OA13182A - Process for preparing anhydrite III stabilized from plaster and hydraulic binder obtained. - Google Patents

Abstract

The invention relates to a method of preparing stabilised calcium sulphate anhydrite III. The inventive method comprises the following successive steps consisting in: a) heating a powdered starting material to a temperature of between 220 DEG C and 320 < DEG >C, so as to form soluble anhydrite III; and b) subjecting the material thus transformed to a thermal quenching process, so as to lower the temperature thereof by at least 150 < DEG >C in order to obtain a temperature that is at least less than 100 < DEG >C and, preferably, less than 80 < DEG >C, in less than 2 minutes and, preferably, in less than 20 seconds, such as to obtain the aforementioned stabilised anhydrite III. The invention is characterised in that the powdered starting material is ground to produce a particle size of less than 200 ñm and, preferably, of less than 150 ñm and mainly comprises calcium sulphate hemihydrate.

Description

"PROCESS FOR THE PREPARATION OF ANHYDRITE III STABILIZED FROM PLASTER AND HYDRAULIC BINDER OBTAINED"

The present invention relates to a novel process for the preparation of calcium sulfate stabilized anhydrite III by heat treatment of a pulverulent material and a novel hydraulic binder obtained.

In the monoclinic crystal structure of calcium sulfate dihydrate, the water molecules are bonded to two oxygen atoms of two different SO4 ion tetrahedra and one calcium atom. At the molecular scale, it has a structure in sheets in parallel layers, structure in which alternates a layer of water and two layers of CaSO4. The water molecules represent a weight proportion of 26.2% of the crystalline structure. The gypsum also contains surface water molecules (hereinafter referred to as "liquid water"), the amount of which depends on the moisture content of the gypsum concerned.

Traditional plaster is obtained by partial dehydration of the gypsum (CaSO4, 2H2O) in hemihydrate or bassanite (CaSO4, VêHjO). Its use as a coating requires rehydration of the hemihydrate powder.

The current commercial plaster is β-plaster obtained by baking gypsum at a temperature below 200 ° C, generally from 110 to 140 ° C, at atmospheric pressure, and then grinding. They consist of ground β hemihydrate of formula CaSO4.0.5H2O which represents a weight proportion of 5.5% of water molecules in the crystalline structure (hereinafter called "solid water"). In commercially available β plasters, β-hemihydrate may be accompanied by soluble anhydrite of formula CaSO 4, eH 2 O and anhydrite with CaSO 4 but in a small proportion, especially less than 20%.

It is known to prepare improved plasters (sometimes referred to as "plasters"). These hemihydrates are obtained by cooking at pressures greater than atmospheric pressure. They have higher mechanical characteristics than those of ordinary plasters. The phenomena which take place during the treatments are poorly known, and the improvement of the mechanical performances is generally attributed to the presence of the crystallographic variety in the products obtained, without the exact proportion of this variety being known in these products. nor the conditions which make it possible to obtain it in a stable and reproducible way.

Further dehydration of the gypsum leads to the transformation of the hemihydrate into less and less hydrated phases such as anhydrite III, also known as soluble anhydrite, and then anhydrite II or β still called insoluble anhydrite. The anhydrite III can be represented by the formula (CaSO4, eH2O) with ε = 0.06 to 0.11 according to the most recent and most commonly accepted data in the technical literature, it being understood that the number of moles of water could be controversial in the technical literature. Anhydrite III is an extremely hygroscopic metastable phase and therefore has a very high rehydration kinetics. It rehydrates spontaneously in hemihydrate depending on the hygrometry of the air. Thus, 100% anhydrite III is never obtained, it being always in combination with compounds comprising among them mainly bassanite (hemihydrate).

Gypsum or calcium sulfate dihydrate heat treatment methods are known that make it possible to obtain stabilized anhydrite III in order to limit its spontaneous rehydration, which essentially comprises the following two stages: a) Firstly, a drying step then dehydration of the gypsum by increasing the temperature to form the anhydrite III. This dehydration must allow drying the surface moisture of the gypsum, and eliminate the two molecules of water of crystallization. b) Then, a thermal quenching in order to stabilize this metastable phase.

The stabilization of anhydrite III blocks the evolution of anhydrite III to other crystallographic forms and prevents its spontaneous rehydration. Stabilized anhydrite III has very interesting properties, such as very high mechanical strength and low thermal and acoustic conductivity.

