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SPECIAL ISSUE
Weathering of volcanic tuff rocks caused by moisture expansion
Wanja Wedekind •Rube
´n Lo
´pez-Doncel •
Reiner Dohrmann •Mathias Kocher •
Siegfried Siegesmund
Received: 18 September 2012 / Accepted: 29 November 2012 / Published online: 28 December 2012
!The Author(s) 2012. This article is published with open access at Springerlink.com
Abstract Moisture expansion in natural building stones
is considered one of the most important factors affecting
their weathering and deterioration. The processes that may
be responsible for the expansion under determinate relative
humidity (hygric dilatation) and water-saturated conditions
(hydric dilatation) are generally attributed to the presence
of swellable clay minerals. In contrast to this assumption,
our investigations show that moisture expansion also takes
place in volcanic tuff building stones almost free from
clay minerals. To provide a deeper understanding of the
processes, swelling and deterioration were performed on
14 volcanic tuffs used as important building stones of
different ages, compositions and weathering stages from
Mexico, Germany and Hungary. The investigations under-
taken include extensive chemical, petrophysical and fabric
analyses. The samples show a wide range of effective
porosity, microporosity, capillary water absorption, mois-
ture expansion, and CEC values. High moisture expansion
does not seem to depend on clay mineral content alone. We
also observed that there is no significant effect on dilatation
if clay minerals are present but only form a thin coat on the
outer shell of bigger pores. Moreover, we identified a
correlation between microporosity, average pore radius and
moisture expansion. The investigations highlight the fact
that moisture expansion cannot only be attributed to
swellable clay minerals, and suggest that the presence and
accumulation of micropores and their average radius and
distribution play an important role for non-clay associated
swelling intensity, which can most probably be attributed
to the disjoining pressure.
Keywords Weathering of tuffs !Hygric expansion !
Hydric expansion !Clay mineralogy !Microporosity !
Disjoining pressure
Introduction
Volcanic rocks have been an important natural building
material since the beginning of time. Their use varied from
monumental sculptures in antique cultures to recent mod-
ern buildings all over the world. The reason these rocks are
so appreciated as natural building stones is mainly due to
three facts; firstly, because some of these rocks and, in
particular, tuffs, are relatively soft and easy to work with.
Secondly, they are available worldwide, because tuff rocks
are present in many places and all geological times; and
thirdly, tuff rocks exist in a great assortment of colours,
composition, grain size, sorting and textures, which makes
these rocks very attractive for construction and artwork
(Fig. 1).
From a geological point of view, tuffs are volcanic rocks
with matrix grain sizes ranging from fine clay minerals up
to silt-sized material (matrix normally present in ash form),
in which sand, crystals or rock fragments up to gravel and
W. Wedekind (&)!S. Siegesmund
Geoscience Centre of the University Goettingen,
Goettingen, Germany
e-mail: wwedekind@gmx.de
R. Lo
´pez-Doncel
Geological Institute, Autonomous University of San Luis Potosı
´,
San Luis Potosı
´, Mexico
R. Dohrmann
Federal Institute for Geosciences and Natural Resources (BGR),
Hanover, Germany
M. Kocher
Department of Restoration, Art Technology and Conservation
Science, Technical University of Munich, Munich, Germany
123
Environ Earth Sci (2013) 69:1203–1224
DOI 10.1007/s12665-012-2158-1
block-sized clasts and lithics (also known as lapilli), as
well as volcanic blocks and bombs, are embedded. The
arrangement between these tuff components and the
lithology itself opens a very wide spectrum of porosities,
very different fabrics (Fischer 1961; Le Maitre et al. 2004)
and normally large amounts of clay minerals in the matrix.
However, at the same time, these characteristics cause tuffs
to be less resistant to deterioration, particularly if they are
exposed to moisture and humidity (Steindlberger 2003;
Ruedrich et al. 2011; Timothy et al. 2011).
Deterioration on buildings is often concentrated in areas
affected by temporary moisture infiltration and leads to
different damage phenomena (Fig. 2). Moisture expansion
is clearly recognized as one of the most important factors
contributing to the weathering and deterioration of building
rocks (Ruedrich et al. 2011; Weiss et al. 2004), and this is
mostly associated with the swelling and shrinking of clay
minerals (Schuh 1987; De La Calle and Suquet 1988;
Snethlage et al. 1995; Jimenez Gonzalez and Scherer 2004;
Dixon and Weed 1989; Moore and Reynolds 1997; Graf
et al. 1995). The intensity of moisture expansion varies
markedly depending on the type of stone. In general,
volcanic stones, and primarily tuffs, have a wide range of
moisture expansion, which, as measured in this study, can
reach a dilatation of up to 6 mm/m (Fig. 3).
Although the swellability of the clay minerals plays a
decisive role in the weathering of building rocks, the dif-
ferent processes that cause this phenomenon, and the
question as to which is the main process during the
observed swelling, are still under discussion. In principle,
there are two mechanisms that can act inside and between
the clay minerals; the inner-crystalline (intra-crystalline)
(Madsen and Mueller-Vonmoos 1989) or the osmotic
(inter-crystalline) swelling (Heim 1990; Auras and
Steindlberger 2005). To understand these mechanisms, it is
very important to have measured the amount and type of
Fig. 1 Volcanites used as building stones and for artwork: aRochlitz
Tuff, RP (portal at monastery Wechselburg/Germany), bHilbersdorf
Tuff, HD (tulip pulpit cathedral of Freiberg/Germany), cCantera
Rosa San Miguel el Alto, CR (cathedral Arandas/Mexico), dBlanca
de Pachuca Tuff, BP (Palacio Postal Mexico-City), eTenayoca
´tetl
Tuff, TY (Aztec sculpture Tlatecuhtli, Templo Mayor, Mexico-City),
fChiluca, CH, and Cantera gris de los Remedios, GR (cathedral of
Mexico-City), gCantera Amarilla Tuff, CA (Santa Monica Church,
Guadalajara/Mexico)
1204 Environ Earth Sci (2013) 69:1203–1224
123
clay minerals present, because 1:1 and 2:1 layered clay
minerals can condition this effect through their cation-
exchange capacity and the water uptake provoked by the
electrolyte concentration in water that caused an osmotic
swelling (Madsen and Muller-Vonmoos 1988,1989; Heim
1990). There is also a ‘‘non-clay associated’’ swelling type
Fig. 2 Different damage phenomena observable in tuffs associated
with moisture expansion of the rock material. aBack weathering of
Cantera Rosa Tuff (CR) of San Miguel de Alto, Jalisco/Mexico.
bExtensive cracking parallel to the bedding of the Bufa Tuff (BT) in
Guanajuato/Mexico. cScaling parallel to the bedding of Loseros Tuff
(LS) found in Guadalajara/Mexico
Fig. 3 Swelling- and moisture
expansion of building stones
(data from different authors and
from this study). Modified from
Kocher (2005): Granite, marble
und sandstone I from
Hockmann and Kessler (1950),
from Snethlage (1984),
limestone from Lukas (1990),
sandstones II from Schuh
(1987), sandstones III from
Snethlage and Wendler (1997),
concretes from Wesche (1977),
mudstones from Madsen (1976)
and Madsen and Nueesch
(1990)
Environ Earth Sci (2013) 69:1203–1224 1205
123
known as disjoining pressure, which is similar to osmotic
swelling and related to the electrolyte concentration of a
solution (Derjaguin and Obukov 1936). This swelling type
is more dependent on the size and amount of micropores in
the rock, as well as on the presence of lithoclasts in
sandstone (Ruedrich et al. 2011).
The disjoining pressure (P
d
) arises from an interaction
between two flat and parallel surfaces. The value of the
disjoining pressure can be calculated as the derivative of
the Gibbs energy of interaction per unit area with respect
to distance. This energy, also known as free energy, is
defined as a thermodynamic potential that measures the
process-initiating work obtainable from a thermodynamic
system at a constant temperature and pressure (Greiner
et al. 1995). The disjoining pressure will be affected when
the distance between them is less than two times the
thickness of adsorbed moisture on a free surface, where
the force needed to keep the forced distance is determined
by the so-called ‘‘disjoining pressure equation’’ (Nielsen
1994).
