What Causes Rust Stains on Concrete (and How to Prevent Them)

What Causes Rust Stains on Concrete (and How to Prevent Them)

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If you just poured and laid down a new concrete foundation for a driveway or a garage, something that you might notice is that over time, it starts to rust. Now, not all concrete is going to do so, but with that being said, much of it does, and there are many reasons for this.

If you notice that your concrete has brown or reddish stains on it, especially if there are cracks, then your concrete is rusting. Let’s figure out what causes this, as well as how to prevent this from occurring in the first place.

4 Causes of Rust Stains on Concrete

As we mentioned above, there are several different causes, so let’s take a look at all of the possible culprits right now.

1. Soft Rock

Perhaps the most common cause of concrete rusting is soft rock. Now, what you need to know is that concrete is made by mixing cement with water, sand, and stone aggregate. That stone aggregate is very important for the binding process.

However, with that being said, when companies mine that stone aggregate out of the ground, there are often other rocks that come along with it, which we know as soft rock. A lot of this soft rock is sifted out, but it is simply impossible to remove all of it from the stone aggregate.

Soft rocks are, as the name implies, quite soft and they tend to absorb a good deal of water. Plus, they might also contain a high amount of minerals. These soft rocks can also contain a lot of metals. Therefore, when it rains or when your concrete gets wet, these soft rocks can absorb water, crack, get exposed to the air, and then eventually rust.

As such, it’s not the actual cement in the concrete that rusts, but the soft rock. You will notice that these soft rocks will continue to rust until they are removed or are just totally deteriorated.

2. Rebar

If you built any kind of concrete foundation or a wall that needs to be reinforced, then chances are that you used rebar for that reinforcement. Rebar effectively provides your concrete with a metal skeleton that will help support it.

Now, with that being said, rebar is made out of metal, and no, it’s not made out of stainless steel. It’s usually made out of steel and/or iron. What is important to note is that rebar is usually buried deep down within the concrete, so it won’t rust. However, if cracks form in the concrete, then water may be able to get to the rebar, in which case it will start to rust.

This is especially bad during the wintertime in colder areas because road salt that is poured on the road to prevent ice from accumulating has the ability to cause concrete to crack much faster than it otherwise would.

As the rebar starts to rust, it actually creates a chemical reaction that causes the cracks in the concrete to get much worse, and this only accelerates the rusting process. The rust can then actually work its way up to the surface and cause the soft rock and mineral aggregates in the concrete to rust as well. This is perhaps one of the most common causes of concrete rusting.

3. Fertilizer

If you happen to have a lawn or garden right next to your concrete, and you often use fertilizer to make sure that those plants grow big and strong, then you can rest assured that the culprit of your concrete rusting is that fertilizer. Fertilizer generally contains a whole lot of iron and other minerals, all of which have the ability to rust exposed to water and oxygen.

You will generally notice these rust stains occur closest to those areas where you use the fertilizer, and they generally look like small orange spots.

Keep in mind that this process, between the time it takes you to fertilize and the time it takes for the rust to occur, can take as little as one day. The good news here is that this type of rust stain is generally not permanent and quite easy to clean. This rust generally only sits right on the surface, and that does make things a bit easier.

4. Exposure to Hard Water

Although this is fairly uncommon, another possibility here is that random waters, whether from your garden hose, rain, or from nearby rivers and streams, may be exceedingly hard. Hard water contains a lot of minerals and may contain high concentrations of iron, which when exposed to the air, will rust.

If you notice rust stains forming on your driveway after it rains or after you excessively water your plants, then hard water may very well be the cause.

5 Ways to Prevent Rust Stains from Forming on Concrete

Now that we know what causes rust stains on concrete, let’s talk about how to prevent them from occurring in the first place.

1. Be Careful with Fertilizers

One tip that we can provide you with here is to be very careful with fertilizer. If you are going to fertilize your garden and lawn then make sure that you don’t get any of that fertilizer on the driveway. Moreover, when you go to water your plants or your lawn, make sure that the runoff from the water does not go onto the driveway. When the fertilizer mixes with water and gets onto your driveway, it will rust, so keeping fertilizer away from it is very important.

2. Remove Metal Objects

Another important thing that you should do in order to prevent your concrete from rusting is to remove any metal objects that are in direct contact with it. Any type of metal that can rust will rust eventually, and that rust will start to transfer onto the concrete itself. Therefore, things like fence posts and other such items should not be planted directly in the concrete, or if they are planted directly in the concrete, make sure that they are made out of a type of metal that is not going to rust.

3. Regular Cleaning

If you are really worried about rust stains accumulating on the concrete, something that you can do is to clean the concrete regularly using a special concrete cleaner. There are many types of concrete cleaners out there, and yes, there are those specifically designed to remove rust stains. Now, even if you don’t already have rust stains that you can see with your naked eye, there might be small amounts starting to accumulate. If you use a special rust removing concrete cleaner about once per month, you can prevent any kind of buildup from occurring.

4. Use a Concrete Sealant

In order to prevent concrete from rusting in the first place, one of the best things that you can do is to use a special type of concrete sealant. A concrete sealant will add a layer of protection overtop the concrete. It will help prevent both air and moisture from getting to the surface of the concrete, and it also helps prevent the concrete from cracking, thus preventing water from getting to any rebar underneath.

5. Regular Maintenance and Repairs

If you notice that you have some cracks in your driveway or in your concrete foundation, then you should definitely repair them right away. The more cracks there are in the concrete, the more of that soft rock is going to be exposed. And if you happen to have rebar in the concrete that will be exposed as well with both the soft rock and the rebar having the potential to rust. Regular maintenance repairs and regular cleaning is often more than enough to prevent concrete driveways from rusting.


Now that you know why concrete rusts, as well as how to prevent it from occurring in the first place, you can work diligently to keep your concrete looking brand new.