Riddle solved: Why was Roman concrete so durable?

Scientists have been trying for decades to learn the secret of Roman concrete, particularly in structures that have endured particularly harsh conditions, such as docks, sewers and seawalls, or structures built in seismically active sites. The material has been indestructible for two thousand years and the question is: ‘why is that?’
An international team of researchers now discovered that the Romans used strategies to make their concrete that provide several important self-healing properties. The trick is in using quicklime during mixing. The researchers traced the Roman process through so-called clasts, a type of lime inclusions that previous research dismissed as the result of careless mixing.
Studying samples of this ancient concrete, he and his team determined that the white inclusions were, indeed, made out of various forms of calcium carbonate. And spectroscopic examination provided clues that these had been formed at extreme temperatures, as would be expected from the exothermic reaction produced by using quicklime instead of, or in addition to, the slaked lime in the mixture. Hot mixing, the team has now concluded, was actually the key to the super-durable nature of Roman concrete.

According to the researchers, ‘hot mixing’ has two important advantages. First, when the overall concrete is heated to high temperatures, it allows chemistries that are not possible if you only used slaked lime, producing high-temperature-associated compounds that would not otherwise form. Second, this increased temperature significantly reduces curing and setting times since all the reactions are accelerated, allowing for much faster construction.
During the hot mixing process, the lime clasts develop a characteristically brittle nanoparticulate architecture, creating an easily fractured and reactive calcium source, which, as the team proposed, could provide a critical self-healing functionality. As soon as tiny cracks start to form within the concrete, they can preferentially travel through the high-surface-area lime clasts. This material can then react with water, creating a calcium-saturated solution, which can recrystallize as calcium carbonate and quickly fill the crack, or react with pozzolanic materials to further strengthen the composite material. These reactions take place spontaneously and therefore automatically heal the cracks before they spread. Previous support for this hypothesis was found through the examination of other Roman concrete samples that exhibited calcite-filled cracks.
The researchers think their discovery can make building with concrete more sustainable and environmentally friendly.

Credits: MIT