Accelerators in concrete serve as admixtures to speed up the hardening process, enabling the concrete to achieve early strength faster. Although accelerators do not necessarily impact the time it takes concrete to set, they reduce this time in practice. A common accelerator is calcium chloride, which is particularly useful for hastening early strength development in cold weather or for rapid repair jobs that require quick heat generation after mixing.
The effectiveness of calcium chloride can be compared to raising the temperature of the concrete by 11 degrees Fahrenheit, with a typical addition amount being 1 to 2 percent of the cement's mass. The distribution of this admixture must be uniform within the mix, achieved by dissolving it in the mixing water.
However, using calcium chloride has drawbacks. It may reduce concrete's resistance to sulfate attack and increase the risk of alkali-aggregate reaction. Furthermore, it can enhance shrinkage and creep and lower the durability of air-entrained concrete against freeze-thaw cycles. There is also a potential for embedded metal corrosion, such as reinforcing steel. As a result, other chloride-free admixtures like calcium formate are considered for acceleration as they are less likely to cause corrosion. However, their long-term effects on concrete are still being evaluated.
An admixture is an additional substance added to the concrete during mixing to include a unique feature.
Admixtures used to accelerate the hardening of concrete are accelerators; these do not have to affect the setting time of the concrete.
During early hydration, an accelerating admixture should enhance the dissolution of the slowest-dissolving cement components, like silicates.
The most prevalent among the accelerators is calcium chloride. This admixture is often applied in cold weather at 35 to 40 degrees Fahrenheit or when rapid repair requires high early strength post-mixing.
Adding anhydrous calcium chloride at 1 percent of the cement's mass can accelerate the hardening process as effectively as an 11 degrees Fahrenheit increase in temperature.
Uniform distribution of calcium chloride throughout the concrete mixture is critical, and is most effectively accomplished by dissolving flakes instead of granule admixtures in the mixing water.
Calcium chloride may reduce cement's sulfate resistance and increase alkali-aggregate reaction risks, as well as shrinkage and creep, but enhances resistance to erosion and abrasion.
繁體中文
