Franzoni E., Sassoni E., Scherer G.W., Naidu S., Artificial weathering of stone by heat-

ing, Journal of Cultural Heritage 14S (2013) e85-e93, DOI:10.1016/j.culher.2012.11.026

ARTIFICIAL WEATHERING OF STONE BY HEATING

Elisa Franzoni1, Enrico Sassoni2,*, George W. Scherer3, Sonia Naidu4

1 Dipartimento di Ingegneria Civile, Ambientale e dei Materiali, Università di Bologna, via Terracini 28, 40131 Bologna, Italy,

Tel: +39 051 2090329, Fax: +39 051 2090322, e-mail: elisa.franzoni@unibo.it Assistant Professor

2 Dipartimento di Ingegneria Civile, Ambientale e dei Materiali, Università di Bologna, via Terracini 28, 40131 Bologna, Italy,

Tel: +39 051 2090363, Fax: +39 051 2090322, e-mail: enrico.sassoni2@unibo.it Ph.D.

* Corresponding Author

3 Department of Civil and Environmental Engineering, Princeton University, Eng. Quad. E-319, Princeton, NJ 08544, USA,

Tel: +1 609 258 5680, Fax: +1 609 258 1563, e-mail: scherer@princeton.edu Full Professor

4 Department of Chemical and Biological Engineering,

Princeton University, Eng. Quad. E-226, Princeton, NJ 08544, USA, Tel: +1 609 258 9089, Fax: +1 609 258 1563, e-mail: snaidu@princeton.edu

Ph.D.

Franzoni E., Sassoni E., Scherer G.W., Naidu S., Artificial weathering of stone by heat-

ing, Journal of Cultural Heritage 14S (2013) e85-e93, DOI:10.1016/j.culher.2012.11.026

1

ABSTRACT

Since the effectiveness of stone consolidants significantly depends on the weather-

ing level of the stone samples on which they are tested, in this study the suitability

of heating stone to high temperature, as an artificial weathering method to induce

controllable microstructural, physical and mechanical alterations, was investigated.

Three lithotypes with different characteristics were used: Giallo Terra di Siena (GS,

a highly porous calcareous sandstone), Globigerina limestone (GL, a highly porous

limestone) and Pietra Serena (PS, a porous quartzitic sandstone with low porosity).

The lithotypes were characterized in terms of mineralogical composition, pore size

distribution and water absorption, as well as dynamic modulus, static modulus,

compressive and tensile strength. They were then heated for 1 hour, in different

conditions: (i) dry samples were heated to 100, 200, 300 and 400°C; (ii) water-

saturated samples were heated to 200°C; (iii) water–saturated samples were heated

to 200°C and, after cooling to room temperature, re-heated to 400°C. After heating,

all the lithotypes experienced an increase in open porosity and water absorption, as

a consequence of the anisotropic thermal deformation of calcite crystals. Corre-

spondingly, GS and GL exhibited an increasing reduction in mechanical properties

for increasing heating temperature. PS, on the contrary, exhibited an increase in

compressive and tensile strength, which was attributed to chemical-physical trans-

formations undergone by secondary mineralogical fractions (clay minerals, etc.) at

high temperature. All things considered, heating proved to be a fairly effective and

reproducible method to cause artificial weathering in stone samples for the testing

of consolidants. However, depending on the microstructural characteristics of the

lithotypes, the effectiveness of heating may vary significantly, which requires a

case-by-case adjustment of the most suitable heating procedure and the develop-

ment of complementary methods for artificial weathering.

KEYWORDS

Artificial weathering; Heating; Microstructure; Mechanical properties; Thermal

deformation; Stone consolidation; Calcite

Franzoni E., Sassoni E., Scherer G.W., Naidu S., Artificial weathering of stone by heat-

ing, Journal of Cultural Heritage 14S (2013) e85-e93, DOI:10.1016/j.culher.2012.11.026

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1. INTRODUCTION

Stones used in sculpture and architecture, as well as rocks in their original loca-

tion, are exposed to environmental weathering, which is responsible for alterations

in their microstructure (e.g. open porosity, pore size distribution, chemical-

mineralogical composition of the phases, etc.), that, in turn, result in a modifica-

tion of physical and mechanical properties.

The open porosity and effective porosity (i.e. the interconnected open po-

rosity between two opposite sides of a specimen) generally increase as a conse-

quence of weathering, responsible for the opening of new micro-cracks, the wid-

ening of existing micro-fractures and the dissolution of the most soluble fractions

[1, 2]. For instance, a sensible increase in effective porosity (+4% in the first 5 cm

from the surface, compared to the underlying unweathered part) was detected in

the case of a tuff-made stonework exposed to environmental weathering for 150

years [3]; more dramatic increases (up to 10 times) were found in the case of

rocks weathered to various degrees [1, 4]. Alongside the modification in porosity,

pore size distribution is generally affected by weathering as well. On the one

hand, the average pore size increases when dissolution of the most soluble frac-

tions occurs, so that smaller pores are internally dissolved and/or collapsed, thus

forming larger pores [1, 5]; on the other hand, the average pore size decreases

when clay minerals, formed through weathering, and/or crystallized soluble salts

partially fill the pores, thus reducing the pore size [1]. As a consequence of the

modification in open porosity and/or pore size distribution, also specific surface

area is affected by weathering, generally increasing with increasing weathering

degree [1]. Due to these modifications in the pore system, the physical properties

of weathered stones are altered as well. In particular, water absorption, responsi-

ble for all the weathering mechanisms related to water (e.g., freeze-thaw cycles,

crystallization of soluble salts, swelling of clays, etc.), was found to increase by

up to 5 times in weathered stones, compared to unweathered ones [1]. Moreover,

the increase in pore amount and size is responsible for significant decreases in

mechanical properties of weathered stones. Ultrasonic pulse velocity and dynamic

Franzoni E., Sassoni E., Scherer G.W., Naidu S., Artificial weathering of stone by heat-

ing, Journal of Cultural Heritage 14S (2013) e85-e93, DOI:10.1016/j.culher.2012.11.026

3

elastic modulus were reduced by about 40% in weathered rocks and stoneworks

[3, 4]. Moreover, weathered rocks exhibit a strong decrease in static elastic modu-

lus (-30-60%), compressive strength (-30-80%) and tensile strength (-20-60%), in

comparison with the same unweathered lithotypes [1, 4].

