What is Accelerator in Concrete?

The senior engineer of Yuanwang Group, a concrete admixture manufacturer, Stella Gao, stated that many concrete construction contractors do not have a clear understanding of what is accelerator in concrete. The following is a training session she conducted on this topic on 1/17/2015.

What is Accelerator in Concrete?

Accelerators in concrete are chemical admixtures for concrete that accelerate cement hydration, shorten the setting time, and improve early strength.

Common concrete accelerators types include inorganic salts (such as calcium chloride and nitrates) and organic compounds (such as triethanolamine and calcium formate).

These accelerators are often used in winter construction to speed up setting and hardening time of concrete in low temperatures. They are also used in projects that require rapid formwork removal and early loading to speed up the construction process.

What does an Accelerator do in Concrete?

Accelerators in concrete act like a “fast-forward” button for the concrete setting process, mainly through two mechanisms.

1. On one hand, they act as helpers for cement hydration.

For example, calcium ions from calcium chloride dissolve into the cement paste, increasing the calcium ion concentration. This speeds up the reaction between tricalcium silicate and water, allowing the cement to harden faster. At the same time, accelerating admixtures provide a “growth template” for the substances formed, acting like nucleation sites, helping them gather more quickly and shorten the time it takes for the concrete to set from soft to hard.

2. On the other hand, accelerators in concrete alter the “personality” of the cement particles. They adsorb to the surface of the cement particles, reducing repulsion between them, which makes it easier for the particles to come together and speed up the hardening process.

For instance, when calcium formate is added to the mix, it releases ions during the reaction between cement and water. These ions energize the hydration in the cement, enabling components like tricalcium silicate to react more quickly with water, generating more substances—like calcium silicate hydrate gel—that strengthen and harden the concrete.

Concrete Accelerators Types

▪️Concrete set accelerator
▪️Concrete hardener admixture

Although both concrete set accelerator and concrete hardener admixture aim to improve early concrete performance, their focus differs.

A concrete set accelerator primarily targets the setting time of concrete. It helps the cement-water mixture transition more quickly from a plastic, flowing state to a stiffening, setting state. This significantly shortens the time required for both initial and final setting. This is especially crucial in winter construction or scenarios that require rapid shaping, such as road repairs in winter, where it prevents concrete from freezing due to prolonged setting times in low temperatures.

On the other hand, a concrete hardener admixture focuses more on the strength development of the concrete, acting like an accelerator for the “growth” of the concrete. It enhances the early strength gain during the hardening process, allowing the concrete to achieve higher load-bearing capacity in a shorter period.

Examples of Concrete Accelerators

Advantages of Accelerators in Concrete

1. Accelerating Construction Progress

For example, in road repair projects, concrete that sets and hardens quickly allows roads to be reopened sooner, minimizing traffic disruptions. In high-rise building construction, accelerating concrete hardening speeds up formwork turnover, improving construction efficiency and enabling faster project completion.

2. Adapting to Special Construction Environments

In cold regions or during winter construction, low temperatures can severely slow down cement hydration, leading to significantly prolonged setting times and slow strength development, which may even result in freeze damage. Accelerators in concrete promote hydration at low temperatures, allowing the concrete to set and harden normally even in cold environments. This reduces the adverse effects of low temperatures on concrete performance, ensuring project quality.

3. Improving Project Quality

Enhancing Early Strength: Accelerators in concrete accelerate the hydration process, helping the concrete form a denser structure early on, thus improving early strength. This is crucial for structural components that need to bear loads early, such as the lower columns of high-rise buildings or bridge piers. Enhancing early strength allows these components to carry the weight of the upper structures more quickly, ensuring the safety and stability of the project.

Optimizing Durability: While promoting early hardening, accelerating admixtures can also improve the pore structure of concrete, refining and evenly distributing the pores. This enhances the concrete’s impermeability, reducing the infiltration of aggressive substances (such as chloride ions and sulfate ions) and thereby increasing durability and extending the service life of the structure.

4. Cost Savings

Reducing Labor Costs: As accelerating admixtures speed up construction progress, the time workers spend on site is reduced, thereby lowering labor costs.
Lowering Equipment Rental Fees: With faster concrete hardening, equipment rental times for machines like excavators and cranes are shortened, leading to lower rental fees.

Which is the Best Accelerator for Concrete?

There is no single “best” concrete accelerator that works for all situations, as different types of accelerating admixtures have their own advantages and disadvantages. The choice should be based on the specific project requirements and conditions. Below is an analysis of the applicability of common accelerating admixtures in various scenarios.

When Should You Add an Accelerator to Concrete?

The decision to add accelerators in concrete is mainly based on construction progress and environmental factors.

In terms of progress, for fast-paced projects such as road repairs, precast component production, and large-scale continuous pouring projects, accelerators can shorten the setting time of concrete, speed up formwork turnover, and ensure construction continuity, significantly improving efficiency.

From an environmental perspective, in low-temperature conditions where cement hydration slows down and concrete is susceptible to freeze damage, accelerators in concrete promote hydration and ensure the quality of winter construction. In humid environments, such as underground or underwater projects, accelerators help accelerate the drying and hardening of concrete, minimizing the adverse effects of moisture. Additionally, for critical parts of the structure that require early strength or projects with special durability requirements, concrete accelerators can meet the necessary performance standards.

How Much Accelerator to Add to Concrete?

Best Practices for Effective Use of Accelerators in Concrete

To make the best use of accelerators in concrete, consider the following tips:

Prepare in Advance: Carefully review the product manuals for accelerating admixtures to understand their characteristics. Before using a new cement or concrete accelerator, conduct compatibility tests to ensure proper integration.

Accurate Dosage: Determine the optimal dosage through concrete mix trials, and then measure it accurately with high-precision equipment. Avoid making arbitrary adjustments to the dosage, as this could affect the concrete’s performance.

Proper Mixing: Add liquid concrete accelerator along with water, while solid accelerators should first be mixed with a portion of the cement before adding the rest. Ensure thorough mixing, and consider extending the mixing time if needed.

Monitor Environmental Conditions: Reduce the dosage in hot conditions and increase it in cold conditions. Keep a close eye on temperature and humidity levels. In damp environments, use waterproof measures; in dry environments, ensure proper moisture retention.

Quality Control: During construction, test the concrete’s performance as required and maintain detailed records of accelerator use and concrete performance. This will facilitate future adjustments and traceability.