Material and Elastic Properties of Al-Tobermorite in Ancient Roman Seawater Concrete

摘要

The material characteristics and elastic properties of aluminum-substituted 11 A tobermorite in the relict lime clasts of 2000-year-old Roman seawater harbor concrete are described with TG-DSC and ~(29)Si MAS NMR studies, along with nanoscale tomography, X-ray microdiffraction, and high-pressure X-ray diffraction synchrotron radiation applications. The crystals have aluminum substitution for silicon in tetrahedral bridging and branching sites and 11.49(3) A interlayer (002) spacing. With prolonged heating to 350℃, the crystals exhibit normal behavior. The experimentally measured isothermal bulk modulus at zero pressure, K_0, 55 ±5 GPa, is less than ab initio and molecular dynamics models for ideal tobermorite with a double-silicate chain structure. Even so, K_0, is substantially higher than calcium-aluminum-silicate-hydrate binder (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 A interlayer spacing is locally associated with phil-lipsite, similar to geologic occurrences in basaltic tephra. The ancient concretes provide a sustainable prototype for producing Al-tobermorite in high-performance concretes with natural volcanic pozzolans.