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Polycarboxylates reduce water and benefit the concrete construction process through their molecular design

Source: | Updated: Jan 24, 2016

The use of chemical admixtures for enhancing concrete performance is a widely accepted practice within the concrete industry. Air-entraining admixtures, accelerating admixtures, and water-reducing admixtures are all valuable additions to the toolbox of materials available for concrete producers. Water-reducing admixtures are particularly helpful to the producer as it allows him to satisfy two sometimes competing requirements: good workability needed during concrete placement and a lower water-cementitious materials ratio (w/cm) needed for durability and other hardened concrete properties.

The term water-reducing admixture has been around for many years. ASTM C 494 categorizes water-reducing admixtures into several classifications:

  • Type A, Water-reducing

  • Type D, Water-reducing and retarding

  • Type E, Water-reducing and accelerating

  • Type F, Water-reducing, high range

  • Type G, Water-reducing, high range and retarding

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Figure 1. Schematic of a polycarboxylate molecule.

Additionally, ASTM covers the use of chemical admixtures for the production of flowing concrete in ASTM C 1017. In this standard, the focus is on treating a concrete mixture with a chemical admixture for the express purpose of producing high-slump concrete while not reducing any of the mix water. In most cases admixtures classified as C 494 Type F or G also would be used in the manner prescribed by ASTM C 1017. These high-range water-reducing admixtures do more than simply reduce water; they disperse cement particles. This dispersive action then allows one to either reduce water, to generate higher slump, or both. Therefore, more flexibility and value is available than the name implies.

Polycarboxylate dispersants

Significant advances in dispersant chemistry have been made in the last decade. This includes the introduction and use of polycarboxylate dispersants across all segments of the concrete industry. Prior to that, most dispersant chemistries had limitations with respect to making modifications to the molecule. However, the introduction of polycarboxylate dispersants has paved the way for developing molecules that will influence performance in specific and tailored ways. This is a tremendous technological advancement for the concrete industry as this enables the use of molecules developed for the sole purpose of dispersing portland cement, whereas previous dispersants were mainly byproducts of other industries.

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Figure 2. Slump retention of three tailor-made polycarboxylate molecules.

Dealing with molecules designed for concrete applications has real advantages for concrete producers. Considering the architecture of a polycarboxlyate molecule allows one to better understand why there is so much promise and flexibility in their application to the concrete industry. First, polycarboxylates are classified as being comb polymers (Figure 1).The name itself implies much about the structure of these molecules in that they are characterized as consisting of a backbone having pendant side chains, much like the teeth of a comb. For these molecules to be effective as dispersants, they must be attracted to the surface of a cement particle. The backbone of the polycarboxylate molecules typically serves two functions: as the location of binding sites (to the surface of the cement particle) and to provide anchoring sites for the side chains of the molecule. The pendant side chains serve as a steric, or physical, impediment to reagglomeration of the dispersed cement grains.

Due to the nature of the processes used to manufacture early synthetic dispersants, a chemist's ability to manipulate their structure was limited. Typically, the structures obtained were complex and the processes were relatively difficult to control from a molecular design point of view.

However, the nature of the chemistry that leads to polycarboxylates is rich with possibilities. It allows a chemist to design a dispersant that is an excellent water reducer versus a dispersant that may maintain high levels of workability over longer periods of time. However, the powerfully flexible chemistry behind polycarboxylates is helpful only if one truly understands the nuances of the other materials in the concrete mixture. Ultimately, differences seen in concrete behavior often can be traced back to mineralogical differences in the cements and aggregates. This opens up the door for chemists to optimize a dispersant's performance based on the predominant mineralogies found in a given material. The design of these next generation dispersants may be based on careful and intelligent manipulation of any of the design parameters for polycarboxylates, or through customized formulations, or both. One thing is for certain—game changing performance is often the result.

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Figure 3. Slump retention comparison between first and next generation high early strength polycarboxylates with and without retarder.

The next generation

The latest generation of polycarboxylate admixtures is based on this cutting-edge molecular design and synthesis. One can now take into consideration a significant number of factors and create a dispersant molecule that is custom designed for that scenario. As has been previously mentioned, the concept of these polycarboxylate dispersants being labeled simply as “water reducers” is somewhat outdated. Polycarboxylates do reduce water, however, additional performance characteristics that benefit the concrete construction process also are possible via molecular design. These additional benefits include previously unreachable levels of slump retention, which can result in significant material and production efficiencies for the concrete producer and the contractor. Figure 2 shows the slump retention comparison between three polycarboxylates tailor made for specific applications.

Impressive levels of high early compressive strength also can be achieved, which will impact production efficiencies for contractors, as well as precast concrete producers. Some of these new molecules provide combinations of the previously mentioned characteristics resulting in never before seen concrete performance. For example, a molecule has been developed that will provide slump retention for 45 to 60 minutes while still providing high early strength. Historically, some level of retardation was required to provide slump retention to high early strength concrete mixtures, however, too much retardation would negatively impact the early compressive strength. Because of this need to balance retardation and strength gain, a compromise was required that would not allow one to capture the full benefits of both slump retention and early compressive strength. Figures 3 and 4 compare a first generation high early strength polycarboxylate molecule coupled with a retarder (for slump retention) versus a next generation polycarboxylate designed specifically for slump retention and high early strength without a retarder. The figures show slump retention, rate of hardening, and 14-hour compressive strength respectively.

These new performance combinations provide significantly greater value to the concrete producer and contractor than previous dispersants. In considering the concrete performance, one must take the next step and relate that to actual value for the industry. Performance characteristics, such as slump retention, can provide the following benefits:

  • Eliminate or reduce retempering at the jobsite allowing for more consistent and efficient concrete placement

  • Improved surface aesthetics due to consistent workability resulting in a reduction in surface patching

  • Overall more consistent and higher quality concrete

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Figure 4. 14-hour compressive strength comparison between first and next generation high early strength polycarboxylates with and without retarder.

These next generation polycarboxylate superplasticizers are being used across North America by those who understand the value of their performance. They aid in the development of new concrete mixture types, providing additional value to the contractor, as well as enhancing production efficiencies. For example, these new molecules are being used to produce high-performance mixtures, such as self-consolidating concrete (SCC).

These well engineered mixtures are being used in a variety of concrete applications and the new polycarboxylates have become an important component in these mixtures. Additionally, several large precast concrete producers have begun using the high early strength/slump retaining polycarboxylates to facilitate the placement of high-performing SCC mixtures. This results in more consistent concrete production as well as further improvements to the surface finish of the elements cast with concrete.

This next generation of polycarboxylate superplasticizers is being recognized as more than just a high-range water-reducing admixture. They are being recognized as “performance admixtures.” The incredible feat of this technology is to allow concrete producers to find new ways of producing concrete as well as creating concrete mixtures with new levels of performance. This is perfect timing for an industry moving toward performance-based concrete. One thing is for sure—this is just the beginning.

 

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