Concrete is a versatile and durable material commonly used for driveways, patios, foundations, and other flatwork around homes. However, it is susceptible to discoloration issues over time that can detract from its appearance. This article provides an overview of the most common types of concrete discoloration, along with tips for prevention and removal.
Discoloration of concrete surfaces is a frustrating problem for many homeowners. Stains can develop for a variety of reasons, ranging from the concrete’s natural tendency to yellow with age to issues like rust deposits from nearby metal objects. Other common culprits include efflorescence, which is a white powdery substance that leaches out of the concrete over time. Organic growth like mould, mildew, and algae can also discolour concrete surfaces, leaving dark, splotchy areas.
Also read: Benefits of Investing in Concrete Repairs
Causes of Concrete Discoloration
Concrete can become stained or discoloured over time due to various factors. Listed below are a few of the most typical reasons why concrete discolours:
Chemical Reactions and Corrosion
Chemical reactions within the concrete or with external elements can lead to staining and discoloration. Alkali-silica reaction (ASR) occurs when the alkali in cement reacts with silica in the aggregates, causing expansive gel formation and cracking. Corrosion of reinforcing steel due to chloride attack or carbonation can also stain concrete.
Efflorescence
Efflorescence appears as white, powdery deposits on the concrete surface. It is caused by water-soluble salts in the concrete migrating to the surface as moisture evaporates. Calcium hydroxide is the most common salt that causes efflorescence.
Also read: Concrete Restoration in Edmonton: Reasons and Process
Organic Growth
Mould, mildew, algae, and other organic growth can stain concrete, especially in damp, shaded areas outdoors. Organic matter provides nutrients for these organisms to spread. Dark green, black, or orange discoloration is common with organic staining.
Rust Stains
Rust stains occur when steel particles or adjacent rusted metal transfer rust onto the concrete through moisture. Rust will cause orange, brown, or reddish discoloration where it oxidizes on the surface. Rebar corrosion can also create rust stains from the inside out.
Preventing Concrete Discoloration During Installation
Proper concrete installation is key to preventing discoloration issues down the road. Here are some best practices:
Mix Concrete Thoroughly
Make sure all the dry ingredients are fully incorporated into the wet mix. Clumping can lead to weak spots and inconsistent curing, which causes discoloration. Continuously mix for at least 3-5 minutes after adding water.
Allow Adequate Curing Time
Concrete continues to hydrate and strengthen for weeks after pouring. Avoid sealing or exposing the slab to traffic until it has cured for at least 4 weeks. This allows full hydration to occur and prevents early discoloration.
Also read: Concrete Driveway Enhancements with Creative Design Ideas
Apply a Curing Compound
Applying a curing compound after finishing concrete will slow moisture loss as it cures. This leads to proper hydration and reduces efflorescence bloom. Use a compound compliant with ASTM C309.
Seal the Concrete Surface
After full curing, apply a concrete sealer according to manufacturer specifications. This helps protect against staining agents and creates an even appearance. Reapply sealers periodically.
Also read: The Best Concrete Sealing Company in Edmonton
Removing Existing Discoloration
Concrete can become stained and discoloured over time from things like rust, organic growth, efflorescence, and more. While prevention is ideal, existing stains can often be removed or minimized to restore the look of concrete. Here are some tips for removing common types of concrete discoloration:
Cleaning Agents
Using cleaning agents is one of the easiest ways to remove stains from concrete. Look for concrete cleaners, degreasers, and multi-purpose cleaners at your local hardware store.
- Apply the cleaner to the stained area, let it sit for 5-10 minutes, and then scrub with a stiff brush before rinsing.
- Repeat as needed for stubborn stains.
- Avoid using acidic cleaners, which can further etch and discolour concrete.
Acid Stains
For rust, mineral, and efflorescence stains, specially formulated acidic cleaners work well. These contain diluted acids like hydrochloric, phosphoric, or sulfamic to dissolve staining. Wet the concrete first, apply the acidic cleaner, scrub with a brush, and rinse thoroughly. Neutralize the concrete with an alkaline solution afterward. Wear protective gear when using acidic products.
Also read: The Hidden Danger of De-icing Chemicals: Why Your Sidewalks and Driveway May Be Crumbling
Power Washing
Power washing uses high-pressure water to blast away dirt, grime, and some types of staining. This works best for surface-level stains.
- Make use of a pressure washer with a minimum of 3,000 PSI.
- Hold the nozzle 6-12 inches away at a 45-degree angle.
- Make use of a pressure washer with a minimum of 3,000 PSI. Let the concrete dry completely when it is finished.
Efflorescence Treatments
Efflorescence is a white, powdery deposit that can appear on concrete surfaces. It is caused by water evaporating and leaving behind salts and minerals in the concrete mix. Efflorescence is harmless, but it can be unsightly. There are a few ways to treat and remove efflorescence:
- Acid washing: This involves applying a diluted muriatic or phosphoric acid solution to the concrete with a pump sprayer. The acid reacts with the efflorescence to dissolve and neutralize it. Acid washing is effective but requires safety precautions like eye protection, rubber gloves, and avoiding skin contact. Rinse the concrete thoroughly after treatment.
