A Closer Look: Accelerators and Retarders

Editor’s Note: This is the fourth article in a year-long series explaining common raw materials used in precast.

Thanks to a wide variety of available admixtures, precast concrete producers can manufacture durable, high-quality products for nearly any project or need.

By Claude Goguen, P.E., LEED AP

Sometimes as humans we seek to slow things down, like the final days of a vacation. But speeding things up can also be preferable, like the time spent sitting in the dentist’s chair. Manufacturing concrete also requires variations in speed. Hot and cold weather concreting, avoiding cold joints between lifts, countering the effects of supplementary cementitious materials (SCMs) and admixtures, and facilitating double pours are some of the situations that necessitate admixtures that can either slow down or speed up production processes.

The setting of concrete can be described with many terms, including hardening, drying, stiffening and more. However, these terms are not synonymous. To help clear up some of the confusion, it’s best to start with the basics.

When cement and water are mixed, hydration occurs. Hydration is the chemical reaction that takes place when water is added to a hydraulic cementitious material. Many factors affect the rate of hydration, which in turn will affect setting time and strength gain.

Setting is defined as the process of hydrated cement (paste) changing from a fluid state to a solid state. According to the American Concrete Institute (ACI), initial set is a “degree of stiffening of a cementitious mixture less than final set, generally stated as an empirical value indicating the time required for the cementitious mixture to stiffen sufficiently to resist, to an established degree, the penetration of a weighted test device.” Final set is defined by ACI as a “degree of stiffening of a cementitious mixture greater than initial setting, generally stated as an empirical value indicating the time required for the cementitious mixture to stiffen sufficiently to resist, to an established degree, the penetration of a weighted test device.” The device they are referring to is most often the Vicat needle that is described in ASTM C403 – Standard Test Method for Time of Setting of Concrete Mixtures by Penetration Resistance.

NPCA file photo

Hardening, or strength development during hydration, is related to the formation of a few constituents which consist of mainly calcium silicate hydrate (CSH) and calcium hydroxide (CH). This process binds the aggregates together to increase the stress-bearing capacity of the concrete. This is different than setting because it involves the concrete as a whole including the aggregate – not just the paste. Slump loss, or spread loss, is the reduction of workability that occurs as concrete naturally sets.

Some of these terms may sound similar, but there are situations when a manufacturer may want to increase strength gain while maintaining usual set times, or vice versa. There are also instances where manufacturers might want to pause the hydration process to allow for the concrete to be placed during an extended pour. Admixtures are available to suit all these purposes. Below, we will discuss set-controlling admixtures along with hydration modifiers.

Cement Particles

The chemical components of cement affect concrete performance. When it is manufactured, four main components (phases) are present in the final cement product: dicalcium silicate (C₂S), tricalcium silicate (C₃S), tricalcium aluminate (C₃A) and tetracalcium aluminoferrite (C₄AF).

C₃S is more soluble than C₂S and therefore hydrates rapidly. It is the main influencer on initial set and initial strength gain and produces desirable CSH and CH in the concrete. C₃A is the more soluble phase in cement and, while it greatly impacts initial set, it does not produce CSH or CH during hydration. Consequently, cement plants add gypsum to temper the rapid hardening effects of the C₃A. C₂S reacts slower and contributes to longer-term strength development producing mainly CSH. Whereas the C₄AF contributes little to set and strength gain.

The five steps of hydration

When a cement particle meets water, the hydration reaction begins on its journey to creating a solid product. Hydration can be split up into five phases. The first 10-20 minutes of the reaction is called the pre-induction period. Here, early reactions of the C₃S and C₃A in the cement produce calcium, silicate and aluminate ions, and the water becomes a pore solution. During this phase, the C₃A reaction also releases a significant amount of heat.

Everything slows down in the next phase, which is the induction or dormant period. At this point, the concrete remains fluid and can still be placed. Varying theories exist surrounding what influences the length of the induction period.

“The cement minerals are very soluble and keep releasing ions into the pore solution,” said Jeff Thomas, senior principal scientist with GCP Applied Technologies. “Eventually it becomes supersaturated and solid hydration products such as calcium hydroxide and calcium-silicate-hydrate nucleate and begin to grow. This then frees up space for more ions in the pore solution and the process continues.

“The hydration products tend to form on top of the dissolving cement grains, eventually forming a layer around them. Since these solid products occupy more space than the original cementitious material, the cement grains actually grow in size as they react.”

Eventually, the hydrated product of several cement grains will connect and form a solid network. This is when setting begins.

Next, the dormant period gives way to the third accelerated phase of hydration. The C₃S in the cement continues to react, and the concrete gains strength as CSH continues to form. The release of heat rises again but then falls during the fourth stage, which is commonly referred to as the deceleration period despite early strength gain still progressing considerably. The final stage is a slow, continuous hydration period that contributes to later strength gain.

