The concrete challenge
- Worldwide, the industry is responsible for up to 10% of human CO₂ emissions.
- Researchers are looking at the use of alternative building materials made from hemp, bamboo or timber.
An incredibly versatile material, concrete has proved essential to the development of urban life as we know it. Roads, bridges, towers, apartment buildings all depend on concrete. And its use is skyrocketing with the rapid growth of cities in developing and emerging countries. Contrary to popular belief, concrete and cement are not the same thing. Cement is a powder, which, when mixed with rock, sand aggregate and water, binds together into concrete. Although cements were used by the ancient Egyptians and Romans, the ordinary Portland cement we now rely on was only developed in the 1800s. Cement is the largest manufactured product on earth by mass, making it second only to water among materials consumed by humankind.
The environmental impact of concrete is heavy, however. The extraction of the sand and gravel we use as concrete aggregate leads to disappearing beaches, erosion and environmental issues like pollution or biodiversity loss. In Indonesia, entire islands have disappeared due to excessive sand mining, and sand mafias from India to Morocco illegally destroy coastal environments to feed the construction industry’s appetite for sand aggregate. The aggregate extracted in 2012 – estimated at 25.9 billion tonnes – would create enough concrete to build a wall 27 metres high and 27 metres wide circling the globe at the equator. But the most glaring environmental cost of concrete is from cement production. The cement industry is estimated to be responsible for up to 10% of global human CO2 emissions, making it one of the world’s largest industrial drivers of global warming.
Ordinary Portland cement is produced in a two-step process: raw materials, including limestone and other minerals are quarried, crushed and ground. This powder is fired in a kiln at 1450° C, where it fuses into small, hard nodules called clinker. The clinker is then ground together with other minerals to produce the powder we know as cement. Clinker production, says Detlef Heinz, professor of Mineral Engineering at the Technical University of Munich, is where the CO2 emissions come in. It’s a process he knows better than most, having previously worked in the cement industry. The clinker kilns not only burn huge quantities of fossil fuels but the chemical reactions involved in clinker formation also emit CO2.
Reducing the amount of clinker
The cement industry has long been aware of the gravity of the problem. In 2009 they released the Cement Industry Roadmap 2009, a plan for transitioning the industry to halve its global CO2 emissions by the year 2050. “There are potential ways to reduce the CO2 from cement production,” explained Heinz, “such as carbon capture and storage [CCS], where you capture the CO2 from the cement production, and then sequester it or use it as a basic material for new products like the synthetic natural gas CH4.” Could sequestering carbon and using carbon-neutral clinker kiln fuels solve the problem?
According to a 2016 report on eco-efficient cements from the United Nations Environment Program (UNEP), probably not. Although the report says CCS may be needed to reduce emissions enough to keep global warming to below 2° C, it points to better solutions in the short term. Heinz agrees: “I think the most important strategy, which Europe has been using for the last couple decades, is the use of secondary material to reduce the cement clinker component.” Using less clinker in cement could significantly reduce the CO2 burden of concrete production. A proven example is the use of fly ash – a byproduct of coal burning – to replace normal cement clinker by up to 60%. Another common alternative is the granulated slag formed in iron and steel blast furnaces, which can cut the requirement for traditional cement clinker by up to 80%.
These advances seem promising, but Karen Scrivener, head of the Laboratory of Construction Materials at the École Polytechnique Fédérale de Lausanne, has doubts about the viability of many of these clinker-substituting substances. Scrivener, who was one of the lead researchers behind the 2016 UNEP report, says there are serious issues with cement mixtures that use fly ash and blast furnace slag, the most common alternatives. “Today, the average rate of replacement worldwide is about 20%. The problem is that the supply is limited,” said Scrivener.
“For blast furnace slag, we have about 8% availability compared to cement production, and the usable levels of fly ash works out to be about 8–10%. And, of course, if you’re talking about fly ash, which comes from burning coal, you have to ask how long we will keep on burning coal, which is a major CO2 contributor.”
Scrivener believes that her team in Switzerland may have found a better solution. With partners in India and Cuba, they have developed a calcined clay cement that can decrease clinker needs by half, cutting overall CO2 emissions by up to 30%. It is just as strong as normal cement, with properties that make it ideal for use in coastal environments where traditional steel-reinforced concrete structures are often subject to corrosion. They are testing the new cement in Cuba, where it will be used in the reconstruction following the devastation wrought by hurricane Irma.
