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Proc Natl Acad Sci U S A. 2014 Dec 30;111(52):18484-9. doi: 10.1073/pnas.1417456111. Epub 2014 Dec 15.

Mechanical resilience and cementitious processes in Imperial Roman architectural mortar.

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Departments of Civil and Environmental Engineering and
Department of Civil and Environmental Engineering, University of Maine, Orono, ME 04469;
DuPont Engineering Research & Technology, Wilmington, DE 19805;
Sovrintendenza Capitolina Beni Culturali di Roma Capitale, Ufficio Fori Imperiali, Rome 00187, Italy;
Departments of Civil and Environmental Engineering and School of Materials Science and Engineering, Southeast University, Nanjing 211189, China; and.
Departments of Civil and Environmental Engineering and School of Transportation Science and Engineering, Harbin Institute of Technology, Harbin, Heilongjiang 150090, China.
Lawrence Berkeley National Laboratory, Berkeley, CA 94720;
Earth and Planetary Science, University of California, Berkeley, CA 94720;
Departments of Civil and Environmental Engineering and.
Department of Civil and Environmental Engineering, Cornell University, Ithaca, NY 14853;


The pyroclastic aggregate concrete of Trajan's Markets (110 CE), now Museo Fori Imperiali in Rome, has absorbed energy from seismic ground shaking and long-term foundation settlement for nearly two millenia while remaining largely intact at the structural scale. The scientific basis of this exceptional service record is explored through computed tomography of fracture surfaces and synchroton X-ray microdiffraction analyses of a reproduction of the standardized hydrated lime-volcanic ash mortar that binds decimeter-sized tuff and brick aggregate in the conglomeratic concrete. The mortar reproduction gains fracture toughness over 180 d through progressive coalescence of calcium-aluminum-silicate-hydrate (C-A-S-H) cementing binder with Ca/(Si+Al) ≈ 0.8-0.9 and crystallization of strätlingite and siliceous hydrogarnet (katoite) at ≥ 90 d, after pozzolanic consumption of hydrated lime was complete. Platey strätlingite crystals toughen interfacial zones along scoria perimeters and impede macroscale propagation of crack segments. In the 1,900-y-old mortar, C-A-S-H has low Ca/(Si+Al) ≈ 0.45-0.75. Dense clusters of 2- to 30-µm strätlingite plates further reinforce interfacial zones, the weakest link of modern cement-based concrete, and the cementitious matrix. These crystals formed during long-term autogeneous reaction of dissolved calcite from lime and the alkali-rich scoriae groundmass, clay mineral (halloysite), and zeolite (phillipsite and chabazite) surface textures from the Pozzolane Rosse pyroclastic flow, erupted from the nearby Alban Hills volcano. The clast-supported conglomeratic fabric of the concrete presents further resistance to fracture propagation at the structural scale.


Roman concrete; fracture toughness; interfacial zone; strätlingite; volcanic ash mortar

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