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CBD-127. The Structure of Porous Building Materials

Originally published July 1970.

Porous materials such as stone, brick, concrete and wood are far from being the simple materials their extensive use suggests. They are much more complex, for example, than sophisticated materials like the silicon wafers used in transistors. Understanding of their structure is essential, therefore, if reliable prediction is to be made of their response to various conditions of exposure.

The constitution of all solid materials is ultimately resolved as an assemblage of atoms of one or more elements. These are organized in particular groupings to form molecules, crystals or particles acting as micro-units from which the physical structure of any solid material is constructed.

All physical and chemical changes in a material originate on the scale of the atom or micro-unit, whereas they are normally observed on the scale of man's visual perception. When any event in a material is observable, it has progressed to a stage where millions of atoms are involved. For this reason it is useful to establish a relative scale for comparison. If a grain of fine sand represents the limit of visual perception (about 100 microns; 1 micron = 1 millionth of a meter), then an atom is so small that about 1 million must be put side by side to make up the grain of sand. The Ångström unit, Å, is used for the atomic scale (1 micron = 10,000 Å). Various assemblages of atoms that form the structural micro-units of materials are usually less than a micron in size and thus must be magnified more than 100 times to be seen.

Atoms are the basic chemical building units. Reactions take place between atoms (as ions) to form molecules, which in turn assemble into crystals or amorphous particles and, finally, into the familiar common materials. For the purpose of this Digest it is sufficient to note that the chemical nature of a material is determined by the basic chemical unit, the molecule. In organic materials the "backbone" of the molecules is a carbon-to-carbon linkage, with side connections of hydrogen as well as carbon, oxygen, and other elements. The particular elements and their arrangement as well as the number of atoms determine the chemical nature of the molecule. For example, a chain of six carbon atoms and a complement of 14 hydrogen atoms form the solvent hexane; a carbon chain of up to 1/4 million carbon atoms forms the plastic, polyethylene. Some general aspects of organic materials are discussed in CBD 117.

In the major inorganic class of materials such as clays and cement the common backbone of the silicates is the silica tetrahedron. This unit forms a tetrahedron shape and permits a great variety of structural configurations by joining at the apex of adjacent units. 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. This includes ceramics, concrete, stone, and brick, as well as organic coatings, wood (wood products) and cellulose products.

Micro-Units

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. Viewed on this scale (magnified 10,000x) where detail of the microstructure can be clearly seen, there is order, complexity and great variety. As building materials include a large number of compounds, they contain a range of crystalline systems composed of various shapes of units, including cubes, plates, needles, and tubes, as well as amorphous (shapeless) particles. 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. The water molecule is only 3.5 Å units in diameter and water can therefore enter the pores whenever the boundary between micro-units is greater than this size. Usually the pores are considerably larger than molecular dimensions and are of different sizes often ranging from very large to micro-size, and are generally interconnected, providing continuous channels to the outside boundary of the material.

In some special cases where a material has a layered structure, as in clay or hydrated cement, water molecules by virtue of chemical affinity can separate the layers even when they are very closely spaced. The special micro-pores thus created are referred to as inter-layer spaces and are not included as porosity. Water molecules as well as other gas molecules may be diffused through material classed as non-porous (i.e. plastics), but the rate is insignificant compared with that through porous materials.

When solids are formed by reactions such as hydration of plaster of paris or cement, with consequent growth of numerous crystals of various size and shape, the result is a structure of microunits making direct contact with each other only at some points on the growing faces of the crystals. The remainder of the surfaces do not come together but leave spaces of varying size and shape. These represent pores, which can be micro-size, as well as larger capillaries and channels.

It will be appreciated that such a system of pores is interconnected and communicates with the outside of the body of the material. The total volume of such space, as a percentage of the total volume of the solid and pores, is defined as the porosity of the material. The porosity of common materials such as plaster is in the range of 50 to 65 per cent, of concrete, 40 to 60 per cent, and of brick, 5 to 50 per cent. Materials that are fused or sintered from an aggregation of particles begin as a highly porous system, but if fusion takes place at high temperature and is accompanied by pressure, as in hot-pressing of ceramics, the resulting porosity can be relatively low.

