Staining Mixes

For stain jobs, you may or may not have any influence over the base concrete. If you do, a low-shrinkage, durable slab mix is the primary goal and there are really only a few things to be cautious about. In his second book, Guide to Stained Concrete Interior Floors, Bob Harris cautions against using calcium chloride accelerator since it can result in "dark, splotchy areas."

Site Sherry Boyd L.M. Scofield Company Wheat Ridge, CO
Stains react with the cement in the mix.
L.M. Scofield in Douglasville, GA

Fly ash in staining mixes may reduce the color intensity a bit by reducing the amount of calcium hydroxide that the metallic salts in the stain react with. Calcium hydroxide is produced from portland cement. "Fly ash is a real benefit," says Bannister, "because it minimizes the free lime and therefore the efflorescence. There's still plenty of cement to react with the stain. But you do like the more reactive cement so you wouldn't use a leaner mix and superplasticizer, instead go for the straight portland mix and reduce the water so there's not a lot of paste."

"I've had better success with a straight cement mix," says Harris. "I find when I get into pozzolanic materials, fly ash or slag, I get uncontrollable reactions. When you stain that with acid stain it can turn out very splotchy and sporadic, kind of like concrete that contains calcium chloride."

Curing is critical with all concrete (see Curing Concrete). For staining jobs, though, sprayed-on liquid curing compounds should not be used, since they seal the surface, preventing stain from penetrating. Cure with curing blankets, especially important if you've gone with the low water (low shrinkage) mix. Water curing is best, but it can promote efflorescence.

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