José Delgado Rodrigues – Decay of granite
EC ADVANCED STUDY COURSE
SCIENCE AND TECHNOLOGY OF THE ENVIRONMENT
FOR SUSTAINABLE PROTECTION OF CULTURAL HERITAGE
Technical Notes for Sessions 14–15
SURFACE AND STRUCTURAL STABILITY FOR THE CONSERVATION OF HISTORIC
BUILDINGS
Giorgio Croci and José Delgado Rodrigues
PART 2. Decay of granite
J. Delgado Rodrigues, Geologist, Laboratório Nacional de Engenharia Civil,
Lisboa, Portugal
1
Granites and their components
Granites are igneous rocks originated in deep-seated zones in the Earth's crust. They have an
acidic composition where feldspar and quartz are the essential constituents. When present in
significant amounts, micas justify specific designations: biotite-granites, muscovite-granites are
some current varieties. Minor components are usually present and they include amphiboles,
pyroxenes, apatite, iron oxides, zircon etc.
Alkali feldspars – sodium-feldspar (albite), potassium-feldspar (orthose and microcline) – and
plagioclases (sodic/calcic) dominate in quantitative terms, but it is quartz, through its higher
hardness and strength that strongly influences the overall behaviour of granites.
Granite is a holocrystalline rock, typically having granular texture, with minerals rarely showing
well-defined crystallographic forms. Granitic rocks exhibiting in greater or lesser degree a
parallel disposition of elongated or tabular crystals of feldspar, mica, etc. can be found. This
disposition confers a certain banding to the rock that is known as a gneissic structure. In some
varieties relatively large crystals of feldspar confer a porphyritic texture to the rock.
Granite occurs in very large masses quite frequently well identifiable in the landscape by
characteristic geomorphologic aspects. When sound, it is a very compact and strong material
and this property is one reason for it having been used since Antiquity as ornamental stone. It is
still nowadays one of the most popular construction material, in spite of the producing costs of
cutting and polishing.
2
Granites and related rocks
Granite is a well-defined petrographic group (Streckeisen, 1986) for its mineralogical
composition and texture. However, for many purposes, several other rock types bear great
similarity to granites: granodiorites, diorites, syenites and monzonites are some of the most
current granite-like varieties. For technological purposes and namely in the field of stone
conservation, they are sometimes included in the generic group of granitic rocks. More
incorrectly, the market of ornamental rocks also includes other plutonic rock types of very
distinct composition, for instance gabbros and feldspathoidic rocks, in this same group.
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José Delgado Rodrigues – Decay of granite
All these rock types have feldspars as main constituents, but quartz is the mineral that imposes
the specific behaviour to these rock varieties. In fact, quartz is harder, stronger, less deformable
and more durable than feldspars and this contrast between the two major components is the
most typical characteristic of granitic rocks.
Under tectonic stresses, quartz behaves as a brittle material while feldspars have a more
ductile behaviour. As a consequence, the large amass of quartz grains appear badly fissured
while feldspars deform more uniformly with less rupture events. This particular property of
granitic rocks brings about important consequences as regards their weathering behaviour and
the cause of most of the decay features detected in these materials can be traced up to that
differential property of quartz and feldspars. This differential behaviour is enhanced by the
increase in grain size, reason why coarse-grained varieties are known as decaying faster than
the fine grained ones.
3
Primary and secondary minerals
Granites are plutonic rocks, designation that signifies that they were consolidated at great depth
within the Earth's crust. They were originated by crystallisation from a molten silicate mass –
the magma – as a consequence of an extremely slow cooling process. These rocks are without
exception holocrystalline and their components may reach considerable dimensions. Minerals
originated in this phase of the rock life are considered primary minerals.