Bond

The microstructure of a material results from the assembly of the micro-units forming the structure. If the assembly of micro-units is to result in a strong body there must be a strong connection or bond between units. In some instances connections are made by virtue of chemical linkage, as occurs in cross-linking of polymer molecules, to make them much stronger. In other cases crystal boundaries merge during their growth to provide an efficient connection, as for metals and, to some degree, plasters. Most often (in the common cementing systems) the bond is simply the physical attraction of one solid for another, the van der Waals' force, which relies on the close proximity of the atoms in the adjacent surfaces to keep them together.

As the size of micro-units decreases and total surface area increases the area in close contact will increase. A strong cementing action results when portland cement hydrates because of the very small size of the particles created by the hydration process. The great increase in total surface area in a confined space brings adjacent surfaces so close that strong bonds are established. Very fine particles in a dry state can be formed into a strong, rigid body if sufficient pressure is applied to bring the particles into close proximity.

Microstructure

When a material is formed, its physical and mechanical properties will be influenced by the nature of the micro-units, their size and shape, the manner of their nucleation and growth, the nature of the boundary formed by the interference of growing surfaces, the area of this boundary, and the extent of surfaces that do not come together but leave spaces represented by porosity.

In metals, special ceramics, and certain rocks the boundary between grains is continuous and the stress on the material is shared proportionately and uniformly by the individual micro-units (Figure 1a). In a similar manner, a microstructure formed from individual crystals or particles that intergrow at points of contact or are joined by fusion represents a potentially strong material if the area of the interface boundary is large (Figure 1b and Figure 1c). Of the many factors that should influence the formation of a microstructure it has not been possible to identify and relate the effect of each on the final properties of the material.

Figure 1a.  Quartz crystals, non-porous system (Courtesy Sperry Rand Research Center).

Figure 1b.  Brick, sintered clay, porosity 40 per cent.

Figure 1c.  Gypsum, hydrated from plaster of paris and water, porosity 30 per cent.

Figure 1d.  Gypsum, hydrated, porosity 30 per cent.

Figure 1e.  Wood cell (Micrograph courtesy State University College of Forestry at Syracuse University).

Only total porosity has so far shown a relation with strength and modulus of elasticity. Figure 2 shows this relation for plaster and cement, but similar semi-logarithmic relations have been found for many materials. Because of this, small changes in porosity can cause large changes in modulus and strength. Concrete can be brought to a strength of over 20,000 psi and cement paste to over 30,000 psi by special techniques of compaction that provide the lowest possible porosity. No particular formulation of the cement is involved. Even for special compaction of clays, similar strengths can be achieved. Hydrated plaster can be as strong as cement if the porosity is decreased below that normal for the material. Figure 1c shows the needle-like crystals intergrowing and forming a network of braces in three directions not unlike a three-dimensional truss.

Figure 2.  Porosity to modulus relation.

The microstructure of cement at early stages of hydration resembles a clay system of plates and rods, except that there is evidence of more positive connections and a three-dimensional network almost like a geodesic structure. When cement has completely hydrated, however, the structure (Figure 1d) has very few of the large pores evident earlier, although the micro-porosity of what appears to be a solid structure is still about 30 per cent. Micropores in this structure are of the order of 10 to 100 Å in size and would require a magnification of at least 30,000 to be visible. Figure 1d shows the typical structure; it has micro-porosity, but at this magnification it cannot be seen.

Nature produces porous organic materials such as wood in which the geometry is quite regular and the micro-units are of a regular shape and uniform arrangement. Wood as a structural material has been discussed in CBD 85, CBD 86 and CBD 88. Figure 1e is a micrograph of a section of wood cell at a right angle to the grain. Total porosity is greater than in any of the previous examples, but the mechanical properties of strength and modulus are higher than would be expected for such a porous system. This is because the micro-units are assembled in the manner of a composite, where the wood cells in the shape of tubes are bonded along their length with other wood cells by the cementing action of the lignin. Wood is perhaps the best example of a natural composite. It also demonstrates the great potential of synthetic composite materials: the fibreglass fishing rod is man's copy of a wood composite.

An inevitable consequence of micro-structure where porosity exists is the fact that the total internal surface of a material (represented by the boundaries of the internal space) will be exposed to the aggressive action of the environment if the space communicates with the outside. It allows foreign agents to penetrate the "heart" of the material. Because the micro-units are usually very small, this internal surface can be very large. A pound of hydrated cement in concrete, which has a layered or folded sheet structure and interconnected space, will have a total surface area of about 5.5 acres. The significance of this will be discussed in a future Digest.