Multi-Analytical Study of Lime-Based Mortars from the 16th-Century Venetian Fortress of Bergamo (Italy)

Mortars taken from the 16th century Venetian Fortress of Bergamo (Italy) were characterized (binder-concentrated fractions and aggregate fractions as well as bulk samples) with a multi-analytical approach using X-ray diffraction (XRD), inductively coupled plasma optical emission spectrophotometry (ICP-OES), optical microscopy (OM), differential scanning calorimetry (DSC) and thermogravimetric analysis (TG). The results showed the presence of calcite, hydrocalumite and hydrotalcite-type compounds, brucite, aragonite, plombierite and a large fraction of amorphous phases (ranging between 14 and 27 wt%) in the binder. Quartz and carbonate-rich sands were used as aggregates. The mortar is a Mg-rich material containing 4–5 wt% brucite. No evidence of magnesite or hydromagnesite was found in any sample, although these phases are frequently detected in the binder of buildings from the Renaissance period that are located in Northern Italy. The large average amount (12–13 wt%) of reactive silicate, such as Mg-containing phyllosilicates that can react with lime, and the presence of carbonate-containing hydrocalumite and hydrotalcite indicate hydraulic interactions between lime and reactive silicate aggregates. The CO2/H2Obound ratio, evaluated from the weight loss referred to the finer fraction (<63 μm), ranges from 1.99 to 2.55, which suggests that the walls of Bergamo were constructed using lime-based mortar with hydraulic properties.