Jackson, M.D.; Moon, J.; Gotti, E.; Taylor, R.; Chae, S.R.; Kunz, M.; Emwas, A.-H.; Meral, C.; Guttmann, P.; Levitz, P.; Wenk, H.-R.; Monteiro, P.J.M.: Material and Elastic Properties of Al-Tobermorite in Ancient Roman Seawater Concrete. Journal of the American Ceramic Society 96 (2013), p. 2598-2606
10.1111/jace.12407

Abstract:
The material characteristics and elastic properties of aluminum-substituted 11 Å tobermorite in the relict lime clasts of 2000-year-old Roman seawater harbor concrete are described with TGDSC and 29Si MAS NMR studies, along with nanoscale tomography, X-ray microdiffraction, and high pressure X-ray diffraction synchroton radiation applications. The crystals have aluminum substitution for silicon in tetrahedral bridging and branching sites and 11.49(3) Å interlayer [002] spacing. The experimentally measured bulk modulus, 55±5 GPa, is less than ab initio and molecular dynamics models for pure tobermorite with a double-silicate chain structure, perhaps because Al3+ substitution for Si4+ produces increased interlayer spacing and weakened bonds at bridging and branching sites. Even so, Ko, is substantially higher than C-A-S-H in slag concrete. Based on nanoscale tomographic study, the crystal clusters form a well connected solid, despite having about 52% porosity. In the pumiceous cementitious matrix, Al-tobermorite with 11.27 Å interlayer spacing is locally associated with phillipsite, similar to geologic occrrences in basaltic tephra. The ancient concretes provide a sustainable prototype for producing Al-tobermorite in high performance concretes with natural volcanic pozzolans.