At Continental, the portland-making process begins with workers mixing water, calcium (from local limestone), silica, alumina (from clay or shale) and iron together to form a slurry, which is fed into the plant's 622-foot-long, fire-brick-lined kiln. This tubular giant, looking something like a massive artillery piece, tilts slightly downward toward the Mississippi River floodplain, rotating at a rate of just over one round per minute.

As slurry pours into the higher end, coal and other fuels fire the lower. A counter-current configuration ensures that as the mineral mixture works its way down the kiln, it grows continually hotter, first driving out moisture, then carbon dioxide, until, near the kiln's end, the minerals fuse into marble-sized pellets of super-heated clinker. The clinker emerges from the end of the kiln and drops into a cooling unit, where it is eventually ground up with gypsum to form cement.

All of this tumbling and grinding produces great quantities of dust. Most of this is reprocessed until it is no longer capable of producing clinker. The rest, approximately 3 to 5 percent of the clinker production rate, is rendered into the plant's pollution control system as kiln dust that must be disposed of.

"This by-product is cement kiln dust, called CKD, basically just ash and other materials left over after they burn limestone and other products to make cement," says Likos. "It also has some heavy metals in it, and it needs to be disposed of in a controlled way. Cement kilns throughout the country produce a staggering amount of this stuff."

The CKD issue is complicated by another factor: Because cement kilns get very hot, sustaining gas temperatures above 3,000 degrees, the federal government years ago approved their use for burning certain hazardous wastes -- among them old motor oil, paint thinner and other solvents -- to supplement traditional fuels. The practice is an effective way to dispose of waste, conserve fossil fuels and lower fuel costs. But it is not without controversy.

Activists in the past have charged that waste-burning kilns were regulated less stringently than commercial hazardous waste incinerators, allowing toxic materials to find their way into both dust and exhaust emissions. Plant officials counter that, in fact, federally approved permits, risk assessments and regulations are in place. They say also that, regulations notwithstanding, Continental has a long track record of working independently to minimize potential hazards.

As he strides across a slightly spongy patch of newly reclaimed ground, Leonard Rosenkrans, a Continental fuels laboratory chemist, gestures with a sweeping motion toward a stand of newly planted maple, black locust and ash trees. The soil in which the trees are planted, he says, is just one example of the company's good faith. "We've maintained a positive image and a good relationship with the city, and this is all part of that," says Rosenkrans, an MU grad whose great-grandfather also worked for Continental.

"We've agreed not to take certain [allowable] kiln fuels that might have a stigma associated with them. We could take PCBs or pesticides -- we could burn those, they're destroyed like any other organic -- but we've chosen not to take those materials." The company's work with MU, he adds, is further evidence of its commitment.

Likos, along with two MU colleagues, David Hammer and John Bowders, both professors of civil and environmental engineering, began working with Continental at the behest of the plant's owners, who asked for a long-term, environmentally sound solution for managing dust. The team suggested they try developing what is known as "artificial soil" from other organic and mineral waste products -- including CKD -- and use it to reinvigorate the denuded floors of Continental's old limestone quarries.