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 time (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 used, Admixtures and type of aggregate used.
Concrete finishing methods, concrete age, and moisture content
Conditions when stain is applied, weather
Efflorescence.
In general, cements that produce larger amounts of calcium hydroxide during hydration will show more stain color. Higher cement contents produce 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. Non-chloride accelerators don’t cause this mottling effect.
If they’re near the surface, calcium-based aggregates, such as limestone, take stain readily and deepen the color of the concrete above them. Siliceous aggregates, such as gravel, do not 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. Colors on troweled surfaces also look richer than those on floated surfaces. “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. Application/reaction should last at least 4 to 6 hours.
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 or even black. Because of the possible color shifts, we advise against using these colors for exterior concrete or in direct sunlight.
Equipment Needed
Acid-etch stain finishes don’t require much equipment for application. For surfaces preparation, you may need a buffing machine equipped with sanding pads. Power-washing equipment is also useful. Any equipment that comes in contact with the staining liquid, such as sprayers, must resist hydrochloric acid. Brushes used to apply or spread the stain should have acid-resistant, uncolored bristles.
Workers should wear the proper safety equipment including acid-proof gloves, goggles, boots, and facemasks to filter acid vapors. Good quality wet vacuums are highly recommended for cleanup. Steel golf spikes are also recommended because shoe marks will show in the finished stain.
When to Stain
The work of other trades can affect staining results. For instance, drywall dust on a surface to be stained will react with the stain, coloring the surface differently wherever the dust is present. Spilled grease, oil and other lime-based materials such as paint or caulk will block the chemical process and produce unwanted color variations. A good relationship with the owner or project manager helps to prevent such mishaps. Project management should ensure that the floor isn’t damaged before or after staining and keep trades away from areas where preparation and staining are in progress.
If an installer other than the concrete contractor applies the chemical stain, the contractor and installer must agree on the following:
Pour schedule: To get a rich color, installers should wait a minimum of 21 days and preferably 28 days before stain application.
Jointing method: For slabs requiring sawed control joints or pattern lines, dust or sawing slurry must be removed before any of it bonds to the slab. Otherwise, stain color
at the joints will differ from the color of the rest of the surface. It is preferred to apply saw cuts after stain application and neutralization.
Finishing Process: Although some applicators like to work on a hard troweled surface, some prefer finishes that more readily accept the stain. Uniform finishing throughout the job helps to ensure more uniform stain penetration.
Curing Method: Do not use plastic sheeting, liquid membranes, or wet curing methods because they can trap moisture and cause efflorescence.
It’s often best to install chemical-stain finishes and a first coat of sealer, and then protect the floor surface with a cover before allowing other trades on the floor. Unfortunately, any protective material will affect the final appearance of the stained floor to some degree, usually by leaving an outline of its shape on the floor and by darkening the surface a bit. Don’t use any cover material that doesn’t allow water vapor to escape. Breathable cloth traps are perhaps the best covers for preventing discoloration caused by the work of other trades.
Surface Preparation
It has been said, “Every step successfully completed leads to the successful completion of the next step.”This is certainly the case with chemical- stain work; proper surface preparation is a vital step.
First throw a little water on the surface in several locations to see if the concrete absorbs water. If it doesn’t, curing agents or sealers may be blocking the entry of stains and must be removed. Also remove any grease and oil, paint drops, taping compounds, caulk, or other surface contaminants.
Scraping, applying solvent or stripping agents, sanding, and grinding are the principal removal methods. To pick up contaminants more easily, use stripping agents that will mix with water. If you choose to grind the surface, avoid making grinding marks that will reflect through the colored finish by using either a cup grinding head with a fine-grit (diamond or black abrasive) or diamond pad. Use a light touch, laying the cup flat on the concrete and moving it in small-diameter circles until the blemish disappears. Do not shot blast a concrete floor that is to be stained.
If a slab must be patched, use acrylic-modified, low-shrinkage materials that will accept stain. These patches will always show in the finished product, and the owner should be made aware of this.
To open up the surface for stain penetration, many installers prefer to sand floors using floor buffing machines with #60, #80, or #100 paper or screen-mesh sandpaper that allows dust to pass the pad. This process can add its own pleasing effect to the final appearance by accentuating high and low areas on the surface. (More material is removed from high spots, giving them a richer color.)
