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

Identification of 19th century Roman cements by the phase composition ....
conference proceedings
Chemical process engineering, Conservation and Restoration, Electron microscopy
The term “cement” in its historic context stands for several groups of highly hydraulic binders which may differ in the temperatures of calcination and hence yield mortars of significantly differing properties. A key to identify the exact type of binder used in a historic cement mortar consists in the textural and mineralogical features of unhydrated residual clinker particles. Employing various techniques of light and scanning electron microscopy allows define fingerprints for given cements, a task hardly addressed in earlier studies on natural cements calcined at low temperature. The present contribution focuses on the most characteristic residual clinker phases found in historic Roman cement mortars, a group of materials widely used in the 19th century urban architecture and civil engineering. These binders were produced from natural marlstones through shaft kiln calcination at temperatures virtually below sintering. High amounts of non-crystalline reactive compounds form in this low temperature regime. Along with fine-grained impure C2S, they have developed into a relatively homogeneous hydrated matrix. In parallel, however, non- or low reactive compounds have assembled together in binder-related nodules characteristic for Roman cement mortars. Most of these compounds are of non-crystalline nature, they comprise solid solution silicate phases as well as crystalline CS (wollastonite), coarse C2S and C2AS (gehlenite). By means of thin section and reflected light microscopy, combined with scanning electron microscopy and X-ray microanalysis, the phase assemblages can be observed and identified in virtually all of the historic mortars. Various classes of residues were distinguished within the above general frames. According to their texture and mineral content, they are classified into overfired or underfired in respect to the optimum temperature of calcination. Optimum clinker assemblages with high amounts of reactive phases may form nodules as well, characterised by their specifically dense hydrate structure. Following a general description of the above mentioned classes, the contribution presents a group of samples from historic façades in the city of Budapest as an example of the usefulness of the classification in the practice of mortar analysis. The data suggest that different brands of Roman cements characterised by specific setting times may have existed and used on purpose for specific mortar applications. The paper aims at providing information useful for the identification and characterisation of historic natural mortars.
Johannes Weber, N. Gadermayer, F. Pintér
Columbia University N.Y.
New York (USA)
published in
Proc.12th Internat Congr.on Det. and Cons. of Stone, New York, 22-26 Oct. 2012