Ancient Roman syntheses of Al-tobermorite in a 2000-year-old concrete block submerged in the Bay of Pozzuoli (Baianus Sinus), near Naples, have unique aluminum-rich and silica-poor compositions relative to hydrothermal geological occurrences. In relict lime clasts, the crystals have calcium contents that are similar to ideal tobermorite, 33 to 35 wt%, but the low-silica contents, 39 to 40 wt%, reflect Al3+ substitution for Si4+ in Q 2(1Al), Q3(1Al), and Q3(2 Al) tetrahedral chain and branching sites. The Al-tobermorite has a double silicate chain structure with long chain lengths in the b  crystallographic direction, and wide interlayer spacing, 11.49 Å. Na+ and K+ partially balance Al3+ substitution for Si4+. Poorly crystalline calcium-aluminum-silicate-hydrate (C-A-S-H) cementitious binder in the dissolved perimeter of relict lime clasts has Ca/(Si+Al) = 0.79, nearly identical to the Al-tobermorite, but nanoscale heterogeneities with aluminum in both tetrahedral and octahedral coordination. The concrete is about 45 vol% glassy zeolitic tuff and 55 vol% hydrated lime-volcanic ash mortar; lime formed <10 wt% of the mix. Trace element studies confirm that the pyroclastic rock comes from Flegrean Fields volcanic district, as described in ancient Roman texts. An adiabatic thermal model of the 10 m2 by 5.7 m thick Baianus Sinus breakwater from heat evolved through hydration of lime and formation of C-A-S-H suggests maximum temperatures of 85 to 97 °C. Cooling to seawater temperatures occurred in two years. These elevated temperatures and the mineralizing effects of sea-water and alkali- and alumina-rich volcanic ash appear to be critical to Al-tobermorite crystallization. The long-term stability of the Al-tobermorite provides a valuable context to improve future syntheses in innovative concretes with advanced properties using volcanic pozzolans.
KAUST Repository Item: Exported on 2020-10-01
Acknowledged KAUST grant number(s): KUS-11-004021
Acknowledgements: This research was supported by Award No. KUS-11-004021, from King Abdullah University of Science and Technology (KAUST) and the Loeb Classical Library Foundation at Harvard University. Data were acquired at beamlines 126.96.36.199, 188.8.131.52, and 12.2.2 at the Advanced Light Source at the Lawrence Berkeley Laboratories supported by the Director of the Office of Science, Department of Energy, under Contract No. DE-AC02-05CH11231, and the Advanced Nanofabrication Imaging and Characterization Laboratories at King Abdullah University of Science and Technology. We thank the ROMACONS drilling program: J.P. Oleson, C. Brandon, R. Hohlfelder, and CTG Italcementi researchers and staff, especially B. Zanga, in Bergamo, Italy; A.D. Kilcoyne and T. Tyliszczak at the 184.108.40.206, 220.127.116.11 beamlines; and S. Clark at the 12.2.2 beamline. T. Teague, D. Hernandez, B. Black, C. Hagen, and C. Kosso provided research support. We thank M. Sintubin, G. Sposito, R-A. Itty, P. Brune, and J. Kirz for critical discussions, and the anonymous reviewers whose comments improved the manuscript. H.-R. Wenk acknowledges support by NSF (EAR-0836402). The Berkeley Research Impact Initiative provided funds for Open Acess.
- Geochemistry and Petrology