Sooner or later after its solidification, plutonic rocks start a long evolution inside the crust that
brings them from their initial deep original places up to the surface where they are found and
used by Man. Since their crystallisation, plutonic rock masses are in contact with fluid solutions
of diversified compositions and origins. Some come from the original magma and others may
be derived from surface waters that have reached deep circulating paths. Both types of fluid
solutions are aggressive for the rock forming minerals and when in contact they may force the
original crystals to undergo successive changes in the search of a thermodynamic equilibrium
compatible with the new environment conditions. The main consequence of this process is the
formation of new minerals more stable in these new conditions. They are designated as
secondary minerals and are considered as playing decisive roles in the performance of the
igneous rocks when exposed to outdoor conditions. Sericitization and argilification of feldspars,
chloritization of biotite are some typical processes leading to the occurrence of secondary
minerals. Clays, in general, are the most typical secondary minerals and one of the most
deleterious occurrences as regards the performance of stone materials.
4
Porous space: pores and fissures
Figure 1 shows the relation between the ultrasonic velocity in granites and in carbonate rocks
for dry and saturated conditions. It shows a clear increase in velocity in saturated granites while
both values are identical in the carbonate stones.
This situation is quite trivial and many authors dealing with this type of test have corroborated
this fact. In broad terms, this different behaviour is typical of fissured materials, in which the
ultrasonic velocity in saturated condition is higher than the corresponding dry one, while in
porous, non-fissured materials, both values are similar. This difference in the stone behaviour
may be successfully used to distinguish between porous and fissured stones (Tourenq et al.
1971).
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José Delgado Rodrigues – Decay of granite
S artificially fissured granites
„ carbonate rocks
Figure 1. Saturation has distinct influence on the ultrasonic velocity of porous carbonate rocks
and of fissured granites
The explanation for this difference lies in the shape of the stone voids and in the different
ultrasonic velocities in the air (dry voids) and in the water (saturated voids), 340 and 1500 m/s,
respectively. Fissures are thin but long and the elastic waves paths have to cross them in order
to progress through the specimen, while pores are equidimensional voids and the elastic waves
follow by-passing paths without needing to cross them. While in fissured stones the waves'
paths cross distinct media for dry and saturated conditions, in the porous stones, the waves
follow always the mineral paths by-passing the voids. Since no voids are crossed both
velocities are similar.
5
Homogeneity / heterogeneity / anisotropy
Heterogeneity means that the rock properties vary from point to point and, then, no single
measurement can assume a complete representativeness of the rock body. Theoretically, this
drawback can be overcome through statistical methods but, for doing this, it is necessary to
have a high number of measurements available and, in most cases, this represents a great
difficulty or even a real impossibility.
Anisotropy is applied to materials whose properties show significant variations with the
measuring direction. The anisotropic character derives from the anisotropic nature of the
constituent crystals themselves, from the occurrence of spatially oriented constituents, from the
existence of layers or zonings of different mineral composition and from the existence of
privileged orientations in the fissuration patterns.
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José Delgado Rodrigues – Decay of granite
To tackle the problems of heterogeneity and anisotropy is a very complex, expensive and time
consuming task and the current works on stone conservation can very seldom afford to pay for
studies where these problems might be adequately approached.
To get rid of these problems is not easy and in many cases it turns a virtually impossible
desideratum. In these circumstances, all the available skills should be asked for, and a high
sense of criticism has to be presented during all the experimental and interpretation phases.
When admissible, comparing results obtained in the same specimen, before and after any
specific intervention can reasonably bypass these obstacles and when replicated specimens
can be used better accuracy levels will be achieved.
In some rock types, for instance with igneous rocks, the material available for testing may
belong to slightly different weathered zones and this fact may introduce meaningful differences
in test results. Figure 2 presents some results obtained in granites varying from sound to
slightly weathered degrees and clearly shows that even very small differences (here assessed
by the specimens' porosity) bring about significant decreases in the stone strength.
Figure 2. Variation of point load strength in granites with the increase in their weathering
degree (after Ferreira, 1990)
It is also important to bear in mind that any test measurement is subject to experimental
uncertainties and multiple measurements should be carried out in order to minimise these
errors. Single measurements are to be avoided as far as possible and replicated specimens
should be taken when precise values are sought for. Figure 3 shows the distribution of porosity
in granite specimens extracted from an apparently homogeneous block. As in many other
experimental determinations, porosity has an apparently normal distribution, and when
identifying such a stone it is certainly more correct to indicate a range of values than a single
figure for this property. The same is to be expected for any other experimental determination.
