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The size and charge of the interlayer cations determines the distance between the layers and the extent to which water can enter between the layers to cause swelling. Dislocation loops cause slip by movement of both edge and screw components Fig. State Bureau of Quality and Technical Supervision. And the compact CaC03 film generated on the surface hinders the further infiltration of the air and prevents the inner water evaporating, which slow down the crystallization of Ca OH 2. The thistle board is mainly used as inner wall, partition wall, and ceiling.
Any standard introductory text in materials science and engineering. Many thermoplastic polymers have low moduli of elasticity and low melting points.
What kind of bonding will be responsible for these characteristics? The Architecture Of Solids Now that we have discussed the nature of bonding, we need to consider how atoms and molecules form the solid materials that we use in everyday life. This depends on the nature of the chemical bond of the molecular structures and on the assembly of different phases in a composite structure.
The possibilities which will be discussed in this chapter are shown in Table 2. The metallic bond is an array of cations bonded by delocalized electrons. The possible arrangements are determined by considering the atoms as a collection of solid spheres of equal radius the ionic radius. Three possible packings exist: These names derive from the unit cells, which are the smallest volumes that describe the structures.
The BCC structure can be visualized as intersecting primitive cubes see Fig. The HCP and FCC structures represent the closest packing of spheres, since each sphere is surrounded by 12 touching neighbors. The BCC structure is less dense; each sphere is surrounded by eight touching neighbors. Most of the remaining one-third of metals have BCC structures.
Many of these are transition metals Fe, Cr, etc. Alkali metals Na, K, etc. It appears that the relatively large thermal vibrations of the atoms at high temperatures see Chapter 1 favor the more open structure. Atoms and ions really behave as "soft spheres," which can be deformed by directional bonding and thermal vibrations. In most instances the cations are smaller than anions because they have fewer electrons and hence fewer filled atomic orbitals.
We can describe ionic crystal structures as packed arrays of anions, with the cations located in holes within the array. If the cation is too small for the holes, the packing will be unstable and must be rearranged to allow the anions and cations to touch each other see Fig. The different stable packings are presented in Table 2. The most stable crystal structures are those in which the ions are surrounded by as many neighbors of the opposite charge as possible, since this increases the total binding energy i.
This extra energy, obtained by packing the ions together, is known as the lattice energy. The size of the ions, as well as their packing, affects the lattice energy. The highest lattice energies are found in crystals containing close-packed arrangements of small ions e.
Structures, Wiley, , Figs. TABLE 2. The stable packing is now octahedral, which remains stable until R. Above R. Top view a Cubic c Tetrahedral The relationships of these packing geometries to crystal structure are shown in Fig. The structure can be considered as interpenetrating lattices of cations and anions. Alternatively, we can think of the cations and anions collectively occupying a single lattice site.
Jons of Different Charge The preceding discussion has been concerned with the effects of differing atomic size, while each anion and cation has the same charge. When the charges are not the Sec. Figure 2. Consider the simplest case of calcium fluoride CaF2 , where there must be twice as many F" ions as ca: The simple cubic packing arrangement is modified to accommodate the extra fluoride ions.
It has a coordination number of 6 in an octahedral arrangement, in which the hydroxide ions are in a close-packed hexagonal array with the calcium ions in the octahedral holes Fig. However, every second layer of octahedral holes is unoccupied to maintain electroneutrality.
As a result, the hydroxide ions on either side of this plane are not held together by strong ionic forces, but only weak van der Waals secondary attractions.
The crystal is therefore readily cleaved along this plane, as can be seen in fracture surfaces of portland cement binder.
This is an extreme example of crystal anisotropy: Most crystals show a degree of anisotropy Anisotropy is observed in ionic crystals but is most marked in covalent crystals, where directional bonds dominate.
Therefore, an alternate view of a crystal structure is to view it as a packed coordination polyhedra Figs. Structures, Wtley, , Fig. I, Academic Press, , Fig. This type of visualization becomes important at low coordination numbers e. A similar type of structure occurs for carbonates, sulfates, etc.
Fully two-dimensional silicate structures are sheet silicates, which are typified by clay minerals. Consider kaolinite, which is an HCP array of oxygen and hydroxide ions Fig. The charge and geometry of the two sheets match perfectly, and each polar sheet combination is bound to the a b c Figure 2.
Four oxygen atoms surround a silicon atom. Thus, kaolinite crystallizes as fiat sheets and is isotropic only in two dimensions: Kaolinite typifies a "two-layer" clay Fig. Clays crystallize as flat plates which cleave easily along the weakly bonded plane. Mica is well known in this regard in that it forms very large crystals.
