Words by Tony Whitehead
Carbon-neutral alternative uses no Portland cement at all
A carbon neutral alternative to cement
The manufacture of Portland cement is a carbon-intensive process, so scientists and suppliers worldwide have looked for ways of reducing its role in the production of concrete. It is now commonplace to find concretes in which around 30% of the cement is substituted with industrial by-products such as ground granulated blast-furnace slag (GGBS) and pulverized fuel ash (PFA), also known as fly ash.
What’s different about geopolymer concrete is that it uses no Portland cement at all, and therefore the “cement” or binding element of the concrete is almost carbon-neutral. Rather than hydrating Portland cement, geopolymer concrete is made by mixing the sand and aggregate with fly ash or slag, and then activating it with an alkali such as sodium hydroxide.
It has been around since the late 1970s, but only recently became commercially available, says Robert Kilgour, principal engineer in materials technology in WSP in Perth. “It is a similar price to standard concrete and its structural properties and the mechanics of strength gain are well understood. Importantly, there is now also a long-term history of using it for a range of applications including bridge beams, pavements and building structures. It is very durable and particularly resistant to chloride and sulfate salts, which makes it ideal for marine or coastal applications.”
The largest project to feature geopolymer concrete
One of the largest projects ever to feature geopolymer concrete is Brisbane West Wellcamp airport in Queensland, built in 2014. A 435mm layer of geopolymer, supplied by Wagners under the brand name Earth Friendly Concrete (EFC), was used to construct much of the apron and pavements. Foundations, wall panels in the terminal building, and a bridge also used geopolymer. In all, 70,000 tonnes were supplied to the project.
Some 330m3 of EFC was also used in the production of 33 large floor beams that form three suspended floor levels in the Global Change Institute building at the University of Queensland, completed in 2013.
Geopolymer concrete is set to become more widely used, but its market penetration will probably remain limited. Kilgour explains: “In Australia, there are two players in the market and they can only produce so much. Longer term, there is also likely to be a problem with the supply of GGBS and PFA.“
Ironically the decarbonization of power production, and the export of steel manufacturing to China, will limit the local availability of PFA and GGBS, waste materials from steel and power production respectively, and essential for this type of low-carbon concrete. However, in those areas of the world where steel production and fossil-fuel power are still going strong, geopolymer concrete could be an increasingly serious option.