Editor’s Note: This is the fifth article in a year-long series explaining common raw materials used in precast.
By Alex Morales, M. Ed.
In the 1930s, Roy J. Plunkett was researching chlorofluorocarbon refrigerants when he produced a white powder as a byproduct of his work. He studied the powder for properties other than refrigeration and found the substance to be heat-resistant, chemically inert and low in surface friction. Plunkett had accidentally invented what we know today as Teflon.1
Water-reducing admixtures have a similar backstory – researchers stumbled upon an admixture’s impact on water demand while trying to evenly disperse pigment in concrete. In 1930, Cabot Corporation had promised to create a black concrete for a local state road, but trial batches yielded a blotchy result. At the same time, Dewey and Almy Chemical Company was experimenting with naphthalene sulfonate formaldehyde condensate (NSFC) as an emulsifier for artificial rubber in sealing compounds. Together, they found that NSFC evenly dispersed the black color throughout the concrete mix.2
They also noticed that the black concrete was considerably stronger than the same mix without NSFC. Further studies revealed that NSFC, while evenly distributing the black color, was also dispersing the cement in the mix. Dispersing the cement particles and preventing cement particle flocculation enables the mix water to more readily access the cement grains, thereby lowering the amount of water required to hydrate the cement. This discovery would spur years of research on how to design admixtures that properly distribute cement throughout a concrete mix, acting as what we now know as water-reducing admixtures.
Water reducers and water demand
Water demand of a concrete mix refers to the amount of water required to create a specified slump. It is a measure of water as it relates to the workability of a concrete mix. Water-reducing admixtures do not magically find water in a concrete mix and eliminate it – no such additive exists. Instead, a water reducer is an admixture that, when added to a fresh concrete mix, will increase its workability without the use of additional water. In the presence of a water reducer, a concrete mix will need less water to achieve the same slump.
A good predictor of hardened concrete quality is the concrete mix’s ratio of water to cementitious materials, or w/c. Lower w/c values – within reason – have long been associated with increased compressive strength, watertightness, durability and more, so less water in a mix is a common goal of designers. In the plastic state, however, conventional concrete made with more water tends to be more workable and easily placed, which is ideal from a production standpoint. Water reducers balance these competing needs, resulting in concrete that is easily placed, consolidated and finished without increasing the mix design’s w/c.
Water reducers today
Today’s water reducers are advanced additives that have evolved from the artificial rubber emulsifier that helped disperse the black concrete coloring agent in the 1930s. Water reducers developed over the years can be categorized as lignosulfonates, hydroxycarboxylic acids, hydroxylated polymers, salts of melamine formaldehyde sulfonates or naphthalene formaldehyde sulfonic acids. The most recent technology, using polycarboxylates, was introduced in the 1990s. The exact formulation of a water reducer is largely proprietary, and suppliers in the industry work with precast producers to optimize the dosing of their particular brand. But how a modern water-reducing admixture works relates to what was discovered when NSFC had its unintended water-reducing effect in the 1930s.
Effect on cement
When cement contacts water, opposite electrical charges at the surface of the cement particles attract one another. This causes a flocculation, or grouping, of cement particles which can increase water demand. Water-reducing admixtures neutralize surface charges on cement particles and cause all particles to carry like charges. Because like charges repel each other, flocculation is reduced and cement particles are better dispersed throughout the mix. Better dispersion of cement particles economizes water during the cement hydration process. Water that would otherwise be trapped within flocculated particles is freed, decreasing the viscosity of the paste and increasing slump. As a result, the amount of batch water required to create a desired slump is reduced.
The effect of a water reducer is dependent on source material, dosage, position in the batching sequence and the water reducer’s molecular weight. For example, if you use a water reducer categorized as a hydroxycarboxylic acid, expect different dosing rates and batching sequence recommendations than for a water reducer that is a hydroxylated polymer. Because the chemistry of each water reducer varies, even when used at the same dosage rate, they will not behave exactly alike.
Ranges of water content
Typically, water reducers are categorized by their impact on water content rather than on the materials used to create them.
Low-range water reducers
Conventional water-reducing admixtures (WRA) are low-range water reducers. Low-range WRAs can reduce water content by approximately 5 to 10% and must comply with ASTM C494 Type A (water-reducing admixtures). They are intended for concretes with 3-to-6-inch slumps. Water reducers in this range are usually made of lignosulfonates, hydroxycarboxylic acids or carbohydrates. When used at high dosages, low-range water reducers can prolong concrete’s set time.
Mid-range water reducers
Mid-range water reducers typically reduce water content by 10 to 15% for concrete with a slump range from 5-to-8 inches. They must usually comply with the requirements of ASTM C494 Type F (water reducing, high range) and Type G (water reducing, high-range and retarding) admixtures. Mid-range water reducers can help reduce stickiness associated with lower water content and improve the finishability of the concrete. This range of water reducers was developed in the 1980s specifically to address the retarding effect seen with higher doses of low-range water reducers. Water reducers in this range are usually made of lignosulfonates and/or polycarboxylates.