Dry clays have a slippery feel due to the plates sliding across one another and can be used as a solid lubricant. Talc is the most familiar example of this. Most clays Sec. Taylor, Chemistry of Cement Vol. The oxygens large circles are in HCP packing; the shaded circles lie between or above the open ones.
Each silicon atom black is surrounded by four oxygens; each tetrahedron shares three of its oxygens with three other tetrahedrons. Notice the hexagonal pattern of "holes" in the sheet. The top layer of hydroxides has a hexagonal pattern.
If the two sheets are superposed as if they were facing pages of a book they mesh, forming kaolinite. Finally, silica tetrahedra can be fully bonded in three dimensions to four other tetrahedra. Several different packings are possible. Cristoballite Fig. Alternate small cubes contain silicon atoms at the corner of a tetrahedron, and these are additional sites in the unit cell. These forms are called polymorphs. Many crystalline compounds exhibit polymorphism. The different packings have different binding energies, and therefore one polymorph is energetically favored for a particular set of conditions e.
All structures can be described by a unit cell, which is the smallest volume that represents the structure. Any crystal is made up of unit cells stacked three dimensionally. A unit cell is an arrangement of points in space a space lattice where atoms are located. Since most civil engineers will not need to study crystal structures in depth, we will not show all the Bravais lattices here.
Atomic planes within crystals or directions within crystals are identified by Miller indexes. Such information is only needed when determining crystal structure, describing an atomic plane, or identifying directions of dislocation movements.
In this text we will discuss dislocations qualitatively without reference to Miller indexes. An introduction to Miller indexes can be found in any basic materials science text.
It must be remembered that although it is possible to grow large single crystals in favorable circumstances, all crystalline structural materials are polycrystalline. As 2The zinc blende structure Fig. The crystals grow until they impinge on neighboring crystals. Both metals and ce- , ramies are composed of relatively large crystals that are irregular in shape and size Fig.
Most crystals of interest do not even come close. Thus, the responses of materials of interest to stress and other properties are determined by how imperfections respond. Table 2. Point defects are points in the crystal lattice which either have an atom missing a vacancy , an extra atom present an interstitial atom , or an atom of a different size replacing the expected atom a substitutional atom. These defects are shown in Fig. The additional strain caused by the distortion raises the energy of the crystal i.
Specific Examples Grain boundaries Pores Tensite stress fields b Point Line Compressive stress fields a before the crystal becomes unstable. Thus, calcium ions ionic radius 0. Point defects can also lead to charge imbalance, most commonly where an ion of one charge substitutes for an ion of a different charge.
Consider the clay mineral kaolin Fig. External cations must thus be located between the layers to balance the net charge. The size and charge of the interlayer cations determines the distance between the layers and the extent to which water can enter between the layers to cause swelling.
This is shown in Fig. Frequently, however, point defects occur in pairs, to avoid disruption of charge balances.
The two most common are a Frenkel defect Fig. A point defect pair also tends to minimize the strain within the crystal lattice caused by the defects. Point defects allow atoms to diffuse more readily through a crystal lattice; atnmic diffusion is an important process in the heat treatment of metals.
Line defects are commonly known as dislocations. There are two distinct types: The dislocation thus forms a line through the crystal and is denoted by the symbol.
L The distortion induced in the lattice by the edge dislocation leads to zones of compressive and tensile strain Fig. The distortion is measured by the a b e Sec. Distortions caused by edge dislocation: Consider a plane cutting through a crystal. The Burgers vector is parallel to the dislocation line, and atomic movement can be regarded as a spiral movement along the line.
The formation of the screw dislocation makes it easier for crystals to grow from solution. The growth of a crystal requires new planes of atoms to be laid down in the required sequence of packing e.
A screw dislocation creates B or C sites in what would normally be a plane of only A sites. In a perfect crystal, a new plane has to form by adsorbed atoms at the surface aggregating to form B or C sites a much more difficult process. This occurs because the atoms are packed in one crystal or grain in one orientation, and must change to a different orientation in the adjacent crystal. As can be seen in Fig. Since it represents the Chap.
Grain boundary segregation may result in second phases often amorphous developing at crystal interfaces. Materials 2nd Edition by S. Materials 5th Revised Edition by Surendra Singh. Wood types Wood. Coarse and fine aggregate. Introduction to the manufacture.. Grout Cellular. Precast Grout.
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Back Matter Pages About this book Introduction This expansive volume presents the essential topics related to construction materials composition and their practical application in structures and civil installations.
This book also: Composition of structural materials Concrete and cement in structures Corrosion prevention and protection Materials in civil infrastructure Polymeric building materials Polymeric matrix composites Structural glue Wooden materials in structures. Editors and affiliations.
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