Green cement a step closer to being a game-changer for construction emissions

If the cement industry were a country, it would be the third-largest emitter of CO₂ in the world.
Joe Mabel/Wikimedia, CC BY-SA

Yixia (Sarah) Zhang, Western Sydney University; Khin Soe, Western Sydney University, and Yingying Guo, UNSW

Concrete is the most widely used man-made material, commonly used in buildings, roads, bridges and industrial plants. But producing the Portland cement needed to make concrete accounts for 5-8% of all global greenhouse emissions. There is a more environmentally friendly cement known as MOC (magnesium oxychloride cement), but its poor water resistance has limited its use – until now. We have developed a water-resistant MOC, a “green” cement that could go a long way to cutting the construction industry’s emissions and making it more sustainable.

Producing a tonne of conventional cement in Australia emits about 0.82 tonnes of carbon dioxide (CO₂). Because most of the CO₂ is released as a result of the chemical reaction that produces cement, emissions aren’t easily reduced. In contrast, MOC is a different form of cement that is carbon-neutral.

Global CO₂ emissions from rising cement production over the past century (with 95% confidence interval).
Source: Global CO2 emissions from cement production, Andrew R. (2018), CC BY

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What exactly is MOC?

MOC is produced by mixing two main ingredients, magnesium oxide (MgO) powder and a concentrated solution of magnesium chloride (MgCl₂). These are byproducts from magnesium mining.

Magnesium oxide (MgO) powder (left) and a solution of magnesium chloride (MgCl₂) are mixed to produce magnesium oxychloride cement (MOC).
Author provided

Many countries, including China and Australia, have plenty of magnesite resources, as well as seawater, from which both MgO and MgCl₂ could be obtained.

Furthermore, MgO can absorb CO₂ from the atmosphere. This makes MOC a truly green, carbon-neutral cement.

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MOC also has many superior material properties compared to conventional cement.

Compressive strength (capacity to resist compression) is the most important material property for cementitious construction materials such as cement. MOC has a much higher compressive strength than conventional cement and this impressive strength can be achieved very fast. The fast setting of MOC and early strength gain are very advantageous for construction.

Although MOC has plenty of merits, it has until now had poor water resistance. Prolonged contact with water or moisture severely degrades its strength. This critical weakness has restricted its use to indoor applications such as floor tiles, decoration panels, sound and thermal insulation boards.

How was water-resistance developed?

A team of researchers, led by Yixia (Sarah) Zhang, has been working to develop a water-resistant MOC since 2017 (when she was at UNSW Canberra).

Adding industrial byproducts fly ash (above) and silica fume (below) improves the water resistance of MOC.
Author provided

To improve water resistance, the team added industrial byproducts such as fly ash and silica fume to the MOC, as well as chemical additives.

Fly ash is a byproduct from the coal industry – there’s plenty of it in Australia. Adding fly ash significantly improved the water resistance of MOC. Flexural strength (capacity to resist bending) was fully retained after soaking in water for 28 days.

To further retain the compressive strength under water attack, the team added silica fume. Silica fume is a byproduct from producing silicon metal or ferrosilicon alloys. When fly ash and silica fume were combined with MOC paste (15% of each additive), full compressive strength was retained in water for 28 days.

Both the fly ash and silica fume have a similar effect of filling the pore structure in MOC, making the cement denser. The reactions 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 (SiO₂) content of silica fume make it an effective binding substance known as a pozzolan. This helps give the concrete high strength and durability.

Scanning electron microscope images of MOC showing the needle-like phases of the binding mechanism.
Author provided

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Although the MOC developed so far had excellent resistance to water at room temperature, it weakened fast when soaked in warm water. The team worked to overcome this by using inorganic and organic chemical additives. Adding phosphoric acid and soluble phosphates greatly improved warm water resistance.

Examples of building products made using MOC.
Author provided

Over three years, the team has made a breakthrough in developing MOC as a green cement. The strength of concrete is rated using megapascals (MPa). The MOC achieved a compressive strength of 110 MPa and flexural strength of 17 MPa. These values are 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. Even in hot water (60˚C), the MOC can retain up to 90% of its compressive and flexural strength after 28 days. The values remain as high as 100 MPa and 15 MPa respectively – still much greater than for conventional cement.

Will MOC replace conventional cement?

So could MOC replace conventional cement some day? It seems very promising. More research is needed to demonstrate the practicability of uses of this green and high-performance cement in, for example, concrete.

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 can be a game-changer for the construction industry.

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The problem with reinforced concrete

The Conversation

Yixia (Sarah) Zhang, Associate Professor of Engineering, Western Sydney University; Khin Soe, Research Associate, School of Computing, Engineering and Mathematics, Western Sydney University, and Yingying Guo, PhD Candidate, School of Engineering and Information Technology, UNSW

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Nitrogen fertilisers are incredibly efficient, but they make climate change a lot worse

Sustainable farming can reduce nitrous oxide emissions.
eutrophication&hypoxia/Flickr, CC BY-SA

Pep Canadell, CSIRO; Hanqin Tian, Auburn University; Prabir Patra, and Rona Thompson, Norwegian Institute for Air Research

Nitrous oxide (N₂O) (more commonly known as laughing gas) is a powerful contributor to global warming. It is 265 times more effective at trapping heat in the atmosphere than carbon dioxide and depletes our ozone layer.

