What is Concrete Densifier?
Concrete densifiers, also known as concrete hardeners, are silica-based compounds that react with lime (calcium hydroxide) in concrete to form additional mineral compounds, filling in the pores and increasing surface hardness. Think of this process as a closing of the pores of the concrete. Closing the pores of the concrete makes the concrete harder and makes it difficult for compounds to enter the concrete, thus minimizing staining too. Densifying concrete is done prior to polishing concrete since concrete in its native state is too soft for polishing.
Early/Traditional Concrete Densifiers
Early densifiers were generally made from silicates such as sodium silicate. They performed the basic task of densifying the concrete, but they were problematic and had a number of drawbacks. Applying these traditional densifiers was time-consuming and labor-intensive. They had to be vigorously scrubbed into the surface for an hour, required overnight curing, and left a gelatin-like residue that had to be scrubbed off and disposed of before polishing could even begin. Inadequate removal of the densifier frequently resulted in an unsightly deposit known as "whiting" which was left on the concrete surface. Lithium silicates eliminated some of the time-consuming aspects of application and removal, but still presented a whiting hazard if excess material were applied, and it was difficult to judge how much was too much. Lithium silicates are also significantly more expensive than other silicates.
Traditional silicates are highly caustic materials with a pH in the range of 11-12. Material with such a high pH poses potential risks in handling, and places an ecological burden on the environment when disposed. The bottom line is that traditional lithium and sodium silicates leave a lot to be desired.
Colloidal Silica Concrete Densifier
The newest and most effective concrete densifiers are now made with colloidal silica. Colloidal silca is a much purer form of silica that is far lower in pH (similar to baking soda). It is faster-reacting, can be applied in about one quarter of the time, does not require overnight curing, and leaves no gelatinous residue to remove. It has virtually no risk of whiting, which is a significant risk-reduction factor for contractors. These time savings enable applicators to keep their costs down and increase their productivity. The elimination of residue disposal is also more eco-friendly and helps make colloidal silica concrete densifier a greener product than traditional concrete densifiers.
Some applicators report achieving equivalent polishing results with one less abrasive polishing step when they use colloidal silica, speeding up schedules and lowering costs further. Applicators also report that colloidal silica produces better results on concrete that is marginal in quality or condition.
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