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Construction workers pour concrete via a pumper truck for part of the foundation of a new building being built in downtown Washington, DC.PAUL J. RICHARDS/AFP/Getty Images

Mary Soderstrom’s latest book is Concrete: From Ancient Origins to a Problematic Future.

A week after the “pause” in construction here in Quebec was lifted in May, the concrete pumper trucks were at work on a construction site near my home. The guys looked delighted to be back on the job – lots of smiles as they guided the fresh slurry into the forms that would support the apartment building they were constructing. The ones working lower down on the building, side by side, were wearing masks and didn’t seem to be too annoyed by the new regulations.

Ah yes, the new normal, I thought. We’re making some changes, COVID-19 oblige. But would you believe we’re back to playing with concrete, even though it’s one of the most environmentally damaging materials on Earth.

Statistics show that construction starts are down in Canada – but not as much as you might expect. Between March and September, $96-billion was invested in residential and non-residential construction in Canada, compared with $102-billion over the same period in 2019. Then there’s China, which was struck with COVID-19 and closed down before any place else did. Projections are that construction equipment sales will actually rise in China by 14 per cent this year.

All this means that concrete – a mixture of cement, water, aggregate and a few other additives – will continue to be a major motor behind climate change. Those clear skies over industrial cities, those mornings when you could hear birdsong in downtown Toronto, those evenings when orcas feeding in the Strait of Georgia weren’t bothered by noise from passing watercraft – none meant real change, all were no more lasting than a jet contrail at sunset.

Concrete, however, sticks around. It is an ancient material, and the Romans were aces at making it. Their constructions – for example, a vast harbour installation built off the coast of what is now Israel in 12-23 BC – are often still strong, unlike many piers, bridges and wharfs constructed mere decades ago. Their secret was to take quicklime mortar – fired limestone and water – and add sand from the Pozzuoli caldera, a volcano near Naples. That changes the chemical reaction and makes a binder that will set in water. Turns out that sand from just about any volcanic rock will work, too. So will ground-up terracotta pots, but this trick was lost after Rome fell, and a modern equivalent wasn’t perfected until the 1820s.

The 19th century saw many experiments with concrete. Among the most important were those designed to overcome concrete’s big defect: the fact that while it resists compression very well, it isn’t all that strong when flexed or bent. Steel reinforcement was the answer, and since that development concrete has been used to build everything from hydroelectric dams and grain silos to luxury high-rises and owner-built concrete block shanties.

Between 1901 and 2000, the U.S. consumed 4.5 gigatons of cement to make concrete. That’s a lot, but the amount is dwarfed by the 6.6 gigatons used by China between 2011 and 2013 as it set out to move approximately 250 million people into urban areas, rehouse much of the existing urban population and provide roads, electricity and water for all of them. The undertaking was immense, even bigger than the rehousing of Europe, North America and the Soviet Union after the Second World War.

The amount of CO2 pumped into the atmosphere to produce that cement was also immense: making each tonne of cement releases 927 kilograms of CO2. About half comes from the fuel burned to cook limestone, while the other half is owing to the chemical reaction that breaks down limestone into cement’s active ingredients. The result: Cement production currently accounts for between 5 per cent and 8 per cent of worldwide CO2 emissions.

Worse, concrete creates collateral damage. More buildings mean more roads, more heat islands, more demand for air conditioning, more fossil fuels burnt, more certainty that our climate will change faster and more severely. Over time, the damage will be much, much worse than what the pandemic is wreaking.

So what to do?

We in North America have no control over what happens in China or other countries, but we can set our own house in order. A strong carbon tax policy would stir cement manufacturers to work harder to cut down CO2 emissions. “Greener” concrete is theoretically possible. A number of companies are fiddling with the chemistry to make cement-like binders at lower temperatures or ones that absorb CO2 under certain conditions. Then there is the increasing use of additives that cut down on the amount of cement necessary for strong concrete. There are even experiments under way to grow concrete with bacteria. Using more wood in construction is also a possibility. Wood, theoretically at least, is carbon neutral, and new techniques can overcome its inherent weaknesses when building tall.

But real change will only come when we, collectively, rethink the sort of cities we want to live in. Despite those beautiful COVID-19 skies, the danger is that we will head toward more urban sprawl, more concrete and more roads because less dense areas may seem healthier in a time of plague.

No, the right thing to do is to favour development along the lines of what I saw on another walk another morning, this time in a neighbourhood of triplexes and street trees. Four guys were adjusting the flow of concrete from a ready-mix truck into forms that were reinforcing the foundation of a century-old three-storey triplex in a neighbourhood that is an example of the much-lauded “15-minute city.” The low-rise community full of tiny gardens and small parks, corner stores and local businesses has a density of 6,238 inhabitants per square kilometre, which is much higher than one of China’s flagship new developments, Pudong, across from Shanghai’s centre, a forest of high-rises with a density of 4,167 inhabitants per square kilometre. Needless to say, there’s a lot more concrete in the latter.

We’re going to need concrete in the future to keep decent housing in repair, to replace infrastructure and to build new housing that is both “green” and inviting. Just how we manage that will have immense repercussions for the future of the planet, including whether we have clear skies from now on.

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