Densifying Concrete  - Why it is important?

With the development of polished concrete there have been a number of new products introduced to assist contractors in the polishing process.  The most significant development has been the introduction of concrete densifiers.

While there are a large number of concrete densifiers available on the market today, they can be grouped into three (3) types:  Sodium silicate, lithium silicate and potassium silicate.

The densifying process helps seal masonry, concrete and other Portland cement based materials into a mass that is solid, rather than porous.  Its penetrating quality provides deep sealing that is effective for the life of the concrete.  The chemical composition or action of a densifier creates a surface that is easy to clean and maintain.  This is done by hardening and strengthening the concrete to protect it from deterioration which includes scratching, scuffing and dusting.  Dusting is the process where the concrete surface begins to wear and deteriorate by the fine dust particles.  Without a densifier application the polished concrete surface wears 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.

Concrete Densifiers or Hardeners - How Do They 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.  The silicate is what actually changes the surface composition of the concrete and the sodium, lithium and potassium are the transport agents 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.  The calcium silicate structure in the pores of the concrete is what increases the strength of the surface.  The transport agents do not have any impact on the surface characteristics of the concrete.

• Sodium Silicates - are applied by flooding the surface with the densifier at a rate of approx. 200 sq. ft./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, and then thoroughly rinse the surface to remove any excess product.  Rinsing is an important step in the process because if any excess is left on the surface it can lead to ASR (Alkali Silicate Reaction - mass cracking on the concrete surface.)  When applied and rinsed properly this is not a concern.
  
• Lithium & Potassium Silicates - are applied by using a low pressure sprayer at a rate of approx. 500 sq. ft./gallon.  They are left on the surface with no rinsing is 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 to the sodium silicates on a price per square foot basis.

Which is Better?

Lithium and potassium densifiers are substantially easier to apply and it’s no surprise that most polishing contractors use 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.  Lithium and potassium densifiers are advertise as having increased penetration over the sodium silicates and claim to have a more evenly distributed surface densification.  Some lithium and potassium densifiers only increase abrasion resistance of a concrete floor by 20-80% depending on the manufacturer.  The penetration of the densifier is irrelevant to some extent as the polished surface you are attempting to maintain is less than 1/64th of an inch.  While some densifiers penetrate well over 1/64th of an inch (used on roads and highways to guard against rock salt damage) their abrasion resistance increases to approximately 40-80%.  When considering a polished concrete floor, the 5-10% difference in the overall price of the floor is negligible when taking into account the increased abrasion resistance of the more expensive densifier.

Sealers - Topical Sealers

All polished concrete densifyers are penetrating.  They penetrate the surface of the concrete and have no topical characteristics.  No other type of surface forming sealer should be used in conjunction with concrete polishing.  The shine from concrete polishing is achieved through a process similar to polishing granite or slate, which is why the shine can last decades or more.  When a topical sealer is applied to achieve shine, the shine will only last as long as the sealer.  Depending on traffic, that may be anywhere from 3 months to just a few years.