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
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.
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.
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.