The mortar of the "Leaning Tower" of Pisa: the product of a medieval technique for preparing high-strength mortars

Thirty mortar samples from the "Leaning Tower" of Pisa were examined. The s pecimens include mortars from both the first (1173-1178) and second (1272-1 278) stages of Tower's construction. The mineralogical, petrographical, che mical and physical data collected on the bulk mortar and its binder show th at the famous "Leaning Tower" of Pisa was built through the constant use of a high-quality hydraulic mortar (average compressive strength about 16 N/m m(2); average binder SiO2 content about 29 %) as the binding agent for the "infill" masonry. Although a great deal of variability exists in the values of each measured property, even amongst samples from the same stage of con struction, the averaged values for the foundations and the above-ground str uctures erected during the two distinct construction stages are highly unif orm. The most conspicuous differences lie in the sand content and granulome try, which are respectively greatest and coarsest in the foundation mortars . Chemical and mineralogical data suggest that the Tower mortar was prepare d by mixing slaked lime, obtained from an almost pure limestone, with sand from Arno and Serchio Rivers alluvium and a diatomaceous earth, probably qu arried at Mt. Amiata, about 180 km to the south of Pisa. It is estimated th at the construction of the Tower from the foundations to the top of the 7(t h) storey (the belfry, added to the main structure only around 1365, is not considered here) required the use of about 1400 tons of slaked lime (assum ing 60% water content), 1350 tons of sand and 400 tons of diatomite.