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Acid washing concrete is a preparatory step to ensure that surface coatings adhere properly. A low-concentration acid solution can also be used to eliminate white mineral deposits (efflorescence) and tough stains. This process poses risks to humans, plants, and metal objects, especially when performed indoors where toxic fumes may accumulate.
Do not confuse this process with acid staining, which involves coloring concrete. Acid washing is not recommended prior to staining concrete.
Steps
Preparation
Remove dirt and grease. Sweep or vacuum loose dirt from the concrete surface. For oil stains, clean using a degreaser or alkali cleaner, then rinse thoroughly with water.
- If water beads up on the surface, the acid wash may not be effective. Degreasing can resolve this issue.
- Avoid using trisodium phosphate (TSP) cleaners, as residual chemicals can react violently with acid and produce hazardous gases.
Select the acid. Choose a cleaning product based on your experience and the work environment:
- Sulfamic acid is the safest option for non-professionals.
- Phosphoric acid produces fewer fumes and is suitable for areas with stainless steel or acid-sensitive metals. It’s also ideal for removing mineral deposits.
- Muriatic acid (hydrochloric acid) is the most hazardous, producing strong fumes. It should only be used outdoors and by professionals.
Follow safety guidelines. These acids are among the most dangerous household chemicals. Always wear acid-resistant gloves, rubber boots, and chemical-resistant goggles. Protect your lungs with an acid-rated respirator and use fans to improve ventilation if necessary. Cover exposed skin with well-fitted clothing, preferably including a face shield, hat, and PVC or butyl apron.
- Keep water nearby to rinse off any chemical spills on skin or clothing. A shower or eyewash station is ideal.
- Have baking soda or garden lime ready to neutralize any acid spills on the ground.
Pour the acid into water in a plastic bucket or container. Unlike metal, most common plastics can withstand acids at this concentration. To avoid violent chemical reactions, always add water to the bucket first, then slowly pour in the acid. Prioritize the manufacturer’s instructions on the acid bottle over general guidelines. Common ratios include:
- Sulfamic acid: 120 g of powder or crystals per liter of water.
- Phosphoric acid: Dilute to a 20–40% concentration.
- Muriatic acid: Mix 3-4 parts water to 1 part acid or follow the product label for a 10% concentration (15% for smooth, hard concrete).
- These solutions are for concrete cleaning. For removing mineral deposits (efflorescence), use a weaker mixture (10:1 or 16:1 ratio for muriatic acid).
Acid Washing the Concrete
Wet the entire area. Spray water onto the concrete until damp but not pooling. Also, wet surrounding objects like wooden plants, shrubs, walls, doors, frames, cabinets, and floor coverings. Move any nearby furniture away.
- Keep the concrete wet throughout the acid washing process. Divide large areas into sections or periodically rewet to prevent drying.
- Protect asphalt, plaster, and tarmac surfaces with plastic sheeting or other barriers.
Apply the acid to the concrete surface. Use a plastic watering can to pour the acid onto the concrete, keeping it close to the ground. Work in small sections, starting with a test area in an inconspicuous spot. Plastic can corrode, sometimes within an hour, so have extra watering cans ready. Follow the label instructions for the required acid amount or use these general guidelines:
- Sulfamic acid: 1 liter per 28 m².
- Phosphoric acid: 3.8 liters per 45-250 m² for mineral deposit removal.
- Muriatic acid: 4.5 liters per 5 m².
Spread the acid evenly. Immediately after applying the acid, use a long-handled broom or paintbrush to spread it into a uniform layer. For larger areas, enlist help—one person can use a floor scrubber while the other works the acid into corners and walls.
- Ensure the surface and surrounding areas remain wet during application. You may need to spray water periodically.
Let the acid sit for a few minutes. Allow 5-10 minutes for the acid to penetrate the concrete. If removing white mineral deposits, wait until they visibly lift from the surface (usually within minutes).
Rinsing the Concrete
Rinse thoroughly. Before the acid dries, scrub the surface with a long-handled brush while rinsing with plenty of water. Leaving acid on the concrete too long can cause damage.
Neutralize the acid. Mix 1 cup of baking soda, garden lime, or household ammonia with 4 liters of water (about 250 ml per 4 liters), or follow the instructions on an acid neutralizer product. Scrub the solution onto the concrete surface and let it sit for at least 10 minutes to fully neutralize the acid. Pay special attention to edges and depressions on the surface.
- At this stage, the acid-washed concrete should have a uniform texture similar to medium-grit sandpaper. If the surface is smoother or still has white mineral deposits, repeat the acid wash.
Rinse thoroughly multiple times. Even after neutralizing, residual liquid on the concrete can leave white, powdery streaks when dry. Spray water, scrub, and repeat the process several times to prevent this. Use a wet vacuum for the final rinse or sweep the water into a drain.
- Use a garden hose instead of a pressure washer, as the latter can drive acid deeper into the concrete.
- For safety, test the pH after the final rinse. A pH below 6.0 indicates too much acid remains, requiring another rinse. (Rarely, a pH above 9.0 means too much neutralizer was used.)
Dispose of leftover acid. If you have unused acid solution, slowly pour it into a wide bucket half-filled with the neutralizer solution you used earlier. Gradually stir in more acid or alkali as needed until the mixture stops fizzing. Once neutralized, it can be poured down the sink or drain. Rinse tools and clothing exposed to acid with water.
- If you don’t plan to use the remaining pure acid, dispose of it as described. Stored acid poses serious risks due to corrosive fumes and spill hazards.
Tips
- Enlist help if possible. The process is faster if one person scrubs while another continuously sprays water.
- Allow the concrete to dry for at least 2 days before applying surface coatings (longer in humid, cool, or poorly ventilated conditions). Even if the surface appears dry, moisture beneath can interfere with coatings.
Warning
- Never pour water into acid. Always add acid to water to prevent dangerous splashes, then stir the solution slowly.
- Keep the entire work area wet throughout the process. This prevents the acid from causing permanent damage to materials. Muriatic acid not only corrodes concrete but also damages metal, wood, and synthetic materials like carpets.
- Keep children and pets away from the work area.
What You Need
- Water hose
- Sulfamic acid, phosphoric acid, or muriatic acid
- 20-liter bucket
- Water
- Long stirring stick (a wooden stick will do)
- At least two spray bottles or plastic watering cans
- Paintbrush or floor scrubber
- Neutralizing solution, garden lime, baking soda, or household ammonia
- Broom
- Wet vacuum
