The weathering of building stone in historical monuments is a slow process of decay. However, na-tural disasters such as earthquakes and human related disasters, e.g. wars can lead to sudden dama-ge. Both forms have a destructive affect on the man-made surface as well as to the rock material.
The decay of building stones proceeds from an increasing loss of strength to the final collapse of the binding forces of the material. The deterioration processes are influenced by both intrinsic and ex-trinsic aspects (Fig. 5). The inex-trinsic aspects are dominated by the texture, the pore space, the parti-cle size and the (chemical) composition that characterizes the material (Camuffo 1995). The extrin-sic factors are all environmental impacts that affect the material, like climate and object-specific environmental conditions. Both factors interact with each other and influence the properties of the material, their resistance and weathering and weathering forms.
Figure 5: Schematic design of intrinsic and extrinsic aspects and factors that characterize the material and influence the weathering as well as the stability of a stone within the environment.
1. 2. 1 Physical deterioration
In general, physical deterioration consists of the interplay of different conditions like expansion or contraction of the stone’s material. This can be done by thermal expansion and contraction, frost disintegration, salt crystallization and hydration, hydric swelling and shrinking and a biogenic phy-sical impact. Usually this is implied by mechanical stress, material fatigue or load exceeding the mechanical resistance of the stone (Fitzner 1978; Snethlage 1984).
Wa n j a We d e k i n d We a t h e r i n g a n d C o n s e r v a t i o n o f M o n u m e n t s
Structural deterioration phenomena related to physical deterioration consist of fissures and cracks and structural disintegration. Phenomena that affect the surface area of the material are sanding, flaking and scaling.
Extrinsic factors that are ascribed to physical deterioration are water, frost and salts as well as heat and cold. Stress takes place by water due to swelling or shrinking during water uptake and shrinking and swelling by drying. A constant alternation of the two conditions can lead to a weakening of the material, especially between zones that are wet and other ones that are still dry. Possible consequen-ces can be the formation of flakes and crusts as is shown in Figure 6.
Figure 6: a) The main wind and rain direction in Rome comes from the west. During a rainfall the front facade of the Nerva temple at the Forum Augustus is completely wet, whereas on the back the water runs down in veins. b) In these zones weathering intensity is the greatest. c) Where the stone is wet, weathering crusts flake away from the dry stone
underneath.
Crystallization of salt or ice is another important extrinsic factor in weathering and deterioration.
For both forms of weathering water as well as changes in temperature is necessary: for salt weathe-ring water acts as a transport media and for ice crystallization it is temperature dependent. For frost action cracking is a typical weathering form (Ruedrich et al., 2011 b). Salt crystallization shows dif-ferent forms of weathering, often sanding and rounding (Charola 2000) (Fig. 7 a) and is related to temporary moisture infiltration and evaporation (Fig. 7 b and c).
For the mechanical disintegration of rock by salt weathering three mechanisms may be involved: 1.
crystallization of salts in pore spaces, 2. thermal expansion of crystals in response to changes in temperature, and 3. the hydration of salts due to changes of relative humidity (Cooke, Smalley 1968). In the case of the last two mechanisms thermal and hydric expansion or shrinking plays a role.
Figure 7: a) Rounding of ancient column bases made from sandstone in Petra due to salt weathering by rising moisture, b) extent of moisture front due to the modern reservoir located to the left of the monument (blue line) and c) leads to the
accumulation of salt at the surface, crystallization and weathering.
1. 2. 2 Thermal expansion and contraction
When materials are heated they expand, as they cool the material contracts. Thermal expansion in natural building stones is considered one of the most important factors affecting their weathering and deterioration. Minerals have different linear thermal expansion coefficients (Fig. 8). Some mi-nerals like calcite also show a negative thermal expansion coefficient perpendicular to the c-axis of the crystal (Fig. 8).
Figure 8: Thermal expansion of different minerals.
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In rocks with many crystals or minerals with a high thermal expansion coefficient like marble (cal-cite), this expansion and contraction can cause mechanical fracturing and cracks like it is shown in Figure 9 a (Siegesmund et al., 1999). Cyclic temperature changes are generally assumed to drive stone decay through the temperature gradients generated within the stone and the thermal expansion mismatch between minerals (e.g. Halsey et al., 1998). In interaction with humidity the effect of ex-pansion decreases (Weiss et al., 2004). Also the color of the stone as well as the density plays a cri-tical role. Black stones with a high density or a closed porosity are susceptible to cracking resulting in fluctuations of the surface temperature.
Good examples where thermal expansion may be the main deterioration process for cracking phenomena are the stair stringers of the Quetzalcotl Pyramid of Teotihuacan, Mexico (Fig. 9 b and c). The main facade of this building is subject to complex deterioration phenomena that include de-cay caused by the presence of water and soluble salts, as well as inappropriate conservation treat-ments carried out in past decades (Villasenor 2006).
Figure 9: a) Cracks formed in a statue of marble in Lugano (Swizerland) probably due to thermal stress, b) massive cracks and material lost at the stair stringers of the Quetzalcotl Pyramid of Teotihuacan (Mexico) and c) the Quetzalcotl
complex.
