Low-Carbon Cement Paves a Development Path (or sidewalk)

Glass-half-full types see carbon-cutting potential here. No other sector has such a high potential for drastic emissions reductions at reasonable cost, according to the United Nations Environment Programme.

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Enter Drexel University materials scientists Alexander Moseson and Michel Barsoum. They've created a low-tech, low-energy, low-cost cement that they hope to move out of the lab and into the real world.

But they face huge hurdles: Entrenched industry, tough building codes, a mindset that says there's only one way to lay a foundation or build a bridge - with Portland cement.

Carbon emissions from cement are set to grow explosively as developing countries such as India create a ‘first-world’ infrastructure. Scientists and entrepreneurs are struggling to push alternative technologies out of the lab and onto the street.

Big CO2 footprint

Officials have a duty to give taxpayers the best, most-durable road or bridge or whatever possible. - Steve Kosmatka, Portland Cement Assoc.

Portland cement's big C02 footprint is generated by its manufacturing process. Limestone, sand, and other metal ores are heated in a kiln - usually coal-fired - to 1,500 degrees Celsius, triggering a chemical reaction that forms granules called "clinker." Clinker is then crushed to form the familiar, powdery cement that is later blended with aggregate and water to make concrete. About one-third of the greenhouse-gas emissions come from heating the kiln, according to Steve Kosmatka, vice president of research and technical services at the Portland Cement Association. The rest come from the chemical reaction.

The problem with any bid to replace it, he and others say, is that Portland cement enjoys economies of scale, which keep prices down. Introduced in the 19th century, it's also familiar: Governments and builders have many decades of "use and comfort" with the product, he said. It is the benchmark for industry codes. "Officials have a duty to give taxpayers the best, most-durable road or bridge or whatever possible," Kosmatka said. "There's risk to trying new things."

Potential demand for lower-carbon building materials has sparked a race to replace Portland cement featuring a handful of manufacturers and scientists. Some claim to sequester carbon within the cement itself. Others use alternative fuels. Still others tap unconventional feedstocks, such as magnesium silicate, that require lower kiln temperatures.

Moseson and Barsoum are trying the latter, mixing recycled iron slag or fly ash with readily available limestone. "We literally used a bag of garden lime from Home Depot," Moseson said. Instead of a coal-fired kiln, they use a bucket with a spoon at room temperature.

Promising alternatives

Such "alkali activated" cements are one of the most promising alternatives to Portland cement, the IEA notes, in part because they rely on ubiquitous materials such as industrial waste.