Novacem is taking a 150-year-old technology – making cement – and giving it a carbon emissions upgrade. For an industry that accounts for 5 percent of the world's greenhouse gas emissions, that could make quite a dent in global warming.

The Imperial College London spinout says it has a technique for making Portland cement using magnesium oxide – the chemical most familiar to consumers as talcum powder – rather than calcium carbonate as a feedstock.

Calcium carbonate has to be heated to about 1,400 degrees Celsius to make cement, and releases its carbon in the process, says Stuart Evans, chairman of Novacem. Magnesium oxide doesn't have carbon in it, and because Novacem only has to heat it to about 800 degrees Celsius to make cement, it can use biomass as a fuel.

Those two main factors could make a Novacem plant carbon-neutral or even carbon-negative, he said. Every ton of standard Portland cement replaced by Novacem cement could result in about a three-quarters of a ton reduction in CO2 emissions, the company claims.

Does it work? Novacem is building a small-scale test plant, to open by year's end, to see, Evans said. It hopes to have a larger-scale plant operating by 2011 and full-scale production underway by 2013, he added.

Some hefty scientific credentials appear to back the idea. Among the investors who put more than £1 ($1.7 million) into the company in August is the Royal Society Enterprise Fund, the first-ever investment for the venture arm of the U.K.'s premiere science institution, the Royal Society. Other investors include the Imperial Innovations Group and the London Technology Fund.

It's far from being the only startup looking at making greener versions of the world's primary building material. One noteworthy entrant is Los Gatos, Calif.-based Calera, the Khosla Ventures-backed startup whose claims of carbon-sequestering cement have come under some controversy (see More Clues in Calera Cement Controversy).

Carlstadt, N.J.-based Hycrete and Newark, Calif.-based CalStar Cement are other startups developing new technologies for building products related to cement – Hycrete with a new waterproofing process that removes the need for plastic liners, and Calstar with bricks made by fly ash, a byproduct of burning coal.

While the U.S. building market has slowed down quite a bit amidst the economic recession, the long-term prospects for green cement abroad are huge, Evans said (see Peddling Green Cement and Concrete Abroad).

Evans pointed to China as a particular example – China uses almost half of the world's supply of the material, he said.

"The Chinese, I think, would be very interested in green cement," he said.

Learn how to differentiate your company through greener product lines at Greening the Supply Chain on September 17 in Boston.