Efflorescence happens naturally as concrete cures.
“I built a retaining wall out of stackable blocks last year, and they are turning white and chalky-looking. When I wet them down, the blocks look great, but when they dry out they look white again. I don’t like what I’m seeing – can you help?”
Those powdery white stains on your blocks are likely caused by efflorescence. As concrete cures, the moisture slowly migrates to the surface of the block where it evaporates into the air. Like a candle wick, the water brings dissolved salts and minerals from within the concrete mixture. As the water evaporates, those minerals are left on the surface of your block, giving it a whitish, hazy or streaky stain.
In most cases, this problem is temporary – eventually the salts will all be removed from the concrete, and the efflorescence will stop. How long this takes depends on the ingredients with which the concrete was made, and how long it takes for the concrete to cure and dry out completely.
However, with landscape retaining walls, your blocks are constantly being exposed to ground water and hydrostatic pressure, which can force moisture (and more salts from the soil) into the blocks to later show up as efflorescence.
Efflorescence on wall at joints in the underground drainage channel.
Here are some tips for dealing with efflorescence:
Efflorescence is ugly, but it does not affect the strength or functioning of the block.
Normal efflorescence should stop once the concrete is completely cured. Ongoing streaky stains or cakey buildup, particularly at joints and cracks, may indicate a problem with water infiltration and drainage.
It’s important to reduce the amount of moisture that is getting into your wall. Mortared walls should be treated on both sides with a waterproofing sealer. Non-mortared, stackable retaining walls should be backfilled with gravel, with water drainage directed off to the sides. This prevents water pressure from building up in the soil behind the wall (and forcing itself into the concrete blocks).
Avoid pressure-washing or overly wetting your blocks. This makes them look nice at the time, but it forces water into the concrete, inviting more efflorescence.
Clean off light stains with a dry, stiff brush, then remove the dust with a damp sponge. If you have heavy stains, try scrubbing with white vinegar. Stubborn stains can be removed with a masonry cleaning solution.
Cleaning solutions are often specialized to address particular mineral stains. Understanding Efflorescence (EaCo Chem, PDF 8.43mb) has an illustrated guide to different types of efflorescence, as well as recommended products to address each problem.