Why is Concrete Changing Color?
Discoloration of the surface of concrete slabs can appear as rough color changes in large concrete surfaces, light or dark spots or spots on the surface, or early light efflorescence. Laboratory tests to determine the effects of different concrete processes and materials show this. No single factor is responsible for all discoloration.
The discoloration factors are calcium chloride mixtures, cement alkalis, hard surfaces, inadequate or insufficient curing, wet subsurface, varying proportions of aqueous cement on the surface, and changes in the concrete mixture. Discoloration from these causes appears very soon after concreting.
Discoloration in later life can result from atmospheric or organic coloring; the concrete is dirty. This discoloration is usually removed by high-pressure washing with high-pressure water and, if necessary, chemical cleaning agents.
The use of calcium chloride on concrete can discolor the surface. Calcium chloride speeds up the hydration process but has a retarding effect on the hydration of the ferrite compound in Portland cement. The ferrite phase usually becomes lighter with hydration. However, the delayed, non-hydrated ferrite phase remains dark in the presence of calcium chloride.
How to Avoid Blotchy Concrete?
Extreme discoloration can result from attempts to firmly glue the surface after it has become too stiff to be properly trowelled. If a surface is vigorously trowelled, gradually compacting it, the point can be reached where certain areas’ water-cement value is drastically reduced. As a result, the armored area is almost always darker than the adjacent concrete.
Waterproof paper and plastic sheets used to wet calcium chloride-containing concrete are known to give flat surfaces a speckled appearance due to the difficulty of placing and maintaining a cover in complete contact with the surface over the entire area. In addition, they are lighter in color than those that are not.
Concrete materials and proportions influence the color of the concrete. Individual types of cement can differ in color. Therefore, replacing one cement with another can change the color of the concrete. Concretes that contain significant amounts of mineral additives, fly ash, fumed silica, metakaolin, or slag, may differ in color from those that do not contain mineral additives.
The color of the sand affects the color of the concrete. High-strength concrete with a low proportion of water-cement is darker than low-strength concrete with a high water-cement ratio.
Discoloration of concrete can be avoided or minimized by
- avoiding the use of calcium chloride additives
- using consistent concrete ingredients provided consistently from batch to batch
- with proper placement, finishing, and timely curing. In addition, concrete practices should not vary as any interruption or change in the concrete mix, formwork, surface treatment, or curing can result in significant and sometimes permanent discoloration.
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How do you remove discoloration?
The first (and generally effective) remedy is a complete and immediate rinse with water. Let the tile dry, then repeat rinsing and drying until the discoloration is gone. Use hot water whenever possible. Weaker acids, such as triacid (vinegar) or 3% phosphoric acid, reduce carbonic acid and stain formation. Treating a dry plate with a 10% caustic soda (sodium hydroxide) solution can merge light spots with a darker background. Strong acids should not be used as they can release the aggregate.
One of the best ways to remove the most stain when other agents have failed is to treat a surface with a 20% to 30% diammonium citrate water solution. However, this chemical is expensive to purchase in small quantities, so consider it last. For these and other treatments and recommendations to help prevent and remove discoloration, according to Surface Discoloration of Concrete Flatwork, RX203; “Discoloration of Concrete—Causes and Remedies,” Concrete Technology Today, PL861.
Coloring discolored concrete with a chemical stain is another way of making color deviations less noticeable. Darker colors generally hide color variations more effectively. Chemical dyes can be used on concrete indoors or outdoors.
Concrete Discoloration Causes
Rare discolorations ranging from beige to red / orange have been reported in Wisconsin, Illinois, Louisiana, and other states. This discoloration is more likely to occur in times of high relative humidity and ambient temperatures. Often with specific types and amounts of wet curing. In addition, fly ash aggravates stains by intensifying the color.
Differences in the hardening and degree of hydration of cementitious surfacing materials under high humidity and ambient temperature conditions likely cause this discoloration.
In particular, the additional hydration of ferritic compounds in cementitious materials reduces iron availability to oxidize and discolor concrete. In addition, research has shown that staining occurs under certain conditions, including oxygen and water availability.
Blotchy Concrete Remedies
Fast-drying of concrete leads to insufficient humidity to produce the color, while continuous immersion does not allow air access. In both cases, there is no discoloration. Therefore, keeping the concrete wet for the required curing time and drying it as quickly as possible is recommended. For example, a wet canvas covered with plastic sheeting should be removed in the morning of a hot day rather than in the evening or before it rains.
These types of stains are difficult to remove. Commercial sodium bisulfate cleaners successfully remove the color; however, the difference between cleaned and stained areas decreases over several weeks.
The following chemicals are highly ineffective in removing this polish from orange stains: hydrochloric acid (2%), bleach, hydrogen peroxide (3%), phosphoric acid (10%), oxalic acid (3%), and diammonium citrate (0.2M).
We hope this technical data will help you avoid many mistakes and eliminate your concrete issues. You can request a Free Estimate from Concrete Contractors near me. Call DUOMIT if you need technical assistance. Read also about Concrete Grinding and Concrete Crazing and Curling. Or what shall you do if you get Asbestos in Flooring?