BG65176B1 - Method for the production of hydraulic bonding agent based on anhydrite iii or alpha anhydrite of gypsum and the produced hydraulic bonding agent - Google Patents

Abstract

The method is applied in particular for synthetizing hydraulic bonding agent based on natural calcium sulphate (gypsum). The calcium sulfate is heated to obtain a high proportion of anhydrite III or alpha and thermal dipping occurs. Said method consists in the following: the calcium sulfate is completely dehydrated and the matter thus dehydrated is cooled in a brutal manner. Preferably, the present invention can be used in the construction industry in order to obtain a hydraulic bonding agent that can be used as cement.

Description

The invention relates to a method for treating powdered starting material based on natural calcium sulfate (gypsum) or synthetic calcium sulfate (sulfogypsum, phosphogypsum and other by-products of the same kind) in order to obtain a new hydraulic binder which may be used as cement based on very high content of anhydrite III or alpha-anhydrite (alpha ASTM classification).

The invention also relates to a binder of said method, which can be used as cement.

BACKGROUND OF THE INVENTION

Gypsum has different crystalline forms; on a molecular scale, it has a layered structure in which a layer of water is alternated with two layers of CaSO ₄ .

Gypsum has been used for thousands of years to produce plaster, one of the oldest building materials known since the 6th century BC.

Gypsum is currently used:

- for the production of cement (as a curing regulator);

- for agricultural use;

- in various industries (chemical, paper, etc.).

Much more important is its use in the production of cement and plaster, through dehydrating gypsum.

While in the cement industry gypsum is introduced at clinker formation and heated at a high temperature of the order of 1400 ° C when plaster is produced, the essential principle is to remove water wholly or partially from gypsum - a complex operation that involves the crystallization phenomenon, which is difficult to control.

Several types of calcium sulfate treatment methods are available for plaster preparation. In particular, an improved plaster (sometimes referred to as an alpha plaster) can be prepared which, once solidified, has mechanical characteristics that are far superior to those of a conventional plaster. The phenomena that occur during such treatments are poorly understood and, above all, the improvement in mechanical properties is explained by the presence of anhydrite III or alpha anhydride in the products obtained, without carefully examining the amount of them or the conditions under which they can be produced. in a reproducible way: it exists only in quantitative traces.

Traditionally, improved plaster is produced from gypsum by humid heating in an autoclave, followed by a hot-drying step carried out in a stream of warm, dry air. The heating is carried out in an atmosphere of saturated water vapor at a pressure in the range of 5 to 10 bars for a period of about 10 hours.

Possibly to overcome the disadvantages of this traditional method of producing improved plaster (a very expensive method with impossible reproducibility), other methods propose to reproduce essentially the conditions of the traditional methods (wet-heat treatment followed by drying with hot air), such as different means and technologies are used (FR-A-2 389 855, FR-A-2 445 940, FR-A-2 572 721, US 2 269 580, US-A-3 145 980).

SUMMARY OF THE INVENTION

The method according to the invention is based on the following observation: when calcium sulphate is treated normally to produce an "improved plaster", the product obtained is actually a mixture of anhydrous forms (gamma anhydrite) or hydrated forms (hemihydrates, dihydrates, ... ). The inventors' research shows that this state of things comes from two factors. First, the heating of the alpha-anhydrite products and other forms, and second, the replacement of the product after heating, with partial transformation, in a special re-hydration. In essence, the idea pursued in the process of the invention is to obtain a stable final product containing a portion of the weight of alpha-anhydrite that is much larger than that contained in the known similar plaster; to this end, the structure of the resulting mixture after heating is preserved by cooling. This significantly blocks the subsequent transformation of alpha-anhydrite formed by heat treatment.

PCT / FR1996 / 000622 discloses that the cooling operation is preferably carried out in such a way that the heated material is at a temperature lower than 100 ° C for a period of 6 to 12 minutes. This can be achieved by using cold, dry, compressed air injected at multiple locations in the moving material, adjusting the air flow rate to obtain an adequate cooling rate.

