Teachers and Employees

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

Roman cement mortars in Europe’s architectural heritage of the 19th century
Conservation and Restoration, Architecture (Fine Arts/Design), Chemical process engineering
Natural cements calcined at low temperatures, so-called Roman cements, formed an important binder material in 19th century construction and façade decoration of many European cities and towns. As a rule, Roman cement mortars appear well preserved. In order to understand their range of composition and properties, a number of samples from historic buildings were collected and analyzed. Microscopical techniques including scanning electron microscopy were employed along with the assessment of physico-mechanical properties. The study shows that the most significant feature of Roman cement mortars is a wide range of differently calcined clinker relicts within each mortar. They were recognized as either underfired or overfired resp. poorly dispersed, and obviously play an important role for the mortar properties. The binder relicts are composed of a number of phases in the system Ca-Si- Al-Fe. C2S and C2AS (gehlenite) are amongst the most frequent compounds, their grain size and microstructure depending on calcination temperatures. In particular, the clinkers produced at lower temperatures show non-equilibrium features such as solid solution systems and zoning by partial diffusion. The aggregates found in the mortars cover a wide range of mineralogical compositions reflecting local geological conditions. Cast and in-situ applied mortars differ in the amount of aggregate which is generally lower for cast elements. There is no correlation between the amount of inert material and the state of preservation indicated e.g. by the occurrence of shrinkage cracks. Historic cast mortars show high compressive strengths at comparably low moduli of elasticity. The total porosity is frequently high. The contribution presents the above-mentioned properties and discusses them in terms of the excellent ageing performance of the historic Roman cement mortars.
Johannes Weber, Gadermayr, Hughes, Kozlowski, Stillhammerova, Ullrich, Bayer, Vyskocilová
JAI - Journal of ASTM International
Washington DC (USA)