Chemical admixtures for concrete can be thought of as the spices that make good concrete even better. Much like a master chef can take a dish and tell a story with the expert use of spices, a supplier can do things with concrete using admixtures that reduce costs, increase product durability, and develop concrete products that thrive in more extreme environments then would have been dreamt of without them. A supplier that understands how to implement chemical admixtures in concrete can offer more options with their product than a supplier who does not. Chemical admixtures are playing a greater role in the construction industry. According to Trent, 2020, “Global concrete admixtures construction chemicals market was valued US$ 10.85 Bn in 2017 and is expected to reach US$ 16.54 Bn by 2026, at a compound annual growth rate of 5.41 % during a forecast period.” This article will provide the history, function, and benefits that can be derived from the use of chemical admixtures in concrete.
Admixtures are liquid or powder-based additives that improve, extend, or otherwise change a characteristic of concrete in its fresh and/or hardened state. The origin of admixtures can arguably be dated back to the Roman era, along with the invention of an early form of concrete. Romans originally mixed many strange things into their concrete, including animal blood and eggs, which in some cases seemed to make the concrete more durable. However, according to Holbrook (1941, p. 118), the first modern use of a chemical admixture in concrete occurred in 1933 when an engineer named Bertrand H. Wait started experiments on blends of Portland cement. Wait was able to make concretes with a scaling resistance against freezing and thawing in a salt solution that was 12 times greater than that of concrete made of pure Portland cement. As a result, in 1934, the first road was built using this blended cement containing about 85% of Portland cement and 15% of Rosendale natural cement—natural cement being an earlier precursor to portland cement that utilized minimally processed cement rock naturally found in the Earth’s crust. In 1937, this blend became the standard for highway construction in New York State and soon thereafter in other states (Jackson, 1944). It was never conclusive what made these blends more resistant to freeze thaw cycles. It could have been a consequence of the natural cement itself, but more likely it was because one out of the two Rosendale cements that were used in the New York State contained a small amount of beef tallow as a grinding aid, which we know today is a natural surfactant that aids in entraining protective air bubbles into concrete. Over the decades, admixtures have evolved from primitive organic ingredients into synthetic lab developed compounds that have more consistent and controllable properties.
Modern admixtures are designed to enhance the properties of regular portland cement concrete. Admixtures extend many different functions in concrete, including reducing the cracking potential, protect against freezing and thawing, and make a more flowable concrete using less water. We will look at three of the most common types of admixtures found in modern concrete; air entrainers, responsible for creating microscopic voids of specific size and distance from each other within concrete, water reducers, which reduce the amount of water needed while keeping concrete fluid, plasticizers, which make concrete even more fluid without adding additional water and shrinkage reducers, which as you may have guessed, reduce the damaged that occurs when concrete shrinks.
The primary function of an air entraining admixture is to generate sufficiently sized, and sufficiently spaced bubbles within the plastic concrete that, when the concrete hardens, leave behind spherical voids within the concrete itself. These voids in the concrete provide space for water to freeze into without causing stresses that break concrete apart over time. The way an air entrainer works is that it operates in the boundary between liquid and gas, so in the case of concrete, between the paste and the air voids trapped within it. Air entrainers use molecules called surfactants that are hydrophobic on one end—they repel against water—and hydrophilic on the other end—they attract to water. Because of this difference in polarity on each end, these molecules arrange themselves in a sphere that encapsulates some of the air void that was within the paste making it smaller and better structured instead of being a random pocket of air within the concrete paste. In fact, many admixtures take advantage of similar characteristics.
Water reducers in their early history were made up of natural materials, like lignin for example. Lignin is the key ingredient that makes cells so robust in trees and because it comes from trees, it is one of the most abundant organic materials on Earth. Many admixture manufacturers are shifting away from lignin in favor of more synthetic polymers which provide greater control over water reduction and more predictable set time behavior. The primary mechanism behind water reducers is the binding of molecules to the surface of cement or cementitious particles to either push particles apart electrostatically by changing the polarization of the surface or to physically push cement particles apart sterically. As you may guess, the cement and cementitious particle composition can play a strong roll in how well water reducers bind to the surface. Different cement morphologies will attract these compounds better or worse. Other compounds like water soluble alkalis dissolved in the fluid between particles can bind with and consume some of the admixture dose while other admixtures themselves can compete for the surface of the cement particle making the water reducer less effective; both issues result in higher doses than would otherwise be necessary for the desired level of performance.