The stabilized anhydrite III content is a function of the heat treatment process. Thus, it is known that the heating temperature, the cooking time and the particle size of the starting gypsum are decisive.

It is known in WO96 33957, a heat treatment process of pulverulent material based on calcium sulfate dihydrate in which a firing step is carried out which raises the temperature of the treated gypsum to a temperature of between 220 and 360 ° C., according to characteristics of the treated gypsum and a thermal quenching operation implemented so as to bring the heated material by cooking to a temperature below 100 ° C in a time between 6 and 12 minutes. This cooling is achieved by means of cold air and dry injected under pressure in the heart of the material. Nevertheless, this process does not make it possible to obtain a very high proportion of stabilized anhydrite III.

This process does not make it possible to convert more than 65% of the calcium sulphate dihydrate contained in the starting material into stabilized anhydrite III.

In FR 2 767 816 there is described a process for converting more than 70% of the calcium sulfate hydrate into a stable anhydrite III in which a faster thermal quenching is carried out by lowering the temperature to less than 80.degree. preferably between 40 and 50 ° C, in less than two minutes to stabilize the anhydrite III in greater proportion. This process makes it possible to obtain a product containing more than 70% or more than 90% stabilized anhydrite III relative to the total weight of the compounds resulting from the transformation of the calcium sulfate hydrate contained in the product of departure. This product also has a higher specific surface area (about 7 mm 2 / g) and improved strength properties compared to the lower-content compositions described in the previous patent WO 96 33957. Thus, the maximum yield obtained from Natural gypsum (Vieujot) is greater than 90% for experimental cooking conditions at 340 ° C for forty minutes with a particle size of 315-400 microns or only 300 ° C with a particle size of 80-100 microns.

In practice, in these processes for the preparation of stable anhydrite III, a pulverulent material based on calcium sulfate hydrate (CaSO 4, 2H 2 O), such as natural gypsum or gypsum, is used as starting material. synthesis, including sulfogypsum, phosphogypsum, borogypsum or titanogypsum.

However, these gypsum must be ground so that their particle size does not exceed 120 microns, preferably 100 microns.

Patent FR 2 804 423 describes an industrial plant and a process for obtaining stabilized anhydrite III, produced under industrial conditions, comprising at least 85%, preferably at least 90%, more preferably at least 95% by weight stabilized anhydrite III relative to the total weight of the compounds derived from the transformation of the calcium sulfate hydrate contained in the starting material.

In FR 2 804 423, the inventor has highlighted the problem posed by prior industrial installations, which is that during the cooling phase the material can be kept in contact with a too large proportion of moist air which can cause spontaneous rehydration of anhydrite III before stabilization.

The method described in FR 2 804 423 makes it possible to reduce the presence of moist air during the cooling or thermal quenching phase, so that the anhydrite III does not turn into a hemihydrate during its stabilization and also reduces the contact of the anhydrite III formed with the moisture in the air during the firing step and until the completion of the cooling step.

For this purpose, in FR 2 804 423, a process for the industrial production of stabilized anhydrite III has been described, by heat treatment of a pulverulent material based on calcium sulfate hydrate (CaSO 4, 2H 2 O), such as natural or synthetic gypsum, in an industrial plant which comprises from upstream to downstream: 1) a dehydration reactor by heating, and 2) a cooling reactor, 3) a passage zone between the downstream end of said dehydration reactor and the upstream end of said cooling reactor, which method comprises the successive steps in which: a) said powdery material is heated in said dewatering reactor (1) at a temperature between 220 ° C and 360 ° C so as to convert the calcium sulphate hydrate to soluble anhydrite III or a, and b) subjecting said material thus transformed to a thermal quench in said cooling reactor (2) in such a way that at least 150 ° C to reach a temperature of at least less than 110 ° C, preferably less than 80 ° C, more preferably less than two minutes, so as to obtain stabilized anhydrite III.

During the firing step a), the heated calcium sulfate dihydrate loses (26.2-e)% water of crystallization (or "solid water"), as well as the surface water (or "liquid water ").

