Decorative concrete is perhaps one of the most stressful aspects of the concrete industry. If even the slightest blemish appears on the surface, the customer could be very unhappy. Some customers have even refused payment until the contractor resolves the aesthetic issues with the concrete. And efflorescence is perhaps the most notorious blemish that can occur – particularly with colored concrete projects. But what exactly is efflorescence- and can it be prevented?
What is efflorescence?
Efflorescence is a result of a chemical reaction between calcium hydroxide – more commonly known as lime – in the concrete and the carbon dioxide in the air. This chemical reaction creates calcium carbonate, which discolors the concrete. Efflorescence is more common in areas of high moisture or when rain or dew is present for extended periods. The water that seeps into the concrete creates a conduit for the lime molecules to travel through the concrete capillaries to the surface. This form of discoloration can also be caused by poor concrete practices, like spraying water on the surface during the finishing process, or even by some admixtures, such as calcium chloride-based accelerators.
Can Efflorescence Be Removed?
Removing efflorescence is possible, depending on how soon you catch it. If you catch it as it is forming it can be removed with a power washer and plain water. Once you have power-washed the surface, it is important to remove as much water as possible from the surface with a water vacuum. If power-washing doesn’t work, the next thing to try would be an acid wash. Acid washes should be executed very cautiously, as some of the stronger, commercial acids can cause further discoloration on the concrete surface.
Can Efflorescence Be Prevented?
There is also a risk of efflorescence when working with cement-based materials, but there are certain steps you can take to mitigate this risk. Preventing moisture absorption is an important step in this process. This means placing a vapor barrier between the concrete and the substrate and then sealing the surface of the concrete with a water-proof sealer. It is also very important to use good finishing practices. So instead of retempering the concrete with water, use a finishing aid or evaporation retarder to maintain the workability of the concrete’s surface. Finally, choose your admixtures carefully. Chloride-based admixtures tend to accelerate or promote the development of efflorescence, so choosing a non-chloride admixture is generally recommended.
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