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Content archived on 2024-04-16

Conservation of granitic rocks : application to megalithic monuments


The objective is to improve knowledge of the degradation phenomena and to support the development of more effective procedures for the conservation of granitic materials in general, and of the specific aspects related to the megalithic monuments in particular.
A study of the decay mechanisms in granitic rock dolmens have demonstrated that exposed stones suffer from accelerated decay and in some pieces, granite is losing the feldspars, plagioclases and mica and is becoming enriched in quartz. Going along with a substantial loss of material, rugosity and fissuration increase, water penetration becomes easier and decay mechanisms are enhanced.

In polluted areas black crust were observed which consisted of 2 main types of dark patinas: a thick gypsum crust and a thin dark patina. Thick gypsum crusts developed in sulphur dioxide polluted urban areas and their morphology is similar to crusts described for carbonate stones. The thin dark patinas are composed of mineral fragments detached from the stone, dust particles, iron rich particles and anthropogenic particulates imbedded in an amorphous iron rich matrix. Biotite is the most likely source of iron. The contacts with the underlying stone are often gradational, with the patinas actively growing at the expense of the stone. Biotite and feldspars are good substrates for lichen growth and lichen thalli can penetrate deep into rock.

The quality and integrity of stone pieces from monuments or outcrops was evaluated by means of nondestructive tests. Recently, trials using tomographic characterization have allowed images of the stone interior to be obtained.

A study of stone conservation treatment was carried out using alkylsilane, polysiloxane, styrene acrylic resin and polyfluorocarbon resin. The harm potential of these products was distinct and measurements of water vapour permeability was adequate to quantify it. There were also modifications in the stone appearance.
The technical work consists of in situ inspections of monuments, identification and description of pathologies and selection of monuments for detailed studies. Laboratory work is carried out on material sampled directly in the monuments and in surrounding outcrops and existing quarries.

In situ trials with nondestructive characterizing methods are performed. Identification of differentially decayed zones, mapping of results and the development of a suitable technique for discriminating zones in different weathering stages in granitic rocks will lead to decisions on conservation operations.

To understand the multiple phenomena involved in stone degradation, some research on the basic principles of moisture and heat transport in porous media and their application to granitic rocks will be carried out. Laboratory studies will be conducted on the identification and description of irreversible phenomena in stones under current environment and service conditions, physical and mathematical modelling of effects induced by heat and fluids acting in the stone aiming at finding explanations for actual decay forms found in granitic rocks.

Biodegradation of granite rocks will also be studied. The classification of colonizing species and their habitats as well as the identification of existing signs of biotic attack and their interpretation, in terms of physical and chemical transformations undergone by crystals, will be carried out. The efficacy of some selected biocides will be tested.

Stone treatment is an important part of this project. Technologies will be developed for: identification and characterization of treatment products by physical and chemical means; the identification of transformation after curing in vitro and when applied in stones; the role of specific mineral components, (feldspars and biotites); and the detection of different adhesion properties to these basic constituents of granites. The work also includes: the characterization of moisture behaviour inside the stones; evaluating the efficacy of some selected products; the definition of the most adequate criteria for performance evaluation; and the assessment of the durability of treated stones in extreme conditions (acid attack, salt crystallization and freezing).

Investigations will be carried out of construction technology used in megalithic monuments and acquisition of information about past interventions. Photographic and topographic surveys of monuments and archaeological digs for investigating possible actions of the ground on the monuments will also be performed.


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101,Av. do Brasil 101
1799-066 LISBOA

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