Civil Engineering Reference
In-Depth Information
they fit into the holes in the close pack structure is determined by the
number of valence electrons in the metallic and nonmetallic elements.
The atoms pack to maximize density, with the constraint that like ions are
not nearest neighbors (Ashby and Jones 1986).
Simple covalent bonds form materials that are highly durable and
strong. Diamond is the preeminent example of an elemental high-strength
material. Diamond is widely used in industrial applications needing wear
resistance, such as cutting tools. However, the most important of the pre-
dominately covalent bond materials used by civil engineers are the silicate
compounds of portland cement concrete. Silicon atoms link with four oxy-
gen atoms to form a stable tetrahedron (Figure 2.19) that is the basic build-
ing block for all silicates. When combined with metal oxides, MO, with a
ratio of of 2 or greater, the resulting silicate is made up of separate
monomers linked by the MO molecules. Figure 2.20 shows the calcium
silicate crystal structure. The primary reaction compounds of portland
cement are tricalcium silicates and dicalcium silicates. Each of these have
two or more metal oxides, CaO, per silicate molecule. An ionic bond forms
between the metal oxide and the silicate.
Ceramics have only about one-fiftieth the fracture toughness of metals.
Due to the nature of the ionic and covalent bonds, ceramic compounds tend
to fracture in a brittle manner rather than to have plastic deformation, as is
the case for metals. Like metals, ceramic compounds form distinct grains as
a result of multiple nuclei forming multiple crystals during the production
of the compound. In addition, due to the production process, ceramic mate-
rials tend to have internal cracks and flaws. Stress concentrations occur at
the cracks, flaws, and grain boundaries, all of which lower the strength and
toughness of the material. As a result, ceramic materials lack tensile strength
and must be reinforced when they are used for structural applications.
MOSiO
2
SiO
4
FIGURE 2.19
Silicate tetrahedron.
Si
-
-
-
O
-
-
-
Ca
2
Ca
2
Ca
2
-
-
-
FIGURE 2.20
Simple inorganic
solid structure (calcium silicate).
-
-
-
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