To restore cohesion between grains and mechanical properties of weath-

ered architectural stones, consolidating treatments are usually applied. The effec-

tiveness of such consolidants significantly depends on the weathering level of the

tested stone [6], so the actual performance of new consolidating materials should

preferably be evaluated on weathered specimens. Nevertheless, environmentally

weathered stone samples are rarely available in sufficient quantity and with suita-

bly constant characteristics, hence the importance of developing methods for arti-

ficial weathering of stone, able to provide altered specimens with uniform and re-

producible characteristics, is evident.

Extensive experimental studies on artificial weathering of stone have been

performed with the aim of reproducing and accelerating environmental weathering

processes occurring in the field, i.e. dissolution in clean and acid rain and for-

mation and dissolution of soluble salts, owing to pollutant dry deposition (see re-

view in [7]). These studies, performed on different types of stone (mainly car-

bonate stones), involved a wide range of environmental conditions, in terms of

temperature, relative humidity, presence of atmospheric gaseous pollutants (such

as SO2, NOx, O3) and particulate matter, as well as presence of acidic solutions

(such as H2SO4, HNO3, H2CO3, HCl) (see, among others, [8-16]). However, those

experimental investigations generally aimed at evaluating the rate of stone surface

recession (determined by measuring stone weight loss and/or the amount of Ca2+

ions dissolved from the stone). Therefore, they provide little guidance regarding

the effects of the artificially induced decay on stone microstructure and physical-

mechanical properties that would be of use in producing weathered stone samples

for testing of consolidants.

A more promising method for obtaining artificially damaged stone speci-

mens could be to adopt some methods described in international standards for as-

sessing stone resistance to various weathering mechanisms (such as ageing by

Franzoni E., Sassoni E., Scherer G.W., Naidu S., Artificial weathering of stone by heat-

ing, Journal of Cultural Heritage 14S (2013) e85-e93, DOI:10.1016/j.culher.2012.11.026

4

SO2 action [17], salt crystallization [18], freeze-thaw [19] or thermal shock [20]).

Such methods can be used to intentionally induce damage in the stone [2, 21, 22];

however, these methods exhibit some drawbacks:

(i) the duration of the weathering procedures: according to the cited European

Standards, a prolonged exposure period is required for determining the re-

sistance to SO2 (21 days, [17]), salt crystallization (up to 15 cycles [18]),

freeze-thaw (up to 168 cycles [19]) and thermal shock (up to 20 cycles, unless

failure happens earlier [20]). Accordingly, in the cited studies 5-32 cycles of

salt crystallization [2, 22], 50-75 cycles of freeze-thaw [2, 21] and 32-50 cy-

cles of thermal shock [21, 22] were repeated to achieve significant weather-

ing;

(ii) the conditions of stone samples at the end of the weathering procedure: artifi-

cial weathering by freeze-thaw, salt crystallization and thermal shock is hard-

ly controllable and capable of being modulated to a desired weathering level,

so that weathered samples often exhibit intense cracking and huge material

loss [2, 22]. In addition, samples subjected to salt crystallization are usually

contaminated by high amounts of salts. Such opening of cracks, detachment

of parts and contamination by salts, although being somehow representative

of decay conditions that may be experienced in the field, complicate the eval-

uation of consolidant performance (in terms of chemical reactivity, ability to

bond grains, etc.).

Therefore, in this study, as an alternative artificial weathering method, the

effectiveness of heating stone to high temperature for obtaining damaged speci-

mens with uniform and reproducible characteristics, but still uncontaminated by

salts, was investigated. Heating was selected as a promising damaging method be-

cause:

(i) crystals of calcite, present in most architectural stones, such as limestones,

marbles and calcareous sandstones, are known to undergo a marked aniso-

tropic thermal expansion, expanding parallel and contracting perpendicular to

crystallographic c-axis (respectively, about 0.19% and 0.04% for a thermal

Franzoni E., Sassoni E., Scherer G.W., Naidu S., Artificial weathering of stone by heat-

ing, Journal of Cultural Heritage 14S (2013) e85-e93, DOI:10.1016/j.culher.2012.11.026

5

variation from 20 to 100°C) [23, 24]; such anisotropic deformations are re-

sponsible of stress that can lead to the opening of micro-cracks at grain

boundaries. As a matter of fact, grain decohesion and intergranular cracking

owing to natural heating-cooling cycles have been recognized as a main cause

of weathering processes that typically affect marbles, e.g. bowing and sugar-

ing [24-28];

(ii) crystals of different minerals, which may be present in heterogeneous stones

such as granite and sandstone, have different thermal expansion coefficients

and hence undergo different deformations, which may result in additional mi-

cro-cracks [29];

(iii) the possible presence of water inside the pores, when stone is heated to high

temperature, may be responsible of further damaging, as a consequence of the

pressure exerted on pore walls [30] and of the chemical reaction between wa-

ter and Si-O bonds on the surface of the newly formed micro-cracks [31].

Given the frequent problems of thermal weathering of marbles, many experi-

mental studies have been carried out to analyze the effects of heating-cooling cy-

cles on marble porosity and/or mechanical properties (see for instance [24, 30, 32,

33]). However, these studies were basically aimed at imitating the thermal varia-

tions in the field, hence they generally involved heating-cooling cycles with a rel-

atively low maximum temperature (rarely exceeding 100 °C). The effects of heat-

ing stone samples to higher temperatures – not representative of realistic thermal

excursions, but useful to induce more dramatic alterations in stone microstructure

– have been considered only in few cases and only for marble and granite [29,

32].

The effect of heating as an artificial weathering method for stone was in-

vestigated for the first time in a previous study [6], where Indiana limestone (a

medium porous limestone) was heated at 100, 200, 300, 400 and 500 °C for 1, 4

and 16 hours. For the heated samples, a decrease in dynamic elastic modulus line-

arly proportional to the heating temperature was found. Samples heated to 300 °C

for 1 hour exhibited a decrease of -43.4% in dynamic modulus and of -27.1% in

Franzoni E., Sassoni E., Scherer G.W., Naidu S., Artificial weathering of stone by heat-

ing, Journal of Cultural Heritage 14S (2013) e85-e93, DOI:10.1016/j.culher.2012.11.026

6

tensile strength. Heating duration longer than 1 hour proved to have no further ef-

fect on the dynamic modulus reduction.

