C - Lublin

POROUS STRUCTURE OF NATURAL BODIES
Hajnos M., Świeboda R.
Porosity is one of more important parameters characterizing porous bodies.
The solid phase of a given body may be either nonporous (cast, monolyte) or may
contain pores - spaces filled by liquid or gas phases. The pores may be open and/or
closed, may have different shape, may be available or nonavailable from the
surface of the body, may have very different sizes. So, the porosity is defined as the
occurrence of internal spaces in solid body. A measure of the porosity is a
volumetric ratio of all free spaces to the bulk of the body. The latter value is called
absolute porosity (m 3 /m 3 ). The other definition, relative porosity (m 3 /m 3 ), is a ratio
of a volume of available (open) pores only to the bulk volume of the body. Both
porosity values may be expressed also as percentage or, which is frequently used,
as volume of pores in a mass unit of the body (m 3 /kg).
Porous structures may be formed in several ways:
- As a result of connection of smaller nonporous units (grains) into larger
aggregates. The porous structure is thus formed from internal spaces between the
grains and the grains themselves form the structure skeleton. Such materials have
granular structure.
- As a result of the removal of some part of the solid from monolytic body (e.g.
leaching, dissolution). Such materials have a sponge structure.
- As a result of both processes together.
For most porous materials one can distinguish the intergranular porosity
(between the grains) which is called bed porosity and intragranular porosity (inside
the individual grains). Solid bodies may have also a geometrically irregular
surface, which is difficult to distinguish from “real” pores. The pore is thus defined
by a convention as the surface cavity having larger depth than the half of its
average size (e.g. the radius for cylindrical pore).
The pores may be classified according to different criteria:
According to their availability from outside one distinguishes:
Unavailable pores (totally closed, isolated) – having any connection with
surface of the solid body (e.g. occluded gas or liquid bubbles in rocks). Such pores
are not detected by most porosimetric methods, however they influence among
others the measurements of solid phase density, shear stresses under high pressures
and may be responsible for a collapse of a solid under high external forces.
Available pores – connected with the surface either indirectly or via other pores
(channels). These pores are subdivided on several groups:
- pores available from one side (one-side open)
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