High-range water reducers
High-range water reducers (HRWR), or superplasticizers, complying with ASTM C494 Types F and G are capable of reducing water content from 12 to more than 30%. Because they are so efficient at reducing water demand and reducing a mix’s w/c, HRWRs often yield denser hardened concrete with improved watertightness and reduced chloride ion permeability. In addition, HRWRs can produce high-performance concretes with strengths in excess of 16,000 psi. HRWRs are usually made of sulfonated melamine formaldehyde condensates, sulfonated naphthalene formaldehyde condensates, lignosulfonates and/or polycarboxylates. These admixtures can be used at higher dosage rates than low-to-mid-range water reducers without the retardation effect associated with high dosages of those admixtures.
A closer look at the materials used to make these admixtures can help you understand how they are sourced, what makes them effective at reducing water demand and, ultimately, why dosing rates can differ for the same WRA used to achieve the same slump or water-reducing effect.3
Sugars make up most of this classification of admixtures. The primary source of these sugars is agricultural. As a result, the base ingredients in these admixtures vary from region to region. Carbohydrate-based admixtures create a film around cement particles and make the particles slick, causing them to slide past each other to prevent flocculation. This creates a water-reducing effect and prevents water from becoming trapped within cement particles that have flocculated. Carbohydrate-based admixtures are primarily hydration stabilizers and in high doses can create a very sticky concrete mix that can be difficult to place and finish.
Hydroxycarboxylic acids are in the carbohydrate family of admixtures, having primarily a set-retarding effect on concrete and, secondarily, a water-reducing effect.4 The most commonly used WRAs are α-hydroxycarboxylic acids like gluconic acids. Gluconic acids are mild organic acids derived from glucose and, like carbohydrates, are an agricultural byproduct primarily derived from plants.
Lignosulfonates are complex polymers that are a byproduct of the wood pulping process.5 During wood pulping, lignin is sulfonated to make it water-soluble and to separate it from the insoluble cellulose. The resulting solution is called lignosulfonate.
Lignosulfonates are water-soluble polyelectrolyte polymers, meaning they are made of a single type of charged monomers. The overall polymer is either negatively or positively charged.6 This property makes their use in water reducers meaningful in relation to dispersing cement particles throughout a concrete mix. Lignosulfonates typically reduce water content by about 10% and have a secondary set-retarding effect on concrete. They can be used with carbohydrate or hydroxycarboxylic admixtures to reduce water demand while balancing retarding effects.
Melamine formaldehyde sulfonates
Melamine formaldehyde is a resin created by condensation polymerization. The melamine monomer and formaldehyde monomer react during this process to create a polymer. Both are chemicals sourced from a variety of industries, including coal tar and petroleum. For use in a water-reducing admixture, the melamine is sulfonated prior to polymerization to create a charge on the melamine particle, which aids in cement dispersion.
Naphthalene formaldehyde sulfonic acids
Naphthalene is a solid hydrocarbon obtained from the distillation of coal tar or petroleum. Like melamine formaldehyde, naphthalene formaldehyde is created by condensation polymerization, and the naphthalene is sulfonated before the polymer is made. Both materials have a high water-reduction ability, capable of reducing water content by 20 to 30%.
Polycarboxylates are the newest type of water-reducing admixture. They consist of specialized polymers created by addition polymerization, opening the door to the advent of self-consolidating concrete. The polymer is called a comb polymer because its backbone – when viewed at a molecular level – includes teeth. The backbone is an acrylic or methacrylic acid while the teeth contain polyethylene oxide or polypropylene oxide. Originally, petroleum feedstock was the primary source of the ethylene oxides and acrylic acids. Today, there are various sources, including the coal industry.
The shape of the polymer contributes to a high flowability while preventing segregation. The backbone is usually already charged when sourced. It sticks to the cement while the teeth help move the rest of the mix along. The backbone-and-teeth comb structure of the polymer can be used to improve workability and other concrete characteristics, but it is the charged backbone of the comb polymer that contributes to the improved dispersion of cement in a mix, reducing water demand.
Raw material sources for each type of WRA – some of which are naturally occurring organics – can vary, and each type of WRA affects water demand in different ways. As a result, a specific type of admixture can affect a concrete mix in different ways depending on the brand’s formula. Moreover, the chemical makeup of cement and batch water (especially if using well water), aggregate composition and even other admixtures will affect the performance of the mix in the presence of a WRA. It is important to run test batches when beginning to use a WRA or change the dose of an existing WRA to understand how the chemicals in the admixture affect the mix design. Rely on admixture suppliers throughout this process to optimize the performance of the WRA for each application. PI
Alex Morales, M.Ed., is NPCA’s director of workforce development.
4 Chaudhari, Ojas & Biernacki, Joseph & Northrup, Scott. (2017). Effect of carboxylic and hydroxycarboxylic acids on cement hydration: experimental and molecular modeling study. Journal of Materials Science. 52. 1-17. 10.1007/s10853-017-1464-0