Can concrete's climate impact be cut by recycling building materials?

The construction sector in the UAE is at the forefront of a drive to use recycling building materials to reduce the damage caused to the environment by traditional methods.

Constructing and running buildings has a major climate impact, accounting for 37 per cent of emissions that cause global warming, according to the UN Environment Programme. Typically, around a quarter of a building’s carbon footprint is accounted for by its construction, with continuing emissions, for heating, cooling and powering appliances, responsible for the remainder.

The UAE, which has a very active construction sector, is involved in efforts to decarbonise buildings. An art and architectural installation called Stoot recently unveiled in the UAE was made with recycled materials from a Swiss company, Oxara, cofounded by Dr Gnanli Landrou, who grew up amid traditional clay-brick and cement houses in Togo in West Africa.

A Dubai design practice, Mula Design Studio, founded by Abdalla AlMulla, an Emirati architect, collaborated in the installation’s creation. Oxara’s patented technology mostly uses construction waste, such as old concrete, brick and excavated material, cutting carbon emissions and the amount of construction waste that ends up in landfill.

"By building Stoot, we proved that the technology and the product is ready to be used, and the performance is positively aligning with what we’re expecting; we’re achieving higher compressive strength. Validation tests are continuing with a local company,” Dr Landrou said.

The material is not suitable for load-bearing parts of high-rise towers, but could be used in non-structural elements in skyscrapers, and for lower-rise buildings.

"This means we’re talking about villas, we’re talking about three or five-storey residential houses, we’re talking about Arabian houses, private villas – that’s where we see the opportunity," Dr Landrou said.

Oxara will enter the Swiss market next year and Dr Landrou said that the technology could be deployed in the UAE within the next six months to a year.

The energy efficiency of buildings is forecast to improve, but a 2023 UNEP report, Building Materials and the Climate: Constructing a New Future, said that efforts to cut emissions from producing and deploying the key things that buildings are made from – cement, bricks, steel and aluminium – have "lagged".

As a result, the embodied carbon of a building will account for a growing proportion of its lifetime carbon footprint, rising from about 25 per cent now to 50 per cent "in the next few decades". There is a need, the report stated, to reduce the "extraction and production of raw materials" by, among other things, reusing building materials.

Making bricks accounts for about 2.7 per cent of global greenhouse gas emissions, mostly from the CO2-generating kilns used to fire them. Cement production is responsible for around eight per cent of emissions because of the kilns that create clinker, a major part of cement, which with water, sand and gravel, makes concrete.

Producing one tonne of cement generates between 561 and 622kg of CO2, according to a study published last year in Renewable and Sustainable Energy Reviews. About 60 per cent of this comes from the decomposition of limestone (which has the chemical formula CaCO3) to calcium oxide (CaO), the study indicated, with a further 30 per cent caused by the burning of fossil fuels to heat the ingredients, and the remaining 10 per cent from powering other equipment.

Sustainable future

Oxara claims that its products cut the carbon emissions associated with standard cement production by up to 90 per cent. What Dr Landrou described as the "massive urbanisation trend" in the global south and emerging economies meant that there was an opportunity in these regions to "define what is the next cement". He indicated that there is enough construction waste in the UAE for the production of the company’s raw materials.

"If we look at Dubai, there’s about 5,000 tonnes of construction and demolition waste that goes to landfill every day. Every year you’re talking about one million tonnes of demolition waste. There’s a huge amount of this resource," he said.

About 70 per cent of solid waste produced in the UAE comes from construction and demolition.

Dr Aseel Takshe, who chairs the Department of Public Health at the Canadian University of Dubai, said that "several promising materials" were being considered for eco-friendly buildings, such as "biomaterials including bamboo, recycled steel and plastics, engineered wood products and precast concrete".

She also said that firms could consider modular and prefabricated construction to reduce waste.

There has been, Dr Takshe said, "a significant push" in the UAE" to strengthen regulations to promote environmentally friendly construction and building design.

"These initiatives demonstrate that the UAE is … encourag[ing] more environmentally construction methods and building design, aligning with global sustainability goals and the country’s vision for a greener future," she said.

The wider picture

Professor Kevin Paine, of the Department of Architecture and Civil Engineering at the University of Bath in the UK, said that within the concrete industry, "there’s a push to clean up their act".

"Everyone I speak to in the concrete industry is very, very keen to be more sustainable, to be as low carbon [as possible]," he said.

He added, however, that he "hadn’t yet seen an alternative that’s good enough at scale" to replace concrete and that reducing its carbon footprint was not easy. If it was, he said that "it would have been done by now".

"I’ve seen technologies that do work in various places that have a good local source of material that will work in a certain way," he said.

"I don’t think there’s a single technology that’s going to replace concrete or change the way concrete is made. There are going to have to be different technologies around the materials that are available."

One issue with the use of recycled building materials to cut the embodied carbon of buildings is limitations in supply where it is needed, he suggested: in fast-developing parts of the world, there may not be enough demolition waste, and shipping such waste in would negate carbon benefits from using it.

At Queen’s University Belfast in Northern Ireland researchers are looking at using waste materials with high silicon dioxide (SiO2) content to replace standard cement, with significant reductions in carbon emissions. Prof Marios Soutsos and colleagues have developed a method to produce sodium silicate powder from waste glass.

"The work we’ve done is with a local waste-collection company," Prof Soutsos said. "The company was keen to convert [the waste glass] into sodium silicate because of the increase in the price of their waste material."

Another way to reduce the carbon footprint of concrete is to use in it a quantity of wollastonite, a calcium silicate mineral that also contains small amounts of aluminium, iron and magnesium.

A study on wollastonite powder published in October in Innovative Infrastructure Solutions found that concrete containing it "exhibited better performance", heralding "a new era of sustainable material for advancing civil engineering infrastructure".

Researchers have also looked at using rice ash husk to replace a portion of the cement in concrete, although the impact on carbon emissions is less dramatic than using recycled materials.