Coal ash is abundant around coal-fired power stations. In fact, that might be considerably understating things – globally, power stations produce around 1.2 billion tonnes annually, and in Australia coal ash accounts for nearly 20% of all waste. It's a staggering figure – and it's also a safe bet that this stuff will remain abundant long into the renewable energy transition.
Hence, it's an enormous potential material resource, and low-carbon concrete manufacturers have been using it as a cement substitute, typically replacing up to 40% of the cement. In an environmental sense, this kills two birds with one stone, making use of a massive waste product while cutting down on cement – which by itself accounts for somewhere around 8% of all global carbon emissions.
A team from RMIT has been working with the Ash Development Association of Australia, and the AGL Loy Yang Power Station, to make better use of this dubious asset, attempting to push the ash content up to replace over 80% of the cement.
To do so, the researchers used a mixture of low calcium fly ash, with 18% hydrated lime and 3% nano-silica acting as strengthening agents, then poured some concrete and started testing its mechanical properties.
The resulting High-Volume Fly Ash (HFVA-80) concrete demonstrated a compressive strength increase from 22 to 71 MPa between days 7 and 450. It achieved flexural strengths of 2.7-8.7 MPa, splitting tensile strengths of 1.6–5.0 MPa and an elastic modulus of 28.9–37.0 GPa. It outlasted regular Portland cement over time when exposed to acids and sulphates for two years.
"Our addition of nano additives to modify the concrete’s chemistry allows more fly ash to be added without compromising engineering performance,” said project lead Dr. Chamila Gunasekara, from RMIT’s School of Engineering, in a press release.
Better still, the team says it sees the technique doesn't require fine 'fly ash' and appears to work about as well with low-grade 'pond ash,' having now created and tested structural concrete beams from the latter, which have passed Australian Standards certification for engineering performance.
“It’s exciting that preliminary results show similar performance with lower-grade pond ash, potentially opening a whole new hugely underutilized resource for cement replacement,” says Gunasekara. “Compared to fly ash, pond ash is underexploited in construction due to its different characteristics.
"There are hundreds of megatonnes of ash wastes sitting in dams around Australia, and much more globally. These ash ponds risk becoming an environmental hazard, and the ability to repurpose this ash in construction materials at scale would be a massive win.”
The RMIT team has also worked with Hokkaido University to develop a pilot computer modeling system forecasting the performance of these new concrete mixtures over time, and the team hopes to use this software to analyze and optimize further new mixes.
Source: RMIT