The final preparation step involves carefully washing the surface with water and detergent. Don’t use acid to clean the surface because it will diminish the effect of the stain. It’s best to scrub with a buffing machine using strip pads (preferably black) and to pick up the
effluent with a good-quality wet vacuum. The surface must be cleaned and free of streak marks, footprints, and all residues. Anything remaining on the surface will affect stain penetration, sawing and pattering.
Decorative cuts and sandblasted patterns achieved with stencils can enhance the appearance of stained surfaces. Timing of these operations, though, depends on the desired effect. When you want the overall stain finish to be as evenly colored as possible, cut lines and patterns after staining is complete. Stains penetrate differently around indentations. If there is to be a color change at a pattern line, cut the line first to form a barrier to stain movement. If sawed joints are to be grouted, complete the staining and sealing before grouting to help prevent grout accumulation on the unprotected stain.
Pattern lines are generally laid out with pencil or chalk. Mark only where you cut, and don’t use chalk colors that are difficult to remove, or adhere lines to the concrete surface using clear fixative sprays. Many tools are available for cutting pattern lines in concrete. Most installers use grinders or hand-held saws with tables that ride against guides. Dry-cutting diamond blades that do minimum damage to the edge of the cut are a good choice. Dust-collection devices that attach to grinders and saw capture almost all of the dust. A 1-1/2 inch extruded aluminum “L” angel, available in most hardware stores, makes a good saw guide.
If you cut patterns before staining, cut them just before cleaning the surface in preparation for the stain. Sawing dust contains free lime that can adhere to the surface, causing distortion. If you cut after staining, do it after the first coat of sealer has been applied.
Dramatic effects also can be achieved by applying stencil to surfaces after staining and then sandblasting to reveal plain or colored concrete in areas not covered by the stencils. These stencils are usually made of plastic materials and have adhesive backings before sandblasting to Improve stencil adhesion.
Stain Application
A new slab must cure for 28 days before work is started. Depending on several variables such as the type of concrete, temperature, etc., stains can be applied in as little as 21 days.
When choosing how to apply stain, keep the following things in mind:
Water drives the chemical-stain reaction. To achieve color consistency, make sure the moisture content of the concrete is roughly the same for every placement colored.
Staining, sealing, and covering finished work before other construction trades return to the area saves on cleanup, achieves a better looking installation, and makes damage repair during the rest of construction easier to handle.
There are many ways to apply stain, with each method, providing a different final appearance. However, there are some general guidelines.
Sprayers are often used to apply stains, but they should be rated for acid and have no metal parts. Acid will quickly destroy metal parts, which can affect the color of the stain.
Use a spray tip with a circular pattern, spraying in a pattern that goes from left to right and then from right to left, with someone scrubbing the stain into the surface using a medium-bristle brush in a circular motion just behind the spray. It’s important to scrub in the stain and not just push it around. An additional spray pass just behind the scrubbing removes brush marks. This method ensures good penetration and minimal marking from either the sprayer or the brush.
Additional water can be use to create different concentrations of stain color. Wetting the concrete before the stain is applied is one way to do this. Following the application with water from a spray bottle is another way.
Stains applied by paintbrush will penetrate well, but care must be taken to minimize brush marks, which are not usually regarded as creative as desirable effects. Whatever stain application method is used, be sure to carefully mask surrounding areas to avoid accidental staining. Acid stains can be difficult, and in some cases, impossible to remove.
The increased interest in chemical-stain finishes is in the direction of more subtle effects. Installers frequently dilute stains with water to produce less intense effects. For example, some contractors often apply the stain with an 80% stain dilution (20% water by volume). In this matter, the contractor can gradually build up color to meet owner expectations. Second and third colors can also be added in the same fashion to create color overlays.
Sometimes a stained overlay is the best solution for concrete surfaces that show damage or has been abused during construction. Commercial overlay materials can be integrally colored, textured, and stained to provide a new range of decorative possibilities. The overlays have high flexural strength and wear resistance. As with everything involving stains, however, it’s wise to create a sample to ensure compatibility of the overlay cement with the stain and to get owner approval for the result.
Clean-up
When most of the chemical reaction is complete, a layer of colored residue with a mildly acidic pH will remain on the surface. This layer must be thoroughly neutralized and cleaned off with a scrubbing machine and Everlast Neutralizer applied liberally so the sealer will bond properly when applied to the surface. Use a white rag after several cleanings to check for residue. Also, the use of a pH tester to test surface of pH is a good way to ensure that conditions are ready for the next step of applying the clear sealer.
