Many solid and powder materials both natural (stones, soils, minerals, etc.) and manufactured (catalysts, cement, pharmaceuticals, metal oxides, ceramics, carbons, zeolites, etc.) contain a certain void volume of empty space. This is distributed within the solid mass in the form of pores, cavities, and cracks of various shapes and sizes. The total sum of these void volume is called porosity. 

The type and nature of porosity in natural materials depend on their genesis (for instance rocks can be of igneous, sedimentary or metamorphic origin) while in man-made materials depend on their manufacturing and generally it can be controlled.

Porosity strongly determines important physical properties of materials such as durability, mechanical strength, permeability, adsorption properties, etc. The knowledge of pore structure is an important step in characterizing materials, predicting their behavior under different environmental conditions.

There are two main and important typologies of pores: closed and open pores. Closed pores are completely isolated from the external surface, not allowing the access of external fluids in neither liquid nor gaseous phase. Closed pores influence parameters like density, mechanical and thermal properties. Open pores are connected to the external surface and are therefore accessible to fluids, depending on the pore nature/size and the nature of fluid. Open pores can be further divided in dead-end or interconnected pores.

Further classification is related to the pore shape, whenever is possible to determine it.

The characterization of solids in terms of porosity consists in determining the following pore parameters:

Pore size

Pore dimensions cover a very wide range. Pores are classified according to three main groups depending on the access size:

• Micropores: less than 2 nm diameter    
• Mesopores: between 2 and 50 nm diameter    
• Macropores: larger than 50 nm diameter

Several techniques are available to investigate pore access dimensions depending of the size and nature of pores.

Specific pore volume and porosity

The internal void space in a porous material can be measured. It is generally expressed as a void volume (in cc or ml) referred to the mass unit (g). 

Pore size distribution

It is generally represented as the relative abundance of the pore volume (as a percentage or a derivative) in function of the pore size.

Bulk density

Bulk density (or envelope density) is calculated by the ratio between the dry sample mass and the external sample volume.

Percentage porosity

It is represented by ratio between the total pore volume and the external (envelope) sample volume.

Specific surface area

The surface area of a solid material is the total surface of the sample that is in contact with the external environment. It is expressed as square meters per gram of dry sample. This parameter is strongly related to the pore size and the pore volume. The larger is the pore volume the larger is the surface. The smaller is the pore size the higher is the surface. The surface area results from the contribution of the internal surface of the pores plus the external surface of the solid or the particles (in case of powders). Whenever a reasonable porosity is present, the contribution of the external surface to the total is very limited.