Compositions: coating or plastic – Coating or plastic compositions – Inorganic settable ingredient containing
Reexamination Certificate
2000-02-04
2002-09-10
Marcantoni, Paul (Department: 1755)
Compositions: coating or plastic
Coating or plastic compositions
Inorganic settable ingredient containing
C106S693000, C106S695000, C106S696000, C106S732000, C106S735000, C106S783000
Reexamination Certificate
active
06447597
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to hydrated calcium aluminate expansive materials and more particularly, the present invention relates to shrinkage compensating admixtures for mortar, concrete and cementitious construction products.
BACKGROUND OF THE INVENTION
Contraction or shrinkage is an inherent characteristic of cementitious materials with concomitant cracking and other related structural failures. Three broad categories characterize shrinkage, namely: plastic, autogenous and drying. In order to minimize shrinkage strains, shrinkage reducing admixtures (SRA) and shrinkage compensating admixtures (SCA) are incorporated in the cementitious materials. Although SRA reduces shrinkage strains, its effectiveness is compromised in freezing and thawing conditions and its use is limited because it is a liquid component. Calcium sulfoaluminate based admixtures are the most effective SCA. These additives promote expansion in the mix that counteracts shrinkage deformations. They are largely used in cementitious mixes, allowing for good flexibility in the formulation of cementitious materials, at a lower cost, and can be dry blended with cement. These materials have utility in certain circumstances, but are limited in freezing and thawing conditions. One of the more commonly used SRA compositions is calcium sulfoaluminate admixture (CSA). This additive is useful for enhancing flexibility in the formulation of cementitious materials at a lower cost and also facilitating the possibility of dry blending the mixture with cement.
In the prior art, Sheikh, U.S. Pat. No. 5,741,357, issued Apr. 21, 1998, provides a hydrated high alumina cement. This reference teaches that the inner core of the particles is the anhydrous alumina bearing material and is accordingly a source of aluminates necessary the formation of ettringite and subsequent expansion. The outer coat of hydrated products prevents the rapid dissolution of the inner core material to control the formation of ettringite and the timing of expansion. Although this technology is quite meritorious, it was somewhat limited in effectiveness of the prehydration process in preventing the accelerated reaction of the high aluminous cement and the inconsistency produced in relation to the degree and timing of expansion.
One of the primary limitations of the technology in U.S. '357 relates to the fact that the particles are ground for the production of the admixture. As will be appreciated, due to the grinding, a mixture of particles having differing amounts of unhydrated product unprotected by the hydrated crust naturally result (FIG.
1
). This subsequently translates into potential inconsistency in the rate and amount of expansion obtained by the admixture. Such a limitation can compromise the performance of the admixture as an effective shrinkage compensating agent.
Having regard to the progress made in this field as outlined in the Sheikh patent, the present application overcomes the admixture homogeneity problem and teaches an admixture that can be dry blended with cementitious material to provide a homogenous final product.
SUMMARY OF THE INVENTION
An objective of all embodiments of the present invention is to provide an improved cementitious additive and mixture incorporating the additive for use in cementitious compounds to alleviate the complications and limitations in the compositions of the known art.
It is an object of one embodiment of the present invention to provide an expansive additive for controlling shrinkage in a cementitious mixture, comprising:
calcium aluminate particles where at least 75% by mass of the particles are hydrates of C
3
AH
6
CAH
10
or a mixture thereof, where C represents CaO, A represents Al
2
O
3
, and H represents H
2
O;
an expansion effecting sulfate-based compound for effecting expansion in the mixture; and
calcium hydroxide.
A further object of one embodiment of the present invention is to provide a cementitious mixture having reduced shrinkage upon curing, comprising:
cementitious material;
calcium aluminate particles where at least 75% by mass of the particles are hydrates of C
3
AH
6
, CAH
10
or a mixture thereof, where C represents CaO, A represents Al
2
O
3
, and H represents H
2
O;
an expansion effecting sulfate-based compound for effecting expansion in the mixture; and
calcium hydroxide.
In the prior art, there is no teaching with respect to the use of a high by mass concentration of C
3
AH
6
hydrates, CAH
10
hydrates or a mixture of these as an addition for a cementitious mixture. In fact, the prior art methods and compounds are focused upon anhydrous particles. Fu, et al., in “Characteristics of Shrinkage Compensating Expansive Cement Containing a Pre-Hydrated High Alumina Cement-Based Expansive Additive”,
Cement and Concrete Research
, Vol. 24, No. 2, pp 267-276, 1994, are concerned with the use of the anhydrous material to effect desirable results.
In the present invention, it has been found that the additive can effectively be customized for specific uses/environments of use and, therefore, is not limited to a specific field of utility, but rather is useful in a host of cementitious mixtures.
A still further object of one embodiment of the present invention is to provide a method for synthesizing hexagonal phase calcium aluminate hydrate particles, comprising the steps of:
i) providing a source of calcium aluminate particles;
ii) hydrating the calcium aluminate sufficiently to form a slurry;
iii) agitating the slurry sufficiently for hydration to occur and form hexagonal phase calcium aluminate hydrate particles;
iv) maintaining the temperature of the slurry to prevent thermal conversion of the hexagonal phase calcium aluminate hydrate particles to cubic phase calcium aluminate particles; and
v) drying the slurry at a temperature suited to prevent thermal conversion of the hexagonal phase calcium aluminate hydrate particles to cubic phase calcium aluminate particles.
Another object of one embodiment of the present invention is to provide a method for synthesizing cubic phase calcium aluminate hydrate particles, comprising the steps of:
i) providing a source of calcium aluminate particles;
ii) hydrating the calcium aluminate sufficiently to effect hydration of at least 75% by mass of the particles in a slurry;
iii) agitating the slurry sufficiently for hydration to occur;
iv) maintaining the temperature of the slurry in a range sufficient to convert any hexagonal phase calcium aluminate particles formed to cubic phase calcium aluminate hydrate particles; and
v) drying the slurry.
By controlling the amount of unhydrated (anhydrous) material present, the inherent variability of the prior art mixtures is avoided. The complete hydration of 75% of the mass to form C
3
AH
6
and CAH
10
allows for full predictability in the final product.
A further object of one embodiment of the present invention is to provide a method for synthesizing cubic phase calcium aluminate hydrate particles, comprising the steps of:
i) providing a source of calcium aluminate particles;
ii) hydrating the calcium aluminate particles;
iii) curing the mixture in a humid atmosphere for a period sufficient to prevent self desiccation at a temperature sufficient to effect conversion of hexagonal phase particles to the cubic phase calcium aluminate hydrate particles; and
iv) drying the mixture.
A still further object of one embodiment of the present invention is to provide a method of forming a cementitious mixture having reduced shrinkage in use, comprising mixing the compounds:
i) cementitious material;
ii) an expansion effecting sulfate compound for effecting expansion in the mixture;
iii) calcium hydroxide; and
iv) presynthesized calcium aluminate hydrate particles formed by the method of claim
14
.
An even further object of one embodiment of the present invention is to provide a method of forming a cementitious mixture having reduced shrinkage in use, comprising mixing the compounds:
i) cementitious material;
ii) an expansion effecting sulfate compound for effecting expansion in the mixture;
Mailvaganam Noel P.
Repette Wellington L.
(Marks & Clerk)
Marcantoni Paul
National Research Council
Sharpe Paul S.
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