Shrinkage reducing admixtures (SRAs) utilize a similar type of molecule that has the same polarity characteristics, except that instead it works in the boundary between liquid and solid within concrete—in this case between water and cement particles. This boundary contains a lot higher surface area, which is why SRA’s tend to require a much higher dose than air entrainers. These molecules cover the surface of a cement particle and reduce the tension applied to that particle that the surrounding water exerts on it. Believe it or not, the “stickiness” or surface tension of water can become a strong enough force to crack concrete when spread over a lot of water and surface area and these admixtures work to minimize that effect.
There are a lot of ways that concrete can benefit from the use of admixtures. Many of the reasons to use admixtures are market specific, for example, in the southwest United States many regions do not have adequate sources of concrete sand available for concrete. This leaves concrete mixes in those regions with higher cement and cementitious contents, adding cost, and concrete with rougher texture that is not able to pump or finish very well. There are at least two ways that admixtures can mitigate this issue. One is a small amount of air entrainment can be incorporated into the mix. The microscopic bubbles introduced by the air entrainment will serve to provide fluidity and finishability to the concrete mix where it would not have had it before. In addition to air entrainment, an admixture called a viscosity modifier (VMA) could be incorporated into concrete mixes as well. VMA’s add body to a concrete mix and act like a thickening agent. The additional body will provide better pumping and finishing characteristics to the mix, which will counteract the grittiness of a mix with poor quality sand.
Incorporating water reducers into concrete has become a commonplace and a lot of areas of the US. ship every yard of concrete with a low range water reducer in it. Low range water reducers can typically reduce the amount of water in a mix by 5%, which in turn will allow a producer to reduce the amount of cement in a mix as the water to cement ratio is the biggest metric for concrete performance. If you can reduce the water, you can reduce the cement and still maintain performance. High range water reducers, which are water reducers providing significant water reduction, are so heavily engineered now, that no one product behaves the same as another. The configuration of the molecules can vary so widely that dosages, combinations of admixtures, cement chemistry, aggregate quality, all can play a role in how useful admixtures can be. No one supplier will likely use the same high range water reducer, and no one supplier will use the same dose, because they can essentially be chosen to suit exact needs. This fact makes it critical to test admixtures to see how they affect set time, strength, flowability, pumpability, on a routine basis to account for variability in all the materials in a concrete mix. Many suppliers will test an admixture prior to adopting it for use, but do not do further testing beyond that, which can cause problems when weather, aggregates, or cement chemistry change as they so often do over time.
The benefits of utilizing concrete admixtures are many, but with their use comes added complexity. It does not help that some of the admixtures working mechanisms are still a mystery and require further study. Some admixtures can produce nasty and even toxic compounds when they come in direct contact with each other. For example, an accelerator that contains calcium nitrite, when exposed to a low pH environment—often a result of mixing with other admixtures, can generate nitrogen oxide gas which can be toxic (Calcium Nitrite - Registration Dossier - ECHA, n.d.). Regardless, admixtures can help improve poor concrete aggregates, extend pumping and finishing, reduce production costs, and minimize cracking and other common defects in concrete. Gone are the days of vanilla cement concrete slabs and upon us now are the days of laboratory engineered concrete designed to produce specific performance characteristics.
Calcium nitrite - Registration Dossier - ECHA. (n.d.). Calcium Nitrate Registration Dossier – ECHA. https://echa.europa.eu/registration-dossier/-/registered-dossier/26779/9
Trent, N. T. (2020, July 16). Concrete Admixtures Construction Chemicals 2020 Global Market Net Worth US$ 16.54 Forecast by 2026 - Press Release - Digital Journal. Digital Journal. http://www.digitaljournal.com/pr/4744677
Holbrook, W. H. (1941). Natural Cement Comes Back. Popular Science, October, 118–120. https://books.google.com/books?id=VCcDAAAAMBAJ&pg=PA118#v=onepage&q&f=false
Jackson, F.H., 1944. And introduction and Questions to which Answers are Thought. Journal of the American Concrete Institute 15(6). In: Concretes containing Air-Entraining Agents-A Symposium, vol. 40. A part of the Proceedings of the American Concrete Institute Detroit, pp. 509–515