Cement hydration is temperature and time dependent. The higher the temperature, the faster the reaction happens. As time passes, strength continues to develop, even beyond initial set, as the hydration reactions continue. Accelerators act as a catalyst for hydration reactions.
The main reason to use accelerators is to achieve early strength gain and early setting. As soon as water comes in contact with cementitious materials, hydration reactions begin. Likewise, most accelerators also begin performing shortly upon their addition to a concrete mix. During the dormant period when the fresh concrete is plastic and able to be handled, prior to initial set, the action of set accelerators can last anywhere from 10 minutes in a hot mix to 2 hours for a cold mix. During the setting period, after initial set but prior to final set, accelerators can provide rapid strengthening for up to 10-12 hours, especially if heat is applied. Lastly, during the hardening period, strength gain is typically very slow and can take many hours to complete. In some cases, however, flash set, or rapid stiffening accompanied by excessive heat generation, can occur due to an insufficient amount of gypsum in the cement. False set, which involves rapid setting and very little heat generation, could also occur.
Typically, accelerators are custom blends of chemicals that do exactly what the precaster wants with the specific cement used. Some have accelerating and water-reducing properties (ASTM C494 Type E chemical admixtures), while some are only accelerators (ASTM C494 Type C chemical admixtures). Some even help improve concrete’s workability while others can help increase corrosion resistance. One way that manufacturers classify accelerators is based on the materials from which they are produced. Ketan Sompura, director of concrete technology and admixture and fiber products manager at Sika, said there are four types of accelerators; soluble inorganic salts, soluble organic compounds, quick-setting admixtures and miscellaneous solid materials. Admixture companies may combine several raw materials from all of these categories.
There are several soluble inorganic salts used for accelerators. The most effective is calcium chloride. Chlorides are the most economical accelerators and perform well. Chlorides, however, promote corrosion of steel, so they aren’t used in steel-reinforced concrete. Other inorganic salts contain nitrates, nitrites and thiocyanates instead of chlorides. These accelerators are slightly more expensive and can be used with steel reinforcement. Soluble organic accelerators use triethanolamines and calcium. Quick-setting accelerators aren’t typically used in precast or ready-mix applications because they cause such a rapid loss of plasticity and workability. Other accelerators may use compounds such as silicates and carbonates.
“Usually, accelerators are a blend of organic and inorganic materials,” said Sompura. “Each raw material has a different reaction time. One may not do anything for 30-60 minutes and another will.
“Manufacturers combine them in different ratios and tailor the start and stop times before they react and provide results so they are suitable for precast applications.”
How accelerators work in concrete
Set accelerators work by acting as a catalyst for hydration reactions. The products of hydration reactions continuously accumulate, and the reactions continue until either all of the cement or all of the water in the mix has been exhausted. However, the products of hydration reactions can also surround unhydrated cement particles and create a barrier for the unreacted cement to come in contact with water. Chloride- and salt-based accelerators weaken the barrier created by these products and allow certain compounds in cement to hydrate, speeding up the hydration process. Accelerators containing triethanolamines act on a different compound in the cement and increase the rate at which the reactions occur with that compound. Triethanolamines can also encourage ettringite formation.
“The majority of accelerators react with tricalcium silicate, the major component of cement,” Sompura said. “Usually accelerators don’t work with dicalcium silicate, and only some of the accelerators work with tricalcium aluminate to make them be very quick setting.”
What’s important about accelerators
Although all accelerators start out as powders, most are supplied in liquid form because it is easier to dispense and evenly integrate into the concrete. Admixture suppliers provide bagged, dry forms to smaller precasters who don’t use high volumes, don’t have liquid dispensers and metering systems, are in remote locations or don’t receive weekly deliveries. Some accelerators, however, must be shipped in liquid form because they would be too dangerous as powders.
Liquids are easy to dispense and a homogenous mix is attained easier with liquids than with powdered additives. Sompura said there are two ways to integrate liquid accelerators into fresh concrete. One uses an automatic dispensing system which directly feeds accelerators into concrete mixtures. This metered batch process that takes liquid from a bulk tank to the mixer is usually used with large volumes of chemical admixtures. For lower volumes, a dispensing tube and a glass dispensing bottle provide visual confirmation for the person in charge of batching.
Admixtures are dosed by weight of the cement in the batch. The dosage of chloride accelerators typically follows the industry standard of 1-to-2% of the cement weight. Non-chloride accelerators have more variable dosages. Each accelerating admixture has a manufacturer-recommended dosing rate on its product data sheet. Sompura said these recommended dosages are guidelines and starting points for precasters to determine how much of the product to add. Most precasters create trial mixes and test a few doses to see which gives their product the required strength while also exhibiting a desirable level of workability and maintaining plasticity long enough to place the concrete and finish the exposed surfaces.
Accelerators are used more frequently in low ambient temperatures because the hydration reactions occur at a slower rate in colder temperatures. Accelerators also have a greater impact in colder temperatures. The warmer the ambient temperature, the less effective the accelerator. Using 2% accelerator by weight of cement at 45 degrees Fahrenheit may provide 5 hours of acceleration, but at 70 degrees Fahrenheit, the same amount of the same admixture may only provide 2-3 hours of acceleration. The accelerators-decreased effectiveness at higher temperatures is a nonissue since the hydration reactions occur at a faster rate at warmer temperatures – making accelerators unnecessary except for special circumstances.
Accelerators can also pose challenges for precasters, such as long-term strength gain and shrinkage. These are easily dealt with as long as the precaster is aware of the challenge and the solution.
“Although accelerators provide higher early strength, many people don’t realize that they lower the ultimate strength,” said Jesse Osborne, precast segment and admixture manager at Euclid Chemical. “Most precasters accept this and over-design a little bit, adding 5-to-10% more cement to reach the specified strength.”
Osborne said many admixture producers also include components that help offset shrinkage, which is a concern for precasters. Expansive agents and surface tension-reducing components can be included in accelerators. ASTM C494 dictates the maximum allowable amount of shrinkage and water reduction caused by the use of accelerating admixtures.
Sompura said accelerators affect the set time, but don’t necessarily affect the strength. Precasters should also be aware of how much time will elapse between mixing and placing the concrete, how the concrete will be transported to the forms, ambient temperature and the mix water temperature. Some precasters use hot water to increase the rate at which the hydration reactions occur. In some cases, if approximately 30 minutes will elapse between mixing and casting, a set accelerator could make the concrete too stiff to place. Instead, an accelerator that doesn’t take effect for the first half hour after mixing should be used.
Combinations of different accelerators mixed in manufacturer-specified ratios could also be used, depending on job specifications and how all the raw materials and chemicals interact. It is important to test different products in test batches to determine which combinations work best with the materials being used.
How accelerators advance precasters’ work in the plant
According to Mark Celebuski, partner at Trinic, a manufacturer supplying powdered accelerating admixtures, precasters often use accelerators to offset the delaying effects of other additives. Some common additives like water-reducing admixtures or supplementary cementitious materials can decelerate the set of concrete. Using accelerators helps hydration occur at an elevated rate and increases the rate of set and strength gain. Precasters can also use accelerators to turn forms faster, which translates into greater efficiency and a reduction in total days and labor costs for production.
Debbie Sniderman is an engineer and CEO of VI Ventures LLC, an engineering consulting company.