Compositions: coating or plastic – Coating or plastic compositions – Inorganic settable ingredient containing
Reexamination Certificate
1999-08-10
2001-06-05
Green, Anthony (Department: 1755)
Compositions: coating or plastic
Coating or plastic compositions
Inorganic settable ingredient containing
C106S709000, C106S715000, C106S718000, C106S719000, C106S720000, C106S721000, C106S722000, C106S724000, C106S725000, C106S726000, C106S727000, C106S728000, C106S732000, C156S039000, C428S703000
Reexamination Certificate
active
06241815
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates generally to the field of construction materials, particularly boards or panels, patching materials, joint compounds, and the like, which are made with gypsum and cement. Such materials may include more gypsum than cement, but still have good water resistance and strength.
Both gypsum and Portland cement (generally hereinafter “cement”) are well known as construction materials. Gypsum (calcium sulfate dihydrate) is the principal component of the familiar wallboard, where it is faced with paper to provide strength and a smooth surface. Cement is used in various applications where its hardness, water resistance, and durability make it valuable, such as in concrete structures. Cement is also used in building panels where its hardness and water resistance are important.
Gypsum is generally produced by the rapid hydration of calcium sulfate hemihydrate, while Portland cement operates mainly by the relatively slower hydration of calcium silicate and aluminate minerals. Consequently, adding calcium sulfate hemihydrate to cement offers the benefits of improving the productivity of facilities which manufacture cement-containing panels, since the mixture hardens rapidly. Gypsum is, however, somewhat soluble in water, and mixtures which include both gypsum and cement are not as water resistant as cement alone or cement containing a minor amount of gypsum. Furthermore, it is well known that gypsum reacts with one of the components of cement, namely, tricalcium aluminate (3CaO.Al
2
O
3
, abbreviated as C
3
A) to form ettringite [3CaO.Al
2
O
3
.(CaO.SO
3
)
3
.32H
2
O also C
6
A{overscore (S)}
3
H
32
], which may cause expansion and undesirable cracking. Formation of ettringite can be useful, provided that it occurs early in the process of making panels (referred to as primary ettringite), since it provides fast setting and early mechanical strength. Once the mixture of gypsum and cement has been solidified, however, the formation of ettringite (referred to as secondary ettringite) is generally not desirable. Consequently, efforts have been made to prevent the formation of secondary ettringite in gypsum and cement formulations. This has been referred to as preventing an internal “sulfate attack,” since it is the reaction of gypsum, CaSO
4
.2H
2
O, with tricalcium aluminate and water, which results in the formation of ettringite. The tricalcium aluminate is quite soluble and cement often includes a small amount of gypsum to react with dissolved C
3
A. A high alumina content does not necessarily mean that a cement is susceptible to sulfate attack, because the reactivity of the alumina-bearing compounds matters more than the total alumina content.
An important approach to limiting the formation of ettringite has been to add “pozzolanic” materials. In general, pozzolanic materials are defined by ASTM C618-97 as “ . . . siliceous or siliceous and aluminous materials which in themselves possess little or no cementitious value but will, in finely divided form and in the presence of moisture, chemically react with calcium hydroxide at ordinary temperatures to form compounds possessing cementitious properties.” For a given pozzolanic material, the finer the material is, the greater the pozzolanic activity. Also, amorphous materials are considered to possess greater pozzolanic activity. Finely divided amorphous silica, such as silica fume, has been found to have substantial pozzolanic activity. A related material, microsilica, is even more pozzolanic than silica fume. A crystalline silica having a large particle size, such as sand, would not be expected to have significant pozzolanic activity. Other naturally derived materials which, when finely divided, have been referred to as pozzolanic include pumice, perlite, diatomaceous earth, tuff, trass, etc. Man-made pozzolanic materials include metakaolin, microsilica, silica fume, ground granulated blast furnace slag, and fly ash.
Differences in pozzolanic activity may be related to the chemical reactivity of the components. That is, the quantity of silica or alumina in a pozzolanic material may not be as significant as the form in which they are found. The literature suggests that the temperatures used in processing of naturally derived or man-made pozzolans may determine whether or not the product is an active pozzolan. Thus, a high silica content may not be necessary, provided that the silica has been activated by its processing. Similarly, the alumina content of pozzolanic materials has been contended to be important. But, a high alumina content may have little effect, provided that the aluminum compounds are not reactive. For example, metakaolin contains less silica and much more alumina than silica fume, but has been found by the present inventor to provide superior mechanical performance in products made with metakaolin. Similarly, blast furnace slag contains less silica, but is more active than fly ash. It may be concluded that the pozzolanic activity of silica and alumina-containing materials should be considered only as potential until the pozzolanic properties are validated by appropriate tests.
In U.S. Pat. No. 4,494,990, Harris disclosed the effect of adding a pozzolanic material to a mixture of alpha gypsum (alpha hemihydrate) and Portland cement. He used a “sulfate reactivity factor” to determine whether the pozzolanic material was useful. This sulfate reactivity factor requires knowledge of the amounts of various components of the cement present and the relative amounts of the pozzolanic material, gypsum and cement. Broadly, the composition of Harris would contain 25-60 wt % of cement, 40-75 wt % of calcium sulfate hemihydrate (typically the alpha form), and 3-50 wt % of a pozzolan, in his examples, silica fume, having a sulfate reactivity factor less than 12.
In their article in Cement and Concrete Research, Vol. 25, No. 4, pp. 752-758, 1995, Singh and Garg discussed their work with a binder made of calcined phosphogypsum, fly ash, hydrated lime, and Portland cement.
In the same journal, Vol. 28, No. 3, pp. 423-437, 1998, Kovler reported on his work with blends of gypsum, Portland cement, and silica fume. Kovler stated “[s]uch blends can possess the advantages of gypsum (early hardening, high early strength, enhanced workability) and Portland cement (improved durability in moist conditions), but are free of the deleterious effect of ettringite and thaumasite, which are formed when gypsum and Portland cement react.” (Note that “gypsum” is often used to refer to hemihydrates, as is done here.)
Bentur, Kovler and Goldman reported on similar compositions in Advances in Cement Research, Vol. 6, No. 23, pp. 109-116, 1994. They tested mixtures of gypsum, Portland cement, and silica fume having more than 92 wt % silica and noted that improved wet strength “ . . . was explained by the reduction in ettringite formation and the development of a microstructure in which gypsum crystals were engulfed by CSH.” By CSH was meant calcium silicate hydrate, according to a shorthand notation commonly used in the field, which constitutes the main constituent of Portland cement.
A high early strength cement was disclosed in U.S. Pat. No. 4,350,533 by Galer et al. of United States Gypsum Company. The high strength was obtained by forming substantial amounts of ettringite from mixtures containing high alumina cement and calcium sulfate (all forms, including gypsum, said to be useful). Pozzolanic materials, such as fly ash, montmorillonite clay, diatomaceous earth, and pumice, were considered optional ingredients, but could replace up to about 20% of the cement. A related and commonly assigned patent is U.S. Pat. No. 4,488,909.
In EP Patent No. 271,329, compositions containing 70% ettringite and up to 30% CSH were made using “non-traditional materials,” including CaSO
4
.
In U.S. Pat. No. 4,661,159, Ortega et al. disclosed a floor underlayment composition which included alpha calcium sulfate hemihydrate (alpha gypsum), beta calcium sulfate hemihydrate (beta gypsum), fly ash, and Portland cement
Green Anthony
Jenkens & Gilchrist
United States Gypsum Company
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