In addition, massive amounts of this calcined clay can be found in mining tailing waste. “This means you don’t have to extract anything,” says Scrivener. “For example, in China, you have 10 million tonnes of it in a spoil heap in one site, two or three times the annual volume of cement use.” Other materials can be recycled and reused in concrete production, including old concrete itself. Lisbeth M. Ottosen, a professor specialising in resource recovery at the Technical University of Denmark (DTU), considers a used resource not as waste, but rather a resource input for the next process. She is a member of ZeroWaste Byg, an interdisciplinary research team that is investigating the reuse of construction materials for a zero-waste society.
Much of their research focuses on the potential of using bio ash from sources like Denmark’s wood-burning power plants. “It’s broader than just the reuse of concrete aggregate,” she says. “I focus my research on the resources which escape from the materials cycle as waste.” Another example is a DTU student research project in Greenland that uses the fibres from discarded fishing nets to replace the virgin polymers in fibre reinforced concrete. The fact remains that the lack of an alternative to sand and gravel aggregate for concrete is a huge problem in parts of the world like China. With around 200 tonnes of sand needed to build an average family house, construction booming and the environment suffering, the question stands – can we recycle concrete? Ottosen says it is common practice to reuse concrete in some European countries, but that this often involves “downcycling”, using it to create an inferior product for roading aggregate or “backfill” in areas like highway noise breaks.
With the EU setting a 2020 goal of recycling 70% of construction waste, it is debatable how much of this reuse will be in a degraded form – or instead “upcycled” into new concrete. Heinz says that using old concrete waste as a source of aggregate for new concrete might be a good solution, but there are still some issues that must be considered. Concrete is only a fraction of the total waste material from construction and only a fraction of this can be reused, especially since old concrete is sometimes contaminated. The recycling process itself also produces CO2.
And the aggregate available from recycling is not enough to meet the construction industry’s demand. “In Germany, you have annually something like 80–90 million tonnes of mineral materials like crushed concrete and bricks from the demolition of buildings,” Heinz says, “but we need up to 500 million tonnes of aggregate primary material for the production of construction materials.”
Buildings made of bamboo and hemp
We should instead consider alternatives to concrete – where applicable and available. “It will be necessary to go back to using renewable materials such as wood in situations where it is technically possible that they can carry out the same construction task as with concrete,” says Detlef Heinz. Although many large buildings require the use of concrete to ensure structural integrity, the argument for renewable or alternative construction materials in some cases is worth considering.
New methods to use renewable resources like timber and bamboo with only minimal concrete for specific purposes such as fire protection are emerging, in addition to alternative construction materials with concrete-like properties, such as rammed earth buildings and hempcrete – a material made from hemp fibres and a lime binder. In France hempcrete has been used in combination with concrete in a public housing project and a seven-story office tower. Petr Hajek, professor of Civil Engineering at Czech Technical University in Prague, says that new types of concrete are also proving important for areas where concrete-use cannot be eliminated. He points towards new variants like self-compacting-concretes, highand ultra-high-performance mixes and concrete reinforced with textile meshes instead of steel. “These new technologies enable construction of more subtle concrete structures using less concrete, with better structural performance and durability, and lower environmental impact.”
Hajek says that even something as simple as optimising the shape of structural support can significantly decrease their weight and thus the amount of concrete needed. Or, going further, the use of non-steel reinforcement in the form of textile mesh made from carbon can enable the creation and use of thin, corrosion-resistant concrete shells. One such example is the Carbon Concrete Composite (C3) developed at Technical University Dresden, a woven-carbon fibre composite manufactured with lower energy consumption and CO2 emissions than traditional reinforcements. Similarly, a bamboo composite material developed by a team at ETH Zürich could replace steelreinforced concrete, and potentially be used for beams, floor slabs and joints. And, as futuristic as it sounds, Hendrik Jonkers at Delft University in the Netherlands has invented a concrete that heals itself. Self-activating, limestone-producing bacteria in the concrete repair cracks, reducing the need for maintenance and thus new cement down the line. Like all the solutions above, there are drawbacks – in this case, higher short-term costs.
Scrivener is adamant that the cement industry must ditch this short-term mentality. “In the long term these are things that can save money, but we just have to get over the introduction barrier, and find incentives to introduce these changes.” Although it’s not clear exactly which path the cement and concrete industry will follow, one thing seems obvious: there is no magic bullet solution for making concrete more sustainable. “From making the cement to mixing it into concrete and using it in the building, in each of these steps we have possible savings. Only in putting together the solutions for each of these steps will we have the reductions we need.”