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Green Concrete

Nanoengineered materials could reduce greenhouse-gas emissions.

Protecting the built environment from the forces of the natural world with dams and seawalls is important work (see “Saving Holland”), as is protecting the natural environment from the engineered world. But the 21st-century engineer should also look to the natural world as a powerful design partner and a source of sustainable solutions. A good place to begin is by studying the way natural materials are constructed at the nano­scale and drawing inspiration from them as we engineer our own materials. Take, for example, the civil engineer’s construction material of choice: concrete, the oldest engineered building material and one of the most widely consumed materials on earth, second only to water.

Each year, 1.89 billion tons of cement–the primary component of concrete–are manufactured, enough to produce one cubic meter of concrete for every person alive. Unfortunately, cement is a major source of atmospheric carbon dioxide–largely because it’s made by burning fossil fuel to heat a limestone and clay powder to 1,500 °C, which changes its molecular structure. When the cement powder is later mixed with water and gravel, the invested energy is released into chemical bonds that form calcium silicate hydrates–the glue that binds the gravel to make concrete. The production of cement accounts for an estimated 7 to 8 percent of all human-generated carbon dioxide emissions.

If we can engineer a novel cement whose manufacture produces only half as much carbon dioxide, we will achieve a significant reduction in total carbon dioxide emissions. And human bone could show us how to do it.

Cement’s strength comes largely from the way the calcium silicate hydrates self-assemble into particles that pack together with the highest density possible for spherical objects. Human bone–or, more precisely, the apatite minerals in bone–achieves a very similar packing density at the nanoscale, yet it is “manufactured” at body temperature with no ­appreciable release of carbon dioxide. At the nanoscale, bone has much in common with concrete: it consists largely of calcium; its strength displays a significant frictional component; and collagen proteins help re­inforce it, much as steel bars reinforce concrete.

Of course, with bone, the hydration and hardening of the apatite minerals takes a month or so, more time than we can afford on construction sites. But if we can find a way to mimic the process and speed it up, we could replicate it to fashion a new building material.

This is but one example of research drawing on the limitless designs offered by the natural world–and extracting fundamental engineering principles from them.

Franz-Josef Ulm, an expert on materials, is a professor in MIT’s Department of Civil and Environmental Engineering.

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