The replacement of concrete’s traditional ingredients with more sustainable, low-energy alternatives has emerged as one of the most effective means for reducing the carbon footprint of the modern world’s most ubiquitous building material.
In addition to serving as modern world's leading building material, concrete is also one of its biggest individual sources of greenhouse gas emissions, accounting for as much as five per cent of the global anthropogenic total.
According to Ali Behnood, a buildings material researcher from the University of Technology Sydney, it’s the two chief ingredients in conventional forms of concrete – Portland cement and coarse aggregate – that account for the outsized carbon footprint incurred by its production.
“Research indicates that 74 to 81 per cent of the emissions from concrete production are the result of cement production, while 13 to 20 per cent is from coarse aggregate,” said Behnood. “These two figures show the importance of reducing the amount of Portland cement in concrete, as well as the amount of natural coarse aggregate.
“Concrete is basically just aggregate, cement and water, therefore in order to enhance its technical properties we use admixtures - sometimes chemical admixtures and sometimes mineral admixtures.”
While a number of materials are already used as replacements for cement in concrete mixtures, such as the fly ash produced as an industrial residue by coal-fired power plants, they do not exist in sufficient abundance to serve as fully viable alternatives.
Scientists around the world are now scrambling to find sustainable, readily-available materials that can serve as replacements for Portland cement or coarse aggregate in the production of greener forms of concrete.
In Switzerland, a team led by Karen Scrivener, head of the Construction Materials Laboratory at the Ecole Polytechnique Federal de Lausanne (EPFL), has investigated the use of ground limestone and calcined clay as a large-volume replacement for Portland cement, both of which possess significant promise as substitute ingredients given their ready availability and comparatively low cost.
In Malaysia, researchers from the Universiti Teknologi MARA in Selangor are even investigating the use of dried and incinerated sludge as a substitute for cement, with a view to solving the problem of waste disposal while also producing more sustainable concrete.
Behnood, alongside his UTS colleagues Professor Bijan Samali and Professor Vute Sirivivatnanon, believe they have found viable substitutes for both Portland cement and natural aggregate that can be readily sourced by manufacturers based in Australia.
They are using copper slag as a replacement for coarse aggregate and ground pumice as a replacement for Portland cement, both which are low-cost materials found in ready abundance within Australia or New Zealand.
“We decided to replace Portland cement with ground pumice because it’s basically (available) almost for free,” said Behnood. “Pumice is volcanic ash, so it’s not that available in Australia, but close to us in New Zealand you have ample sources of pumice because of the country’s volcanic activity."
The copper slag used to replace coarse aggregate is even easier to source for producers in Australia, given the abundance of copper mines in various parts of the country.
“In Australia was have a huge number of copper mines – six per cent of the world’s economic copper resources are in Australia, mainly in South Australia and Queensland. This puts us third after Chile and the United States," Behnood said. “So we have a lot of ingredients for concrete replacement in Australia and New Zealand - that’s why we believe there are applications for this material here.”
Sourcing sustainable low-cost materials as replacements for coarse aggregate or Portland cement is just the start of the development process, however. The next step for researchers is to determine whether the technical properties and real-world performance of the new concrete are strong enough to make it a viable building product.
“There are concerns that admixtures can reduce the properties of concrete,” said Behnood. “So we have to prove that the product has nearly the same or even superior technical properties.
“According to our test results, we have achieved a similar or even better performance by using copper slag, silicon fume and pumice together as replacement admixtures.
“This new concrete can also be categorised as a green concrete based on the GBCA’s ranking system.”
The UTS researchers are now looking to team up with partners in industry to trial the product, which they believe is best suited as a tougher concrete for more heavy-duty environments.
“Copper slag is a little bit heavier than limestone, but it’s also more resilient, so we are looking for certain, specific applications like industrial concrete floors, which require more abrasion resistance,” Behnood said.