3. 3. 1 Introduction
The old mining city of Guanajuato in central Mexico belongs to a long list of important historical cities in Latin America and preserves one of the most important historical legacies in colonial buil-dings. The city is located at 21°0'N, 101°16'W in the southern Cordillera between the Mesa Central and the Transmexican Volcanic Belt (TMVB) with an altitude of 1999 m above sea level (Fig. 56).
Figure 56: The city of Guanajuato is situated in a valley in the mountains of the southern Cordillera.
The establishment of this city in New Spain was basically due to the discovery of silver and gold deposits in 1548 that initiated the beginnings of the mining industry. This resulted in the legal foun-dation of the town of Santa Fe de Guanajuato in 1570. During the seventeenth, nineteenth and early twentieth centuries a number of buildings were erected that would play important roles in Mexico’s battle for independence like the Alhóndiga de Granaditas. Guanajuato became an important econo-mic, cultural and religious center in Mexico as reflected in the remarkable growth that has occurred during the twentieth century. This is evident in the increasing preservation of old churches, the con-struction of notable buildings such as the Teatro Juarez, the building of the University of Guanajua-to and the Iglesia de la Compañía de Jesús Church (Fig. 1A, 1B). In 1988 the city was declared a World Heritage Site by the UNESCO.
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3. 3. 1. a) Climate and environmental conditions
Guanajuato has a humid subtropical summer climate that is mild with dry winters, mild rainy sum-mers and moderate seasonality. This climate is usually found in the highlands of some tropical countries with the classification Cwb after Köppen-Geiger (Fig. 3).
The annual average temperature is 18.1 °C, whereas the average monthly temperatures vary by 7.2
°C (Fig. 57). In the winter time records indicate temperatures by day reach 22.1 °C on average fal-ling to 7.7 °C overnight. In spring time temperatures climb to 27.9 °C generally in the afternoon with overnight lows of 12 °C. During the summer average high temperatures are 26.7 °C and avera-ge low temperatures are 14.3 °C. Come autumn temperatures decrease and achieve averaavera-ge highs of 24.5 °C during the day and lows of 11.5 °C generally shortly after sunrise. Total annual precipitati-on averages 698 mm (Fig. 15). According to the Holdridge life zprecipitati-ones system of bioclimatic classifi-cation, Guanajuato is situated in or near the warm temperate dry forest biome (Fig. 14).
Figure 57: Climate Graph for Guanajuato.
3. 3. 1. b) Geological setting and utilization of the rock material
The geology around the city of Guanajuato is specially interesting because of the great diversity of the rocks, the quality of the outcrops, and the large number of clearly exposed structures (Aranda-Gómez et al., 2003). The most important building material found in Guanajuato is the Loseros Tuff and the Bufa rhyolite Tuff located in a distance of around 5 to 10 km in north east direction to the
historical town (Buchanan 1979). Important querries for the Loseros Tuff and Bufa Tuff can be found in Cerro Tepozan (Salazar-Hernández 2015).
Practically all the colonial constructions were built with natural stones from the surrounding region.
These include the greenish to reddish volcanite, called the Loseros tuff and the reddish to grayish rock known as the Bufa rhyolite tuff (Fig. 53 b).
The utilization of the Loseros Tuff and the Bufa rhyolite as natural building materials include filler rocks for roads, walls, bridge facades and especially the construction of a complex system of under-ground tunnels that cross the city of Guanajuato.
When the Loseros Tuff is cut along the lamination (X-axis), or it is finely reworked perpendicular to the lamination it is used as fine masonry for many important cultural, religious and governmental buildings as well as for decoration elements of ordinary houses (Fig. 58 a). In the nineteenth centu-ry and early twentieth centucentu-ry the Loseros Tuff was widely used as a popular material especially for tomb monuments in central Mexico. Examples can be found in Guadalajara (Fig. 58 c) as well as in Mexico City.
Figure 58: a) A 19th century house in the historical city of Guanajuato with decorative parts made from Loseros Tuff. b) The portal of the San Diego Church in Guanajuato mainly made from Bufa Tuff c) Monumental tomb at the Mazquitlan
Cemetery in Guadalajara made from Loseros Tuff.