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José Delgado Rodrigues – Decay of granite
Figure 3. Frequency distribution of porosity values in 158 specimens taken from a
single bloc
6
Weathering in natural environments
The geomorphology of granitic areas is usually dominated by the occurrence of boulders,
normally consisting of a corestone surrounded by a large number of roughly concentric sheets
of weathered rock. Boulders vary in size and their type and frequency of occurrence depends
on the rock textural features, on the tectonic history of the rock mass and on the regional
climatic conditions. Boulders are quite frequently used for extraction of blocks and this
operation is a tradition that has been valid since very ancient times.
Granites arrive at the surface environment by the erosion of the upper layers of the Earth's
crust. During this process of "migration" through the crust, granites undergo their first decay,
namely of a chemical nature (deuteric alteration) promoted by solutions circulating at great
depths and of a physical type promoted by the unloading and cooling processes. The presence
of secondary minerals and a more or less intense fissuration degree are the direct
consequences of this evolution.
Once at the surface or in its vicinity, granites undergo another type of evolution during the
process of adaptation to the environment conditions prevailing in this new situation. This
adaptation, designated as weathering, may be defined as the process through which rocks are
broken down and decomposed by the action of external agents such as water, temperature
variations, plants and bacteria. This process affects the rock in situ and the transformations
induced by it may bring important consequences in behaviour of the rock materials once
extracted from their original emplacement and used in man-made constructions.
Conventionally, weathering processes are divided into physical and chemical types. In reality
both occur together but one may prevail according to specific environment conditions. Physical
weathering is brought about chiefly by temperature changes (e.g. differential expansion of the
rock minerals), by mechanical loads (unloading and tectonic stresses), by internal loads due to
special decay agents, such as freezing, hydric expansion and salt crystallisation and external
agents such as man, wild animals and plants. Chemical weathering is mainly brought about by
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José Delgado Rodrigues – Decay of granite
the action of water and the other components transported in solution. Chemical weathering acts
mainly through hydrolysis, dissolution and oxidation reactions.
Fissuring, porosity and permeability enhancement as well as the occurrence of secondary
minerals are ubiquitous and important phenomena in natural environments. Kaolinite and
halloysite are the most common secondary minerals of granites, although montmorillonite
and/or illite may be formed in poorly drained environments. At the surface of well-drained
profiles, gibbsite may also be present. Amorphous products are also found. The mineral
composition is determinant in the evolution of this rock type but, besides the solid matter, two
other inherent components, the pore space and texture, play decisive roles in its behaviour.
Quartz is virtually inert from the chemical viewpoint but it is determinant when thermal or
tectonic actions come into action. Feldspars are susceptible to hydrolysis and the contrast of
their mechanical properties with those of quartz makes them the key components in the rock
weathering. Ferromagnesian minerals may play significant roles and biotite, one common
constituent, may influence the overall behaviour of the rock largely beyond what could be
expected from its weight percentage.
Weathering of granites in natural environments has a large tradition of research and abundant
literature can be found on this subject (see e.g. Pedro (1964, 1993), Robert (1970, 1971),
Pérami (1971).
For their specific characteristics, namely as regards their mineralogical composition and texture
on which a more or less complex tectonic history is superimposed, granitic rock masses
undergo surface and near-surface weathering at distinct rates that lead to non-uniform spatial
distribution of the alteration patterns and consequently of the stone quality.
Quarrying of granitic materials in regions of temperate and humid climates, where rock
weathering reaches higher depths, may turn into a hazardous task and, side by side with
sound, strong and durable materials, others more weathered, weak and perishable are
frequently found.
8
Decay in man-made constructions
When perfectly sound, granites are very hard and resistant and quarrying them is a very difficult
and expensive task. Very often, to overcome this drawback, quarriers tend to favour the
exploitation of slightly or moderately weathered zones (or even highly weathered) but this
choice has dangerous consequences as regards the subsequent stone behaviour, namely
because the presence of secondary minerals in large amounts may lead to very fast
degradation, once the rock is extracted from its natural environment.
Fissuration, porosity and permeability are increased in the course of the natural weathering
processes, and the formation of clay minerals is also a ubiquitous and important consequence
of these processes. Special attention should be given to clay minerals because their presence
in significant amounts may lead to very fast degradation rates once these clay-bearing
materials are extracted and exposed to outdoor, aggressive environments.
