The Surprising Science of Slag

This blog is written by John Macdonald, a lecturer in geology and environmental geoscience at the University of Glasgow. Outside geoscience, he plays the bagpipes and raquet sports (not at the same time) and enjoys hillwalking. You can find him on Twitter @DrJohnMacUoG and Instagram @johnmacdonald181

The world is facing a climate crisis. Global temperatures are increasing at alarming rates and extreme events such as wildfires, storms and flooding are expected to become more common. Climate change will affect huge numbers of people around the world and will cost governments billions.

Climate change is happening because human activity has increased the amount of greenhouse gases in the atmosphere. These gases trap heat at the Earth’s surface and cause global warming. Carbon dioxide (CO₂) is an important greenhouse gas, because it remains in the atmosphere for a long time. Burning fossils fuels (coal, oil and gas) for electricity, transport and heating is a big source of CO₂ in the Earth’s atmosphere.

*Graph showing how the amount of CO₂ in the atmosphere has increased through time since the industrial revolution (source: NASA).*

How can we reduce the amount of CO2 in the atmosphere?

The biggest thing we can do to fight increasing CO₂ levels is reduce our use of fossil fuels. This means fewer cars on the road, less flights, more renewable energy and less plastic.

At the same time, we can take steps to remove CO₂ that we have already put into the atmosphere. It is well known that trees and plants take in CO₂ naturally as they grow, and that by planting more trees we can help to fight climate change.

What you may not know is that some by-products of industry can also take in CO₂ from the atmosphere, through natural reactions. And that brings us to the topic of this blog – the surprising science of slag.

What is slag?

Steel is a material that is used in most building projects, from your local school or factory to skyscrapers in the world’s major cities. Steel is made in a giant oven at very high temperatures from natural raw materials like iron ore and limestone. The steel making process doesn’t use all of this material, and the bits left over are called slag. The global steel industry creates around half a BILLION tonnes of slag every year.

Slag forms chunks of black or dark grey rock that have a bubbly texture. Poor old slag isn’t the most attractive of industrial waste, and it often ends up piled in heaps and covered with soil, grass and trees to make it look nicer.

However, this isn’t the end of the story. Slag is one of these special industrial materials that can remove CO₂from the atmosphere. By simply observing pieces of slag that have been sat on heaps for years or even decades, we can see how they have captured CO₂.

Slag starts out as black or grey in colour but over time it becomes a paler grey colour. This is because of the tiny white crystals that form on its surface. These crystals are made of the mineral calcite, which has the chemical formula CaCO₃. The calcite forms when Ca (calcium) in the slag reacts with CO₂ from the atmosphere. Rather than being a gas in the atmosphere contributing to climate change, the CO₂ is now locked away in the solid calcite crystals.

Typical slag from steelmaking. This would have been a black or dark grey colour when first made. Slag turns to this pale grey colour eithin weeks or months of coming into contact with atmsopheric CO₂

Understanding slag’s benefits

Scientists who observed this chemical reaction conducted laboratory experiments to try to increase the amount of CO₂ that could be captured by slag. Crushing up the slag into fine powder, adding lots of water, and artificially increasing the amount of CO₂ in the air all helped to increase the amount of CO₂ the slag could take in. The research showed that the natural process can be artificially enhanced, but it comes at a cost- a lot of energy is needed to crush up the slag and, if fossil fuels were used as the energy source, this would create CO₂ and minimise the benefit. Water is also an increasingly valuable commodity, which would be better used for drinking supplies.

Rather than artificially altering the reaction, there is increasing interest in allowing slag to passively capture CO₂ without any human intervention. UK researchers at the are now investigating how quickly slag can capture CO₂on its own, and how much it can capture. In order to work this out, we use a type of CT scanner – a bit like the ones used in hospitals – to look inside the slag and see where the calcite is forming and how much there is.

*A computer-generated representation of a piece of slag from XCT scanning. The pink colour is the slag and the green is the calcite (containing the locked up CO₂)*

CT scanning does not affect the slag, so we can keep on scanning it over time to work out how quickly the CO₂is being captured. The aim is to work out the extent to which slag can contribute to our overall goal of limiting the amount of CO₂ in the atmosphere.

Slag used to be viewed as an ugly problem that needed to be covered over with soil and plants. However, ongoing research by geoscientists is helping to understand how slag could help to tackle climate change. We are likely to always need steel, so slag will always be produced – we might as well put it to good use!