Accelerators

Accelerators are admixtures added to concrete to speed up the hardening process and enable early strength development. Unlike setting time modifiers, accelerators enhance the dissolution of slow-dissolving cement components, particularly silicates, during early hydration. This allows concrete to achieve strength faster, making accelerators essential for cold weather construction and rapid repair applications.

Calcium chloride is the most prevalent accelerator because it effectively accelerates hardening at a low dosage. Adding anhydrous calcium chloride at just 1 percent of cement's mass produces results equivalent to raising concrete temperature by 11 degrees Fahrenheit. This makes it particularly valuable in cold weather conditions or when rapid repair requires high early strength post-mixing.

Uniform distribution of calcium chloride throughout the concrete mixture is critical for effectiveness. This is most effectively accomplished by dissolving calcium chloride flakes in the mixing water rather than using granule admixtures. Proper dissolution ensures consistent acceleration across the entire concrete batch and prevents localized concentration issues.

Calcium chloride has several significant drawbacks. It may reduce cement's sulfate resistance and increase alkali-aggregate reaction risks. Additionally, it can enhance shrinkage and creep, lower durability of air-entrained concrete against freeze-thaw cycles, and cause embedded metal corrosion, particularly in reinforcing steel. These limitations have prompted consideration of chloride-free alternatives.

Calcium formate is a chloride-free admixture considered as an alternative accelerator because it is less likely to cause corrosion of reinforcing steel and other embedded metals. However, the long-term effects of chloride-free accelerators on concrete durability and performance are still being evaluated, making them a developing option for projects where corrosion prevention is critical.

Calcium chloride acceleration is most beneficial in cold weather conditions, typically at 35 to 40 degrees Fahrenheit, where natural hydration is slow. It is also valuable for rapid repair projects requiring high early strength development quickly after mixing. In these scenarios, the accelerator's ability to enhance early strength and reduce construction delays outweighs typical dosage considerations.

Accelerators specifically enhance the hardening rate and early strength development without necessarily affecting setting time, distinguishing them from other admixtures. While additives and fillers in concrete serve various purposes like workability or durability, accelerators focus on speeding hydration of cement components. This targeted function makes them distinct from waterproofing and anti-bacterial admixtures in concrete that address different performance needs.