Discolored Concrete: 3 Causes and What You Can Do

Giovanni Valle is a licensed architect and LEED-accredited professional and is certified by the National Council of Architectural Registration Boards (NCARB). He is the author and managing editor of various digital publications, including BuilderSpace, Your Own Architect, and Interiors Place.

You’ve just finished having your driveway done and are happy with the initial results. However, after a few days, you start to notice that something isn’t right. Some parts of your concrete start to become discolored, and now your driveway is covered with blotchy stains.

Concrete becomes discolored because of differences in the mix of two different batches. Concrete batches must match in levels of moisture, timing, and composition. What can be done to avoid discolored concrete is to try to keep the mixes consistent and not rush the placing and drying process.

With that being said, concrete is pretty simple to mix and place. However, if your goal is to create a consistent and beautiful concrete slab, then there are many factors during the mixing, placing, and drying process that you need to be careful with.

Why does Concrete Lose its Color?

Concrete is usually thought of as only mixing cement powder and water together. Which is not wrong, but is also not entirely right. There are different types and brands of concrete mixes, each of them containing different levels of gravel and sand. Also, there are cement replacements such as slag, which are entirely different from traditional concrete.

When water is added, the right mixture between water and air needs to be made. Different levels of any of these components can result in varying degrees of discoloration once the mix dries up. As a rule of thumb, the more moisture in a blend, the lighter it will be. Therefore, using a less hydrated mix on a more hydrated one can create the appearance of dark spots.

The usage of water after the cement mix has been placed and left to dry, as the water will not be spread equally throughout the project, and some parts of the cement tend to dry before others do, making them more prone to absorption.

Another factor that could change the coloration of your cement project is a difference in the mixture of your cement. This can occur in two ways:

One is a change in the provider of the cement mix. Cement mixes, although similar, tend to have naturally different colors. This can cause inconsistencies in the color of your cement project.
Secondly, the usage of calcium chloride in an attempt to speed up the setting process of the concrete can lead to discoloration. Calcium chloride can significantly increase the chances of your cement project turning dark.

Lastly, there’s the issue of poor workmanship. This can occur in any of the following forms:

Environmental factors such as dry/humid weather are not taken into consideration
Trowel burning the concrete, which would basically ‘burn’ your cement into a darker color
Finally, improper mixing of water and cement when using two different batches for the job

These are the main factors that could cause your cement to decolorate either into a lighter or darker color and could create an uneven and unappealing image on your concrete project. However, let us move on to what can be done to avoid said discoloration.

How to Keep Concrete from Turning Dark

As the saying goes, “better to prevent than to cure.” Preventing your concrete from turning dark is absolutely possible. It merely requires you to be knowledgeable and continuously careful and aware when going through the steps of laying down a cement mix.

Your main enemy will be moisture, and as previously mentioned, moisture can come from many different sources. These include disparities in the water levels of two separate batches, the weather, and poor workmanship. When it comes to your cement turning dark, what you want is for it not to lose moisture.

Ways of avoiding this are by keeping a good record of how much water was used in the first batch when making multiple batches for one project. Alternatively, you could just make sure you can finish the whole project with a single, homogeneous batch. If what you want is for your cement to be light, use more water, but be careful not to make your mix too porous.

When it comes to the weather turning your cement darker, you’re looking at dealing with the effects of dry weather since it can take away the moisture from your mix. What can be done to avoid the impact of dry weather is to moisturize the subgrade (the floor underneath the project) before placing a slab. Additionally, windbreakers and evaporative retardants could be used.

Now, to avoid turning your concrete dark through improper technique, it is essential that you do not use calcium chloride. Calcium chloride is commonly used to accelerate the drying process of a concrete slab, but it is basically begging for a darker color to appear in your cement.

Another common practice that could result in a darker color for your cement is trowel burning. Trowel burning will basically evaporate the water out of your cement mix, thus turning it darker as a result. And lastly, the most effective way of dealing with poor workmanship is to get a professional contractor that knows how to handle and lay cement properly.

These are some of the main ways of avoiding color discoloration for your cement project. However, if the damage is already done and your concrete has turned dark, then fret not. It is, in fact, possible to remove the dark color from your concrete.

How to Remove Discolored Concrete

There are a couple of different methods that can be used after your concrete had been discolored from using calcium chloride. However, discolorations, such as the one resulting from trowel burning, are too complicated or even impossible to get rid of. These methods apply primarily to concrete flatwork.

One way to undo discoloration is through brushing and scrubbing the concrete slab with hot water. It could also be flushed and brushed and let to dry overnight, and, ideally, the discoloration should be gone.

If this doesn’t work and the discoloration is still there, then an acidic solution might work. Mix a diluted solution (1% concentration) of hydrochloric acid or dilute solutions (3% concentration) of weaker acids that can be used. Make sure you dampen the concrete before applying the acid so that it doesn’t seep into the cement.

You could also try using a 20 to 30% water solution of diammonium citrate. It should be left on the concrete for about 15 minutes, and then the whitening gel should be thoroughly rinsed and brushed away. This method is commonly used for severe cases of concrete discoloration.

There are also some pre-made solutions in the market, such as Wet & Forget, which promise to lighten up your concrete while causing minimal to no damage to your lawn or clothes.

How to Properly Lay Down Concrete Without Dark Spots

Now that we know the leading causes for discoloration, both light and dark, on cement, how to prevent them, and how to possibly correct them, let’s go over the process of successfully laying down concrete without any dark spots.

