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The fight over tire burning in Lyons mirrors a growing national debate. There are approximately 82 facilities in the country <br />using tire-derived fuel, 36 of them cement kilns, and the number is growing. In response to increasing public concerns <br />about these activities, grassroots groups in 18 states have formed the National Citizens Alliance, a coalition opposing <br />cement plants and aggregate companies burning hazardous waste in their kilns. They say cement and aggregate kiln <br />incinerators are the least-regulated and cheapest type of thermal treatment for combustible wastes in the country. <br />All of these concerns are compounded by the fact that Cemex doesn't have a good track record handling its current <br />operations, says Richard Cargill, executive director of the St. Vrain Valley Watchdogs. <br />"How can we trust this company to have good combustion controls, when they don't seem to have good controls over <br />anything else?" says Cargill. "Is this the kind of company we want doing anything in Boulder County-burning tires, making <br />cement or anything else?" <br />Respond: letters@bo uldenueekly.com <br />Cement 101 <br />Take a look around you-chances are you'll notice something made out of concrete. Concrete is the most widely used <br />man-made building material in the world, and there's more of this versatile and powertul material around all the time. For <br />example, every 18 months New York City adds about 4.25 million cubic tons of concrete to itself-enough to make another <br />Hoover Dam. <br />While people often use the terms concrete and cement interchangeably, cement is actually the main hardening ingredient <br />in concrete. Manufactured cement was invented by English bricklayer Joseph Aspdin in 1824, who named his creation <br />portland cement because its color was similar to limestone from the isle of Portland. <br />The creation of cement begins in the rock quarries where raw materials are extracted. Usually these materials are <br />primarily limestone, but in Lyons both shale and limestone are used. At the cement factory these materials are crushed <br />into powder and run through a kiln, a huge rotating, horizontally sloped furnace, the world's Largest piece of moving <br />industrial equipment. As the mixture approaches temperatures of 2,700 degrees Fahrenheit, it calcinates, losing its carbon <br />dioxide, and emerges as a red-hot material called clinker. The clinker is first cooled, then ground into a fine powder and <br />mixed with gypsum at a ratio of 95 percent clinker to 5 percent gypsum to make portland cement. The Cemex plant <br />outside Lyons alters the chemical composition of their cement to make seven different types of portland cement. <br />Dry cement is then shipped to concrete producers, which combine the cement with water, sand and gravel to make <br />concrete. Concrete is then transported to construction sites in revolving drums. Concrete doesn't just dry like clay. Water <br />initiates a chemical process in the cement called hydration, locking together with compounds of calcium, silicon, aluminum <br />and iron. Dry concrete is a wholly distinct material from wet concrete, a material than can gain strength indefinitely. <br />18 <br />