concrete sustainability

The Cape Times reported on 3 January 2007 that 10.7 million tons of cement were consumed in South Africa in 2004 – that’s 250 kg per capita – and that annual consumption is expected to increase to 17 million tons by 2010. (According to the NPCA there is a higher per capita rate of consumption in the USA at 300+ and Germany at 400+.)

Cement uses a lot of energy in its manufacture, so just how bad is it for the environment? A 2004 research paper, How Sustainable is Concrete? presents a nice summary of the answer.

Portland cement is usually manufactured by heating limestone and shale in a kiln to around 1500 degrees Celsius, then grinding the resulting clinker with gypsum to form a fine powder. This process gives the cement its high embodied energy, and the reaction also produces CO2. The research found the energy consumed in production of Portland cement to be 4.88 MJ/kg. (That’s about 1.2 GJ per capita in South Africa in 2004.) Producing concrete from Portland cement consumes 2.07 GJ/m3 or 0.89 MJ/kg. Replacing 10% of the Portland cement with fly ash reduces this to 1.94 GJ/m3.

Interestingly, a concrete beam of a specific strength embodies less energy than a steel I-beam of the same strength. For the beam designed in this research, concrete manufacture required 109 MJ and steel manufacture required 237 MJ. A more complex calculation is required to consider the broader sustainability issues related to solid waste, air and water pollution and resource consumption. The ATHENA software package was used to demonstrate that concrete has a lower environmental impact than steel when compared in structures designed for the same engineering function.