Concrete is a familiar substance. Its durable nature and versatile applications have made its usage ubiquitous throughout our cities. However this primary building material is also extremely energy intensive to make and transport, and produces a significant amount of the world’s greenhouse gas emissions. Can the omnipresent grey substance ever be reconciled as a green building material?

Concrete’s impact on the environment starts when limestone is blasted in quarries to make cement - the binder, or substance that sets and hardens it into a useful building material. Cement accounts for 7 to 15% of concrete’s total mass by weight and is made by superheating (in coal-fired kilns) a mixture of limestone and clay and then grinding the resulting substance into a powder. When this power mixes with water, it forms strong calcium-silicate-hydrate bonds that can bind other particulates, like sand or gravel, to make concrete. The cement-to-water ratio determines the strength of the concrete.

Once limestone has been blasted and mined it is then transported to a cement plant, where the fuels used by the plant and machinery produce CO2 emissions. Next the limestone, or calcium carbonate, releases CO2 when it is heated to make the cement. Forty percent of CO2 emissions from the cement plant come from the combustion process and Sixty Percent of CO2 emissions come from the calcination process, according to the Cement Sustainability Initiative report produced by members of the concrete industry. The report also says that since calcination is intrinsic to the process, they must focus on reducing energy use associated with the manufacture of concrete.

So what does concrete have going for it?

It lasts. This is the stuff the Romans built their empire with. Concrete is highly resistant to heating and thawing. It’s impermeable to air and wind-driven rain. And concrete is inedible, so bugs and vermin can’t gnaw at it. This durability means that a building can preserve its concrete foundation or concrete exterior while replacing less durable parts like windows, insulation and plumbing (you get a point from LEED if you reuse a building).

A building with exterior concrete walls can also be energy-efficient, especially in climates that have daily temperature fluctuations. Even though concrete provides little insulation, it creates thermal mass that can store warmth or cold, reducing indoor temperature fluctuation. White concrete also reflects heat and can mitigate the urban heat island effect.

A Locally-Sourced Material: Another reason concrete is so popular is because the raw materials to make it are prevalent in most of the world. For the eco-conscious builder, this means it can be locally-sourced, reducing CO2 emissions from transportation. Of course, “local” is a relative term – advocates in the cement industry claim that “the cement, aggregates, and reinforcing steel used to make concrete and the raw materials used to manufacture cement are usually obtained or extracted from sources within 300 miles of the ready mixed, precast concrete, or masonry plant.”

Concrete can also be be recycled – to a point. The concrete industry web site concretethinker.com says: “Most concrete in urban areas is recycled as fill or road base and not placed in landfills. Concrete pieces from demolished structures can also be reused to protect shorelines, for example in gabion walls or as rip rap.” Used concrete can also be reused as the aggregates in new concrete. Concrete’s recyclability is limited because its chemical properties change over time and with each processing. Other materials can be recycled as aggregates in concrete.

Canadian environmental consultants, Carbon Sense Solutions, may have just developed a carbon storage method that could reduce global CO2 emissions by as much as 1% a year. Their new method called, CO2 Accelerated Concrete Curing, accelerates the curing process and stores carbon dioxide at the same time. This method applies only to precast concrete, but has the potential to make a huge impact on the world.

The method, also known as concrete carbonation, actually occurs naturally as concrete cures, but up until now has not been considered economical. Carbon Sense Solutions says it has developed a faster way to store more CO2 in concrete, using off the shelf technology, which uses dramatically less energy. They also claim the concrete is more durable, more resistant to shrinking and cracking, and less permeable to water.

As concrete is used more than any other man made material on earth, (the Chinese alone consume 40%!), and concrete is responsible for upwards of 5% of global CO2 emissions, any amount of increase carbon storage in concrete would make a difference. So if Carbon Sense can really deliver as they say it can in Technology Review, the process “has the potential to sequester or avoid 20% of all cement-industry carbon dioxide emissions.”
That’s some carbon sucking concrete.
as seen in inhabitat