3. 3. 1. c) Main weathering forms
Although both volcanic rocks are widely used, they show significant deterioration and weathering effects, first of all by delamination (Fig. 59 a), contour scaling (Fig. 59 b), and crumbling (Fig. 59 c and d). These destructive phenomena are mostly found in areas of the building where moisture and water are permanently or temporarily present like in columns, fountains, balconies or external stair-cases.
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Figure 59: Appearance of the different damage and deterioration types observed in the Loseros Tuff. a) Delamination at the stairway of the main building of the university. b) Contour scaling perpendicular to the lamination at a column of a tomb in Guadalajara. c) Flaking and crumbling in the lower part of a wall affected by moisture infiltration and d) at a basin wall of a fountain in Guanajuato.
A detailed view of the rock deterioration in the construction clearly shows that in the Loseros Tuff the horizons formed by coarser grain sizes are more affected than those of finer fractions (López-Doncel et al., 2012). Furthermore, coarser horizons have an apparent higher porosity because the pores reach the grain size of sand, or even larger. Also the binding cement, relocation processes and the concentration of this cement near the surface seems to play a role especially in the forming of scales. On the other hand, the Bufa rhyolite exhibits deterioration caused mainly by structural cracking and crumbling (Fig. 60 b, c, and d).
Figure 60: Massive deterioration of the Bufa Tuff at the Compañía de Jesús Church. a) The church under construction, b and c) structural cracks in detail at a pillar in the facade and d) the weathering situation at the facade.
3.3. 2 Rock materials
3. 3. 2. a) Bufa Tuff
The Bufa Tuff (BT) is a grayish, light pink/red to orange porphyritic rhyolitic tuff, which has around 10% quartz and sanidine phenocrysts, together with isolated well-flattened pumice (Fig. 61 a).
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 in color. Subhedral to euhedral quartz, plagioclase, and biotite flakes and dispersed pyroxenes and olivine crystals are present in a microcrystalline and partially devitrified glassy matrix (Fig. 61 b). The fine-grained matrix is composed of calcite, hema-tite (principle cause of the pinkish, reddish color) and small amounts of illite plus illite-smechema-tite mi-xed layer of different ordering types.
Figure 61: The Bufa Tuff of Guanajuato. a) hand specimen, b) thin section in polarized light and c) SEM micrograph.
Table 7: Pore space properties of the Bufa Tuff used for the Compañía de Jesús Church.
Effective porosity [Vol.-%] 18.39 Particle density [g/cm -3] 2.61 Bulk density [g/cm -3] 2.13 Average pore radius [µm] 0.06
Micropore porosity [%] 82
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3. 3. 2. b) Loseros Tuff
The Loseros Formation belongs to the Cenozoic volcanic rocks that form the Sierra de Guanajuato Area. This area is divided into two main rock successions. A succession that represents the base-ment composed principally of volcanic and sedibase-mentary sequences of Jurassic to Upper Cretaceous in age. These rocks are the oldest units that crop out in this region. These Mesozoic rocks belong to the so-called ‘‘Mesozoic Basement or Basement Complex of the Sierra de Guanajuato’’ (Ortiz-Hernández et al., 1992; also called the ‘‘Guanajuato Volcanic Arc’’ by Monod et al., 1990). A se-condsuccession overlies the Mesozoic sequence. More than 2,500 m of Tertiary to Quaternary vol-canic rocks are exposed, which show diverse chemical compositions varying from basaltic to
ande-Tabble 8: Water transport properties and hygric and hydric dilatation of the Bufa Tuff used for the Com-pañía de Jesús Church.
