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How to meet the challenges of concrete’s carbon footprint

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If there is a single material the modern world relies upon, it’s concrete. Over the past hundred years, the human race has made, poured, and often demolished 100 billion tonnes of the core ingredient, Portland cement. It continues, according to the trade association for the industry, to pour some 14 billion cubic metres every year.

Even the Covid pandemic put barely a dent in the production of concrete. According to the International Energy Agency (IEA), 2020 saw just a 4 per cent fall in cement and steel production, though the crackdown on speculative construction in China might arrest the rampant growth of concrete jungles across that country, which now has an estimated 90 million empty apartments.

In 1980, Europe and North America accounted for 40 per cent of global cement production of 900 million tonnes. By 2017, that share had dropped to less than 7 per cent, according to US environmental charity ClimateWorks Foundation. By that time, China and India accounted for more than 60 per cent of cement production, churning out some 2.6 billion tonnes annually. Analysts expect China’s status as number-​one consumer will slip relative to India and other developing nations as they move towards much more urban development. But by 2030, the industry could easily be making some 4.8 billion tonnes of cement a year.

“We will need to build in the future one New York City every month,” Claude Loréa, cement director of the Global Cement & Concrete Association (GCCA), claimed at a conference held last October to announce the association’s plans for concrete’s big problem: its prodigious output of carbon dioxide. Against that backdrop, the 8 per cent of human society’s carbon dioxide emissions that concrete generates annually may seem entirely expected, but it is an 8 per cent that is not at all easy to cut.

is wood the answer

Is wood the answer to cutting concrete carbon emissions?

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The chemistry of cement makes greenhouse-gas emissions inevitable. When mixed with clay and heated to temperatures over 1,400°C, calcium carbonate in the limestone turns into calcium oxide, with the carbonate component released as carbon dioxide gas, and then reacts with silica in the clay to form a complex mixture of silicates in a product called cement clinker. This clinker then reacts with water to first form a sticky mortar that quickly hardens even underwater. The cement does not dry out. Instead, much of the water takes part in a polymerisation reaction that binds both it and the other rocky materials in the mixture into a solid block.

Figures from the GCCA showed that in 2019, each tonne of clinker resulted in the production of 800kg of carbon dioxide, excluding the emissions used by power generation. Because of the intense heat, that power component is also large at just over half that of the chemical process, based on recent figures. A shift to hydrogen burning or electric heating could conceivably remove the emissions from the energy inputs. But that still leaves the chemistry problem. Yet the GCCA expects cement production to be net-zero by 2050.

Green MP Caroline Lucas asked a representative of the UK’s concrete industry in a select-committee hearing on sustainable buildings in October: “How much heavy lifting do you think the little word ‘net’ is doing when we talk about becoming net-zero?”

It may wind up doing more than even the GCCA expects, as it may make sense for this to be one of the last industries to move to energy sources that do not generate carbon dioxide, though there are a number of levers the industry can use alongside trading carbon sinks against its own sources.

The emissions of clinker are not entirely set in stone. The limestone-and-clay combination can be used in comparatively low ratios with other minerals, such as the slag from steelmaking furnaces. The variation in emissions from production, according to data collated in 2020 by Jane Anderson and Alice Moncaster at the Open University, can swing from less than 300kg CO2 equivalents per tonne for some forms of the CEM III grade, which can contain more than 50 per cent blast-furnace slag and ash, to almost 1,200kg for the white cements often used to face buildings.

Conventional wisdom has for the past couple of centuries regarded adding large amounts of fly ash and slag as leading to weakened cement. CEM III does not get used for structures that need high strength. Weakened concrete is not inevitable; Roman architects realised this long before Joseph Aspdin came up with the recipe for Portland cement in widespread use today.

Despite its ubiquity, the chemistry of cement and its ramifications for structural strength are not that well understood, largely because the chemistry involves the interaction of so many different elements even when the material has hardened. The demand for greener concrete has reignited work on comparing the properties of ancient cements to see if there are secrets to be gleaned – and it seems there are.

