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? Read on for our in-depth report.
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.
Concrete producers also say that as concrete ages, it carbonates and reabsorbs all the CO2 released during calcination – but this process takes hundreds of years.The general consensus is that cement manufacturing produces about 5% of global CO2 emissions generated by human activity, and 3% of global emissions of all greenhouse gases. By comparison the transport sector is responsible for about 15% of global greenhouse gas emissions, so concrete has a pretty hefty share of the pie considering it’s just one material.
The problem stems from volume: According to the World Business Council for Sustainable Development, concrete is the most widely used substance on Earth apart from water. Approximately 2.35 billion tons of concrete are produced each year. That works out to a cubic meter, or about a 3.2- by 3.2- by 3.2-foot cube of concrete for each person on Earth every year, according to researchers at MIT.
And Demand is on the Rise: Concrete production has nearly quadrupled since 1970. Global cement output may reach 5 billion metric tons by 2030 due mainly to growth in China and India, according to a December report by the global conservation organization WWF. (The same report estimates the concrete industry’s share of global emissions at 8%, and claims the industry is capable of reducing its emissions by 90% with current technology.)
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. LEED also awards points if you build concrete walls and a concrete ceiling with no coating.
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.
Some progress has been made in reducing the amount of emissions in the concrete manufacturing process. The English company Novacem says it has created a cement (patent pending) that uses magnesium silicates, which do not emit CO2 during combustion. Its production process also runs at lower temperatures and the cement absorbs far more CO2 as it hardens – enough to more than offset the amount generated during manufacturing. A California-based company, Calera, claims it has discovered a cement-manufacturing process with a negative carbon footprint. Calera’s technique, remniscent of how some corals produce reefs, is to filter carbon dioxide emissions through seawater to create a chalky carbonate byproduct. This cement substance is then mixed with aggregate and water to create concrete, sequestering carbon emissions and avoiding the need to heat the ingredients, the company says.
Is It Green?
The reality is that even if new technology truly achieves a carbon-negative manufacturing process, greener cements (like ones requiring complex facilities to shoot emissions through seawater) won’t be as affordable or accessible in the developing parts of the world that are consuming concrete most voraciously. Significant CO2 reductions won’t be possible without heavily subsidizing the industry in poor countries.
The importance of durability should not be trivialized, but today’s concrete is not green. A combination of new technology and efforts to decrease energy used in manufacturing is needed so that perhaps tomorrow’s concrete can be.
Lead photo by Jeff Kubina