In this study, for three lithotypes with different microstructural characteris-

tics, the effectiveness of heating to high temperature (up to 400°C) to induce con-

trollable microstructural, physical and mechanical alterations, has been systemati-

cally investigated. In particular, the effects of heating have been evaluated in

terms of modifications in porosity and pore size distribution, water absorption and

sorptivity, dynamic and static modulus, compressive and tensile strength.

2. EXPERIMENTAL PROCEDURE

2.1 Stones

Three lithotypes with significantly different microstructural characteristics were

selected:

(i) Giallo Terra di Siena (GS), a highly porous calcareous sandstone traditionally

used in architecture in Tuscany (provided by Il Casone S.p.A., Italy). It is

mainly composed of carbonatic granules of algal fragments, shells, fragments

of bivalves and micro-crystalline calcite and may contain low amounts of

quartz, feldspars and fragments of magmatic and metamorphic rocks; it has

fine to very fine grains, partially oriented, bonded by carbonatic cement that

does not completely fill the voids between the grains;

(ii) Globigerina Limestone (GL), a very highly porous limestone, typically used

in modern and historical architecture in Malta (provided by Xelini Skip Hire

and High-Up Service, Malta): it is mainly composed of calcite crystals and

fossils, including globigerinae (from which the name derives), shells and sea

urchins; it may contain low amounts of quartz, feldspars and clays; it has fine

grains, bonded by carbonatic cement that does not completely fill the pores;

(iii) Pietra Serena (PS), a quartzitic sandstone with low porosity, typically used in

historical architecture in Florence (provided by Il Casone S.p.A., Italy): it is

mainly composed of quartz, feldspars, micas and fragments of silicate and

carbonatic rocks; it has medium grains, oriented parallel to the bedding planes

Franzoni E., Sassoni E., Scherer G.W., Naidu S., Artificial weathering of stone by heat-

ing, Journal of Cultural Heritage 14S (2013) e85-e93, DOI:10.1016/j.culher.2012.11.026

7

and bonded by carbonatic cement that almost completely fills the voids be-

tween the grains.

2.2 Stone characterization

2.2.1 Microstructural characteristics

The mineralogical composition was determined by powder X-ray diffraction

(XRD, Philips Diffractometer PW 1840, 40kV/20mA, Cu Kα radiation) and the

carbonate content was assessed on duplicate samples by the Dietrich-Frühling gas

volumetric method (based on the measurement of the CO2 volume released by re-

acting the powdered sample with HCl). The morphological characteristics were

evaluated by observation on a scanning electron microscope (SEM, Philips

XL20). The total open porosity (OP), the pore size distribution and the specific

surface area (SSA) were determined by mercury intrusion porosimetry (MIP,

Fisons Macropore Unit 120 and Porosimeter 2000 Carlo Erba).

2.2.2 Physical properties

The rate of water sorption by capillarity (sorptivity) was measured according to

EN 15801 [34] using 5 cm cubes. To avoid any alterations due to sample drying in

an oven, even at relatively low temperature, the samples were preliminarily dried

at room temperature using a fan until constant weight. To evaluate the effect of

heating on stone sorptivity, the same samples used for the unheated condition

were dried again at the end of the test and then heated to different temperatures,

then the sorptivity of the heated samples was determined (§ 2.4): in this way any

possible uncertainty owing to stone microstructural variability was avoided. Since

all the three lithotypes exhibited some anisotropy owing to bedding planes (§ 3.1),

the sorptivity was measured with the water flux in the direction parallel to the

bedding planes. This orientation was chosen because anisotropic stones are usual-

ly placed in the building with the bedding planes horizontally (i.e. perpendicular

to the loads, so that the strongest direction is stressed), hence, the penetration of

water inside the stone by capillarity (for instance in case of a rain event) happens

in the direction parallel to the bedding planes.

Franzoni E., Sassoni E., Scherer G.W., Naidu S., Artificial weathering of stone by heat-

ing, Journal of Cultural Heritage 14S (2013) e85-e93, DOI:10.1016/j.culher.2012.11.026

8

The water absorption by capillarity (WA) was determined on triplicate

samples (5 cm side cubes) as the total amount of water penetrated after 24 hours,

in the same conditions as the sorptivity test.

2.2.3 Mechanical properties

Dynamic elastic modulus (Ed) was determined on 2 cubes (5 cm side) for each

lithotype and calculated according to the formula Ed = ρV2, where ρ is the bulk

density and V the ultrasonic pulse velocity, measured by means of a commercial

instrument (Matest) with 55 kHz transducers (transmission method), using a rub-

ber couplant between the transducers and the sample. Given the anisotropy of the

lithotypes, the Ed was calculated as the average for the values measured in the

three directions.

Static elastic modulus (E) and compressive strength (σc) were determined

by axial compressive tests perpendicular to the bedding planes on 2 cubes (5 cm

side), using an Amsler loading machine (maximum load 600 kN) at a constant

loading rate of 2.5 kN/s. Strains were measured by two LVDT coupled to the

plates of the loading machine. The reliability of this system had been preliminari-

ly validated by comparing to results obtained in parallel measurement with strain

gauges and an LVDT. The static modulus was calculated as the slope of the

straight part of the stress-strain curve.

Tensile strength (σt) was measured by performing the Brazilian test on 6

cylinders (5 cm height and 2 cm diameter, core-drilled perpendicular to the bed-

ding planes), using an Amsler-Wolpert loading machine (maximum load 100 kN)

at a constant displacement rate of 4 mm/min.

2.3 Artificial weathering by heating

Cubic and cylindrical samples of each lithotype were artificially weathered

according to the following procedures:

(i) heating in a furnace at 100, 200, 300 and 400 °C for 1 hour in dry conditions;

Franzoni E., Sassoni E., Scherer G.W., Naidu S., Artificial weathering of stone by heat-

ing, Journal of Cultural Heritage 14S (2013) e85-e93, DOI:10.1016/j.culher.2012.11.026

9

(ii) heating in a furnace at 200 °C for 1 hour after water saturation, reached by

partially immerging the samples into deionized water and letting them soak

for 24 hours (hereinafter, this condition is referred to as WS+200°C) ;

(iii) heating in a furnace at 200 °C for 1 hour after water saturation (reached as

above) and then, after cooling to room temperature, re-heating to 400 °C in

dry conditions (hereinafter referred to as WS+200+400°C).

All the samples were inserted into the furnace once it had reached the desired

temperature.

