What Causes Concrete Staining?

Stains are ugly and undesirable. They make the concrete look old and dirty, and thus make the room or house look the same way. There are many reasons as to why concrete would stain, but let’s focus on the less obvious ones. Why concrete would have oil stains is a pretty obvious answer, but sometimes, even well-preserved concrete will stain out of nowhere. Here are some of the reasons why this would happen. 

• How the Concrete Was Mixed  

How the concrete was mixed plays a huge impact on how it looks over the years. One of the biggest reasons concrete stains is the addition of calcium chloride. Calcium chloride accelerates cement hydration and reduces setting time by over 50 percent. However, the downside to using calcium chloride is that it stains the concrete because of its chlorine content. 

Too much water in the concrete mixture also causes stains in many ways. If there’s too much water, the sealant and the water never bond together, eventually resulting in the sealing agent evaporating faster than the water. Too much water can make the resin in a sealant bubble up, causing the concrete to stain while developing a frothy texture.  

• Rust Stains 

Rust stains can be caused by two things: fertilizer and soil. Fertilizers contain minerals that have a chemical reaction when exposed to water. It’s very common for concrete near plants to have rust stains due to oxidative fertilizers. The mineral content in these fertilizers can also be transported via the homeowner’s shoe inside the house, where they will stain indoor concrete such as garage floors. Cars also bring in harsh salts and metals on the tire, where the concrete stains once it gets wet. 

North Carolina’s state soil is known as Cecil soil. It has a brown loam topsoil, but the rest of it is red clay soil. Red soils are rich in metals such as iron, which oxidize and give the soil its iconic red color. Unfortunately, the soil isn’t the only thing that turns rust red. The concrete slabs supported by the soil are also stained.