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Understanding Colloidal Silicates
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Understanding Colloidal Silicates

Products based on colloidal silicates have been used for decades as a way to increase the strength of concrete and other cementitious materials. Now being marketed as a way to stave off common plaster problems in swimming pools, it’s helpful to take a clear look at what these products can and cannot do.

By Paolo Benedetti

Colloidal Silicates (CS) have been used in the general construction industry since the 1950’s.  Yet, they are only recently being adopted by the swimming pool industry as way to improve the performance of pool plaster. 

Truth is they are nothing new to those of us who work beyond the horizons of the pool industry. Unfortunately, some of those unfamiliar with CS densifiers, incorrectly promote them as waterproofing. Once you understand the science behind colloidal silicates, you’ll understand why they do not “waterproof” the concrete. Still they do deliver potential benefits nonetheless.


CS are pozzolans that densify concrete or plaster. One of the things that sets them apart from other materials in the same category of admixtures is their particle size.

Common pozzolans, such as fly ash, are measured in microns, while colloidal silicates are measured at the nano scale, i.e., billionths of a meter. Because of this extremely small size, CS can penetrate the matrix of even aged concrete. However, here is the rub, the denser the concrete the less that CS can penetrate.

In the context of the pool industry, this presents a conundrum. CS can only penetrate multiple inches if the shell is porous and of inferior quality. And, at that level of porosity, even CS densifiers may not be of any meaningful benefit. 

Still, make no mistake, properly placed and dense concrete can still benefit from CS, as it densifies the surface, and slows water migration. As we all know, preventing water from invading concrete helps prevent a host of potential problems. The same is true of plaster, where water penetrating the surface leads to a variety of issues from discoloration to delamination, and others.

Here’s how they work: CS react with the calcium hydroxide in the concrete matrix and the hydration water to form calcium silicate hydrates (CSH). CSH is the family of compound that gives concrete its strength. Other pozzolans such as fly ash, metakaolins and silica fume also react with the calcium hydroxide to form CSH. Excess calcium hydroxide is converted into CSH by these pozzolans, densifying and strengthening the concrete. 

Because the CS particles are so small, they have a lot of surface area compared to their weight.  This results in a highly reactive and efficient process in the conversion of calcium hydroxide into CSH, when compared to simpler pozzolans.


The two most commonly available liquid CS products are sodium- or lithium-based. Given a preference, based on my experience, I prefer the lithium-based products. However, the lithium products carry with them a higher cost due the cost of the raw materials. From a performance standpoint the lithium-based products perform better and have a higher degree of penetration than their counterparts.

Sodium based products do work well with weaker, less dense and porous concrete. This is all a function of molecule size. The smaller lithium molecules can penetrate deeper into the concrete. Also, sodium-based silicates tend to raise the pH of the concrete, while the lower pH lithium silicates have less of an effect.

How are they used?

CS can be applied to concrete in a number of ways. The most common is the spray application onto fresh concrete by the use of an aerosol sprayer. Compressor-driven or hand-held bug-sprayers are used to apply a coat of CS to the fresh concrete. 

Flat work concrete contractors have the option of utilizing a CS finishing aid, which is troweled into the surface during the final finishing. In addition to densifying the surface, it also increases the available cement paste on the surface, thereby easing finishing and reducing the need for additional water.

Sprayed onto aged concrete, CS can still densify the surface, reduce or eliminate dusting and can even push embedded stains and contaminates to the surface.

No matter how it’s applied, when CS is used on fresh concrete, it will immediately densify the surface of the concrete. This denser layer retards the evaporation of hydration water, protecting the concrete from rapid water loss. The problems resulting from rapid water loss are reduced, including shrink cracks, slab curl, checking and crazing. As such, this surface densification promotes even curing and slows the water loss from within the concrete.

This is why many concrete curing agents are simple formulations of sodium-based colloidal silicates.

The over application of colloidal silicates can create a haze or clouding of the concrete.  These areas of over application may resist the adherence of subsequent finish materials.  Additionally, colloidal silicates sprays are known to etch glass and plastics, so these surfaces must be diligently protected from overspray.  I’ve lost more than one pair of valuable sunglasses from my inattention to the direction of the overspray and mist!

The resulting concrete surface profile (CSP) of CS-treated concrete is paramount, especially if subsequent layers of finishes are going to be installed. The densified concrete surface simply will not develop the microscopic adherence required for bonding because the “pores” of the concrete are tighter. 

Therefore, a rough CSP is required to develop a mechanical bond.


Laboratory testing has demonstrated that CS surface treatments dramatically increase the density of the surface, reduces surface permeability, and vapor transmission. Still, it’s important to keep in mind that CS is only a surface treatment and the benefits do not extend throughout the concrete or to the reverse side.

Anyone who has ever polished concrete treated with CS can attest to the density of the surface and its resistance to even acid etching. CS-treated concrete can be polished to an almost glass like appearance.

This denser surface is more abrasion resistant than untreated concrete. The increased density also reduces the permeability of liquids that routinely permanently staining concrete, such as motor oil, food, and grease.

Technical References

For anyone interested in learning more about CS, you can find a tremendous amount of readily available information with just a few clicks. Here’s a partial list of resources I used in developing this discussion.

Concrert Decor: Basic facts About Colloidal Silica Densifier

Construction Specifier: Better Concrete Starting at the Finish Long-Term Benefits of Colloidal Silica-based Finishing

Engineering News: Colloidal Silica Post Pozzolan Technology Solves Many Challenges Regarding Concrete

National Academy of Sciences: Alkali-Silica Reactivity, Overview of Research

Paolo Benedetti founded Aquatic Technology in 1993. He is an instructor for Watershape University and and has authored a myriad of articles on the finer points of pool construction and design. He is a pioneer in the field of aquatic design, constantly pushing the envelope, creating a number of firsts that spawned new trends in the industry.

Opening image by Yurchanka Siarhei | Shutterstock

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