It is estimated that cement manufacture produces 5 – 10% of the world’s global carbon dioxide emissions. Yet mankind manufacturers 20 billion tons of concrete every year. Our current idea of modern cities cannot exist without concrete.
To reduce this impact on the planet mankind has three options:
- stop building
- build without concrete (for example Tokyo’s bamboo skyscraper)
- Find a replacement raw material for cement.
Fortunately, a team of research chemical engineers from Rice University have found a way to replace Portland cement in concrete with fly-ash, the by-product of coal-fired power plants.
While it is currently common practice to add a small amount of of silicon- and aluminum-rich fly ash or microspheres to concrete, to date, no one has been able to replace Portland cement from concrete’s list of raw materials. Despite centuries of trying, mankind has barely improved the original Roman recipe.
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The new concrete binder can be made from microspheres (a.k.a. cenospheres) and so would remove the need to consume large amounts of energy for the manufacture of cement. As the microspheres are made from a waste by-product of energy production it but would also be part of the circular economy.
The research was led by Rouzbeh Shahsavari, Asst Professor at Rice’s Multiscale Materials Modeling Lab, who noted that, “Previous attempts to entirely replace Portland cement with a fly ash compound required large amounts of expensive sodium-based activators that negate the environmental benefits.” Adding that, “And in the end it was more expensive than cement.”
Reporting on the breakthrough, the online scientific journal, Phys.org, notes how, “The researchers used Taguchi analysis, a statistical method developed to narrow the large phase space—all the possible states—of a chemical composition, followed by computational optimization to identify the best mixing strategies.”
As a result, the team were able to improve the mechanical and structural properties of the new microsphere-base composites. This led to them finding an optimal balance of calcium-rich fly-ash, nanosilica, and calcium oxide which contains less than 5% of a sodium-based activator.
The team has now published their results in the Journal of the American Ceramic Society, where they, “propose an unprecedented binder composite with the promising early‐age strength, which is cost‐effective and reduces the CO2 footprint compared to Portland cement. The major constituent is fly ash occupying 76.4‐80.3% by the total mass of the constituents, while calcium oxide, nanosilica and the minimum amounts of sodium‐based activators are added to induce the early‐age strength development.”
The report continues to note that after the first seven days the new binder’s strength, “… is comparable to that of conventional Portland cement.” Adding that, Overall, the promising early‐age strength coupled with the significantly decreased amount of sodium‐based chemicals and the reduced CO2 footprint will lay a foundation for development of low‐cost, environmentally‐friendly binder in diverse industries.”
“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 haslow 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.
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“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. Future work will assess such properties as long-term behaviour, shrinkage and durability.”
While it is evident that further testing is needed before we can begin to build with Portland cement-less concrete, it is incredible that this work has proven that a comparable binder during ‘early day’ testing is possible. Even more interesting, is that cement’s replacement can be made from a waste by-product such as microspheres.
Today, Shahsavari is suggesting that other industrial waste products, such as rice husks or blast furnace slag, could also be turned into ‘environmentally friendly cementitious materials’.
Construction raw materials without cement? Not even the Romans could manage that!
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Photo credit: Phys.org, RiceUniversity, BnProducts, Wired, Alibaba & Phrmg