Ruedrich et al. (2011) presented a detailed overview of
the different types and causes of the moisture expansion.
Whatever the cause of the moisture expansion may be, the
water uptake and distribution into the rock is the principal
mechanism allowing water (moisture and humidity) to
interact with the clay minerals present and is only realiz-
able through the porosity. The porosity, therefore, is one of
the most important parameters that must be known and
determined.
As seen above, microporosity plays an important role
not only for the disjoining pressure, but also because, by
reaching the critical pore size of 5 lm, the pore water is
trapped and cannot be drained out of the stone (Larsen and
Cady 1969). Pore water hosted in the micropores can
contribute to the dissolution of certain minerals in the
matrix or to the acceleration of the swelling of clay min-
erals. Once the rock has been affected by internal weath-
ering, the effective porosity ratio has been increased
(Tugrul 2004), allowing major water circulation in all its
variants into the rock.
This phenomenon also occurs with all the other types of
porosity, and their size and distribution within the rock are
similarly decisive for stone durability (Benavente et al.
2004; Siegesmund and Duerrast 2011).
The factors clay mineral content and porosity are two
parameters that are certainly important in tuff rocks,
because they can occur in a wide range of amounts and
forms. Likewise, the chemical and mineralogical compo-
sition of tuff rocks is very diverse, which also influences
their durability and resistance against weathering and
deterioration.
The main goal of this investigation is to contribute to the
existing knowledge about moisture expansion of tuffs used
as building stones. This study used 14 important volcanic
tuff rocks used in construction from Germany, Hungary,
and Mexico of different ages, compositions, and states of
weathering. Hydric and hygric wetting experiments con-
ducted on the tuff rocks, as well as chemical and miner-
alogical analyses of the clay fraction, which included XRD
(X-ray diffraction) of oriented slides and CEC analysis,
were performed in order to determine the total amount of
swellable clay minerals and to clarify what role the pres-
ence of 1:1 or 2:1 layered clay minerals plays in tuff and its
relationship to moisture expansion.
Examined rock material
Fourteen total volcanic tuffs were investigated in this
study, while eight tuffs were investigated in greater detail
because of their particular properties. From these eight
volcanites, four are from Mexico: Cantera Blanca Tuff
(BP), Tenayoca
´tetl Tuff (TY) Bufa Tuff (BT) and Cantera
Amarilla Tuff (CA); three are from Germany: Habichts-
wald Tuff (HW), Rochlitz Tuff (RP) and Hilbersdorf Tuff
(HD); and one from Hungary: Eger-Demje
´n Tuff (ED)
(Figs. 4,5).
Additionally, another six volcanites were investigated:
Weibern Tuff (WB), Loseros Tuff (LS), Chiluca Tuff
(CH), Gris de los Remedios Tuff (GR), Cantera Rosa Tuff
(CR), and Cantera Formacio
´n Tuff (CF).
Methods
The open (effective) porosity, as well as the density, of six-
and-a-half centimetre long cubic stone samples were
measured using hydrostatic weighting (DIN 52 102). The
water-saturated mass, the buoyancy mass of the samples
measured after water saturation under vacuum, and the dry
sample mass were used to calculate the porosity.
The moisture expansion by hygric and hydric wetting of
the volcanic rocks was determined using cylindrical sam-
ples ([15 mm 9100 mm). To evaluate hydric moisture
expansion, these stone cylinders were measured under
water-saturated conditions. The hygric moisture expansion
was performed in a special measurement environment
connected to a climate chamber. The moisture expansion
was measured step by step, beginning from 20 % relative
humidity (RH) and gradually increasing up to 95 % at a
constant temperature of 30 "C. For water-saturated condi-
tions, the samples were completely immersed in distilled
water. The resolution of the displacement transducer is
0.1 lm and the accuracy is about 0.5 lm. The measure-
ments were carried out on all samples parallel to the X,Y,
and Zaxes.
1206 Environ Earth Sci (2013) 69:1203–1224
123
Fig. 4 Illustration of the volcanic rock samples described in detail. From top to bottom: Cantera Blanca Tuff, Tenayaca
´tetl Tuff, Bufa Tuff and
Habichtswald Tuff. (a,d,g,j) Polished section, (b,e,h,k) thin section under polarized light, (c,f,i,l) SEM micrograph
Environ Earth Sci (2013) 69:1203–1224 1207
123
Fig. 5 Illustration of the volcanic rock samples described in detail. From top to bottom: Rochlitz Porphyr Tuff, Hilbersdorf Tuff, Cantera
Amarilla Tuff and Eger-Demje
´n Tuff. (a,d,g,j) Polished section, (b,e,h,k) thin section under polarized light, (c,f,i,l) SEM micrograph
1208 Environ Earth Sci (2013) 69:1203–1224
123
Pore size distribution was measured by MIP (mercury
intrusion porosimetry).
The capillary water absorption was measured with
respect to the three principal directions (X,Yand Z) on
sample cubes 65 mm in length. The cubes’ bottom plane
was placed into water to measure the weight increase over
time.
The water vapour diffusion characterizes the diffusion
resistance of a porous material compared to an equally
dimensioned inactive air film and is one of the most
important water transport mechanisms. The measuring
method gives information about the network of the porous
system, the drying behaviour and the transport mechanism
controlled by the pore size distribution (Ruedrich and
Siegesmund 2006). Slices of the different stones were
prepared in all directions (X,Y,Z) with a diameter of 4 mm
and a thickness of 10 mm and were attached as covers on
Teflon cups filled with water (100 % RH). These cups were
then placed into a climate chamber at 50 % RH and a
temperature of 20 "C. The weight losses of the cups were
measured every 24 h by calculating the moisture flow
through the material.
Pressure will develop if the strain in the sample’s dila-
tation is mechanically inhibited during water uptake (cf.
Kocher 2005). Ruedrich et al. (2011) shows that, for
sandstone, there is a clear correlation between moisture
expansion and swelling pressure.
Swelling pressure was measured for five selected
cylindrical samples with a diameter of 5 cm and a high of
2.5 cm. Samples with a very high moisture dilatation
(in this case the HD) were chosen, one with a high dila-
tation (LS), two samples with a medium dilatation (BT
and BP), and one with moderate expansions (CA). The
samples were measured in the Zdirection while a preload
0.021 MPa was given. Two techniques were used: a
universal testing machine and a commercial odometer
(Kocher 2005).
The splitting tensile strength (STS) was determined by
means of the ‘‘Brazilian test’’, which involves disc-shaped
specimens 40 mm in diameter and 20 mm in length. In
order to calculate the average value, a minimum of six
samples was used, while the normal amount was 10 sam-
ples. The splitting tensile strength was measured perpen-
dicular to the XY and XZ planes, where XY is the bedding
plane. Tensile strength tests were conducted under dry and
water-saturated conditions.
For the compressive strength measurements, tests were
performed on standard specimens of 50 mm in diameter
and 50 mm in length. The load was applied to the end-
faces of the specimen with a strain rate of 1,000 N/mm
2
/s
until failure using a universal testing machine. The
uniaxial compressive strength was measured under dry
conditions.
Petrography and mineralogy
The tuff rock ages studied range from the Permian to the
Recent, with chemical compositions that vary from basalt
to dacite tuff up to rhyolitic ignimbrite tuff. For optical
microscopy, the mineralogical composition of bulk sam-
ples and clay fractions were also analyzed using X-ray
diffraction and cation-exchange capacity (CEC) determi-
nations, respectively. The mineralogical composition,
including clay minerals, is listed in Table 1.
