It's Chemistry, not Magic

Debunking more Myths & Misconceptions about Chemical Hardeners

-Phil Smith, P.E., FACI, ASTM

A lot of misinformation is being circulated about the use and effectiveness of chemical hardeners and densifiers. Some of the information-or may I say, misinformation-is unfortunately little more than a pseudo-science mixture comprised of half truths, things that they want to believe, magic, and some good old fashioned sales puffery. A lot of smoke, not much light.

The products that we are talking about are liquid chemical treatments that are applied to the surface of cured, hard troweled or diamond polished concrete in order to harden and densify it. These treatments are commonly used on "bare concrete" floors that may be found in a wide range of exposures from industrial, warehousing, retail, educational and other commercial establishments. L&M manufactures two such products: Seal Hard and FGS Hardener Plus.

Cement chemistry revisited
To better understand these products we must gain an understanding of some of the basic principles of portland cement chemistry. There are many chemical compounds in dry portland cement, but two of the most significant ones are tricalcium silicate (C3S) and dicalcium silicate (C2S).

The first compound, C3S, reacts as soon as water comes into contact with cement, and continues reacting (hydrating) for the first 3 days or so. About 7 days after the concrete has been placed, the C2S starts to react and continues for many months thereafter. When the concrete is 28 days old, about 95% of the hydration has taken place. (See Linda Hill's excellent article on cement hydration in Concrete News, Summer 2005). The hydration of these two compounds produces the same primary reaction product-namely, calcium silicate hydrate (C-S-H). It is C-S-H that bonds with the small and large aggregates to form concrete.

How does C-S-H form? When water comes into contact with cement the C3S compound releases large amounts of calcium ions and hydroxide ions. Also released are large amounts of silica, specifically SiO2. With the release of hydroxide ions the pH of the concrete rapidly increases from basically neutral to very alkaline pH of 12 to 13 (on a scale of 1 to 14). In this very alkaline environment calcium ions react with hydroxide ions to form calcium hydroxide. Calcium hydroxide then reacts with the SiO2 to form the main cementing material, C-S-H.

We know this about calcium hydroxide: it has a very high pH; it is soluble in water; and it has very little ability to bond aggregates together. After curing for 28 days, 15% to 20% of the hydrated portland cement paste is calcium hydroxide. Therefore, there is large amount of calcium hydroxide normally available that can be converted to C-S-H. All that is needed to make this happen is a source of silica.

Understanding this property of cement, consider that it is a now common practice to add fly ash (a byproduct of burning coal that is rich in silica) to concrete mixes for this purpose. The silica in fly ash reacts with the calcium hydroxide in the cement paste and produces C-S-H, thereby increasing the strength of the concrete. For more information on Fly Ash and its effect with C-S-H see 5 Minute Classroom in this issue).

How do chemical hardeners and densifiers work?
This same principle has been used for many years to improve the surface properties of concrete; namely, liquid chemical hardeners provide a source of soluble silica, SiO2 . Two things occur when a liquid silicate solution is applied to a hardened concrete surface: First, the liquid silicate solution reacts with available calcium hydroxide and forms C-S-H. Secondly, by converting some of the calcium hydroxide into C-S-H, the pH is decreased.

Now that we have completed our cement chemistry lesson, we can use this information to debunk some of the misinformation that is out there.

How does one know if the use of a chemical hardener and densifier beneficial to the concrete? And, are they all equal?
Some naysayers say that chemical hardeners just don't work; that these products are no more than band-aid treatments for distressed, low performance concrete. Experience proves otherwise.

There are a number of systems, silicate and non-silicate, that can be used to harden and densify the surface of concrete. Construction Technology Laboratories (CTL), known worldwide for its impeccable reputation for accuracy in the performance testing of concrete and the reporting of its findings, collected data which proves that SEAL HARD, a popular formula which contains a blend of proprietary ingredients, increases the long term durability and abrasion resistance of treated concrete over that of untreated, well-cured concrete nearly 2-1/2 times.

Comparative testing by CTL also proves that not all silicate solution products are created equal. SEAL HARD outperformed it closest rivals by a significant and dramatic margin.

How about floor safety of treated concrete surfaces?
The next challenge comes from those who say that when a chemical hardener is applied to a concrete surface the coefficient of friction is reduced and the floor becomes slippery. Testing proves otherwise. The National Floor Safety Institute (NFSI) has tested L&M treated floors before and after the application of our products, showing the coefficient of friction did not change. In fact, the static coefficient of friction on both wet and dry treated concrete consistently remains above ADA requirements and earns our chemically densified floors an NFSI rating of "high traction." Again, data is on file with L&M.

Can the use of a chemical hardener raise the pH of concrete and have a detrimental effect?
Another recent point of attack is that chemical hardeners raise the pH of concrete, thereby making concrete more vulnerable to Alkali Silica Reaction (ASR) attack. Some aggregates become soft in an alkaline environment and can absorb water and expand, causing surface pop-outs. As stated earlier, the pH of relatively new concrete is generally 12 to 13. When a chemical hardener is applied to the surface of concrete a substantial amount of the calcium hydroxide is converted to C-S-H, thereby lowering the pH of the concrete, but only at the surface. It should be pointed out that once a silicate compound comes into contact with portland cement paste, it no longer exists in its original state as a silicate, but now becomes a part of the cement paste in the form of C-S-H.

