The source of 5 percent of global carbon dioxide emissions is hiding in plain sight, in the sidewalk beneath our feet. It is cement, a key ingredient of concrete, the most widely used building material on the planet. And manufacturing conventional "Portland" cement releases nearly a ton of the greenhouse gas for every ton produced - some 3 billion tons in 2010.

And that figure is about to balloon.

You've found a way to bake bread without the oven.
- An investor to Alexander Moseson

As developing countries strive for "first world" living standards, they will be building more sidewalks, roads and housing, the International Energy Agency predicts. In many places, the growth spurt is already underway: Construction in India alone has jumped 10 percent per year for the past decade, according to the World Green Building Council.

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.

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.

Tests showed that the Drexel duo's cement is as durable as Portland but emits 95 percent less CO2. "You've found a way to bake bread without the oven," Moseson said one impressed investor remarked. That energy-saving trick means this cement could cost about 50 percent less to produce, according to their calculations.

Alkali-activated cements are not new. Research suggests that ancient Egyptians used a lime-based concrete to build the Pyramids, and industry has been trying to develop practical versions since the 1950s.

But market issues, lack of environmental awareness, inconsistent ingredients, and limited knowledge of cement chemistry restricted alkali cement to a niche market. The cement industry came up with viable alkali cements 20 years ago but found few customers, Kosmatka said. "It was a product ahead of its time."

Now, proponents say, the time for more widespread application of alternatives may be right.

Push for green standards

Governments in Asia and elsewhere are kick-starting green building industries, opening doors for alkali-activated and other green cements. China's newest cement standards, for instance, require a 15 percent reduction in energy use. India's green-building standard takes a life-cycle approach and emphasizes recycling and pollution reduction.

Ultimately, green cement is "a social and political issue," concluded Barsoum.

"The technology may be the simplest part of the equation."

By Miranda C. Spencer
for The Daily Climate

Photos courtesy Alexander Moseson, Drexel University.

Miranda Spencer is a freelance journalist based in Philadelphia.

This article was first published on My Green Australia. My Green Australia is a resource of information about living green and healthier in Australia. We want to protect and cherish our country and preserve the natural beauty of Australia for many generations to come.