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 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 ºC. 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.

We report various synchrotron radiation laboratory based techniques used to characterize cement based materials in nanometer scale. High resolution X-ray transmission imaging combined with a rotational axis allows for rendering of samples in three dimensions revealing volumetric details. Scanning transmission X-ray microscope combines high spatial resolution imaging with high spectral resolution of the incident beam to reveal X-ray absorption near edge structure variations in the material nanostructure. Microdiffraction scans the surface of a sample to map its high order reflection or crystallographic variations with a micron-sized incident beam. High pressure X-ray diffraction measures compressibility of pure phase materials. Unique results of studies using the above tools are discussed—a study of pores, connectivity, and morphology of a 2,000 year old concrete using nanotomography; detection of localized and varying silicate chain depolymerization in Al-substituted tobermorite, and quantification of monosulfate distribution in tricalcium aluminate hydration using scanning transmission X-ray microscopy; detection and mapping of hydration products in high volume fly ash paste using microdiffraction; and determination of mechanical properties of various AFm phases using high pressure X-ray diffraction.

This study aimed to investigate the chloride-binding capacity of alkali-activated fly ash (denoted as FSW) and slag (denoted as SSW) samples and their synthesized Cl-bearing phases, which are capable of binding and immobilizing chloride when seawater is used as the mixing water. This study also examined the progressive changes in the pore structures of the FSW and SSW samples over time. The results show that the SSW sample is significantly more effective in the uptake of chloride ions compared to the FSW sample at 28 days of curing. While the FSW sample forms Cl-bearing zeolites (Cl-chabazite and Cl-sodalite) (possibly with similar types of geopolymeric gels), the SSW sample synthesizes Cl-bearing, layered double hydroxides (LDH) (Cl-hydrocalumite and Cl-hydrotalcite). Although both samples involve Cl-binding phases, the FSW sample is likely to be less efficient because it largely produces zeolites (or similar geopolymeric gels) with no Cl-binding capability (i.e., zeolites X and Y). Meanwhile, the SSW sample produces Cl-bearing LDH phases as well as C-S-H(I), which can physically adsorb chloride. The SSW sample exhibits both pore-size refinement and porosity reduction over time, while the FSW sample only exhibits pore-size refinement. Therefore, the SSW system is more advantageous in the use of seawater because it more effectively prevents Cl ingression due to greater impermeability.