Colloidal Silica Densifiers

Concrete Densification

Concrete is a critical building material in many structures that house commercial, industrial, and institutional operations. The longevity of concrete surfaces must be maximized, especially those that encounter high traffic, such as concrete floors. Concrete densifiers help prolong the life of concrete surfaces as densification increases abrasion resistance and reduces permeability to liquids that may cause staining, corrosion, or mold growth. Polished concrete may also benefit from densifiers by gaining sheen from a denser, more uniform surface after polishing. 

Concrete 101

It is important to understand concrete composition first to understand how concrete densifiers work. Concrete is primarily composed of two components: aggregate and paste. The aggregate consists of materials such as sand, gravel, and stone. It is held together by the paste, commonly a mixture of portland cement and water. When portland cement is hydrated with water, it forms calcium silicate hydrate (CSH). CSH is a chemical responsible for providing concrete with its density and strength. 

How Do Concrete Densifiers Work?

When portland cement is hydrated in the paste, calcium hydroxide, also known as hydrated lime or portlandite, is formed as a byproduct. Calcium hydroxide that remains in the hardened cement paste without reacting to form CSH is known as free lime, which does nothing to contribute to the strength of the concrete. 

Silica-based concrete densifiers work because they contain nano-sized silica particles that react with the free lime in the hardened cement paste within the concrete to form additional CSH. Concrete floors, slabs, and surfaces that do not have densifiers applied are very porous and have many open voids that weaken compressive strength. Thus, the densifier creates additional CSH, increasing the density of the cement paste by filling open voids, thereby increasing the cement’s overall strength. 

Below is a diagram illustrating how a silica-based densifier reacts with calcium hydroxide to form CSH and water. 

Silica Densifiers

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 silica densifiers are available, and they vary in the form of reactive silica that is utilized. 

Silicate Densifiers

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 the silica in silicate densifiers is relatively large and not carefully controlled, which can present a challenge in terms of 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 off to prevent hard deposits and discoloration on the surface of concrete. 

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 for better penetration of colloidal-silica-based densifiers to deliver silica deeper into the concrete pores than silicate-based densifiers. 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, adding 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, unlike silicate densifiers, colloidal silica densifiers do not leave behind surface mineral deposits that must be cleaned, saving time during application. 
 

LUDOX® Colloidal Silica for Densifier Formulations

LUDOX® colloidal silicas from Grace are commercially available in various grades, ranging 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 recommended grades are below.
 

Summary 

Silica-based concrete densifiers are ideal for enhancing the strength and durability of concrete while improving its appearance after polishing. The silica particles within these densifiers react with calcium hydroxide to form calcium silicate hydrates, which are responsible for improvements. Densifiers using LUDOX® colloidal silica are among the most effective because their particles are very small and feature a high surface area. This allows them to penetrate deeply into porous materials and have high reactivity to increase densification efficiency. 
 
LUDOX® is produced to the highest quality standards and is ideal for concrete densifier formulations. Contact us below to discuss and sample LUDOX® for your concrete densification application.