Petrographical analyses of each tuff sample were per-
formed on oriented thin sections under a polarisation
microscope.
The Blanca Pachuca Tuff is a light grey to white
pyroclastic rock with clearly acid composition (rhyolitic
tuff). It is characterized by an almost total lack of macro-
crystals and a fine, porcelain-like appearance (Fig. 4b).
Only a few platy anhedral feldspar phenocrysts and some
scarce idiomorphic quartz crystals were observed macro-
scopically. Little angular (anhedral) quartz crystals
([0.2 mm), sometimes grouped in fine laminaes, scattered
biotite and hornblende crystals are also present. The tuff
likewise shows a hydrothermal alteration causing epidoti-
sation and argilitisation. The matrix is composed of glass
together with significant amounts of smectites (montmo-
rillonite) and zeolites (mordenite–clinoptilolite).
The Tenayoca
´tetl Tuff is a greyish to red vulcanite with
a characteristic porphyritic fabric (Wedekind et al. 2011).
The tuff has a basic chemical composition and exemplifies
a welded ash tuff with a small portion of lapilli fragments.
The fine-grained, glassy matrix (around 60 % of the rock)
contains approximately 40 % of clasts and crystals in grain
sizes about 0.2–5 mm. The matrix shows flow direction
with intercalated light and dark-coloured bands and stripes
(Fig. 4d). Altered crystals of plagioclase, hornblende,
andesine, oligoclase, labradorite and augite are observable
(Wedekind et al. 2011). The glassy matrix could be partly
recrystallised as indicated by XRD. TY contains significant
amounts of smectite (10–20 %).
The Bufa Tuff is a greyish, light pink/red to orange
porphyritic rhyolitic tuff, which has around 10 % quartz
and sanidine phenocrysts, together with isolated, well-
flattened pumice. More important in this tuff are the
angular to subangular, abundant lithic components
(15–20 % of the rock), which can be up to 15 cm in
diameter. The lithic fragments are basically red to dark red
coloured. Subhedral to euhedral quartz, plagioclase, biotite
flakes and dispersed pyroxenes and olivine crystals are
present in a microcrystalline and partially devitrified glassy
matrix (Fig. 4h). The fine-grained matrix is composed of
calcite, hematite (principal cause of the pinkish, reddish
colour) and small amounts of illite plus illite–smectite
mixed layer of different ordering types.
Environ Earth Sci (2013) 69:1203–1224 1209
123
The Cantera Amarilla Tuff is a yellowish to orange-
brownish porphyritic tuff (hypercrystalline to aphanitic
texture) with clasts of very different sizes. The clasts are
mainly poor flattened light brown to brownish-reddish
pumice and other lithoclasts (basalts and other volcanic
fragments), which can have diameters of a few mm (sand
grain size) up to 10 cm and sometimes more (Fig. 5g, i).
The crystals are practically only platy and tabular, sub to
euhedral, well-twinned alkali feldspar phenocrysts, and
seldom-rounded reworked quartz and unidentified opaque
crystals. The Cantera Amarilla Tuff is rich in K-feldspar
and plagioclase; quartz is absent but cristobalite, tridymite,
and clay minerals are accompanying minerals. Smectite,
kaolinite and halloysite were identified in separated clay
fractions.
The Habichtswald Tuff shows a wide spectrum in grain
sizes of the fragments, ranging from several millimetres up
to 5 cm. Its characteristic dark greyish, glassy to micro-
crystalline groundmass contains a great amount of mont-
morillonite. This tuff has a hypocrystalline texture with
euhedral olivine-pyroxene crystals, while plagioclase,
amphibole and biotite are also recognizable (Fig. 4k). Clasts
of pre-existing basic rocks are also present as vitreous
fragments (peridotite and basaltic clasts; Stueck et al. 2008).
The Hilbersdorf Tuff shows different varieties in grain
size and colour. It can be pale pink to brick red, light or
dark purple or greenish, and often shows a peculiar
alteration with light pink or greenish speckles, spots or
strips (Fig. 5d). In the fine-grained matrix of the tuff
(around 60–70 % of the rock), lapilli inclusions were
observed; however, the average grain size varied from 0.5
to (rarely) coarser than 2 mm. Larger cavities from several
mm to cm, mostly amygdaloyd-type, were partially filled
with clay minerals. It consists of mono- and polycrystalline
quartz, alkali feldspar and seldom of lithic grains of
metamorphic rocks (sericite-rich), quartzite and gneiss-like
xenoliths in a hypocrystalline to eutaxitic texture (Fig. 5e).
In addition to quartz and feldspar, XRD proved the pres-
ence of approximately 20 wt % muscovite/illite. SEM
(scanning electron microscope) images of the matrix of
the Hilbersdorf Tuff (Fig. 5f), additionally demonstrated
the presence of dickite, a clay mineral that belongs to the
kaolinite-group.
The Rochlitz porphyry has a red rhyolitic groundmass
composed of quartz, feldspars, thin biotite crystals and elon-
gated, flattered porous glass fragments in different grades. The
fine-grained to glassy, sometimes granular and well-porous
reddish to dark red-brownish matrix exhibits a moderate flow
fabric (Fig. 5b). The pore space is mostly filled kaolinite
(around 50 %). Within the matrix (40–70 %), up to 4 mm
rounded to euhedral quartz crystals and 1–3 mm white,
tabular to columnar relictic alkali feldspars (completely
replaced by clay minerals) occur as phenocrysts. The reddish
colour of the tuff is caused by hematite present in the matrix.
Table 1 semi-quantitative analyses of clay minerals and the CEC-value of the chosen samples
Sample Smectite Chlorite K-S
(R0)
I-S
(R0)
I-S
(R3)
Muscovite/
illite
Biotite/
phlogopite
Kaolinite Halloysite Quartz Cristobalite/
tridymite
Glass
BP XX ?XX
TY XX XX
BT XXX ?XXX
CA xxxX
HW XX X XX
HD XX XX XXX
RP ?XXX
ED X X x XXX
Sample Calcite K-feldspar Plagioclase Hornblende Analcime Clino-ptilolite Diopside Augite Olivine Hematite Meq/100 g
BP XX X ?XX 15.6
TY XX XXX x ?x 17.6
BT x X XXX ?? ? 3.0
CA XXX XXX 4.2
HW XX ?XX XX 16.8
HD XXX 0.7
RP ?1.1
ED XX XX x 10.4
Semi-quantitative: ?traces, x minor component, X 5–10 %, XX 10–20 %, XXX [20 %
1210 Environ Earth Sci (2013) 69:1203–1224
123
The Eger-Demje
´n Tuff is a greyish to light brownish
ignimbrite, which has clasts of very different sizes. The
clasts, measuring between 1 and 30 mm, are mainly elon-
gated, flattened pumice fragments, in addition to which
large plagioclase and hornblende phenocrysts can be rec-
ognized (Fig. 5k). Microcrystalline porphyritic fabric in a
eutaxitic to glassy texture with flattened well-welded vitr-
oclast can also be observed. The main constituents are
euhedral to subhedral plagioclase crystals, which show a
very impressive zonal texture, together with plagioclase,
subhedral quartz, corroded hornblende and altered biotite
(partially altered to chlorite; Stueck et al. 2008). Smectite
occurs in relatively large amounts, approximately 10 wt%.
The Weibern Tuff is a yellowish to light brown phono-
litic volcanic rock that consists principally of 85 % matrix
and 15 % clasts. The matrix is rich in analcime, muscovite/
illite, quartz and calcite. The clasts include rock fragments,
euhedral to anhedral crystals and pumice, and its grain
sizes range from 0.5 up to 10 mm. The thin section shows
that the rock fragments are sandstones, shales and basalts
and other unidentified volcanic glass fragments. The
crystals are predominantly reworked quartz from sedi-
mentary rocks, olivine, pyroxenes and feldspars. Flattened
pumice occurs likewise embedded in the porphyritic–
hyalopilitic groundmass.