2.4 Evaluation of heating effects

The modifications in porosity, pore size distribution and water absorption were

evaluated for samples heated in the WS+200°C condition, at 400°C and in the

WS+200+400°C condition. To prevent variability in stone microstructure from

confounding the evaluation of the heating effects, the samples for the MIP test af-

ter heating were taken very close to the sampling point used for MIP test on the

unheated stone. As for the water absorption test, the same samples used for char-

acterizing the unheated stone were used, after they were dried and then heated in

the three conditions reported above.

Finally, the modification in sorptivity was evaluated by repeating the sorp-

tivity test on the same sample that had been used to characterize the unheated

stone, after it was re-dried at room temperature and heated to 400°C.

3. RESULTS AND DISCUSSION

3.1 Stone characterization

The mineralogical composition and the carbonate content of the three lithotypes

are reported in Table 1. Since both calcite and dolomite are present in sample PS,

the carbonate content, measured by the Dietrich-Frühling method, accounts for

both carbonate minerals and is hence indicated as equivalent calcium carbonate

content in Table 1. The SEM images illustrating the lithotype morphology are re-

ported in Figure 1. The pore size distributions and the results of water absorption

Franzoni E., Sassoni E., Scherer G.W., Naidu S., Artificial weathering of stone by heat-

ing, Journal of Cultural Heritage 14S (2013) e85-e93, DOI:10.1016/j.culher.2012.11.026

10

and sorptivity tests are reported in Figure 2, while the values of OP, average pore

radius, SSA and WA are summarized in Table 2. The stress-strain curves obtained

from the axial compressive test are reported in Figure 3 and the average values of

Ed, E, σc and σt are summarized in Table 4.

GS is a highly calcareous sandstone (CaCO3 = 84.3%), also containing

quartz and Na-feldspar. Its highly porous microstructure can be noted in the SEM

image and from MIP results (OP = 20.12%). It exhibits a quite coarse average

pore radius (rav = 3.5 μm), resulting in moderately high sorptivity and a total water

absorption of 4.0%. GS exhibits a dynamic elastic modulus Ed = 35.3 GPa with

some anisotropy (Ed measured perpendicular to the bedding planes is about 10%

smaller than that parallel to the bedding planes). The static elastic modulus, E, is

sensibly lower, since it amounts to 11.8 GPa. It is typical for Ed to be higher than

E, which has been attributed to the difference in strain amplitude between dynam-

ic and static measurements, the possible presence of fluid inside pores, and the

presence of cracks, clays, and planes of weakness and foliation [35, 36]. Com-

pressive and tensile strength of GS amount to 43.6 MPa and 6.2 MPa, respective-

ly.

GL is a limestone mainly containing calcite (CaCO3 = 91.4%) and small

amounts of quartz. The calcium carbonate is largely owing to fossil micro-

organisms, as can be seen in the SEM image. It exhibits a highly porous micro-

structure (OP = 40.06%, rav = 3.0 μm) resulting in a very high sorptivity and high

water absorption (WA = 14.6%). As a consequence of the very high porosity, GL

exhibits low values of both dynamic and static modulus (Ed = 15.2 GPa and E =

6.5 GPa), E and Ed being quite different for the reasons discussed above. GL

shows a slight anisotropy as well, the Ed perpendicular to the bedding planes be-

ing about 6% higher than Ed measured parallel to them.† Compressive and tensile

strength show pretty small values (σc = 27.5 MPa and σt = 3.1 MPa).

† This is opposite to the typical anisotropy, where the direction normal to the bedding is less stiff; however, the result is reproducible: parallel to the bedding planes Ed = 14.8 and 14.7 GPa, while in the perpendicular direction Ed = 15.6 GPa. After water saturation, Ed decreases in all directions (Ed = 13.1 and 13.0 in the two parallel directions, and Ed = 14.2 in the perpendicular direction), but the sense of the anisotropy is preserved.

Franzoni E., Sassoni E., Scherer G.W., Naidu S., Artificial weathering of stone by heat-

ing, Journal of Cultural Heritage 14S (2013) e85-e93, DOI:10.1016/j.culher.2012.11.026

11

PS is a highly quartzitic sandstone, mainly containing quartz and feldspars,

alongside calcite and dolomite (CaCO3 = 12.8%) and small amounts of micas and

pyroxenes. It exhibits a very dense microstructure (OP = 6.05%) and a small av-

erage pore radius (rav = 0.029 μm) and consequently the sorptivity is very slow

and the total water absorption limited to 1.3%. Consistently, mechanical proper-

ties are high (Ed = 32.2 GPa, E = 11.8 GPa, σc = 109.4 MPa, σt = 8.0 MPa). A

marked anisotropy was detected: Ed measured perpendicular to the bedding planes

is about 35% lower than that measured parallel.

The three lithotypes selected in this study hence show a significant varia-

bility in mineralogical composition and porosity, which was chosen to evaluate

the heating effects on stones with different microstructural characteristics.

3.2 Heating effects

The effects of heating on the porosity, water absorption and sorptivity of the three

lithotypes are illustrated in Figure 2 and summarized in Table 2. The changes in

Ed, E, σc and σt after heating are reported in Table 3 and the modifications in com-

pressive stress-strain curves are illustrated in Figure 3.

In the case of lithotype GS, heating proved to have a significant effect on

both microstructural-physical and mechanical properties. Indeed, an increasing

reduction in mechanical properties for increasing temperatures was experienced,

as suggested by the results reported in Figure 3 (top) and Table 3. In particular,

both dynamic and static modulus exhibited a maximum decrease after heating in

the WS+200+400°C condition (ΔEd = -32.5% and ΔE = -23.5%). Compressive

strength also decreased for increasing temperatures, reaching a minimum after

heating at 400 °C (Δσc = -14.0%), while tensile strength experienced an equal re-

duction (Δσt = -14.3%) after heating at 400 °C and in the WS+200+400°C condi-

tion.

The decrease in mechanical properties can be attributed to the opening of

micro-cracks consequent to the stress induced by calcite anisotropic thermal de-

formation during heating [24, 33]. This is confirmed by the increase in porosity

detected for increasing heating temperatures (Figure 2). The maximum increase in

Franzoni E., Sassoni E., Scherer G.W., Naidu S., Artificial weathering of stone by heat-

ing, Journal of Cultural Heritage 14S (2013) e85-e93, DOI:10.1016/j.culher.2012.11.026

12

porosity (ΔOP = +15.7%) was found after heating at 400 °C, i.e. in the condition

that caused the greatest decrease in compressive and tensile strength. After heat-

ing at 400 °C, the amount of small pores increased in absolute terms (pores having

radius r < 1 μm increased, for instance, from 18.1 mm3/g to 21.2 mm3/g after heat-

ing to 400 °C).

