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- Title
Unlocking the secrets of Al-tobermorite in Roman seawater concrete.
- Authors
JACKSON, MARIE D.; SEJUNG R. CHAE; MULCAHY, SEAN R.; MERAL, CAGLA; TAYLOR, RAE; PENGHUI LI; EMWAS, ABDUL-HAMID; MOON, JUHYUK; SEYOON YOON; VOLA, GABRIELE; WENK, HANS-RUDOLF; MONTEIRO, PAULO J. M.
- Abstract
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 Q²(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 [020] 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 thickBaianus 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 seawater 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.
- Subjects
CALCIUM silicate hydrate; ALUMINUM; HYDROTHERMAL deposits; SILICA; CONCRETE research; POZZUOLANAS
- Publication
American Mineralogist, 2013, Vol 98, Issue 10, p1669
- ISSN
0003-004X
- Publication type
Article
- DOI
10.2138/am.2013.4484