How Do Concrete Stains Work?

That depends in large part on the type of stain you're using. Generally, there are two categories of concrete stains--reactive and nonreactive--according to Chris Sullivan, vice president of sales and marketing for ChemSystems Inc. In the first category are chemical stains--water-based acidic solutions containing metallic salts that react with the concrete's lime content. Once the chemical reaction takes place, the stain forms a permanent bond with the concrete and won't chip off or peel away. 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.

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 show more stain color; higher cement contents yield more intense colors.

In the nonreactive category are water-based acrylic stains that don't rely on a chemical reaction to impart color. Instead, they are formulated to penetrate the concrete surface and deposit their pigment particles in the open pores. Sullivan says that nonreactive stains have grown in popularity over the past few years because they come in a much broader palette of colors than acid stains and are easier to apply. The downside: They won't produce the same variegated, translucent color tones characteristic of acid stains. The color effects tend to be more opaque and more uniform.

To learn more, read What Is a True Concrete Stain? and The Concrete Network's Shop Smart Guide to Buying Concrete Stains and Dyes.

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