According to the process described in FR 2 804 423, the humidity level in the ambient atmosphere in contact with the anhydrite material III obtained in step a) is reduced before and during step b). by aspirating the ambient atmosphere in contact with said transformed material, with suction means located near, preferably just upstream, said passage zone, at the downstream end of said dehydration reactor, and sending a flow of cold and dry gas, preferably air, on said transformed material, counter-to the displacement of said material, in said passage zone between step a) and step b) , and in said dehydration reactor during step b).

Thus, the material dehydrated in step a) is not likely to rehydrate before performing step b).

Advantageously, a flow of cold and dry gas is sent on said material transformed in counter-sense with respect to the displacement of said material during step b), and said flow of dry gas against the direction is evacuated with the humid atmosphere of step a). Thus it is avoided that the moist hot air released in step a) is discharged upstream of the reactor of step a) and is in contact with said material before it is completely heated, which would increase the moisture content of said material and affect the performance of step a). The dry air is injected at a pressure studied to discharge the air under low pressure so as not to hinder the movement of the gypsum to the outlet of the reactor.

This method requires the implementation of an industrial installation) of heat treatment of a pulverulent raw material based on calcium sulfate hydrate to obtain stabilized anhydrite III, comprising from upstream to forward a) a dehydration reactor by heating and, b) a cooling reactor, characterized in that the plant comprises - suction means for the gas contained in said dehydration reactor, located in proximity, preferably just in upstream of said passage zone, at the downstream end of said dehydration reactor, and

I - means for injecting dry gas counter-to the movement of said material, preferably air, so that the passage zone between the downstream end (12) of said dehydration reactor and the upstream end of said reactor cooling circuit is traversed by said flow of dry gas against the direction of displacement of said material, and - means for injecting cold and dry gas against the direction of displacement of said material, located in said cooling reactor, of preferably at the downstream end of said cooling reactor, so that the gas flows through the entire upstream downstream cooling reactor.

More particularly, said means for injecting dry gas are located downstream of said suction means, preferably downstream of said cooling reactor and said injected dry gas is sucked by said suction means located upstream of said passage zone. preferably in said dehydration reactor.

The experiments carried out show that the step of pre-drying the gypsum to reduce the free water content to a level preferably lower than 3%, the natural gypsums having a free water content which varies from 4 to 10% and the gypsum industrial plants with free water contents of 10 to 60%, is very important because this free water content is detrimental to the good start of cooking good dehydration of crystalline molecular water. Nevertheless, it is observed that part of the product is entrained by water vapor during its evaporation on the one hand, and, on the other hand, that gypsum grains stick together and against the walls of the dehydrating reactor causing clogging, both of which adversely affect the yields of the reaction.

The object of the present invention is to provide a process for the preparation of stabilized anhydrite III which is simpler and less expensive to produce, especially in terms of industrial installation.

Another object of the present invention is to provide a process for the preparation of stabilized anhydrite III with a better yield and providing a weight ratio of stabilized anhydrite III, preferably at least 90%, preferably at least 95%, or even at least 98% in the product obtained and of better quality, especially in terms of mechanical property, and such as compressive strengths (Rc) and flexural strength (Rf). The inventor has now surprisingly discovered that it is possible to obtain the industrial production of stabilized anhydrite III in a large proportion and of better quality, in terms of mechanical property, in particular compressive strengths (Rc) and bending (Rf) using, as raw material powder, comminuted β hemihydrate or commercially available plaster β, particle size less than 200 microns, preferably less than 150, or even less than 100 microns, that is to say to say to a commercially available plaster β, and by performing the same successive stages of firing and thermal quenching without, however, requiring a pre-drying step insofar as the plaster β of the trade is itself dry, and pre-grinding stage of step to the extent that it is implemented milled plaster β of suitable particle size.

The subject of the present invention is therefore a process for the preparation of stabilized calcium sulphate anhydrite III of formula, in which the following successive steps are carried out: a) heating of a powdery starting material at a temperature of between 220 ° C. C and 320 ° C, so as to form soluble anhydrite III or a, and b) is subjected to said material thus transformed a thermal quenching, so as to lower its temperature of at least 150 ° C to reach a temperature at least less than 100 ° C., preferably less than 80 ° C., in less than 2 minutes, preferably in less than 20 seconds, so as to obtain said stabilized anhydrite III, 9 process characterized in that said material starting pulverulent powder is milled to a particle size less than 200 μπι and preferably less than 150 pm and mainly comprises calcium sulfate hemihydrate (CaSO4, 0.5H2O).

