017 / Green Concrete
By Khaled Abou Alfa • Published 27th December, 2019 • Join the Conversation
When you walk through the entrance of the Pantheon in Rome, you walk under the inscription:
M. AGRIPPA L. F. COS. TERTIUM FECIT
which translates to, ‘Marcus Agrippa, son of Lucius, in his third consulate, made it’. Except of course he didn’t. Agrippa was responsible for the first incarnation of the temple, in 27 B.C. That building was burnt in one of the fires that occurred in the decades after it’s construction. The building that stands there today wasn’t built and completed until 123 A.D. We can trace the origins of it’s longevity back to one fateful evening, when Rome burned.
The ceiling of the Pantheon.
With a full moon in the sky, on the 19th of July, 64 AD, a fire started that lasted for six days. Ten out of Rome’s fourteen districts would be completely engulfed and destroyed. Although the origins of the fire remain in dispute, the Emperor Nero is often portrayed as the villain of this story, playing his fiddle while Rome burned 1. Regardless of how or even why the fire started, the outcome remained the same, large parts of the city had to be rebuilt. Following the fire, Nero would solidify Rome’s presence in the architectural world forever.
Nero introduced new building codes that aimed to mitigate against fires. His new urban laws featured wider streets, restricted heights of buildings, required fire fighting platforms and the use of more fire resistant materials. As a result the use of opus caementicium, so called Roman concrete, proliferated.
This type of concrete, 2000 years old now, has allowed many structures to remain standing. After the fall of the Roman Empire, this material and its use in construction was all but forgotten. It wasn’t until around the 17th century, when concrete was rediscovered, for its ascension to begin anew. While modern concrete is similar to Roman concrete, there are differences in their properties. Roman concrete exhibits exceptional longevity, but is of much weaker strength. While we have the ingredients of Roman concrete, based on the writings of Vitruvius’ On Architecture text, we’ve not been able to completely recreate this type of concrete2.
The story behind Roman concrete teaches us several important lessons. Not that the Romans had better technology than us, rather that the science behind concrete has room to develop. There is room for the material to grow in very different directions to the established ones we have now.
All we need to do is find them.
Concrete continues its dominance within the construction industry as the material of choice. It’s success, attributed to it’s considerable versatility, wide availability and transferability to different regions of the world (where production is not available). Its use is a necessary tool for enabling communities to lift themselves out of poverty and fuel their economic growth.
Without concrete, many necessary projects would not be possible. The dichotomy of this material is that it’s current production method is contributing significantly to the detriment of the planet. The production of cement accounts for around 8% of all carbon emissions.
While looking for completely different materials (such as timber, see issue 007) is a valiant endeavor, this represents a small part of the solution. A more environmentally considered solution and production chain needs to first be developed. It will then need to overcome entrenched practices before spreading widely.
Concrete has four main components:
- Air (up to 8%)
- Water (14-21%)
- Portland Cement (7-15%)
- Aggregate (60-75%)
It is the final two components that need special consideration. Their production and mining inflict the most damage to our world and environment.
Portland cement takes its name from Portland stone, quarried on the Isle of Portland in Dorset, UK. Portland cement forms the binding agent when mixed with water, is predominantly made from lime and silica. These components, (along with others such as shells, chalk, shale, clay, slate, iron ore), goes through a main kiln. This process produces clinker. Clinker is then ground to produce Portland cement3. Clinker production is at the core of the challenge ahead for the industry.
While electricity is used throughout the process, the cement kiln uses an additional thermal intensive process. This thermal process, carried out using oil or coal, is necessary to reach the necessary temperatures (up to 1450°C) to create clinker. Other forms of energy for this process, used to a lesser degree, include natural gas, waste and other renewables and Biomass. To meet global environmental targets alternative forms need to increase, while our dependence on carbon based fuels needs to decrease.
The clinker-to-cement ratio is the measure of how much clinker is needed to create cement. By lowering the amount (and subsequently increasing the amount of cement constituents), means lower emissions and lower energy use. The IEA’s (International Energy Agency), has developed an SDS (Sustainable Development Scenario) which outlines that the global average should be around 64%. (in order to meet the three of the main energy-related UNSDGs). Considering it’s the largest user of concrete (by some margin), the silver lining being that China has one of the lowest ratios at 60%. Currently the EU27 average is 73.7%. The US has one of the lowest ratio at around 90%. Clearly there is much work for the industry to achieve its current potential, much less more ambitious targets and estimates.
Encouragingly in an industry not celebrated for it’s R&D efforts, there is movement around environmentally considered commercial production of cement. As the effects of climate change become part of our lives, solutions to these issues become increasingly important.
- The CemZero project, considers the possibilities of electrifying cement production. The feasibility study was completed in early 2019 and is now looking to build a pilot plant.
- Oxy-fuel capture technologies is increasingly being considered for inclusion into cement plants. This technology modifies the manner in which fuel is burned within the plant, by using oxygen instead of atmospheric air. The system recycles some of the byproducts and produces captured CO2 which can be utilized in other applications.
- While still at a very early stage in the development, there are several efforts using solar energy to carry part of the burden of cement production.
- Meanwhile, researchers at MIT, are proposing using Electrochemical synthesis to create cement. While capturing the purer CO2 released from the production.
Water, water, everywhere,
Nor any drop to drink.
― Samuel Taylor Coleridge, The Rime of the Ancient Mariner
Desert Sand. Image courtesy of Finite.
Aggregate describes a broad category of coarse to medium grained particles which include sand, gravel or crushed stones. Our hunger for aggregate is so insatiable that it is the most mined material in the world. While there is a colossal global network, monitoring and reporting of aggregate extraction is limited and the purview of developed countries. The amount in use can be calculated by proxy, as the use of sand correlates to the use of cement (which is well documented). While the percentages will vary regionally, globally 85-90% of this sand is extracted from quarries and sand & gravel pits (this likely includes off shore sand extraction), however 10-15% is being extracted from rivers and sea shores. Even at this level of activity considerable environmental and social damage is being caused.
Beach Sand. Image courtesy of Finite.
It might seem that we have an abundant supply of sand in the form of desert sand. Except that this type of sand is not a viable option for construction purposes. The size of the wind swept particles are too fine and smooth to bind the building materials together. The complete lack of regulation and consideration has meant that sand mining mafias have moved in to exploit the situation.
Unfortunately the 24 Indonesian sand islands that have been relocated and become part of Singapore will not return. There is still hope for the rivers that have seen even the most aggressive of mining. Given time, these rivers can replenish themselves. The issue is bringing in the necessary controls. In order to do this we need viable alternative solutions. Similar to the cement industry, there is ongoing development to find alternatives.
- Manufactured Sand (M-Sand). Manufactured sand is produced by crushing hard granite stone.
- Finite. The startup Finite has developed a material that will allow the use of desert sand in construction. The material is currently going through regulatory approvals for use in permanent structures.
- Industrial byproducts such as slags. This is a temporary solution as the world moves towards a decarbonized future.
- Recycled sands from construction and demolition waste.
Our dependance on concrete shows no signs of slowing down. There are cracks appearing in the supply chain. Both the economical and environmental costs of energy and sand continue to rise. With perseverance, the days of unrestricted use of carbon based fuels for the production of cement and the unregulated nature of sand mining are numbered.
The fiddle wasn’t invented till the 11th Century and Nero was 50km away when the fire began.↩
Knowing the ingredients isn’t the same as knowing the recipe. There has been some development in this field by the likes of Marie Jackson who believes that unlocking this recipe will allow us to use it in specific marine applications.↩
Gypsum, slag and fly ash can also be added at this stage to control the properties of the final product.↩