Directions (X, Y, Z) / Anisotropy (A) X Y Z A % Average ∅
w value [kg/m2 √h] 0.18 0.21 0,17 16 0.186
µ value 16.7 14.08 19.28 15.5 16.68
Sorption 95 % rh [g/cm3] 0.054
Hydric dilatation [mm/m] 0.361 0.55 0.79 55 0.53
Hygric dilatation 95% RH [mm/m] 0.18 0.203 0.33 45 0.24
Table 9: Ultrasonic velocity of the Bufa Tuff in dry and water-saturated conditions
Ultrasonic velocity [km/s] dry water-saturated
Tabel 10: Petro-mechanical properties of the Bufa Tuff used for the Compañía de Jesús Church
Directions (X, Y, Z) / Anisotropy (A) X Y Z A [%] reduction [%]
Compressive strength dry [N/mm2] 66.69 64.34 59.59 11 nd
Splitting tensile strength dry ßSZ [MPa] 6.04 -- 6.95 7 Splitting tensile strength water-saturated ßSZ [MPa] 3.65 -- 4.57 20 37.6
Surface hardness dry [HLD] 574
Surface hardness wet [HLD] 509 11.3
sitic to rhyolitic. The extrusions of these Cenozoic volcanites are associated with the extensional tectonism at the end of the Laramide Orogeny in western and central Mexico (Nieto-Samaniego et al. 1992). The Loseros Tuff is a felsic volcanoclastic rock that consists of well-sorted, sand-sized crystals and detrital rock fragments, which are embedded in an ash-rich altered groundmass. The Loseros Tuff appears in a wide variety of color shades, which can range from reddish brown, pink, green and even white, but the green variety is the most requested and used rock.
Figure 62: a) Macroscopic view of the coarse Loseros Tuff. b) Thin section in polarized light and c) overview under the SEM. d) Macroscopic view of the fine-grained Loseros Tuff. e) Thin section and f) overview under the SEM.
The grain size can also vary locally from gravel (granule), up to the clay fraction but the sand grain size dominates. Lithic fragments also found in the sand fraction are the reason why some authors classify the Loseros Tuff as a sandstone (Salazar-Hernández et al., 2015). Loseros Tuff is a volcanic pyroclastic rock (Fisher 1961; Fisher, Schmincke 1984; Le Maitre et al., 2004) with a significant amount of epiclastic detrital material. In the field this tuff exhibits protruding pseudo-stratification with gradational beds composed of 5 to 50 cm thick layers (locally accretional lapilli layers are thi-cker than 1 m). Together with the lamination a series of very characteristic sedimentary structures are observable, such as cross-bedding, ripples, flame and cut-and-fill structures. Edwards (1956) noted that the majority of the grains are quartz, plagioclase and volcanic lithic fragments. In thin
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section the Loseros Tuff shows a glassy matrix, which in polarized light appears almost completely opaque and its texture ranges from hypocrystalline to holohyaline. Crystals consist of altered angu-lar to subhedral plagioclase, anguangu-lar quartz, and also altered biotite flakes (Fig. 62 b and e). Under the microscope ‘‘fissile-like’’ laminae are also recognizable, which result from intercalations of fine sand and clay. XRD analysis showed a significant amount of CaCO3 (calcite) that under the micro-scope is recognizable as cement in the matrix. Thin section evidence shows that the greenish colora-tion of the tuff comes from a large number of small albite crystals with an altered appearance (initi-al stage of transformation to clay miner(initi-als, Fig. 63 a and b).
Figure 63: Thin section photomicrographs of the studied tuff. a) Green coloration of an altered albite crystal (centre of image, compare with Fig. 7a) is observable in transmitted light. The matrix consists of poorly recognizable calcite, vol-canic glass and small quartz crystals. b) Observable under polarized light is a matrix consisting of opaque components (volcanic glass) with small crystals of calcite (crystals with third-order colors) and the altered plagioclase. These show rare interference colors. c) Diffractograms showing the primary mineralogy of the two tuff varieties: coarse Loseros
(blue) and fine-grained Loseros (red).
The matrix is also made up of volcanic glass, which shows different stages of devitrification. The green color was not recognized in the matrix. Buchanan (1980) attributes the greenish coloration in this tuff to alteration (chloritization) of the lithic fragments, but the geochemical analyses do not show the presence of chlorite in the Loseros Tuff. Wedekind et al., (2013) noted: ‘‘the matrix has more than 20 % calcite and also contains kaolinite.
However, most common are the dioctahedral clay minerals like illite plus R3-ordered illite (0.95) - smectite mixed layers, which add up to a CEC value of 7 meq/100 g’’. No mineralogical difference is evident between the finer and the coarser varieties of this tuff. The XRD patterns of both show only a major amount of albite and calcite in the coarser fraction (Fig. 63 c). On the other hand, the SEM indicates that the tuff is relatively dense with very fine-grained feldspar and illite–smectite
particles (Fig. 62 c and f).