Preferably, said ground powder starting material consists of β-plaster preferably comprising at least 95% β-calcium sulfate hemihydrate.

More preferably, said starting pulverulent material is a comminuted calcium sulfate hemihydrate compound.

However, it is possible to use, as said ground pulverulent starting material, a material consisting mainly of plaster a, preferably consisting of plaster a comprising at least 95% hemihydrate calcium sulfate.

The process according to the invention is advantageous in several ways. Firstly, it makes it possible to obtain a product comprising a high content of 85 to 95%, by weight, of stabilized anhydrite III, the remainder of the constituents consisting mainly of β-hemihydrate. Secondly, the yield of the reaction is greater because there is more sticking of the pulverulent material on the walls of the cooking reactor and there is less loss of product during the evaporation of water vapor.

In addition, the method according to the invention can be implemented in a simplified thermal installation that does not require the use of moisture evacuation means, such as those used in previous industrial production processes. stabilized anhydrite III described above, and the heat treatment is faster. In total, the process and the simplified thermal installation make it possible to save energy and to invest considerable material.

In a particular embodiment, the two steps of the process according to the invention can be carried out successively in the same reactor, in particular a jacketed reactor, the cooking and cooling then being obtained by circulation of a hot or cold fluid respectively in the double envelope.

The result of the process according to the invention is surprising since it had not been demonstrated, in the previous processes, that the anhydrite III gypsum pass was made by an intermediate step of converting all the gypsum into hemihydrate, and the complex mechanisms of the dehydration of crystalline water did not allow to assume that the change of the crystalline form of the starting pulverulent material would make it possible to obtain stabilized anhydrite III having the same characteristics as those obtained from heat treatment of gypsum.

It must indeed be remembered that the firing temperature and the heating time as determined in the previous patents were based on an experimental design methodology based on a mathematical and statistical approach called "uniform network theory DOHELERT which made it possible to limit and then fix the temperature and the cooking time by a limited number of manipulations from a polynomial written in adimenational coded variables. On the other hand, it should be pointed out that the dehydration of the gypsum is not regular with the increase of the temperature and that the phases of evacuation of the free water of surface and the crystal water of dehydration and the Residual surface water is considered to be concomitant and interacting, whereas ground plaster is a product without "liquid water".

In a preferred embodiment of the process according to the invention, use is made, as starting pulverulent material, of the hot β plaster ground after leaving the β-plaster manufacturing reactor from gypsum, that is to say a β-plaster. at a temperature of 110 to 140 ° C. Of course, the change of starting raw material results in a reduction in the duration of the firing step of about 50% compared to a process involving starting material of natural or synthetic gypsum. In this embodiment, it is therefore possible to provide simplified mixed installations producing both plaster and a stabilized anhydrite III-based sulfatic binder obtained from plaster, which makes it possible to improve the economic operating conditions. and manufacturing said products.

In the process according to the invention, when commercial β-plaster is used, it has a homogenized particle size of 0 to 150 microns, preferably 0 to 100 microns, guaranteed and dry, which also explains why in part the best yield of the process according to the invention compared to a process in which the powdery starting material is gypsum because the gypsum crystals has a particle size and crystallographic forms of greater heterogeneity and, therefore, the hemihydrate formed during cooking is not only in a wetter atmosphere, but also, in a less homogenized particle size, which adversely affects both the reaction yield and the quality of the binder and anhydrite III that it contains obtained.

Advantageously, according to an alternative embodiment, said starting pulverulent material comes from a natural or synthetic gypsum heat treatment, and has an initial temperature, before the firing step a), of 100 to 150 ° C, preferably 110 to 150 ° C, preferably at 140 ° C. This new process also allows plasterers, to achieve stabilized anhydrite III by an inexpensive modification of their current installation and, therefore, provides a new economical industrial process knowing that the addition of gypsum in plaster brings a very clear improvement of the mechanical characteristics.

According to another embodiment, said starting powder material is at a temperature below 100 ° C, preferably at room temperature.

Another advantage of the process according to the present invention is that, in the step of heating the starting pulverulent material mainly containing β calcium sulfate hemihydrate, anhydrite III can be obtained at a temperature of 220 to 320.degree. C much more easily and much more rapidly by heating by direct contact with a hot gas, especially hot air, preferably said starting powder material being transported cocurrently and heated by said hot gas in the form of a fluidized bed .

