by Elana Manasse-Piha
Only surpassed by the energy and agricultural sectors, concrete is the third largest producer of CO2 emissions in the world, but is so commonplace, most people never even consider it. Most concrete used today is made by using a paste of Portland Cement (cement is an ingredient of concrete) and fresh water to “glue” small rocks together; the resulting product is concrete. This manufacturing process produces large amounts of CO2 and uses a significant amount of freshwater — a limited resource. Another factor to consider is that modern concrete starts deteriorating within 10-20 years and has a total lifespan of only 50-100 years. Therefore, it must be replaced relatively often, creating even more waste, this time in the form of decaying concrete. This short period of use forces concrete’s environmentally hazardous lifecycle to start over again.
There is, however, a form of concrete that requires significantly less CO2 to produce, uses seawater instead of freshwater, and lasts exponentially longer than Portland concrete. Surprisingly, this improved recipe for concrete was created by the Romans over 2,000 years ago, and the structures made from it are still in amazing shape today! Recognizing the obvious advantages of Roman concrete, scientists have been working diligently for decades to recreate it.
Scientist taking a drill sample of Roman concrete. Photo source: https://www.sciencemag.org/news/2017/07/why-modern-mortar-crumbles-roman-concrete-lasts-millennia
The basic recipe for Roman concrete involves mixing volcanic ash, lime, and seawater to make mortar and then adding small bits of volcanic rocks as aggregate to make the final product. Sounds easy to reproduce, right? Unfortunately, the exact proportions, as well as the precise method for mixing the ingredients have been lost to history. On top of that, the specific ash and type of volcanic rocks used can greatly affect the final product.
Made entirely out of Roman concrete in the 2nd century A.D.,the Pantheon has weathered earthquakes, storms and invasions, to still stand proudly in modern day Rome. Photo source: Wikipedia
The reason Roman concrete is so special is due to its chemistry. A chemical reaction between the mortar (cement) and the aggregate volcanic rocks creates mineral intergrowths that prevent cracks from lengthening. Additionally, when seawater penetrates the concrete, it dissolves components of the ash used in the mortar, acting as a catalyst that allows new minerals to grow. One of those minerals, called aluminous tobermorite (Al-tobermorite), is exceptionally rare and especially beneficial. As Al-tobermorite forms, it grows into thin, interlocking, plate-shaped crystals that naturally reinforce the concrete in which they form. Due to the formation of Al-tobermorite, Roman concrete becomes stronger in saltwater. In contrast, modern concrete erodes even faster than normal when exposed to seawater. Even when small cracks do form, Roman concrete has some self-healing ability, as the formation of new Al-tobermorite crystals will fill the cracks and reinforce the concrete.
Drill core of Roman concrete. Inset is a scanning electron microscope image of Al-tobermorite crystals. Photo source: https://als.lbl.gov/learning-from-roman-concrete/
There are a few downfalls to Roman concrete; it takes years to develop its full strength and ultimately has less compressive strength than typical Portland concrete. While not the definitive material to replace all current concrete structures, Roman concrete is the ideal material for many projects that must not only be durable, but also long-lasting. Marine structures used to capture renewable energy—such as offshore wind turbines and hydropower stations—are ideal candidates for Roman concrete. These structures need to be sturdy and enduring since they are based in moving water. In addition, the reduction of CO2 used to create these structures would reduce the time needed for them to accrue net gains in producing clean energy as well as greatly extend the lifetime of said structures, allowing these gains to increase over time. Due to its low CO2 emissions during manufacturing, as well as its staggering longevity, the use of Roman concrete around the world would be an amazingly impactful climate solution.
So what do you think, are there any projects in your area that could benefit from this ancient Roman technology? Let us know in the comments!