The Gris de los Remedios Tuff was described as a lapilli
tuff with an andesitic chemical composition (Wedekind
et al. 2011). The colour of this well-welded tuff is grey to
light grey. The matrix contain phenocrysts of up to 10 mm
large prismatic hornblende and other, smaller melanocratic
minerals (dark), idiomorphic plagioclase crystals and large
lithic fragments (light), mainly pumice. The texture of this
tuff is clearly hypocrystalline. The bulk sample also con-
tains significant amounts of cristobalite, K-feldspar,
smectites and traces of muscovite/illite and quartz.
The Chiluca Tuff is a compact andesitic, porphyritic tuff
with a hypocrystalline texture. The tuff is grey in colour
with dark and light inclusions of mafic and felsic minerals.
The rock does not show a visible orientation of the crystals,
nor are flow textures present. In the thin section, euhedral
and subhedral well-zoned and twined plagioclase, idio-
morphic amphibole (hornblende) and large tabular biotite
are observed. Pyroxene and olivine are scarce. The clay
fractions show illite and an illite–smectite mixed layer
mineral, however, the amounts are small.
The Loseros Tuff is a felsic volcanoclastic rock with
rhyolitic composition that consists of very well sorted, fine-
to-medium, sand-sized crystals and detrital rock fragments,
which are embedded in an ash-rich and strong, altered
groundmass. The Loseros Tuff can appear in different
colour tones like greenish, reddish, purple and greyish, but
the greenish variety is characteristic and most frequently
used in construction (Buchanan 1980; Lo
´pez-Doncel et al.
2012). The majority of the grains are quartz, plagioclase
and volcanic lithic fragments (Edwards 1956). The crystals
are angular to subhedral plagioclase, angular quartz, and
strong altered biotite flakes. In thin section, it is possible to
recognize a microlamination (laminae), due to the varia-
tions in the frequency of the sand-sized fragments and the
matrix. The Loseros Tuff comes in different varieties. The
difference is related to the grain size and lime content.
Within the matrix, between 5 % to more than 20 % calcite
can be found. The matrix also contains kaolinite; however,
dioctahedral clay minerals illite plus R3 ordered illite
(0.95)-smectite mixed layers are the most common.
The Cantera Rosa Tuff is a pinkish to reddish rhyolite–
dacite tuff (rhyodacite). CR is an attractive building stone
because of the fine and homogenous groundmass contrast
with the presence of centimetre to decimetre sized, more or
less altered pumice clasts. It is likely that the presence of
olivine and a little bit of biotite give the tuff its charac-
teristic pinkish-reddish colour. The clasts are characteristic
because their matrix is normally a darker reddish colour.
The clasts occur in different sizes and shapes, from angular
and flattened to sub-rounded, but they are primarily ori-
ented in the flow direction. When the pumice clasts are
altered and eroded, the tuff looks porous. The fine hypo-
crystalline groundmass (completely opaque) consists of
quartz and feldspar, which are intercalated with little glass
shards (vitroclasts). The XRD pattern shows a feldspar-rich
matrix with anorthite, orthoclase, sanidine, and in lesser
amounts, quartz and traces of cristobalite. The clay fraction
contains smectite and also rarely found R0 ordered kao-
linite (0.85)–smectite mixed layer minerals.
The Cantera Formacio
´n Tuff is a pinkish to reddish,
massive rhyolitic ignimbrite tuff (hypocrystalline and par-
tially eutaxitic, porphyritic texture), with plenty of non-
collapsed pumice at an average size of 2 cm, with 15–20 %
of quartz, sanidine and less abundant biotite. The main
components are euhedral sanidine (40 %), anhedral to
euhedral quartz (40 %), weakly zoned oligoclase (4 %),
biotite and magnetite usually altered to hematite (1 %).
The matrix (75–80 % of the rock), shows vitroclastic tex-
ture in outlying areas to the phenocrysts and near them is
good eutaxitic foliation. In general, the matrix displays a
weak birefringence, suggesting that it devitrified to a fel-
sitic cryptocrystalline aggregate. XRD analysis indicates,
that this tuff is poor in clay minerals, only containing illite
and poorly ordered 1:1 clay minerals (kaolinite and hal-
loysite were proven by IR, infrared spectroscopy).
Petrophysical properties
All the measurements were done in all three dimensions of
the different rocks. The direction parallel to the bedding
Environ Earth Sci (2013) 69:1203–1224 1211
123
and lamination is defined as X, the direction perpendicular
to the lamination as Yand the direction perpendicular to the
bedding as Z(compare also Fig. 6).
Porosity
Four different groups can be distinguished according to
their effective porosity. Group 1: porosities ranging from 0
to 10 vol%; Group 2: from 10 to 20 vol%; Group 3: from
20 to 30 vol%; and Group 4 with porosities between 30 to
more than 40 vol% (Table 2).
Within group 1, TY with 4.7 vol% and HW with 8.8
vol% have the lowest and highest, effective porosity,
respectively. The other tuffs of this group (LS and CH)
range between 7 and 8 vol% (Table 2).
Within group 2, three tuffs show porosities ranging
between 12.8 and 18.4 vol% (CF, BP, BT, see Table 2).
Only two tuffs (ED, RP) have a porosity between 25 and up
to 27 vol% (Table 2). The last group comprises four tuffs
Fig. 6 Four samples illustrated as examples for the general texture of
the investigated rocks and their division into different directions:
aclastic material, like brownish pumice inclusions, is found oriented
parallel to a possible flow direction of the Cantera Amarilla Tuff. The
direction with an elongated accumulation of clasts is defined as the
Xdirection. The adjustment of the clasts in a more or less horizontal
position describes the bedding of the tuff. The direction perpendicular
to the bedding is defined as the Zdirection. bLong extended
aggregates of different minerals describe the flow direction of the TY
tuff (Xdirection). cThe fine lamination of the Loseros Tuff also
describes the bedding structure of the material. dExtended, dark-
coloured, iron-rich minerals show a lenticular shape and are defined
as the Xdirection. The direction perpendicular to these minerals is
defined as the Zdirection
Table 2 Pore space properties, moisture transport, and retention properties of the different tuffs
Stone type WB ED HW RP HD BT LS CR CA TY CH GR BP CF
Effective porosity (vol%) 37.1 24.56 8.76 27.13 30.24 18.39 7.06 40.8 41.56 4.74 7.95 31.03 15.26 12.83
Particle density (g cm
-3
) 2.43 2.29 2.59 2.64 2.63 2.61 2.34 2.56 2.53 2.38 2.58 2.55 2.53 2.6
Bulk density (g cm
-3
) 1.52 1.48 2.38 1.92 1.83 2.13 2.18 1.53 1.48 2.26 2.37 1.82 1.84 2.26
Average pore radius (lm) 0.412 0.495 0.004 0.335 0.145 0.06 0.095 0.988 0.554 0.303 0.358 1.285 0.035 0.804
Micropore porosity (%) 16 14 70 21 51 82 95 13 18 47 27 15 97 15
wvalue (kg/m
2
Hh)
X16.01
a
0.291 3.1
a
4.8
a
0.163 0.18 0.026 6.86 3.34 0.11 0.39 8.30 0.147 0.113
Y– 0.355 – – 0.197 0.21 0.048 6.78 3.78 9.16 0.34 9.08 0.134 0.110
Z15.77
a
0.226 2.82
a
4.6
a
0.175 0.17 0.045 6.43 0.51 0.1 0.38 6.21 0.073 0.066
Average [15.89 0.290 2.96 4.7 0.178 0.186 0.039 6.69 2.54 3.12 0.37 7.86 0.118 0.096
Anisotropy (%) 1.5 36 9 4 17 16 46 32 87 99 13 13 50 42
lvalue
X9.25 19,45 22.33 12.88 13.83 16.7 64.5 9.89 7.27 22.27 88.97 10.04 15.37 78.28
Y9.38 12.38 20.42 12.44 12.88 14.08 40.93 10.2 7.58 36.43 59.86 9.75 16.51 70.61
Z9.39 24.42 24.02 15.47 17.30 19.28 66.7 10.35 7.72 80.51 118.56 10.35 21.52 130.27
Average [9.34 18.75 22.25 13.59 14.67 16.68 57.37 10.14 7.52 46.40 89.13 10.04 17.8 93.05
Anisotropy (%) 1.5 49 15 19.5 25.5 15.5 39 4 6 72 49.5 6 27 46
Sorption 95 % rh
(g/cm
3
)
0.043 0.067 0.120 0.026 0.045 0.054 0.090 0.0085 0.039 0.066 0.0084 0.047 0.130 0.0061
a
Stueck (2006)
1212 Environ Earth Sci (2013) 69:1203–1224
123
with a porosity ranging between 30 and up to 42 vol% (HD,
WB, CR, and CA).
