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Arizona Concrete

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Arizona Concrete

Post  MBAstudent on Tue Feb 23, 2010 1:09 am

According to the Mine Faculty at the University of Arizona, cement is
manufactured primarily from suitable limestone and shale rocks. Arizona had two
dry-process cement plants in 1969, namely the Arizona Portland Cement Company
plant in Pima County, near Tucson, and the American Cement Corporation plant at
Clarkdale, in Yavapai County (52-53).
The use of cementing materials goes back to the ancient Egyptians and
Romans, but the invention of modern portland cement is usually attributed to
Joseph Aspdin, a builder in Leeds, England, who obtained a patent for it in 1824.
Currently, the annual world production of portland cement is around 700 million
metric tons (Danbury).
Many people use the words concrete and cement interchangeably, but
they're not. Concrete is to cement as a cake is to flour. Concrete is a mixture
of ingredients that includes cement but contains other ingredients also (Day 6-
7).
Portland cement is produced by pulverizing clinker consisting
essentially of hydraulic calcium silicates along with some calcium aluminates
and calcium aluminoferrites and usually containing one or more forms of calcium
sulfate (gypsum) as an interground addition. Materials used in the manufacture
of portland cement must contain appropriate proportions of calcium oxide, silica,
alumina, and iron oxide components. During manufacture, analyses of all
materials are made frequently to ensure a uniformly high quality cement.
Selected raw materials are crushed, milled, and proportioned in such a
way that the resulting mixture has the desired chemical composition. The raw
materials are generally a mixture of calcareous (calcium oxide) material, such
as limestone, chalk or shells, and an argillaceous (silica and alumina) material
such as clay, shale, or blast-furnace slag. Either a dry or a wet process is
used. In the dry process, grinding and blending operations are done with dry
materials. In the wet process, the grinding and blending are done with the
materials in slurry form. In other respects, the dry and wet processes are very
much alike.
After blending, the ground raw material is fed into the upper end of a
kiln. The raw mix passes through the kiln at a rate controlled by the slope and
rotational speed of the kiln. Burning fuel (powdered coal, oil, or gas) is
forced into the lower end of the kiln where temperatures of 2600°F to 3000°F
change the raw material chemically into cement clinker, grayish-black pellets
about the size of 1/2-in.-diameter marbles.
The clinker is cooled and then pulverized. During this operation a small
amount of gypsum is added to regulate the setting time of the cement. The
clinker is ground so fine that nearly all of it passes through a No. 200 mesh
(75 micron) sieve with 40,000 openings per square inch. This extremely fin gray
powder is portland cement (Kosmatka and Panarese 12-15).
Dany Seymore of Show Low Ready Mix said that the cement used by Show Low
Ready Mix is trucked in by Apex Freight Company and comes from the cement plant
in Clarkdale, Arizona, now know as Phoenix Cement. Their aggregate comes from
Brimhall Sand and Rock in Snowflake, Arizona. Show Low Ready Mix uses Fly Ash
from the A.P.S. power plant just out side of Joseph City, Arizona, in their
cement. The mixtures they use are as follows:

Silicia Dioxide Cement 21% Ash 62%
Aluminum Trioxide Cement 4% Ash 23%
Ferric Oxide Cement 3% Ash 6%
Calcium Oxide Cement 64% Ash 3.5%
Mag. Oxide Cement 2.5% Ash 1.2%
Sulfur Trioxide Cement 3% Ash .2%

These combine to make:

1. Tricalcium silicate C3S
2. Dicalcium silicate C2S
3. Tricalcium aluminate C3A
4. Tetracalcium aluminoferrite C4AF

1 and 2 make up 75% of cement. 1 and 2 plus H2O equal CSH (Calcium Silicate
Hydrate) which is the glue. Fly Ash is C3S plus C2S which equals Calcium
hydrazide which is a white stuff and water soluble. Calcium Hydrazide and Fly
Ash equal CSH.

The winter and summer mixtures are different due to the weather
conditions. For winter, Fly Ash is not used because it inhibits the set time of
the concrete. Also used is accelerators to help the concrete set faster. A
material called Fibermesh is used in the concrete for reinforcement and to
control cracking as the concrete sets. Mr. Seymore also states that heat and
moisture are the main components to make concrete set up.
The concrete is mixed out of the plant into the truck so the materials
can be feathered together and mixed up properly. The PSI ratings are determined
by the mixture of sand, aggregate, cement, water, and chemical additives that
are mixed together. The most common mixtures for residential are 2500 to 3000
PSI.
Concrete cannot be delivered any where that is more than 90 minutes away
from the batch plant, unless a chemical inhibiter is used to put the concrete to
sleep until it reaches the sight of delivery. Then another chemical is added to
activate the concrete.
Show Low Ready Mix mixes approximately 25,000 to 30,000 cubic yards of
concrete in Show Low per year. That is only 70 to 75 percent of the total
concrete poured in Show Low. There are a few other companies that also handle
the Show Low area.
Concrete is basically a mixture of two components: aggregates and paste.
The paste, comprised of Portland cement, (the term APortland cement@ pertains to
a calcareous hydraulic cement produced by heating the oxides of silicon, calcium,
aluminum, and iron.) Water binds the aggregates (sand and gravel or crushed
stone) into a rocklike mass. The paste hardens because of the chemical reaction
of the cement and water.
The paste is composed of Portland cement, water, and intrapped air or
purposely entrained air. Cement paste ordinarily constitutes about 25% to 40% of
the total volume of concrete. Since aggregates make up about 60% to 75% of the
total volume of concrete, their selection is important. Aggregates should
consist of particles with adequate strength and resistance to exposure
conditions and should not contain materials that will cause deterioration of the
concrete.
Aggregates are generally divided into two groups: fine and coarse. Fine
aggregates consist of natural or manufactured sand with particle sizes ranging
up to 3/8 inches; coarse aggregates are those with particles retained on the
No.16 sieve and ranging up to 6 inches. The most commonly used maximum aggregate
size is 3/4 inch or 1 inch. A continuous gradation of particle sizes is
desirable for efficient use of the cement and water paste.
For any particular set of materials and conditions of curing, the
quality of hardened concrete is determined by the amount of water used in
relation to the amount of cement . Some advantages of reducing water content
are: increased compressive and flexural strength, lower absorption, increased
resistance to weathering, better bond between successive layers and between
concrete and reinforcement, less volume change from wetting and drying, and
reduced shrinkage cracking tendencies. The less water used, the better the
quality of the concrete, provided it can be consolidated properly.
The freshly mixed (plastic) and hardened properties of concrete may be
changed by adding admixtures to the concrete, usually in liquid form, during
batching. Admixtures are commonly used to: adjust setting time or hardening,
reduce water demand, increase workability, intentionally entrain air, and adjust
other concrete properties (Kosmatka and Panarese 1-2).
After completion of proper proportioning, batching, mixing, placing,
consolidating, finishing, and curing, hardened concrete becomes a strong,
noncombustible, durable, abrasion-resistant, and practically impermeable
building material that requires little or no maintenance. Concrete is also an
excellent building material, because it can be formed into a wide variety of
shapes, colors, and textures for use in almost unlimited number of applications.

MBAstudent

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