When perfectly sound, the constitutive primary minerals are very stable from the chemical point
of view, and at the scale of human life, only physical actions, such as loads and temperatures,
may introduce significant changes with practical consequences. Consequently, non-weathered
igneous rocks have high durability and even some very high porosity basalts may have
excellent behaviour in aggressive environments.
Substantial impairment of stone performance may occur even for incipient weathering stages
and this fact makes igneous rocks particularly difficult to study. Decay mechanisms may be
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José Delgado Rodrigues – Decay of granite
very specific to them and many testing methods are not sensitive enough to detect the usual
range variation of the parameters involved.
When looking at stonework built of granite and related rocks, two main decay processes are
currently identified: the detachment of large pieces of the stone surface with shapes more or
less regular and parallel to the original surface of the stone, and the loss of small particles more
or less similar in shape and dimensions to the mineral components of the stone. They are
commonly called plaque formation and sand disintegration, respectively. Plaques, plaquettes
and scales differ in size and all of them are very devastating decay processes. The destruction
of the stonework by plaque detachment is a discontinuous process, but once formed, the
plaque, and consequently the entire surface, may be lost in a very short period. Sand
disintegration is a bulk process and usually the entire stone is affected by its occurrence. It is a
continuous and progressive process and the evolution rates may be faster than those found in
plaque detachment.
The published material on granite decay in man-made constructions is not very abundant
although this panorama has improved in the last decade [e.g. Castro et al. 1991; Casal et al.
1992; Cooper et 1991; Haneef et al. 1992; Nord & Ericcson 1993; Smith et al. 1993]. This
contrasts with the attention dedicated in recent years to decay processes affecting soft, highly
porous stones such as limestones and calcareous sandstones in monuments and buildings.
Besides the influence of the intrinsic characteristics (namely, porosity and fissuration state,
nature and amount of secondary minerals), several external factors have been pointed out as
relevant contributors to the granite decay.
Gypsum has been currently found in urban environments and it is considered one significant
decay factor (Casal 1989). The sulphate anion may be supplied by the atmospheric pollutants
or by solutions ascending from the building foundations. Mortars used in joints may also
constitute a relevant source for gypsum (Cooper et al. 1989). Mortars are also important
sources for calcium, particularly when sulphur comes from the atmosphere, since calcium is not
easily available in granites. Some authors accept that granite decay by plaque formation is
related to the occurrence of gypsum precipitation in the outermost layers of the stone (Casal et
al. 1989).
Sand disintegration, on the other hand, is sometimes related to the occurrence of highly soluble
salts, namely on coastal environments (Casal et al. 1989, Silva et al. 1994) although the
amount of these salts might be extremely small. In some circumstances, soluble salts may be
absent in zones with sand disintegration thus suggesting that other decay mechanisms may be
in action. Thermal and moisture cycles are other possible alternatives for explaining these
decay forms.
7.1
The role of inherited alteration
Unlike other common stones, moderate to highly weathered granite is quite frequently quarried.
In fact, during the geological history of this rock type weathering modifications are brought
about by natural causes that affect the overall conditions of the entire rock masses that are the
emplacements of quarries. Quite frequently, these weathering modifications show gradual
variations from the topsoil down to the fresh granite rock with all possible intermediate stages.
When quarrymen approach such types of outcrops, they naturally tend to favour the less
resistant varieties, since the sound granite is extremely difficult to extract.
This was the current tradition of old quarriers and still today this is a normal way outside the
modern market of ornamental rocks.
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José Delgado Rodrigues – Decay of granite
Slightly to moderately weathered stones are thus common in old buildings and monuments and
granite that may have porosity below 1% when sound may be found in old constructions with 2
to 5 %. This fact turns them highly vulnerable to decay mechanisms and many monuments
display evident signs of degradation clearly associated to this inherited feature of the
construction material. In general these stones show fast decay rates, especially when they are
subject to humidity and temperature cycles or when salt crystallisation is acting.
