Compositions: coating or plastic – Coating or plastic compositions – Inorganic settable ingredient containing
Reexamination Certificate
1999-06-14
2001-07-24
Marcantoni, Paul (Department: 1755)
Compositions: coating or plastic
Coating or plastic compositions
Inorganic settable ingredient containing
C106S810000, C106S823000
Reexamination Certificate
active
06264737
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the use of esters of carbonic acid as set accelerators and thixotropic agents in cementitious compositions. More particularly, the invention pertains to dry, flowable cement admixtures wherein such esters are adsorbed on a solid particulate support which is non-reactive toward the ester. The invention also pertains to methods of synergistically increasing the viscosity of a cementitious composition by the addition of both silica fume and a carbonic acid ester.
BACKGROUND OF THE INVENTION
Organic esters of carbonic acid, particularly cyclic alkylene carbonates such as propylene carbonates and ethylene carbonates, are known hydraulic cement set accelerating agents. Such agents decrease the time necessary to harden a cementitious composition comprised of water, hydraulic cement, and other optional components such as aggregate. The use of carbonic acid esters for such purpose is described in detail in JP 58-135166 (Chemical Abstracts, Vol. 100, No. 2, 1983, Abstract No. 11845p), JP 61-247648 (Chemical Abstracts, Vol. 106, No. 22, 1987, Abstract No. 181621d), and U.S. Pat. No. 5,453,123. The last named patent additionally teaches that carbonic acid esters function as thixotropic additives in cementitious compositions. That is, carbonic acid esters have a thixotropic effect if incorporated into a mixture of hydraulic cement and water.
Many carbonic acid esters, however, are liquids or low melting solids which hydrolyze rapidly when placed in contact with water in a highly acidic or basic environment. In many end-use applications such as grout or concrete repair, cement is sold in dry, bagged form and simply mixed with water immediately before use at the construction or repair site. Adding the carbonic acid ester set-accelerating agent together with the water at the time of mixing is technically possible, but not favored because of the problems associated with securing and maintaining a separate supply of the carbonic acid ester and adding the correct dosage to the cementitious composition. Most end-users of bagged cement products are not currently prepared to handle liquid or low-melting organic admixtures. Premixing the carbonic acid ester with the dry cement may also be problematic due to the fact that the cement, which is highly basic in character, may tend to catalyze the premature decomposition of the carbonic acid ester during the time the bagged cement pre-mix is stored (particularly if the pre-mix is exposed to moisture). An alternative method of handling the carbonic acid ester would be in the form of concentrated aqueous solutions or dispersions, which could then be combined with the dry cement and water at the construction site. However, many carbonic acid esters such as propylene carbonate have limited solubility in water. Maintaining stable dispersions of the carbonic acid ester during storage may also be difficult.
SUMMARY OF THE INVENTION
The present invention provides a dry, flowable cement admixture comprised of an ester of carbonic add and a solid particulate support which is non-reactive towards said ester. In a preferred embodiment, the cement admixture is comprised of silica fume and from 1 to 100 parts by weight of a cyclic alkylene carbonate per 100 parts by weight silica fume.
The invention also furnishes a cementitious composition comprised of (a) hydraulic cement (e.g., Portland cement) and (b) the aforedescribed dry, flowable cement admixture. The cementitious composition may additionally be comprised of aggregate (e.g., sand, gravel, crushed stone). When ready to be used, the cementitious composition is mixed with water and applied using conventional techniques. The viscosity of a mixture comprised of hydraulic cement and water is synergistically increased by adding both silica fume and a carbonic acid ester.
A method of accelerating the set of a cementitious composition is also provided by the present invention wherein the aforedescribed dry, flowable cement admixture is admixed with hydraulic cement, water, and (optionally) aggregate to form the cementitious composition, the amount of the cement admixture being effective to reduce the cementitious composition set time obtained in the absence of the cement admixture.
Surprisingly, adsorbing the carbonic acid ester onto a solid particulate support prior to mixing with hydraulic cement and water does not interfere with the ability of the carbonic acid ester to effectively accelerate the curing of the resulting cementitious composition. That is, the carbonic acid ester appears to be readily released from the support when the admixture is mixed with water and cement such that the setting characteristics of the cementitious composition are very similar to those obtained if the ester is added separately from the support. Moreover, the use of such supported admixtures permits stable, set accelerated cement formulations to be packaged and sold in dry, bagged form which need only be mixed with water prior to use at the construction or building site. In addition, certain solid supports, particularly silica fume, have unexpectedly been found to synergistically enhance the thickening characteristics of carbonic acid esters without interfering with the desirable thixotropic effects of such additives.
DETAILED DESCRIPTION OF THE INVENTION
Preferably, the ester of carbonic acid which is contained in the inventive admixture is at least partially soluble in water. The ester may be either a diester of carbonic acid, a monoester of carbonic acid, or a salt of a monoester of carbonic acid. This invention is particularly well-suited for esters or mixtures of esters which are liquids at room temperature or which melt only slightly above room temperature (e.g. having a melting point of 40° C. or less). It has been found that surprisingly high levels of such substances may be incorporated into the cement admixture while maintaining the flowable (e.g., non-clumping) properties of the particulate support.
Preferred esters of carbonic acid are unsubstituted or substituted aliphatic esters, which may have either an open chain or a cyclic structure. Specially preferred esters include cyclic aliphatic diesters such as alkylene carbonates. Specific examples of such preferred cyclic aliphatic diesters include propylene carbonate (4-methyl1,3-dioxolan-2-one), ethylene carbonate (1,3dioxolan-2-one) and mixtures thereof.
The support selected for use in combination with the aforedescribed carbonic acid ester is a solid substance which is in the form of discrete particles. While the precise size, shape or surface area of such particles is not critical, it is preferred to employ a support having a relatively high surface area in order to increase the maximum amount of the carbonic acid ester that can be loaded onto the support. It may also be desirable for the physical properties (e.g., particle size and shape, density) of the support be similar to that of the cement particles with which the cement admixture will eventually be combined.
The support must be non-reactive with the carbonic acid ester during normal storage conditions. That is, the support should not catalyze or otherwise cause decomposition or other reaction of the ester. For this reason, the surface of the support should be essentially neutral, i.e., neither highly acidic nor highly basic. The support should also not bind the carbonic acid ester irreversibly; when the cement admixture is combined with cement and water, the carbonic acid ester must be able to impart the desirable set acceleration and thixotropic effects to the resulting cementitious composition. The support should additionally be selected from among those materials which will not adversely affect any of the desired properties of the cementitious composition such as compressive strength, slump, air entrainment, shrinkage, or the like.
Silica fume is a particularly, desirable substance to employ as at least a portion of the solid, particulate support. An amorphous by-product commercially obtained by conventional silicon and ferrosilicon metal alloy productio
Liotta, Jr. Frank J.
Schwartz Steven A.
Arco Chemical Technolgy, L.P.
Long William C.
Marcantoni Paul
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