How To Remove Rust Stains from Concrete

If you’re staring at the eyesore of a rust mark on your cerement patio or walkway we know the feeling. Here’s how to remove rust stains from concrete yourself, plus how you can prevent it from happening again in the future.

Given how hardy it is, and the smooth finish it can feature without too much work, concrete is an element that can be found in a great variety of areas around the house. Patios, garages, driveways, any location with practical use will benefit from this material. That said, concrete can be affected or blemished by exposure to certain substances, and rust is a fairly common one. In this article, we will discuss what may cause these stains, the possible effects, and multiple ways for removal.

Is It Dangerous?

In a word: no. This is more of an aesthetic problem. Rust can develop on any area with concrete regardless of specific location (more on this shortly), which includes your driveway, for example. Depending on where it is, it may turn to be a rather glaring eyesore, which is why it is convenient to know how to get rid of it and, also, how it could be prevented.

What Causes Rust on Concrete?

Most of us know what rust is: what happens when iron reacts to oxygen exposure, in the presence of water (such as moisture in the air). From here, it may be tempting to assume that rust appearing on concrete is rare and easily avoidable. It’s not so simple, however, and proper resolution of the problem may very well depend on correctly identifying (and addressing) the underlying cause.

Fertilizer

Surprised? Well, it’s true!, and it may very well be the cause for those stains appearing seemingly out of nowhere. Many fertilizers incorporate minerals, such as iron, as part of their formula. This is usually of no consequence (it’s what plants crave, after all), but if your fertilizing happens too close to a portion of concrete—or right on top, such as with potted plants—, those minerals may accumulate and rust, causing little orange stains.

It’s also possible for fertilizer to be dropped in transit on top of the concrete, or blown by the wind in that direction, forming stains further in. Generally, these blemishes are fairly easy to clean—unless the fertilizer happens to seep into the concrete’s pores, which will be harder to deal with.

The recommendation: Aside from keeping your gardening locations as far away as possible, always sweep any concrete areas after using fertilizer, especially if you expect the weather to become more humid. Metals present in fertilizer can begin to rust (and, therefore, stain) in as little as a day after being exposed to rain.

Presence of Metal

As the saying goes, the simplest answer is often the correct one. Fences, grates, bolts, shelves, brackets, any bit of metal that is too close to concrete can cause stains to develop. It can be as easy as forgetting a tool which then gets rained on; this, in particular, creates what is called a spill, i.e. rusty water seeping out of the tool and onto the concrete surface.

If this is allowed to happen for long enough, the stain may set in, making its removal more difficult (though not impossible).

It should be noted, the metal in question may be well out of sight—which doesn’t prevent it from causing issues, as you will see in our next item.

Rusty Rebar

A popular bit of trivia: crocodiles’ bite force is tremendous—3,700 psi, to be specific—, but their muscles for opening their jaws are, by contrast, so weak, it’s possible to keep their mouths shut with some electrical tape (don’t try this at home).

Concrete is similar, in that its compressive strength (ability to bear weight) is formidable, but its tensile strength—or resistance to pulling forces—is much, much lower. Imagine a concrete beam, placed horizontally between two pillars: if there is no support in the middle, and one of the pillars happens to tilt far enough away, said beam would snap, just like that.

The solution to this shortcoming is rebar made of steel, which gives a supporting framework to concrete, allowing it to be used for giving shape to just about anything—including bridges and overpasses. The one caveat: it can develop rust. Normally this isn’t a problem, as the rebar is situated to deep into the concrete for any exposure to happen. However, any cracks that develop on the concrete may allow water to seep in, coming into contact with the rebar within.

If this happens, it can lead to a slowly cascading chain of events. First, the rebar rusting will create oxide that will expand within the concrete, eventually causing cracks; this will increase exposure for the rebar, which in turn will cause more rust, and oxide. Eventually, the rust will make its way to the surface, visible as a stain.

Soft Rock

This may seem far-fetched—rock, in my smooth walls and walkways?—, and as such requires an explanation. To create concrete, you begin with cement; then comes sand, to provide texture. What really gives the mixture its well-known strength, however, is a stone aggregate. When mining the stone for this aggregate, soft rocks are also extracted. While a great portion of this unintended component may be filtered out, it is essentially impossible to remove every bit of it. For this reason, concrete manufacturers are allowed a certain amount of soft rock in their batches.

Soft rock is more porous, and comes with a higher iron content. If it happens to be too close to the exterior after the concrete has been applied, then it will absorb water, which will make it expand, push further out, and expose its iron to the elements. This, in turn, causes rust to develop.

A related issue: popouts. This happens when there is enough iron within the concrete to react to the water outside, which, similarly to soft rock, makes it expand, push into the surface and, as its name implies, pop out.

Given the many metaphorical planets that must align, issues related to the concrete’s very composition are not among the most common causes for rust development; however, they can be harder to tackle, as it usually entails removing the affected spot altogether. The best preventative measure is the same as with the rebar: sealing every possible vulnerability so nothing will be unduly exposed.

How to Clean Rust Stains From Concrete

Here we are. We have identified the cause. We have, hopefully, taken measures so it (hopefully) won’t happen again. Next on the agenda, dealing with the stains themselves. Before beginning, it is usually recommended to wash the area with soap and water, for better results. Another thing to keep in mind is that it may take several washes to be rid of the stain.

The following solutions are sorted by stain difficulty, from easier to harder.

DIY Cleaning Options

Dealing with rust requires some form of acid, which even the stronger cleaners will incorporate in some form. The cleaner you pick can be poured on the area if the surface is horizontal, or sprayed for walls and similar locations.

For simpler stains, you can try with lemon juice. Let it sit for about 10 minutes, and then scrub in small circles, to help remove as much of the rust as possible. Rinse with warm, soapy water. If that doesn’t work, you can upgrade to white vinegar, which should also be allowed to rest there for several minutes before moving on to scrubbing and rinsing.

Detergent can be used as well, but it will require a boost. To achieve the cleaning power we need, mix one part of baking soda with two parts of detergent. Apply to the stain, let it sit for a while, then scrub and rinse.

If the surface is just too delicate, and any stiff brush could cause scratches, you can also use a sponge dipped in a vinegar solution (one cup of vinegar and half a cup of water). Dab the surface gently, and perform several passes until the stain goes away.

If none of this works, it may be time to move on to more powerful alternatives.

Commercial Cleaners

With this sort of compounds, it is also often recommended to perform a test on an area that is not so visible. As always, one should follow the manufacturer’s instructions when using.

We’ve Done It: No More Rusted Concrete!

We now know that rust stains on concrete are no terrible danger—which doesn’t mean they should be allowed to stay. We are now prepared to get rid of them, and to deal with the underlying causes so they will hopefully never return. May your concrete walls and surfaces remain healthy and spotless for many, many years.