Why is Ancient Cement Better than Ours?

The Atlantic Cities:

But if the dazzling concrete curves and cantilevers of modern architecture have matched the Romans' for style and structure, today's standard recipe, 2,000 years later, remains in some ways inferior.
New research into Pozzolanic cement, so named for the corner of the Bay of Naples where the ash of Mount Vesuvius facilitated its creation, shows the advantages of the Roman method. Their mixture for hydraulic concrete, a blend of volcanic ash and lime, has a tougher molecular structure than its modern equivalent. It's unusually resistant to fragmenting and nearly immune to the corrosion caused by salt water. That's why Roman jetties and port structures have weathered two salty millennia, while our maritime concrete creations degrade within a matter of decades.
So why aren't we doing as the Romans did?...One problem with ordinary Portland cement, which is used, along with pebbles and sand, to make most of the modern world's concrete, is that it requires an enormous amount of energy. The lime and clay mixture must be heated at over 2,500 degrees Fahrenheit, making the manufacture of cement the third-largest source of greenhouse gas emissions in the U.S., and responsible for 5 to 10 percent of carbon dioxide production worldwide  .In contrast the Roman method, recorded by Vitruvius, requires 10 percent less lime by weight and two-thirds the baking temperature, greatly reducing its environmental impact. It also has a molecular structure that ensures strength and longevity, meaning that, like the piers of the great Roman harbors, it seldom requires replacement. Aluminum-rich ash, reacting with lime and seawater, forms a highly stable mineral called tobormorite, which modern cement can only imitate.

Now, I'm not a geologist, nor am I a chemist, nor am I a process engineer.  But it would seem to me that one reason the cement the Romans used required less energy to make than today's Portland  cement does is because the Earth did a lot of the cooking already before it got blown out of the volcano.  Anyway, the mystery of the tobormorite is intriguing.