Although some new types of concrete might be taking on ethereal traits, traditional concrete continues to weigh very heavily on the planet. Each year, the United States alone lays down 500 million tons of concrete. “That has a tremendous impact on the environment,” says Columbia University’s Christian Meyer. Sand, gravel, or crushed stone are extracted from natural resources. “Entire mountains can be taken away to satisfy the voracious appetite of the concrete industry,” says Meyer.
Another enormous toll on the environment comes from the carbon dioxide—one of the main global warming gases—released during cement production. Heating limestone and clay at high temperatures requires burning a substantial amount of fossil fuel. Also, carbon dioxide is released from a chemical reaction that occurs when the limestone and clays are combined.
The production of a ton of Portland cement—the most common type—releases about a ton of carbon dioxide into the atmosphere, says Meyer. The cement industry contributes 7 percent of the global human production of carbon dioxide, he adds.
Meyer has been active in promoting the so-called green building movement. Much of his research focuses on ingredients, mainly recycled materials that can substitute for the traditional components of concrete. For instance, his group has made a glass-based concrete by replacing all or part of the sand and gravel elements with crushed recycled glass.
Normally, when glass is mixed with cement, the cement’s alkali reacts with the glass’ silica. This creates a gel in the final product that swells in the presence of moisture and cracks the concrete. The Columbia group replaced part of the cement with the clay mineral metakaolin, which absorbs the alkali ions so that they don’t react with the silica. Wausau Tile of Wausau, Wis., licensed Meyer’s concrete-glass invention and is now producing colorful floor tiles made with recycled glass.
To replace the entire cement component of concrete, Meyer is looking at yet another unlikely source: dredged material from New York City harbor. The Port Authority scoops out material from the harbor to keep the shipping lanes open. Instead of dumping vast amounts of the material in a landfill, Meyer is working on ways of blending the dredged products into concrete after treating the hazardous ingredients to render them harmless.
Surprisingly, his group has found that various properties of the clay minerals in dredged harbor sediment are superior to those of clay minerals on land. In preliminary tests, the researchers found that concrete mixed with dredged material could survive 100 times as many freeze-thaw cycles as concrete without the dredged substance can.
The modern church Dives in Misericordia located in Rome, is made from a self-cleaning concrete that breaks down atmospheric pollutants that would otherwise darken the surface over time.
New forms of concrete might also abate environmental pollution. Scientists at the Italcementi Group in Bergamo, Italy, have developed a self-cleaning concrete that keeps buildings from turning black from pollutants in the atmosphere. Luigi Cassar and his colleagues at the research branch of Italcementi made the concrete by adding particles of the white pigment titanium dioxide to the cement component.
When titanium dioxide absorbs ultraviolet light, it becomes highly reactive and breaks down pollutants that come into
contact with the concrete’s surface. The reactive material can kill bacteria and fungi and also break down pollutants
such as nitric oxide, sulfur dioxide, and many volatile organic compounds that contribute to concrete’s darkening.
The self-cleaning concrete has already been used in several new buildings, including a modern church in Rome called the Dives in Misericordia. “The goal was to create a material for a church that would last, say 1,000 years, and to have a surface that remains the same color,” says Cassar.
The material has applications beyond keeping concrete surfaces bright. Cassar’s group has found that the concrete can actually clean the air. The company is investigating coating buildings and roads with the photocatalytic material.
Computer models of the material and urban pollution predict that covering 10 to 15 percent of the roads and building surfaces in a city such as Milan, Italy could reduce air pollution by 40 to 50 percent, Cassar’s group calculates.
The pursuit of improved concrete materials continues. How much of the world adopts these new types of concrete
depends on numerous factors, including whether the materials meet technical needs, how much they cost, and whether big-time architects and designers adopt them.
The efforts to bring concrete to new heights of function and form, however, is almost certain to transform the traditional perception of concrete as a cold, drab, low-tech material. Its use is likely to extend as far into the future as it reaches into the past.
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