Of the vesicular Tezontle basalt (I) fresh material of a quarry nearby Mexico City was investigated. Macroscopically it shows a typical cellular fabric. The voids are predominantly 8 mm in diameter. Locally, partially melted rock fragments from older volcanic rocks are incorporated. These fragments show irregular shapes with a red colored rim, which can be traced back to iron oxide minerals.
Microscopically the cell walls of the cellular rock fabric show a cryptocrystalline fabric with acicular plagioclase crystals. Only isolated small pyroxene and amphibole crystals as well as hematite impregnations occur (Fig. 6I). X-ray fluorescence analyses after Acevedo-Dávila et al. 2007. The Tezontle basalt geochemically contains approximately 60% SiO2, 12% Al203, 8% MgO, 5% Fe2O3, 4% CaO and 3% Na2O.
The material is permeated by fractures and small cracks. Its porosity in non-weathered conditions ranges from 35 to 66% with an average value of 52 vol.%. This strong variation of the porosity can be traced back to its heterogeneous cellular structure. Due to the high porosity the bulk density with a value of 1.4 g/cm3 is very low. The particle or matrix density reaches a value of about 2.71 g/cm3. Possibly mafic minerals like pyroxene and amphibole possibly cause this relatively high value. The average w value only reaches 0.40 kg m−2 h−1/2. Active capillary pores amount to 76.2% of the pore radii distribution by a considerable outbalance of larger capillary pores (>10 μm = 43.7%) and macro pores (Table 1). Furthermore, the basalt exhibits no measurable hygric swelling but with 7.0 10−6 K−1 a relatively high thermal dilatation coefficient (Table 3). A residual strain after five cycles could not be detected.
Under dry conditions, the compressive strength ranges from 12 to 32 N/mm2. The highest values are reached in Z direction averaged with 27 N/mm2. The lowest values were measured in Y direction averaged with around 21 N/mm2. The anisotropy amounts nearly 21% (Table 4). The modulus of elasticity ranges from 3.57 to 8.34 kN/mm2 with an anisotropic behavior of around 13%. The uniaxial compressive strength is with 23.8 N/mm2 very low. Under dry sample conditions, the strength varies between 21.33 and 26.90 N/mm2. In contrast, the uniaxial compressive strength values of the water-saturated specimens are 3% higher and range between 27.76 and 22.31 (Table 4). This is caused by the rock heterogeneity, and thus the rock strength does not depend on the water content. The Young’s modulus of the Tezontle rock is around 5.7 kN/mm2 (Table 4).
Tenayocátetl rhyolitic ignimbrite
The volcanic rock from tenayocátetl (II) is grayish to red in color and is characterized by a porhyritic fabric. By mineralogical classification, the rock exhibits a rhyolitic composition and represents a welded ash tuff with a small portion of lapilli fragments. This rock shows the characteristic foliation, which is caused by flow structures. Microscopically the fine-grained ash matrix cannot be further detected. The lapilli fragments mainly show rounded quartz.
Altered crystals of plagioclase, hornblende, andesine, oligoclase, labradorite and augite are observable. Within the fine matrix, small acicular crystals define the flow direction (Fig. 6, third row). This matrix is colored in layers from light red to dark red because of kaolinite and hematite.
Because of the limited amount of original historical material available, compressive strength tests could only be done in the fabric directions parallel (Y) and perpendicular (Z) to the flow direction under dry conditions.
Tenayocátetl has a porosity of 7.3% and a density of 2.45 g/cm3 (Table 1) The w value shows extreme anisotropic behavior of nearly 99%. While the w values in the Z and X direction only reach 0.10–0.11 kg m−2h−1/2, the Y direction with a value of 9.16 kg m−2h−1/2 is nearly ten times higher. (Table 2). The pore radii distribution shows an amount of nearly 53% as sum of all fractions of capillary active pores (0.1 to >10 μm) as well as a proportionate dominance of micropores (0.01–0.1 μm) of nearly 38% (Table 1). Thermal dilatation reaches 9.71 mm/m with a very low anisotropy of 2.2% (Table 3). Also for the rhyolitic Ignimbrite a residual strain after five heating-cycles could not be detected.
