Chemical densifiers/hardeners have been used in industrial applications for almost 100 years. They have evolved like most other things in our culture from products that were difficult and labor intensive to products that are relatively easy to use. Although they have been used extensively and are believed to improve the performance of the concrete, the mechanisms by which they improve the performance of the concrete are not completely understood. Part of the problem is the complexity of concrete. It is generally understood that every concrete slab is different and therefore performance of these types of products will vary from slab to slab. A chemical hardener that works well on a tight, dense slab may not work well on an open, porous slab, and vice versa.
How they work
Chemical hardeners are believed to work through three different mechanisms. First, we know they react with calcium hydroxide, which is a soft, water-soluble material that is produced during the hydration of the cement. When calcium hydroxide reacts with silica it is converted into calcium silicate hydrate. This new compound is hard and is not water soluble. Only 20 percent of the cement is converted into calcium hydroxide so it is not believed that this reaction fully explains the increase in performance characteristic of the concrete. Second, it is believed that some of the chemical hardeners swell and block the pores or capillaries of the concrete. Third, others deposit an insoluble solid in the pores of the concrete.
There are a variety of different types of chemical hardeners — magnesium fluorosilicate, sodium silicate, sodium silicinate, potassium silicates, lithium silicate and amorphous silica. There are variations in the metallic salts and there are also differences in the silica ratios as well. Some of the chemical hardeners are acidic and others are alkaline. Some must be scrubbed into the floor while some can be sprayed on and left. Some are large and some are small. All of these materials have one thing in common — they are using silica, silicate or a silicinate to react with the calcium hydroxide. The metallic salts sodium, lithium and potassium are just a vehicle to get the silica to the receptor.
Chemical hardeners work on the surface of the concrete. They improve the wear surface of the concrete. It is important to keep as much of the material on or near the surface as possible so it can have the greatest impact. Conversely, if we can fill the area below the surface and not become diffused in the concrete we can have a greater impact on densifying the slab.
Silicinates are the largest of the silicas that are used for hardening concrete. Silicas and some of the silicates are some of the smallest. A higher percentage of solids in solution or suspension will have a tendency not to penetrate as well into a dense slab. On the other hand, a higher percentage of solids will work better to plug an open and porous slab. The inverse of this is also true; a lower percentage of solids with a smaller particle size will do a better job on more dense concrete. There are a number of other factors that can affect the depth of penetration into the slab including temperature, moisture content of the air and also of the slab, mechanical force used during application and rate of application, and dwell time.
When the temperature is high it can affect the surface temperature of the slab. If it is too hot it will cause the chemical hardener to dry on the surface and not penetrate into the slab. If you wet the slab to cool down the surface you have increased the relative humidity of the slab. If the air has a lower level of relative humidity, now the moisture will be leaving the slab and moving into the air. If the chemical hardener is applied at this point the movement of the water out of the slab will inhibit the penetration of the chemical hardener. It is better to allow the surface to dry completely before applying the chemical hardener.
Sodium silicates and sodium silicinates are both large and difficult to get into the slab. All of the chemical hardeners benefit from the use of surfactants. Wetting agents make things wetter thus allowing them more dwell time and may allow them better penetration into the slab. Mechanical scrubbing is effective in aiding penetration of these two types of chemical hardeners into the concrete. A lower concentration may also benefit these types of hardeners in trying to get them into the concrete.
It may be beneficial in aiding penetration of chemical hardeners to apply them in the afternoons when temperatures begin to fall and concrete is at its lowest relative humidity and relative humidity in the air is climbing.
The final piece of application is coverage rates. In order to be effective chemical densifiers have to be used to rejection. When the slab cannot take any more material it is full. If it still has the capacity to accept more chemical hardener then it has not reached saturation or its full potential. If they are applied at coverage rates in excess of the recommended application they have little long-term affect on the slab. All of these products get into the concrete due to capillary actions. The longer they stay on the slab the more product you can get into the concrete. If you increase the coverage rate you decrease the dwell time. This results in more product sitting on the surface and achieving less penetration into the slab.
Even though we do not completely understand how they work, we do know that when chemical hardeners are applied correctly they can improve the performance of a concrete slab. They will not make bad concrete good, but they can make good concrete better.