On earth and on its satellite moon, there exists spherical particles as fine as portland cement. So there is no reason not to believe the particles exist on other planets. On our planet, there are naturally occurring particles as well as man-made ones—on other celestial bodies uncontaminated by human activity, they are all natural.
The natural particles are mostly spherical, although sometimes tear shaped, and typically glassy. Synthesized varieties are similar but occasionally contain portland cement-like minerals such as calcium aluminates, calcium silicates, and alkali sulfates, plus carbon from the incomplete combustion of coal from which they were derived. Known as fly ash, today it is frequently used as a supplementary cementing material—or as some people would have it, a complimentary cementing material—to help make concrete sustainable.
The chemical composition of man-made fly ash varies depending upon the coal source. Common chemical components include silica (SiO2), alumina (Al2O3), iron oxide (Fe2O3), lime (CaO), magnesia (MgO), and alkalies (Na2O, K2O). On the flip side are very trace materials that at certain limits are considered hazardous, including arsenic (As), mercury (Hg), chromium (Cr), cadmium (Cd), and lead (Pb).
Fly ash is derived from combustion of powdered coal—natural fly ash from fiery magmas where lava-eject forms telltale spheres, sometimes tear-shaped, and known as Pele's tears after the Hawaiian goddess of fire.
Some six decades ago, when fly ash began to belly up to concrete, the industry was very cautious to introduce this newcomer as acceptable. While its attributes were always broadcast up front, its potential problems were less publicized. However, once standards such as ASTM 618 for physical and chemical composition, suitable mix designs, and an understanding about it came forth, its use blossomed. Today, due to cost advantages, the sustainability movement, past research and development, and successful applications, fly ash fills ready-mix plant silos, is used in blended cements, and is included in many mix designs.
Ultra-high-strength concrete would be more difficult to make without its pozzolanic properties that tighten paste porosity and consume calcium hydroxide (Ca(OH)2). Some other benefits include increased late strength, better workability, reduced permeability, lower shrinkage, reduced creep, less heat of hydration, reduced bleeding, and better concrete pumpability. By reducing permeability, it increases durability. Some disadvantages are increased setting times, lower initial strengths and slower rates of strength gain (particularly at lower temperatures), and delayed finishing—the increased setting time can be advantageous during hot-weather concreting.
Fly ash costs less than portland cement and saves energy by replacing energy-consumptive portland cement, thereby reducing carbon dioxide emissions. Concrete consumes what would be a lingering waste product and encapsulates everything considered potentially toxic.
There recently has been a move to designate fly ash as a hazardous waste. If so classified, this greatly valued material will negate much of the industry's efforts to make gray concrete green, force construction to stumble along the road of sustainability, and set us back in reducing concrete's carbon footprint.
There must be a better way of resolving this situation. A hybrid rule has been proposed, which would designate fly ash as hazardous when disposed in landfills but nonhazardous when beneficially reused—a wonderful compromise. The fate of fly ash, which has done so much good in the past and can continue to do so in the future of our industry, this country, and for civilization, may well rest in your hands. Play an active and positive role in its fate. Visit www.sourcewatch.org/index.php?title=Fly_ash.
William Hime was a principal with Wiss, Janney, Elstner Associates and began working as a chemist at PCA 58 years ago.