The Pantheon looks pretty good for a 1900 year old building, considering that it is the largest unreinforced concrete dome in the world. Perhaps it's because it was not reinforced, so there was no iron to rust and expand, or perhaps because Roman concrete was different than the stuff we use today. TreeHugger has noted before that Roman concrete was a whole lot greener than today's mixes; now a new study by researchers at the Berkeley Lab shows that the concrete actually gets stronger over time.
Unlike modern concrete which actually shrinks, opening up tiny cracks that propagate and let moisture in, Roman concrete, made with volcanic ash instead of portland cement, is actually self-healing as a crystalline binder forms and prevents the concrete from cracking any further. According to Marie Jackson of UC Berkeley:
The mortar resists microcracking through in situ crystallization of platy strätlingite, a durable calcium-alumino-silicate mineral that reinforces interfacial zones and the cementitious matrix. The dense intergrowths of the platy crystals obstruct crack propagation and preserve cohesion at the micron scale, which in turn enables the concrete to maintain its chemical resilience and structural integrity in a seismically active environment at the millennial scale.
So not only would concrete made with volcanic ash have a much lower carbon footprint, It would last a lot longer. Jackson continues in a more comprehensible tone: