Page 445 - Carrahers_Polymer_Chemistry,_Eighth_Edition

408 Carraher’s Polymer Chemistry

12.3 OTHER CEMENTS

There are a number of cements specially formulated for specifi c uses. Air-entrained concrete con-
tains small air bubbles formed by the addition of soap-like resinous materials to the cement or to
the concrete when it is mixed. The bubbles permit the concrete to expand and contract (as temper-
ature changes) without breaking (since the resistance of air to changes in the concrete volumes is
small). Light-weight concrete may be made through the use of light-weight fillers such as clays and

pumice in place of sand and rocks or through the addition of chemical foaming agents that produce
air pockets as the concrete hardens. These air pockets are typically much larger than those found in
air-entrained concrete.
Reinforced concrete is made by casting concrete about steel bars or rods. Most large cement-
intense structures such as bridges and skyscrapers employ reinforced concrete. Prestressed con-
crete is typically made by casting concrete about steel cables stretched by jacks. After the concrete
hardens, the tension is released, resulting in the entrapped cables compressing the concrete. Steel
is stronger when tensed, and concrete is stronger when compressed. Thus, prestressed concrete
takes advantage of both of these factors. Archways and bridge connections are often made from
prestressed concrete.
Concrete masonry is simply the name given to the cement building blocks employed in the con-
struction of many homes, and it is simply a precast block of cement, usually with lots of voids. Precast
concrete is concrete that is cast and hardened before it is taken to the site of construction. Concrete
sewer piped, wall panels, beams, grinders, and spillways are all examples of precast cements.
The cements cited above are all typically derived from Portland cement. Following are non-
Portland cements.
Calcium-aluminate cement has a much higher percentage of alumina than does Portland cement.
Furthermore, the active ingredients are lime, CaO, and alumina. In Europe it is called melted or
fused cement. In the United States it is manufactured under the trade name Lumnite. Its major
advantage is its rapidity of hardening, developing high strength within a day or two.
Magnesia cement is largely composed of magnesium oxide (MgO). In practice, the MgO is mixed

with fillers and rocks and an aqueous solution of magnesium chloride. This cement sets up (hardens)

within 2–8 h and is employed for flooring in special circumstances.
Gypsum, or hydrated calcium sulfate (CaSO • 2H O), serves as the basis of a number of prod-
4
2
ucts, including plaster of Paris (also known as molding plaster, wall plaster, and fi nishing plaster).
The ease with which plaster of Paris and other gypsum cements can be mixed and cast (applied)
and the rapidity with which they harden contribute to their importance in the construction field as a

major component for plaster wall boards. Plaster of Paris’ lack of shrinkage in hardening accounts
for its use in casts. Plaster of Paris is also employed as a dental plaster, pottery plaster, and as molds
for decorative fi gures. Unlike Portland cement, plaster of Paris requires only about 20% water and
dries to the touch in 30–60 min giving maximum strength after 2–3 days. Portland cement requires
several weeks to reach maximum strength.

12.4 SILICATES
Silicon is the most abundant metal-like element in the earth’s crust. It is seldom present in pure ele-
mental form, but rather is present in a large number of polymers largely based on the polycondensa-
–4
tion of the orthosilicate anion, SiO as illustrated following:
4
–4
–6
SiO R Si O + O −2 (12.3)
4
7
2
(1) (2)
–6
–6
–4
Si O + SiO R Si O + 2O −2 (12.4)
3
4
9
2
7
(3)
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