How do these products work with polished concrete?
Diamond polished concrete is one of the newest innovations in the concrete industry. Diamond polishing concrete is a process in which existing, worn concrete surfaces can be brought back to a serviceable condition and to an attractive appearance. Some companies misrepresent the mechanical grinding and polishing process as one that mechanically "closes up" the concrete. Actually, the opposite is true. A concrete slab that has been polished has a surface that is porous and comparatively soft.

Most concrete when originally placed was hard-troweled to produce a very hard and physically dense wearing surface. An existing concrete floor that is a candidate for diamond polishing will more than likely have had a measurable amount of its dense, hard troweled surface worn away over the years. Furthermore, the grinding and polishing of the process will remove still more of it, exposing a softer, more porous concrete underneath.

Our FGS Hardener Plus has been designed to address the depth of penetration issues in the more porous surfaces produced by diamond polishing, whereas our SEAL HARD has been formulated for newer concrete surfaces that have been hard troweled and remain in good condition. FGS Hardener Plus hardens and densifies the surface of diamond polished concrete. It does this better than any other L&M product, and better than any competitive product. Without the hardening effects of FGS Hardener Plus, the surface of a diamond polished concrete floor will soon lose its shine and become dull.

Wet versus dry polishing-pluses and minuses.
There are two methods by which concrete can be diamond polished: the wet, slurry-producing method and the dry, dust-free method. The wet slurry method is older, and was developed by terrazzo contractors long before heat resistant diamond pads and highly efficient vacuum systems were available. The wet process requires water to be placed on the surface of the concrete during the grinding and polishing process. During the process, a wet slurry of grinding debris is produced and deposited on the surface of the concrete. This must be safely and completely removed. Depending upon how well that is done, the presence of the slurry potentially leaves the small capillary pores of the concrete filled with water and slurry debris, thus interfering with the chemical treatments.

The steps of the wet process necessitate the application of the chemical hardener over already-wet surfaces. This condition interferes with its penetration and reaction, during the intermediate grinding steps. It is like adding more water to an already wet sponge (albeit a concrete sponge). Subsequent grinding and polishing steps will remove part of the treated surface as the wet process steps continue. In addition, the chemical treatments on this process must compete with the water and slurry for space in the pores of the concrete. It is fair to ask, "With the wet process, how much actual long term benefit am I getting from the chemical treatment?"

In contrast, the FGS PermaShine system employs a patented dry process which uses a vacuum system that leaves the air virtually dust free, the surface of the concrete dry, and the capillary pores ready to absorb the proprietary chemical hardener, FGS Hardener Plus. This assures maximum densification into what is known as the "near surface wear zone," the top 1/16 to 1/8 inch of the concrete surface. In the FGS process, two applications are installed to provide maximum benefit to the owner.

Silver bullet products: A new generation of confusion.
Some of the wilder claims about silicate solutions are made on behalf of the new "silver bullet product", lithium silicate. One wild claim that is made is that when other forms of silicate solutions are placed on a concrete surface they form or leave an expansive gel that will harm the concrete and results in floor surface distress, map cracking and aggregate popouts. However, absolutely no scientific evidence of these claims has been published by any reliable source or proven by any scientific, independent testing.

Proponents of these products go on to say that lithium ions are smaller than either sodium or potassium ions, therefore the lithium silicate solution penetrates into the concrete surface better and deeper than sodium or potassium silicate based solutions. While it is true that lithium ions are smaller than sodium or potassium ions, this is only true when the products are in the dry state. Once in a water solution, as they all are, the lithium's radius of hydration is much larger than either sodium or potassium, making it more difficult for the lithium silicate solution to penetrate into the concrete surface than either the sodium or potassium silicate solutions.

Once the real science is removed from the equation, all they are left with is the "depth of penetration" argument.

Independent, scientific testing proves that the importance of penetration is only important to a limited extent, the extent of the near surface wear zone (the top 1/16 to 1/8 inch of the top surface). This critical surface area of the concrete floor is subjected to the most abuse. That is why independent testing proves-and it's why we say, "L&M Treatments Provide Protection Where it Counts."

Not miracles. Just good, solid chemistry.
One thing that should be remembered when it comes to chemically treating concrete surfaces: there are definite benefits to using the right product under the right conditions. While not miracle products, with proper installation and the right product, chemical hardeners and densifiers do significantly improve the durability and life expectancy of concrete floor surfaces well beyond the initial cost of installation.

Beware the half-truths coming from the pseudo-scientists. As much as they may want to, no one can rewrite chemistry. And real science is not a hat trick or a shell game.

For more about Myths & Misconceptions, refer to L&M Concrete News, Fall 2001, or click here.

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© 2006 L&M Construction Chemicals, Inc. | ConcreteNews Summer 2006.

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