The fact is that concrete is a very porous substance that readily absorbs liquids into its pores and capillaries allowing it to stain very easily. Some liquids such as acids literally dissolve the concrete while others like oils leave unsightly stains which are very difficult to remove.
Another major enemy of concrete is freezing water which expands inside the pores of the concrete collapsing the walls of the pores which then forms bigger holes which eventually leads to cracking and whole sections of concrete popping out which is known as spalling. The threat of freezing water or freeze-thaw damage is made even worse when road salts are added into the equation accelerating the concrete damage.
What can I do to protect my concrete surfaces?
To protect concrete from these problems it is necessary to apply a concrete sealer which leads to the next question which is; what type of sealers do I need to apply to my concrete?
There are many different chemical types of concrete sealers including:
The Concrete Network is a good resource to find suppliers for all of these different types of sealers.
These can be split into two basic categories of film formers (epoxies, polyurethanes, polyesters, waxes, and acrylics) and chemically reactive sealers or penetrants (silicates, silanes, siloxanes). Some of these products can increase the longevity of your concrete by years.
Firstly, let’s start with the film formers, which all have various advantages and disadvantages. These all form a protective barrier on the surface of the concrete and the main disadvantage with that is that they can easily be damaged causing peeling, cracking and scraping of the surface film. Wax based products are the cheapest, very short lived and are only intended to be temporary products that are easily removed. Acrylics are the next cheapest type of sealer, they provide protection against water but wear away quickly and require frequent re-application. Polyurethanes are nearly twice as thick as acrylics, they provide excellent resistance to abrasion and chemicals. However, most polyurethanes are moisture intolerant until they cure which means if any water is present on the surface when the sealer is applied, a chemical reaction will occur that results in foaming. Epoxies and polyesters also produce a hard, long-wearing, abrasion-resistant finish. They bond well to concrete and cement-based overlays and are available in clear or pigmented colors. However, epoxies have a tendency to yellow with UV exposure, so they generally are limited to interior applications.
Secondly, let’s look at the penetrating reactive type of sealers. Silanes and siloxanes are impregnating sealants that penetrate into the pores of the concrete and then chemically react with themselves in a polymerization reaction to form a gigantic molecule. This results in the concrete becoming waterproof and stain resistant. These products are UV stable(non-yellowing) and are highly effective stain repellants that can be used in decorative, aggregate or stamped concrete. They are available in two types which either leave a natural finish or a wet look. Silicate based sealers have been used extensively for over 50 years with very successful results. These products actually chemically react with the free lime in the concrete to form a new chemical compound which is calcium silicate. This new chemical compound makes the concrete physically harder and stronger and is commonly referred to as a densifier. It increases the abrasion and crush resistance of the concrete and also reduces salt penetration reducing freeze-thaw damage and they stop efflorescence. In indoor situations, they can be used to cure wet and damp basements and reduce radon penetration which is known by the EPA to cause lung cancer. Silicate based sealers are permanent and last for the life of the concrete, they are available in two different concentrations, a regular strength for new concrete which is less than 10 years old with no signs of damage and a more concentrated version for older, damaged or more porous concrete.
To summarize, the best types of sealers to strengthen and harden concrete and to protect it from salt damage are the silicates and the most effective and reliable sealers for stain protection and water resistance are the silanes and/or siloxanes.