Preparation

First, you will need to be careful when gathering your materials. You will want to be particularly cautious when choosing your first provider/brand of cement mix. Some factors to look at when doing this are color, price, and composition. This is important because you will need to stick to that very same provider/brand throughout the remainder of the project to avoid inconsistencies.
Next will be choosing who will carry out the project. Mixing cement and placing it is not necessarily rocket science, an inexperienced person can learn the steps quite easily. However, the value of experience can reduce the chance of mistakes occurring, so you must decide if you will do the whole project or hire an experienced handyman to help you or do it for you.
Next is the area where you will place the concrete. Factors that you will need to be aware of are the weather, mainly of the dry and hot variety. If your area experiences warm and dry weather, then you should try getting the subfloor wet before you begin. Additionally, you will want to make sure the surface is even and stable.
Try to check the surface for any moisture deposits or foreign materials that could interfere with the coloring or, most importantly, the integrity of your concrete project.

Placement

Now that you’ve got your materials and preparations ready, it is time to get your hands dirty. You will need to put the cement mix in the mixer with a particular amount of water, sand, and gravel that you will need to remember if you are planning on using an additional batch for the same job.

Try to avoid cement replacements such as “green cement” alternatives like slag, since these are mostly made out of unstable or multiple materials that can discolor after placement. That is if your sole goal is to have an even-colored cement project. If you are using it for environmental reasons, then green cement is just as good as traditional cement.

You might be tempted to add calcium chloride to your mix to get the drying process done faster. However, this would be a terrible mistake, since calcium chloride dramatically increases the probability of your cement becoming discolored.

Make sure your mix is moist to avoid discoloration but not too wet to the point where the cement can become porous. Pour the cement on the desired area and then trowel it to even it out. However, hard troweling must be avoided, since the high temperature will evaporate the moisture from the cement, turning it dark.

Drying

Now that your cement has been adequately mixed, laid, and troweled, it is time to let it dry. Ideally, you did not use calcium chloride in your mix, which would have made your cement more likely to turn dark.

Avoid sprinkling the finished project with water. This would only further moisturize some parts of your cement, which would end up making some spots lighter than others. In summary, let your cement dry on its own and protect it from the heat with windbreakers.

You might choose to use a polyethylene sheet to cover your cement project, which is not a wrong thing to do. However, make sure that the plastic sheet is entirely even since leaving folds or wrinkles on the sheet will create air bubbles that could trap moisture in them, which will essentially give you lighter spots.

It is also imperative to note that your cement project will be darker when first placed and throughout the first 7 to 10 days of drying and curing.

Are there any Advantages to Having Dark Cement?

You are most likely reading this article to find ways to avoid or get rid of the dark color of your cement. Still, before you read the conclusion and go through with your plan to have a lighter cement, it is essential to note the advantages of darker cement, too.

There aren’t necessarily any significant advantages or disadvantages when it comes to darker concrete color. This has more to do with moisture. As previously mentioned, darker cement colors arise from lower water content in the cement mix. Likewise, light concrete colors occur from a higher concentration of water in your mix.

The difference that moisture makes, aside from color, is how porous and how solid your concrete project will be. Add too much water, and the concrete project will be very porous and sensitive to breaking. Add too little, and it will be hard to work with, and the improper mixing might also make it unstable.

Therefore, a darker color might actually signify that your concrete is more on the solid side, as long as the mix properly absorbed all the materials. Also, it is essential that we define what ‘dark’ concrete is.

No concrete should turn dark (unless you’re paving a road). A ‘dark’ color might result from darker spots being next to notably lighter spots, making the dark spots appear relatively ‘dark.’ Thus, having darker spots shouldn’t necessarily mean that the spots are somehow less desirable than lighter spots.

Dark spots or a darker overall discoloration resulting from spills of foreign material can be a problem since they could signify that your concrete surface has been compromised.

Are there any Alternatives to Concrete?

There are a few alternatives to concrete. “Green concrete” is one alternative. It is a mix of recycled materials intended to minimize the environmental impact of construction.

These materials include slag, which is a combination of many recycled materials mixed together to form a concrete-like mix. However, using cement replacements next to an existing pour is usually not a good idea. In the case of slag, a greenish or bluish tint can appear on your concrete after about a year, and it won’t look as pretty as it sounds.

Primarily, for this reason, you might want to stick to conventional concrete if your objective is to have a homogenously colored cement project.

In Summary

Concrete can become discolored for a myriad of reasons. These reasons range from moisture levels to foreign materials being spilled over the surface. Many factors can result from the builder’s behalf, such as in the case of negligence.

Therefore, it is essential that when preparing to start your project, that you carefully choose the cement mix provider/brand that you will use throughout your entire project. It is also important that you choose someone who has experience on the subject to help you if you do not trust yourself enough with working alone on the project.

You must also make sure that you take subfloor and hot weather into consideration since these can affect the moisture levels of your mix. Do not try to rush the process by adding calcium chloride and do not hard trowel the project. Both can result in darker colors and, worse, make it uneven.

Dark concrete can, in fact, be removed through the usage of many liquid mixtures, most of them involving acid. However, they are not guaranteed to work, particularly with the case of hard troweling. Alternatives to concrete should not be used if the priority is even coloration since “green concrete” can have unpredictable discoloration.

There are no clear advantages or disadvantages to having darker spots or overall coloration on your concrete project unless they were caused by foreign chemical spills on its surface. Darker concrete can actually be more solid when it is a result of an adequately low water mixture. ‘Dark’ concrete is mostly a relative term.

In summary, be careful throughout the concrete project and do not rush things. Lack of moisture will be your worse enemy if you want a lighter concrete, and it is always much better to take steps for prevention than to cure.

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