Based on the macroscopic observation a marked difference in the type and form of damage was identified. The hypothesis is that the integrity of the tuff is probably affected by the particle (grain) size and by the apparent porosity. In order to analyze these differences two different varieties of the tuff were studied: (1) a coarse-grained specimen, which has been separated into seven different ho-rizons and designated 1g–7g (Fig. 66). And (2) a fine-grained specimen, that was separated into five horizons labeled 1f–5f (Fig. 67). All the coarse-grained horizons have a grain size that ranges from fine sand-size (1g and 2g), sand-size (3g and 6g), and coarse sand-size (5g and 7g) to very coarse sand-size, and to very fine pebbles (4g). The horizons of the fine-grained variety vary from very fine sand (4f and 5f) to silt-size (1f and 3f), and locally even clay fractions (2f).
Figure 64: a) Typical large pore of relict crystals in the finer tuff embedded in a dense matrix as observed under SEM.
b) Tenfold magnification of the central region indicates the presence of very fine-grained albite crystals. c) Clay mine-rals (illite–smectite) are densely packed and d) identified under high magnification.
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The petrographic analyses were performed on oriented thin sections of the coarser and finer varie-ties utilizing a polarizing microscope. Mineralogical and geochemical analyses were performed using XRD (whole rock samples, and oriented slides of clay fractions smaller 2 µm, along with XRF, elemental carbon and sulphur analysis, and CEC analyses (compare Ruedrich et al., 2011 a, b).
Figure 65: a) Larger pore with plenty of relict crystals or zeolites in the coarser tuff embedded in a dense matrix as ob-served under SEM. b) Feldspar crystal showing kaolinisation and c) the kaolinite booklets. d) Booklets in detail.
Hydrostatic weighing was carried out to determine the matrix and bulk density as well as the poro-sity of each horizon. Water uptake coefficient (w value) was determined with the help of the capilla-ry suction in a closed cabinet while weighing. The water vapor diffusion resistance value (l) was measured using the wet-cup method. The pore radii distribution was determined using mercury in-jection porosimetry (Brakel et al., 1981, see also Siegesmund, Snethlage 2011).
The hydric and hygric expansion of each horizon was measured on cylindrical samples (diameter 15 mm, length 100 mm). For hydric expansion measurements the cylinders were completely immersed in distilled water (water-saturated). For hygric dilatation analysis an initial relative humidity (RH value) of 20 % was used, which was increased gradually to a RH value of 95 %. The temperature was kept constant at 30 °C during the whole experiment. Cylindrical samples with co-planar end-faces of 50 mm in diameter and 50 mm in length and 40 mm in diameter and 20 mm in length, re-spectively, were used for the compressive and tensile strength tests. The compressive strength load was realized with the help of a servo-hydraulic testing machine with a stiff testing frame (3.000 kN/
mm2) and a load range up to 300 kN. The tensile strength measurements were determined by means of the ‘‘Brazilian test’’.
Figure 66: Appearance of the coarse Loseros Tuff variety and its separation into the different studied horizons.
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Figure 67: Appearance of the finer Loseros Tuff variety and its separation into the different studied horizons.
3. 3. 2. c) Horizontal petrophysical properties
Analyses of each of the studied horizons were performed, in order to determine the density and po-rosity of both, the fine (g1–g5) and the coarse fraction (g1–g7). The results of the determination of the porosity, bulk density, particle density and average pore radius are presented in Table 11. As shown in the table, fine-grained horizons have a greater particle density than the coarse-grained ones, with an average of 2.63 and 2.37 g/cm3, respectively. The same occurs with the bulk density.
Contrary to expectations, the fine-grained varieties have a higher porosity and in some horizons even a larger average pore radius than the coarser ones. Sedimentary rocks such as siltstones and claystones (similar to the f1–f5 samples) usually have much lower porosities than sandstones (equi-valent to g1–g7 samples) because decreasing grain sizes typically correlate with decreasing pore sizes. The studied tuffs were deposited principally in a similar way (as pyroclastic and epiclastic rocks), so they should have a similar behaviour, which in this case does not occur. This phenome-non is related to the type and distribution of the porosity (micro or capillary porosity), which will be discussed below.