Researchers at the University of California at Berkeley concluded in 2017 that the Roman recipe is superior, thanks to the same kind of ash that buried Pompeii. The UC Berkeley team used X-rays to analyse 2,000-year-old mortar in the supports of a pier that remains at Orbetello, halfway between Pisa and Rome, to find that the inclusion of volcanic ash along with seawater in the original did not just help to harden the construction but to continue to do so over time.

addition of volcanic ash

The addition of volcanic ash to this 2,000-year-old pier at Orbetello, Italy, has aided its structural integrity and longevity

Image credit: J.P.Oleson

Pliny the Elder, who later died in a close encounter with ash from the AD79 eruption of Vesuvius, noticed the effect: “As soon as it comes into contact with the waves of the sea and is submerged, [it] becomes a single stone mass, impregnable to the waves and every day stronger.”

Salts in the seawater and pulverised volcanic rock (pozzolana) seem to have promoted a second chemical reaction around the aluminium and silica hydrates it contained: those compounds led to tiny hard fibres and plates growing in the mortar that act as a reinforcement.

By working on recipes that take advantage of similar processes to those in the pozzolanic Roman recipes, chemists could find novel ways to reduce the proportion of limestone in clinker and in turn bring overall emissions down. However, it is this tendency of cements both old and new to sustain solid-state chemical reactions over many years that makes the analysis of concrete’s overall carbon footprint harder to assess than it might at first seem.

The calcium oxides and silicates of set cement are less chemically stable than the carbonates in the original limestone rock. That instability lets concrete repair some of the damage caused by its manufacture: the cement reabsorbs carbon dioxide as the oxide changes to a carbonate. One study published five years ago argued as much as 40 per cent of the carbon dioxide emitted during production could be reabsorbed by buildings over an 80-year period.

As part of its programme for achieving net-zero carbon emissions, the GCCA has assumed a more conservative figure of 6 per cent of emissions from the complete production life-cycle, which would equate to a recovery up to 15 per cent of the carbon released in the calcium reactions in the kiln.

Recycling of concrete is also unlikely to be a big contributor to emissions savings. Demolition inevitably leaves a pile of undifferentiated rubble that needs to be sorted in order to find concrete suitable for incorporating into buildings, and the end result may not have the same strength as virgin material, which will limit its use to sectors where timber may be the more sustainable option.

There seems to be some room to reduce the carbon contribution of materials that go into roads in general – though not bridges and flyovers – by focusing alternative materials and concretes with higher slag ratios on lesser-used highways, reserving the higher-quality cements for motorways. Taking China as an example, this would make close to two-thirds of the volume of material destined for the country’s roads as being open to substitutes. The ratio is even higher in the US, where freeway construction is far less intensive than in the past.

Roadbuilding, however, represents a relatively small proportion of concrete usage, especially in the developing world where cement demand is increasing the fastest. Even in the freeway-centric US, residential construction on its own during the 2008-2010 mortgage-led financial crash proved to be a larger consumer of concrete, according to data collated by ClimateWorks.

Sequestration and recycling will lead to some savings in life-cycle emissions. But the GCCA sees the biggest component to a net reduction as being carbon capture and storage during production, with more emphasis on the capture than storage.

Magali Anderson, chief sustainability and innovation officer at Swiss materials supplier Holcim, said at the GCCA’s net-zero launch event: “I see carbon as a resource and that resource can be transformed, helping other industries to decarbonise.

“The pricing of the carbon, the economic model will be absolutely crucial in coming years,” she added, pointing to the supply of carbon dioxide to producers of aviation fuels and plastics with green hydrogen as the energy source as one prime target.

One big advantage that cement production has over other industries when it comes to carbon capture is that the chemical reaction produces a higher concentration of the gas than is found in the exhaust of gas turbines or even ironmaking furnaces. That makes it possible to use cheaper and simpler methods to extract the carbon dioxide. Research performed a little over five years ago by researchers at Imperial College, London found that capture from ironmaking and oil refining could easily cost twice as much as that from cement kilns, though changing technology may reduce this differential.

