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Weber, Johannes ao. Univ.-Prof. Dr.phil.

Methods of Polarising Microscopy &SEM to Assess the Performance of Nano-Lime...
conference proceedings
Electron microscopy, Conservation and Restoration
Attempts to evaluate the efficacy and harmlessness of a consolidation treatment for porous mineral materials have to deal with the task to measure relevant properties at sufficient in-depth resolution. Non- to low-invasive methods, such as drill resistance or ultrasound velocity measurements, prove useful in this context, but need eventually to be complemented by other means of analysis which provide more precise topographic and micromorphologic information. In view of this, the present study, performed in frame of the EU-project STONECORE, aimed to assess some of the relevant features related to a stone consolidant based on nano-lime by methods of microscopy. Focus is put on the in-depth distribution and the bonding properties of the consolidant after evaporation of the solvent, an issue of specific interest for the final result of a treatment. The treatments tested were performed on a sieve fraction of crushed stones and mortars from various sources. Thus, textural characteristics of the different materials could be largely eliminated, so that emphasis was put on the precipitation of the consolidant in dependence to the chemical and mineralogical nature of the substrates at a given mode of treatment. The analyses included polarising microscopy (PL) on thin-sections and scanning electron microscopy (SEM). By use of these methods, the consolidant could be well traced in the pore system of all samples, and quantitative data by digital image analysis on the rate of pore filling at varying depth from the surface of treatment could be calculated. The results show that, despite full penetration of all samples, the precipitation of the consolidant was partly governed by its backward migration towards the surface. Reducing the rate of evaporation could significantly contribute to achieve a more even distribution. For the given mode of treatment, substrates rich in quartz had especially high gradients of pore filling from the surfaces inwards. No clear impact of the zeta potentials on this effect could be established. In addition to the above, since the rate of carbonation of the consolidating lime was another issue of interest, an approach is presented to identify, again at high spatial resolution, the conversion of the calcium hydroxide to carbonate. Both PL and SEM proved useful in that respect. They can be employed to replace, in a more precise and significant way, the usual check of pH by liquid indicators. It was thus shown that moisture plays a key role in the formation of calcium carbonate from the hydroxide.
Johannes Weber, E.Ghaffari, T. Köberle
Columbia University N.Y.
published in
Proc.12th Internat Congr.on Det. and Cons. of Stone, New York, 22-26 Oct. 2012