Pozzolanic reaction of volcanic ash with hydrated lime is thought to dominate the cementing fabric\ud and durability of 2000-year-old Roman harbor concrete. Pliny the Elder, however, in first century CE\ud emphasized rock-like cementitious processes involving volcanic ash (pulvis) “that as soon as it comes\ud into contact with the waves of the sea and is submerged becomes a single stone mass (fierem unum\ud lapidem), impregnable to the waves and every day stronger” (Naturalis Historia 35.166). Pozzolanic\ud crystallization of Al-tobermorite, a rare, hydrothermal, calcium-silicate-hydrate mineral with cation\ud exchange capabilities, has been previously recognized in relict lime clasts of the concrete. Synchrotron-based\ud X-ray microdiffraction maps of cementitious microstructures in Baianus Sinus and Portus\ud Neronis submarine breakwaters and a Portus Cosanus subaerial pier now reveal that Al-tobermorite\ud also occurs in the leached perimeters of feldspar fragments, zeolitized pumice vesicles, and in situ\ud phillipsite fabrics in relict pores. Production of alkaline pore fluids through dissolution-precipitation,\ud cation-exchange and/or carbonation reactions with Campi Flegrei ash components, similar to processes\ud in altered trachytic and basaltic tuffs, created multiple pathways to post-pozzolanic phillipsite and\ud Al-tobermorite crystallization at ambient seawater and surface temperatures. Long-term chemical\ud resilience of the concrete evidently relied on water-rock interactions, as Pliny the Elder inferred. Raman\ud spectroscopic analyses of Baianus Sinus Al-tobermorite in diverse microstructural environments\ud indicate a cross-linked structure with Al3+ substitution for Si4+ in Q3\ud tetrahedral sites, and suggest\ud coupled [Al3++Na+\ud ] substitution and potential for cation exchange. The mineral fabrics provide a geoarchaeological\ud prototype for developing cementitious processes through low-temperature rock-fluid\ud interactions, subsequent to an initial phase of reaction with lime that defines the activity of natural\ud pozzolans. These processes have relevance to carbonation reactions in storage reservoirs for CO2 in\ud pyroclastic rocks, production of alkali-activated mineral cements in maritime concretes, and regenerative\ud cementitious resilience in waste encapsulations using natural volcanic pozzolans

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