However, this cooling is not effective enough to produce such an amount of anhydrite III or alpha-anhydrite that is really important. According to the prior art, a very high percentage of anhydrite III or alpha-anhydrite, namely 90%, cannot be produced, therefore, no hydraulic binder can be produced that can be used as cement.

The method according to the invention can produce such a product.

It is an object of the invention to refine the process for producing a large amount of stable, soluble anhydrite III or alpha-anhydrite.

The process according to the invention can transform more than 90% of pure calcium sulfate into anhydrite III or alpha-anhydrite.

To this end, the present invention provides a process for the preparation of a hydraulic binder based on natural calcium sulfate (gypsum) or synthetic calcium sulfate (sulfogypsum, phosphogypsum, titanogypsum, etc.), including heating of calcium sulfate:

- a hydraulic binder that can be used as cement based on anhydrite III or alpha-anhydrite, comprising, according to the invention, more than 70% stable, soluble anhydrite III or alpha-anhydrite and which consists of:

- a heating step which is carried out at the temperature of the treated gypsum from room temperature to 220 ° C to 350 ° C, depending on the characteristics of the treated gypsum;

- a step of abruptly cooling the resulting product, reducing its temperature from 220 ° C to 350 ° C to less than 80 ° C in less than two minutes in order to stabilize alpha-anhydrite by crystallographic blocking and fixation.

In one embodiment of the method, the heating temperature is 300 ° C - 310 ° C.

In a preferred embodiment, the temperature of the sharp cooling is in the range of 40 ° C to 50 ° C.

In a preferred embodiment, the temperature rises over a period of 10 to 40 minutes depending on the type and size of the gypsum grains.

Before treatment, the treated gypsum comprises from 0 to 20% water, and the size of its grains in the range from 0 to 30 mm.

In a preferred embodiment, the treated gypsum comprises from 5 to 15% water, and the size of its grains in the range from 0 to 10 mm.

A hydraulic binder that can be used as cement based on anhydrite III or alpha-anhydrite obtained by the method described is characterized in that it contains more than 70% stable and soluble anhydrite III or alpha-anhydrite.

A hydraulic binder that can be used as cement based on anhydrite III or alpha-anhydrite obtained by the above method is characterized in that it contains more than 90% stable and soluble anhydrite III or alpha-anhydrite.

A hydraulic binder that can be used as cement based on anhydrite III or alpha-anhydrite obtained by the method according to the invention, characterized in that the mechanical strength is:

MPa at 24 h;

MPa at 8 days;

more than 40 MPa per 14 days.

The present invention also includes a hydraulic binder that can be used as cement obtained by the method described.

It is an essential inventive concept of the invention to increase the amount of alpha-anhydrite in the product as a means of limiting product change after heating by rapid cooling. To further increase this portion of alpha-anhydrite, the inventor also applies optimization of the heating operation to obtain as much of this variation as possible after heating.

When heated, gypsum produces batches of hydrated or anhydrous products.

At about 100 ° C, alpha- or beta-hemihydrates are obtained (depending on whether pressurized steam or free air is used) as determined in the reaction:

CaSO ₄ , 2H ₂ O - CaSO ₄ , 1/2 Н ₂ О + 3/2 Н ₂ О monoclinic rhombohedral

At about 300 ° C, anhydrite III or a very soluble but permanently unstable anhydrite is obtained, which is immediately rehydrated to hemihydrate in contact with water vapor:

CaSO ₄ , 1 / 2CO — CaSO ₄ ffl (or alpha) +1/2 CI rhombohedral hexagonal

At about 300 ° C for alpha-hemihydrate and 350 ° C for beta, anhydrite III (or alpha-anhydrite) becomes stable anhydrite II (overheating):

CaSO ₄ III (or alpha) - CaSO ₄ II hexagonal orthorhombic Anhydrite III is slowly rehydrated to contact with water.

At about 1230 ° C, a new transformation reaction takes place:

CaSO II - CaS0 ₄ 1 orthorhombic wall-centered cubic

Only anhydrite I CaSO _{4 is} difficult to rehydrate.