Table 11: Porosity and density of the investigated tuff horizons
3. 3. 2. d) Comparative compressive and splitting tensile strength
The compressive strength tests show that the coarser type of the Loseros Tuff has values that range from 74.3 to 58.0 N/mm2, where the largest value occurs in the X-axis and the lowest value in the Z-axis. This condition is associated with the lamination of the tuff. The anisotropy in the coarser Loseros is 22 % and its modulus of elasticity ranges from 6.3 to 10.0 kN/mm2 with an anisotropic behaviour of around 37 % (Tab. 12).
The uniaxial compressive strength of the finer Loseros Tuff ranges from 42.3 to 57.3 N/mm2, with an anisotropy of 26 %. In contrast to the coarser variety, the greatest value of compression occurs in the direction of the Z-axis and the smallest value in the direction of the Y-axis (Tab. 12). This can be explained by the much finer lamination in the Loseros Tuff, and therefore the sample appears more homogeneous. Measured tensile strength values follow the same trend as the compression test, whe-re the higher values occur in the coarser variety. The tensile stwhe-rength values of the coarser Loseros Tuff range from 50.1 to 66.0 Mpa, with an anisotropy of 24 %. The fine-grained Loseros shows a maximum value of 59.2 and a minimum value of 30.6 Mpa with an anisotropy of 48 %.
N a m e Porosity [Vol %] B u l k d e n s i t y
Table 12: Tensile and compressive strength, surface hardness as well as the elastic modulus of the studied
Fine Coarse Fine Coarse Fine Coarse Fine Coarse
X dry 5.919 6.6 50.30a 74.26 501 [Ø]
Y water saturated nd nd nd nd nd nd
Z water saturated 2.707 4.107 nd nd nd nd
Anisotropy [%] 54 30.6 - - - - -
-Reduction Ø [%] - 18.8 - 14.2 - - -7.1 -12.4 -
-Table 13: Ultrasonic velocity of the Loseros Tuff varieties under dry and water-saturated conditions Ultrasonic velocity
[km/s] Fine (dry) Coarse (dry) Fine (ws) Coarse (ws)
X 3.711 3.918 3.469 3.102
Table 14: Pore radii distribution of each of the studied horizons
The distribution of the pores in both varieties is unequal unimodal (Figs. 68, 69). Most of the studi-ed horizons are dominatstudi-ed by micropores with pore sizes ranging from 0.001 to 0.1 lm (Table 14;
Figs. 68, 69). Horizons 1f and 2f only show a microporosity of 76.4 and 78.9 %, respectively. These are also the horizons with the finest grain sizes (Fig. 68). Moreover, in the horizons of the coarse-grained variety (1g–7g) the microporosity also dominates; however, absolute percentages are smal-ler. Horizons 4g and 7g reach high values of about 30 % capillary porosity and as expected these horizons have the coarsest grain size (Tab. 14; Fig. 61). Horizon 3g has the widest pore size distri-bution, including more than 3 % of pores showing sizes of 10 µm and the horizon 4g includes more than 5 % of capillary pores in the size range of 1–10 µm]. Sànchez-Gonzàlez (2004) reported water absorption values in the fine-grained Loseros Tuff of around 0.8–2.4 % and for the coarser type the values vary from 6.3 to 6.5 %.Wedekind et al. (2012 a) reported absorption values of 0.1 kg/m2 √h for the fine-grained tuff (see also Wesche 1996).
The capillary water absorption measured is dependent on the three principal directions (X, Y, Z) on sample cubes of 65 mm length. The cubes were set with the bottom plane into water and the weight
S a m p l e P o r e r a d i i d i s t r i b u t i o n [%]
Average ∅ 13.53 71.41 13.10 0.94 1.008
Coarse-grained Loseros Tuff
increase over time was measured. On average both varieties have a low water uptake coefficient of 0.04 kg/m2 √h for the coarse variety and 0.057 kg/m2 √h for the fine variety. The coarse variety shows an anisotropy of 46 % and the fine variety 43 %. Both varieties have different vapor diffusi-on resistances. The coarse variety has a resistance that is more than a factor of two greater than the
increase over time was measured. On average both varieties have a low water uptake coefficient of 0.04 kg/m2 √h for the coarse variety and 0.057 kg/m2 √h for the fine variety. The coarse variety shows an anisotropy of 46 % and the fine variety 43 %. Both varieties have different vapor diffusi-on resistances. The coarse variety has a resistance that is more than a factor of two greater than the