Why Is My Wet Concrete Orange?

Fertilizer and Discolored Concrete

• The trace mineral iron, which is sometimes included in fertilizer formulas, appears as orange stain on concrete when the surface becomes wet. The reaction results from the oxidation of the metal present in the plant food. It is the same as what occurs when an iron bar rusts after getting wet. According to the company Cooper Concrete, the rust stain eventually disappears. Prevent the problem by broadcasting fertilizer only over soil. Sweep up any granules that spill on neighboring solid surfaces as soon as possible.

Mold and Orange Concrete

• Mold belongs to the fungus family and propagates quickly in moist environments. The orange that you see on wet concrete are spores, mold’s reproductive cells. They appear in different colors, including orange.

Recognizing Mold

• An orange substance on wet concrete isn't always mold. Before you treat it as such, the Canada Mortgage and Housing Corporation recommends that you apply a drop of bleach to the stain. If the area treated loses its color, it is likely that mold is growing on your concrete surface.

Treating Mold

• Suck the mold spores off the concrete with a vacuum cleaner fitted with a HEPA filter. HEPA filters are designed to trap very fine particles such as mold spores. Follow up by scrubbing the area with detergent and rinsing it with water. In severe cases when the surface remains visibly moldy after this treatment, use trisodium phosphate. Wear gloves and goggles to apply the caustic substance, commonly known as TSP and available at hardware stores. Let the chemical sit on the moldy area for 15 minutes before rinsing it with water. After treatment, dry the concrete and take steps to keep it moisture-free to avoid mold recurrence. If the surface is outdoors, clean it on a sunny day. If it is indoors, invest in a dehumidifier to keep the room dry.