TALK LIKE A MATERIALS SCIENTIST AND PROCESS ENGINEER

By-product — unnecessary product produced during a process

Carbon capture and storage (CCS) — the capture and storage of CO₂ emissions to prevent the gas entering the atmosphere

Cement — a powdery substance made from lime and clay, that can be used to make concrete and mortar

Circular economy — an economic system based on the reuse and reutilisation of materials and products

Clinker — the hard product made from processing raw materials, such as limestone and clay, at high temperatures (around 1500 °C). Making clinker is the first step in the conventional cement-making process

Concrete — a building material made of cement, water and aggregates (for example, sand and gravel) that hardens into a stone-like mass

Decarbonise — reduce or eliminate CO₂ emissions from a process

Lime — a powder created by heating limestone, and an intermediary substance in the creation of cement

Limestone — a naturally occurring rock, mostly composed of calcium carbonate, used as a core raw material for the manufacture of cement

Mortar — a material made of cement, sand and water used in construction to bind bricks or stones together

“Cement is the most manufactured commodity in the world,” says Dr Theodore (Theo) Hanein, from the University of Sheffield. “Current demand is at 4 billion tonnes every year, which is more than one kilogram of cement every day for every person on the planet!” Cement is vital for society as it is a core material of concrete and mortar, which are essential components in the buildings and infrastructure that surround us. We cannot live without it.

However, all this production has a serious impact on the planet. “Cement production requires vast amounts of natural resources, and is responsible for 8% of our carbon dioxide (CO₂) emissions, which are changing the climate,” explains Theo. In line with global climate commitments, governments are investing in ways to reduce CO₂ emissions, and the cement industry is a prime area of interest. “The UK cement industry is set to cut 4.2 megatons (Mt) of CO₂ emissions per year by 2050 – and about half of these savings could be through resource efficiency,” he says.

The chemistry of cement

It is the use of resources, especially raw materials, that forms the current focus of Theo’s research. “Unlike other manufacturing processes that emit CO₂ through their use of energy, emissions from cement production are mainly from the conversion of the raw material into clinker, a key process in cement manufacture,” he says. While some energy-intensive industries can decarbonise through switching to renewable energy inputs, it is not so simple for cement. By far, the most common type of cement produced today – and for the past 200 years – is Portland cement, which relies on the following reaction to begin production:

“Not only does this emit CO₂, but almost half of the limestone’s solid mass is lost in the process,” says Theo. “The UK cement industry uses about 12.5 Mt of limestone every year, so we’re interested to see if this natural raw material can be substituted with by-products from other sectors.” This could be by-products from steel manufacturing or mineral mining, or even the reprocessing of old cement from construction or demolition sites. Developing this process would have the double benefit of cutting down cement the emissions from cement production, as well as using the waste produced by other sectors.

The contamination challenge

The main issue with such reprocessing efforts is that any alternative materials will contain other elements that could change the underlying chemistry of the cement manufacturing process and the end product. “The effects of these contaminants across the service life of the new cement needs to be studied and understood completely to ensure that the cement can be used safely,” explains Theo. Cement that has previously been used in construction, for instance, has generally been mixed with other materials, such as sand and gravel, to give it the properties needed, but that can interfere with the repurposing process which needs to be done for the cement to be reused.

A particular project of Theo’s, called FeRICH, is investigating this challenge when utilising by-products from steel manufacture. “While these by-products contain the key elements that are essential to cement production, they also have an unusually high amount of iron,” says Theo.

“FeRICH aims to replace natural raw materials used in traditional Portland cement by upcycling iron-rich by-products, but our understanding of the reactions that take place when we do this is still incomplete.” The introduction of iron leads to the production of calcium (alumino)ferrites (compounds made from iron and one or more metallic element), such as Ca2(Al,Fe)2O5. Theo is interested in learning what this means for the qualities of the end product, especially its durability and how it reacts with water.

“The FeRICH project will establish the best cement-making conditions to achieve maximum efficiency in both production and performance,” says Theo. “Calcium (alumino)ferrites containing contaminants (also known as dopants) could, potentially, also give the cement electromagnetic properties, and we’ll be examining how this could bring benefits or disadvantages to the end product.” By establishing these traits of ferrite-rich cement, the project can help reduce the environmental impact of both the cement and steel industries in the UK. This would also increase their economic competitiveness in the global market, bringing a boost to the UK and, as other nations follow suit, the rest of the world’s cement and steel industries.

CO₂ and the circular economy

While these processes can help increase the efficiency of cement production, the production of CO₂ as a by-product remains inevitable. To address this, a lot of investment is taking place into carbon capture and storage (CCS) technologies, and Theo has developed CCS technology to separate CO₂ from limestone without it being released as a gas. “CCS may be able to capture CO₂ from the exhausts of cement kilns, but we need to find a safe way to then store it,” he says. There are various projects around the world that are finding ways to pipe compressed CO₂ underground or underwater – for example, into empty oil or gas fields (areas where these natural resources have been found and extracted for use as a fuel). However, Theo thinks that using chemical reactions to change the CO₂ into something else is likely a better solution. “Currently, the safest and most developed method of CCS is probably via mineralisation,” he explains. This involves the reaction of CO₂ with other materials to produce solid minerals. “It would be most beneficial if the generated minerals could be reused in the construction or mining industries, to avoid the storage of large amounts of material.”