MAKING ANCIENT MORTARS HYDRAULIC. HOW COMPOSITION INFLUENCES TYPE AND CRYSTALLINITY OF REACTION PRODUCTS

The hydraulic characteristics in a lime-based mortar are primarily determined by the occurrence in the raw materials of a variable amount of reactive silica (SiO2) and alumina (Al2O3) that, in presence of water, interact with lime (CaO) to form different hydrate products (C-A-H; C-A-S-H; C-S- H). At certain conditions, other hydrate 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 for characterizing samples from the sites of Aquileia (Northern Italy), Nora (Sardinia) and Pompeii (Naples), having different composition and collected from structures with different function. The results we obtained demonstrate the occurrence of different hydraulic phases in mortar-based materials of the investigated sites. The crystallinity degree of hydraulic phases primarily depends on the composition of raw materials used in the compounds. The accuracy devoted in the treatment of raw materials and in the making of the compounds are other factors influencing the hydraulic properties of the resulting mortars, as well as the pH of the system.