Is Roman concrete better? And if so, why aren't we using it?

The Pantheon: Constructed of Roman Concrete, without reinforcing steel

Roman concrete has stood the test of time (the Pantheon is still standing and in use and it is built of Roman concrete).  Modern concrete is a mix of sand, gravel and Portland cement.  Portland cement is mostly baked pulverized limestone.  The Romans used a mixture with far less Portland, substituting volcanic fly ash and pumice, and what cements they did used did not have to be baked as long.  The materials the Roman's used are common world wide.  Beyond arguably creating less carbon emissions, the Roman concrete is extremely durable.   I am a skeptic on climate change, but any method that involves less energy to make and results in a better more durable product is definitely worth looking at.  

As noted in this Wikipedia article on Roman Concrete:

Volcanic dusts, called Pozzolana or "pit sand", were favored where they could be obtained. Pozzolana makes the concrete more resistant to salt water than modern day concrete.^[4] The pozzolanic mortar used had a high content of alumina and silica.

Which makes sense.  Why make Portland when volcanoes around the world have essentially made a ready substitute?

A core of Roman concrete from some ancient concrete sea breakers just north of Naples

The Pantheon: constructed of Roman concrete by Marcus Agrippa during the reign of Augustus (it was substantially rebuilt by Hadrian).
Still the world's largest unreinforced concrete dome structure.  

Update:  Maetenloch at AoSHQ had this link.