When you look around a modern city today, beneath the slick glass facades lies a material that dates back to ancient Rome. The versatility and availability of concrete has led it to be one of the most consumed materials around the world. In 2014, it was found to be the 2nd most consumed material, only beaten by water. This had been the trend for some decades, only levelling off in recent years with China producing more cement than the rest of the world combined. The second and third largest producers were India and USA respectively, lagging someway behind.

Global production of cement in millions of tonnes, split between the 3 largest producers and the Rest of the World (RoW)

Source: USGS

Concrete is made up of many components, with cement acting as the binder that holds everything together. Typically cement makes up 8-15% of concrete by volume depending on the strength required. However, the majority of greenhouse gas emissions come from the production of cement, which is why it usually ends up being the focus of conversations around the sustainability of concrete.

Close to 75% of the carbon footprint of concrete is from the cement production. Part of this is due to the energy sources used in the process predominantly being fossil fuel based, however a large portion is also released chemically during the production of clinker, a precursor to cement. Various clays and limestones are baked at high temperatures (1450^oC) and ground to form a fine powder and combined with other mineral components. Furthermore, the calcium carbonate within the limestone reacts during the baking and forms carbon dioxide.

Breakdown of the GHG emissions (in %) from the production of cement where chemical emissions are those from the reaction of calcium carbonate, thermal emissions are from the fossil fuels needed to heat the kiln, and the remaining encapsulate all other emissions from the production process

Source: Chatham House

Combining the huge production quantities of cement with its moderate carbon footprint, results in an industry that is responsible for 8% of global greenhouse gas emissions.

Source: Nikkei Asia

2000 years ago, the Romans created a concrete, using chemically similar ingredients to construct the Pantheon and the Colosseum. Whilst the concrete itself may be weaker than modern day mixes, these structures that still stand today and will continue to in the future thanks to the durability of the mix the Romans had used.

The increasing demand of concrete is by no accident, as the function it fulfils can be seldom replaced by other materials. Some of the largest concrete structures such as the Three Gorges Dam (pictured above) or the Hoover Dam would not be possible using other materials. There is even speculation that the concrete within the Hoover Dam is still setting , some 90 years after its initial construction, a testament to the pure mass of material used, and the research and development required to realise the structure.

Within our cities too, concrete is used everywhere from infrastructure projects, structures that sit above ground and the foundations that lay below. Beyond the cost effectiveness and availability, it also imparts increased thermal mass to a building, allowing it to normalise diurnal temperature variations and lower the energy demand of buildings to regulate internal temperature.

Fast forward to today, and concrete is being reinvigorated with modern technology; houses (below) are being printed to benefit from unique builds-ups and reduced wastage. Other mixes incorporate passive agents such as titanium dioxide that limit the build-up of dirt and mould, reducing the required maintenance to keep the surface clean.

Source: 3D Printed House

Concrete has found itself to be a cornerstone of modern urbanised areas and doesn’t seem to be going anywhere anytime soon. However, with imposing decarbonisation goals to reduce anthropogenic emissions, this multi-billion-pound industry is facing scrutiny as to how it will adapt.

The environmental impact of concrete is no secret to the construction industry, and there have been many technological developments to address this issue. As expected, they generally focus on reducing the cement component in concrete mixes.

One method utilises by-products from other industries, which can be used to reduce the cement content. Iron is typically produced through a blast furnace, a large kiln that heats the ore and other ingredients. As part of this process, a slag is produced which is filtered off and generally considered a waste product. If this slag is allowed to harden, and ground, it has a chemical consistency which works well as a supplement to cement, and is known as ground granulated blast furnace slag (GGBS). Similarly, fly ash is collected from the flues at coal plants, which can then be ground into a powder to supplement cement, known as pulverised fly ash (PFA). As both these substances are readily being produced in existing systems, they are considered to have an extremely low carbon footprint. There are also codified grades which take into account these cement replacements making it extremely accessible to specify and use. However, the replacement alters the consistency of the final concrete, and as such there are limits to how much cement they can replace. Furthermore, both substances are derived from industries that will likely shift away from their established processes.

As such there is now a rise of more innovative solutions that look to push what is possible. Geopolymer concrete aims to replace the cement component entirely with an alternate binder with much smaller GHG emissions.

Geopolymer floor for the turning node at Brisbane West Wellcamp airport

Source: BFT International

Solidiatech takes it a step further and use carbon dioxide in the setting process to sequester carbon during the curing process. Whilst this process still uses cement, it can find use for carbon dioxide captured through other processes.

These are just some of the technologies that are being researched. However, questions are still raised as to whether incremental changes are enough, and how soon these technologies can be implemented. Whilst concrete will still be used through our urban environment, there is still some hope that it will be able to clean itself up and start to cut down on its GHG emissions.

For more information on this topic, the Guardian has a great write up on the state of concrete from a year ago as part of their Concrete Week investigation.