More particularly, the heating is effected by means of an annular thermal flash device comprising a hot-air drying column in which the hot air is able to transport cocurrently and heat said powdery material along a helical ramp.

The tests carried out by the inventors have made it possible to demonstrate that by carrying out the process under the same conditions of industrial installation, by changing the starting material constituted from pre-dried and ground gypsum, on the one hand, and On the other hand, plaster β of the same particle size, can increase the weight ratio of anhydrate III or stabilized obtained in a significant proportion.

The process according to the invention makes it possible to obtain stabilized anhydrite III produced in an industrial installation and having a degree of purity of at least 90% by weight, preferably at least 95% or even 98% by weight. total of the compounds from the transformation of the calcium sulfate hydrate from the starting material.

Such high levels were not obtained in previously described treatment methods.

More particularly, the stabilized anhydrite III, obtained according to the present invention, is associated with a low calcium sulphate hemihydrate (or bassanite) content, so that the weight ratio of the hemihydrate to the stabilized anhydrite is less than 10/90, preferably less than 9/91, more preferably less than 5/95.

In the case where the starting material comprises at least 95%, preferably at least 98% of calcium sulfate dihydrate, stabilized anhydrite III can be obtained with a degree of purity which represents at least 90%, preferably at least 95% relative to the total weight of the product obtained.

Crushed plaster being a product without "liquid water" and having a greater homogeneity of particle size as well as crystallographic, especially in terms of crystal shape, the industrial or synthetic gypsum, stabilized anhydrite III obtained a better quality in terms of of granulometric and crystallographic homogeneity with crystals of thicker and shorter shape than stabilized anhydrite III crystals obtained from gypsum heat treatment according to the prior art.

As a result, the hydraulic binders according to the invention have improved mechanical properties of compressive strength.

The present invention therefore also relates to a hydraulic binder comprising stabilized anhydrite III obtained by the process according to the invention.

More particularly, a hydraulic binder of the present invention comprises greater than 95% or even 98% of stabilized anhydrite III by weight.

Even more particularly, a hydraulic binder according to the invention confers an increased compressive strength, especially Rc of at least 45 MPa, preferably at least 50 MPa at 28 days in the air according to the tests carried out according to the standard EN- 196.1.

More particularly, the binder according to the invention comprises additives of mineral fillers chosen from slags, fly ash, pozzolan, silica fumes, fine limestone, lime and by-products and industrial waste of mineral or organic nature.

The present invention also relates to materials formulated from a binder according to the invention, preferably in the form of mortar, concrete, plaster or grout, particularly useful for the manufacture or production of panels, bricks, blocks and cappings. . Other features and advantages of the present invention will become more apparent in light of the following detailed embodiments.

Comparative Example 1:

In a furnace of rotary type with double jacket, indirectly heated with the aid of a hot fluid flowing in the jacket, 1000 kg of crushed industrial gypsum said sulpho-gypsum, particle size 0 - 100 μηι, are prepared. dried so as to have a pre-drying moisture content of not more than 3%, containing at least 98% of calcium sulfate dihydrate and 2% of impurities (MgO, chloride, Na2O, SO2). The cooking is carried out at a temperature of 230 ° C., this temperature being reached in 40 minutes.

Thermal quenching is then performed by lowering the temperature to 80 ° C. in 4 seconds. This cooling quench is performed in a reactor located in a double-jacketed rotating cylinder cooled with cold water at less than 20 ° C.

700 kg of hydraulic binder containing 70% to 80% of stabilized anhydrite III and 20 to 30% of other constituents, mainly comprising hemihydrate (approximately 15 to 20%), as well as products such as that MgO, chloride, Na2O and SO2, etc .... I During the process, there is a significant loss of product and therefore processing efficiency because, on the one hand, the bonding of gypsum grains between them and on the surface of the rotary furnace causing clogging and, secondly, that the product is entrained with the water vapor removed. In addition, reaction conditions in a very humid atmosphere interfere with the stabilization of the anhydrite III and promote its conversion into hemihydrate. *

Example 2 according to the invention

In the same oven as in Example 1, the same quantity of 1000 kg of commercially available dry plaster, in particular of the Lafarge Prestia Selecta® type, having a particle size of 0 to 100 μm, is cooked in the same oven as the product of the Example 1. Said dry β plaster used contains at least 98% calcium sulfate hemihydrate and 2% residual compounds.