Though the use of waste fuels and biomass for cement production has grown in order to move the energy part of the equation to net-zero, it may make sense to put more emphasis on carbon capture from natural gas in the short to medium term. The use of waste and biomass for fuel is still below 20 per cent, even in the US where producers began to adopt it more than 30 years ago. China now heats about 10 per cent of its kiln throughput using waste fuels but the rest is almost entirely coal. A big problem for any of the heat-intensive industries is that carbon pricing will increase competition for these fuels, which will naturally force prices up. The US aviation industry on its own could easily consume the most readily convertible waste for its own sustainable fuels (see E&T July 2021) and be more able to absorb higher feedstock prices.

The industry could respond by continuing to rely on fossil fuels for longer than others and focus even more on carbon capture. Research is proceeding on methods to carbonate the slags and other industrial wastes that might be mixed with the clinker. These materials do not quickly carbonate under normal conditions, but heated in an atmosphere of almost complete carbon dioxide they will take up the gas.

The big problem with carbon capture as a strategy is that if a market for carbon dioxide does not develop for synthetic fuels and chemicals, long-term storage may prove to be uneconomic for cement makers at the scale required.

Other recommendations, such as those made by ClimateWorks, put far more focus on downstream efficiency compared to the GCCA’s focus on capture. In a ‘whole system’ scenario, where the end markets adapt to use less concrete and substitute it with materials such as engineered timber, carbon capture’s contribution becomes far less significant. However, there are few other materials that can compete with the ability to be poured into place to form roads, bridge supports or supporting walls for tall buildings.

Taking China as an example. Climate­Works’ analysis of consumption showed that easily two-thirds of the country’s recent annual consumption of 690 million tonnes of cement has been for large urban concrete-and-steel buildings rather than smaller constructions. As it’s a more efficient use of the material, concrete per square metre of liveable area, the rapid rise of city living in the developing world suggests these countries will build out in a similar way.

Researchers in the Resource Efficient Built Environment Lab (REBEL) group at Edinburgh Napier University concluded that cross-laminated timber flooring could cut concrete use and carbon emissions in the construction of tall buildings. This is where other considerations come into play. Sam Liptrott, director at fire and risk consultancy OFR, warned the Commons select committee in October that in the wake of the Grenfell Tower disaster there is a reluctance in the UK to use timber so extensively in large buildings until safety legislation is clarified.

For emissions specifically, timber itself has a possible sting in the tail: burning or disposal in landfill leads to at least some of the wood releasing greenhouse gases into the atmosphere. Landfill can be worse because that can result in the cellulose decomposing to methane, which has a worse effect than the carbon dioxide that would mostly be produced by burning. Even with the effects of decay, which will be exhibited by the dead trees in forests as well, the release will at least be over significantly longer timescales than those seen with concrete manufacture. But it again provides a further complication to the problem of determining which outcome is better.

Questions over land use will also come into play if builders step up their use of lumber. Countries where timber is relatively rare, such as China, will have to import significant quantities, whereas they can make enormous quantities of cement locally. The effect these decisions have on emissions is hard to gauge.

Taking spruce growth in the south of Sweden as an example, Michele De Rosa of LCA Associates and colleagues at Aarhus and Aalborg universities took life-cycle assessments under different assumptions, such as when emissions occur within the life cycle, and found that they delivered wildly different results ranging from effective sequestration of 24kg of carbon dioxide per cubic metre of timber to more than three tonnes of emissions per cubic metre.

Time continues to play a major role if we zoom out and consider the big picture. Using UK statistics from 2008, in their report for Chatham House, Johanna Lehne and Felix Preston showed that concrete’s role in emissions pales in comparison to the emissions generated during the lifetime of a building through heating and air conditioning and that even in the construction phase, the high energy-demand of steel and glass and the long-distance transport of specialised materials all take a toll. The biggest impact could well come from taking a look at the financial excesses that made a lot of China’s building splurge so pointless and damaging, therefore doing far more to make both banks and developers think twice about whether it is a good idea to pour any concrete in the first place.

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