The Role of Densifiers in Polished Concrete

Comparing the Four Types of Silicate Hardeners

by Dan Farmer, Michael Brady Inc

The concrete polishing process consists of mechanically abrading the surface of the concrete with grinders to remove material from the surface, smoothing and flattening the surface and in many cases exposing the larger aggregate to form a decorative surface. This process can remove as little as ¹/32 of an inch to as much as ¼ inch or more of the surface material of the concrete. Integral to this process is the application of chemical agents to harden and close the surface pores, sealing it against the penetration of moisture. The contaminants borne by this moisture cause discoloration, staining, and in some cases, deterioration of the concrete itself. The chemical application fills the pores, increasing the density of the surface and making a smoother, flatter surface, which allows light to be reflected evenly rather than being diffused by empty pores. It also increases the surface abrasion resistance by 25 percent or more. This finished polished concrete surface is the result of the reflective characteristics of the concrete itself and not a surface film or membrane, thus it does not peel, rubber tire burn or require recoating.

Silicate hardeners (also called densifiers) are water based chemical solutions that react with the calcium hydroxide in the concrete to produce calcium silicate hydrate (CSH); the same material that is produced in the reaction between Portland cement and water, giving the concrete much of its strength. Four basic materials are used as the active ingredients in these products: sodium, potassium, lithium and magnesium. Manufacturers add wetting agents and other substances to their silicate products to improve penetration and enhance the chemical bond. Each product type has its proponents and many argue long and loud about the merits of one over the other, though they all work in the same basic way.

Sodium silicate is the most common and the least expensive. Potassium silicate is probably next in terms of popularity. Lithium silicates are gaining popularity because they may offset the effects of reactive aggregates which may be present in concrete. Magnesium floro-silicates can react with both calcium hydroxide and calcium carbonate to form insoluble by-products within the pores of the concrete.¹

When the hydration reaction occurs in curing concrete, silicate compounds within the cement react with water to produce calcium silicate hydrate (CSH). It is CSH that bonds with the small and large aggregate to form concrete. An important consideration in this reaction is that significant amounts of calcium hydroxide are left over from the reaction and the resulting concrete is very alkaline (12- 13 on a scale of 1 – 14). Calcium hydroxide is very high in pH, is soluble in water, and has very little ability to bond aggregates together. At the 28 - day cure point, 15 to 20 percent of the hydrated Portland cement paste is calcium hydroxide. Thus, there is a large amount of calcium hydroxide that can be converted to CSH, strengthening the concrete. All that is needed to make this happen is a source of silica.

Two things occur when a liquid silicate solution is applied to cured concrete. First the liquid silicate solution reacts with the available calcium hydroxide and forms CSH, strengthening the concrete. Also, by converting some of the calcium hydroxide into CSH, the pH is reduced². Liquid hardeners should not be placed on freshly poured concrete in order to maximize their effectiveness. During the first days after placement, the pores of the concrete are saturated with water not absorbed by the hydration reaction. This free water aids the ongoing hydration process but also causes the pore structure to be swollen, reducing the size of the capillary pores. This significantly reduces the ability of the applied liquid hardener to penetrate into the near surface regions of the concrete. By the end of seven days, free water in the pores is greatly reduced, making the pore structure more open and allowing greater penetration of applied liquids into the concrete. Additionally, the longer the hydration reaction has been ongoing, the more calcium hydroxide will be available to react with the silica. Under normal conditions, most concrete will achieve about 60 – 65 percent of its hydration within seven days and produce a proportional amount of calcium hydroxide.

As soon as the hardener is applied to the surface of the concrete, the silicate immediately begins reacting with the calcium hydroxide present in the concrete, densifying the concrete but also filling the pores of the concrete limiting the penetration of the hardener. The total penetration of the hardener into the concrete is limited to approximately ¹/8 inch. With this in mind, the application of hardener is best coordinated with and accomplished during the polishing process. These hardeners should be applied by saturating the surface with hardener and working it into the pores of the concrete using power driven scrubbers. This forces the reactive ingredients into the pores of the cement paste and removes impurities from the near surface allowing more contact between the chemical hardener and the calcium hydroxide compounds, increasing the effectiveness of the hardner.³ The surface should then be flooded with water to remove residual materials and chemicals to avoid discoloration of the surface. Additionally, silicates left on the surface react with carbon dioxide in the atmosphere to form a white residue (carbonation) which is difficult to remove. As mentioned earlier, grinding and polishing remove the material at the surface of the concrete, often removing ¹/8 inch or more. Thus the application of liquid hardeners should be scheduled between the preliminary grinding and the final polishing steps of the polished concrete process.

1.  "Using Liquid Hardeners to Enhance the Diamond Polishing Process" Joe Nasvik, Concrete Construction 9/1/2004.

2.  "Its Chemistry, Not Magic; Debunking more Myths and Misconceptions About Chemical Hardeners" Phillip Smith: L&M Construction Inc. News, Summer 2006.

3.  "Myths and Misconceptions About Concrete Hardeners" Phillip Smith: L&M Construction Chemicals, Inc. News, Fall 2001.