Materials being readied for a final pour of concrete on the Auckland City Rail Link. File photo / Supplied
Materials being readied for a final pour of concrete on the Auckland City Rail Link. File photo / Supplied
COMMENT
In her Weekend Herald column, Michelle Dickinson (Nanogirl) urges the building industry to think up ways to reduce carbon emissions, in particular those associated with concrete, while delivering the increased infrastructure that society wants.
The New Zealand cement and concrete industry is very aware of this challengeand is, to the best of our abilities, committed to finding solutions.
Michelle's article is thoughtful and considered; stating "concrete has created the foundations for modern life". The benefits of concrete are indeed well known. It is durable, offers thermal mass, fire and flood resilience and acoustic separation, as well as being infinitely recyclable and easily repurposed. Without concrete we couldn't enjoy the convenience and comfort of our built environment.
Michelle also points out the manufacture of cement, the essential component of concrete, involves significant CO2 emissions. This is a point well made. Manufacturing cement, which accounts for about 7-15 per cent of the concrete mix, is an energy intensive process. Concrete production, however, is much less so.
During the manufacture of cement, around two thirds of CO2 emissions are from the "decarbonisation" of limestone. These are essentially the gases released from heating limestone, one of cement's main raw ingredients. Emissions reduction in this area remains challenging because the manufacturing process is inextricably tied to a chemical reaction.
The other third of direct CO2 emissions are from burning fossil fuels to fire the kiln. Typically, this is coal. In New Zealand, wood waste is substituting a significant proportion of fossil fuel. This "bio-fuel" maximises the recovery of energy from waste, reduces methane emissions, and saves landfill space by using a product that would otherwise not have a purpose. Another fossil fuel substitution programme soon to get underway will be to use waste vehicle tyres to fuel the kiln.
Global industry initiatives include finding substitutes for cement itself. Examples are waste from steel manufacture (ground granulated blast furnace slag) and waste from coal combustion (fly ash). These materials can result in environmental benefits, improved concrete performance, and long-term cost advantages.
Woodwaste could be burned for cement production instead of coal. Photo / Supplied
In addition, New Zealand is fortunate to have access to an abundant and stable supply of pumice, a type of natural pozzolan, from the North Island Volcanic Plateau. This pozzolan can potentially substitute up to 25 per cent of cement in concrete production. Industry is currently completing trials and will be working to update building standards to enable the use of this natural pozzolan as a cement replacement. This will be a significant step forward in helping to reduce the CO2 emissions associated with cement, and in turn concrete.
As Michelle points out, concrete has major benefits. One often not understood, is concrete can absorb CO2 from the atmosphere during its service life, and secondary life following recycling. This carbonation process sequesters CO2 to offset emissions from cement manufacture.
Rob Gaimster. Photo / Supplied
In New Zealand, the cement and concrete industry is also taking a broader approach to the carbon issue with a host of enhanced environmental practices including waste minimisation, recycling and the use of synthetic vehicle fuel. Globally, the industry is also exploring new technologies in a concerted effort to improve overall production and performance.
We are committed to reducing CO2 emissions and doing our part in contributing to the goal of a society that is carbon neutral.
Thank you to Michelle for her informative article.
• Rob Gaimster is chief executive of Concrete New Zealand