This inherited weathering is translated into the rock characteristics through the modification of
the chemical composition of feldspars and mica, the increase in the density of the fissure
network and in the increase of the overall porosity. The occurrence of secondary minerals,
namely of the clay type, is also currently a consequence of that inherited weathering.
7.2
The role of other inherited features
Some special features may also be inherited from the original rock mass emplacement. For
instance, fractures and fissures networks derived from tectonic stresses and from unloading
may influence the water penetration rates and mechanical strength. Since they are surfaces of
virtually nil adhesion strength quite frequently they give rise to detachment of more or less thick
plaques of very devastating consequences.
It should not be excluded that sound blocks extracted from tectonically active belts might carry
important virgin stresses that afterwards through the mechanism of stress release may cause
excessive deformations and accelerate spalling and other physical mechanisms.
7.3
Decay forms
The observation of monuments built of granitic materials shows two main decay forms: plaque
formation and sand disintegration. Both are very devastating mechanisms and once they are
installed no easy remedial measures are available to fight against.
In many places, granites are covered with soiling substances and in some cases their
appearance may not be substantially different from those found in carbonate rocks and
sandstones. Chemically and structurally they are distinct and therefore a specific study should
be carried out whenever any coherent conservation intervention is required.
A brief explanation of these decay forms may be of some help for understanding the possible
ways of tackling them.
a) - Plaque formation and sand disintegration and their mechanisms
Plaques, plaquettes and scales are very frequent and damaging decay forms found in the
historical buildings built of granitic stones. Normally they occur in the first 3 to 4 ashlar layers,
up to 1.5 to 2m in height. They may be scarce in the first row and abundant at the other levels.
In the upper levels, occasional occurrences can be found but they are generally well correlated
with the existence of architectonic features that lead to the accumulation of water, such as
cornices, parapets, etc.
These decay features may occur in all orientations but the north facing walls and in general the
shadowed places are particularly prone to their occurrence. They also occur in semi-closed
spaces such as cloisters and arcades. These forms are more frequent and well defined in fine
to medium grained granites and the most frequent thickness is in the order of 3 to 5mm.
Occasionally, plaques as thick as 1cm and even more are found.
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José Delgado Rodrigues – Decay of granite
In some reported cases, the chemical analysis of soluble salts extracted from plaques,
plaquettes and scales reveal the presence of gypsum with absence of any other salt in
significant amounts. This salt is therefore currently associated to these decay forms but the
exact action mechanism is still poorly understood.
Sand disintegration is a common decay form and it may be considered one of the most
damaging decay mechanisms in numerous monuments located near the seacoast but it may
also be very relevant outside these particular environment conditions. When the stone is
relatively porous intense sand disintegration can be found, particularly in the walls facing the
sea. In sheltered areas, plaquettes and scales may also be found. Samples collected in the
decay products usually show that soluble salts occur in important amounts.
Chlorides and nitrates are the main soluble salts present and they are usually associated to the
sand disaggregated stones and therefore the origin of this decay form is attributed to the
presence of these salts and to conditions of intense evaporation that allow their crystallisation.
These results agree with previous findings for other rock types (Pauly, 1975, Arnold and
Zehnder 1988).
b) - Black crusts in granite substrates
Building stones that have been transferred from their "natural" habitat into environments
modified and polluted by anthropic activities quite frequently exhibit surface patinas displaying
very dark colours and, according to their surface patterns, two main types can be observed: the
thick sulphate rich crusts and the thin iron rich patinas.
Gypsum is the most common "alien" crystalline component of the thick crusts.
Gypsum crusts on granite appear similar to the ones developing on carbonate stones and, as is
the case of building limestones, they are made up of a framework of gypsum crystals with
imbedded particulate material derived from air pollution, soil dust and stone fragments
(Schiavon, 1994). In the case of granite, though, the depth of penetration within the sound
stone of the weathering front crystallising the gypsum is much lower; this can be explained by
taking into account the lower original porosity of granites which on average ranges between 15% in volume (as compared to porosity values of up to 30% for limestones). In this respect, it is
important to note, however, how the process of gypsum crystallisation itself is increasing the
internal porosity of the granite by creating new fractures and widening pre-existing ones, thus
allowing the weathering to progress with time deeper into the stone.