Hygric swelling shows an extreme anisotropic behavior of 51.4%: hygric swelling in the Z direction with 5.68 mm/m reaches a value nearly two times higher than in the other fabric directions (Table 3).
Compressive strength attains a value of 50.83 N/mm2 by isotropic behavior (Table 4). The e-module ranges from 12.13 to 14.81 kN/mm2 with a moderate anisotropic behavior of almost 17%.
While the stone shows nearly the same values according to its mechanical properties, the water uptake capacity and the hygric dilatation show extreme anisotropic values (Tables 2, 3). Water suction only seems to be possible through the direction of lineation (Y). Hygric expansion shows an anisotropic value of 51.4%. The expansion perpendicular to the flow direction is almost two times higher than parallel to the direction of foliation or lineation.
The thermal expansion shows height values and isotropic behavior (Table 3). Thermal expansion is reaching 5.6 mm/m at 60°C and 9.8 mm/m at 90°C.
Remedios tuff (Gris de los Remedios)
The volcanic rock of Remedios represents a lapilli tuff (III), supported by an ash matrix. The color of this tuff is gray to light gray. Two types of lapilli fragments are distinguished, one is white in color, and the other is dark. The ash matrix is more or less gray. Within the matrix, dark spots occur that can be traced back to mafic minerals. The mafic minerals often show prismatic crystal shapes and represent mostly idiomorphic developed hornblende crystals. The white lapilli fragments are dominated by pumice.
Microscopic examinations revealed the presence of minerals and compounds like cristobalite, clays, sodium plagioclase, hornblende, and sparitic particles. While the crystals within the inclusions are unaltered, the crystals within the matrix show alteration. Both the matrix and the inclusions are closely connected. Between these two components, no pore space is observable (Fig. 6III). The porosity and density of the matrix and the inclusions differ at around 45%. The pore radii distribution also differs in the same dimension (Fig. 6, third row). The matrix (a) shows a distinct concentration of capillary active pores (1–10 μm), while the inclusions (b) show nearly the same volume of capillary active macropores and micropores (0.001–0.1 μm). The inclusions define a flow orientation, whereas an orientation of the single crystals within the matrix cannot be discerned.
Secondary precipitated calcite is located in small cracks and in the pore space. Chemical analyses indicate that the calcite comprises 0.8 wt%.
The porosity of Gris de los Remedios ranges from 24.5 to 31.5%, whereas their bulk density is around 1.78–1.92 g/cm3. The average w value reaches 7.8 kg m−2 h−1/2 with an anisotropy value of 30.8% (Table 2). The amount of capillary active pores comprises 84.7%. From the capillary active pores, a 55% fraction ranges between 1 and 10 μm (Table 1). Thermal dilatation averaged reaches 6.14 mm/m with a moderate anisotropy of nearly 15% (Table 3). A low residual (irreversible) strain during the five cycles of measurement could be detected.
Hygric swelling averages show a value of 5.67 mm/m with an anisotropy of 20.4% (Table 3).
Under dry conditions, the compressive strength of single samples ranges from 15.8 to 19.3 N/mm2. The anisotropy amounts to 8.29% (Table 4). The elastic modulus ranges from 3 to 5 kN/mm2 with an anisotropic behavior of around 26%. The Remedios variety has an average compressive strength of 17.3 N/mm2 and an e-module of 8.29 kN/mm2.
At water saturation, the compressive strength decreases down to 10.17 N/mm2 by a moderate anisotropy of 10.6% (Table 4). Compressive strength decreases to 41.2% according to the measurements under dry conditions (Table 4).
The stone has a high w value (7.8 kg/m2) and a high hygric dilatation by reaching an average value of 5.6 mm/m. Both values clearly show anisotropy. The w-value is nearly 31% and the hygric dilatation 20% (Tables 2, 3). Average compressive strength values only reach 17.3 N/mm2 at dry conditions and decreases to 10.1 N/mm2 in the water-saturated state, a reduction of 41% according to the measurements under dry conditions (Table 4).