The same baking temperature of 230 ° C is obtained in 10 minutes.

Thermal quenching is carried out subsequently under the same conditions as in Example 1 and 920 kg of a hydraulic binder containing 96% of stabilized anhydrite III are obtained.

No phenomenon of grain bonding between them or on the walls of the cooking reactor is observed and the absence of an over-saturated humidity atmosphere makes it possible to increase the yield of binder and stabilized anhydrite III obtained.

The method, according to the invention, for obtaining anhydrite III stabilized by dehydration from the plaster and therefore of a product ground to a homogeneous particle size, provides an improvement in the quality of the anhydrite III in terms of homogeneity of the crystallographic form and the weight content of stabilized anhydrite III (from 95 to 98% instead of from 70 to 80% for the product of Example I). On the other hand, considerable energy saving is achieved with a cooking time ranging from 40 minutes to 10 minutes.

The heat treatment according to the invention can be carried out with existing plaster manufacturing installations, thus considerably limiting material investments.

An annular heat flashing device consisting of a hot air drying column, explained below, is cooked in the same quantity and quality of plaster β as in Example 2 at the same baking temperature of 230.degree. ° C.

This heating step is performed in a few seconds, in particular less than 5 seconds.

The drying column comprises a helical ramp capable of receiving at its base said pulverulent material, which may be transported cocurrently by hot gas also injected at the base of the ramp of the column, the said pulverulent material being transported to at the top of the column, at the top of said helical ramp.

The thermal flash heating device functions more precisely as follows: the product to be dried is introduced at the base of a drying column in which it is transported at a speed of more than 2 m / s by the heated air pulsed from a hot gas generator circulating cocurrently along a helical path within said column, so that the heating of the pulverulent material in the form of a fluidized bed is by direct contact of the product with the With hot air, the flow rate and the length of the column are such that, with hot air sent at a temperature of 280 to 320 ° C., the pulverulent material reaches a temperature of 230 ° C. at the top of the column. a secondary heating air coming from this same heat source is introduced punctually at different levels over the entire column, which makes it possible to: • dry the product according to its particle size by modifying the angle of the helical trajectory of the particle to dry and therefore its residence time, • maintain the energy potential throughout the column by heat input. - The heavy elements are recovered at the base of the column and possibly evacuated by a hatch. the product is discharged at the top of the column where it has reached the temperature of 230 ° C. and separated from the transport and drying air in a cyclone and / or a filter. - A fan allows the evacuation of this air to the atmosphere. - the recovered product is itself evacuated at the base of the cyclone through a lock.

Thermal quenching is carried out subsequently in the same conditions as in Examples 1 and 2 and the same quantity and quality of hydraulic binder containing stabilized anhydrite III as in Example 2 is obtained.

Thermal quenching can also be performed with a cold air cooling device in an installation in which said powdery material is in the form of a fluidized bed. The advantage of this annular thermal flash heating system is: - the high thermal efficiency; - the low thermal load; ) - the small size of the installation; and - the speed of drying.

Example 4: crystallographic morphology

The hydraulic binders obtained in Examples 2 and 3 according to the invention were observed by electron microscopy and microphotographs.

The stabilized anhydrite III crystals are in the form of short, thick rods of more homogeneous sizes than the stabilized anhydrite III crystals obtained from gypsum according to Example 1, the photomicrographs of which show Needles thinner and longer than the short and thick rods of the binders of Examples 2 and 3.

Example 5: Resistance to Compression: Comparative tests of compressive strength also made it possible to demonstrate the advantageous mechanical properties of a hydraulic binder obtained by the process according to the invention.

A binder according to Example 1, hereinafter called "Gypement" and a binder according to Examples 2 and 3 hereinafter called "Hydrocim" were tested.

The following data report the values of Rc = compressive strength in MPa (mega Pascal) measured according to EN-196.1 "test method of cements - part I: determination of mechanical strengths".

Binding Gypement: Rc at 28 days in the air = 40 MPa

Binder Hydrocim: Rc at 28 feet in the air = 47 MPa. - The setting time of the Hydrocim binder is slightly lower than that of the Gypement, ie about 3-4 minutes for the Hydrocim instead of 4-5 minutes for the Gypement.

