Making Ancient Mortars Hydraulic. How to Parametrize Type and Crystallinity of Reaction Products in Different Recipes

Abstract

The hydraulic features of lime-based mortars are primarily determined by the occurrence in the raw materials of variable amounts of reactive silica (SiO2) and alumina (Al2O3) that, in presence of water, interact with lime (CaO) to form different hydrated products (C-A-H; C-A-S-H; C-S-H). Under certain conditions, other hydrated products, based on the interaction of magnesium with silica (and sometimes also with alumina) can occur (M-S-H/M-A-S-H). The target of this research is the analysis and comparison of the characteristics and structure of calcium-based silico/aluminate hydrates and magnesium-based silico/aluminate hydrates in ancient mortars and concretes. We adopted a multianalytical approach, involving Optical Microscopy (OM), X-Ray powder diffraction (XRPD), scanning electron microscopy and energy-dispersive microanalysis (SEM-EDS) and magic-angle spinning solid-state nuclear magnetic resonance (MAS-SS NMR), for characterizing samples with different composition and structural function from the sites of Aquileia (Northern Italy), Nora (Sardinia) and Pompeii (Naples). The results we obtained demonstrate the occurrence of different hydraulic phases in the mortar-based materials, whose development primarily depends on the array of raw materials used in the manufacturing of the compounds. However, the observed hydraulic phases present a prevalent gel-like structure, that was not adequately parametrized and quantified adopting the analytical approach traditionally employed for the characterization of ancient mortars (i.e. OM, XRPD, SEM-EDS). In this perspective, the utilization of MAS-SS NMR resulted crucial in characterizing the chemical environment of hydraulic phases, and C-S-H in particular, by discriminating the degree of polymerization of the silicate tetrahedra even in disordered structures.