Roman Concrete Could Improve Sustainable Modern Cement

Ancient Romans may have left us valuable lessons in producing sustainable concrete capable of surviving for thousands of years.

A fresh study has closely examined the ingredients and energy requirements behind the ancient formula, identifying potentially useful approaches for improving modern cement.

Roman concrete and its environmental cost

Unexpectedly, the researchers discovered that making Roman mortar and concrete consumes more water and creates more greenhouse gas emissions than Portland cement, the cement type most commonly used in concrete today.

Even so, although its upfront energy burden can be higher, Roman concrete recipes may be more sustainable over time because they might require replacement or repairs less frequently.

An international group of environmental engineers calculates that Roman concrete mixes would have to last at least 41 percent longer than modern versions to produce equivalent cumulative building emissions.

For structures with shorter service lives, including roads and motorways, Roman concrete would need to endure at least 29 percent longer.

"When we take concrete's service life into consideration, that's when we start seeing benefits," says lead author and engineer Daniela Martinez of the University of the North in Columbia.

"There's a lot of lessons that we can draw from the Romans. If we can incorporate their strategies with our modern innovative ideas, we can create a more sustainable built environment."

Durability and self-healing Roman concrete

Assessing the lifespan of Roman concrete against modern concrete is difficult, as many current concrete buildings and structures are reinforced with steel.

When steel corrodes, it expands and causes the surrounding concrete to crack and deteriorate, sometimes within as little as a decade. Ancient Roman concrete, in contrast, can 'self-heal': when fissures develop in the cement, leaking water naturally bonds them back together.

This helps explain why ancient Roman sea walls have lasted for millennia despite continual wave impacts and corrosive salt water.

It remains uncertain whether modern concrete can perform in the same way.

"Corrosion of steel reinforcement is the main cause of concrete deterioration, so comparisons should be made with great care," explains author and engineer Paulo Monteiro of the University of California, Berkeley.

Martinez and colleagues accept that cement structures with longer lifespans "can only partially offset increased demand for new material," although Roman production practices could offer additional environmental advantages.

Cement ingredients and air pollution

Roman cement and Portland cement share the same primary ingredient. Heating limestone produces quicklime, a highly reactive and caustic powder; once mixed with water, it creates a powerful mortar.

Concrete mixtures across the ancient Roman Empire varied considerably, but they typically combined this mortar with locally available volcanic rock instead of the gypsum used in modern concrete.

Roman concrete also releases substantially fewer hazardous air pollutants, perhaps because limestone was fired with oak and fir wood rather than fossil fuels.

Recent modelling suggests that applying Roman cement methods could cut nitrogen oxide and sulphur oxide emissions by up to 98 percent compared with current techniques.

"Using biomass and other alternative fuels to fire kilns may prove more effective in decarbonizing modern cement production than implementing Roman concrete formulations," says Martinez.

Today, concrete is the world's second most widely used material, behind only water. Producing it is responsible for around 8 percent of global human-caused carbon emissions, or roughly one quarter of all carbon emissions from industry.

Arizona State University materials scientist Narayanan Neithalath says that, if cement manufacturing were treated as a country, it would rank as the world's third-largest carbon dioxide emitter, after China and the United States.

"If we stand a reasonable chance of staying below the two degrees Celsius warming target set by the Paris Agreement, cement-related emissions will have to fall by more than 20 percent in the coming decade," Neithalath explained in 2023.

Research into ancient Roman concrete could assist efforts to meet these urgent climate targets.

The study appeared in iScience.