Tests were also carried out on mortars comprising 35% binder + 65% of EN 196.1 standardized sand and prepared under the following conditions: - dry mixing for 30 seconds, - introduction of water for 30 seconds while maintaining the mixing at low speed, - the water / binder ratio is set at 0.7, - mixing at low speed for 90 seconds, - mortar with binder Gypcement: Rc at 28 days in air = 35 MPa, - mortar with Hydrocim: Rc at 28 days in the air = 45 MPa, the setting times of a mortar as described above with a Hydrocim binder are of the order of 10 to 15 minutes, instead of 12 to 17 minutes with a binder Gypement.

The flexural strengths (Rf) measured according to the same standard are of the same order with the Hydrocim binder as with the Gypcement binder but are higher than those obtained with the cements, ie generally of the order of Rf = 1.5 x rc.

These high mechanical properties of flexural strength give the binder Hydrocim mortars and properties including seismic properties.

Claims (17)

A process for the preparation of stabilized anhydrite III of calcium sulphate in which the following successive steps are carried out: a) heating a pulverulent starting material at a temperature of between 220 ° C. and 320 ° C., forming soluble anhydrite III, and b) subjecting said material so transformed to thermal quenching so as to lower its temperature by at least 150 ° C to reach a temperature of at least less than 100 ° C, preferably less than 80 ° C, in less than 2 minutes, preferably in less than 20 seconds, so as to obtain said stabilized anhydrite III, characterized in that said starting pulverulent material is milled to a particle size of less than 200 μm and preferably less than 150 .mu.m and predominantly comprises calcium sulfate hemihydrate. 2. Method according to claim 1, characterized in that said ground starting pulverulent material is constituted by β-plaster preferably comprising at least 95% β-calcium sulfate hemihydrate. 3. Method according to claim 1, characterized in that said ground raw powder material consists of a hemihydrate compound β calcium sulfate. 4. Method according to one of the preceding claims, characterized in that the particle size of the starting pulverulent material is homogenized in a range of values from 0 to 100 pm. 5. Method according to one of claims 1 to 4, characterized in that said starting pulverulent material comes from a natural or synthetic gypsum heat treatment, and has an initial temperature, before step a) cooking, 100 to 150 ° C, preferably 110 to 140 ° C. 6. Method according to one of claims 1 to 4, characterized in that said starting powder material is at a temperature below 100 ° C, preferably at room temperature. 7. Method according to one of claims 1 to 6, characterized in that one carries out the heating step a by direct contact with a hot gas, in particular hot air, preferably said powder starting material being transported cocurrently and heated by said hot gas in the form of a fluidized bed. 8. Method according to claim 7, characterized in that the heating step a) is carried out using an annular thermal flash device comprising a hot air drying column in which the hot air is able to transport cocurrently and heat said powdery material along a helical ramp. 9. Method according to one of claims 1 to 6, characterized in that the two steps a) and b) are performed in the same reactor, preferably double jacket. 10. Method according to one of the preceding claims, characterized in that at the end of step b), one obtains said stabilized anhydrite III with a degree of purity of at least 90% by weight, relative to total weight of the compounds from the transformation of the starting pulverulent material. 11. The method of claim 10, characterized in that said stabilized anhydrite III obtained has a degree of purity of at least 95%, h preferably 98%, by weight relative to the total weight of the components from the transformation of the powdery starting material. 12. A method according to claim 10 or 11, characterized in that said stabilized anhydrite III obtained is combined in admixture with little or no hemihydrate calcium sulfate β so that the weight ratio of said hemihydrate to stabilized anhydrite III, is less than 9/91, more preferably less than 5/95. 13. Hydraulic binder comprising stabilized anhydrite III obtained by the process according to one of claims 1 to 12. 14. Hydraulic binder according to claim 13, characterized in that it comprises more than 95% or even 98% stabilized anhydrite III, by weight. 15. hydraulic binder according to claim 13 or 14, characterized in that it has a compressive strength (Rc) of at least 45 MPa at 28 days in the air. 16. hydraulic binder according to one of claims 13 to 15, characterized in that it further comprises mineral filler additives selected from slag, fly ash, pozzolan, silica fumes, fine limestone and lime and sub- industrial products and wastes of mineral or organic nature. 17. Material formulated from a binder according to one of claims 13 to 16, preferably in the form of concrete, mortar, coating or grout.