Australian researchers have developed a new magnesium-based “green cement” by overcoming water resistance issues that have traditionally limited its use.
The engineering academics have produced a new version of magnesium oxychloride cement (MOC), which they believe has the potential to replace – or partially replace – conventional cement used in construction materials, resulting in a reduction of emissions.
According to the study, published in The Conversation, MOC is produced by mixing magnesium oxide powder and a concentrated solution of magnesium chloride. The magnesium oxide powder, also called light burnt magnesia, is produced from magnesium mining, while magnesium chloride can be obtained from sea water.
The product can achieve much higher compressive strength than regular cement, while its fast setting benefits and early strength gain are advantageous for construction.
Additionally, magnesium oxide can also absorb carbon dioxide from the atmosphere, making MOC a green and environmentally friendly cement with lower CO2 emissions.
However, the use of MOC has previously been restricted to indoor applications such as floor tiles, decoration panels, sound and thermal insulation boards, due to its tendency to degrade in strength after prolonged exposure to water and moisture.
Now, the research team led by Yixia (Sarah) Zhang, Associate Professor of Engineering at Western Sydney University, has developed a method to overcome this by adding industrial by-products such as fly ash and silica fume, as well as chemical additives.
Fly ash, a by-product of the coal industry, was found to significantly improve the water resistance of MOC. When combined with silica fume, the full compressive strength of the MOC was retained after being soaked in water for 28 days.
“Both the fly ash and silica fume have an effect of optimising the pore structure in MOC, making the cement denser,” Zhang told Quarry.
“The reactions of fly ash and silica fume with the MOC matrix form a gel-like phase, which contributes to water repellence. The extremely fine particles, large surface area and high reactive silica content of silica fume make it an effective binding substance known as a pozzolan. This helps give the concrete high strength and durability.”
Although this particular MOC product demonstrated excellent resistance to water at room temperature, Zhang said it rapidly weakened when being soaked in warm water. The team later solved this by adding phosphoric acid and soluble phosphates.
Overall, the MOC achieved a compressive strength of 110 megapascals (MPa) and flexural strength of 17 MPa at 28 days, a few times greater than those of conventional cement.
The MOC can fully retain these strengths after being soaked in water for 28 days at room temperatures and, in hot water (60˚C), the MOC can retain up to 90 per cent of its compressive and flexural strength after 28 days.
“There is a lot of potential for industry application. Firstly, this material will improve its current indoor applications significantly and expend its outdoor applications,” Zhang said.
“In the longer run, if it can be applied in producing concrete to replace or practically replace the conventional cement used, that would be a big thing to happen for the sector.
“We would like to work further with government and industries to develop new MOC-based products and tailor the mix design to suit the industrial needs.”
Zhang added: “When concrete is the main structural component, steel reinforcement has to be used. Corrosion of steel in MOC is a critical issue and a big hurdle to jump. The research team has already started to work on this issue.
“If this problem can be solved, MOC could be a game-changer for the construction industry.”