Better Roads Digital Magazine
Issue link: https://read.dmtmag.com/i/85916
RoadScience As fly ash generally is half the price of cement, replacement rates of 15 to 25 percent can reduce the price of concrete per ton equivalently, with even higher replace- ment rates and savings in mass pours. In April, a new nanostructural component of a proprietary, ultra-fast setting non-portland hydraulic cement mix with fly ash – with the working name of BescherBalls – was articulated by CTS Cement Manufacturing Corp. Calcium sulfoaluminate cements are used in ultra-high early strength cements, and in green "low-energy" cements. Its physical properties – such as expansion or rapid curing, valuable for pavement repairs under traffic or emergency bridge structural repairs – are obtained by adjustment of the availability of calcium and sulfate ions. Manufacturing energy requirements are lower because of the lower kiln temperatures required for reaction, and the lower amount of decarbonated limestone in the mix. Discovered in mixtures of coal fly ash and hydrated cal- cium sulfoaluminate cement, BescherBalls consist of micron- sized glass spheres upon which needles have grown radially. Whereas these needles usually grow randomly in CTS Rapid Set cement, in fly ash/cement mixes they organize themselves as spines on round fly ash particles. It is possible these struc- tures grow and develop only in calcium sulfoaluminate/fly ash mixtures. According to Dr. Eric Bescher, CTS vice president for cement technology, this is the first time these complex structures have been seen. "We are excited about discovering these new self- organized inorganic architectures," Bescher said. "Think of these structures as micron-sized sea urchin shells embedded in cement paste. We have some indications that they may play a beneficial role in the reinforcement of concrete or in shrinkage mediation. Our work is in progress and we are investigating the influence they could have on other properties of construc- tion materials." Another mineral admixture, silica fume, also called micro- silica, provides a big improvement in durability of concrete structures exposed to deicing salts. Silica fume makes a more durable concrete, but too much will make concrete brittle, as a number of state DOTs found in the early 1990s as bridge decks containing large amounts of silica fume began to disintegrate. An industrial byproduct of glass manufacture, silica fume admixture operates at the nanoscale. Because the silica fume particles are much, much smaller than the cement particles – with a surface area in the neighborhood of 20,000 sq. m./kg. – they can "pack" between the cement particles and provide a finer pore structure. In the early stages of hydration, silica fume can help acceler- ate the hydration process because its tiny particles provide nucleation sites for hydration, much the same way that micro- fine dust particles, or cloud seeding, induce formation of rain droplets. In the nucleation process, a silica fume particle provides a site on which material in solution can "nucleate" or "center," which helps the material precipitate sooner than it might otherwise do. And once it precipitates, the concentration of that material in solution is reduced, which tends to get more material into solution from elsewhere, speeding the process. And like fly ash, silica fume can reduce bleeding, but for a different reasons: the silica fume introduces a lot of surface area per particle into the mix, which helps hold the water in place. 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 perme- ability 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 con- comitant expansion of the steel within the concrete, which ultimately causes cracking and spalling. 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 unpreten- tious 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 re- sulting 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. Better Roads May 2012 17