Why Does Concrete Change Color?

In most cases, concrete is poured in an area where it must be both durable and visually attractive. Whether it’s a sidewalk, guardrail, or patio, you expect that the concrete will look good, or at least uniform in color.

The leading cause of concrete discoloration is inconsistency. There can be inconsistency in the mixture, in the materials, or from one mix to another. Changes in moisture levels and weather can also have an effect.

Concrete Discoloration: Types and Causes

Concrete can discolor in a few different ways. It can be blotchy, spotted, striped, or may just differ in color from one area to another. These variations often depend on the cause of the discoloration.

There are several causes of concrete discoloration. Read on to learn more about each of the reasons.

Mixture

Concrete is an admixture of water, aggregate, and cement. Different ratios between these materials can affect the color of concrete. In particular, increasing the amount of water will lighten the concrete. 

When a concrete slab is poured in multiple batches, any variation in the mixture ratio will result in some discoloration. The ratio must be precisely the same for each batch to ensure a consistent color.

pouring concrete slab

Materials

The cement and aggregate used in concrete are made up of several different materials. These materials can vary from batch to batch.

There is an art to getting the exact same materials in the exact same ratio if more than one load of concrete is needed.

Water

While the amount of water can affect the color of concrete, when and how it is added also plays a role. 

If too much water is added or it isn’t properly mixed, it can result in blotchiness.

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While blotchiness is unsightly on its own, it becomes even more of a problem when there is more than one pour.

If one pour is carried out without the addition of water, while another pour comes with added water, there will be a definite discrepancy in color between the two slabs.

Calcium Chloride

In some cases, contractors will add calcium chloride to the concrete mixture if they are trying to get the concrete to set quickly. If there is a low percentage added – less than 2% is ideal – then there shouldn’t be an issue.

However, many contractors add too much calcium chloride, leading to dark concrete. If the concrete isn’t mixed correctly, the discoloration is even more pronounced, and there may be blotching.

Weather

Weather is another factor that can cause concrete discoloration and curling. Pouring concrete in summer usually doesn’t cause problems, but winter pouring can sure be an issue.

Winter tends to bring lower temperatures and high humidity, which can slow the hardening process.

Crystallization can occur, giving the concrete a darker color near the surface. One solution is to increase the concrete’s temperature. You may also want to try using insulation in the forms to warm up the concrete. 

Workmanship

In some cases, it comes down to the contractor or the crew. Inexperienced crew members may not employ the best finishing methods. Even improper troweling techniques can lead to a discolouring effect.

There can also be many unknowns that occur during a pour, such as rapid changes in weather.

An inexperienced contractor may not know how to respond to each of these issues as they come up, and may not know the techniques to mitigate problems. Some may even worsen the problem rather than remedy them.

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Pouring

Contractors will sometimes try to rush the pour, especially if they need to get a job done quickly when bad weather is coming. They don’t allow enough time for proper placement of the concrete.

By trying to pour too quickly, the concrete will not harden correctly, and discoloration may result.

concrete pouring

Curing

Curing methods can sometimes discolor concrete. If the curing isn’t even, it can affect the concrete’s appearance. Some contractors use polyethylene sheets for curing. The plastic can lead to streaks on the surface. 

How Do You Fix Concrete Discoloration?

The easiest way to remove discoloration is to use hot water and a brush to scrub the area.

In some cases, the discoloration will simply disappear.

If that doesn’t work, you can next consider an acid wash solution. It’s essential to use the weakest concentration possible because the acid could damage the concrete.

Start with a 1% concentration and don’t go over 3%. Make sure to wash with water after applying one coat of acid, and before you apply the next. 

If scrubbing and acid don’t work, your last option is to seal, coat, or paint the concrete. A sealant will give the top of the concrete a uniform color and help protect it against damage from warm air and water.

Sealants come in a variety of colors and are reasonably easy to apply.