Concrete sealers are applied to concrete to protect it from corrosion. They either block the pores in the concrete to reduce absorption of water and salts or form an impermeable layer which prevents such materials from passing.
Extensive research from concrete authorities in North America – American Concrete Institute, Portland Cement Association, National Ready Mix Concrete Association – confirm that almost all damage to concrete is attributable to moisture intrusion: alkali-silica reaction (ASR), chemical intrusion, freeze/thaw, and corrosion of reinforcements.
There are two main sealer categories: topical sealers (film-forming) and penetrating sealers (reactive).
Penetrating sealers should be properly matched with substrate porosity in order to penetrate the surface, clot, and form a barrier. Penetrating sealers generally do not significantly modify substrate appearance. They are chemically reactive and bond with minerals in cement reducing the amount of free silica available for ASR and reduce moisture required to induce ASR. As well as blocking surface moisture they block subterranean moisture and can reduce efflorescence. Lastly, penetrating sealers can harden and increase the density of concrete, increasing its strength as measured in pounds per square inch (PSI). The Lifespan of these sealers can be nearly permanent.
Today, five major compounds are commonly used in concrete sealers:
Acrylic resins form a topical thin film membrane. Acrylics are affordable and generally simple to apply. They are well known to increase perceived visual enhancement and generally provide good UV protection for colored substrates. They can make substrates slick when wet, sometimes requiring the addition of anti-skid materials. Despite being the softest and least lasting of the major sealer categories, price and convenience make acrylics a very popular choice for residential applications such as stamped concrete and exposed aggregate.
Epoxy. Because epoxy coatings are so strong and durable they trap moisture under the service which will cause the epoxy to bubble and crack. The water it is trapping on the other side wants to come in, and eventually it will! Epoxy sealers are also not permeable to water vapor.
Silicates. Silicates are deep penetrating sealers capillary that stop the wicking process in concrete. The silicate reacts with the free lime in concrete and forms and expanded crystalline structure beneath the surface. These crystals fill the pores of the concrete and prevents the penetration of radon and moisture infiltration by capillary action. The barrier stops water and radon.
Silane/Siloxane. Silane/siloxane sealers are clear water-based, low VOC, ready-to-use, breathable penetrating siloxane/silane water repellent for dense concrete and masonry surfaces – brick, cast concrete and stucco. Silane/siloxane sealers disintegrate quickly from traffic and UV-rays, darken the sealed service and can turn yellow, aren’t paintable and can be very slippery when wet.
Polyester Sealers. Polyester sealers are low viscosity, rapid curing, penetrating sealers that anchor lacquers, urethanes, polyesters, vinyl esters and most epoxies to a variety of substrates. Polyester sealers are strong and effective against water but they do not completely stop the intrusion of vapors and gases.
Several of the above listed sealers can be used in combination. Silanes, silicates and siliconates, which must be applied directly to the substrate, can be very receptive to topical coatings and can be used first in a multi-component system, followed by acrylics or epoxy/urethane systems. Silicates can also be applied as a second step with siliconates as the smaller molecules can provide additional pore closure. Siloxanes and silicones are too topical in nature to be reliably used in conjunction with topical coatings.