One cubic metre of concrete has a carbon footprint of 350kg of CO2, making this staple material in the construction industry one of the biggest contributors to greenhouse gases in the world.
But for the construction of South Africa’s 80km Gautrain mass rapid transit railway system, the cement was mixed with recycled materials to reduce the carbon footprint by more than 30%.
The Gauteng rail network links Johannesburg, Pretoria, Ekhuruleni and O. R. Tambo International Airport. It was built to relieve the traffic congestion in the Johannesburg–Pretoria traffic corridor and offer commuters a viable alternative to road transport, as Johannesburg has limited public transport infrastructure.
For the development of Gautrain, the contractors asked materials technology scientist Cyril Attwell (Rice University) to design the concrete and oversee the production of it for the project.
From an original estimate of 340,000 tonnes of cement, he was able to reduce the required amount to 210,000 tonnes. The outcome was that approximately 130-million kilogrammes of carbon dioxide were saved. His approach was to replace more than 30% of the cement with flyash from mine dumps.
“The original estimation of cement usage would have needed 4.5km x 3km of rainforest, alive for 40 years, to counteract the carbon dioxide reduced in making the cement for the Gautrain. Due to the technology we applied, we reduced that by 2.5km x 3km,” explained Attwell.
Using the molecular integration of the human skeleton as inspiration, they also managed to achieve 74MPa for the material.
“Even more unique was the flexural strength increase,” said Attwell. “The standard relationship between compressive strength and flexural strength is you have 10% flexural strength of compressive, but because of the crystal formations we had, we were able to increase that to 22%.”
Attwell has also worked on the development of the Portside Tower, which is not only the tallest building in Cape Town, but also largest autoclaved aerated concrete structure in the world currently. In producing the concrete for the five-star Green Star certified building, 65% of the material constituted waste from the steel industry, reducing carbon output by 58%.
For the construction of the 146,000m2 of paving at the City Deep Container Terminal dry port in Gauteng, 100% replacement waste material was used. Along with a 92.7% reduction in carbon emissions and increased durability – at its peak the terminal handles 300 trucks a day – the project saw a 30% reduction in material costs, and a 25% reduction in labour costs.
“This site, to date, is the only site where you broke up 77,000m3 of concrete and used 100% of it – no other site has done this in South Africa yet… We either utilised it on site, or sold it to our other sites and used it there,” explained Attwell.