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