adding water-reducing admixtures can help create a condition where uniform bleeding occurs. He also says he doesn't worry about adding superplasticizers to concrete in order to increase the placing slump.
Do's and don'ts
Generally speaking, acid staining new concrete installations gets a better reaction than old ones. Stratton says older concrete surfaces have more carbonation that can lead to an imbalance between the acid in the stain and the high level of pH needed in the concrete to bring the metallic salts in stains out of solution. If you see a lot of fizzing when you apply stain, it's the reaction between the acid in the stain and the carbonation (calcium carbonate) on the surface. If no fizzing occurs, then no carbonation is present (except with carbonate aggregates). You might think that adding more acid to the stain would help but Stratton cautions against this practice because it further defeats the reaction. For the same reason, do not acid etch a concrete surface before a stain application—another unhelpful practice because it depletes the reactive CH component.
Finally, don't hard-trowel slabs intended for acid etch stain finishes because it makes the surface less permeable. The goal is to construct floors that have good wear resistance and at the same time have good capillary structures for stain reactions.