Optical and Electronic Microscope for Minero-Petrographic and Microchemical Studies of Lime Binders of Ancient Mortars

Abstract

In this paper, the advances in the use of optical and electronic microscope for study of the minero-petrographic and microchemical features of lime binders of ancient mortars are discussed for various case studies. Mortars belonging to several historic periods and with different functions in building structures and archaeological sites were selected in order to verify the complementarity of optical and electronic microscope analyses applied to these artificial materials. The data obtained with the application of optical and microscope analyses were able to provide detailed and more precise information on the composition, structure, and texture of lime binders, highlighting the features of air hardening calcitic lime binder, air hardening magnesian lime binder, natural hydraulic lime binder, and air hardening binders with materials providing hydraulic characteristics added. Furthermore, a complete analysis and classification of the lime lumps was determined.

The San Giovanni Baptistery in Florence (Italy): Assessment of the State of Conservation of Surfaces and Characterization of Stone Materials

“…In the case of marbles, the petrographic observation allowed for the identification of MGS (maximum grain size), the fabric type (homeoblastic or heteroblastic), the main features of microstructure, and the crystal boundaries shapes, identified as the most relevant petrographic parameters for provenance determination [6,8]. For mortars, the petrographic approach allowed for a complete characterization of each component (binder, aggregate, and inorganic additives), as well as the identification of raw materials and technologies used for their production [17,19,20]. In the case of bricks, the optical characteristics of groundmass, the presence and composition of framework, and the quantification of macroporosity could be obtained [21,22].…”

“…A calcium carbonate polymorphous, vaterite, was individuated in samples S7 (most superficial sample), S12, and S13. Vaterite was typically found in the Florentine mortars, associated with the production of natural hydraulic binders obtained by burning marly limestone [17,19,37]. The presence of portlandite (Ca(OH) 2 ) in the deepest sample of S4 was noted, linked to an incomplete carbonation of binder.…”

“…All mortar samples suffered a strong recrystallization of the binder in the form of spathic calcite. In different depths of S5, some differences in the aggregate characteristics were present: S5 (7-12) cm was composed of abundant binder/aggregate (B/A 1/3) and fine grain size predominant, while S5 (12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22)(23)(24)(25)(26) cm and S5 (26)(27)(28)(29)(30)(31)(32)(33)(34)(35)(36)(37)(38) cm were characterized by less binder/aggregate (B/A 1/2-1/3), and a fine-medium size was predominant (Figure 9e). Furthermore, a different amount of carbonate rock fragment was registered, with respect to the attic mortar samples (a higher entity in S5 samples).…”

“…The TGA results confirm the hydraulic behavior, as suggested by the presence of lumps of unburnt Alberese limestone in petrographic observations. The firing of marly limestone provides natural hydraulic lime, typically produced in the Florentine area [19,[37][38][39]. Finally, the mortar samples show, mainly, the presence of lumps which are typical of Alberese limestone, suggesting the use of this stone to produce lime.…”

“…The sample, collected from the mosaic masonry, is a hydraulic lime mortar used for the addition of cocciopesto. This artificial material, obtained by crushing bricks or ceramic, reacts with the air hardening lime binder, developing amorphous hydrated calcium silicates and hydrated calcium aluminates which are able to set under conditions of high humidity or under water, thereby allowing the preparation of mortars with good mechanical characteristics and durability [19,20]. In the past, cocciopesto was mainly used for the preparation of mortars with different functions (floors, plasters, tanks, aqueducts) when natural pozzolans were unavailable, and has also been found in the preparation of mosaic layers [40].…”