Density
Bulk density ranges between 1.38 and 2.38 g/cm
3
(Table 2). Only two samples have a low bulk density
between 1.38 and 1.48 (ED, CA). Five samples range
between 1.52 and 1.92 g/cm
3
(WB, CR, GR, HD, and RP).
The other six have a density from 2.13 up to 2.38 g/cm
3
(BT, LS, TY, CF, CH, and HW).
Microporosity
The tuff rocks investigated have a low (up to 50 %) or a
high amount (up to 100 %) of micropores measured by
mercury porosity (Fig. 7). The microporosity is defined as
the pores between 0 and 0.01 lm, the capillary active
pores range between 0.01 and 10 lm after Klopfer
(1985). After Klopfer three clear regions can be differ-
entiated depending on water transport and other mecha-
nism involved. For microprobes pore diameters \0.1 lm
water will condense at relative humidity values below
99 % RH. In pores between 1 lm and 1 mm capillary
suction takes place (Siegesmund and Duerrast 2011). The
microporosity of the investigated rocks ranges between 13
and 97 %. All results are listed in Table 2and illustrated
in Fig. 7.
As pertains to the eight rock types explained in detail,
BP has the highest microporosity with 97 %, followed by
BT with 82 %, HW with 70 % and HD with 51 %. The
tuffs with a low microporosity are ED with 14 %, CA with
18 %, RP with 21 % and TY with 47 % (Fig. 6).
Pore radii distributions
Pore size distribution measured by mercury porosimetry
can be put into groups of ideal pore size types like
unimodal equal and unequal, and bimodal unequal (Sieg-
esmund and Duerrast 2011). The investigated tuffs show
diverse pore types including transitional ones.
The group with largely equal porous structure and a
limited spectrum of pore sizes consists of BT and BP
(Figs. 7,8). Both nearly only have microporosity
(0.01–0.1 lm pore size) with 78 and 90 %, respectively.
Their pore size distributions proceed in a uniform pat-
tern with a low tendency to an unequal unimodal structure
Fig. 7 Overview diagram of the main pore space properties and structural characteristics measured by hydrostatic weighting and MIP
Environ Earth Sci (2013) 69:1203–1224 1213
123
(Figs. 7,8). Both tuffs also are comparable in porosity and
density (Table 2).
Both RP and LS belong to the group of equal pore size
types with a wider spectrum of pore sizes. 52 % of the
pores in RP are between 0.1 and 1 lm, but in general, both
tuffs have pores between 0.0015 and 10.1 lm. This group
is represented by RP, shown in Fig. 8.
The group of unequal pore size types with a unimodal
spectrum of pore sizes consists of CA, CF, CR, WB, and
ED. This group is represented by CA and ED (Fig. 8).
The tuffs with a large amount of capillary active pores
(1–10 lm) are CA with 82 % and CR with 87 %. Conse-
quently,the micropores range between 13 and 18 % (Table 2).
Eight of the tuffs investigated show an unequal pore size
type with a bimodal spectrum of pore sizes (Fig. 7). Three
tuffs have a poorly developed bimodal spectrum and the
two remaining tuffs only show a low tendency to a bimodal
pattern. The majority of the eight selected tuffs have a
higher amount of capillary pores than micropores and only
two of them (HW and HD) contain more micropores than
capillary pores.
TY and HD have practically the same amount of cap-
illary and micropores (around 47 %). The Habichtswald
Tuff mostly has micropores (72 %), but also contains some
large capillary active pores and even macropores. RP has a
ratio of 1:2 between micropores and capillary active pores,
with a low tendency to a bimodal pore size spectrum. CA,
with 83 %, has the highest amount of capillary active pores
of the selected tuffs (83 %) and has also some macropores.
Macroscopic/textural aspects
The arrangement and the spatial distribution of the com-
ponents into a tuff (matrix and fragments) seem to play an
important role in the behaviour of their petrophysical,
mechanical and moisture properties. Moreover, some tuffs
have no remarkable array components and appear to have
no major differences between fragments and matrix, giving
them a rather homogeneous and massive appearance, while
most samples exhibit an orientation of the clasts with
lamination or layering. Of the 14 rocks investigated, six
show well-developed layering (BP, TY, HW, BT, CA, and
RP, Figs. 4,5). On the other hand, six tuffs show marked
differences between the size, shape and amount of the
fragments and their contents in matrix, giving them a very
inhomogeneous (unsorted) appearance (Fig. 7). ED and the
HD show an almost homogenous, well-sorted structure.
Fig. 8 Pore size distribution of the eight investigated tuffs described in detail, measured by hydrostatic and MIP
1214 Environ Earth Sci (2013) 69:1203–1224
123
Our observations allow us to suggest that the spatial
distribution of the different fragments, like their layering,
and their proportion with respect to the matrix seem to have
a significant effect on moisture uptake, sorption, and
mechanical properties, and are determinant for the mois-
ture expansion often associated with a clear anisotropy.
Moisture expansion
The hydric expansion shows values that range from
1.4 mm/m for LS up to near moisture contraction for CR
and CF (Fig. 9).
There are notable differences in the values of the
expansion under water-saturated conditions. The LS
showed the largest hydric expansion with 1.4 mm/m, CR
even a reduction in length with -0.03 mm/m (Fig. 8).
Some horizons of HD (not included in this study) can show
a hydric expansion of up to 6 mm/m. In general, the
expansion perpendicular to the layering is the largest.
Six tuffs have a moisture expansion between 0.5 and
1 mm/m and another seven samples showed values ranging
from 0.4 mm/m to above zero (Table 3).
Apart from LS, CA shows the highest values of hydric
dilatation perpendicular to the lamination (Zaxis) with
0.9 mm/m and an anisotropy of 82 %, followed by BT with
0.8 mm/m and 55 % anisotropy, and 0.6 mm/m for HD
and BP, with anisotropy values of 24 and 8 %, respectively
(Table 3). TY has a moisture expansion of 0.5 mm/m and a
high anisotropy of 71 %, while the RP reached a hydric
dilatation of 0.42 mm/m with a low anisotropy of 8.1 %.
HW and ED both show a moderate hygric and hydric
dilatation and a high similar anisotropy between 50 and
60 %.
