Staining concrete is one of the most popular applications for transforming concrete slabs. Often referred to as colored concrete, homeowners, designers and builders are drawn to stained concrete because of the unique outcome that can be achieved combining colors, application techniques, etc. on cement flooring and other substrates. The results are limited only by the creativity of those involved in the stained concrete process. Because of concrete’s porous qualities and neutral tone, it is the perfect blank canvas for topically applied color. Using acid-based chemical stains, decorative concrete contractors have been able to achieve rich, earth-toned color schemes resembling natural stone, marble, wood, or even leather, giving a completely custom look to cement floors, concrete driveways, patios, walkways, pool decks, concrete walls and more.

Most acid stains are a mixture of water, hydrochloric acid, and acid-soluble metallic salts. They work by penetrating the surface and reacting chemically with the hydrated lime (calcium hydroxide) in the concrete. The acid in the stain lightly etches the surface, allowing the metallic salts to penetrate more easily. Once the stain reacts, it becomes a permanent part of the concrete and won’t fade, chip off, or peel away. Like stains for wood, acid-based stains are translucent and the color they produce will vary depending on the color and condition of the substrate they are applied to. Each concrete slab will accept the stain in varying degrees of intensity, creating natural color variations that bring character and distinction to each project. What acid stains don’t offer is a broad color selection. You’ll mostly find them in a limited array of subtle earth tones, such as tans, browns, terra cottas, and soft blue-greens. But newer products on the market—such as water-based penetrating stains and water- and solvent-based concrete dyes—are greatly expanding the artist’s palette with colors ranging from soft pastels to vivid reds, oranges, yellows, and purples.
Chemical stains can be applied to new or old, plain or colored concrete surfaces. Although they are often called acid stains, acid isn’t the ingredient that colors the concrete. Metallic salts in an acidic, water-based solution react with hydrated lime (calcium hydroxide) in hardened concrete to yield insoluble, colored compounds that become a permanent part of the concrete. Several companies manufacture chemical stains that are variations of three basic color groups: black, brown, and blue-green.

Malay Tan Cola Stain Black Stain Aqua Blue Stain English Red Stain Golden Wheat Stain Green Lawn Stain Vintage Umber Stain

The acid in chemical stains opens the top surface of the concrete, allowing metallic salts to reach the free lime deposits. Water from the stain solution then fuels the reaction, usually for about a month after the stain has been applied. Other factors that affect the outcome include:

Cement properties and amount
Admixtures used
Type of aggregate used
Concrete finishing methods
Concrete age and moisture content when stain is applied
Weather conditions when stain is applied
Efflorescence
In general, cements that produce larger amounts of calcium hydroxide during hydration will show more stain color, and higher cement contents pro-duce more intense colors.

Air-entraining or water-reducing admixtures don’t pose a problem. However, calcium-chloride accelerators can cause very mottled, darkened areas, and for this reason aren’t recommended.

Nonchloride accelerators don’t cause this mottling effect.

If they’re near the surface, calcium- based aggregates, such as lime-stone, take stain readily and deepen the color of the concrete above them. Siliceous aggregates, such as gravel, don’t react with the stain.

Open finishes achieved by floating followed by minimal troweling take more stain and produce denser colors than do hard-troweled surfaces. However, open finishes lose color faster because the concrete wears away. Because of this, Gary Jones, president of CP Concrete Systems in Burnaby, British Columbia, Canada, prefers staining hard-troweled surfaces because the stain color lasts longer. "Colors on troweled surfaces also look richer than those on floated surfaces," says Jones. "But you have to sand the surface or use a higher acid concentration to ensure adequate stain penetration."

Slabs placed in wet weather result in a richer stain color if the concrete is stained soon after it’s placed. However, wet slabs are more likely to effloresce, lightening the color and causing a more mottled effect in areas where the stain doesn’t take because efflorescing salts hinder penetration. On sunny days, the concrete can become hot and dry, and the stains won’t penetrate as deeply into the concrete.

The continued presence of water will cause the reaction to continue for a long time, and concrete stained blue-green will gradually turn brown or even black. Initially, this provides nice variation to the appearance, but eventually, nearly all the blue-green color may change to brown and black. Because of the possible color shifts, some manufacturers advise against using these colors for exterior concrete. Interior slabs must be placed on a well-drained base or sub-grade and have a low moisture content before stain is applied. Jones believes the brown-colored "flowering" of blue-green stains is caused by oxidation of a copper component resulting from water vapor passing through the slab. Others believe the brown color is caused by a fungus, which can be eliminated by using sealers containing a fungicide.