After 1250 ° C, anhydrite I decomposes: CaSO ₄ 1 - CaO + SO ₂ + 1/2 O ₂

Conventional industrial plaster applications only use:

- alpha-hemihydrate;

- beta-hemihydrate; and

- anhydrite II (insoluble or excessively hot).

Anhydrite III or the alpha-anhydrite of the present invention cannot be used because of its high instability.

The change in the properties of the plaster in structures and the numerous disadvantages in terms of strength, water resistance, adhesion to known substrates, etc., is also known.

The present invention relates to a hydraulic binder obtained by heat treatment of a specific and new gypsum comprising two substantive phases. The first, dehydrating phase, produces a large percentage of CaSO ₄ anhydrite III (or alpha anhydrite). The second, rapid cooling, phase-locking of crystallography, gives stable anhydrite III (or alpha-anhydrite) CaSO ₄ and makes it good to use.

This rapid or abrupt cooling, in a dry atmosphere, could never be accomplished in the production of plaster.

The commercial plaster is obtained only by dehydrating and heating the gypsum, without any abrupt cooling, which formulates the main aspect of the invention.

The technical conditions for this new hydraulic binder according to the invention include:

A dehydrating phase comprising raising the temperature of the gypsum with low humidity (3% to 15% water) to be treated; the temperature rise takes place over a period of 10 to 40 minutes, up to a temperature of 220 ° C to 350 ° C depending on the nature of the gypsum, or more precisely between 300 ° C and 310 ° C

2. Rapid or abrupt cooling in a dry atmosphere.

This rapid or abrupt cooling, never predicted by the gypsum production and stabilization of the very soluble anhydrite III (or alpha-anhydrite) CaSO ₄ anhydrite, blocks its crystallization by thermal shock, which should lower the temperature by 220-350 ° With less than 80 ° C in less than 2 min.

The high percentage of stable and soluble anhydrite III (or alpha-anhydrite) CaSO ₄ (more than 70%, or often 90%) results in a remarkable hydraulic binder that can compete favorably with most conventional binders.

This new binder has the following characteristics:

- fire resistance: flammability, Mo category according to the French standard NF P 92-507;

- excellent curing in seawater;

- remarkable adhesion to all pads; and

- Hardening at very low or high temperatures, etc.

The industrial applications of this binder are of interest in the production of cement, concrete and plaster, in the production of waste or inert industrial by-products, in the manufacture of mixtures with materials which combine well with calcium sulfate, in the construction of housing in developing countries. countries and more.

This hydraulic binder can be produced by the application of known techniques: low temperature heating (220 ° C to 350 ° C) and cooling. This may be under normal factory conditions.

In addition to its non-geotechnical properties, this hydraulic binder has:

- economic importance;

- importance in terms of energy saving;

- ecological significance: not polluting.

Upon completion of dehydration, the percentage of CaSO ₄ anhydrite III (alpha anhydrite) is above 50%, or often 70% to 80%, while rapid cooling causes crystallization of form III (alpha anhydrite) CaSO ₄ and makes it stable and soluble and enables it to be used industrially.

The following conversion conditions appear to deliver the best results. The amount of moisture in the starting material is the first obstacle and thereafter it is adjusted, if necessary, to a value in the range of 12% to 20%; thereafter, heating is carried out, resulting in the formation of a powder material at a certain temperature, being heated under conditions sufficient to raise the gas temperature above the material layer to a value in the range of 350 ° C to 550 ° C, and to reach an average temperature at the core of the material above 220 ° C and below 350 ° C. The heating may, for example, be accomplished using infrared heaters located above the material layer, the power of said heaters being adjusted depending on the duration of exposure of the material.

One possible explanation for the best results obtained under these operating conditions is the following.

The temperature of 220 ° C-350 ° C in the core of the material layer is ideal for the preparation of alpha-anhydrite, but not of other forms. The extracted water is discharged from the material layer into an atmosphere that is warmer and the temperature is higher than the critical (365 ° C). Thus, a supercritical state rapidly preventing or limiting re-hydration and changes to the surface of the material is rapidly achieved, so that when the heating is complete, the amount of alpha-anhydrite is very large (precision proportions cannot be obtained in samples taken before cooling).

