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Research Article

Mechanical resilience and cementitious processes in Imperial Roman architectural mortar

View ORCID ProfileMarie D. Jackson, Eric N. Landis, Philip F. Brune, Massimo Vitti, Heng Chen, Qinfei Li, Martin Kunz, Hans-Rudolf Wenk, Paulo J. M. Monteiro, and Anthony R. Ingraffea
  1. Departments of aCivil and Environmental Engineering and
  2. hEarth and Planetary Science, University of California, Berkeley, CA 94720;
  3. bDepartment of Civil and Environmental Engineering, University of Maine, Orono, ME 04469;
  4. cDuPont Engineering Research & Technology, Wilmington, DE 19805;
  5. dSovrintendenza Capitolina Beni Culturali di Roma Capitale, Ufficio Fori Imperiali, Rome 00187, Italy;
  6. gLawrence Berkeley National Laboratory, Berkeley, CA 94720;
  7. iDepartment of Civil and Environmental Engineering, Cornell University, Ithaca, NY 14853;
  8. eSchool of Materials Science and Engineering, Southeast University, Nanjing 211189, China; and
  9. fSchool of Transportation Science and Engineering, Harbin Institute of Technology, Harbin, Heilongjiang 150090, China

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PNAS first published December 15, 2014; https://doi.org/10.1073/pnas.1417456111
Marie D. Jackson
Departments of aCivil and Environmental Engineering and
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  • ORCID record for Marie D. Jackson
  • For correspondence: mdjjackson@gmail.com
Eric N. Landis
bDepartment of Civil and Environmental Engineering, University of Maine, Orono, ME 04469;
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Philip F. Brune
cDuPont Engineering Research & Technology, Wilmington, DE 19805;
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Massimo Vitti
dSovrintendenza Capitolina Beni Culturali di Roma Capitale, Ufficio Fori Imperiali, Rome 00187, Italy;
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Heng Chen
Departments of aCivil and Environmental Engineering and
eSchool of Materials Science and Engineering, Southeast University, Nanjing 211189, China; and
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Qinfei Li
Departments of aCivil and Environmental Engineering and
fSchool of Transportation Science and Engineering, Harbin Institute of Technology, Harbin, Heilongjiang 150090, China
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Martin Kunz
gLawrence Berkeley National Laboratory, Berkeley, CA 94720;
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Hans-Rudolf Wenk
hEarth and Planetary Science, University of California, Berkeley, CA 94720;
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Paulo J. M. Monteiro
Departments of aCivil and Environmental Engineering and
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Anthony R. Ingraffea
iDepartment of Civil and Environmental Engineering, Cornell University, Ithaca, NY 14853;
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  1. Edited by David J. Killick, University of Arizona, and accepted by the Editorial Board October 27, 2014 (received for review September 12, 2014)

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Significance

A volcanic ash–lime mortar has been regarded for centuries as the principal material constituent that provides long-term durability to ancient Roman architectural concrete. A reproduction of Imperial-age mortar based on Trajan’s Markets (110 CE) wall concrete resists microcracking through cohesion of calcium–aluminum–silicate–hydrate cementing binder and in situ crystallization of platey strätlingite, a durable calcium-aluminosilicate mineral that reinforces interfacial zones and the cementitious matrix. In the 1,900-y-old mortar dense intergrowths of the platey crystals obstruct crack propagation and preserve cohesion at the micron scale. Trajanic concrete provides a proven prototype for environmentally friendly conglomeratic concretes that contain ∼88 vol % volcanic rock yet maintain their chemical resilience and structural integrity in seismically active environments at the millenial scale.

Abstract

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
  • volcanic ash mortar
  • fracture toughness
  • interfacial zone
  • strätlingite

Footnotes

  • ↵1To whom correspondence should be addressed. Email: mdjjackson{at}gmail.com.
  • Author contributions: M.D.J., E.N.L., P.F.B., M.V., H.-R.W., P.J.M.M., and A.R.I. designed research; M.D.J., E.N.L., P.F.B., M.V., M.K., H.-R.W., and A.R.I. performed research; M.D.J., E.N.L., P.F.B., H.C., Q.L., M.K., and A.R.I. analyzed data; and M.D.J., E.N.L., and P.F.B. wrote the paper.

  • The authors declare no conflict of interest.

  • This article is a PNAS Direct Submission. D.J.K. is a guest editor invited by the Editorial Board.

  • This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1417456111/-/DCSupplemental.

Freely available online through the PNAS open access option.

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Resilience of Imperial Roman architectural mortar
Marie D. Jackson, Eric N. Landis, Philip F. Brune, Massimo Vitti, Heng Chen, Qinfei Li, Martin Kunz, Hans-Rudolf Wenk, Paulo J. M. Monteiro, Anthony R. Ingraffea
Proceedings of the National Academy of Sciences Dec 2014, 201417456; DOI: 10.1073/pnas.1417456111

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Resilience of Imperial Roman architectural mortar
Marie D. Jackson, Eric N. Landis, Philip F. Brune, Massimo Vitti, Heng Chen, Qinfei Li, Martin Kunz, Hans-Rudolf Wenk, Paulo J. M. Monteiro, Anthony R. Ingraffea
Proceedings of the National Academy of Sciences Dec 2014, 201417456; DOI: 10.1073/pnas.1417456111
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