There has been a great deal of controversy over the past couple of decades about the effectiveness of liquid chemical treatments to harden and densify the surface of concrete floor slabs. Widely used as far back as the late 1940s, these products were included in a section titled Hardeners in the original 1969 publication of the American Concrete Institute (ACI) document 302.1R, then called the Recommended Practice for Concrete Floor and Slab Construction.
That document stated, “Liquid surface treatments should be considered to be emergency measures for treatment of deficiencies. They are not intended to provide additional wear resistance in new, well designed, well constructed floors, nor to permit the use of concrete of lower quality.” Some of this advice remains valid today, but since the mid-1990s, using liquid chemical surface treatments has become common. Today, they are applied to most industrial floors during or shortly after construction. Are these materials being used correctly, or has the industry lost sight of what they do and what they don’t do? Let’s revisit the guidance of the original ACI 302 and re-examine their capabilities and limitations.
What densifiers do
As portland cement hydrates, two of the main compounds formed are calcium silicate hydrate (CSH), which accounts for the strength gain of concrete, and calcium hydroxide, which does not contribute to the strength or durability of concrete. To optimize the formation of CSH, the concrete must be kept hydrated for a minimum period of time, typically five to seven days.
Chemical surface treatment products are generally forms of silicate compounds such as sodium silicate, potassium silicate, lithium silicate, and magnesium fluorosilicate. On hardened concrete, where the hydration reaction has basically stopped, when surface treatments are applied the silicate component reacts chemically with the calcium hydroxide in the concrete to form additional CSH. Since CSH is harder than calcium hydroxide, the concrete becomes “hardened.” Also, since the CSH generally forms in the empty pores of the cement paste, the concrete becomes “densified.” It is because of these basic reaction results that chemical surface treatment products are commonly called hardener/densifiers.
But how much harder and denser does the concrete become? That depends on how much calcium hydroxide is available to react with the silicate. Unfortunately, chemical treatment products are not the only materials that react with calcium hydroxide. Pozzolans such as fly ash and slag also react with available calcium hydroxide. If the concrete mix contains a pozzolan, the application of a surface treatment may be less effective than desired. This is especially true with the more recent “green” concrete mixtures that contain higher amounts (50% replacement or more) of pozzolans. Especially troubling with these mixes, if the treatment reacts with the calcium hydroxide first, the pozzolan will be left unreacted which may result in lower strength than anticipated.
In addition to pozzolans, normal carbonation consumes near-surface calcium hydroxide. In carbonation, carbon dioxide from the air reacts with calcium hydroxide in the presence of moisture to create a mild form of carbonic acid. Once this occurs, the applied silicate cannot be converted to the desired CSH and, therefore, the product doesn’t harden or densify the concrete. Similarly, once a silicate product has been applied, a second application is not effective unless the surface has been worn or ground to expose additional unreacted calcium hydroxide.
But even when there is calcium hydroxide in the concrete, the reaction only occurs if the silicates come into contact with it. While these treatments are sometimes referred to as “reactive penetrants,” their depth of penetration is dependent on the porosity of the concrete. The porosity of hard-troweled slab surfaces is generally low so the depth of penetration is minimal, especially with burnished slabs. Claims of deep penetration are questionable on most industrial floors.
But how deep do they need to penetrate? If they only penetrate 1/32 to 1/16 inch, they can harden and densify the immediate slab surface and that is all the user should be concerned with. And by decreasing small surface pores (filling them with CSH), floors typically become more attractive and easier to clean. With successive scrubbing, surface treatments improve floor shine. Successive scrubbing is required because the silicate compounds don’t form as smooth reaction products. The scrubbing slowly polishes the micro-texture of the newly formed compounds and blends them into the surrounding smooth surface, thus, enhancing the appearance.
So how can we determine their effect? Probably not by measuring the floor’s abrasion resistance as suggested by George Garber in this article. The silicates only react with available calcium hydroxide and calcium hydroxide does not occur as a layer at the slab surface. Therefore, chemical treatments do not result in a hardened layer consistently across the slab surface. At best, ignoring pozzolanic and carbonation reactions, they create additional CSH at the surface. They do not improve the existing CSH or harden the aggregates. Therefore, there may be very little difference between the tested wear of treated surfaces (where there is now additional CSH) and untreated surfaces (with the existing CSH that was originally present). And the more effective the surface curing was, the less apparent the difference will be between treated and untreated surfaces with equal finishes.
But just because their effect may not be quantifiable doesn’t mean silicates are ineffective. If calcium hydroxide is available within the depth of penetration, the surface will be densified. Perhaps the improvement should be measured with a gloss meter and not an abrasion tester.