The increases in porosity and amount of small pores led to increases in wa-

ter absorption and sorptivity (Figure 2). Samples heated at 400 °C experienced the

highest WA increase (ΔWA = +20%). Correspondingly, after heating at 400 °C the

results of the sorptivity test exhibited basically no modification in the initial part

of the graph (whose slope depends on the amount and size of the coarser pores,

which were not substantially altered by heating) and an increase in the slope of the

second part of the graph (which is more influenced by the smaller pores, which

increased in number after heating), leading to a higher amount of water absorbed

after 24 hours.

In addition to the effect of calcite crystal anisotropic deformation, some

contribution to stone damage from the presence of water during heating (and dur-

ing microcrack opening) seems present [30, 31], since both Ed and E exhibited an

additional decrease after heating in the WS+200°C condition, compared to 200°C

in dry conditions, and in the WS+200+400°C condition, compared to 400°C in dry

conditions (Table 3).

GL exhibited a decrease in mechanical properties after heating to high

temperature, as shown in Figure 3 (medium) and Table 3. In this case, the maxi-

mum decreases in dynamic and static elastic modulus were found after heating in

the WS+200+400°C condition (ΔEd = -28.2% and ΔE = -14.8%). However, the

reduction in compressive strength in the same heating condition is smaller (Δσc =

-9.8%) and, more significantly, a definite trend of reduction for increasing tem-

peratures cannot be clearly recognized (Table 3). In the case of tensile strength, a

reduction Δσt = -13.7% was found after heating both at 400 °C in dry conditions

and in the WS+200+400°C condition.

The reduction in mechanical properties of GL seems overall to be less pro-

nounced than that experienced by GS, which may be explained considering that the

Franzoni E., Sassoni E., Scherer G.W., Naidu S., Artificial weathering of stone by heat-

ing, Journal of Cultural Heritage 14S (2013) e85-e93, DOI:10.1016/j.culher.2012.11.026

13

very high open porosity of GL (OP = 40.06%, basically double that of GS) can al-

low some deformation of calcite crystals without causing stress in the stone and mi-

cro-cracks opening. Consistently, no significant systematic modification in pore

size distribution was observed (Figure 2), the difference in total open porosity be-

tween unheated and heated samples being more likely attributable to the intrinsic

stone variability than to modifications occurring after heating. However, new pores

and cracks at the nano-scale, not detectable by MIP measurement, may have devel-

oped after heating and be responsible for the decrease in mechanical properties.

According to the small variation in the pore system, the variations in

sorptivity and water absorption are quite small (Figure 2). The highest increase in

WA, experienced after heating in the WS+200+400°C condition, is +7.5% (Table

2), while only small changes in sorptivity were found after heating at 400°C.

In the case of lithotype PS, some significant and partly unexpected modifi-

cations in mechanical properties were found (Figure 3, bottom, and Table 3). Dy-

namic elastic modulus steadily decreased for increasing heating temperatures,

reaching a minimum for the WS+200+400°C condition (ΔEd = -21.3%). However,

for the same heating condition, the decrease in the static elastic modulus only

amounts to 3.4% and no definite trend for increasing temperatures exists (actually,

E value after heating to 400°C is slightly higher than that of unheated samples).

The fact that, for increasing heating temperatures, Ed steadily decreases while E on-

ly shows little changes can be explained considering that, as a consequence of heat-

ing, some micro-cracks opened in the stone (OP increased from 6.05% to 8.58% af-

ter heating at 400°C, Table 2). Such micro-cracks, although causing an increase in

the time required for the ultrasonic wave propagation (hence a decrease in the dy-

namic modulus), apparently had a limited effect on the static modulus, which also

depends on the relative position and orientation of the cracks [36].

Moreover, in spite of the decrease in Ed and insignificant change in E, a

sharp increase in both compressive and tensile strength was measured (Table 3).

Although unexpected, such increase in stone strength after heating is however

consistent with some experimental findings reported in the literature [37, 38].

Those studies report that compressive strength, tensile strength and elastic modu-

Franzoni E., Sassoni E., Scherer G.W., Naidu S., Artificial weathering of stone by heat-

ing, Journal of Cultural Heritage 14S (2013) e85-e93, DOI:10.1016/j.culher.2012.11.026

14

lus of sandstone at high temperature (300 °C in [37] and 400 °C in [38]) exhibit an

increase that was attributed to the “hot-melt effect” of some minerals [38]. To val-

idate the hypothesis that the increase in compressive and tensile strength observed

in this study could be a consequence of some chemical-physical transformation

undergone by secondary mineralogical fractions (e.g., clay minerals), the possible

presence of clays, although not detected by XRD, was further investigated. The

morphology of some fractions observed by SEM suggested that clay minerals

could actually be present (Figure 4, left) but, to have a further confirmation, com-

pressive strength test was performed on dry and water-saturated samples. As a

matter of fact, clay bearing stones are known to undergo a dramatic decrease in

static modulus when they are tested in water-saturated conditions [39], hence a

comparison of the static elastic modulus of dry and water-saturated PS triplicate

samples was performed. Considering that (i) a decrease ΔE = -17% was found for

the water-saturated PS samples (Figure 4, right); (ii) stones containing high

amounts of clays are reported to undergo reductions in ΔE from -45% to -120%

[39]; (iii) fired clay bricks, not containing any clay mineral, underwent reductions

in ΔE of 3-5%, the presence of small amounts of clay minerals seems confirmed.

The increase in compressive and tensile strength found in PS samples after heat-

ing at high temperature could therefore be attributed to some chemical-physical

transformation undergone by clay-type minerals, which led to a more effective

bonding of stone grains.

As a consequence of heating, PS underwent a progressive increase in po-

rosity and amount of smaller pores with increasing temperature. After heating to

400 °C, OP increased by +41.8% and the average pore radius decreased from

0.029 μm in the unheated sample to 0.008 μm. This increase in porosity is due to

the opening of new micro-cracks, owing to the anisotropic thermal deformation of

the calcite crystals contained in the stone cementing fraction and to the differential

thermal deformation of the different minerals. The opening of micro-cracks is also

favored by the very dense microstructure of the stone, which (contrary to the case

of GL) does not allow any deformation of grains without causing stress at the

grain boundaries.

