Technology▼Solutions Dredged sediments in cement
© 2007 American Chemical Society
in the kiln because they are exposed to temperatures as high as 1450 °C for 2–3 hours. Gardner estimates that dredged sediment from the New York–New Jersey harbor could replace as much as 14% of raw material. Authorities in the Northeast are experimenting with other innovative ways to manage contaminated R AYMOND BERGERON, CABLE ARM, INC.
Every year, U.S. authorities manage several hundred million tons of sediments dredged from the bottom of coastal waterways and harbors. They dump clean sediments in the ocean to replenish beaches or use them as landfill at abandoned industrial sites to make parking lots. But managing contaminated sediments—one-half to three-quarters of the material dredged in the northeastern U.S.—is much more difficult. The solution to disposal of contaminated sediments is simple if you consider the sediments’ main components: silica, alumina, calcium, and iron oxides, says Kevin Gardner, a civil engineering professor at the University of New Hampshire. These raw materials are used in portland cement and cost cement companies hundreds of thousands of dollars. Replacing some of them with dredged sediments would lower costs for the companies while reducing mining, he says. Portland cement companies, which use alternative materials such as coal fly ash, could easily incorporate dredged material into their manufacturing process, Gardner says. Dredged sediments, like any geological materials, have a range of concentrations of different elements, and cement companies already handle such fluctuations. “Every 2 hours they’re adjusting their feed rate of different things so that they get a [consistent] mix,” explains Gardner. The cement manufacturing process is also well suited for handling contaminants. Cement facilities are equipped to trap pollutants such as lead, mercury, and arsenic found in mined raw materials and in waste products such as coal fly ash. Although Gardner and his research team have not performed tests to prove this, Gardner believes that organic contaminants, such as PCBs and oil, will be completely oxidized
Researchers at the University of New Hampshire believe that contaminated sediments dredged from U.S. waterways could be used at existing cement plants to make portland cement.
sediments. In the Boston harbor, the U.S. Army Corps of Engineers filled confined aquatic disposal cells—pits excavated in the floor of the harbor—with contaminated sediment and capped them with clean sand. But this is not an effective long-term solution, says Gardner. “They’ve got a finite amount of space that’s suitable for this at the bottom of the harbor.” The U.S. EPA leads a program for developing technologies to use dredged sediments from the New York–New Jersey harbor. One technology, a high-temperature process that decontaminates sediments and converts them into constructiongrade cement, shows promise at a commercial level and is undergoing full-scale tests, says Eric Stern, the EPA program’s manager. Unlike Gardner’s idea, this process requires a dedicated facility to manufacture the cement additive, requiring a capital
cost of millions of dollars. Gardner’s idea has potential and “is certainly worth continuing on, because it’s looking at retrofitting existing cement manufacturing plants,” Stern says. Several cement companies have shown interest in using dredged sediments, Gardner says, but they are unwilling to fund research for a full-scale demonstration. According to Stern, private funding is the only way these technologies will come into widespread commercial use. Gardner’s idea “has a lot of attractiveness if you can marry the two industries [dredging and cement] together,” but that is not simple, says Norman Francingues, an independent technical consultant to the Port of Baltimore, which is now evaluating the feasibility of shipping contaminated sediments to cement facilities. It means working out logistics such as how to treat, transport, and store the sediments as well as paying for the associated costs. Just shipping a ton of sediment for a mile costs 5 cents, he says, which adds up when you are transporting hundreds of thousands of tons. The question becomes, “Does the cement facility need the dredged material, or does the port need to do something with the dredged material?” Francingues says. The current abundance of raw or other alternative materials gives cement companies little incentive to consider dredged sediments; this means the port would have to bear the costs. State tax incentives for the cement companies to use contaminated sediments might help, he says. If these logistics and policies can be resolved, Gardner believes that his solution will be cost-effective and sustainable in the long term. It is ironic, he says, that we are dredging navigation channels to bring in shiploads of mined material while “we’re wondering what to do with the dredged material.” —PRACHI PATEL-PREDD
JUNE 1, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY ■ 3795
