Short answers, pulled from the story.
Roman concrete is a building material used in ancient Rome, also called opus caementicium. Like modern concrete, it combined a hydraulic-setting cement with an aggregate, but it was typically laid rather than poured because the aggregate components were larger.
Roman concrete benefits from several mechanisms that modern concrete lacks. Seawater reacts with volcanic ash and quicklime in the mix to form aluminous tobermorite crystals, which resist fracturing. Lime clasts in the concrete also self-heal cracks by producing reactive calcium that allows new calcium carbonate crystals to seal the damage. Modern concrete exposed to saltwater deteriorates within decades, while Roman marine concrete has survived in usable condition for two thousand years.
Roman concrete was in widespread use from about 150 BC, though some scholars believe it was developed a century earlier. Vitruvius documented its use and proportions around 25 BC in his Ten Books on Architecture, specifying a ratio of one part lime to three parts pozzolana for buildings and one to two for underwater work.
Research in 2023 found that lime clasts in Roman concrete, previously considered a sign of poor mixing technique, are central to its self-healing ability. When water seeps into cracks, the lime clasts react to produce reactive calcium, which forms new calcium carbonate crystals that reseal the damage. The clasts were most likely created by hot-mixing quicklime rather than using slaked lime.
The Pantheon dome is the world's largest and oldest unreinforced concrete dome. It demonstrates Roman builders' sophisticated use of graded aggregate density: the upper dome region uses alternating layers of light tuff and pumice at 1350 kg per cubic meter, while the foundation uses travertine at 2200 kg per cubic meter, reducing the load at the dome's most vulnerable point.
Corporations and municipalities in North America are exploring Roman-style concrete that replaces volcanic pozzolana with coal fly ash, which has similar properties. Proponents say concrete made with fly ash can cost up to 60% less than conventional concrete because it requires less cement, and it has a smaller carbon footprint due to its lower cooking temperature and longer lifespan.