Gypsum-rich Portland cement

Details Gypsum-rich Portland cement Gypsum-rich Portland cement

2000-03-30

2001-03-06

Green, Anthony (Department: 1755)

Compositions: coating or plastic

Coating or plastic compositions

Inorganic settable ingredient containing

C106S718000, C106S721000, C106S735000, C106S773000, C106S788000

Reexamination Certificate

active

06197107

ABSTRACT:

FIELD AND BACKGROUND OF THE INVENTION
The present invention relates generally to cementitious compositions and, in particular, to gypsum-containing cementitious construction materials for high strength concrete, blocks, grout, floor underlayments, road-patching materials, backer boards, fiberboard and roofing tiles.
Ordinary Portland cement (OPC) is the main cementitious material of the building industry. It is the main binder in concrete blocks, roofing tiles, grouts, fiberboard, mortar, tile adhesives, etc. Disadvantages OPC include low early strength and high shrinkage on drying.
Gypsum (CaSO
4
.2H
2
O) is an inexpensive, available material that is commonly used as an additive for OPC and for the production of gypsum-based products such as plasterboard, fiberboard, plaster, etc. Calcined gypsum, (calcium sulfate hemihydrate—CaSO
4
.½H
2
O) forms gypsum upon wetting and sets within minutes, displaying excellent early-strength characteristics. The set gypsum has very low strength relative to OPC. In addition, the solubility of gypsum in water (2 g/l), along with high porosity, gives the material poor water-resistance.
Attempts to improve the water-resistance of gypsum boards by mixing Portland cement and calcined gypsum have met with limited success because of ettringite (3CaO.Al
2
O
3
.CaSO
4
.32H
2
O) formation. It is known that a major factor in the long-term deterioration of concrete is the formation of ettringite. This results from the reaction of tricalcium aluminate (3CaO.Al
2
O
3
) present in OPC with sulfate. The formation of ettringite increases the volume of the concrete, causing splitting, cracking and crumbling. Although mixtures of OPC and calcined gypsum have previously been used and are in some cases advantageous, the interaction between the tricalcium aluminate and the sulfate has greatly limited the use of such mixtures. Their application has been primarily as “quick patches”, which are known to be of poor long-term durability.
Much work has been focused on inhibiting the formation of ettringite. To this end, U.S. Pat. No. 3,852,051 deals with special formulations of Portland cement having low concentrations of tricalcium aluminate. Such formulations are expensive, however, and exhibit low ultimate compressive strength.
U.S. Pat. No. 4,494,990 to Harris discloses a cementitious composition containing OPC and alpha gypsum. The composition also includes a pozzolan source, such as silica fume, fly ash or blast-furnace slag. The Harris patent teaches that the pozzolan blocks the interaction between the tricalcium aluminate and the sulfate in the gypsum.
U.S. Pat. No. 4,661,159 discloses a floor underlayment composition that includes calcium sulfate alpha-hemihydrate (alpha gypsum), calcium sulfate beta-hemihydrate (beta gypsum), fly ash, and Portland cement. The patent also discloses that the floor underlayment material can be used with water and sand or other aggregate to produce a fluid mixture that may be applied to a substrate.
A cementitious composition useful for water-resistant construction materials is disclosed in U.S. Pat. No. 5,685,903 to Stav, et al. The composition includes beta gypsum, OPC, silica fume, and pozzolanic aggregate as filler. The OPC component may also contain fly ash and/or ground blast slag.
U.S. Pat. No. 5,685,903 teaches that the silica fume component is an extremely active pozzolan that prevents the formation of ettringite. It is further disclosed that compositions according to the invention that include both a pozzolanic aggregate and a finely divided pozzolan result in cementitious materials in which the transition zone between the aggregate and a cement paste is densified and thus produces a cured product of higher compressive strength than compositions which utilize a pozzolanic aggregate alone or a finely-divided pozzolan alone.
It is believed that the mechanism which causes changes in the microstructure of compositions according to the invention to result in higher compressive strengths is associated with two effects: 1) a pozzolanic effect in which the surfaces of the pozzolanic aggregate react with free lime to form calcium silicate hydrate (CSH) which becomes part of the product matrix: 2) a microfiller effect due to the fine size and spherical shape of the silica fume.
A cementitious binder composition useful for water-resistant, high-strength construction materials is disclosed by Stav, et al. in U.S. Pat. No. 5,858,083. The binder includes calcium sulfate beta-hemihydrate, a cement component comprising Portland cement, and either silica fume or rice-husk ash. The silica fume or rice-husk ash component is at least about 92% amorphous silica and has an alumina content of about 0.6 wt. % or less.
According to U.S. Pat. No. 5,858,083, the silica fume component is an extremely active pozzolan and prevents the formation of ettringite. The silica fume component includes at most 0.6 wt. % alumina in the form of aluminum oxide. U.S. Pat. No. 5,858,083 cites Malhotra, M., and Mehta, P. Kumar,
Pozzolanic and Cementitious Materials, Advances in Concrete Technology,
Vol. 1, who report typical oxide analyses of silicon fumes made from the ferrosilicon alloy industry having SiO
2
amounts of as low as 83% and Al
2
O
3
amounts from between 1.00% and 2.5%. Oxide analyses of certain North American blast-furnace slags have SiO
2
amounts of as low as 33% and Al
2
O
3
amounts as high as 10.8%. U.S. Pat. No. 5,858,083 concludes that not all pozzolans, and specifically, not all silica fumes, are acceptable for use according to the invention.
In all of the above-mentioned patents, methods have been developed for preventing the formation of ettringite. However, the prior art teaches that the formation of ettringite can actually be beneficial to cementitious material improving the compressive strength in the early stages.
The role of ettringite in expansive cements is related in U.S. Pat. No. 4,255,398. It is taught that ettringite should be precipitated on the surface of solids already present, and not from the liquid phase during cement hydration.
In
Cement and Concrete Research,
(Vol. 26, No.3), Singh and Garg report on the properties of a gypsum-based binder containing portland cement, calcined phosphogypsum, ground granulated slag and an organic retarder. The physical properties of the blended gypsum binder are compared with those of plain gypsum plaster. The superior behavior of the blended gypsum binder to water is attributed to the filling of voids and pores of the gypsum matrix with ettringite and CSH. The improvement of compressive strength of the blended gypsum binder over the 28-day test period is ascribed to the filling of the matrix with ettringite and tobermorite. The problems associated with late ettringite formation are not dealt with.
In
Material Science of Concrete,
Lawrence surveys and summarizes the topic of delayed ettringite formation (DEF). Based on extensive experimental investigations in the literature, the author teaches that correlation between expansions of siliceous sand mortars and the chemical composition of OPC indicate the importance of the sulfate level in the cement: where the sulfate level of a cement showing expansion has been increased by the addition of CaSO
4
or Na
2
SO
4
, the final expansion is increased. Added fly ash, blast furnace slag, or microsilica tends to reduce the expansions. This conclusion is supported by the work of Stav et al. (U.S. Pat. No. 5,858,083) for pozzolanic materials containing at most 0.6 wt. % alumina in the form of aluminum oxide. Higher levels of aluminum oxide promote DEF expansion, causing long-term deterioration of OPC.
U.S. Pat. No. 4,350,533 to Galer et al. discloses a cementitious composition containing high-alumina cement, calcium sulfate, and Portland cement and/or lime. The reaction is rapid, and the only significant factor contributing to strength during the very early stages of hydration (i.e., a few minutes to a few hours) is the formation of ettringite. Portland cement is not a necessary component of the composition and can be replaced by lime.

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