To understand how concrete densifiers work it's important to first understand how concrete works in general. Concrete is primarily composed of two components, aggregate and paste. The aggregate consists of sand, gravel, and stone. This aggregate is then held together by the paste, which is commonly composed of portland cement and water. When portland cement is hydrated with water it forms calcium silica hydrates (CSH). This CSH is what is primarily responsible for providing the concrete with its density and strength.
How Do Concrete Densifiers Work?
When Portland cement is hydrated to form CSH, calcium hydroxide (also known as hydrated lime or portlandite) is also formed as a byproduct. This calcium hydroxide remains in the hardened cement paste and does nothing to contribute to the strength of the concrete. In a sense, the calcium hydroxide is useless. Silica based concrete densifiers work by reacting with the calcium hydroxide in the hardened cement paste to form additional calcium silica hydrate within the paste. This additional CSH created increases the density of the cement paste and thus the strength of the cement.
When you break it down into the simplest of terms, the goal of all silica based concrete densifiers is to deliver reactive silica via a liquid carrier (usually water) into the pores of concrete to react with calcium hydroxide in cement to create more CSH. The various products used to do it, however, vary in the form of reactive silica 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. The particle size of silicate densifiers typically very greatly and they usually only 60-70% of their active weight is silica. When potassium silicate and sodium silicate densifiers release silica to form CSH, mineral salts are left behind. These salts must be scrubbed from the concrete surface to prevent hard deposits and discoloration form occurring. Lithium silicate densifiers pose less of a challenge but they can still leave deposits if over applied.
Colloidal Silica Densifiers
Colloidal silicas are dispersions of amorphous silica particles in water. The particle size in colloidal silicas vary from 5-50 nm. Because of this small particle size colloidal silica based densifiers can deliver pure silica into the pores of concrete to form new CSH from calcium hydroxide. Once bonded to form CSH, colloidal silica particles can also form bonds with other silica particles to further increase concrete density and strength, silicate densifiers can't do this. Another benefit to colloidal silica densifiers is that they leave behind no mineral deposits that must be cleaned. This allows for faster application and time savings.
LUDOX® Colloidal Silica for Densifier Formulations
LUDOX® colloidal silicas from Grace are commercially available in a wide variety of grades ranging in particle size, silica sol charge, pH, and stabilizing counter ion. LUDOX® is produced to the highest of quality standards and is highly recommended for use in concrete densifier formulations and as a troweling aid. Contact us today to discuss your application and find a grade that's right for you, or click here to view all available grades.