Franzoni E., Sassoni E., Scherer G.W., Naidu S., Artificial weathering of stone by heat-

ing, Journal of Cultural Heritage 14S (2013) e85-e93, DOI:10.1016/j.culher.2012.11.026

15

As a consequence of the increase in porosity, sorptivity and water absorp-

tion increase for samples heated to high temperature (for instance, ΔWA = +23.1%

after heating to 400 °C).

All things considered, heating to high temperature caused significant

changes in microstructural, physical and mechanical properties of the studied

lithotypes: the alterations in OP, WA and mechanical properties after heating at

400 °C (which caused the most significant changes in most of the cases) are sum-

marized in Table 4. Such alterations can suitably resemble alterations owing to

environmental weathering, which generally causes increases in OP and WA and

decreases in mechanical properties. However, in the case of PS, in spite of an in-

crease in porosity and water absorption, heated samples exhibited an increase in

strength, which suggests that preliminary tests should be carried out on each spe-

cific lithotype to set-up the most effective heating procedure.

4. CONCLUSIONS

The results of the present study indicate that heating is a fairly effective and reproduc-

ible method to cause artificial weathering in stone samples that can later be used for

the testing of stone consolidants. Indeed, by using artificially weathered stone speci-

mens, where micro-cracks and grain decohesion have been induced, a more signifi-

cant evaluation of a consolidating treatment’s effectiveness, in terms of binding ac-

tion and strengthening effect, can be achieved, compared to the use of unweathered

stone specimens.

In particular, heating at 400 °C proved to be the most effective weathering

condition in most of the cases. In fact, especially for lithotypes containing high

amounts of calcite (such as GS and GL), the increases in porosity, sorptivity and wa-

ter absorption and the decrease in mechanical properties after heating at 400 °C suita-

bly resemble those experienced by stone subjected to environmental weathering.

Moreover, heating has the advantage of being a controllable method, which provides

samples “weathered” to a desired degree, by choosing the appropriate heating condi-

tion.

Franzoni E., Sassoni E., Scherer G.W., Naidu S., Artificial weathering of stone by heat-

ing, Journal of Cultural Heritage 14S (2013) e85-e93, DOI:10.1016/j.culher.2012.11.026

16

However, depending on the nature and microstructure of the stone subject-

ed to artificial weathering by heating, the effectiveness of the treatment may vary

significantly. In fact, in spite of the satisfactory artificial damage induced in cal-

careous lithotypes (GS and GL) by heating at 400 °C, the same weathering condi-

tion proved to cause no mechanical damaging in the quartzitic sandstone analyzed

(PS). Consequently, the effects of heating on the stone type to be used for testing

of consolidants should be assessed by means of preliminary tests and the most ap-

propriate heating conditions hence tailored to the specific stone type. Further

complementary methods for artificial weathering are anyway worth of investiga-

tion.

ACKNOWLEDGEMENTS

Dr. Gabriela Graziani and Dr. Enzo Padula are gratefully acknowledged for their

collaboration on stone characterization. Dr. Francesco Fusi is gratefully acknowl-

edged for the SEM observations.

REFERENCES

[1] A. Tuğrul, The effect of weathering on pore geometry and compressive strength of selected rock types from Turkey, Eng Geol 75 (2004) 215-227

[2] D.T. Nicholson, Pore properties as indicators of breakdown meachanisms in experimentally weathered limestones, Earth Surf Process Landforms 26 (2001) 819-838

[3] T. Esaki, K. Jiang, Comprehensive study of the weathered condition of welded tuff from a his-toric bridge in Kagoshima, Japan, Eng Geol 55 (1999) 121-130

[4] G. Vasconcelos, P.B. Lourenço, C.A.S. Alves, J. Pamplona, Ultrasonic evaluation of the physical and mechanical properties of granites, Ultrasonics 48 (2008) 453-466

[5] M.G. Orkoula, P.G. Kousoukos, Dissolution effects on specific surface area, particle size and porosity of pentelic marble, J Colloid Interf Sci 239 (2001) 483-8

[6] E. Sassoni, S. Naidu, G.W. Scherer, The use of hydroxyapatite as a new inorganic consolidant for damaged carbonate stones, J Cult Herit 12 (2011) 346-355

[7] E. Franzoni, E. Sassoni, Correlation between microstructural characteristics and weight loss of natural stones exposed to simulated acid rain, Sci Total Environ, 412-413 (2011) 278-285

[8] K.M. Neal, S.H. Newnam, L.M. Pokorney, J.P. Rybarczyk, Elemental analysis of simulated acid rain stripping of Indiana limestone, marble and bronze, In: R Bobain (Ed), Materials deg-radation caused by acid rain, ACS Symposium Series; Washington, DC, 1986. p. 285-300

Franzoni E., Sassoni E., Scherer G.W., Naidu S., Artificial weathering of stone by heat-

ing, Journal of Cultural Heritage 14S (2013) e85-e93, DOI:10.1016/j.culher.2012.11.026

17

[9] R.J. Cheng, J.R. Hwu, J.T. Kim, S.M. Leu, Deterioration of marble structures – The role of acid rain, Anal Chem 59 (1987) 104A-106A

[10] J.B. Johnson, S.J. Haneef, B.J. Hepburn, A.J. Hutchinson, G.E Thompson, G.C. Wood, Labo-ratory exposure systems to simulate atmospheric degradation of building stone under dry and wet deposition conditions, Atmospheric Environment, 24A, 10 (1990) 2285-2592

[11] J.B. Johnson, M. Montgomery, G.E. Thompson, G.C. Wood, P.W. Sage, M.J. Cooke, The in-fluence of combustion-derived pollutants on limestone deterioration: 1. The dry deposition of pollutants gases, Corros Sci 38 (1996) 105-131

[12] J.B. Johnson, M.,,Montgomery, G.E. Thompson, G.C. Wood, P.W. Sage, M.J. Cooke. The in-fluence of combustion-derived pollutants on limestone deterioration: 2. The wet deposition of pollutants species, Corros Sci 38 (1996) 267-278

[13] E. Zendri, G. Biscontin, A. Bakolas, G. Finotto. Simulated study on the chemical and physical decay of the acid rain on carbonate stone, In: J. Riederer (Ed), Proceedings of the 8th Interna-tional Congress on Deterioration and Conservation of Stone, Berlin, 1996, p. 273-279

