A number of green concrete additives have been developed that reduce the amount of cement needed to make concrete, from plastics to iron slag and .
One of the most common additives is fly ash, a by-product of the coal energy production process, which currently can replace up to a maximum 20-30 per cent of cement, we heard in last year’s vent.
Now researchers from Rice University in the US think they’ve found a way to boost that closer to 80 per cent, potentially removing carbon-intensive cement from the concrete equation entirely to create a truly green concrete.
The breakthrough could have significant ramifications for the building and construction industry, with 20 billion tonnes of concrete laid each year, contributing between five and 10 per cent of total global carbon emissions.
“The industry typically mixes five to 20 per cent fly ash into cement to make it green, but a significant portion of the mix is still cement,” Rice materials scientist Rouzbeh Shahsavar said.
A problem with trying to completely remove cement is that large amounts of expensive sodium-based activators are typically needed, which negate the environmental benefits, according to Shahsavar.
“And in the end it was more expensive than cement.”
The new breakthrough used a statistical method called Taguchi analysis and computational optimisation to identify the best concrete rising strategies. Shahsavari said this led to cement being entirely replaced by as mixture of calcium-rich fly ash, nanosilica and calcium oxide, with less than fivepercent of a sodium-based activator required.
The cementless fly ash binder would also be able to be sourced from brown coal stations, such as those in Victoria.
“A majority of past works focused on so-called type F fly ash, which is derived from burning anthracite or bituminous coals in power plants and has low calcium content,” Shahsavari said.
“But globally, there are significant sources of lower grade coal such as lignite or sub-bituminous coals. Burning them results in high-calcium – or type C – fly ash, which has been more difficult to activate.
“Our work provides a viable path for efficient and cost-effective activation of this type of high-calcium fly ash, paving the path for the environmentally responsible manufacture of concrete.”
Another climate positive is that the cementless fly ash binder does not require the high-temperature processing of Portland cement, however still showed the same compressive strength after seven days of curing.
“Future work will assess such properties as long-term behaviour, shrinkage and durability,” Shahsavari said.
He also said the same strategy could be used so other industrial waste – such as blast furnace slag and rice hulls – could be turned into cementitious materials without the need for cement.
The research has been published in the .