Is Roman concrete more sustainable? It’s complicated.

In ancient Rome, builders crafted remarkable structures like aqueducts and bridges using a unique form of concrete. The question arises: is Roman concrete a more sustainable option compared to today's commonly used Portland cement? A recent study published in the journal iScience sheds light on this complex issue. The findings indicate that while Roman concrete generates a similar amount of CO2 as modern methods, it produces significantly fewer air pollutants. Both types of concrete share a fundamental composition, combining a semi-liquid mortar with aggregate materials. However, the creation process differs notably. Portland cement is produced by heating limestone and clay, alongside other materials such as sandstone and iron, in a kiln. This process yields a clinker that is finely ground and mixed with gypsum to enhance surface smoothness. In contrast, Roman concrete utilized fist-sized pieces of stone or bricks as aggregate. The durability of Roman concrete has fascinated scientists for years, sparking ongoing research in the field. For instance, a 2017 analysis of concrete from ancient sea walls along Italy’s coast revealed that these structures have withstood the test of time for over two thousand years, even in harsh marine conditions. This study uncovered that the unique recipe of Roman concrete included rare crystals and a porous mineral, leading to chemical reactions with seawater that formed aluminum tobermorite crystals from phillipsite, a mineral commonly found in volcanic ash. These crystals effectively bonded the rocks, mitigating cracks that could compromise the integrity of the structures. As the debate on sustainability continues, Roman concrete stands as a testament to ancient engineering prowess, prompting modern builders to rethink their materials and methods.