Durability of crystalline monumental stones in terms of their petrophysical characteristics
Creators: name: Kovacs, Timea <1982>
Publisher: name: Alma Mater Studiorum - Università di Bologna
Year: 2009
Subjects: CHIM/12 Chimica dell'ambiente e dei beni culturali

description: The durability of stone building materials is an issue of utmost importance in the field of
monument conservation. In order to be able to preserve our built cultural heritage, the thorough
knowledge of its constituent materials and the understanding of the processes that affect them
are indispensable.
The main objective of this research was to evaluate the durability of a special stone type, the
crystalline stones, in correlation with their intrinsic characteristics, the petrophysical properties.
The crystalline stones are differentiated from the cemented stones on the basis of textural
features. Their most important specific property is the usually low, fissure-like porosity. Stone
types of significant monumental importance, like the marble or granite belong to this group. The
selected materials for this investigation, indeed, are a marble (Macael marble, Spain) and a granite
(Silvestre Vilachán granite, Spain). In addition, an andesite (Szob andesite, Hungary) also of
significant monumental importance was selected. This way a wide range of crystalline rocks is
covered in terms of petrogenesis: stones of metamorphic, magmatic and volcanic origin, which
can be of importance in terms of mineralogical, petrological or physical characteristics.
After the detailed characterization of the petrophysical properties of the selected stones, their
durability was assessed by means of artificial ageing. The applied ageing tests were: the salt
crystallization, the frost resistance in pure water and in the presence of soluble salts, the salt mist
and the action of SO2 in the presence of humidity.
The research aimed at the understanding of the mechanisms of each weathering process and at
finding the petrophysical properties most decisive in the degradation of these materials.
Among the several weathering mechanisms, the most important ones were found to be the
physical stress due to crystallization pressure of both salt and ice, the thermal fatigue due to cyclic
temperature changes and the chemical reactions (mostly the acidic attack) between the mineral
phases and the external fluids.
The properties that fundamentally control the degradation processes, and thus the durability of
stones were found to be: the mineralogical and chemical composition; the hydraulic properties
especially the water uptake, the permeability and the drying; the void space structure, especially
the void size and aperture size distribution and the connectivity of the porous space; and the
thermal and mechanical properties.
Because of the complexity of the processes and the high number of determining properties, no
mechanisms or characteristics could be identified as typical for crystalline stones. The durability
or alterability of each stone type must be assessed according to its properties and not according
to the textural or petrophysical classification they belong to.
Finally, a critical review of standardized methods is presented, based on which an attempt was
made for recommendations of the most adequate methodology for the characterization and
durability assessment of crystalline stones.