- Efflorescence removers: Chemical removers are also available that contain detergents, wetting agents, and acids formulated to dissolve efflorescence. These are typically applied with a brush or sprayer. It is safer than acid washing since the acids are diluted. Rinse the area after the recommended dwell time. If efflorescence is obstinate, it might take several applications.
Rust Stain Removal
Rust stains are one of the most common types of discoloration found on concrete surfaces. They occur when metal particles come into contact with concrete and rust due to moisture exposure. The rust leaches out and leaves behind a reddish-brown stain.
Rust stains can be removed in a few different ways.
- Rust removers: Specialty rust remover products are available that can help lift and dissolve rust stains. Look for ones that contain oxalic or phosphoric acid. Apply the remover per the manufacturer’s instructions, let it sit for 5-10 minutes, then rinse thoroughly. For stains that are difficult to remove, several applications might be necessary.
- Oxalic acid: For tough rust stains, oxalic acid crystals can be effective. Mix 2-4 tablespoons of oxalic acid powder with 1 cup of warm water until dissolved. Let the stain sit for 15 to 20 minutes after applying this solution, then rinse. Be very careful handling oxalic acid, as it is toxic.
- Scrubbing with an abrasive: For light rust staining, sometimes physical abrasion can help scrub the stain away. Use a stiff bristle brush or abrasive sponge pad to scrub the affected area. Try to avoid damaging the surface. Test in an inconspicuous spot first.
- Sandblasting: In severe cases, sandblasting by a professional may be required to remove deep-set rust staining. This should be left to concrete repair experts.
Also read: Repairing Concrete Steps: A Step-by-Step Repair Process by The Concrete Doctors
Organic Growth Removal
Concrete surfaces are prone to organic growths like mould, mildew, algae, and fungi. These can leave unsightly stains and damage the concrete over time. Bleach, hydrogen peroxide, and deck cleaners are effective solutions for removing organic growth from concrete.
Bleach
Bleach contains sodium hypochlorite, which kills organic growth.
- Mix 1 part bleach with 3 parts water and apply it to stained areas.
- Let it sit for 10 minutes before scrubbing with a stiff brush.
- Rinse thoroughly.
- Repeat if needed for stubborn stains. Bleach may lighten concrete colour so test in an inconspicuous spot first.
Hydrogen Peroxide
As an alternative to bleach, hydrogen peroxide can also eliminate organic growth in concrete.
- In equal parts, combine water and 3% hydrogen peroxide.
- Apply it and let it sit for 10 minutes before scrubbing and rinsing.
- Hydrogen peroxide is less likely to affect concrete colour.
Deck Cleaners
Look for deck cleaners that contain sodium hypochlorite or hydrogen peroxide as the active ingredient. These are formulated to remove organic stains from outdoor surfaces like concrete patios, driveways, and sidewalks. Follow the product instructions for application. Deck cleaners are typically more convenient to use than mixing your own bleach or peroxide solutions.
Thoroughly rinse concrete after any chemical cleaning to remove residue. Wear protective gear like gloves, long sleeves, and eye protection when working with bleach, peroxide, or deck cleaners. Repeat applications may be needed for stubborn organic stains on concrete.
Also read: The Importance of Proper Subgrade Preparation for Concrete
When to Call a Professional
Concrete discoloration can be frustrating to deal with as a homeowner. While many common stains can be removed through diligent cleaning, bleaching, and the use of specialty products, there are certain situations where it’s best to call in a professional:
Severe Staining
Some stains penetrate deep into the concrete, beyond the surface level. Examples include oil spills that have soaked into the concrete over time. Attempting to remove stains yourself may only partially lift the blemish. Professionals have access to more powerful cleaning agents and techniques to thoroughly lift stubborn stains.
Etched Concrete
An etched concrete surface has a rough, pitted texture. This occurs when acidic substances eat into the concrete. Examples are spills of acidic liquids like juices, vinegar, or chlorine. Attempting to polish away etching yourself is difficult without industrial tools. Professionals can mechanically grind down the surface to restore smoothness.
Large Projects
For large exterior concrete surfaces, like driveways and patios, full-scale cosmetic restoration is a massive undertaking. From deep cleaning to pressure washing to resurfacing with new colouring and sealants, professionals have the right equipment and workforce. Concrete repair experts can refresh the entire surface for a consistent, even appearance.
Also read: Precast vs Poured-in-Place Concrete Steps: A Comparison of Durability, Cost, and Customization
Final Words
Addressing common concrete discoloration issues can significantly enhance the appearance and longevity of your concrete surfaces. By understanding the causes – such as chemical reactions, efflorescence, organic growth, and rust stains – you can take preventive measures during installation and employ effective cleaning techniques for maintenance. Whether using cleaning agents, acid stains, power washing, or specialized treatments, homeowners can tackle most discoloration challenges. However, for severe staining or large projects, consulting a professional like The Concrete Doctors can ensure the best results, restoring your concrete to its optimal condition and preserving its aesthetic appeal.
