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Chemical Admixtures for Concrete

Source: | Updated: Jan 19, 2016

Chemical admixtures can either accelerate or slow the curing process, reduce the water content, or introduce air bubbles into the mix—called air-entraining admixtures. Each type of admixture is introduced to the concrete during the batching process, targeting a specific concrete challenge:

Accelerators — Accelerators reduce the time it takes your concrete to cure. Accelerators also increase early strength gain, which is especially important for projects that require a quick turnaround. Accelerators are often used when pouring concrete in cold weather.Calcium chloride is the most common and cheapest accelerator, although non-chloride accelerators are preferable if corrosion might be an issue. For example, while a residential driveway could use a calcium chloride additive, a sub-basement built to store toxic waste would require a non-chloride accelerator.

Retardants — If concrete sets too fast after pouring, you may not have time to level it and high temperatures can escalate your timeline. Retarding admixtures slow down the curing process, allowing you more time to smooth and ready your concrete surface. Retardants are most commonly used when pouring concrete during hot weather.

Water Reducers — Water-reducing admixtures cut down on the amount of water that is needed to bring your concrete to pouring consistency, which is especially helpful in hot weather. Heat makes concrete set too quickly, but adding too much water to keep it pourable can weaken the concrete.The less water in your concrete mix, the stronger your concrete will be.A water-reducer is one of the best types of concrete admixtures when you need increased strength and durability. Concrete for foundations, for example, often has water-reducing admixtures added to its formula.

Air-Entraining Admixtures — Adding an air-entraining chemical to your cement is recommended for any project subject to consistent freezing and thawing. The admixture creates microscopic bubbles that make the concrete more workable.These bubbles lend concrete the flexibility to survive the contraction and expansion of freezing and thawing that would otherwise cause cracking. Air-entraining admixtures are particularly useful for projects like sidewalks, driveways, and parking lots.

Mineral Admixtures for Concrete

Mineral concrete admixtures include fly ash and silica fume:

Fly Ash — A less-commonly used admixture, fly ash is a by-product produced when coal is burned. When added to wet concrete, it slows down the curing process, which makes the concrete pliable for a longer period. Fly ash also helps prevent thermal cracking, making it useful in hot, dry climates.

Silica Fume — Silica fume is produced when silicon metal is smelted. When added to wet concrete, it fills the voids naturally created by water. This process is called particle packing, and it makes the mixture denser and stickier.Concrete with silica fume added has greater strength and increased longevity. It is often utilized in areas where concrete will suffer aggressive wear—city streets and sidewalks, for example.

Admixtures are affordable, easy to introduce into your mix, and make working with concrete a more efficient process. Discuss all of your admixture options with your local concrete provider before you begin your next construction project.

 

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