The Truth Behind the Densifier Façade
A perspective by Jeff Lawrence, President, Polished Concrete Company
What is a Densifier and Why is it Important?
With the development of polished concrete we have seen a number of new products introduced to assist contractors with the polishing process. The most significant development in concrete polishing has been the introduction of concrete densifiers which aid in the polishing process. There are a large number of concrete densifiers available on the market today, and choosing the right densifier for your project can seem like a very daunting task when considering all of the available options. Here we will try to shed some light on the subject of densifiers, and provide you with some information in determining the best product to use on your floor.
First, there are three different types of densifiers on the market with regard to the chemical composition. Sodium silicate, lithium silicate, and potassium silicate are the different compounds available and the densifier is typically comprised of one of these types of silicates and water. The higher the solids content of the densifier, the more silicate it contains on a per gallon basis. Most densifiers range in solids content from as little as 10% to as high as 40% depending on the product. The higher the solids content is the more reactive material the densifier contains.
The densifier is applied to the floor to create a denser surface structure and increase the longevity of the polish on the floor. The primary way in which they do this is by increasing the abrasion resistance of the floor making the surface less susceptible to scratching, scuffing, and dusting. Dusting is the process of the concrete surface deteriorating and creating fine dust particles on the surface as it wears. This is the most common and most detrimental issue with polished concrete in that it wears the polished surface away and exposes the unpolished surface below. The primary purpose for applying a densifier to a polished concrete surface is to prevent this from occurring.
How Do Densifiers Work?
All densifiers work in the same way in terms of the chemical reaction that occurs and the final composition of the surface. You will notice that all densifiers are a type of silicate compound. This is because the silicate is what actually changes the surface characteristics of the concrete, the sodium, lithium, and potassium just acts as a transport agent for the silicate. The densifier reacts with the free calcium carbonate in the concrete to create a crystalline structure in the surface pores of the concrete referred to as calcium silicate hydrate or C-S-H. The calcium silicate structure in the pores of the concrete is what increases the surface density and thereby the strength of the concrete surface. The byproducts of this reaction depending on which type of densifier you use are sodium carbonate, lithium carbonate, or potassium carbonate. None of which have any impact on the surface characteristics of the concrete.
With sodium silicates you apply them by flooding the surface with the densifier at a rate of approx. 200²ft./gallon and agitating the product with a broom or low speed scrubbing machine. You then wait, allowing the densifier to completely react with the surface of the concrete ensuring that the surface remains wet with densifier for a minimum of 30 minutes, and then thoroughly rinse the surface to get the excess product off. Maintaining a wet surface ensures that the concrete has been “densified to rejection”, meaning it will not absorb any additional product. As the more porous areas of the slab dry out or absorb completely, you add additional densifier to maintain a wet surface for the specified period. This ensures that even the most porous and problematic areas of the slab have been densified to the point of rejection. It also promotes complete reactivity of the free calcium in the slab. This is extremely important as those areas are most likely to deteriorate and create dull areas in your finished product.
Lithium and potassium silicates are applied in a different fashion. They are sprayed on the surface at a rate of approx. 500²ft./gallon or higher using a low pressure sprayer and are left on the surface with no rinsing required. These products are generally quite a bit more expensive on a per gallon basis, but when applied at the rates suggested by manufacturers they tend to be comparable in price with the sodium silicates on a price per square foot basis.
Which is better?
This is a question that’s been asked of me by a number of architects, GC’s, and industry professionals. Lithium and potassium densifiers are substantially easier to apply and it’s no surprise that most polishing contractors push these types over the sodium silicates. There is no rinsing or surface agitation required and therefore it creates far less work for the polishing contractor resulting in significantly lower labor costs. Lithium and potassium densifiers advertise having increased penetration over the sodium silicates and claim to provide more evenly distributed surface densification.
The concern with these types of densifiers is that based on the manufacturers recommended application procedures there is a high likelihood that the surface has not been treated to the point of rejection. The result is that there may still be a large amount of unreacted calcium in the surface resulting in a reduction in durability (particularly in the most porous areas). The penetration of the densifier is irrelevant to some extent as the polished surface you are attempting to maintain is at the very surface of the concrete. With that in mind, if the top 1/16” wears away, the polished surface is no longer there. This is why it is so critical that the wear surface of the concrete has complete reactivity with the densifier being applied and depth of penetration is of very little concern.
All densifiers create the same crystalline structure in the pores of the concrete which is where the increased durability comes from. With that being said, the primary difference comes from the application procedure used by the contractor. A flood coat application could be utilized with any of the densifiers discussed and would provide comparable results. Flooding the surface to the point of rejection has provided the most significant increases in abrasion resistance in our experience and has been the only method that performs consistently in every application. However, not all products have specified this type of application process. For this reason we recommend only using densifiers that require a flood coat type application which is most commonly specified by sodium silicate manufacturers.
How do I decide on a contractor?
A concrete floor is a significant investment. The best way to ensure you get a quality contractor is to do your research and ask questions. It’s always a good idea to request a list of completed projects and go see the contractors work in person. With regard to the densifier, inquire about the contractors’ installation process and verify that they will be flood coating the surface to the point of rejection. This may end up costing a little more in the end but the benefits will far outweigh the marginal cost difference. There is also “hybrid” or “blended” densifiers on the market today that consist of multiple types of silicates and siliconates. You should always verify that the densifier contains a true silicate of some type. SIliconates are often advertised as densifiers, but they do not provide the same hardening characteristics as silicates do and are more of a sealer than a true hardener. A good contractor should know the difference.
For more information or if you have any remaining questions regarding the spec, please feel free to contact us.