What Solutions Work Best For Dark Spots in Concrete?
- Vinegar and Baking Soda: Mix equal parts of vinegar and baking soda to form a paste. Apply the paste to the stains and scrub with a stiff brush. Rinse thoroughly with water.
- Bleach Solution: Mix one part bleach with three parts water. Apply the solution to the stains and let it sit for about 15 minutes. Scrub with a brush and rinse thoroughly with water.
- Hydrogen Peroxide: Apply hydrogen peroxide directly to the stains and let it sit for 10-15 minutes. Scrub with a brush and rinse with water.
How to Fix Concrete Discoloration:
To get black marks off the concrete, follow these steps:
- Identify the Source: Determine if the marks are due to the above factors.
- Choose the Right Cleaner: Use a concrete cleaner, degreaser, or a mixture of water and mild detergent for general stains. For mold and mildew, a bleach solution may be more effective and for iron deposits use Hydrogen Peroxide.
- Scrub and Rinse: Apply the chosen cleaner to the black marks, wait 15-30 minutes, scrub with a stiff brush, and rinse thoroughly with water.
- Repeat if Necessary: Some stains may require multiple treatments to completely remove.
| Cause | Best Solution We Found to Work |
| Left Over Contaminants | Degreaser/ Rust remover/ Bleach + Scrub in for 5 minutes + Rince ( repeat till lightened/ removed) |
| Uneven mix of concrete | Muriatic Acid + Scrub + Rince |
| Iron Deposits | Rust remover + Scrub+ Let it sit for 15 minutes + Rince |
| Uneven Cleaning From Pressure washing | Degreaser/ Bleach/ Hydrogen peroxide + Scrub in for 5 minutes + Rince ( repeat till lightened/ removed) |
Where to buy supplies: Degreaser, Rust Remover, Bleach, Muriatic Acid, Hydrogen Peroxide.
What are the Black Spots on Concrete After Pressure Washing?
Black spots on concrete after pressure washing are typically caused by mold, mildew, or algae that have grown on the surface. These organisms thrive in moist environments and can leave dark spots in new concrete. Other causes include oil and grease that have penetrated the concrete, iron deposits that have oxidized, and residual dirt or contaminants drawn out by the pressure washing process.
Left Over Contaminants
Contaminates reacting with chemicals in the concrete: Usually before pressure washing a chemical cleaning solution in applied to the concrete to make it easier to remove dirt, debris, oil, mold or algae from the concrete. Unfortunately the cleaning chemical may react with various contaminates such as left over metal in the concrete after a metal object such as a table, chair or gardening tools are placed on top of it. The reaction between the cleaning solution and contaminate of the concrete can lead to some dark spots on the concrete and can be really apparent after pressure washing the concrete.
Uneven mix of concrete
Deep Saturation: Pressure washing forces water into the concrete’s pores. As the water evaporates, it can draw out dirt and contaminants from deep within, leaving dark spots.
Iron Deposits
Rust Stains: Iron particles in the concrete can oxidize when exposed to water, resulting in rust stains that appear as dark spots. This can happen if you pressure wash a surface that recently had metal on top of it such as chairs, tables, or canapes.
Uneven Cleaning
Inconsistent Pressure: Variations in pressure or missed areas during washing can lead to uneven cleaning, making some areas appear darker in contrast to cleaner sections.
Our client used their pressure washing with an improper amount of pressure causing permeant damage. That we were only able to reduce but not completely fix.
We applied the following process to lessen the visual effect of the stain on the concrete. First, we applied a layer of degreaser and scrubbed it in for a solid 3-5 minutes using a scrub brush. Then, we applied a layer of bleach solution and rinsed it off. Finally, we added a layer of hydrogen peroxide to finalize the process.
Preventing Dark Spots During Pressure Washing
Post-Treatment: Use a Degreaser: Apply a degreaser or concrete cleaner to break down oil, grease, and any other potential contaminants after pressure washing.
Consistent Technique: Even Strokes: Maintain consistent pressure and motion while pressure washing. Do not overlap your strokes to prevent streaks.
Thorough Rinsing Remove Residues: After pressure washing, rinse the concrete thoroughly to wash away loosened contaminants and prevent them from resettling.
