GB2073729A - Gypsum/Synthetic Anhydrite Binders - Google Patents

Abstract

A binder consisting of a mixture of at least one residual gypsum (e.g. a phosphogypsum, or a desulphurisation gypsum emanating from thermal power stations) with at least one synthetic anhydride, (i.e. fluoranhydrite, or phosphoanhydrite).

Description

SPECIFICATION Gypsum/Synthetic Anhydrite Binders This invention relates to new binders obtained by simply mixing at least one residual gypsum (i.e.

a gypsum which is a by-product in chemical industry or of processes used in thermal power stations) with at least one synthetic anhydrite.

In the past it has been virtually impossible to find a satisfactory use for those gypsums which are by-products in chemical industry, such as phosphogypsums, or for gypsums obtained from the desulphurisation of combustion gases from thermal power stations.

Those methods which aim to obtain from these gypsum by-products a semihydrated calcium sulphate suitable for use as a binder in the construction industry have the disadvantage of requiring both purification of the residual gypsum by-products and a calcining operation.

In European patent application No. 0 007 610 it has been proposed to mix gypsums resulting from the desulphurisation of the combustion gases from thermal power stations, after partial or total dehydration, with a dehydrated natural gypsum, or to dehydrate the mixture of residual gypsums and natural gypsum simultaneously. This method also entails an energy-intensive dehydration stage.

We have found, unexpectedly, that very high quality binders can be obtained by simply mixing at least one residual gypsum of chemical origin or obtained from the desulphurisation of combustion gases from thermal power stations, with at least one synthetic anhydrite binder. In the present process no purification or calcination of the residual gypsum is necessary, By the term "synthetic anhydrite binder" used in this specification is meant a binder conforming to German standard DIN 4208, obtained from fluor-anhydrite, a b±product of the production of hydrofluoric acid, or from phosphoanhydrite, which is obtained, for example, by the method described in our French Patent No. 2 423 463. Natural anhydrite cannot be used in the preparation of the binders according to the invention.

The residual gypsums from the chemical industry, such as phosphogypsums, which can be used according to the invention, contain at least 90%, preferably at least 95%, calcium sulphate dihydrate CaSO4. 2H2O. They are contaminated with various impurities, such as calcium fluoride, silica, iron oxides, alumina, and phosphates. Their maximum particle size is generally less than 0.3 mm, whilst the average particle size is of the order of 30 to 40 Mm.

The residual gypsums from thermal power stations, which may also be used according to the invention, are obtained in the course of the desulphurisation of the combustion gases issuing from the power stations, by washing with an aqueous solution of lime. First, a mixture of neutral calcium sulphite and acidic calcium su!phite is formed, which is oxidised by air to form calcium sulphate dihydrate. This crude gypsum contains at least 90% by weight of CaSO4 . 2H2O. It is contaminated with a large number of impurities, such as sulphites, chlorides, carbonates, silica, alumina, magnesium oxide, and iron oxides. Its maximum particle size is generally less than 0.2 mm, whilst the average particle size is of the order of 30 to 40 ,um.

The respective proportions of residual gypsums and synthetic anhydrite in the mixtures constituting the new binders according to the invention may vary within fairly wide limits, i.e. from 9010% to 1 0-90%. From the economic point of view, it is advantageous to introduce the largest possible amount of cheap residual gypsums into the mixtures; from the point of view of technical performance, mixtures rich in synthetic anhydrite are the best. The best compromise is obtained with mixtures containing equal parts by weight of residual gypsums and synthetic anhydrite.

According to the invention, it is also possible to use a number of residual gypsums of different origins in admixture with the anhydrite binder.

The new binders according to the invention may be used as a pure paste or may be combined with granulates to obtain mortars or concretes. Additives may be added to the pure pastes or to the mortars or concretes in order to modify the properties of the mixture before it sets or the properties of the material after it has hardened.

The present binder is suitable for use with any material with which a binder is used, especially in the building industry. In particular, by fluidification, self-lavelling coverings of pure paste or mortar can be produced. It is also possible to produce cellular materials by expanding the pure paste or mortars, for example using aluminium powder.

In the two examples given hereinafter in order to illustrate the invention, pure pastes were prepared, and the amount of water added for mixing was adjusted so that the diameter of spreading of the paste, determined according to the German standard DIN 1 164, was 150+5 mm. The beginning and completion of the setting process are also measured according to this same standard DIN 1 164.

The flexural tensile breaking stress and the compressive breaking stress are measured on 4x4x 16 cm testpieces according to the standard NF P 15-451.

Example 1 A phosphogypsum having the following characteristics is used as the residual gypsum: a) particle size: between 0.7 and 200 ,um; average 33 ym, b) chemical analysis: CaSO4. 2H20 95.5% CaF2 2.0% SiO2 0.8% Fe203 0.2% Al203 0.3% Water-soluble P2O5 0.1% water-insoluble P205 0.5% 800 g of this phosphogypsum are mixed with 800 g of fluoro-anhydrite in accordance with German standard DIN 4208 and 176 g of water. The times for the beginning and completion of setting and the mechanical strengths after 7 and 28 days are indicated in the Table given below.

Example 2 A residual gypsum from a thermal power station having the following characteristics is used as the residual gypsum: a) particle size, determined by screening of a product dried at 1 000C: Mesh size of screens Material rejected ( m) (% by weight) 37 88 74 10 90 1.4 100 0 b) chemical analysis: 95.5% CaSO4. 2H2O 95.5% SiO2 SO2 0.3% Fe203 0.1% MgO 0.1% Co2 0.1% H20 3.4% 800 g of this desulphurisation gypsum are mixed with 800 g of fluoroanhydrite in accordance with German standard DIN 4208 and 168 g of water. The times of the beginning and completion of setting and the mechanical strengths after 7 and 28 days are given in the following Table.

Table

Flexural Bulk tensile Density strength Compressive (g/cm3) (bars) strength Beginning Completion of of 7 28 7 28 7 28 Example setting setting days days days days days days 1 > 6h > 12h 1.68 1.66 26 52 115 220 2 65mn 105 mn 1.78 1.76 74 76 270 275

Claims (10)

Claims

1. A binder consisting of a mixture of at least one residual gypsum and at least one synthetic anhydrite.

2. A binder according to Claim 1, wherein the residual gypsum is a phosphogypsum.

3. A binder according to Claim 1, wherein the residual gypsum is one obtained from the desulphurisation of the combustion gases of thermal power stations.

4. A binder according to Claim 1, 2 or 3, wherein the synthetic anhydrite is a fluoroanhydrite.

5. A binder according to any one of Claims 1 to 4, wherein the synthetic anhydrite is a phosphoanhydrite.

6. A binder according to any one of Claims 1 to 5, wherein the amount of residual gypsum is from 10 to 90% by weight.

7. A binder according to any one of Claims 1 to 6, wherein a number of residual gypsums of different origin is used.

8. A binder according to any one of Claims 1 to 7, wherein a number of synthetic anhydrites of different origin is used.

9. A binder substantially as hereinbefore described in Example 1 or 2.

10. A building material incorporating a binder as claimed in any one of the preceding claims.