Reactive binder and aggregate interfacial zones in the mortar of Tomb of Caecilia Metella concrete, 1C BCE, Rome

Integrated spectroscopic analyses and synchrotron X‐ray microdiffraction investigations provide insights into the long‐term reactivity of volcanic aggregate components and calcium‐aluminum‐silicate‐hydrate (C‐A‐S‐H) binder in mortar samples from the robust concrete of the sepulchral corridor of the Tomb of Caecilia Metella, 1st C BCE, Rome. The results of innovative micrometer‐scale analytical maps indicate that Pozzolane Rosse tephra components–scoria groundmass, clinopyroxene, and leucite crystals–contributed to pozzolanic production of C‐A‐S‐H binder and then remained reactive long after hydrated lime (Ca(OH) 2 ) was fully consumed. The C‐A‐S‐H binding phase is reorganized into wispy halos and tendril‐like strands, some with nanocrystalline preferred orientation or, alternatively, split into elongate features with short silicate chain lengths. These microstructures apparently record chemical and structural destabilization of C‐A‐S‐H during excessive incorporation of Al 3+ and K + released through leucite dissolution. Resistance to failure may result from the intermittent toughening of interfacial zones of scoriae and clinopyroxene crystals with post‐pozzolanic strätlingite and Al‐tobermorite mineral cements and from long‐term remodeling of the pozzolanic C‐A‐S‐H binding phase. Roman builders’ selection of a leucite‐rich facies of Pozzolane Rosse tephra as aggregate and construction of the tomb in an environment with high surface and ground water exposure apparently increased beneficial hydrologic activity and reactivity in the concrete.