DE2818169C2 - Process for the production of low-pore plaster of paris molded products - Google Patents

Description

The present invention relates to a method for producing low-pore molded plaster of paris, in particular cladding panels, by shaping comminuted natural gypsum double hydrate CaSO ₄ · 2H ₂ O mixed with water and subsequent heat treatment and solidification of the molded product. It is primarily about cladding panels for the exterior design and interior finishing of buildings or building parts and finishing elements in the construction industry.

Plaster products are usually made from plaster of paris (hemihydrate CaSO ₄ · 0.5 H ₂ O). This in turn is obtained from natural gypsum double hydrate, which is broken in ball mills and crushed into powder. This is then subjected in particular gypsum boilers with stirring and at a temperature of about 180 ⁰ C to a heat treatment for 4 to 8 hours duration. After cooling, the gypsum obtained is packaged in a moisture-proof manner and stored in this form and passed on to the production facilities for the manufacture of the gypsum products.

To produce the molded products themselves, the building plaster hemihydrate CaSO ₄ · 0.5 H ₂ O is moistened with water for a few minutes before the molds are filled; the water / gypsum weight ratio is 0.4 to 0.6. The plaster of paris hardens in the mold for 5 to 15 minutes. After the shuttering has been removed, the product produced is dried, depending on its size, at a temperature of 50 to 60 ° C for 8 to 16 hours

The products manufactured by this known method have a high porosity and water absorption capacity and are therefore only suitable for interior construction, e.g. as flooring, visible surfaces or Soundproofing panels. Added to this are the difficulties of storing and transporting the is building plaster.

DE-AS11 69 355 describes the production of granules known from fine-grained gypsum-containing industrial waste, whereby non-settable and settable calcium sulphate in the form of hemihydrate or anhydrite with or without additives an exciter is granulated. Depending on the process management during the subsequent heating in rotary kilns, Deck furnace or other firing units are obtained granules, which in turn are bindable or neither are. However, these cannot be viewed as a molded product, but rather serve as stucco or Screed plaster.

From the SU inventor's certificate a process for the production of plaster products is known in which natural gypsum double hydrate CaSO ₄ · 2 H ₂ O is broken up to the formation of particles with a specific surface area of 2500 to 2800 cm ² / g and the particles are then broken with water are mixed, the water-gypsum weight ratio being 0.1 to 0.2. The mixture prepared in this way is poured into molds and compacted by shaking. The molded components are subjected to a hydrothermal treatment in a pressure vessel in a steam atmosphere under a pressure of 7.0 to 7.5 bar, in which the double hydrate is dehydrated to form building plaster hemihydrate. To solidify the products through hydration, they are immersed in water until they are saturated

The disadvantage of this method is the high moisture content of the mixture to be shaped, which in connection with the subsequent heat-moisture treatment leads to a porous structure of the products; the water absorption capacity is 18 to 20%. Also, the frost resistance of 10 to 15 cycles is insufficient, wherein said freezing is to a temperature of -15 to -2O ⁰ C in the course of 4 hours and thawing in water understood in the course of 4 hours under a cycle. Finally, the implementation of the heat treatment in a pressure vessel makes it impossible to produce the cast products in a flow process and to automate the process as a whole.

From the SU Inventor's Certificate 62 785 a method for producing plaster products is known in which the crushed natural gypsum double hydrate CaSO ₄ 2 H ₂ O is moistened to 6 to 10% during molding by pressing or tamping with the addition of fillers. When the products are formed by pouring or shaking in, a larger amount of water is used so that the mass becomes mushy. The components shaped in this way or that way are subjected to a hydrothermal treatment in a pressure vessel for 3 to 6 hours, the steam under a

Pressure of at least 1 bar is supplied. It goes the natural gypsum double hydrai even at low pressures (1 to 1.5 bar) into the building plaster hemihydrate, whereby Adhesive forces occur between the crystals of the hemihydrate, which give the products considerable strength impart immediately during hydrothermal treatment.

The water vapor is then slowly released. After hydrothermal treatment and cooling a hydration process occurs due to the water remaining in the pores in the products of building plaster hemihydrate, which is accompanied by an additional increase in strength.

The products manufactured by this process also have the disadvantages already described. Because of the high water content of the mixtures of plastic consistency, they cannot achieve high values have frost and water resistance. But also In the case of products formed by pressing, the hydrothermal steam treatment in the pressure vessel produces a Porosity of less than 14 to 15% and sufficient frost resistance (35 cycles and more) are not achieved.

The object of the present invention is to create a method for producing molded plaster of paris with low pores with increased water and frost resistance.

On the basis of the generic presupposed method mentioned at the beginning, this object is achieved according to the invention in that the shaped products are subjected to a heat treatment known per se in an air atmosphere at a temperature of 100 to 200 ^° C. and the temperature rise up to the value mentioned at a rate of 2 up to 3 degrees / min is carried out, after which the heat-treated products are cooled to a temperature of not more than 50 ° and are solidified by moistening.

