What it is, How It's Caused, & How to Eliminate It
In our line of business, we come across many homeowners who express concern about a white, flaky substance that's appearing on their concrete foundation walls and floors. Often, the homeowner will ask us if the substance is a type of mold or other fungi, if it's harmful, and if there's anything they can do to eliminate this problem and prevent future issues.
What is that Flaky, White Powder on my Concrete?
If the white powder is building up on your concrete surfaces only, then not likely to be mold. Instead, this powder is a mineral salt that's commonly referred to as efflorescence. This builds up on concrete walls and floors in crawlspaces and basements often, occurring as part of the process of moisture coming from the earth, passing through the porous concrete, and passing into your basement air.
Concrete is porous, and it absorbs groundwater like a sponge. When concrete is being used as part of a basement or crawl space, it soaks up moisture from the earth around it continuously. As it does, the moisture can pass through to the inside surface of the foundation walls (known as the "negative side"), where it's added to the air inside the building.
As water vapor passes into the air, it leaves any mineral deposits carried with it behind, which forms a saltlike coating on the concrete. The ugly white powdery buildup is the efflorescence you see on your basement walls.
How to Clean Concrete Efflorescence
Cleaning efflorescence is best done with a power washer -- you can find one at a local hardware store. They will also have a variety of chemicals that you can use in combination with the washer that will quickly and effectively clean your concrete surfaces.
For a simpler method, however, simply combine one part bleach with ten parts water. Use a sturdy push broom, and scrub those walls thoroughly.
Phosphoric acid has also been known to be effective against efflorescence. However, this should be used with extreme care, and it should never be combined with ammonia products, as these two create a poisonous gas when mixed. Phosphoric acid will have the added benefit of being able to eliminate other stains as well, such as those from rust or oil. Be sure to consult with a professional firsthand before using this substance.
The best way to stop efflorescence is to halt the advancement of water vapor through the walls and into your home. One excellent way to do so is by using a concrete sealant on the inside, thereby preventing it from making its way through into your home. This has the added benefit of creating a vapor barrier that will seal away water vapor that would otherwise contribute to the indoor humidity levels of your below grade spaces.
When choosing a concrete sealing product, be sure not to use concrete paint. These types of products are notorious for being pushed off the walls over time by both the pressure of the moisture and by buildup of efflorescence behind the coating. Instead of these, it's very important to find a product that will seal the concrete by bonding deep within its pores, creating a solid barrier that prevents water from passing through without the long-term blistering, peeling, and flaking associated with waterproofing paint and other products.
When you're looking for a concrete sealer, we recommend Concrete Treat: Concrete Sealer and Blanket. When applied, this product creates a glasslike silicate bond deep within the pores of the concrete. The bond seals away humidity and prevents efflorescence buildup on cleaned concrete surfaces. It's usable on both cured and newly placed concrete, and it is also appropriate for outdoor concrete use.