Sometimes the Fly ash present in the concrete substrate can cause an epoxy coating not to bond to the concrete substrate. The 100% solids epoxy coatings require a primer to be applied before applying the coating. The primer can be water based or solvent based epoxy primers. It has been observed that water based primers are rejected by the flyash concrete.
Concrete is typically composed of gravel, sand, water, and Portland cement. Fly ash, a by-product of coal-fired electric generating plants that might otherwise end up in a landfill, can be substituted for more than 25 percent of the Portland cement portion of the concrete mix.
Two types of fly ash are available: Class C, which is typically light or tan colored and is produced from burning lignite or sub-bituminous coal; and Class F, which is dark gray and is produced from burning anthracite or bituminous coal. Fly ash consists mostly of silica, alumina and iron; its tiny, sphere-shaped particles fill small voids and produce a denser concrete. So using fly ash concrete offers the benefit of a stronger, more durable product while reducing the environmental impact of concrete manufacturing and providing a beneficial re-use for fly ash. Using fly ash concrete can help a project achieve LEED credits for recycled content.
Why epoxy coatings fail to bond Fly ash concrete?
Because coal and oil are essentially the same product—they’re just different ages— it is possible that the fly ash might impart oily characteristics to the concrete.
And this we know to be true: A surface with an oily residue will always present a challenge to water-based coatings. Therefore, a solvent based epoxy primer is recommended to be used as a primer before apply applying 100% solids epoxy coatings. The use of pozzolanic metakaolin which is hydrophilic, reactive and lighter color instead of the flyash should have no adhesion problems with coatings.