Concrete sealant technology

The number one question posed to me at the home show was regarding sealants. everyone wants to know if concrete is porous. It is very obvious that the silestone people have been doing a great job playing up the porosity of other materials and trying to make them sound unsafe.

Raw concrete, without a sealant, is inherantly porous. But just like granite, it is just a matter of the sealant that you use. Many different methods seal the surface. We use a newer hybrid technology and there are some critical reasons why.

Absorbtion is the first thing that must be sealed against. Raw concrete can absorb moisture by the fact that there are small pores on the surface. Filling these pores is critical, and once it happens, the health concerns which have been beaten into our heads by the ad campaigns of artificial surface manufaturers are no longer valid. This has been done many ways in the past, most notibly by the use of acrylic based sealers, many of which can be found on the sehlf at home depot (price $10-$60/gallon). however, these sealers have a tendancy to wear off and require re-sealing at regular intervals (one or twice per year). if you get too much of these sealers on the piece, then they can actually form a chaulky colored shell that can be chipped off.

Acrylics also dont protect against the second issue that plagues concrete and natural stone....ACID. acids are found in a lot of the more fun things in life...like lemons, wine, vinegar, etc. This is historically where waxes come in. People that dont like the acrylics, love wax. It protects briefly against the acids by acting as a sacrificial layer. the acid eats it before it gets to the concrete. but it gives you just enough time to clean the offending material off before it gets to the surface below. hence, wax needs to be replaced fairly often. its a good thing that they are less expensive. However, you have to get a food-safe version of these waxes...car wax has petroleum in it.

But the question is, what are the acids attacking? concrete is based upon a crystaline structure of Calcium Silicate Hydrate (CSH) between pieces of natural crystaline stone. the acids primarily attack a byproduct of the cement hydration process, namely Calcium Hydroxide. This byproduct is basic (similar to baking soda) and reacts quickly with acids. As soon as acids react with this chemical, it leaves the surface free to be stained. There are two ways to combat this reaction. One is to protect the Calcium Hydroxide, which is what the waxes do, and one is to remove it.

The sealer that we typically use is based upon Lithium Silicate. Lithium Silicate prrovides a ready source of silicate Ions, which react with the Calcuim Hydroxide to form more of our favorite crystal, CSH. The beauty of this reaction is threefold:

First, it removes the calcium hydrate from the surface, eliminating the targets for these acids.
Secondly, The additional crystals means additional strength and sealing of pores.
Thirdly, it continues to react for years. in fact, forever. Concrete is a curing reaction, which slows over time but never stops, which means that the production of Calcium Hydroxide never stops. those free silicate Ions remain, constantly forming more CSH.

If you dont get all the chemistry, then dont worry about it. Just know that it pays to have a guy with a chemical engineering degree building and sealing your concrete.