Concrete is a fairly straightforward mixture of cement, sand and water that is used in driveways, basement slabs, sidewalks and even streets. While it generally cures in a uniform fashion, sometimes cement will have dark splotches that show on the surface as the material cures, or dries. There are numerous reasons for this, ranging from finishing methods to mixing methods and specific ingredients.
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One of the most common reasons for cement to have different colored areas during the curing process is that the cement was mixed differently in one batch versus another. It might have been an extra 15 minutes of curing, a little extra sand, a little extra cement or something else, but if the mixture is off by even a little bit, it can cause discoloration as the concrete cures over the weeks.
Too much water is another common reason that cement mixtures take on dark splotches during the drying phase. As the water evaporates to the surface and soaks into the cement that is an ingredient of concrete, it leaves behind darker spots where the cement contains more moisture. These will fade over time as the water eventually soaks into or evaporates out of the slab.
If you live in a tropical section of the world or have a high humidity rating, it can sometimes affect the drying time of cement. As a result, the water takes longer to evaporate and soak into the various elements within the concrete, leaving behind dark splotches similar to the ones that appear when excess water is present in the mix.
Potable Water Versus Groundwater
The presence of extra minerals in the overall mixture can create lighter sections that contrast against the darker portions of cement mixed in with the concrete. For this reason finish concrete should always be mixed with potable water rather than groundwater so you can control the presence of minerals present in the overall cement mixture. Too many extra minerals can stain the color of the concrete as it cures.