These patinas are similar in terms of composition, origin and dramatic decay effects to the
sulphate crusts already reported on other lithologies such as limestone and marble. As in those
cases, their origin is due to the reaction between SO2 and Ca-bearing building material and is
then directly connected with anthropogenic activities, such as oil and coal combustion for
domestic, vehicular and industrial needs, also confirmed by the inclusion of anthropogenic
airborne particles and soot within the gypsum framework.
c) – Iron rich black patinas
Thin black patinas are quite distinct from those described above, namely because their
sulphate content is minimal and gypsum is virtually absent in this type of surface decay. These
black to reddish thin patinas appear to form a fairly continuous coating on the granite surfaces,
in places reaching 40-50µm in thickness. Compositionally, the patinas are made up of
particulate fragments in a very fine-grained matrix. EDS analysis shows that, in comparison
with the unweathered mineral substrate, the patinas show higher concentrations of Fe, P, Ca,
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José Delgado Rodrigues – Decay of granite
S, and Cl; Fe abundance may reach almost 50% in elemental weight. Iron has also been found
as micron size spherical iron particles.
Thin iron-rich patinas on granitic surfaces recently studied (Schiavon 1994) are similar to soiling
layers developing in polluted areas on low porosity building material such as quartz-cemented
sandstones, brick (Nord and Ericsson 1993) or even on bronze artefacts (Bernardini et al.
1992). Although containing evidence of a strong anthropogenic contribution to their composition
(soot is again one of the main components), these patinas do also show features consistent
with biological activity. Iron-rich soiling layers, though not showing the same degree of damage
associated with sulphate-affected granite, are also interacting via dissolution episodes with the
stone substrate and should be removed with a suitable cleaning technique which would take
into account their complex composition.
d) - Granite biodeterioration
Numerous authors consider biodeterioration processes as important contributions to stone
weathering. In principle their role as decay inducers is understandable since stone substrata, in
natural outcrops and in the exterior building materials, offer attachment surfaces, wetting and
light conditions and, especially, a great variety of essential nutrients to living organisms.
Algae, lichens and mosses are commonly found growing on external surfaces of buildings,
particularly where design features or maintenance faults result in frequent wetting of the
surface. When, to these already propitious conditions, a non-polluted atmosphere is added,
biological colonisation may proliferate in large extent and, in normal circumstances, following
the ecological succession presented in Table 1.
Table 1. Ecological succession, requirements and main effects of the organisms that colonise
exterior building materials (P. Romão, n.d.)
Colonisation
Stage
Organisms
Requirements
Nutrients
Light
Main Effects on the Minerals
1st
Bacteria
Various
Unnecessary
Chemical transformations (e.g.
oxidation-reduction reactions)
2nd
Fungi
Decaying
organic matter
(saprophytes)
Unnecessary
Chemical transformations (e.g.
complexation of metal cations)
3rd
Algae
Mineral salts
Necessary
Water retention
Hole formation
4th
Lichens
Mineral salts
Necessary
Crystals fragmentation
Detritus accumulation
Proto-soil formation
Chemical transformations
5th
Mosses and
other
Bryophytes
Organic material
and mineral salts
Necessary
Crystals fragmentation
Soil formation
Chemical transformations
6th
Higher plants
Mineral salts
Necessary
Crystal fracturation and disintegration.
Chemical transformations (e.g. ionic
change, acidification and complex
formation)
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José Delgado Rodrigues – Decay of granite
All these organisms need a fairly high level of moisture for active growth, but once established
they can exist under a wide range of moisture conditions and many of them can withstand
severe drying conditions. In addition to moisture and as considered in Table I, all organisms
require nutrients; light is needed only for the photosynthetic ones. Sometimes the appearance
of these biological growths has been regarded as protective, beautiful, even to be encouraged particularly when they can be used as monitors for air quality control.
