Historical hydraulic mortars can comprise different reactive phases involved in the early- and mid-age development of binding
properties such as pozzolana, brick powder or other reactive silica and alumina. However, the specific nature of the reactive
phases in such binders is rarely clearly identified, particularly when different degradation or weathering patterns, frequently
related to the carbonation of the binder, interact with dissolution and precipitation reactions. The question of which binder
and aggregate components have contributed to the hydraulicity of a given historic mortar, a key to a better understanding
of ancient technologies, is frequently not investigated in the case of mortars which underwent the above mentioned alteration
processes. Microscopy provides a powerful analytical technique to determine the type of binders and differentiate between
primary and alteration phenomena.
By the example of three groups of binders, namely Roman opus caementitium/cocciopesto mortars from Ephesus (Asia Minor) and
Vindobona (Roman Vienna), a 16th C. Ottoman horasan masonry mortar from Budapest, and 19th C. highly hydraulic lime filling
mortars from Switzerland, the present article discusses two phenomena frequently observed when dealing with ancient hydraulic
mortars: firstly as to the source(s) of hydraulicity of the mortar and secondly, that after a long period of time exposed
to moist environments, the binder tends to present an inhomogeneous composition of an impure silica gel separated from calcium
carbonate, accompanied by leaching and precipitation of binder constituents.
These two phenomena are illustrated by polarized light microscopy (PLM) and scanning electron microscopy combined with energy-dispersive
x-ray spectroscopy (SEM-EDX) and discussed in light of both conservation and understanding of ancient structures as well as
how it can be applied to the current issues of alkali-aggregate reaction (AAR) and leaching in modern concretes.