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 to 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 more dense, uniform surface after polishing.

Concrete 101

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

How Do Concrete Densifiers Work?

When portland cement is hydrated in a paste to create CSH, calcium hydroxide, also known as hydrated lime or portlandite, is formed as a byproduct. This calcium hydroxide remains in the hardened cement paste and does nothing to contribute to the strength of the concrete.

Silica-based concrete densifiers work by reacting with the calcium hydroxide in the hardened cement paste within the concrete to form additional calcium silica hydrate. Concrete floors or slabs that do not have densifiers applied are very porous and have many open voids that weaken overall compressive strength. Thus, the additional CSH created by the densifier increases 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.

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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 densifiers are available, which 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 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:

Grade Silicate Content Particle Size (nm) pH Counter Ion
HS-40 40% 12 9.7 Na
SM 30% 7 10 Na
AS-30 30% 12 9.4 NH4+
SM-AS 25% 7 9.5 NH4+
AM 30% 12 9 (but stable from 4-10) Na

LUDOX® is produced to the highest quality standards and is ideal for use in concrete densifier formulations.  Contact us below to discuss and sample LUDOX® for your concrete densification application.  

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