Geology 101, Section 12262
20 November 1996
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.
ACement and concrete.@ The 1996 Grolier Multimedia Encyclopedia. CD-ROM. Danbury: Grolier, 1996.
Day, Richard. The Home Owner Handbook Of: Concrete and Masonry. New York: Bounty Books, No Copyright Date.
Kosmatka, Steven H., and William C. Panarese. Design and Control of Concrete Mixtures. Skokie, Ill.: Portland Cement Association, 1990.
Seymore, Dany. President of Show Low Ready Mix. Personal interview. 11 November 1996.
College of Mines Faculty, University of Arizona. Arizona: Its People and Resources. Tucson, AZ.: The University of Arizona Press, 1972.