Nineteenth Century “Novel” Building Materials: Examples of Various Historic Mortars Under the Microscope

Starting in the late eighteenth century, the age of industrialisation brought about a number of new binder systems of hydraulic nature meant to answer the demand for improved mortar strength even at moist conditions. Such mortars form an important part of today’s architectural heritage and are therefore frequently encountered, either as primary materials or as historic restoration mortars, when historic objects are studied in the course of restoration. This paper deals with the basic features of a selection of those cementing materials which were “novel” at their time: Sorel cements, iron hammer scale mortars, and natural Roman as well as early Portland cements. The analytical approach followed is based on light and scanning electron microscopy, believed to provide primary tools to identify the mortars and to understand some of their key properties. Sorel cement mortars were studied at examples of flooring and stone repair mortars, respectively. Both were frequently encountered applications of this binder prepared from a mixture of caustic magnesia and magnesium chloride. In both types of mortar, the binder appears composed mainly of magnesium oxychloride hydrates, whereas the matrix porosity varies from highly porous to dense. The aggregates range in composition from wood fibres to stone fragments, reflecting the capacity of Sorel cement to yield strong mortars with almost any kind of filler. Hammer scale mortars were prepared by adding files of metallic or oxidic iron to lime and sand. They were used for fillings or joints in masonry where they developed significant levels of hardness and strength. The observations by microscopy show that migration of rust products from the iron scales into the surrounding lime matrix is a key factor contribution to the high strength properties of such mortars. Nineteenth century Roman cements were produced from argillaceous limestone at temperatures as low as about 900°C. In their mineralogical composition they differ significantly from NHL and Portland cements. Consequently, Roman cements follow a specific path of hydration, generally yielding highly porous mortars of considerable strength. Whilst light microscopy is a good tool to identify Roman cement mortars by the abundant presence of binder-related particles, the SEM observations help understand their properties, as the calcium silicate hydrates formed in a Roman cement paste are of a coarse nature with card-house-like intergrowth. Early Portland cement mortars produced in the second half of the nineteenth century form the last group of materials addressed in this contribution. By the coarse size of their clinkers they can be easily identified in thin sections, however the lower amount of alite, as compared to modern Portland cements, is obvious. This observation is discussed in terms of lower temperatures of calcination as compared to modern successors.