Using a naturally hydraulic lime is important for creating weaker joints, but it doesn't save that much CO2. You only heat the limestone a few hundred degrees c

Using a naturally hydraulic lime is important for creating weaker joints, but it doesn't save that much CO2. You only heat the limestone a few hundred degrees cooler (though for longer) and you don't pulverize it as much, but hydraulic lime is using the minerals mixed with it to harden just like OPC (which is why it can harden in such thick pieces like the base of the Pantheon), not pull CO2 from the atmosphere like a non-hydraulic lime.