1).
The Armenian tuffs show a broad range of petrophysical properties. The tuffs of this study have a high effective porosity of 21 % to 36 %. However, the Hoktemberyan tuff is characterized by a much lower ratio of micropores (9 %) and bulk density (1.6 g/cm3). Mechanical properties display the Armenian tuffs as moderately strong with maximum tensile strength values of 5 MPa. While Golden Armenia suffers from a distinct strength decrease under saturated conditions, the Hoktemberyan tuff does not seem to be affected by water in its mechanical properties.
The porosity of the Mexican tuffs ranges from 18 % to 36 % and the density values vary from 2.3 g/cm3 to 2.6 g/cm3, respectively. The pore-size distribution of the studied tuffs are unimodal and bimodal, in addition the average pore radius fluctuates from 0.15 µm to 4.02 µm, dominating the capillary pores. SLP volcanic rocks are also very soft to moderately hard, depending on the degree of welding and showing average values of tensile strength in dry conditions of around 8 MPa, falling extremely to values down to 1.33 MPa in water-saturated conditions. Well-welded ignimbritic tuff samples of SLP can have uniaxial compressive strength of 90 MPa (Wedekind et al. 2013).
Water saturation reduces the strength (both uniaxial compressive and tensile strength) of the tuffs with one exception (Hoktemberyan) (Table 1). The strength of the saturated samples can be as low as one-fourth, but in general, the value is half or two-thirds of the dry one. The porosity of the tuffs is in between 18 to 37 vol%, while matrix densities are relatively uniform with 2.3 to 2.6 g/ cm3. The P-wave velocity of the studied tuffs is in between 2.3 and 3.9 km/s. However, these values do not necessarily indicate differences in porosity (Table 1).
Durability
The durability of the Hungarian rhyolitic least porous welded tuffs may reach 90 freeze-thaw cycles and 20 salt crystallization cycles with minor or no decay in structure and technical properties. However, the most porous softest varieties, which have found a larger application in the historical built heritage, can withstand only few weathering cycles, before disintegration, erosion, and cracking. The magnitude of the differences in porosity, and consequently in the absorption of water and salt solutions, controls primarily the diverse durability.
135Secondary factors are pore-size distribution especially in the range 0.1–10 µm – critical for ice and salt crystallization damage. Considering the textural features, the most significant discriminating factor is the proportions between crystals and the weaker pumice and groundmass.
Table 1: Physical properties of studied tuffs (data of Hungarian rhyolitic tuffs is from Germinario and Török 2019, Hungarian andesite tuff is from Török 2007, data of the German and Armenian tuffs are from Pötzl et al. 2018a and Pötzl et al. 2018b, data marked by * is from López-Doncel et al. 2016; and marked by + is from Wedekind et al. 2013).
The Hilbersdorf tuffs withstand 11 to 21 cycles before complete disintegration due to its high porosity, bimodal pore radii distribution and high amounts of micropores.. The hydric and thermal expansion of these tuffs is greatly influenced by their clay mineral content and ratio of micropores. Maximum hydric expansion can reach values of 7.5 mm/m (Pötzl et al. 2018a).
Armenian tuffs also show great variation in their durability and weathering behaviour, dependent on their petrophysical properties. The resistance against salt attack varies from 16 to way over 150 cycles. Even considerably soft tuffs, like the Hoktemberyan varieties, are not strongly attacked by the salt crystallization and withstand more than 150 cycles. The Golden Armenia, on the other hand, disintegrates after 34 cycles. The main difference between both tuffs is the clay mineral content as well as the much higher ratio of micropores in Golden Armenia.
Regarding the durability of the Mexican tuffs, the conducted salt bursting tests show that after 14 cycles the most porous sample was completely destroyed and two samples, both with a bimodal pore distribution, have resisted at the end of the test 47 and 48 cycles, respectively. All SLP samples 136were tested for hydric and thermal expansion and all of them did not show any expansion, and one sample showed even hydric contraction with values around 0.03 mm/m.
Conclusions
The tuffs are broadly used in the monuments of Europe, Asia and North-America. The physical properties of the studied mostly acid tuffs are very different. Despite the high porosity (from 17 to nearly 37 vol%), tuffs can have relatively high tensile strength in dry conditions. This is significantly reduced with water saturation. The loss in strength can be one-fourth of the original one, but for most of the studied tuffs, it is half or two-thirds of the initial dry value. Not only water saturation but also freeze-thaw cycles and salt crystallization lead to a loss in strength or even an ultimate disintegration. There are tuff lithologies that can survive more than 100 salt crystallization cycles or 90 freeze-thaw cycles, while others disintegrate after few cycles. These differences in durability are attributed not only to porosity but also to pore-size distribution and micro-fabric, and are influenced by formation processes such as welding. Despite these findings and variations of durability, the usage of tuffs is still common, but at several sites the tuff elements show signs of deterioration.
Acknowledgements
ÁT acknowledges the financial support of National Research, Development and Innovation Fund of Hungary (K 116532). CP was supported by the German Federal Environmental Foundation (AZ20017/481).
References
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