[14] L.Tecer, Laboratory experiments on the investigation of the effects of sulphuric acid on the deterioration of carbonate stone and surface corrosion, Water Air Soil Poll 114 (1999) 1-12

[15] C.M. Grossi, M. Murray, Characteristics of carbonate building stones that influence the dry deposition of acidic gases, Constr Build Mater 13 (1999) 101-108

[16] M.J. Thornbush, H.A. Viles, Simulation of the dissolution of weathered versus unweathered limestone in carbonic acid solutions of varying strength, Earth Surf Proc Land 32 (2007) 841-852

[17] European Standard EN 13919, Natural stone test methods - Determination of resistance to ageing by SO2 action in the presence of humidity, 2004

[18] European Standard EN 12370, Natural stone test methods - Determination of resistance to salt crystallization, 2001

[19] European Standard EN 12371, Natural stone test methods - Determination of frost resistance, 2010

[20] European Standard EN 14066, Natural stone test methods - Determination of resistance to ageing by thermal shock, 2004

[21] H. Yavuz, Effect of freeze-thaw and thermal shock weathering on the physical and mechani-cal properties of an andesite stone, Bull Eng Geol Environ 70 (2011) 187-192

[22] J. Martìnez-Martìnez, D. Benavente, M.A. Garcìa-del-Cura, Spatial attenuation: The most sensitive ultrasonic parameter for detecting petrographic features and decay processes in car-bonate rocks, Eng Geol 119 (2011) 84-95

[23] S. Siegesmund, K. Ullemeyer, T. Weiss, E.K. Tschegg, Physical weathering of marbles caused by anisotropic thermal expansion, Int J Earth Sci 89 (2000) 170-182

[24] J.M. Logan, Laboratory and case studies of thermal cycling and stored strain on the stability of selected marbles, Env Geol 46 (2004) 456-467

[25] G. Royer-Carfagni, Some considerations on the warping of marble façades: the example of Alvar Aalto’s Finland Hall in Helsinki, Const Build Mater 13 (1999) 449-457

Franzoni E., Sassoni E., Scherer G.W., Naidu S., Artificial weathering of stone by heat-

ing, Journal of Cultural Heritage 14S (2013) e85-e93, DOI:10.1016/j.culher.2012.11.026

18

[26] A.M. Ferrero, M. Migliazza, A. Spagnoli, Theoretical modelling of bowing in cracked marble slabs under cyclic thermal loading, Constr Build Mater 23 (2009) 2151-2159

[27] S. Chatterji, Aspects of generation of destructive crystal growth pressure, J Cryst Growth 277 (2005) 566-577

[28] V. Shushakova, E.R. Fuller Jr, S. Siegesmund, Influence of shape fabric and crystal texture on marble degradation phenomena: simulations, Environ Earth Sci 63 (2011) 1587–1601

[29] S. Chaki, M. Takarli, W.P. Agbodjan, Influence of thermal damage on physical properties of a granite rock: porosity, permeability and ultrasonic wave evolutions, Constr Build Mater 22 (2008) 1456-1461

[30] A. Luque, E. Ruiz-Agudo, G. Cultrone, E. Sebastiàn, S. Siegesmund, Direct observation of microcrack development in marble caused by thermal weathering, Environ Earth Sci 62 (2011) 1375-1386

[31] S.M. Wiederhorn, S.W. Freiman, E.R. Fuller Jr, C.J. Simmons, Effects of water and other di-electrics on crack growth, J. Mater. Sci. 17 (1982) 3460-3478

[32] K. Malaga-Starzec, J.E. Lindqvist, B. Schouenborg, Experimental study on the variation in porosity of marble as a function of temperature. In: Siegesmund S, Weiss T, Vollbrecht A (eds) Natural stone, weathering phenomena, conservation strategies and case studies. Geo-logical Society Special Publication 205 (2002) 81-88

[33] K. Malaga-Starzec, U. Åkesson, J.E. Lindqvist, B. Schouenborg, Microscopic and macro-scopic characterization of the porosity of marble as a function of temperature and impregni-ation, Constr Build Mater 20 (2006) 939-947

[34] European Standard 15801, Conservation of cultural property - Test methods - Determination of water absorption by capillarity, 2010

[35] E. Fjær, Static and dynamic moduli of a weak sandstone, Geophysics 74, 2 (2009) WA102-WA112

[36] J. Martìnez-Martìnez, D. Benavente, M.A. Garcìa-del-Cura, Comparison of the static and dy-namic elastic modulus in carbonate rocks, Bull Eng Geol Environ (2011) 1-6, DOI: 10.1007/s10064-011-0399-y

[37] Rao Qiu-hua, Wang Zhi, Xie Hai-feng, Xie Qiang, Experimental study of the mechanical properties of sandstone at high temperature, J Cent South Univ Technol (2007) s1-0478-06

[38] Zhang LianYing, Mao XianBiao, Lu AiHong, Experimental study on the mechanical proper-ties of rocks at high temperature, Sci China Ser E-Tech Sci 52 (2009) 641-646

[39] I. Jimenez Gonzalez, G.W. Scherer, Effect of swelling inhibitors on the swelling and stress re-laxation of clay bearing stones, Env Geol 46 (2004) 364-377

Franzoni E., Sassoni E., Scherer G.W., Naidu S., Artificial weathering of stone by heat-

ing, Journal of Cultural Heritage 14S (2013) e85-e93, DOI:10.1016/j.culher.2012.11.026

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Table 1. Mineralogical composition of the lithotypes: C = Calcite; D = Dolomite; Q = Quartz; A = Albite (Na-feldspar); S = Sanidine ((K,Na)-feldspar); M = Mi-crocline (K-feldspar); P = Phlogopite (Mica); B = Bustamite (Pyroxene); CaCO3 = equivalent calcium carbonate content (+ + + = dominantly present; + + = present; + = traces; - = not present)

C D Q A S M P B CaCO3 wt.%

GS + + + - + + - - - - 84.3 (±0.6)

GL + + + - + - - - - - 91.4 (±1.6)

PS + + + + + + + + + + + - + - 12.8 (±1.0)

Franzoni E., Sassoni E., Scherer G.W., Naidu S., Artificial weathering of stone by heat-

ing, Journal of Cultural Heritage 14S (2013) e85-e93, DOI:10.1016/j.culher.2012.11.026

