It’s inescapable: our roads, buildings, dams, and homes are made of concrete. It makes sense that concrete is the material of choice: it’s dirt cheap (literally), durable, easy to source, and extremely simple to work with.

“Building something out of stone or brick, you really need skilled tradespeople to do that,” says Vince Beiser, author of The World in a Grain, a book about sand. “But concrete comes out in a liquid form, so all you need to do is build a form of what you want it to be, pour the concrete, let it harden, and boom: you’ve got a wall.” Plus, sand — a key ingredient in concrete — is found in virtually every part of the world, so it’s easy for construction companies to source.

“Concrete has enabled us to do all kinds of incredible things,” says Beiser. “If you need to build housing for tens of millions of people really fast, it’s hard to do that with wood or bricks, but you can do it with concrete.”

In its most basic form, concrete is made up of aggregates (a loosely compacted mass of fragments or particles, usually sand or gravel), water, and cement. Cement is manufactured from lime, silica, alumina, magnesia, sulfur trioxide, alkalis, iron oxide, and calcium sulfate. Cement and water are mixed together to make a paste to which the aggregates are added.

Today, twice as much concrete is used in construction as all other materials combined.

When the mixture hardens, it binds the aggregates into a hard, rock-like mass. That’s the most basic recipe, though Beiser notes that there are “probably thousands” of types of specialized concrete formulated for different purposes: faster or slower drying times, building types, flexibility, environmental temperature, etc. Modern concrete is also reinforced with steel rods to make it stronger. “Pretty much all the concrete in the world today has steel rods or mesh inside of it,” says Beiser.

A brief history of concrete

Concrete has a long history. According to Beiser, both the ancient Egyptians and Romans used the material. “[The Romans] used it all over their empire,” he says. They built roads, ports and public buildings with concrete. The most famous example of Roman concrete use is the Pantheon in Rome — it’s a 2,000-year-old building with an arched concrete roof that is still standing.

When the Roman empire fell, concrete use stopped as “people sort of forgot how to make it,” continues Beiser. It wasn’t until the late 18th century that British bricklayer Joseph Aspdin accidentally burned powdered limestone and clay to create Portland cement, which is the basis of modern cement. Concrete took its current form and became widespread after Ernest Ransome, a San Francisco engineer, saw potential in concrete and perfected the process of reinforcing the material with iron (later steel) rods.

Ransome’s innovation has stood the test of time: his formula is the basis of the concrete we use now. But, when he first figured it out, he had a hard time convincing people to build with it. Back then, people were using bricks, stone, wood, and steel, and no one wanted to take a gamble on an untested material. Even so, Ransome managed to get a few concrete buildings built around the US, a couple of them in San Francisco. When the 1906 earthquake hit, the ensuing fire destroyed much of the city — but Ransome’s concrete buildings remained standing. Since then, the world has embraced concrete. Today, twice as much concrete is used in construction as all other materials combined.

Are we building our environmental downfall?

While concrete’s strength, durability, and ease of use is impressive, perhaps the most mind-blowing thing about it is its huge environmental impact. Manufacturing cement, one of the main ingredients in concrete, is responsible for 8% of global carbon dioxide (CO2) emissions. A 2020 Princeton University report notes that the cement industry contributes 4 billion metric tonnes of CO2 annually. All of these emissions contribute to climate change and worsen its effects: hotter temperatures, extreme weather, rising sea levels, and a melting Arctic.

Then, there’s the damage that sourcing sand is doing to the planet. The UN Environment Program estimated in 2014 that more than 40 billion metric tonnes of aggregates are used every year, and their extraction is causing massive environmental damage to rivers, lakes, and shorelines. We mine so much sand — not just for concrete, the glass we use for windows and screens is also made of sand — that the world is actually facing a sand shortage. Most of the sand used to make cement is from the ocean or beaches, since the type of sand found in deserts is actually too small to use for cement. Dredging the ocean for sand damages coral reefs, tears up marine habitats, makes flooding worse, and affects aquatic life.

Architects are concerned we won’t reach climate targets as long as we continue to use concrete.

On top of the environmental harms of sourcing and producing concrete, now that we’ve paved much of the Earth with concrete, it’s worsening the impact of global warming. Urban heat islands — areas or regions where the temperature is higher than that of the surroundings — can be blamed on the stuff that cities are made of: concrete. If you’ve ever walked across an unshaded parking lot, you know that they get hot as the concrete soaks up the sun’s heat. That’s what’s happening in cities on a large scale.

“Concrete retains heat and keeps putting it back out into the atmosphere, even after the sun’s gone down,” says Beiser. “So when we’re talking about a world where we keep getting hammered by heat waves, concrete is making it worse.”

Greener grey stuff?

Luckily, there might be some greener alternatives to concrete. One Halifax-based company called CarbonCure is replacing some of the cement that goes into concrete with carbon. The company injects captured carbon dioxide into its concrete mix, which then chemically transforms into calcium carbonate and strengthens the cement. Not only does this reduce the carbon footprint of concrete, but it removes carbon dioxide from the atmosphere.

CarbonCure’s product costs the same as conventional concrete (or, even less, in some cases), according to the company’s website. In early 2021, the technology was adopted by a US concrete business that has used it in residential and commercial projects, including the Kansas City Zoo’s aquarium.

Green solutions aside, disincentivizing concrete-use should be step number one.

In Vancouver, BC, construction business Nexii has created Nexiite, a concrete alternative made with water, sand, and a proprietary mix of materials that, according to company studies, has a 30% smaller carbon footprint because it is made without Portland cement and lime. Nexii has struck deals with chains like A&W and Starbucks and was valued at $1 billion in September 2021.

Despite promising developments in greener concrete, architects are concerned we won’t reach climate targets as long as we continue to use concrete — even if it’s more carbon-efficient. Some climate activists are calling for abandoning concrete altogether and replacing it with massive timber, which is created by fusing pieces of soft wood (usually from pine, spruce, or fir) together to make larger pieces of wood. It’s durable, cost-effective, and less carbon-intensive than concrete. However, there’s concern that the mass adoption of timber as a building material might drive up demand for wood and accelerate deforestation.

In the meantime, using less concrete can help cut down on its damaging impacts. One big way to use less concrete is to discourage car use and encourage other forms of transportation, to reduce the need for roads, driveways, and parking lots. So, green solutions aside, disincentivizing concrete-use should be step number one. As for the future, it’s hard to know exactly what concrete’s role will be as we battle the climate crisis. What’s certain is that, to build a better future, we need to think deeply about how we build our world.