The exothermic transformation of alpha-anhydrite to basanite is very rapid and is blocked by cooling, which stabilizes alpha-anhydrite. Further, cooling seems to completely block the change in alpha-anhydrite to a basanite plaster, which is only found as quantitative traces in the final product (as opposed to known binders containing much of this form).

The product is obtained when the binder according to the invention shows (without filler) that it withstands flame resistance tests in accordance with the French standard NF P 92-507 (samples 0.30 m / 0.40 m subjected to heating with a constant heat source). The definition of the four indices specified in the standard (flame index, refinement index, maximum flame length index, flammability index) enable the product to be placed in the MO category, the highest of the six categories defined in the standard.

Furthermore, strength tests are carried out in accordance with the standard to obtain the following results:

- compressive strength: 40 MPa; and

- flexural strength: 10 MPa.

In addition, the quality tests performed with the samples involved show that the product's strength remains good in such a situation.

The study below, by describing calcium sulfate dehydration, enables the process of the invention to be refined to produce a true hydraulic binder that can be used as cement.

The accompanying drawings are given by way of guidance and are not inherently limiting. They represent a preferred embodiment of the invention and make it easy to understand.

Figure 1 is a curve of thermal dehydration for gypsum.

Figure 2 shows a two-dimensional Doehlert matrix.

Figure 3 shows an even coverage of the area.

Figure 4 shows a constant three-dimensional Doehlert matrix.

Figure 5 is an image showing the X-ray overlay.

A. Calcium sulfate dehydration:

The study concerns the processing of calcium sulfate powder, a basic material for the production of a new hydraulic binder with interesting characteristics pertaining to a group of norms, and known physical properties (thermal conductivity, mechanical strength, acoustic insulation, etc.).

The method consistently involves two types of processing:

- heating the powder material to a temperature in the range of 220 ° C to 350 ° C to form alpha-anhydrite (or anhydrite III) (see Figure 1);

- abrupt cooling of the heated material to stabilize this metastable phase. Thus, the evolution in other crystallographic variants is blocked and re-hydration to basanite, the main phase of the plaster, is avoided.

B. Experimental methodology:

The collection of experiments is performed empirically on natural gypsum and on phosphogypsum, titanogypsum, borogips and desulfogypsum.

However, the inventor uses experimental types of techniques to determine the effects of factors that may be involved in the alpha-anhydrite formation phase and to limit the number of operations.

Therefore, the temperature changes from 280 ° C to 320 ° C, the duration of the constant temperature step from 10 min to 70 min, and 3 groups of grain sizes are used: <80; 80 - 100; 100 - 200 micrometer.

The methodology for the experimental strategy, based on a mathematical and statistical approach, uses a model for the phenomenon starting from polynomially written dimensionless coded variables. The natural variable C corresponds to each of these variables X., in accordance with the equation:

U ^a = U ⁰ + X ^a x deltai.

111 ^ 1 keto i = factor;

Ts ^a = the value of the natural variable i for experiment a;

U® = value of the natural variable i in the center of the experimental zone;

_{and (+} 1) _{+ and} (.1)

1 μ ° = --------- 2

Xj ^a = value of the coded variable i for experiment a;

delta = value of change in natural variable i:

U. (+ i) + u. * · »

1 Delta !!. ⁰ = -------------- 2

When this new hydraulic binder is obtained, the strategy is to use a Doehlert fixed matrix, a strategy that allows the coefficients of the polynomial model to be estimated (by the mean squaring rule).

In the coded interval [(-1) (+1)] for the variables studied, the general equation is given below:

+ fb _ii X? + fb _s X _i X _j i = li = l icj> 2 where y = measured effect (production parameter)

B _o = average effect value b. = main effect of variable i;

b _{; i} - squared effect of variable I;

B. = effect of interaction between variables I and j.

In the two-variant embodiment shown in Figure 3, the seven experiments performed are arranged in a centered regular hexagon inscribed in a circle of radius 1.

