Contractors often apply chemical stains on concrete earlier than manufacturers specify. Some contractors even apply them the same day the concrete is placed, just after initial set occurs. However, most manufacturers recommend that the concrete cures for 28 days before staining is attempted. For the contractor, there are several issues to sort out when deciding when it is the right time to stain.
As portland cement hydrates, 20% to 25% of it becomes calcium hydroxide (Ca(OH)2), a somewhat water-soluble compound residing in capillaries created when excess mix water migrates to the surface of freshly placed concrete. An average mix design converts between 100 to 140 pounds of portland cement to calcium hydroxide per cubic yard of concrete. This process starts shortly after hydration and is the major component responsible for concrete's high pH. As mentioned, it's soluble in water and can be transported to the surface of a slab as water moves through it. Because it's also a very reactive material, it combines with atmospheric carbon dioxide (CO2) to produce calcium carbonates; pozzolans, such as fly ash and metakaolin, to produce calcium silicates (the material that gives concrete its strength); surface hardeners, such as sodium silicate, to produce calcium silicates; and chemical stains to yield metallic hydroxides. The metallic hydroxides are the desired permanent colors that make chemical stain applications so attractive.
There are several factors that affect the color intensity: different portland cements produce different amounts of calcium hydroxide, the water-cement ratio (w/c) of the concrete placed affects both the availability and the amount of calcium hydroxide produced, admixtures can affect the reaction, carbonation (the combination of calcium hydroxide with atmospheric CO2) on the surface of slabs uses up calcium hydroxide that would otherwise be available to chemical stains, how hard a surface is troweled, the application of curing agents and sealers before stains are applied, and the debris left by other trades as they do their work.
Making informed decisions
Manufacturers generally take the position that the safest time to apply chemical stains is after 28 days when most hydration is complete. At this time, moisture conditions in a slab tend to be steady and most of the calcium hydroxide has been produced. It's also the safest time to apply green and blue stains, which are particularly sensitive to moisture. At first they produce the desired colors, but with the continued presence of moisture in a slab, the reaction continues eventually producing dark brown and black colorations that most owners dislike. It's wise to check the relative humidity of the concrete to be safe.
When there are several concrete placements, batched over a period of time, there can be variations in color intensity between placements. This is primarily due to moisture differences between the placements as they cure. But after 28 days, when most of the curing has occurred, moisture levels between placements tend to even out.
Ideally, the best time to apply chemical stains, clean off the residue, and apply sealer is before other trades work on a slab. Sometimes general contractors or owners insist upon completing slab work quickly after concrete is placed and decorative contractors make decisions with this in mind. At this time preparation work is minimal and the problems of paint, drywall taping compound, or shadow affects from storing materials on a slab don't alter the final result. And the cost to prepare an area for stain, which contractors may or may not recover, is minimal.
When chemical stains are applied within a day or so after initial set occurs, the contractors dilute them with water to achieve the same color intensity. Some contractors add as much as 90% water and then build up layer after layer of coloration in order to control the intensity of color and achieve more variation. This provides customers an opportunity to check the results after each application to decide on the look they want.
A final thought
There are many factors to consider with regards to when stain work should begin, including a legal issue. When you decide to install a manufacturer's product under different conditions than their specifications stipulate, they may have no legal responsibility for the outcome.