Better Roads Staff
As the production of iron is a controlled process, the resulting GGBF slag is a uniform product that meets rigorous quality standards. The value-added, carefully manufactured slag provides concrete with a low heat of hydration, increased compressive and flexural strengths, inhibition of ASR and resistance to sulfate attack, and reduced permeability to help protect rebar from chloride ion penetration.
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 predominant 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 plasticizing admixtures usually are salts, modifications and derivatives [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 refining, or in the case of napthalene, coal tar.
Water-reducing admixtures are surfactants that improve the quality of concrete and allow development specified 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 increase 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 definitive publication, Design and Control of Concrete Mixtures. They also may be used as plasticizers to enhance concrete 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 finishability, pumpability and placeability of concretes containing silica fume and other supplementary cementing materials.”
HRWRs, on the other hand, can greatly reduce water demand 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 admixtures,” 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 stabilized and entrained; they should be specified 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.”
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