While most of the decorations are made from tuff, the stair stringers are made from black basalt containing non-cross linked gas pores. During the daytime in April and May the temperature can reach around 27 °C and can cool down to around 7 °C at night. Surface temperature on the dark co-lored stone varieties can reach nearly 70 °C because the main facade is oriented towards the west and heats up until the afternoon. At around 3:00 pm the lower part of the stair stringer facade is in shadow because another building is located right in front of it (Fig. 9 c). This leads to a significant decrease of surface temperature and probably leads to the shrinking, cracking and scaling of the stone (Fig. 9 b).
1. 2. 3 Chemical deterioration
Chemical weathering is caused by rainwater, organic or inorganic acids as well as atmospheric ga-ses, reacting with minerals in rocks to form new minerals (clays) and soluble salts (Carroll 2012).
The most common types of chemical weathering are minerals altered by dissolution, oxidation, hy-drolysis, carbonation and biogenic chemical impacts. Some elements leach out, whereas other mine-rals disintegrate by altering the geochemistry (Colman 1982). Mobilization of chemical elements derived from the leachable minerals depends on the intensity of weathering, which is controlled by the climatic impact (Middelburg et al., 1988). Chemical weathering of rock begins at the very sur-face and penetrates into the material over time, forming a rind in the case of most volcanic and se-dimentary rocks (Ogburn et al., 2013).
Figure 10: a) Discoloration by iron oxidation and contour scaling at the Phnom Bakheng Temple in Angkor (Cambo-dia) and b) zonation of the weathering crust. ) Granite weathering of a ancient column in Jerash (Jordan) and d)
weathe-ring of biotite and feldspar crystals. e) Weatheweathe-ring crusts at a Lycian monument in Myra (Turkey) and f) amorphous precipitations and discolorations probably due to micro-biological growth.
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Chemical weathering processes need water, and occur more rapidly at higher temperature, so warm and damp climates like in tropical environments are the most suited. Tuffs and volcanites contain many amorphous leachable minerals and sandstones sometimes contain feldspar and calcite.
Biotite and/or amphibole undergo alteration paths from hydrolysis to form clay, and oxidation to form iron oxides often forming weathering crusts red in color (Lee 1993) (Fig. 10 a). Na-feldspar and K-feldspar undergo hydrolysis to form kaolinite (clay) and Na+ and K+ ions. Both weathering forms are often found in granites by the weathering of selected minerals (Fig. 10 c & d). Through the so-called “carbonation weathering”, minerals are structurally changed, dissolved and precipita-ted or else dissolved completely (Press, Siever 2003). Precipitation can create different forms of crusts and amorphous crust-like structures on the surface of the rock material (Wedekind et al., 2016 a, Fig. 10 e & f).
1. 2. 4 Biological deterioration
The impact of biological deterioration on the weathering process will not be discussed in detail in this study. First of all biological growth has an asthetic impact and often leads to an discoloration like it is shown in Figure 11 a and d (Hallmann et al., 2013). However, even microbiological activi-ty can enhance physical weathering shown in Figure 11 b and c (Papida et al., 2000), but most dete-rioration is traced back to biochemical processes (Caneva, Altieri 1988; Jones et al., 2000). Chemi-cal action that leads to degradation is exercised by the acidity of the rootlets and by excretion of in-organic and in-organic compounds, with aggressive or chelating capabilities. If these compounds con-tain acids, especially when reacting with carbonaceous stone material, their weathering may increa-se like it is shown in Figure 11 e - f (Tioano 1995). The weathering form of pitting is often associa-ted to dissolution by acids (Kumar, Kumar 1999). Organic acids produced from some bacteria and fungi can also form metal organic complexes with cations dissolved from the crystal grid of mine-rals (Palmer et al., 1991). While these complexes stay stable, the metal ions remain diluted and pre-cipitate as ions (Press, Siever 2003). This may enhance the hydrolytic weathering of feldspar found in sandstone, tuffs and other silica rocks.
The colonization with lithotrophic bacteria and fungi can produce ionic compounds and salts. By the oxidation of inorganic substances calcium sulfate dihydrate can be formed (Fassini 1988; Zap-pia et al., 1998). Nitrifying bacteria oxidize nitrous gases (NOx-components) to nitric acid, which again leads to dissolution and the deterioration of the stone material (Sand, Bock 1991).
In general, the presence of micro-organisms indicates a higher amount of humidity which may en-hance deterioration processes (Wihr 1986). This humidity is only a relevant factor in several life zones and is closely connected to microclimatic conditions like shaded areas like in the case of some temples in Angkor shown in Figure 11 a and b (Wedekind et al., 2016 a). Microbiological
growth is not always visible, but it can be tested quite easily by enzymatic indicators (Warscheid et al., 1990). However, due to climatic changes the relative humidity increases in some regions like in northern Europe, which can also have an influence on physical deterioration processes such as hyg-ric and hydhyg-ric dilatation (Schubert, Wedekind 2014).
Figure 11: a) Massive biological growth on decorations and b) overgrowth of mangrove trees at the Ta Phrom Temple in Angkor and c) of a tomb et the Ek Balem cemetery in Guadalajara (Mexico). d) Massive microbiological growth on a
marble statue at the Bartholomew cemetery in Goettingen (Germany). e, f. & g) Micro- and macroscopic observations of a tomb athe Albani cemetery in Goettingen (Germany). e) Microscopic foto of an undamaged surface and
g) of a damaged surface with microbiological growth.