When Admixtures Turn Us Into Chemists
Admixture: ingredients (other than water, aggregate, hydraulic cement and fiber reinforcement) added to a batch of concrete or mortar before or during its mixing.
The world of building and technology collide with the evolution of concrete admixture. This is due to the rise of chemical admixtures, those compounds which produce chemical reaction with the components inside concrete slabs. The installer’s goal is to produce high quality concrete with lower setting times. However, chemical reaction is a chemist’s world. Admixtures play an increasing prominent role in concrete installation, and that makes chemists out of building contractors.
“How Beneficial Is an Admixture?”
The answer is changing as quickly as the new admixture ingredients. However, we can generally observe the main effects of those most widely applied to concrete.
Superplasticizers are popular in admixtures which aim to reduce moisture content (MC) in concrete slabs. This, in turn, speeds drying time. However, the various ingredients of superplasticizers produce different results in concrete moisture content.
Superplasticizer admixtures containing lignosulfonate are successful in lower doses. Higher doses actually slow concrete set times and lead to concrete bubbles, known as air entrainment. Lignosulfonate hybrid mixes result in variable compressive strength readings in concrete slabs.
Superplasticizer admixtures produced with polymer compounds also produce mixed results. If the main goal is to more efficiently disperse moisture content, researchers conclude that polymers with lower molecular weight outperformed admixtures with higher molecular weight.
Many other factors influence concrete installation: cement composition, its alkali content, and particle distribution to name a few. Ultimately, admixtures are only successful when they assist in the equilibration of concrete moisture content. While admixtures can speed the curing process, they do not always benefit the drying process of a concrete slab. It’s only as water vapor migrates from the bottom of the slab to the surface that concrete dries. That is the goal.
The Influence of Relative Humidity (RH)
Changes in RH cause concrete to absorb and release water vapor, and this alters drying times. Admixtures may provide a chemical means of speeding up moisture dispersal, but at a chemical cost to the concrete itself. If concrete has not reached equilibration moisture content (EMC), slab performance and longevity are compromised.
Chemical admixtures may also be incompatible with flooring applications. The pH levels of concrete can break down flooring adhesives, and incompatible admixtures can further complicate matters. It is wise to investigate all thoroughly before attempting such feats of flooring chemistry.
Concrete moisture content can be measured to adapt for changes to RH. Wagner Meters has developed the Rapid RH® 4.0 in situ test method for regulatory compliance as defined by the American Society for Testing and Materials (ASTM), Standard F2170.
It may not resolve the technical evolution of admixtures, but moisture content management remains at the core of every successful concrete slab installation.
ACI Education Bulletin E4-03 Chemical Admixtures for Concrete. Farmington Hills, MI: American Concrete Institute.
Ramachandran, V.A. (Ed.) Concrete Admixtures Handbook: Properties, Science, and Technology, 2nd Ed., Ottawa, Ontario: National Research Council Canada. Web.