The two variables are temperature (280 ° C to 320 ° C) and time (10 to 70 min). The experimental matrix (for coded variables) and the experimental strategy (for natural variables) are shown in Table 1.

Experimental matrix Experimental strategy The effect No XI X2 HZ T ° (° C) time (min) grain size (mm) % a anh t ° endo rx * Qya ₂ anh Θ 1 1 0 0 320 40 80-100 86.1 127.5 • 2 -1 0 0 280 40 80-100 89.7 120.7 ο 3 0.5 0.866 0 310 70 80-100 90.3 119.8 • 4 -0.5 -0.866 0 290 10 80-100 87.3 118.6 0 5 0.5 -0.866 0 310 10 80-100 84.2 130.5 • 6 -0.5 0.866 0 290 70 80-100 86.8 120.2 • 7 0.5 0.829 0.816 310 50 100-200 89.4 122.0 • 8 -0.5 -0.829 -0.816 290 30 <80 74.7 135.5 ο 9 0.5 -0.829 -0.816 310 30 <80 89.9 118.2 • 10 0 0.577 -0.816 300 60 20 <80 83.3 130.8 • 11 -0.5 0.829 0.816 290 50 100-200 81.5 132.9 • 12 0 -0.577 0.816 300 20 100-200 86.5 120.9 • 13 0 0 0 300 40 80-100 90.9 110.1 •

Table 2: Experimental matrix, experimental strategy and analysis results of products starting with natural gypsum • large • small 30 • very small

- high / prolonged heating temperature / time

- low / short heating temperature / time

- endothermic basanite reaction - alpha 35 anhydrite

- ® part of alpha ₂ anhydrite in the mixture of basanite and alpha-anhydrite.

Manipulations 13,3 and 9 show that the best percentage of alpha-anhydrite 40 (90%) is obtained at a heating temperature of 300 ° C to 310 ° C. It was also noted that these good results correspond to the minimum endothermic reaction temperatures for basanite (110 ° C in the case of Experiment 13). 45

Figure 6 shows qualitatively and semiquantitatively the phases present in the processed products. If the treated product is a mixture of alpha-anhydrite and basanite, the 2.80 A line has a 100% intensity. 50

Conversely, the large amount of alpha ₂ anhydrite in the product causes an increase at the line up to 3.49 A, which then reaches 100% intensity and essentially the appearance of the 2.85 A line for alpha ₂ anhydrite. At the same time, the relative intensity of the lines 6.02 A, 3.01 A and 2.80 A decreases. Overlaying the X-ray diffraction spectrum shows an increase in the fraction of alpha _2- anhydrite with a temperature mixture of alpha _2- anhydrite and basanite.

According to the invention, a product can be obtained; which is a true hydraulic binder containing more than 70% stable and soluble alpha-anhydrite; (or often 80% to 90%) with amazing properties and applications:

- rapid curing: up to 3 min;

- accelerated hardening;

- high strength (35 MPa per 7 days);

- thermal and sound insulation;

- treatment of industrial residues: inert or clogging;

- facade coatings;

- building templates;

- pre-fabrication of dwellings, etc. In addition to these technical qualities,

Encoded Variables (X;) Naturally Variable (C) No XI X2 Temperature (° C) Time (min) 1 I 0 320 40 2 -1 0 280 40 3 0.5 0.866 310 70 4 -0.5 -0.866 290 10 5 0.5 -0.866 310 10 6 -0.5 0.866 290 70 7 0 0 3 (X) 40

Table 1: Experimental Matrix and Experimental Strategy for Doehlert Invariant Two-Dimensional Matrix

This model is characterized by five temperature values and 3 temperature levels.

