Historic mortar and plaster in Austria

Historical buildings are constructed by a bounded system of geo-materials, mortar and plaster of various compositions. Mortar and plaster are man-made materials. The composition comprises information about the historical environment with respect to the provenance of the materials, the processing, and the specific applications. The present study is focussed on carbonate and gypsum mortar and plaster of roman, medieval, and early modern (pre-industrial) times in Styria (Austria). The respective materials are well dated and characterised by archaeological investigations, but great gaps exist with regard to the respective mineralogical and chemical composition as well as to the microstructure. Modern methods of material science, geochemistry, and isotope chemistry will be applied for fundamental research on the aspects of historic environment of manufacturing and on secondary processes as re-crystallisation and weathering reactions. Powerful parameters are obtained by the investigation of the pore space system, by chemical and mineralogical analyses of the cements and aggregates, and by the distribution of sulfur, carbon, oxygen, and strontium isotopes. The specific signatures of the cements and aggregates are essentially related to the composition of the primary raw material and the historic manufacturing. Potential provenances of natural materials and a reconstruction of historic routes of transport are aligned to the local geology and the respective geographical position. Otherwise, the analysed composition and microstructure of historic mortars and plasters will be used to decipher the kind, quality, and durability of manufacturing under specific historic and economic aspects. Beyond natural deposits, which may be used as former raw materials, burned gypsum and limestone as well as the respective mortar and plaster will be analysed. These solids are freshly prepared by manufacturing and processing with reverse engineering of historic kilns and former recipes. Knowledge about historic recipes and preparation techniques as well as about the present structure and composition of mortar and plaster is required for an appropriate reconstruction and restoration of historic buildings.

Mortar and plaster of roman, medieval, and pre-industrial buildings as well as nearby natural limestone were collected in Styria/Austria. The aim is to acquire knowledge about the provenance of materials used, historical techniques, kind of processing, quality of burning, setting behaviour, age of formation, and secondary changes during history for historical mortar and plaster. Results from mineralogical, microstructural, geochemical and isotopic characterization can be used for fundamental research in geoscience and archaeology as well as for restoration under cultural heritage aspects. The historical mortar and plaster mostly consist of calcite as cement with aggregates of limestone, dolomite, quartz, and silicate fragments. Romans favoured calcitic limestone for burning and "dry" slaking for cement production, whereas in later periods of history dolomite and "wet" slaking were increasingly used. Aggregate types are mostly related to local available deposits. Provenance may be deduced by the occurrence and characteristics of fragments and minerals like quartz, dolomite, garnet, rutile, and zircon. In plaster small limestone aggregates, whereas in mortar and screed larger quartz and silicate fragments are most frequent. Romans often used brick fractions and flour which mostly caused high solidities due to its latent hydraulic behaviour. Historical epochs of formation can be infrequently distinguished by aggregate to cement ratios (A/C). Variation of A/C reflects individual historical processing e.g. useful for tailored restoration. Burning of limestone in reconstructed historical kiln furnitures and setting experiments show no significant effect on metal to calcium, 87Sr/86Sr, and Rare Earth Element distribution. Accordingly, such signatures can be successfully applied to decipher or exclude local deposits used for manufacturing and various construction periods. Stable isotopes of carbon and oxygen turned out to be suitable proxies to evaluate setting and secondary effects. Historical carbonate cements are mostly "isotopically lighter" compared to natural limestone and show an ideal linear isotopic relationship with an enrichment of the light isotope at the exterior versus interior mortar layer. 13C12C- and 18O/16O-distribution depend on kinetic fractionation caused by diffusion and hydroxylation of CO 2 and by mixture of oxygen isotopes from CO 2 and OH - , respectively. Setting and precipitation experiments confirmed a continuous enrichment of 13C and 18O of gaseous CO 2 along the cement setting path. Range and systematic correlation of isotopes reflect setting behaviour, burning quality, origin of CO 2 and H2 O, and re- crystallization. Selection of suitable cements for radio carbon dating was successfully done by the above stable isotope signatures. In several cases, archaeological ages were confirmed even if organic fragments were not available.