From recycled bricks to buildings made of cob: How construction sector can reach net zero

Cement and brick production is carbon intensive, but new building methods could help to reduce emissions

Green methods of construction, such as bricks made from recycled materials, could hold the key to the sector reaching net zero. Photo: Kenoteq

Concrete production is among the largest contributors to climate change, accounting for about eight per cent of global carbon dioxide emissions.

That is why it is a priority to find less environmentally harmful building materials if the world is to achieve net zero.

A study in the Applied Clay Science journal shows brick manufacturing accounts for 2.7 per cent of global carbon emissions, with the kilns where bricks are fired having a significant footprint.

Producing concrete releases large amounts of carbon because of the process that generates clinker, a key ingredient of cement that, alongside water and the likes of sand and gravel, is a main component of concrete.

Carbon dioxide is given off during the production of clinker.

The International Energy Agency states that, along with other measures, using alternatives to clinker will play a significant role in helping the cement sector to achieve net zero by 2050.

Researchers at the American University of Ras Al Khaimah are analysing the properties of concrete when 5 per cent, 10 per cent or 15 per cent of the cement is replaced with rice ash husk.

Many other research groups around the world have investigated the use of this silica-rich material, which offers strength and stability.

Another approach has been taken in Germany by the Technical University of Dresden and an architectural company called Henn, who said last year that they constructed the world’s first building using "carbon concrete", in which the concrete was reinforced carbon fibre, instead of steel.

The carbon fibre is only a quarter of the weight of steel but is six times stronger, Henn said.

Other ways to strengthen concrete are being examined, with the Rensselaer Polytechnic Institute in the US working to develop reinforcing bars made from natural fibres such as hemp and flax, bound up with plastic.

The production of these bars is more environmentally friendly than that of steel reinforcing bars – and they act as a carbon store.

Biochar, a carbon-rich charcoal-like substance, is also involved in carbon storage and is used as a component of the external cladding of buildings, something that has been tested in Germany.

In the UK, Cambridge Carbon Capture has developed a method to produce lightweight bricks made by bubbling air through a slurry of magnesium oxide.

Carbon dioxide from the air combines with the magnesium oxide to create magnesium carbonate, which can be dried to produce the bricks. This method represents a way of actually taking carbon dioxide out of the atmosphere.

Another brick in the wall

Kenoteq, a company in Scotland that was spun out of Heriot-Watt University, which has a campus in Dubai, aims to cut carbon emissions by producing a brick using recycled materials including plasterboard, brick, mortar, rubble or stone.

Lucy Black, head of business development at the company, said the embodied carbon of its K-Briqs is less than five per cent that of standard bricks.

"The K-Briq is comparable in terms of technical performance to a traditional clay and concrete brick, but our unique selling point is the fact it’s made out of recycled material and has a very low carbon footprint," she said.

"Most building materials are made from raw materials and subject to extraction and mining. That’s a hugely energy and carbon-intensive process. Additionally, our raw materials are finite and the Earth is beginning to run out."

The process uses materials that would often have ended up in landfill, with Ms Black saying that construction waste accounts for about 40 per cent of all the world’s waste.

The bricks, made in a range of colours, are currently available for interior use. The company is in the final stages of securing certification for external applications in the UK, Europe and the US.

Local solutions

Local production processes could be set up under licence so that local waste could be used, rather than materials that have to be shipped long distances. This would also provide a solution to the local waste problem and provide jobs, the company said.

"We’ve been out at Cop28 and had discussions in the UAE and there’s a lot of interest there," Ms Black said.

Kenoteq supplied bricks that Dubai Holding used for its exhibition space at the climate change summit last year and Ms Black said the display had now been transferred to the UAE company’s headquarters.

In another recycling project, researchers at the University of Cambridge have developed a method in which used cement is employed in place of lime flux in electric arc furnaces, which recycle steel.

At the end of the process, the used cement – said to have similar characteristics to new cement – can be recycled into concrete.

"The proposed process may be economically competitive, and if powered by emissions-free electricity, can lead to zero-emissions cement while also reducing the emissions of steel recycling by reducing lime flux requirements," the researchers wrote this year in the Nature journal.

The final straw

As well as new high-tech methods for building construction, efforts are being made to revive older technology. At the University of Plymouth in the UK, Prof Steve Goodhew leads the CobBauge project, which has developed a new way to use cob, a traditional building material made from soil, straw, water and lime.

The aim, he said, is to "take the material into the 21st century" by giving it the insulation properties required by modern building regulations.

"There’s an awful lot of anecdotal evidence from people who live in these type of buildings that the material is warm in winter and cool in summer," Prof Goodhew said.

The approach, tested on two demonstration buildings in Normandy and at the university’s campus, involves having a layer of cob (with about 2.5 per cent straw) to provide structural support, and a light earth layer (a mix of earth and the inside part of the hemp stalk) to moderate heat loss or gain.

By reducing the amount of heat that passes through the wall, the thermal light earth layer helps the cob meet building regulations that, in turn, means contractors can use the material in construction projects.

The embodied carbon of buildings made from cob is far less than that of a standard concrete or brick building, researchers said.

There is a long tradition of using cob in the Middle East. In Yemen, some Unesco World Heritage List buildings that are up to seven storeys tall are made from the material, Prof Goodhew said. Yemeni cob buildings are made from relatively thin sun-dried blocks put together with earth mortar and are often finished with a lime rendering, he explained.

"The only issue is that the walls have to be thicker and in Yemen that’s the case," he said. "As the buildings get taller, the walls taper towards the upper storey so the structural bearing capacity can be maintained."

Cob tends to be unsuitable for very tall buildings, due to the thickness of the walls necessary to support the increased loads, but it is an appealing option for low-rise construction in areas with clay-rich soils.

If properly constructed and maintained, cob buildings can last as long as standard masonry construction, as shown by the continued presence in south-west England of 16th-century cob-based dwellings.