The amount of time needed for each phase depends on many factors, such as: temperature; type of cement; w/c; presence of SCMs; and use of admixtures.

NPCA file photo
When casting large products such as bridge beams, a set retarder may be used to avoid cold joints.

Accelerators

Accelerating admixtures work to reduce the induction or dormant period of hydration. Precasters’ productivity depends on how quickly forms can be turned over while still allowing the concrete to gain essential hardened qualities. Accelerating admixtures, which are designated as Type C admixtures under ASTM C494 / C494M Standard Specification for Chemical Admixtures for Concrete, help expedite the process. Chloride accelerators are not recommended in steel reinforced concrete due to a risk of accelerated corrosion to the steel. Several non-chloride accelerators are available for use in concrete. Many different formulations exist, but they mainly consist of organic compounds or inorganic salts.

Inorganic salts can consist of sodium and calcium salts of formate, nitrate and thiocyanate. Calcium nitrates are commonly used in the precast industry. Organic compounds include triethanolamine (TEA), which can be used on its own or mixed in a formulation of water reducers. Typically, accelerating admixtures are customized to fulfill a specific need in the presence of the type and quantity of cement used. Some have accelerating and water-reducing properties (Type E admixtures), while some are only accelerators (Type C admixtures).

Retarders

To slow down setting time, precasters use a retarding admixture. Some precasters may remember the days when sugar was added to the concrete mixture to slow down set. Sugar and other forms of sucrose – such as corn syrup – along with hydroxylated carboxylic acids or their salts, and lignosulfonates are used predominantly in concrete set reducers. These admixtures are designated as Type B under ASTM C494 and can be applied to form surfaces or added to the mix to delay the set of paste from just a short time to several hours.

Retarders work to extend the period in which the concrete remains plastic. They reduce the chemical reaction speed by slowing the dissolution of the cement phases and inhibiting the nucleation of CSH. Lignosulfonates, for example, work by adsorbing to the C₃S and C₃A and forming a coating that slows down its dissolution. This results in slowing set and in lower early strength gain. With proper quality production practices, long-term strengths should still be equivalent or higher than the same concrete made without a retarder. Most set retarders will also act as a water reducer.

Hydration stabilizers

The technology behind hydration stabilizers has advanced significantly in recent years, allowing them to essentially become retarders on steroids. These powerful stabilizers suspend the hydration of cement for extended periods of time.

“Typical set retarders are harder to predict,” Marc Sinicrope, director of technology at Master Builders (formally BASF), said. “But hydration stabilizers are more predictable and useful in certain situations. For example, some cements may have very high C₃A levels, which can accelerate set – hydration stabilizers can hold off slump loss until the concrete is placed, giving the precaster more working time.”

Hydration stabilizers can be categorized as Type B retarding and/or Type D water-reducing admixtures under ASTM C494. Depending on how the hydration stabilizer is used, it’s impact on long-term strength would range from no impact to an increase similar to that obtained by using a conventional retarder.

Workability-retaining admixtures

To help maintain slump or spread, workability-retaining admixtures are effective. These admixtures do not retard set or significantly affect early-age strength gain, which can be especially beneficial when using self-consolidating concrete. Workability-retaining admixtures are classified as Type S under ASTM C494 and are typically formulated with polymers.

New technologies

Various evolving technologies, many linked to nanotechnology, are resulting in new products that can affect setting, hydration, strength gain and workability. One such technology involves using seeding crystals to provide nucleation sites for hydration products. Recently introduced to the North American market is a product that uses CSH seeds.

Nanoparticles consisting of synthetically produced crystalline CSH are added to provide sites for nucleation of hydrated product. This results in early and late strength gain due to improved hydration. The addition of nucleation sites is referred to as “seeding” and is showing promising results in many studies.¹ The use of CSH seeds can also reportedly permit reductions in total cementitious content, which can contribute to a lower environmental impact.

Speeding up to slow down

Thanks to the many admixture suppliers to the industry, precasters have access to a wide assortment of amazing products that can fit any condition, resulting in a high-quality product. Some may avoid considering the use of these products for fear of increasing initial costs per yard, but a consultation with an admixture specialist may reveal that that the long-term costs and benefits outweigh the initial investment.

Claude Goguen, P.E., LEED AP, is NPCA’s director of technical education and outreach.

References:
1 Influence of Nucleation Seeding on the Compressive Strength of Ordinary Portland Cement and Alkali Activated Blast Furnace Slag – https://rosap.ntl.bts.gov/view/dot/35152/dot_35152_DS1.pdf?
2 https://www.zkg.de/en/artikel/zkg_The_acceleration_of_cement_hydration_by_seeding_influence_of_the_cement_1579652.html