The hydration of tricalcium silicate (C3S) in the preacceleration stages was studied. The C3S particles carry a positive charge during the early stages of hydration. Following a rapid hydrolysis of C3S, calcium ions adsorbed on the Si-rich surface of C,S particles, greatly reducing their further dissolution, thus initiating the induction period. The [Ca"] and [OH-] continue to increase at lower rates and, because Ca(OH), crystal growth is inhibited by silicate ions, become supersaturated with respect to Ca(OH),. When the supersaturation reaches a value of == 1.5 to 2.0 times the saturation concentration, nuclei are formed, and rapid growth of Ca(OH), and C-S-H is initiated. These products act as sinks for the ions in solution, thus enhancing the further dissolution of C3S.
Coal-burning powerplants, which supply more than half of U.S. electricity, also generate coal combustion products, which can be both a resource and a disposal problem. The U.S. Geological Survey collaborates with the American Coal Ash Association in preparing its annual report on coal combustion products (Kalyoncu, 2000). This Fact Sheet answers questions about present and potential uses of coal combustion products.
Graphite is the soft, gray‐black form of crystalline carbon; the other forms of crystalline carbon are diamond and fullerenes. Graphite occurs naturally as crystalline flake, mined as flat hexagonal flakes in usually weathered metamorphic rock, as amorphous in former coal beds subjected to additional heat and pressure, and as lump or high‐crystalline graphite occurring in hydrothermal veins in its host rock in Sri Lanka. Natural graphite is not mined in the United States. It is usually mined by open‐pit and beneficiated (crystalline flake) by flotation. Its principal sources are China and Brazil, then Canada, Mexico, and Madagascar.
Flake graphite can also be recovered from a steelmaking waste called kish. Competition within China sets the world price. Natural graphite is used principally in graphite‐containing refractories for steelmaking that are thermally conductive and resist high temperatures, in automotive brake linings, and in batteries, pencils, lubricants, and crucibles. It may come to be used in fuel cells to power nonpolluting vehicles.
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