The secret of Roman concrete

The splendor of Greek temples and theaters is not only visible to the onlooker but resides in the architectural technique itself, i.e., fitting the stone blocks together without using concrete. 

We don’t know who is responsible for the idea, but we think the Romans are the actual inventors of concrete – a modern material! Whether walking through the remains of cities like Pompeii, visiting amphitheaters, or stopping at the many aqueducts around the Mediterranean, we’ll see sturdy brick walls held together with layers of cement. In the glory days of Rome, these walls were all covered with marble slabs that were reused after the decline of the empire. 

To make their cement, the Romans mixed lime, shale, clay, and aggregate rocks to create a substance that we call cement today.

The material turned out to be so strong that it defied time (and, I dare say, their own expectations). Striking Roman buildings such as the Coliseum and the Pantheon in Rome (see: Linking the Pantheon in Rome with ancient Mausoleums) are still standing after two thousand years. I wonder if any of our modern buildings will have such a long life. 

A recent study revealed that Roman concrete contained calcium-rich deposits, known as “lime clasts,” which are considered impurities in modern concrete and thus removed. These clasts are a kind of millimeter size white chunks of rock that provide a self-healing capability. 

To find out how these lime clasts influenced Roman concrete, scientists produced samples of both Roman and modern cement using what is called “hot-mixed concrete.” After the material dried and hardened, they cracked the samples and poured water through the cracks. Two weeks later, it appeared that the cracks in the Roman example had completely restored themselves, and no water flowed through the material – a self-healing process. The modern concrete, lacking the lime clasts, never healed, and the water kept running through the cracks. 

Today’s environmentalists highly acclaim that the Roman type of concrete could reduce the impact of cement production on our environment and cut down carbon emissions. 

We had to reinvent the wheel! The Romans may not have been aware of air pollution, but instead of walking in their footsteps, we invented replacement procedures that are detrimental to our world.