Capillary water absorption
The calculated water absorption coefficient (wvalue) is
given in Table 2for all directions. The average wvalue of
Fig. 9 Hydric expansion under
water-saturated conditions of
the different tuff samples
Table 3 Hygric expansion in mm depending on relative humidity and water saturation, the difference in percent for the three directions (X,Y,
Z) and its anisotropy
Stone type WB ED HW RP HD BT LS CR CA TY CH GR BP CF
X95 % RH 0.57 0.11 0.34 0.02 – 0.18 0.14 0.29 0.18 0.27 0.08 0.39 0.6 –
Xwet 0.241 0.145 0.248 0.360 0.467 0.361 0.314 0.0006 0.12 0.17 0.286 0.022 0.45 -0.051
Xdiff. (%) 58 24 27 94 – 50 55 99.8 33 63 72 94 25 –
Y95 % RH 0.48 – 0.29 0.01 – 0.203 0.254 0.28 0.16 0.28 0.08 0.42 0.73 –
Ywet 0.351 0.201 0.295 0.379 0.578 0.442 0.555 0.0003 0.11 0.27 0.283 0.021 0.56 -0.01
Ydiff. (%) 27 – 15 97 – 54 54 99.9 31 4 72 95 23 –
Z95 % RH 0.65 0.19 0.29 0.02 – 0.33 0.33 -0.001 0.17 0.44 0.09 0.32 0.65
Zwet 0.585 0.353 0.235 0.427 0.618 0.798 1.428 0.0012 0.90 0.54 0.302 0.031 0.55 -0.026
Zdiff (%) 10 46 19 95 – 59 77 81 19 70 90 15 –
Anisotropy 95 % (%) 26 42 15 50 – 45 75 71 11.1 39 11.1 24 18 –
Anisotropy wet (%) 49.3 59 49.3 8.1 24.4 54.7 78 32.7 82 71.4 5.2 32.2 8.1 65
Average 95 % 0.56 0.15 0.30 0.016 – 0.24 0.24 0.19 0.17 0.33 0.083 0.38 0.66 –
Average wet 0.40 0.23 0.26 0.366 0.55 0.53 0.78 0.0007 0.38 0.33 0.29 0.025 0.52 -0.029
Environ Earth Sci (2013) 69:1203–1224 1215
123
the volcanites varies between almost 16 kg/m
2
Hh for WB
and 0.04 kg/m
2
Hh for LS. Four groups can be divided
according to their water absorption coefficients. Group 1
from [0 to 0.2 kg/m
2
Hh, group 2 from \0.2 to 1.0
kg/m
2
Hh, group 3 from\1.0 to[10 kg/m
2
Hh and group 4
from\10 kg/m
2
Hh. Group 1, with the lowest water uptake
rate, consists of five tuffs ranging from LS to 0.19
kg/m
2
Hh for BT with an anisotropy from 16 to 50 % and a
porosity from 7 to 30 %, respectively. The second group
contains ED Tuff with 0.29 kg/m
2
Hh (anisotropy of 36 %
and porosity of 25 %) and CH with 0.37 kg/m
2
Hh (13 %
anisotropy and porosity of 8 %). The third group contains
five tuff varieties from 2.54 kg/m
2
Hh for CA to 7.86
kg/m
2
Hh for GR. This group has an anisotropy ranging
from 99 % for TY to 4 % for RP and porosities from 5 to
42 %, respectively. The last group consists of only one
rock, WB, with a high average absorption value of more
than 15 kg/m
2
Hh. This tuff has an anisotropy of 1.5 % and
a porosity of 37 %.
Water vapour diffusion
The values of water vapour diffusion range between 130.27
and 7.27. The highest water vapour resistance was mea-
sured for CF with an average value of 93.05 and an
anisotropy of 46 %, followed by CH with nearly 90 and an
anisotropy of 50 % and TY with 46.4 and anisotropy val-
ues of 72 %. The anisotropy values seem to be affected
most probably by the clear layering of the rock (Table 2).
Eight tuffs range between 20 and 10 and only three samples
have a water vapour diffusion value \10 (Table 2).
For the chosen rock samples, the highest water vapour
resistance (l) was measured for TY with 46.4, accompa-
nied by a high anisotropy of 72 %. Only one of these
samples has a vapour resistance value [20 (HW) and a
moderate anisotropy of 15 %. Five of the samples (ED,
HW, RP, HD, BT and BP), comprising the majority, have l
values between 10 and 20 with an anisotropic behaviour
between 15.5 and 49 %. Only CA has a lvalue \10 with a
low anisotropy of 6 %.
Swelling pressure
LS reached the lowest swelling pressure of only 0.005 MPa.
On the other hand, two samples reached medium swelling
pressure values of 0.015 (BT) and 0.03 MPa (BP). However,
CA, with the lowest dilatation value, does not show the
lowest swelling pressure value with 0.011 MPa. All results
show a clear correlation, the swelling pressure measure-
ments suggest that there is relationship between the obtained
moisture expansion results and those obtained by the
swelling pressure experiments. A particular case represents
LS with low swelling pressure values but contrasting relative
high hydric dilatation (Fig. 9).
In general, the swelling pressure is relatively low and
well below the splitting tensile strength (between 0.005 and
0.03 MPa). It can be assumed that swelling and contraction
as a result of drying and wetting has an effect on deterio-
ration even when the pressure induced is low.
Sorption and saturation degree
Hygroscopic water sorption was measured between 20 and
95 % RH by 30 "C in a climatic chamber. The measure-
ments were carried out on drilling core slides with a
diameter of 20 mm and thickness of 50 mm, respectively.
The amount of absorbed water is given per g/cm
3
of the
tested material. We can divide the measured samples into
three groups: low, middle and high hygroscopic water
sorption. By far the highest level is shown by the samples
from BP, LS, and TY. The first two stone types also show
the highest amount of micropores with more than 90 %
(Table 2). The middle group includes the majority of the
investigated tuffs (0.02–0.05 g/cm
3
, Table 2). The last
group contains only three tuffs, which have sorption values
of less than 0.01 g/cm
3
. CF, with only 0.006 g/cm
3
, shows
the lowest saturation at 95 % RH.
A clear tendency of correlation is observable between
the amount of micropores and the saturation degree
(Fig. 10). The highest sorption is shown by the samples
that have the highest number of micropores. All volcanites
show their highest saturation degrees at 95 % RH.
Splitting tensile strength and compressive strength
Under dry conditions, all of the splitting tensile strength
(STS) values vary between 0.99 and 10.65 MPa (Table 4).
Fig. 10 Correlation of sorption by 95 % RH and the amount of
microporosity
1216 Environ Earth Sci (2013) 69:1203–1224
123
The water-saturated samples reached values between 0.49
and 8.71 MPa.
The resistance against tensile stresses of all kind of
rocks, including the tuffs, is one of the most important
parameters for all physical weathering processes. Espe-
cially if the stones have distinct layering and spatial ori-
entation of any foliation or sedimentary layering, splitting
tensile strength tests can help understand the role of
binding forces between different components and
expandable minerals, like some clay minerals (Siegesmund
and Duerrast 2011). Of the 14 investigated tuffs, six have a
clearly recognizable laminated (foliated) pyroclastic fabric
(CA, BP, RP, BT, TY, and LS). The influence of this
layering on mechanical stress is given by the anisotropic
behaviour according to the splitting tensile strength. While
the decrease in the measured values for RP with 33 %,
followed by LS (24 %), HD (20 %), WB (17 %) and BP
Tuff (15 %) alone reached critical values, the impact of
anisotropy by water saturation changed dramatically for
some samples, e.g. HD reached 63 %, HW 31 %, BP 22 %
and BT 20 %. The last two rocks show a clear layering.
The highest splitting tensile strength reduction is observed
for HD and CA (both 49 %) followed by BP (44 %) and
the BT (37 %). Both lithic fragments and large porphyric
crystals can also have an effect on the structural break-
down and the fractured surface. In the case of the Bufa
Tuff, the fractures along the XY plane (parallel to the
bedding) took place along the material boundary between
the clearly oriented lithic components and the fine micro-
crystalline matrix. Lithic fragments can also control the
occurrence of fractures in Cantera Amarilla Tuff. These
fragments, mostly pumice, have a lower hardness than the
matrix and are only partly connected to it (Ja
´uregui et al.
2012).
It is well known that water-saturated tuffs as well as
other porous stone types have a lower strength than under
dry conditions and that there is a correlation between the
modulus of elasticity and the compressive strength (Toe-
roek et al. 2007). An average reduction of around 30 %
(ranging from 12–75 %) was observed in the 14 different
tuffs (Table 4).
