Riddle solved: Why was Roman concrete so durable? By hot mixing at high temperatures and several self healing features

Update of 1/10/2023: Here is an MIT article covering this subject.

Amazing stuff! When in Rome do as the Romans do! 😊

"The ancient Romans were masters of engineering, constructing vast networks of roads, aqueducts, ports, and massive buildings, whose remains have survived for two millennia.
Many of these structures were built with concrete: Rome's famed Pantheon, which has the world's largest unreinforced concrete dome and was dedicated in A.D. 128, is still intact, and some ancient Roman aqueducts still deliver water to Rome today. ...
Now, a team of investigators ... has made progress in this field, discovering ancient concrete-manufacturing strategies that incorporated several key self-healing functionalities. ...

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. ...

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.

To prove that this was indeed the mechanism responsible for the durability of the Roman concrete, the team produced samples of hot-mixed concrete that incorporated both ancient and modern formulations, deliberately cracked them, and then ran water through the cracks. Sure enough: Within two weeks the cracks had completely healed and the water could no longer flow. An identical chunk of concrete made without quicklime never healed, and the water just kept flowing through the sample. ..."

From the abstract:

"Ancient Roman concretes have survived millennia, but mechanistic insights into their durability remain an enigma. Here, we use a multiscale correlative elemental and chemical mapping approach to investigating relict lime clasts, a ubiquitous and conspicuous mineral component associated with ancient Roman mortars. Together, these analyses provide new insights into mortar preparation methodologies and provide evidence that the Romans employed hot mixing, using quicklime in conjunction with, or instead of, slaked lime, to create an environment where high surface area aggregate-scale lime clasts are retained within the mortar matrix. Inspired by these findings, we propose that these macroscopic inclusions might serve as critical sources of reactive calcium for long-term pore and crack-filling or post-pozzolanic reactivity within the cementitious constructs. The subsequent development and testing of modern lime clast–containing cementitious mixtures demonstrate their self-healing potential, thus paving the way for the development of more durable, resilient, and sustainable concrete formulations."

Riddle solved: Why was Roman concrete so durable?

Hot mixing: Mechanistic insights into the durability of ancient Roman concrete (open access)

Fig. 1. Collection location and distinctive features of the ancient Roman concrete samples used in this study.