Efflorescence vs. Mold on Concrete — How to Tell in Katy TX

The Visual Test

Efflorescence is white, gray, or light tan and crystalline or powdery in texture. It typically appears in consistent patterns — along cracks, at the slab perimeter, or in areas of known moisture movement. Mold is usually darker (black, green, gray) and has a fuzzy or irregular texture. It often appears in corners and shaded areas where moisture collects. Both can appear white in early stages, which is where the confusion arises.

The Bleach Test

Apply a drop of household bleach to the white substance. If it disappears or lightens significantly within a minute, it's likely mold — bleach kills organic growth. If it remains unchanged, it's likely efflorescence — bleach has no effect on mineral deposits. This simple test is reliable for distinguishing the two. Note: it doesn't tell you if both are present simultaneously (which can happen).

When Both Are Present

In humid climates, mold and efflorescence can coexist. Moisture moving through a slab creates both mineral deposits at the surface and wet conditions for mold growth. Treatment sequence: address the moisture source, treat mold with biocide, acid wash efflorescence, allow to dry completely, diamond grind the affected area, then apply moisture-barrier primer. Both conditions must be resolved before coating.

Why Correct Diagnosis Matters

Treating efflorescence with biocide (the mold treatment) does nothing — biocides don't affect mineral deposits. Treating mold with acid wash (the efflorescence treatment) kills the surface mold but doesn't address the root cause (moisture) and doesn't prevent regrowth. Starting with the correct diagnosis saves time and money and ensures the treatment actually works. When in doubt, we do both treatments in sequence — it's faster than diagnosing wrong.