The compressive strength ranges from around 8.2 N/mm
2
for CA to 98.0 N/mm
2
for CF (see Table 5). For ten of the
investigated samples, all directions were taken into con-
sideration. Four of these ones only have a very low
anisotropy, between only 0.1 and 8 % (Table 5).
Table 4 Splitting tensile strength (STS) in dry and water-saturated conditions and the respective strength reduction
Stone type Direction WB ED HW RP HD BT LS CR CA TY CH GR BP CF
Splittinge tensile strength dry
ß
SZ
(MPa)
X1.46
a
3.30
b
2.43
a
1.91
a
3.70 6.04 6.29 3.94 0.99 5.43 5.13 2.27 6.90 10.65
Z1.76
a
3.23
a
2.85
a
4.62 6.95 8.25 4.02 1.05 5.71 5.61 2.24 5.89 9.89
Anisotropy
(%)
17 – 25 33 20 7 24 2 6 5 8.5 1 15 7
Splitting tensile strength
water saturated b
SZ
(MPa)
X1.25
a
2.78
b
2.10
a
1.45
a
1.14 3.65 5.82 2.61 0.49 3.94 4.56 1.39 3.99 8.23
Z1.25
a
3.07
a
1.77
a
3.1 4,57 6.69 3 0.56 4.12 4.79 1.58 3.11 8.71
Anisotropy
(%)
0 – 31.5 18 63 20 13 13 12.5 4 5 12 22 5.5
STS reduction (%) [22 37 9 32 49 37 14 30 49 26 13 34 44 18
Reduction (%) X29 – 5 38 16 24 7 34 43 16 11 39 42 23
Reduction (%) Z14 – 13.5 24 75 47 19 25 53 28 15 29 47 12
a
Stueck (2006)
b
Stueck et al. (2008)
Table 5 Uniaxial compressive strength (UCS) in dry conditions (N/mm
2
)
Direction WB ED HW RP HD BT LS CR CA TY CH GR BP CF
X18–19.3
a
– 14–60
b
56
c
37.24 66.69 74.26 31.9 9.45 50.82 91.94 17.25 66.07 95.60
Y– 39.84 64.34 66.9 29.94 8.21 50.86 83.2 16.58 62.23 94.30
Z– 32.04 59.59 57.98 32.56 9.72 50.82 90.58 18.08 53.67 98.04
Anisotropy (%) – – – – 20 11 22 8 15 0.1 21 8 19 4
a
Mueller (1987)
b
Haberland (2006)
c
Vereinigte Porphyrbrueche auf dem Rochlitzer Berge GmbH
Environ Earth Sci (2013) 69:1203–1224 1217
123
Discussion and conclusion
Contrary to many other rock types, most of the volcanic
rocks are formed during a relatively short period of time.
The development can be divided into two main processes:
lava cools down at the surface or sedimentation of volcanic
ash and rock fragments are ejected during a volcanic
eruption (tuffs).
The matrix of the investigated tuff rocks consists prin-
cipally of fine sand up to clay sized pyroclastic material
(mostly volcanic ash and pumice) distributed and packed in
very different ways, depending on the extrusion type. In the
case of the extrusive igneous rock types, the material cools
down so quickly that crystallisation does not occur or is
incomplete. The resulting ash particles and glass shards of
the tuffs show mostly irregular sharp edges, angular to very
angular grain and fragment shapes and consequently a
poorly or weakly welded matrix (Figs. 4,5). This fact
allows the formation of a very complex system of pore
networks defined by macroporosity and microporosity. The
different constituents of pyroclastic rocks in particular can
show a layered and laminated texture, which induced a
distinct anisotropy in terms of its petrophysical, mechanical
and moisture properties. Thus the rock fabric plays a very
important role in the behaviour of the rock to moisture and
weathering and controls its petrophysical properties
(Siegesmund and Duerrast 2011).
Water transport and uptake took place due to the cap-
illary active pores. Compared with the microporosity, the
averaged wvalue should show a weak correlation (Fig. 11).
The microporosity and the wvalue are compared in
Fig. 11a. In general, samples with a high microporosity
have a lower wvalue.
Swellable clay minerals content based on XRD
and CEC results
In spite of the comprehensive investigations we carried out,
the real cause for the moisture expansion observed, either
under hygric or hydric conditions, remains unclear. It is
most likely due to an interaction between different pro-
cesses (Ruedrich et al. 2011).
XRD quantification of clay minerals, particularly in
volcanic rocks, is demanding work (Moore and Reynolds
1997; Dohrmann et al. 2009). The application of the
Rietveld method is complicated when unknown amounts of
glass are present. Nonetheless, the CEC method could be
used as an approximation to determine if expandable clay
minerals are dominant or not (Dohrmann and Kaufhold
2010). Ruedrich et al. (2011) discussed the different CEC
values of the individual clay minerals and the use of the
measured CEC values of the rock samples for the semi-
quantitative approximation of the smectite contents. The
unit of the CEC value is milliequivalent per 100 g (meq/
100 g).
The CEC values listed in Table 1for the eight selected
samples range from 0.7 (HD) to 17.6 (TY).
The TY, ED, BP, HW, and CA samples (Fig. 12) con-
tain smectites, as verified by ethylene glycol intercalation
(EG) of separated\2lm fractions. BT also expanded upon
EG treatment, however, the smectitic interlayers are part of
two different types of illite–smectite mixed layers, as
verified by thermal treatment (375 "C: all interlayers were
collapsed to 10 A
˚). HD contains very small amounts of
smectitic interlayers, the dominating clay minerals are very
close to pure illite (R3 illite(0.95)-smectite mixed layers).
RP, on the other hand, did not expand at all after EG
treatment.
The CEC is highest for the TY, HW, and BP tuff
(16–18 meq/100 g), reflecting the highest smectite content
of approximately 15–20 wt% at least for TY and HW. The
large CEC of BP can partly also be explained by clinop-
tilolite, a mineral of the zeolite group. Zeolites are also
known for their large CEC values (for discussion and
problems with these CEC values see Dohrmann et al.
2012).
ED has also a high CEC value of 10 meq/100 g
reflecting approximately 10 wt% smectite. Smectite was
Fig. 11 a Correlation of the wvalue and the average pore.
bCorrelation of the wvalue and the microporosity
1218 Environ Earth Sci (2013) 69:1203–1224
123
also confirmed by EG expansion in CA, which contains
based on the CEC approximation ca. 5 wt% smectite. BT
has lower amounts of smectitic layers (ca. 3 %) in mixed
layer minerals of different ordering types, whereas the
amounts of smectitic layers are very low in the HD sample
(\1 wt%). RP does not contain smectite at all and the low
CEC value of 1 meq/100 g can be explained by the large
kaolinite content.
Moisture expansion as a function of the CEC value
In the present study, there is no clear correlation between
the CEC values and the moisture expansion (Fig. 13). No
correlation between the whole set of CEC values and the
hydric expansion could be identified, however, for samples
with CEC values\10 meq/100 g (low smectite contents) a
trend is visible (Fig. 13). Some of these samples show a
pronounced anisotropic behaviour, in the case of water
transport properties or mechanical strength, which can
probably be traced back to the fabric characteristics, e.g.
the large clasts of CA and the layering in BT. BP, HW, and
TY have the highest CEC values, between 15.6 and
17.6 meq/100 g (Fig. 13), but they do not really show
significant expansion, although they have a high amount of
smectite. These three tuffs showed medium to high aver-
aged splitting tensile strength values between 2.83 and
6.39 MPa, which probably influence the expected higher
moisture expansion (Table 4). Two of the investigated tuff
rocks (GR, ED) have a substantial CEC value (around
10 meq/100 g), but a similarly low hydric dilatation
(Fig. 13).
