Preliminary studies of the “Artificial Stones” utilized in Florence between the XIX-XX centuries

Some historic buildings in Florence have ornamental elements (for instance stringcourse or coats of arms), ashlars, or the complete façade, realized in "artificial stone"; this is a characteristic of the period between the XIX-XX centuries, contemporary to the increasing use of the modern hydraulic binders (cements). The "artificial stone" is a mortar constituted by lime or hydraulic binders mixed with aggregate and water; this mortar can reach the hardness and appearance of a natural stone (for instance sandstone). The word "artificial stone" represents either a mortar worked directly in façade or an ornamental element prepared in mould. The main purpose of this work has been to characterize the mixture utilized to produce the "artificial stone" used in some historic palaces of Florence with a particular care to the determination of the kind of binder used for its realization. The following analytical methodologies have been used both on the total sample (binder + aggregate) and on the binder fraction: ultrasonic waves to separate the binder from aggregate considering the fraction <63 micron as belonging to the binder; stereomicroscopy to give a macroscopic description of the samples; thin section observations under polarized light for a mineralogical-petrographical characterization; XRD analysis to determine the mineralogical composition; thermal analysis (TG) to reveal thermal transformation (for instance dehydration and decomposition) that give informations on the compounds of the binder (the diagram CO2 versus CO2/H2O permits the classification of the mortars in different groups, as typical lime mortars, hydraulic lime mortars etc...); XRF analysis for the determination of the chemical composition in major elements, to calculate the Hydraulicity Index. The preliminary results show that all the mortars are characterized by a brown binder with a granular aspect and low birefringence. The aggregate is made by a well sorted siliceous sand (diameter 500micron). The ratio binder/aggregate ranges between 3/2 and 1/3. In some samples the presence of Larnite, a calcium silicate characteristic of cement mortars, has been surveyed. Moreover in the binder analysis a strong weight loss has been detected near the temperature of 600°C, caused by the decomposition of calcite present as a carbonation product of the Ca(OH)2 which on its turn comes from the hydration reaction of C3S,and C2S. The Idraulicity Index is higher than that of Portland cement. These data represent important preliminary results about the “artificial stone” utilized in Florence (XIX-XX centuries). However it’s necessary to sample other florentine palaces and to carry out other specific analyses for a better characterization, as: SEM/EDS image analyses, calculation of physical characteristic (total open porosity), DRMS (Drilling Resistant Measurement System) etc...