According to Bruce Seelig, a water quality specialist affiliated with North Dakota State University, people frequently misidentify white mould on concrete. A white growth on concrete is caused by mineral deposits, the result of a high water table in areas such as basements. The process, whereby white mineral deposits accumulate on concrete, is known as efflorescence, which becomes most apparent on dark shades of concrete and mortar.
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Soluble salts dissolve in water. When rain penetrates soil, it dissolves existing salts in the ground. If there are mortar cracks in a building foundation or basement, water will seep through the concrete. As the water dries, the salts will be left behind, causing them to stick onto concrete. Over time, these white deposits can mar the appearance of concrete, mimicking the look of white mould.
There are two types of soluble salts that can accumulate on concrete. Chloride salts are the most soluble. Commonly found throughout America, they cause light powdery coverings on concrete. Sulphate salts may also dissolve during heavy rain, but they have a different appearance than chloride deposits. Sulphate deposits typically look like filaments, or long fibres, much like white mould. Carbonates, like lime, rarely cause problems for a homeowner, because these materials are relatively insoluble.
Efflorescence looks like white mould. The unsightly concrete features long white lines, starting where water is seeping through cracks in the mortar. These white crystals can be scraped away with a fingernail, having a powdery texture. Unlike mould, which has a musty smell, mineral deposits emit little scent. While mould can grow rapidly, mineral deposits on concrete grow slowly over time.
Preventing mineral deposits on concrete requires the proper application of mortar to concrete structures, basements and foundations. Efflorescence occurs as a result of cracks in mortar, which let water seep through the concrete. Even worse, pools of standing water can break apart the internal structure of concrete, in the case of sulphate salts. Installing a sump pump or drains will prevent salty water from maintaining constant contact with concrete, reducing the severity of mineral accumulation.
Since mineral deposits on concrete are water soluble, a pressure washer will remove most traces of calcium and sulphate salts if the problem is caught early. Dry the area after washing. Deposits will convert into carbonates after time, making them much harder to remove. Larger white mould spots may be removed with muriatic acid, which must be diluted, or acetic acid, which is vinegar at 5 per cent concentration. If using acid, make sure to use baking soda on the concrete after removing the efflorescence or else the concrete may be damaged.
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