Rust Stains on Concrete: Causes, prevention, and repair info


The stark gray of concrete porches, sidewalks, and driveways become much less pleasant when those dark oranges splotches and stains start to appear. Out of nowhere, it seems as though the whole slab has become tinted, but these rust spots and stains can easily be solved and understood.

Rust stains appear on concrete due to deposits of iron and water. When it rains, iron from soft rocks within the concrete, fertilizers, metal decorations or tools, rebar, and battery acid can all be causes of rust spots. Stains can be removed with cleaners and prevented with sealant.

To truly understand and get a handle on these pesky spots, it is important to understand where they come from, how to repair them, and how to prevent them from happening again or in the first place.

What Causes Rust Spots

Metal, iron, magnesium, copper, and zinc are the main mineral culprits of these rusty stains. When introduced to water, the chemical reaction causes stains and spots. Objects and materials with lots of iron and other minerals, when mixed with the water from the sprinkler, rain, or even high humidity will cause dark orange spots on your sidewalk or porch.

Soft Rock

Soft rocks are more porous and higher in iron than other basic concrete materials like limestone. They are often near the quarries where the limestone and other materials for concrete are mined and end up in concrete mixtures. “Concrete suppliers are allowed a certain amount of this type of stone within their batches as it is found in the same areas as the other stone used to make concrete.” Source.

The soft rock usually absorbs more water than the rocks around it and pushes out of the concrete becoming exposed to the weather. Once it comes in contact with the water, and air of the atmosphere it begins to rust. It will continue rusting until the rock is completely deteriorated, leaving rust on the concrete for months or even longer.

Fertilizer

Potted plants often have fertilizer within them, and when watered, the minerals and iron from the fertilizer can seep onto the concrete causing little orange spots. Even spreading or spraying fertilizer and allowing it to stay on the concrete for times of high humidity or rain can cause these spots to occur.

These spots offer no safety problem to your concrete and can be left if the appearance is not a problem. When you fertilize, just remember to sweep off your concrete before it rains to prevent those minerals from seeping into the concrete!

Rust In Steel Reinforcements

When laying concrete, as an extra measure of resilience and strength, steel reinforcements may be laid as well. Normally, this would provide for longer life, but that is not the case if rusting happens.

Steel bars are usually too far from the surface to wear from the weather and atmosphere outside, but not when salt is concerned. Salt, when used to slippery surfaces in the winter, penetrates the porous concrete more quickly than normal water and dirt and can cause erosion. When a steel bar is exposed to salt, and then exposed to water and other minerals, rusting starts to occur.

“When steel corrodes, it creates iron oxide that expands inside the concrete. This expansion generates stress, sometimes called oxide jacking, and is the one of the primary causes of concrete deterioration.” Source. This can cause rust to appear from the bars, or concrete to break and crack, revealing rusty steel bars.

Sadly, this often can lead to a phenomenon in concrete called spalling. It is like a pressure cooker of steel, and rust just waiting to explode, leaving your concrete disfigured and broken.

Metal Objects and Others

Those tools you were using yesterday before the rainstorm to work on your lawnmower, or those bolts you accidentally dropped on your way up the walk all contain iron. A little water and time and you once again are seeing rust spots. These are just a few of the reasons, but below are some other culprits of such stains.

  • Though not actual rust but an acid burn, battery acid leaking from vehicles or other equipment will also cause stains that look like rust. After the use of some vehicles, especially golf carts, the battery bubbles over causing these burns on the concrete. But most methods mentioned will always get rid of this kind of stain.
  • Another culprit may be any iron high substance, including soil in some places throughout the world. If rain and wind bring iron dense soil onto concrete, it can also cause spots to occur and rusting to begin.

Prevent Rust Spots

A great way to prevent rust from happening in the first place is to coat and/or seal your concrete upon installation. This puts something in between your concrete and the elements.

Coatings are often applied depending upon the intended use of the concrete. For garage floors or driveways frequented by vehicles, thicker and more protective coats can apply. Coatings put a visible protective film over the concrete to keep oils and other stains from coming in contact with the concrete.

On the other hand, generally, a sealant does the trick. “Sealers do not leave behind a film on the surface, instead they work within the pores to prevent liquids from being absorbed deep into the concrete.” Source. Applying sealant regularly will help allow the soft rock from coming in contact with so much of the elements. Some other preventative methods are:

  • Sweep off all the scattered fertilizer
  • Remove metal tools
  • Apply protective coating to metal furniture or decorations or move them.

Remove and Repair Rust Spots

Getting rid of these spots can be as easy as a simple scrub, taking time to buy specific cleaners, or organizing a time for professionals to come and handle the cleaning. Overall, they can be removed, and your concrete can be restored to its original look. Depending on the rust and your circumstances, you can use a homemade cleaner or purchase a rust-specific cleaner.

Before you begin trying one of these methods, be sure to clear the concrete of any debris or leaves, as well as the hard-flaking parts of rust. This will help to reveal the root of the rust so it can be fixed. While these rust stains cause no damage to the concrete, they can be a real eyesore that can be removed. If it is not indoor or fragile concrete, power washing is effective in prepping the area for repair.

Chemical Cleaners

For those tough stains that do not seem to budge, chemical cleaners are often most effective and can be found at a local hardware store or online. Below is a list of a few, while there are much more available.

  • Iron Out Rust Remover
  • Klenztone Concrete Cleaner
  • CLR Cleaner
  • Singerman Laboratories Rust Remover
  • Krud Kutter Rush Remover
  • Rid O’ Rust
  • Iron Out Rust and Stain Remover

When deciding on which cleaner to use, the following video shows a comparison of many of these products. After a trial, the video claims the best option to be Singerman Laboratories Rust Remover to be the most effective, while others are still good options.

Often, cleaners containing trisodium phosphate or oxalic acid are highly effective, but they are toxic and should be handled with great care. These cannot be used around children or pets and are not the safest in enclosed spaces. Many of these can also strip the concrete of its paint or stain, so research is necessary. Be careful when removing stains and do not use chemicals that will cause more!

Natural Cleaners

There are many discussed remedies that can be made at home to handle rust on concrete. While a professional cleaner would be most effective, if you are up to give it a try yourself, many options have been suggested.

  • Baking Soda
  • Baking Soda & Vinegar
  • Vinegar & Lemon Juice
  • Dish Detergent
  • Laundry Detergent & Baking Soda

All these remedies must be applied and allowed to soak in for 10 to 30 minutes before a good long scrub with a hard-bristled brush. For some rust stains, these will quickly remove them but for others, it may not be the same case. If it seems to be working but does not quite it get it all, reapplying the same remedy multiple times can help. Overall, the more acidic the substance, the more likely it is to remove the rust.

Hopefully, these tips and tricks will help to restore the shine and cleanliness of your driveway, sidewalk, or porch. In summary, it is smart to start by cleaning off the surface first and trying a natural remedy that can be made at home. If that is not effective, buy a cleaner from a local hardware store or call for the aid of a professional. These stains can be removed and prevented with effort and attention!

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