Preventing Efflorescence

Question:
White residue is forming on the surface of my countertops. What is this, and how do I prevent it?

Answer:
This whitish residue is called efflorescence -- calcium carbonate deposits that form on the surface of concrete and masonry. There are two kinds of efflorescence: primary and secondary. Primary efflorescence occurs when bleedwater dries on the surface of the concrete. Secondary efflorescence occurs when calcium hydroxide leaches out of cured concrete.

In order for efflorescence to form, three ingredients must be present: calcium hydroxide, water and carbon dioxide. Calcium hydroxide is a byproduct of cement hydration. When calcium hydroxide in the concrete leaches onto the damp surface of the concrete and reacts with atmospheric carbon dioxide, calcium carbonate forms.

Eliminating one or more of these three ingredients will prevent efflorescence. Water and carbon dioxide are ever present, so they can only be minimized, not eliminated. However, it is possible to eliminate calcium hydroxide in the concrete by consuming it, either as it forms or after formation. Some calcium hydroxide consumers are:

• -Metakaolin

• -Other pozzolans (fly ash, silica fume, etc.)

• -Internal hardeners (sodium silicates, lithium silicates)

• -External hardeners/densifiers (sodium silicates, lithium silicates)

Any of these substances have benefits for concrete other than efflorescence reduction, but when used as recommended by the manufacturer they will also consume calcium hydroxide. However, they react at different rates and have different levels of effectiveness. Metakaolin, for instance, reacts very quickly, and at a 20% cement replacement level has been shown to remove over 99% of calcium hydroxide.

Externally applied hardeners/densifiers are typically used for polishing concrete. They work by soaking into the concrete and reacting with the already-formed calcium hydroxide, creating more calcium silicate, which is what makes cement hard.

If efflorescence does form, the best way to remove it is to simply scrub with a fine-grit sanding pad and then apply a densifier to consume calcium hydroxide still remaining in the concrete.















Extreme efflorescence on a masonry block wall.