Compositions: coating or plastic – Coating or plastic compositions – Inorganic settable ingredient containing
Reexamination Certificate
2000-06-13
2004-05-04
Wood, Elizabeth D. (Department: 1755)
Compositions: coating or plastic
Coating or plastic compositions
Inorganic settable ingredient containing
C106S695000, C106S775000
Reexamination Certificate
active
06730162
ABSTRACT:
The present invention relates to a new hydraulic binder resulting from mixing a binder of the cement or clinker type with a sulfate binder of the plaster or improved plaster type as obtained by applying heat treatment to gypsum, including Gypcement®.
BACKGROUND OF THE INVENTION
Traditional plaster is obtained by partially dehydrating gypsum (CaSO
4
, 2H
2
O) into a hemihydrate or bassanite (CaSO
4
, ½H
2
O) of &bgr; crystal form. In order to be used as a coating the hemihydrate powder must be rehydrated.
Further dehydration of gypsum leads to the hemihydrate being transformed into artificial anhydrites, specifically III or &agr; anhydrite also known as soluble anhydrite, and then II or &bgr; anhydrite also known as insoluble or dead-burned anhydrite. III or &agr; anhydrite can be represented by the formula (CaSO
4
, &egr;H
2
O) where &egr; lies in the range 0 to 0.5, and in particular the range 0.06 to 0.11. III or &agr; anhydrite is a metastable phase that is extremely hygroscopic and thus rehydrates very rapidly. It rehydrates spontaneously into hemihydrate as a function of the humidity of the air. 100% III or &agr; anhydrite is thus never obtained, since it is always associated with bassanite (hemihydrate).
&bgr; plasters are obtained by firing gypsum at a temperature below 200° C. at atmospheric pressure. They are constituted by &bgr;-CaSO
4
, ½H
2
O hemihydrates which, in ordinary plasters, can be accompanied by soluble anhydrite CaSO
4
, &egr;H
2
O and dead-burned CaSO
4
in very small quantities.
Improved plasters (sometimes known as “&agr; plasters”) can be prepared which, once they have set, present mechanical characteristics that are better than those of ordinary plasters. The phenomena which occur during treatments are poorly understood and the improvement in mechanical performance is generally attributed to the presence of the &agr; variety of crystal in the resulting products, without it being known exactly what proportion said variety occupies in such products nor what conditions enable it to be obtained in stable and reproducible manner.
Traditionally, such &agr; plasters are made from gypsum by subjecting it to a firing stage at a temperature below 200° C. using a wet method in an autoclave under a pressure of about 5 bars to 10 bars for a duration of about 10 hours, and then subjecting it to a hot drying stage using a flow of hot dry air.
Other methods have been proposed in attempts to mitigate the defects of that traditional method of making improved plaster (extremely expensive to implement, poor reproducibility).
Heat treatment methods are known that serve to obtain stabilized III or &agr; anhydrite so as to limit its spontaneous rehydration, which methods essentially comprise the following two steps:
a) Firstly a step of drying and dehydrating the gypsum by increasing its temperature to form III or a anhydrite. This dehydration should enable the surface moisture of the gypsum to be dried off and should eliminate the two molecules of crystallization water.
b) Subsequently, a step of temperature quenching to stabilize the metastable phase.
Stabilized III anhydrite makes it possible to obtain materials having properties that are most advantageous, such as high mechanical strength, in particular high early mechanical strength, and properties of thermal and acoustic insulation that can be better than those of plaster or cement. Furthermore, III anhydrite can be obtained at costs that are lower than the costs of obtaining plaster or cement.
The stabilized III or &agr; anhydrite content is a function of the heat treatment method: temperature, firing time, and initial gypsum grain size are the key factors.
WO 96/33957 discloses a method of applying heat treatment to a powder material based on dihydrated calcium sulfate in which a firing step is performed to raise the temperature of the treated gypsum to a temperature lying in the range 220° C. to 360° C., depending on the characteristics of the gypsum being treated, and an operation of temperature quenching implemented so as to bring the material heated by the firing to a temperature below 100° C. in a period of time lying in the range 6 minutes (min) to 12 min. This cooling is performed by means of cold dry air injected under pressure into the core of the material. The method enables the dihydrate to be transformed into as much as 70% stabilized III or &agr; anhydrite.
In French patent application FR 2 767 816, a method is described that enables more than 70% of hydrated calcium sulfate to be transformed into stable III or a anhydrite in which the temperature quenching is performed more quickly by lowering the temperature to below 80° C., and preferably to a temperature in the range 40° C. to 50° C. in less than 2 minutes in order to stabilize a larger fraction of the III or &agr; anhydrite. That method makes it possible to obtain a product containing more than 70% or even more than 90% stabilized III or &agr; anhydrite relative to the total weight of the compounds obtained after transforming the hydrated calcium sulfate contained in the starting material.
In French patent application FR 00/01335, the Applicant describes a method which enables stabilized III or &agr; anhydrite to be supplied on an industrial scale with purity of at least 85%, but capable of reaching 90% or even 95% or more relative to the total weight of compounds derived from transforming the hydrated calcium sulfate in the starting material.
That method of industrially producing stabilized III or &agr; anhydrite by applying heat treatment to a powder material based on hydrated calcium sulfate (CaSO
4
, 2H
2
O), preferably natural or synthetic gypsum, comprises successive steps in which:
a) said powder material is heated to a temperature lying in the range 220° C. to 360° C. so as to transform the hydrated calcium sulfate into soluble III or &agr; anhydrite; and
b) said material transformed in that way is subjected to temperature quenching so as to lower its temperature by at least 150° C. so as to reach a temperature that at worst is below 110° C., and preferably is below 80° C., and to do so preferably in less than 2 minutes, so as to stabilize the III or &agr; anhydrite.
During firing step a), the calcium sulfate dihydrate loses surface water and 26.2% crystallization water. In that method, ambient moisture in contact with the material and transformed into III or &agr; anhydrite in step a) is removed, in particular the moisture given off by said material heated in step a), with this removal taking place before and during cooling step b). More particularly, in said method, the moisture is removed by sucking out the moist ambient atmosphere in contact with said transformed material. In an advantageous implementation, steps a) and b) are performed in different reactors and the moist atmosphere of step a) is removed by suction means situated upstream from the reactor for step b). Advantageously, a flow of dry gas is applied to said material transformed in step a) as a counterflow relative to the travel of said material between step a) and step b), said gas being preferably dry air. Thus, the material dehydrated in step a) does not run the risk of rehydrating before step b) has been performed. Also advantageously, a flow of cold and dry gas is applied to said transformed material as a counterflow relative to the travel of said material while step b) is taking place. Preferably, said counterflow of dry gas is removed together with the moist atmosphere of step a). This prevents the hot moist air given off in step a) being returned upstream from the reactor for step a) where it would come into contact with said material before it reaches the required temperature, which would have the effect of increasing the moisture content of said material and decreasing the efficiency of step n).
The dry air is injected at a pressure that is determined so that the travel of the gypsum towards the outlet from the reactor is not impeded by the air.
In a preferred implementation of the method, prior to step a), the surface moisture of the gypsum is red
Couturier Jean
Hornain Hugues
Li Guanshu
Cohen & Pontani, Lieberman & Pavane
Wood Elizabeth D.
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