Methods of microscopy to identify and characterise hydraulic binders in historic mortars - a methodological approach

Abstract eng:
The contribution focuses on microscopic techniques for use in the identification and characterisation of historic mortars, with the assumption that the key information to understanding historic mortar is contained in the binder. While bulk chemical or gross phase analytical tools may provide preliminary information on the binder type used, imaging methods such as light and electron microscopy offer a more detailed assessment due to the possibility to study simultaneously mineral compounds, textures and microstructures of reacted and unreacted binder components, as well as their interaction with the other constituents of a mortar. This holds not only for traditional air lime based mortar systems, but most specifically also for all those containing cementitious compounds. The use of standard techniques of polarising light microscopy (PLM) and scanning electron microscopy (SEM) on thin sections to identify the binder constituents of hydraulic mortars is discussed. Residual cement grains indicative of high temperatures of formation (typical in Portland cement mortars) can be observed and classified by reflecting light PLM on polished sections eventually supported by staining techniques, even if present in only small amounts. On the contrary, natural or Roman cement mortars, in which the binder was calcined at low temperature, require the use of thin section PLM with transmitted light, possibly complemented by SEM-techniques. For hydraulic lime mortars, either of the above approaches can be optimal, depending on whether they were naturally or artificially mixed. The contribution presents a few examples of mortars made from each of the above binders and addresses additional observations, e.g. the leaching of the matrix by weathering agents.

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