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Table 2. Microstructural and physical properties of the lithotypes: OP = open po-

rosity; rav = average pore radius; SSA = specific surface area; WA = water absorp-

tion

OP [%] rav [μm] SSA [m2/g] WA [wt. %]

GS

Unheated 20.12 3.516 1.51 4.0

WS + 200 °C 20.81 3.508 1.33 4.2

WS + 200 + 400 °C 22.83 3.832 1.74 4.5

400 °C 23.28 4.563 1.76 4.8

GL

Unheated 40.06 3.087 1.77 14.6

WS + 200 °C 37.41 3.607 1.40 15.1

WS + 200 + 400 °C 39.89 3.055 1.57 15.7

400 °C 36.50 3.104 1.18 15.6

PS

Unheated 6.05 0.029 1.54 1.3

WS + 200 °C 6.52 0.030 1.65 1.3

WS + 200 + 400 °C 6.63 0.022 1.60 1.7

400 °C 8.58 0.008 2.73 1.6

Franzoni E., Sassoni E., Scherer G.W., Naidu S., Artificial weathering of stone by heat-

ing, Journal of Cultural Heritage 14S (2013) e85-e93, DOI:10.1016/j.culher.2012.11.026

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Table 3. Mechanical properties of the lithotypes: Ed = dynamic elastic modulus; E

= static elastic modulus; σc = compressive strength; σt = tensile strength (Ed, E and

σc values are averages for 2 samples; σt values are averages for 6 samples; stand-

ard deviations in brackets)

Ed [GPa] E [GPa] σc [MPa] σt [MPa]

GS

Unheated 35.3 (±1.0) 11.8 (±0.1) 43.6 (±0.2) 6.2 (±0.5)

100 °C 33.8 (±1.1) 10.7 (±0.2) 42.9 (±3.1) -

200 °C 32.8 (±1.2) 10.7 (±0.6) 40.3 (±0.0) -

WS + 200 °C 31.0 (±0.1) 10.6 (±0.3) 41.4 (±2.8) 5.6 (±0.5)

300 °C 27.8 (±0.0) 10.6 (±0.1) 38.2 (±0.9) -

400 °C 24.9 (±0.7) 9.4 (±0.1) 37.5 (±0.0) 5.3 (±0.8)

WS + 200 + 400 °C 22.7 (±1.6) 9.0 (±0.0) 40.1 (±0.9) 5.3 (±0.3)

GL

Unheated 15.2 (±0.1) 6.5 (±0.2) 27.5 (±0.3) 3.1 (±0.2)

100 °C 14.5 (±0.0) 6.8 (±0.1) 25.3 (±1.6) -

200 °C 14.2 (±0.1) 6.5 (±0.0) 24.4 (±0.2) -

WS + 200 °C 13.6 (±0.4) 6.2 (±0.0) 26.0 (±1.6) 2.9 (±0.3)

300 °C 13.2 (±0.1) 5.8 (±0.1) 23.7 (±4.1) -

400 °C 11.3 (±0.5) 5.7 (±0.1) 26.6 (±1.2) 2.7 (±0.2)

WS + 200 + 400 °C 11.0 (±0.0) 5.5 (±0.1) 24.8 (±0.5) 2.7 (±0.2)

PS

Unheated 32.2 (±0.3) 11.8 (±0.0) 109.4 (±0.0) 8.0 (±0.4)

100 °C 30.6 (±0.0) 10.6 (±0.0) 102.1(±0.0) -

200 °C 30.6 (±0.7) 11.2 (±0.3) 102.4 (±6.6) -

WS + 200 °C 27.7 (±0.2) 11.5 (±0.1) 112.1 (±3.1) 8.0 (±1.1)

300 °C 28.3 (±0.4) 11.2 (±0.6) 106.1 (±10.6) -

400 °C 27.2 (±0.4) 12.0 (±0.0) 122.6 (±2.3) 8.8 (±0.9)

WS + 200 + 400 °C 25.0 (±0.2) 11.4 (±0.5) 114.8 (±0.6) 8.6 (±0.4)

Franzoni E., Sassoni E., Scherer G.W., Naidu S., Artificial weathering of stone by heat-

ing, Journal of Cultural Heritage 14S (2013) e85-e93, DOI:10.1016/j.culher.2012.11.026

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Table 4. Changes in microstructural, physical and mechanical properties of the

lithotypes after heating to 400°C, compared to unheated condition (OP = open po-

rosity; WA = water absorption; Ed = dynamic elastic modulus; E = static elastic

modulus; σc = compressive strength; σt = tensile strength)

ΔOP [%] ΔWA [%] ΔEd [%] ΔE [%] Δσc [%] Δσt [%]

GS +15.7 +20.0% -29.5 -20.3 -14.0 -14.5

GL -8.9 +6.8% -25.7 -12.3 -3.3 -12.9

PS +41.8 +23.1% -15.5 +1.7 +12.1 +10.0

Franzoni E., Sassoni E., Scherer G.W., Naidu S., Artificial weathering of stone by heat-

ing, Journal of Cultural Heritage 14S (2013) e85-e93, DOI:10.1016/j.culher.2012.11.026

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Figure 1. SEM images of the three lithotypes GS (top), GL (medium) and PS (bottom)

Franzoni E., Sassoni E., Scherer G.W., Naidu S., Artificial weathering of stone by heat-

ing, Journal of Cultural Heritage 14S (2013) e85-e93, DOI:10.1016/j.culher.2012.11.026

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Figure 2. Pore size distribution, water absorption and sorptivity of the three lithotypes GS (top),

GL (medium) and PS (bottom) (the graph y-axis has a different scale for different lithotypes to al-

low a better readability)

Franzoni E., Sassoni E., Scherer G.W., Naidu S., Artificial weathering of stone by heat-

ing, Journal of Cultural Heritage 14S (2013) e85-e93, DOI:10.1016/j.culher.2012.11.026

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Figure 3. Compressive stress-strain curves for the three lithotypes GS (top), GL (medium) and PS

(bottom) (the graph axes have a different scale for different lithotypes to allow a better readability)

Franzoni E., Sassoni E., Scherer G.W., Naidu S., Artificial weathering of stone by heat-

ing, Journal of Cultural Heritage 14S (2013) e85-e93, DOI:10.1016/j.culher.2012.11.026

26

Figure 4. SEM image of sample PS (left) and comparison between static elastic modulus of PS in

dry conditions and after water saturation (right)