When was the last time you thought about the Roman Empire? It’s a common meme that men spend an inordinate amount of time and mental bandwidth on Rome.
A 2023 study by Ipsos tried to wrangle some data on this social media trend and concluded that men are considerably more likely to think about the Roman Empire than women, with 16 per cent thinking about it at least once a week.
As a historian, I spend quite a bit of time ruminating on past societies, even if Rome doesn’t feature particularly often. However, last week I visited the Mithraeum in London, which, being a Roman temple, necessarily entailed some consideration of its ancient architects.
One of the most interesting parts was an incredibly well-preserved piece of a hypocaust, a Roman central-heating system. This concrete flue tile had held its shape after almost two millenniums in wet London clay and made me ponder the durability of Roman engineering.
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We tend to imagine ancient materials as crude or primitive, but Roman concrete was more sophisticated than anything in use until the modern era. Roman architects mixed lime with volcanic ash and aggregate to produce a material that not only endured but, in some cases, got stronger over time.
The Pantheon in Rome, with its 43m span of unreinforced concrete, is still the largest of its kind in the world; a masterpiece of engineering that has survived earthquakes, storms, subsidence and almost 1,900 years of urban life.
Our current concrete recipes use cement, sand, and coarse aggregates in varying proportion and are, by comparison, extremely carbon intensive.
Cement alone is responsible for around seven per cent of global greenhouse gas emissions. Most of this comes from the calcination process: heating limestone to 1,450 degrees, which releases CO₂ from fuel and the chemical reaction itself.
A single cubic metre of conventional concrete can embody 200–400kg of CO₂, and so, even with renewable electricity, the chemistry makes zero-carbon cement difficult.
This is where Roman concrete becomes more than a historical curiosity. It was made at far lower temperatures and used locally sourced volcanic minerals called pozzolans.
Recent studies from Massachusetts Institute of Technology (MIT) suggest that not just the materials but also the manufacturing technique were significant. Roman concrete contains ubiquitous white crystals called lime clasts, formed by quicklime mixing at high temperatures.
These were originally viewed as a sign of poor-quality materials or improper mixing, but the study showed that the lime clasts dissolved in rainwater and then recrystallised in gaps, allowing the material to self-heal.
Irish engineers are hard at work to improve the five million tonnes of concrete that we produce annually, not least because of a Government mandate to reduce the amount of embodied carbon in concrete used for state-funded projects.
Irish firm Ecocem is a market leader in reduced-carbon cement and has been particularly successful in France. Using supplementary cementitious materials from industrial byproducts, such as blast furnace slag from steel production or fly ash from coal, they reduce the amount of clinker, which is responsible for 90 per cent of cement’s carbon footprint.
Researchers from Queen’s University Belfast have also identified an opportunity to use biochar, pyrolysed organic material similar to charcoal, in cement mixes.
A range of inputs can be used to make biochar: straw, slurry solids, forestry thinnings, and even digestate from anaerobic digestion plants. And it is the latter that is most significant.
Agricultural runoff is a key contributor to pollution on Lough Neagh, and so there is a desperate need to reduce its impact. Slurry separation can split solid and liquid fractions on farms, with the liquid applied as a crop fertiliser while the solids can be used for biomethane production in anaerobic digesters.
The biomethane can then be used in place of natural gas, while the resulting digestate can be converted to biochar and then used in construction, locking up embodied carbon for centuries.
This is a rare positive feedback loop: more valourised waste, less aquatic pollution and reduced carbon footprints in construction. Ireland’s agricultural sector, which generates vast quantities of low-value organic residue, provides precisely the kind of feedstock that makes these experiments economically viable.
So the next time you think about the Romans and what they’ve done for us, you can add inspiring materials science to the list.
- Stuart Mathieson is research manager with InterTradeIreland
