To summarize, the goal of all silica-based concrete densifiers is to deliver reactive silica via a liquid carrier, usually water, into the pores of concrete and react with calcium hydroxide in cement to create more CSH. Different types of densifiers are available, which vary in the form of reactive silica that is utilized.
Silicate densifiers are salt dispersions in which the anion of the salt is silica and the cation is typically a monovalent metal, such as sodium, potassium, or lithium. Silicate-based densifiers are relatively inexpensive but have several shortcomings. For example, the particle size of silica within silicate densifiers is quite large and can vary greatly, which can be a challenge for penetrating deep enough into concrete to maximize effectiveness.
Furthermore, mineral salts are left behind when potassium silicate and sodium silicate densifiers release silica to form CSH. These salts must be scrubbed from the concrete surface to prevent hard deposits and discoloration.
Colloidal Silica Densifiers
Colloidal silicas are dispersions of amorphous, spherical-silica particles in water. They perform better than silicate-based densifiers discussed previously for several reasons.
First, the particle size of colloidal silicas typically only ranges from 5 to 50 nm. This small particle size allows colloidal-silica-based densifiers to deliver silica deeper into the concrete pores than silicate-based densifiers to form more CSH. Since colloidal silica particles are very small and spherical, the surface area is very high. Thus, the bonding sites for reacting with calcium hydroxide are maximized, which adds further efficiency to the densification process.
Additionally, colloidal silica particles can form bonds with other silica particles to further increase concrete density and strength, whereas silicate densifiers do not have this capability.
Finally, colloidal silica densifiers do not leave behind mineral deposits that must be cleaned like silicate densifiers do, saving time through a faster application.
LUDOX® Colloidal Silica for Densifier Formulations
LUDOX® colloidal silicas from Grace are commercially available in a variety of grades that range in particle size, pH, and stabilizing counter ion. As mentioned, particle size is important for increasing penetration and reactivity to create CSH. The pH and stabilizing counter ion of colloidal silica should also be considered when LUDOX® is blended with other additives to create densifier or sealer formulations. Some grades that can be utilized for densifiers are as follows: