By Elsabe Kearsley
Every year the world uses 4 billion tonnes of cement to make the concrete that goes into buildings and other infrastructure. It leaves a huge carbon footprint, and comes with other costs too.
Civil engineering professor Elsabe Kearsley, who researches cement and concrete, tells us more about this basic ingredient of modern life and explains some of the work that’s being done to find alternatives.
What is cement? How is it made, and where?
Portland cement was first patented in 1824 in Leeds (UK) by a bricklayer. He discovered that it was possible to glue bricks together using a mixture of clay and limestone that was heated and ground to a fine powder and then mixed with water.
In chemical terms, limestone mainly consists of calcium carbonate (CaCO₃). When it is heated, carbon dioxide (CO₂) is released into the atmosphere, leaving calcium oxide (CaO) behind. This can chemically react with water to solidify into a strong binder.
Modern Portland cement consists of a combination of limestone and clay. The quality of the cement manufactured depends on factors such as the consistency of the limestone mined, the maximum temperature reached in the kiln that is used to heat the raw materials and how finely the material is ground. Finer cement and cement manufactured at higher temperatures are usually stronger.
Portland cement is still used as the main binder to glue together sand and stone to make concrete. Concrete is the most widely used modern building material in the world. Multi-storey buildings, dams, bridges, water purification works, stormwater pipes and railway sleepers mainly consist of concrete reinforced with steel.
In many African countries there are no suitable sources of limestone to manufacture Portland cement. The countries that produce the most cement are China, India and Vietnam.
Early research results suggest that the lateritic soils that are abundant in large parts of Africa can be used as a binder in concrete by heating the materials to relatively low temperatures to form calcined clays. These materials can be ground with limestone or with limestone and Portland cement to form limestone-calcined clay or limestone-calcined cements. They would be strong enough to build affordable, durable infrastructure in developing areas.
What are the drawbacks of cement?
Up to a tonne of CO₂ can be emitted for every tonne of Portland cement produced. The environmental footprint of cement production is further increased by the energy required to heat the raw materials to temperatures as high as 1,500°C to form the clinker that is then ground into the fine powder sold as cement.
It is estimated that nearly 2 tonnes of concrete (page 131) are cast annually for every person living on the planet. Although cement typically makes up only about 10% of the volume of concrete, the world’s annual global cement consumption is in excess of 4 billion tonnes.
The cement industry is responsible for emitting about 2.5 billion tonnes of COâ‚‚ per year, which is about 7% of global COâ‚‚ emissions.
There can thus be no doubt that it is necessary to decarbonise the cement industry. Large international cement producers undertook to limit COâ‚‚ emissions when the Kyoto Protocol was signed in 1997. Since 1997, these companies have made significant investments in reducing their environmental footprints.
It’s also costly to produce good quality cement that can be used to consistently produce strong concrete infrastructure that will last for decades without requiring significant maintenance.
The large South African cement manufacturers follow European standards of cement manufacturing. In South Africa, building regulations require that the National Regulator of Compulsory Specifications keep a list of manufacturers and importers authorised to sell different cement blends. They have to adhere to cement strength and composition requirements, quality control, environmental, mining and labour laws as well as economic empowerment rules. All of these have product cost consequences.
What other options are there?
There are a few ways to reduce cement’s carbon footprint:
- using alternative energy sources to generate heat for making cement – including waste materials such as rubber tyres
- milling the materials finer and pre-heating the limestone
- placing filters in chimneys to limit emissions
- blending the Portland cement with materials that are by-products from other manufacturing industries to reduce the volume of limestone.
Blended cements contain by-products such as ground blast furnace slag from the steel manufacturing industry and fine ash collected in the chimneys of coal-fired power stations. With reliable quality control, blended cements can improve the strength and durability of concrete infrastructure (chapters 15, 21 and 24).
Current international research is focused on alternative binder types.
Most cement sold in South Africa now is blended, containing varying percentages of fly ash and slag.
What’s the state of the cement industry in Africa?
The South African cement suppliers have the capacity to manufacture 22 million tonnes of cement annually, but due to the lack of local infrastructure spend since 2019, the local demand has dropped. Only about 13 million tonnes was produced in 2022.
South Africa is the only southern African country with a well-established local cement industry. The continued low demand for cement, combined with the large volumes of illegally imported or labelled cements sold below the cost price of locally manufactured cement, is placing the industry at risk.
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Lacking suitable sources of limestone, many African countries have to pay high prices for imported cement that is transported over large distances. A lack of local building regulations and testing facilities has made it possible for unscrupulous importers to bring substandard products into these markets.
In the recent past China, India and Vietnam have been able to produce and export relatively cheap cement, as they do not have to comply to the stringent manufacturing requirements, labour, mining and environmental protection laws that most developed countries enforce. China has only recently started implementing environmental protection laws.
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