About Acid Stain

Acid Stained floors are growing in popularity.  Many people are looking to stained floors as an alternative to carpet, tile and wood.  The information below is an introduction to Acid Stained floors.

Acid Stains are not a paint or coating or a sealer.  Acid Stained Concrete is a coloring process involving a chemical reaction on a cementitious material.  A solution made with water, acid and inorganic salts reacts with minerals already present in the concrete (All concrete has calcium hydroxide as a biproduct. This is slaked limed. This picks up carbon dioxide from the air and becomes calcium bicarbonate. This is effluorescence.  These are the chemicals that the stain reacts with).  Acid stains are made from hydrochloric acid, wetting agents and metallic ions. When this solution is placed on concrete it colors the concrete by chemically combining the metallic ions with the particles in the concrete to form oxides; the result of this reaction is color.  Chemical stains can be applied to new or old, plain or colored concrete surfaces. Although they are often called acid stains, acid is not 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.  There are many manufactures of Acid Stain and most produce stain in 8 colors that are variations of three basic color groups: black, brown, and blue-green.  The basic 8 colors are:

Acid Stains give concrete a mottled, variegated, marble-like look.  Never expect Acid Stain to be uniform or have an even tone, you will get different reactions from slab to slab, and even on the same job you may see different coloration patterns. Variations of colors and mottling are to be expected and enjoyed.  It is the random mix of tones and shades that gives an acid stained floor it's unique beauty.

Some stain manufactures will use adjectives such as Vintage or Antique to describe their version.

How Stain works

Acid stains are made from hydrochloric acid, wetting agents and metallic ions. When this solution is placed on concrete it colors the concrete by chemically combining the metallic ions with the particles in the concrete to form oxides. The finish won't fade or chip,it is permanent. The acid in chemical stains opens the top surface of the concrete (this may be referred to as "etched"), allowing metallic salts in the mixture to reach the free lime in the concrete. Water from the stain solution then fuels the reaction between the Lime and the Metalic Salts.  Stains will normally be applied to a surface for 4 or more hours.  However, the surface will continue to develop its patina - an appearance of something grown beautiful especially with age or use; established character for several more hours.

Other factors that affect the outcome include:

• Cement properties and Mix Design 
• 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 produce more intense colors.  If they are near the surface, calcium-based aggregates, such as lime-stone, take stain readily and deepen the color of the concrete above them. Solid aggregates, such as gravel, dont react with the stain.