Lichens are two-component organisms: one alga and a fungus live together and interact in a
symbiotic or almost symbiotic (since the fungus is normally predominant) relationship. Syers
and Iskandar (1973) defined the pedogenetic significance of lichens in terms of biogeophysical
and biogeochemical processes. For the first process, they described two important
mechanisms: rhyzine penetration and thallus expansion and contraction. In the biogeochemical
weathering, those authors consider two contributions: the production of carbon dioxide, oxalic
acid and other so-called "lichen substances" (a group of families of weak organic acids) during
lichen's metabolism, and, mainly, the capacity to form soluble complexes with metallic cations
exhibited by these substances.
Three main points must be emphasised when approaching the biological colonisation of granitic
rocks: 1) - due to their mineralogical content, granitic rocks are richer and more diverse in
nutrients for colonising organisms than calcareous rocks; 2) - quartz and feldspar are
translucent, properties that allow a deep endolithic colonisation by photosynthetic organisms; 3)
- quite often, when extracted for building materials, granitic stones are already weathered.
These facts favour the installation of organisms and the proliferation of lichens on granitic
building materials is easy, despite the hardness of these rocks.
References
Arnold, A. and Zehnder, K. 1988 - Decay of stone materials by salts on humid atmosphere.
Proc. 6th Int. Cong. Deterioration and Conservation of Stone, Torun, pp. 138-148.
Bernardini, G.P.; Corazza, M.; Olmi, F.; Sabelli, C; Squarcialupi, M.C. and Trosti-Ferroni, R.
1992 - The "Incredulita' di S. Tommaso" by Verrocchio: a mineralogical study of alteration
patinas. Science and Techn. for Cultur. heritage 1, pp.177-189.
Casal Porto, M.; Delgado Rodrigues, J., and Silva Hermo, B. 1992 - Construction materials and
decay problems of Salomé Church in Santiago de Compostela. 7th Int. Cong. on Deterioration
and Conservation of Stone, Lisbon, pp. 3-10.
Casal Porto, M. 1989 - Estudio de la alteración del granito en edificios de interés histórico de la
provincia de La Coruña. Tesis Doctoral, 1989. Universidad de Santiago de Compostela.
Castro, E. (1979) - Évaluation de l´hygroscopicité des pierres. 3éme Cong. Int. sur la
Déterioration et la Preservation de la Pierre, Venise, pp. 183 -194.
Castro, E.; Delgado Rodrigues, J. and Cravo, M.R.T. 1991 - Study on the alteration and
conservation of the Torre dos Clérigos. Laboratório Nacional de Engenharia Civil (LNEC),
Lisbon, Internal Report.
Cooper, T. P.; Dowding, P.; Lewis, J.O.; Mulvin, L.; O'Brien, P.; Olley, J. and O'Daly, G. 1989 Contribution of calcium from limestone and mortar to the decay of granite walling. Proc.
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José Delgado Rodrigues – Decay of granite
European Symp. on Science, Technology and European Cultural Heritage, Bologna, pp. 456461.
Cooper, T. P.; Duffy, A.; O'Brien, P.; Bell, E. and Lyons, F. 1991 - Conservation of historic
buildings at Trinity College, Dublin. Workshop "Alteración de Granitos y Rocas Afines,
empleados como materiales de construcción (Deterioro de Monumentos Historicos), Ávila, pp.
59-65, published in 1993.
Costa D. and Delgado Rodrigues, J. 1994 - Assessment of colour changes due to treatment
products in heterochromatic rocks. Proc. Workshop on Degradation and Conservation of
Granitic Rocks in Monuments, Santiago de Compostela, Nov. 1994 (in press).
Delgado Rodrigues, J. Costa D. ,1994 - "Assessment of the efficacy of consolidants in
granites". Proc. Workshop on Degradation and Conservation of Granitic Rocks in Monuments.
Santiago de Compostela, Nov. 1994 (in press).
Delgado Rodrigues, J.; Costa, D.; Sá da Costa, M. and Eusébio I. 1994 - Behaviour of
consolidated granites under ageing tests. Workshop on Degradation and Conservation of
Granitic Rocks in Monuments, Santiago de Compostela, Nov. 1994 (in press).
Delgado Rodrigues, J.; Costa, D. and Schiavon, N. 1994 - Spatial distribution of consolidants in
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Advanced Study Course Technical Notes
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José Delgado Rodrigues – Decay of granite
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Advanced Study Course Technical Notes
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