Ordinary Portland cement gains strength quickly. That makes it the natural choice for columns, beams and slabs where you want to strike formwork and keep the frame moving.
Portland pozzolana cement swaps about a third of the clinker for fly ash. It sets a little slower and runs cooler, and the concrete ends up denser and more resistant to water and chemicals.
A simple rule works on most sites: use OPC above ground where speed matters, and PPC below ground, in mass pours and in plaster. If you are unsure, send us your drawings and an engineer will mark them up.