Better Roads Staff
Chemically, if time and moisture are allowed to do their job, silica fume has a very strong pozzolanic reaction, so that when the cement grains hydrate and generate calcium hydroxide, the silica fume will react to that and create more calcium silicate hydrate within the concrete.
In that instance, more space is filled up within the concrete, which lends much more strength, and improves resistance to chloride ions. Microsilica provides radically reduced permeability to water, thus reduced diffusivity to chloride ions. This impacts the migration of dissolved chloride ions (from road deicing salt or marine spray) through the concrete and onto imbedded reinforcing steel. That’s a benefit as the presence of chloride ions will accelerate oxidation (rusting) and concomitant expansion of the steel within the concrete, which ultimately causes cracking and spalling.
From Steel, GGBF Slag
Yet another mineral concrete admixture, ground granulated blast furnace (GGBF) slag, is formed when molten iron blast furnace slag is rapidly cooled.
As it is linked to the steelmaking industry in the eastern and Midwestern United States, GGBF slag largely is available on a regional basis. Despite its limited distribution, and unpretentious origin as crushed dolomite (magnesium carbonate) used to absorb nonferrous materials from molten iron ore, GGBF slag has an important role as a concrete admixture.
Blast furnace slags are the product of reduction of iron ore in a blast furnace, and should not be confused with steel slag, which is unsuited for concrete admixtures. Granulation – opposed to air-cooling – changes the morphology of the slag. While made from the same elements, air-cooled slag is roughly crystalline, and granulated slag is amorphous. It’s this amorphous form that makes GGBF slag valuable.
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.”
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