Material and Elastic Properties of <scp> <scp>Al</scp> </scp> ‐Tobermorite in Ancient Roman Seawater Concrete

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 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) Å interlayer (002) spacing. With prolonged heating to 350°C, 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 Å interlayer spacing is locally associated with phillipsite, 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.