The thermal dilatation also reaches critical values of around 4 mm/m at 60°C and around 6 mm/m at 90°C. The curve does not follow a linear gradient. When heating up to 60°C, a small depression is recognized. This reduction is probably caused by the dehydration of crystal water within the material. After five cycles, a residual strain could not be detected.
The investigated material was coming from the quarries of Los Remedios located in the mountains northwest of Mexico City.
Chiluca: fresh and altered (Cathedral)
Two varieties of the pyroxene andesite tuff Chiluca have been investigated, an altered sample from the cathedral (described as Cathedral, IV), and a fresh sample from a quarry (described as Chiluca, V). The altered Chiluca tuff sample was removed from the cathedral during the current restoration campaign. The properties of the Chiluca type of the Cathedral differ from the fresh sample.
The Chiluca tuff represents an ash tuff with a high proportion of fragmented feldspar single crystals, giving the rock a porphyritic appearance. The tuff is gray in color with dark inclusions of mafic minerals, which is probably amphibole. Feldspar single crystals occur in a fine-grained matrix. Microscopic examinations show that both stone varieties have a vesicular phenoporphyritic texture and microcrystalline matrix consisting of andesite constituents (around 85%). In both varieties, hornblende crystals are present (around 15%) (Martínez and Martínez 1991).
Some crystals show alteration (Fig. 6, second line) in the sample from the cathedral, whereas the crystals within the matrix of the new stone from the chiluca quarry mostly are still intact (Fig. 6, second line). The Chiluca variety also shows isolated augite crystals and plagioclase. An orientation within the fine matrix cannot be discerned by standard microscopic techniques. Macroscopic observations detected an orientation in the elongated hornblende crystals.
The porosity measured in the altered cathedral variety is 10.6% with the density being 2.68 g/cm3 (Table 1). The Chiluca variety comes with a porosity of 8% and a density of 2.58 g/cm3 (Table 1). Both varieties differ in porosity and density by only 2%. The w value of the cathedral variety averaged 0.33 kg m−2 h−1/2 with an anisotropy of nearly 19%, whereas the Chiluca variety shows a w value of 0.37 kg m−2 h−1/2 with a moderate anisotropy of around 13% (Table 2). Active capillary pores in the case of the altered cathedral variety reach 37.6% and in the sample from Chiluca 73.2% (Table 1). The highest amount of pore fractions (56.3%) within the pore radii distribution in the cathedral variety can be found for micropores with a pore radii size of 0.01–0.1 μm, whereas the Chiluca variety has a concentration of 40% for the pore radii fraction ranging from 0.1 to 1 μm (Table 1). Thermal dilatation of the altered variety averaged around 7.73 mm/m with a low anisotropy of 7.3% (Table 3). Only for the samples from the cathedral, a low residual strain could be detected.
Hygric swelling shows a value of 3.02 mm/m with a very low anisotropy of only 2.3% (Table 3). In the quarry fresh Chiluca, the value of thermal dilatation averaged 7.69 mm/m and is comparable to the altered stone, but it has a clear anisotropy of around 19% (Table 3). Hygric swelling reaches an average of 3.1 mm/m and is slightly higher than the sample from the cathedral as well as showing an anisotropy of nearly 7% (Table 3).
Both varieties have a high compressive strength. The altered variety averaged 98.2 N/mm2 with an anisotropy of 12.5%, whereas the Chiluca variety shows a value of 88.6 N/mm2 with an anisotropy of 21% (Table 4).
The modulus of elasticity of the cathedral variety ranges from 8.45 to 10.2 kN/mm2 with an anisotropic behavior of around 17%, whereas the Chiluca only shows an anisotropy of 11% and a averaged e-module of 13.7 kN/mm2. Because of the limited amount of the original historical material, compressive strength by water saturation could only be measured for the fresh Chiluca variety. By compressive strength tests at water saturation, a decrease of nearly 20% took place in the this variety (Table 4).