Human-driven N₂O emissions have been growing unabated for many decades, but we may have been seriously underestimating by just how much. In a paper published today in Nature Climate Change, we found global emissions are higher and growing faster than are being reported.

Read more:
Nitrogen pollution: the forgotten element of climate change

Although clearly bad news for the fight against climate change, some countries are showing progress towards reducing N₂O emissions, without sacrificing the incredible crop yields allowed by nitrogen fertilisers. Those countries offer insights for the rest of the world.

N₂O concentrations (parts per billion) in air from Cape Grim Baseline Air Pollution Station (Tasmania, Australia) and air contained in bubbles trapped in firn and ice from the Law Dome, Antarctica. N₂O concentrations from these two sites reflect global concentrations, not local conditions. Source: BoM/CSIRO/AAD.

The Green Revolution

There are a number of natural and human sources of N₂O emissions, which have remained relatively steady for millennia. However, in the early 20th century the Haber-Bosch process was developed, allowing industry to chemically synthesise molecular nitrogen from the atmosphere to create nitrogen fertiliser.

This advancement kick-started the Green Revolution, one of the greatest and fastest human revolutions of our time. Crop yields across the world have increased many times over due to the use of nitrogen fertilisers and other improved farming practices.

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But when soil is exposed to abundant nitrogen in its active form (as in fertilizer), microbial reactions take place that release N₂O emissions. The unrestricted use in nitrogen fertilisers, therefore, created a huge uptick in emissions.

N₂O is the third-most-important greenhouse gas after carbon dioxide and methane. As well as trapping heat, it depletes ozone in the stratosphere, contributing to the ozone hole. Once released into the atmosphere, N₂O remains active for more than 100 years.

Tracking emissions from above

Conventional analysis of N₂O emissions from human activities are estimated from various indirect sources. This include country-by-country reporting, global nitrogen fertiliser production, the areal extent of nitrogen-fixing crops and the use of manure fertilisers.

Our study instead used actual atmospheric concentrations of N₂O from dozens of monitoring stations all over the world. We then used atmospheric modelling that explains how air masses move across and between continents to infer the expected emissions of specific regions.

We found global N₂O emissions have increased over the past two decades and the fastest growth has been since 2009. China and Brazil are two countries that stand out. This is associated with a spectacular increase in the use of nitrogen fertilisers and the expansion of nitrogen-fixing crops such as soybean.

We also found the emissions reported for those two countries, based on a methodology developed by the Intergovernmental Panel on Climate Change, are significantly lower than those inferred from N₂O levels in the atmosphere over those regions.

This mismatch seems to arise from the fact that emissions in those regions are proportionally higher than the use of nitrogen fertilizers and manure. This is a departure from the linear relationship used to report emissions by most countries.

There appears to be a level of nitrogen past which plants can no longer effectively use it. Once that threshold is passed in croplands, N₂O emissions grow exponentially.

N₂O emissions from agriculture estimated by using the emissions factors approach of the IPCC (blue), the calculated emission factor in this study (green), and the average of the atmospheric inversions in this study (black).
Thompson et al. 2019 Nature Climate Change

Reversing the trends

Reducing N₂O emissions from agriculture will be very challenging, given the expected global growth in population, food demand and biomass-based products including energy.

However, all future emission scenarios consistent with the goals of the Paris Agreement require N₂O emissions to stop growing and, in most cases, to decline – between 10% and 30% by mid-century.

Interestingly, emissions from the USA and Europe have not grown for over two decades, yet crop yields across these regions increased or remained steady. Both regions have created strong regulations largely to prevent excess accumulation of nitrogen in soils and into waterways.

These areas and other studies have demonstrated the success of more sustainable farming in reducing emissions while increasing crop yields and farm-level economic gains.

A whole toolbox of options is available to increase nitrogen use efficiency and reduce N₂O emissions: precision applications of nitrogen in space and time, the use of N-fixing crops in rotations, reduced tillage or no-tillage, prevention of waterlogging, and the use of nitrification inhibitors.

Read more:
A new way to curb nitrogen pollution: Regulate fertilizer producers, not just farmers

Regulatory frameworks have shown win-win outcomes in a number of countries. With intelligent adaptions to different nations’ and regions’ needs, they can also work elsewhere.The Conversation

Pep Canadell, Chief research scientist, CSIRO Oceans and Atmosphere; and Executive Director, Global Carbon Project, CSIRO; Hanqin Tian, Director, International Center for Climate and Global Change Research, Auburn University; Prabir Patra, Senior Scientist, Dy. Group Leader, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), and Rona Thompson, Senior scientist, Norwegian Institute for Air Research

This article is republished from The Conversation under a Creative Commons license. Read the original article.