The chemical nature of the material is the final result of the chemically bonded atoms of the different elements forming the compound. In some materials such as metals the bulk is comprised of a single element, with only trace amounts of other elements as impurities or intentionally added alloying constituents. Most of the metals used in construction are produced from the molten state by solidifying them into micro-crystals whose growth terminates at boundaries of neighbouring crystals. These solids are usually non-porous in the sense that the boundaries are close enough to exclude the possibility that foreign atoms can penetrate the interior. This Digest is concerned mainly with the class of materials designated as porous.
All building materials are mixtures of a number of chemical compounds identified as molecules.
The compounds either crystallize into various definite shapes or solidify from the molten state as formless particles to make up the micro-units. These are the physical building blocks from which all building materials are constituted. Having recognized that micro-units are the physical building blocks, it follows that physical and mechanical properties must be determined by these units, their arrangement, and connections. Usually a specific material will be a mixture of several compounds and consequently a mixture of differently shaped particles, with a wide range of sizes. Porosity Porosity can be defined as any space between micro-units that is greater than normal atomic dimensions so that foreign molecules such as water can penetrate it.