Physical Properties of Stone
Stone is a heterogeneous substance characterized by a wide range of mineral compositions, textures, and rock structures. Consequently, the physical and chemical properties and the resulting durability are quite variable. The suitability of a stone for a g
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Stone is a heterogeneous substance characterized by a wide range of mineral compositions, textures, and rock structures. Consequently, the physical and chemical properties and the resulting durability are quite variable. The suitability of a stone for a given building can be easily tested in the laboratory. Although some tests are expensive and consume considerable amounts of rock material, others are simple, inexpensive, and nondestructive (see App. A).
2.1 Rock Pores and Porosity Porosity is the ratio of the volume of pore space to the total volume in percent. The definition by Walker et al. (1969) of porosity for concrete also applies to stone, with the apparent or effective porosity n = Vy/Vb x 100,
where n is the total effective porosity, V y the total pore volume determined by the mercury intrusion at up to 1000 atm, and Vb the bulk dry volume of the sample in cm 3 . The average porosity comprises most of the diameters as the sum porosity between all the diameters (Bayly 1968). The modulus is the cumulative sum of the penetration of fluid per gram of rock, of predetermined diameters, expressed in cm3 /g. The porosity number p is calculated as p
=B
- A/V x 100,
where A is the weight of the dried specimen, B the weight of the soaked specimen, and V the total volume of the sample. The porosity increases rapidly in clastic sediments with decreasing grain size, thus clays may have a porosity well over 50%, according to Bayly (1968). The water in micropores is not readily available and cannot migrate. The porosity of rock and stone helps to determine the strength and durability, but also permits estimates of the content of moisture and travel through the masonry. A full understanding of the size and character of such channel ways is therefore important (see Sects. 6.3-6.5). E. M. Winkler, Stone in Architecture © Springer-Verlag Berlin Heidelberg 1997
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Rock Pores and Porosity Table 2.1. Porosity of sandstones and carbonates (Choquette and Prey 1970) Properties
Sandstone
Carbonates
Original porosity Uncemented Cemented Pore sizes
25-40% 15-30% Related to particle size
Pore shape Semiquantitative evaluation Porosity-permeability relation
By particle shape Easy Consistent: depends on sorting, particle size
40-70% 5-15% Little related to particle size and sorting Varies greatly Variable Vugs, channels unpredictable
2.1.1 Origin of Rock Pores
2.1.1.1 Igneous Rocks The tight fit of the mineral fabric in all igneous rocks formed under great pressure and temperature permits very little open pore space. The anomalous behavior of the quartz grains, contracting more than half their volume during the process of cooling, causes extensive cracking across and around the quartz grains. This leads to a microcrack porosity of about 1.0% of the total volume, twice the amount of quartz-free igneous rocks (see Figs. 10.2-10.4).
2.1.1.2 Sedimentary Rocks The porosity and pore space distribution is subject to great variation. A nearly unlimited variety of pore sizes and shapes are characteristic for sedimentary rocks,
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