September 13-20, 2004

Researchers probe use of concrete products to sequester CO2

Researchers at McGill University in Montreal are collaborating with the cement industry on a compelling new approach to reducing carbon dioxide (CO2) emissions which will deliver both environmental benefits and economic gains through significant process and product improvements.

Led by Dr Yixin Shao, a professor in McGill's department of civil engineering and applied mechanics, the project involves sequestering CO2 at the point source (i.e. the cement plant), and transferring the gas to a pressurized chamber where it would be used to accelerate production of high-performance concrete products through carbonation curing.

"We believe there is a good opportunity to use concrete products as carbon sinks for CO2 emissions from the cement industry," says Dr Shao. "Our early research has shown that we can achieve 14% absorption of CO2 by weight of cement in the concrete mix, but we think an uptake rate of 20% is possible." The CO2 is absorbed by concrete in the form of calcium carbonates.

Ultimately, he says, the process of sequestering CO2 for accelerated curing of concrete could lead to a significant reduction in greenhouse gas (GHG) emissions by the cement industry, although the precise amount has yet to be modeled. Worldwide, the cement industry accounts for about 5% of total CO2 emissions.

On average, the production of one tonne of cement generates about 800 kilograms of CO2. The greenhouse gas is produced from two sources: the furnace fuel to heat the cement kiln to over 1,450 degrees C and the cement process itself, which produces CO2 from the decomposition or de-carbonization of limestone.

"Dr Shao's project is unique and of great interest to us because eco-efficiency is at the core of our business" says Gilles Bernardin, director of energy recovery at Montreal-based St Lawrence Cement. Bernardin says the McGill project complements the company's three-pronged strategy to reduce CO2 emissions. It is focusing on more fuel-efficient kiln processes, greater use of fuels from waste materials (such as wood, biomass, and recycled tires), and increasing the concentration of mineral wastes (such as fly ash from coal-fired utilities and slag from steel operations) in cement mixtures.

St Lawrence Cement is providing cash and in-kind contributions to the McGill team, including compressed flue gas from its kiln. The other industry partner in the project is CJS Technology, of Burlington, Ontario, a specialist in cement-mesh panels. The firm has a keen interest in the accelerated curing technology. Major funding for the three-year initiative is provided from the Natural Sciences and Engineering Research Council (NSERC) under its Research Partnerships Program, which promotes university-industry interaction.

Dr Shao says the accelerated curing of concrete products in a CO2-saturated atmosphere is already yielding encouraging results. "Using this CO2 injection method, we can achieve curing in about two hours compared to 24 hours for curing concrete products using conventional steam methods."

He acknowledges, however, that the CO2-curing method would not be suitable for steel-reinforced concrete. With atmospheric carbonation, there is a reduction in the pH value (increase in acidity) of the concrete, leaving steel more vulnerable to corrosion.

Even without steel reinforcing, however, carbonated concrete products can have superior strength and durability compared to steam cured concrete, because they contain much less calcium hydroxide. Products suited to this application include concrete bricks and blocks, fiber-mesh cement boards and pre-cast concrete units with non-metallic reinforcement.

"If we can demonstrate the process is commercially viable," concludes Dr Shao, "then the production of concrete itself will become more environmentally friendly because CO2 curing, compared to conventional steam and autoclaving, is much more energy-efficient."

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