Investigation of the concrete’s binder from the Augustus’ Bridge through the micro-LIBS analysis of the thin sections

The Augustus's Bridge was built by the Romans in the 27 b.C. along the Flaminia way to cross the river Nera, during the works of enlargment of the consular way. The arches the piers and the abutments are made by a framework of well cut travertine ashalars filled with high quality mortars characterized by relative low porosity high hydraulicity and strong mechanical characteristics. An accurate knowledge of the concrete's binder composition is therefore essential both for restoration purpose and for enabling a substantial improvement of the modern binders used at the present. However the determination of the binder's composition is often difficult due to the nature of the mortars, constituted by a microscopic mixture of aggregate and binder which can be hardly selectively separated. In this work a new approach was attempted by applying the Laser Induced Breakdown Spectroscopy (LIBS) technique onto concrete's thin sections from the Augustus's Bridge, in particular to evaluate the hydraulic index. To this aim, a novel micro LIBS apparatus was arranged where Nd:YAG LASER PULSE (1064 nm, 10 ns FWHM) was focussed on the thin section surface by means of a microscope 10X objective, which allowed a tight focussing of the laser beam. The choice of a laser energy was determined by a compromise between a realtive small damage on the sample (spot diameter 30 micron) and acceptable LIBS signal obtained. The microscope equipped with polarizer filters, allowed a visula inspection of the focussing conditions during the measuring process. A feasibiliy study of such approach was made by optimizing the experimental setup, by building calibration curves for Si, Mg, Fe, and Al (necessary for the calculation of the hydraulicity index) and by comparing the obtained quantitative LIBS results with those provided by SEM-EDS analyses.