Moisture expansion as a function of the microporosity
and average pore radius
In contrast to the relation between the CEC value and
moisture expansion, there is an obvious correlation
between the microporosity and the hydric expansion
(Fig. 14a). In particular, a correlation can be detected for
the hydric expansion on the Xand Yaxes, probably because
the swelling process is not affected by layering and lami-
nation (Fig. 14b).
Most of the studied rocks show a correlation between
the average pore radius and the hydric moisture expansion
values. In general, it can be observed that a decrease of the
hydric dilatation occurs with the increase of the average
pore radius, which is clearly recognizable in Fig. 15.
Softening properties as a function of moisture content
Weathering often occurs in the base area of historical
constructions erected of volcanic stones (Yavuz 2006;
Wedekind et al. 2011; Ja
´uregui et al. 2012). One reason
for deterioration is due to the decrease in strength upon
wetting of the rock, described by Hirschwald (1908) as
‘‘softening’’. The softening degree varies among the dif-
ferent lithotypes. For crystalline rocks, such as granites and
Fig. 12 X-ray diffraction patterns of whole rock tuff samples
Fig. 13 Moisture expansion and the CEC value do not show a clear
correlation
Environ Earth Sci (2013) 69:1203–1224 1219
123
gneisses, the softening effect is minimal. However, this
effect can be significant in porous rocks such as sandstones
and tuffs, and can reach a strength decrease of up to 50 %
(Siedel 2010; Ja
´uregui et al. 2012). Morales Demarco et al.
(2007) found a positive correlation between the decrease of
the compressive strength and the magnitude of moisture
expansion in sandstones. This correlation has been verified
for the tensile strength of sandstones in Ruedrich et al.
(2011).
For the weakening and decay of stones, the stress
induced by weathering processes has to exceed the rock
strength. One of the most useful methods to determine this
strength is the tensile strength, because this property is
significantly lower compared to the compressive strength.
This strength is mainly controlled by the porosities as
presented in Fig. 16 for the Zand Xdirection.
The analyzed tuffs show a weak correlation between the
moisture expansion and the reduction of the splitting ten-
sile strength, especially for the Xdirection (Fig. 16b). In
this case, the dilatation due to the effects of moisture
causes a weakening and softening of the rock, particularly
under water-saturated conditions, which is shown for the
Xand Zaxes (Fig. 16a, b). Moisture expansion and the
STS-value in the Zdirection give evidence of a weak
correlation for most of the samples (Fig. 17a). The graph
for the strength versus the expansion in the Xdirection only
shows an undefined point cloud (Fig. 17b).
Moisture expansion related to the physical properties
The fabric of a rock affects the petrophysical properties
(Siegesmund and Duerrast 2011). One key factor is the
pore size distribution, which affects the stone’s durability
(Punuru et al. 1990; Fitzner and Basten 1994; Benavente
et al. 2004). Ruedrich et al. (2011) found that the effect of
the disjoining pressure is relevant in rocks with a high
amount of micropores. In fact, tuffs like HW, HD, BT, LS,
TY and BP have a significant portion of micropores (50 %
or more). These samples also show the highest values of
hydric dilatation, especially on the Zaxis (Fig. 14a).
Especially HD, which has a very low CEC value, shows an
important dilatation. In this case, the dilatation is controlled
exclusively by the microporosity and its dense and com-
plex pore network that is associated to the layering of the
tuff. This observation points to the disjoining pressure as
a major factor affecting moisture expansion (Fig. 5,
Hilbersdorf Tuff, c).
Our investigations demonstrate that, besides the clay
minerals, the microporosity has a significant effect on
swelling and moisture expansion. Ruedrich et al. (2011)
suggest the importance of the relation between clay mineral
content, porosity and the location of the clay minerals
within the pore space. The influence of the swelling
depends on the clay minerals if they are oriented along the
grain boundaries. However, if the tuffs contains high
amounts of clay minerals but the pores are too large, i.e. the
pore walls are only coated by the clay minerals, then the
swelling pressure induced by the clay minerals will not
have any significant effect on the moisture expansion, as
Fig. 14 a Moisture expansion and the microporosity show a distinct
tendency of correlation. bMoisture expansion on the Xand Yaxes
shows a clear correlation
Fig. 15 Moisture expansion and the average pore radius show a
tendency of correlation
1220 Environ Earth Sci (2013) 69:1203–1224
123
was clearly demonstrated by SEM-observations made of
BP, HW, TY, and ED (Fig. 18). Reyes-Zamudio et al.
(2011), and Ruedrich et al. (2011) were also able to show
that rocks with a higher amount of clay minerals and larger
pores do not show higher moisture expansion. BT seems to
be a special case. The SEM images show other iron-rich
minerals besides the presence of some clay minerals
(Fig. 19a, b). The observed dilatation in this tuff may also
be associated with an expansion due to the oxidation and
crystallisation of these minerals (Fig. 19c).
In general, hydric and hygric swelling in porous rocks
were discussed due to swellable clay minerals. Different
hypotheses, like inner-crystalline or osmotic swelling, are
the leading hypotheses. However, both theories would
require that the clay minerals be responsible for a higher
CEC value, and lastly, that they be correlated to greater
moisture expansion.
However, our extensive investigation on tuffs with a
broad variation in mineralogy and rock fabrics shows that
the above-mentioned correlation cannot be supported. At
first, the total amount and type of clay minerals are also of
critical importance. A detailed clay mineral study is
absolutely necessary to explain the observed moisture
expansion.
1. We were able to show that the amount of micropo-
rosity has a significant impact on the intensity of the
moisture expansion. Tuffs with a higher amount of
microporosity show a higher moisture expansion, even
when the swellable clay minerals are lacking.
2. The study also demonstrated that there is a clear
correlation between the amount of moisture swelling
and the softening of the tuffs, as well as the pore size
and the degree of water saturation.
3. There is no doubt that the clay minerals can signifi-
cantly contribute to moisture expansion, but not all of
the moisture-related expansion can be attributed to the
presence and amount of the minerals of this group. The
influence of clay minerals is noticeable in the case of
fine laminated varieties showing a high anisotropy
on moisture transport properties and mechanical
strength.
Fig. 16 Correlation between the splitting tensile strength (STS) and the effective porosity. aCorrelation on the Zaxis and bon the Xaxis
Fig. 17 The splitting tensile strength and moisture expansion show a weak correlation ain the Zdirection and bin the Xdirection under dry
conditions only (b)
Environ Earth Sci (2013) 69:1203–1224 1221
123
4. The importance of the disjoining pressure may have
been overlooked in the literature on moisture expan-
sion. A high moisture expansion was clearly
demonstrated in the case of some tuffs, which were
more or less devoid of any clay minerals showing CEC
values lower than 1.0 meq/100 g.
Fig. 18 a SEM images of Blanca Pachuca Tuff (BP) with partly
filled pores. bThe Habichtswald Tuff (HW), with a clay rich pore
space coating out of smectite. cInner structure of the Tenayoca
´tetl
Tuff (TY) shows altered minerals covered with a fine coating of
smectite (plagioclase in the middle)dEger-Demje
´n Tuff (ED) with
long drown-out pores
Fig. 19 a Bufa Tuff (BT) with oriented iron-rich minerals. bRecrystallisation of Fe-oxide on the surface. cClose-up view with well-crystallized
Fe-oxides
1222 Environ Earth Sci (2013) 69:1203–1224
123
In summary, it is undoubtedly clear that not only clay
minerals may affect the swelling pressure, and that quiet
complex processes of mineralogical aspects and rock fab-
rics, like microporosity and the interaction of moisture
films, have to be considered.
Acknowledgments We are grateful to Detlev Klosa of the BGR for
the SEM images, and Ch. Gross for their help and useful discussion.
Our work was supported by the DAAD, German Science Foundation
(DFG Si-438/44-1) and the Hans-Boeckler-Foundation.
Open Access This article is distributed under the terms of the
Creative Commons Attribution License which permits any use, dis-
tribution, and reproduction in any medium, provided the original
author(s) and the source are credited.
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