What property allowed Roman concrete to be so durable?

Investigators from MIT, Harvard University, and laboratories in Italy and Switzerland, have discovered ancient Roman concrete-manufacturing strategies that incorporated several key self-healing functionalities. For many years, researchers have assumed that the key to the ancient concrete’s durability was based on one ingredient: pozzolanic material such as volcanic ash. Under closer examination, the ancient samples also contain small, distinctive, millimeter-scale bright white mineral features. These white chunks, often referred to as “lime clasts,” originate from lime, another key component of the ancient concrete mix. Previously disregarded as merely evidence of sloppy mixing practices, or poor-quality raw materials, the new study suggests that these tiny lime clasts gave the concrete a previously unrecognized self-healing capability.

Source: MIT News