This was one of those fascinating science news stories that just shows up every now and then. The question that was being addressed was why seaside structures built by the ancient Romans (piers, jetties, harbor breakwaters, etc.) have lasted thousands of years, while modern concrete structures, presumably made with better "modern" construction methods, fall apart a lot faster.
The answer, apparently, is in the recipe that the Romans used. It leads to some fascinating chemistry. They combined volcanic ash (available all around Italy), quicklime, volcanic rock -- and the key ingredient, seawater. The dissolved ions in seawater fostered the growth of a crystal, aluminous tobermorite. I have to admit, that's a new one for me.
The growth of the crystals strengthen the Roman concrete structures, and they continued to get stronger as the crystals grew.
Now, I admit I was wondering why if they knew in 2013 that Al-tobermorite was what held the Roman concrete together, why there was another article in 2017 about it. It looks like the researchers went a long way in four years in determining the reaction pathways (there's more than one) that lead to Al-tobermorite crystallization, rather than just knowing what the end product was.