Compositions: coating or plastic – Coating or plastic compositions – Inorganic settable ingredient containing
Reexamination Certificate
1999-10-15
2002-10-29
Marcantoni, Paul (Department: 1755)
Compositions: coating or plastic
Coating or plastic compositions
Inorganic settable ingredient containing
C106S745000, C106S772000, C264S333000
Reexamination Certificate
active
06471767
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to the manufacture of cement and, more particularly, to the use of gypsum and gypsum-based materials used in the manufacture of cements.
Cement generally refers to any material that initially has a plastic form and which is strongly adhesive after hardening. Cement, in the building and engineering context, usually refers to a fine, powdery substance that is processed so that it will adhere together and with other matters and hardens after being mixed with water. Most modem construction cements utilize gypsum, plaster or portland cement. Portland cement is named for a British cement maker, Joseph Aspdin, because of the resemblance that concrete made with his cement to portland stone, a greyish stone that was widely used in England for building construction. Aspdin's early portland cement was made from lime and clay, or shale, that were heated until they formed cinders and then ground into a fine powder. Those early cements, as with modem portland cements, were mixed with aggregates in the form of sand, or gravel, and water to form concrete, which is the most widely used construction material in the world.
Typically, portland cements are primarily mixtures of tricalcium silicate (3CaO.SiO
2
), tricalcium aluminate (3CaO.Al
2
O
3
), and dicalcium silicate (2CaO.SiO
2
). These three constituents are used in various proportions along with small amounts of magnesium and iron compounds. When such a cement is mixed with water, hydration occurs in the tricalcium silicate to form a gel-like hydrated silica and calcium hydroxide that eventually crystallizes and binds together with the particles of sand and gravel added to the cement to form a hard mass. This result in hard mass is typically referred to as concrete. Where the mixture contains only a fine aggregate, such as sand, the end product is referred to as portland cement mortar and where the mixture contains both fine and course aggregate, the later being in the form of stone or gravel, the end product is referred to as portland cement concrete.
Each element of the cement and concrete mix affects the overall chemistry of the end product, e.g., how fast it sets, its resistance to chemicals, etc. The tricalcium aluminate used in the cement largely produces an initial set of the mixture but does not contribute overall to the ultimate hardening of the mixture. The dicalcium silicate acts in a similar, deliberate manner. The tricalcium silicate causes rapid hardening of the cement or concrete. Special cements and concretes with special properties may be made by adding or decreasing the proportion of these compounds (and others) to obtain specific properties in the end products.
Gypsum is one such component and an important one at that, because of its effect on the retarding of the hardening process of cements/concretes. Gypsum is hydrated calcium sulfate (CaSO
4
.2H
2
O). Gypsum is a cost-effective component that retards the hardening process in portland cement and portland cement concrete significantly to permit the cement or concrete to be delivered, formed and worked while it is in a plastic form. Gypsum acts to reduce the initial rate of heat generation and reduces the rate of hydration of the tricalcium silicate. As an example of its important retarding properties, portland cement that does not use gypsum in its mix will set in about approximately four minutes, while cements/concretes that use gypsum will set in about four hours.
Gypsum is a widely distributed form of a sedimentary rock and it associates with saline deposits such as those formed by the precipitation of calcium sulfate from sea water, as well as with limestone and shale. Gypsum also occurs naturally in volcanic regions and in some clay regions where naturally occurring sulfuric acid has reacted with limestone. Gypsum is mined and crushed at mine locations into aggregate so that it may be eventually transported, usually by rail or truck, to its end user such as a cement plant or a sheet rock manufacturing plant. One by-product of this mining and manufacturing process is the production of gypsum “fines.” Fines are very small particles that may pass through 100 mesh sieves. A 100 mesh sieve, as understood in the art and as used in this application is a sieve having 100 openings per inch.
The mass of these fines is comparable to their size and, as such they are very difficult to transport and utilize. Gypsum fines are usually mixed with water to form a slurry and then piped to a disposal area, such as a storage pit. These fines are too small to use in the manufacturing process of portland cement because they are difficult to transport. Additionally, these fines exhibit a natural tendency to clump together and form deposits, or clumps, that agglomerate during their travel through the cement-making process.
In the manufacture of portland cement, raw materials are mixed from deposits of limestone, cement rock, shale, clay, etc., and are crushed into chunks or rocks of the first processing size which are approximately 5 inches in general diameter. These chunks are then crushed a second time down to a suitable storage size which ranges to about 0.75 inches in diameter for separate storage. From there, the storage materials are sent to a grinding mill where they are mixed in appropriate proportions of approximately 60% lime, 19% silica, 8% alumina, 5% iron, 5% magnesia and 3% sulfur trioxide. This mixture is then conveyed to a grinding mill where the matter is ground into a powder and further stored.
This mixed raw material is then heated in a processing kiln in order to form cinders or, as referred to in the cement industry, “clinker.” The kilns used for this task may be as long as 500 feet and have a diameter of approximately 12 feet. The kilns are slightly tilted in a horizontal plane. Raw materials are introduced into the upper end of the kiln, either in the form of dry rocks or as a wet paste, and as the kiln rotates, this raw material slowly progresses down to the bottom of the kiln, where an array of burners are located. Hot gases from these burners rise up the kiln to heat and dry the mixture as it progresses down the kiln. As the mixture approaches the base of the kiln, the raw material begins to fuse together to form the aforementioned clinker. In this process, water and carbon dioxide are driven off from the raw material by the kiln temperatures which typically will range from approximately 2700° F. to 2900° F. Once formed, the clinker is then cooled quickly and ground into a fine powder of about 3000 to about 5000 Blaine, where it may be conveyed by blowers, or the like, to storage silos. This finely ground product is a base portland cement. As mentioned before, the tricalcium silicate in this base cement would rapidly hydrate and harden in anywhere from about 4 to about 10 minutes. Gypsum is typically added at this point of the cement processing to retard the hydration of the cement when it is used in a mix. This gypsum is ground into the clinker during initial clinker grinding in proportions of anywhere between about 3% to about 10% by weight.
The gypsum used by cement plants is usually naturally mined gypsum. In order to control the addition of gypsum to the clinker in the correct proportions, cement plants need gypsum delivered to it in sizes that are easily crushable, for example, in rocks, or chunks, from about 1 to about 3 inches in diameter. These size chunks are easy to transport as compared to gypsum fines. More importantly, chunks or rocks of about this size are easily crushed by the cement processing equipment and because they approximately match the size of the clinker, they are more readily and reliably processed in the final grinding and mixing stage of cement processing.
Synthetic gypsum may also be used in the manufacture of cement. This synthetic gypsum is produced as a waste material during fossil-fired power generation. Stringent air pollution laws mandate limits in the amount of combustion products that are released into the atmosphere by a power generating plant. These
American International Materials, Ltd.
Marcantoni Paul
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