sustainability

A concrete problem

The material’s almost as old as civilization, and it’s a major carbon culprit

By ANNEMARIE SCHUETZ
Posted 7/14/21

IT’S EVERYWHERE — Concrete is that stone-like substance out of which buildings, bridges and sidewalks are made.

It’s the most widely used building material. “It is used …

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sustainability

A concrete problem

The material’s almost as old as civilization, and it’s a major carbon culprit

Posted

IT’S EVERYWHERE — Concrete is that stone-like substance out of which buildings, bridges and sidewalks are made.

It’s the most widely used building material. “It is used in just about all types of infrastructure,” says concrete inspection company Subsurface Imaging in a 2018 blog post. “In fact, estimates show that for each person there is on earth, there are three tons of concrete. This adds up to twice as much as the total of all other building materials, including wood, plastic, steel and aluminum.”

So, what’s in it?

Cement. Aggregate. Water. Some air.

To produce Portland cement, the most common form, a mixture of crushed limestone and aluminosilicate clay is cooked in a kiln. The temps get up to 1,500 degrees. The high heat turns the mixture into clinker nodules that are then ground into a powder. A massive amount of carbon dioxide (CO2) is produced every time the kiln does its job.

The clinker-making process causes about sixty percent of the emissions in cement manufacture. The remaining forty percent are from the energy used to make this happen, and from the remaining concrete manufacturing process.

Enough cement goes into concrete to bind it all together. The water helps the cement do the job. The aggregate (sand and stone) helps with strength and stability, and depending on what you choose, you can minimize wear. And the air creates little chambers for water to expand into when the concrete freezes.

The advantages

Concrete’s usually durable, it doesn’t rust or rot, it can be molded into any shape—that’s called “workable”— and maintenance costs are low. Rediscover Concrete, a Canadian company whose website says that they focus on sustainability in the cement and concrete industry, argues that when combined with carbon-lowering improvements, it can be the “lowest carbon building material over the life cycle of a structure or pavement.”

Depending on where you live, it’s often locally produced (the ingredients aren’t necessarily hard to find), so it doesn’t have to travel far, which would generate more emissions.

It’s not surprising that such a useful material is used so often. But why does it have people so concerned?

The disadvantages

The concrete industry is responsible for five to eight percent of total global carbon emissions. Here’s another way of saying that: It produces nine-tenths of a pound of CO2 per pound of cement. A 3,900-pound cubic yard of concrete emits 400 pounds of CO2. (The numbers look strange because cement is just a fraction of the concrete.)

The process of making cement consumes up to half of all energy use in emerging nations. And three trillion liters of freshwater are used for concrete production every year.

Water helps bind the aggregate together, the Department of Materials Science and Engineering at the University of Illinois Urbana-Champaign says. The cement-to-water ratio is critically important for making the concrete “strong and workable.” Too wet and it won’t hold together; too dry and you can’t make it into stuff.

The water must be pure to avoid reactions that might weaken the concrete, but that’s a lot of water that people could be drinking or using in the garden. Mining the raw materials and transporting them add to the carbon count. And then there’s the sand situation.

In 2012, 26 billion metric tons of sand went into manufacturing concrete. And demand for sand is growing as more countries urbanize. “With sand extraction regulated differently around the world, important regions for biodiversity and ecosystems are made more vulnerable,” says Sand and Sustainability, a UN report from 2019. Rivers are being dredged for their sand. The illegal sand trade and those who run it, says the IMF, have reportedly killed hundreds of people in India in recent years.

The possibilities

Few markets are large enough for changes to have a massive impact, but concrete, Solidia Technologies says, is one of them. “Market size of the global concrete industry is $1 trillion, while the cement industry itself represents a $300 billion market.”

Given how rapidly some countries are building—and how the United States, for one, needs to improve its infrastructure—sustainably producing concrete is key.

As they worked on ways to reduce emissions in cement manufacture, Project Drawdown found that if you change the ingredients in the cement, that makes a difference. Look at the clinker stage in cement manufacture.

Project Drawdown focuses on the reduction of greenhouse gases (aka “drawdown”) and offers resources to make it happen.

They recommend switching out some of the clinker for materials like “volcanic ash, certain clays, finely ground limestone, ground bottle glass and industrial waste products [like] blast furnace slag... and fly ash.”

Clinker substitution would avoid up to 440 million tons of carbon dioxide emissions annually.

The industry has argued that materials aren’t tested, but Matthew Adams, a civil engineering professor from the New Jersey Institute of Technology, counters that the materials have been used regularly and for a long time.

In a 2020 video from the OpenAir Collective, Adams discussed different materials and the impact their use—rather than the current ones—would have on concrete’s carbon footprint.

All the new ingredients come with pluses and minuses. Calcium aluminate cement and calcium sulfoaluminate cement are alternatives in use now. Magnesium phosphate or geopolymer cement are up-and-coming; geopolymers, in particular, “are basically made from waste material and they have very low embodied carbon,” Adams said. But availability is limited.

So that brings us to fly ash, slag and so on. They can serve as significant alternatives, and their downsides can be offset with other changes, Adams said. Ground glass can be more durable but much depends on the type of glass used.

Ultimately, mixture design can reduce the amount of cement needed, he said.

Another problem, which Project Drawdown addresses, is that as we move away from fossil fuels, fly ash will be less available. They think clays, limestone and similar materials could replace the fossil fuel production byproducts.

And a further issue: current specs. In a 2018 video, Tyler Ley outlines how the industry controls cement ingredients by setting the requirements itself; this is the “prescriptive approach.” It focuses on how the materials and cement are to be formulated and constructed, he says.

An alternative would be a performance-based approach, which would allow for more innovation. Maybe that would give sustainable concrete more of a chance.

“Materials are critical for creating sustainable and responsible concrete designs,” engineer Tarun Naik said in a 2008 paper. But in the end, “To manage natural resources, humanity must obey the rules of nature: Use only what you need and never use a resource faster than nature can replenish it.”

If you want a quick, fun summary of the problems with concrete, you can’t go wrong with a video by Wisconsin Girl Scout troops 1477 and 1952. The Girl Scouts explain the serious sustainability issues with concrete as it’s usually made and offer some forward-thinking possibilities. There are even Playmobil figures discussing concrete!

Watch the video at www.carbonleadershipforum.org/concrete-climate-ideas.

To view the OpenAir Collective discussion by Matthew Adams, an excellent and approachable discussion of sustainable concrete, visit www.bit.ly/openair28.

View Tyler Ley’s video at www.bit.ly/tyley28.

Concrete is used a lot: 

www.concreteinspectors.com

For lots of info on cement and concrete, plus formulae and demonstrations: 

www.matse1.matse.illinois.edu

On sustainable concrete manufacture and general concrete information:

www.drawdown.org

www.cement.org

www.solidiatech.com

Tarun Naik paper:

courses.washington.edu

Sand and sustainability

www.unep.org

On sand theft

www.imf.org

www.nationalgeographic.com

On the importance of climate change on infrastructure, and other information in light of the Champlain Towers South collapse:

www.scientificamerican.com

concrete, materials,

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