As an example, the temperature is:

320 + 280

C ° = --------- = 300 ° C, where

280 ° C U. ⁽⁺ '> = 320 ° C

320 + 280

Delta!}. = ---------- = 20 ° C and X. = 0.5

This strategy has two advantages:

- If, after conducting the seven experiments with the first centered hexagon shown in Figure 4, only four of the results (the percentage of alpha-anhydrite represented on the solid lines in Figure 4) are significant, only three manipulations are necessary to construct a second correctly centered hexagon. Such an approach makes it possible to construct a third hexagon.

Thus, the surface is covered evenly and the optimal conditions for the production of alpha-anhydrite are determined.

- Further, to investigate the impact of a third factor (for example, grain size, only six additional experiments are required (three larger than those used to conduct the first seven experiments and three smaller size) , in a symmetrical manner).

The three-dimensional fixed Doehlert matrix is shown by a sphere centered with a hexagon in a position perpendicular to the 25 major axis. This is characterized by five temperature values, seven time levels and three grain size ranges (see Figure 5).

Thus, the experimental model allows the number of manipulations to be significantly reduced. The experimental matrix, experimental strategy and results are shown in Table 2.

C. Results

The inventor has performed thirteen manipulations, starting with natural gypsum, as determined by the experimental strategy shown in Figure 5.

Thermogravimetric analysis (TGA) and di40 differential scanning calorimetry (DSC) make it possible to determine the percentage of alpha-anhydrite. X-ray diffractometry is used to detect the presence of the phases and to distinguish between two 45 variants of alpha-Ca ₈ O ₄ , which will be referred to as alpha] and alpha ₂ (see Figure 6).

The results are shown in the following table:

this hydraulic binder is with:

- economic importance;

- importance on energy savings;

- ecological importance: it does not pollute nature.

The process according to the invention can convert more than 90% of pure calcium sulfate into anhydrite III or alpha-anhydrite.

All types of gypsum contain pure calcium sulfate plus undesirable impurities.

Depending on the purity of calcium sulfate, different types of cement are obtained.

With respect to the final product, the new binder contains less than 10% gypsum.

Claims

Claims (9)

1. A method for the production of a hydraulic binder based on anhydride III or alpha-anhydride of natural calcium sulfate (gypsum) or synthesized calcium sulfate (sulfogypsum, phosphogypsum, titanium gypsum), comprising heating said calcium sulfate: - a hydraulic binder which can be used as cement based on anhydrite III or alpha-anhydrite, characterized in that it contains more than 70% stable, soluble anhydrite III or alpha-anhydrite, and which method consists in: - a heating step which is carried out at a temperature of the treated gypsum from room temperature to 220 ° C to 350 ° C, depending on the characteristics of the treated gypsum; - a step of abruptly cooling the resulting product, reducing its temperature from 220 ° C to 350 ° C to less than 80 ° C in less than 2 minutes, in order to stabilize alpha-anhydrite by crystallographic blocking, and fixing. A method according to claim 1, characterized in that the heating temperature is 300 ° C-310 ° C. Method according to claim 1, characterized in that the temperature of the sharp cooling is in the range 40 ° C to 50 ° C. A method according to claim 1, characterized in that the temperature rises over a period of 10 to 40 minutes depending on the type and size of the gypsum grains. Method according to claim 4, characterized in that before treatment, the treated gypsum contains 0 to 20% water and the grain size is in the range 0 to 30 mm. Method according to claim 5, characterized in that the treated gypsum contains 5% to 15% water and the grain size is in the range 0 to 10 mm. Hydraulic binder, which can be used as anhydrite III or alpha-anhydrite cement based on the method according to any one of claims 1 to 6, characterized in that it contains more than 70% stable and soluble anhydrite III or alpha-anhydrite. Hydraulic binder, which can be used as cement based on anhydrite III or alpha-anhydrite obtained by the method according to any one of claims 1 to 6, characterized in that it contains more than 90% stable or soluble anhydrite III or alpha-anhydrite. A hydraulic binder that can be used as cement based on anhydrite III or alpha-anhydrite, obtained by the method according to any one of claims 1 to 6, characterized in that the mechanical strength is: 22 MPa for 24 h; 30 MPa for 8 days; more than 40 MPa in 14 days.