Better Roads Digital Magazine
Issue link: https://read.dmtmag.com/i/85916
RoadScience Interaction of cement grains (left) and microsilica fume (right) takes place at the nanoscale. Chemical Admixtures: Surfactants Value-added chemical admixtures are added to concrete in small amounts mainly for the entrainment of air, reduction of water or cement content, plasticization of fresh concrete mixtures, or control of setting time. Important among these are the surfactants, used for water reduction and air entrainment in concrete mixes. As we will see next month, surfactants also are essential to manufacture of today's modern asphalt emulsions. Surfactants are long-chain organic molecules that may be hydrophilic (water-loving) at one end, and hydrophobic (water-hating) at the other. "The surfactants become adsorbed at the air-water and the cement-water interfaces with an orientation of the molecule that determines whether the pre- dominant effect is the entrainment of air or plasticization of the cement-water system," writes P. Kumar Mehta in Concrete: Structure, Properties and Materials. Surfactants used as air-entraining admixtures generally consist of salts of wood resins, petroleum acids, and some synthetic detergents, Mehta writes. "Surfactants used as plasti- cizing admixtures usually are salts, modifi cations and deriva- tives [of] lignosulfonic acids, hydroxylated carboxylic acids and polysaccharides. Superplasticizers or high-range water- reducing admixtures...consist of sulfonated salts of melamine or napthalene formaldehyde condensates." What that means is that these air-entraining and water reducing agents are organic polymers derived from either the wood, pulp and paper industries, petroleum refi ning, or in the case of napthalene, coal tar. Water-reducing admixtures are surfactants that improve the quality of concrete and allow development specifi ed strength at lower cement content. Because less water is required, a lower- slump (stiffer) concrete mix is produced. Mid-range water reducers increase concrete strengths. But counter-intuitively, high-range water reducers (HRWRs) or superplasticizers in- crease slump, resulting in a more pourable, pumpable concrete. Conventional water reducers can reduce a mix's water content by 5 to 10 percent, reports the American Concrete Institute in its defi nitive publication, Design and Control of Concrete Mixtures. They also may be used as plasticizers to enhance con- crete workability. Mid-range water reducers can reduce water content by 6 to 12 percent without the retardation that accompanies high doses of conventional water reducers, ACI says, adding, "Mid-range water reducers can be used to reduce stickiness and improve fi nishability, pumpability and placeability of concretes contain- ing silica fume and other supplementary cementing materials." HRWRs, on the other hand, can greatly reduce water de- mand and cement contents, and make low water-cement ratio, high-strength concrete with normal or enhanced workability, and generate slumps greater than 6 in., ACI says. "A water reduction of 12 to 40 percent can be obtained using these ad- mixtures," ACI says. The result is a concrete with compressive strengths in excess 10,000 psi, increased early strength gain, and reduced chloride ion penetration, ACI says. Air-entraining admixtures also are surfactant-based. These organic additives enable bubbles in a concrete mix to be sta- bilized and entrained; they should be specifi ed when concrete will be exposed to freeze/thaw cycles, deicing salt applications or sulfate attack. "At the air-water interface, polar groups are oriented toward the water phase, lowering surface tension, promoting bubble formation, and counteracting the tendency for the dispersed bubbles to coalesce," writes F.M. Lea in The Chemistry of Cement and Concrete. "At the solid-water interface, where directive forces exist in the cement surface, the polar groups become bound to the solid with the non-polar groups oriented towards the water, making the cement surface hydrophobic so that air can displace water and remain attached to the solid particles as bubbles." According to BASF Master Builders, makers of Micro-Air air- entraining admixture, the benefi ts of retained air bubbles in the concrete include the following: thawing; When cement hydrates and concrete sets or cures, a good deal of water remains unused in the concrete matrix. This wa- ter eventually evaporates to the environment, leaving behind tiny fi ssures. During cold, wet weather, these fi ssures refi ll with water via capillary action, and the water will expand when frozen. Entrained bubbles provide "escape" voids that permit that water to freeze and expand without damaging the cured concrete. Better Roads May 2012 19