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| 02/01/17

Solving the thorny problem of cement emissions

TCE

Canadian researchers competing for a US$20 million clean tech prize have developed potential solutions to the thorny issue of emissions from cement manufacture that go beyond reducing the carbon footprint to actually making it negative—although fresh carbon accounting of cement suggests the product may not be the climate villain it has long thought to be.

Cement is one of the trickier climate mitigation problems. This one sector is responsible for five percent of global greenhouse gas emissions. Yet is not actually the kiln heating part of the process that produces the bulk of its GHGs. Instead, it is the chemical reaction of turning limestone into quicklime, the basis of cement, which produces most of the carbon dioxide (CO2).

This means that even a complete replacement of fossil fuel combustion with some other, clean source of heat would only solve part of the problem.

British Columbia’s carbon tax has been costly for the province’s cement industry, putting it at a disadvantage with tax-free competitors. As a result, the provincial government in 2015 ponied up a $22 million subsidy to help the industry to develop cleaner options, largely by replacing the fossil fuels used for heat with scrap wood and residuals from plastic recycling.

At the same time, without some way of reducing emissions more substantially, society is caught in a bit of a bind: we need to eliminate the emissions from cement entirely, but we also need cement for so many purposes.

This is where some Canadian materials and civil engineering researchers come in. The Carbon X Prize, a five-year, global competition bankrolled by Canada’s Oil Sands Innovation Alliance and US energy firm NRG, hopes to inspire technologies that can convert CO2 into valuable products that do not return the gas back to the atmosphere. Kicking off in 2015, the event organisers announced 27 semifinalists last year. A winner will be chosen in 2020.

One of the semifinalists, a group of McGill researchers, have developed a product they call Carbicrete that replaces Portland cement as the main binding agent with steel slag, a waste product of the steel industry (which is also very carbon intensive), reducing emissions. But they also cure the Carbicrete with CO2 gas, a process that also sequesters the CO2 in the material, thus overall resulting in a net carbon-negative product.

They say the product is also more durable than conventional concrete, and more resistant to cycles of freezing and thawing.

The place of cement in global carbon accounting may however be changing. Last November, a paper appeared in Nature Geoscience that for the first time assessed the natural reversal of CO2 emissions from cement that happens when cement is poured. The surface of fresh concrete reacts with CO2 in the air, drawing it down and sequestering it as calcite (CaCO3).

The researchers concluded that on balance, cement materials over their life cycle represent a large and growing net carbon sink, standing at about 0.25 Gt of carbon per year as of 2013 and not currently taken into account in GHG emissions inventories.

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