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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.
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 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.
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