On the binder of some ancient mortars

About 90 samples of mortars from historical buildings in Pisa have been ana lysed and compared with some samples of ancient mortars characterized by po zzolanic aggregates. Chemical (XRF), mineralogical (XRD) and petrographical (optical microscopy) data have been collected on bulk samples. An X-ray en ergy-dispersive system (EDS) attached to a SEM was used to determine the ch emical compositions of binder, clots and pozzolanic grains (whenever presen t). The binder components which could not be directly assessed (CO2, H2O+) have been indirectly derived through a computation method. Data collected suggest that the main stock of analysed samples (i.e., morta rs from Pisa monuments) contains two principal types of binder: the first t ype corresponds to a common carbonated lime; the second type is a hydraulic lime, where the carbonate component is associated with an important silica te fraction, which constitutes 13 to 89 wt% of the total binder ton average 47%). This hydraulic type had a widespread application in the construction of ancient monuments in Pisa; for example, it was constantly employed in t he building of the famous "Leaning Tower". This study shows that the silica te fraction of such hydraulic mortar, which is amorphous to X-ray diffracti on, is made up by a hydrated calcium alumino-silicate having a very low Al2 O3/ SiO2 ratio ton average 0.125). On the basis of chemical and optical characteristics of hydraulic mortar bi nders from Pisa and the results of laboratory tests, as well as through com parison with the characteristics of ancient hydraulic mortars having pozzol anic aggregates, it is concluded that such mortars were prepared most likel y by mixing slaked lime with a reactive, highly siliceous material such as a diatomaceous earth.