Such a procedure results in plaster cast products with the following unusual properties: Water absorption 7 to 15%, frost resistance 40 to 50 cycles and compressive strength of the products 17 to 30 MPa.

To have a monolithic structure and sufficiently high strength of the plaster products during their To ensure solidification, it is expedient to reduce this to 90 to 95% by volume of the natural gypsum double hydrate to dehydrate molded products.

In a further development of the invention, it is useful if the water / natural gypsum double hydrate weight ratio is used when molding the products from crushed natural gypsum double hydrate Is 0.05 to 0.1. Then the value of the water absorption of the finished molded products will be below 7 to 10%.

If, according to a further proposal, 40% by weight of the natural gypsum double hydrate with a Fraction from 1 to 10 mm and 60% by weight with a fraction with a specific surface area of the particles from 1700 to 1800 cmVg are used, molded products are used, the parts made of marble, granite and can replace other building blocks.

When finally the shaping of the products by pressing at a specific pressure of at least 6 MPa is carried out, the products get a denser structure and better mechanical properties and frost resistance.

The invention is further explained below by the description of exemplary embodiments.

In all cases, natural gypsum stone is used as the starting point, ie the double hydrate CaSO <· 2 H2O, which is prepared in a jaw or hammer crusher to a specific surface area of the particles of 100 to 200 cm ² / g. The actual size of the specific surface area of the starting material depends on the thickness of the molded product to be produced. So it is z. B. expedient to break the gypsum stone to a specific surface area of 600 to 900 cm ^{2 / g for products with a thickness of 15 to 30 mm.} The thinner the products to be manufactured, the larger the specific surface of the broken stone should be.

In the manufacture of plaster products, building elements made of marble, granite and other building blocks

If you want to replace it, it is best to start from a mixture of two fractions of the natural gypsum double hydrate, one of which has a particle size of 1 to 10 mm and the other of which has been ^{ground to a specific surface area of the particles of about 2000 cm 2} / g. It is useful to take 40% by weight of a fraction with a particle size of 1 to 10 mm and 60% by weight of a fraction with a specific surface area of the particles of 1/00 to 1800 cmVg.

The na-

Turgips double hydrate CaSO ₄ · 2 H ₂ O is moistened with stirring until a mixture is formed in which the water / Naiurgips double hydrate weight ratio is 0.04 to 0.15. The specific value of this ratio depends on the terms of the subsequent

jo shaping and the characteristics of the used Fixtures dependent; so should the water / natural gypsum double hydrate weight ratio when molding the products on two-stage presses with a specific pressure of 7.5 to 25 MPa, preferably 0.05 to 0.07. It should also be used for the production of plaster products whose water absorption capacity is 7 to 15%, 0.05 to 0.1.

The molds are filled with the prepared mixture, / .. e.g. by pressing or shaking.

When the products are standardized by pressing, the Pressure should not be less than 6MPa, because then products with a denser structure and better mechanical properties Properties are obtained.

The molded plaster products are a plant

for heat treatment, e.g. B. fed to a continuous dryer or a tunnel dryer, where the heat treatment takes place in an air atmosphere. The temperature rise in the dryer from room temperature up to the final temperature between 100 and 200 ° C takes place at a rate of 2 to 3 degrees / min.

The maximum temperature of the heat treatment and its duration depend on the thickness of the product. For products 6 to 7 mm thick, the maximum temperature value is 150 to 160 ^° C., for example, the product being held at this temperature until at least 90 to 95% by volume of the natural gypsum double hydrate of the molded products are dehydrated. At this level of dehydration is the conversion

bo practically completed and keeping the products in the dryer would be economically inexpedient.

During the heat treatment of the products the natural gypsum double hydrate CaSO4 ■ 2 H2O goes under

b5 Excretion of both mechanically bound and also crystallized water in the building plaster hemihydrate CaSÜ4 · 0.5 H.iO over, whereby the initial crystal structure because of the recrystallization from the double hydrate to

^{5 6}

Hemihydrate is destroyed. In the case of a certain condition, a drying cabinet undergoes heat treatment.

degree of sealing when the mixture is formed, during which the temperature

during the heat treatment a certain destruction rate of 2 ° C / min up to 160 "C is increased,

The natural gypsum double hydrate particles take place. At this temperature the product is left for 1.5 hours

a constant release of water contributes

The contact area of the individual particles is larger. The minutes cooled down to a temperature of 47 ° C

subsequent solidification of the products during loading. The product is then immersed in

Humidification already runs in a changed coarse - a water bath is humidified to saturation and then

structure of the product in which the crystallization of dried in the open air

Doppe & ydrat an increase in strength and a stronger io The compressive strength of the finished product is

A decrease in the porosity of the product favors 11.5 MPa, the frost resistance 50 cycles, the water

It should be noted that there is inappropriateness of 9%.
the temperature of the heat treatment would be moderate

to above 200 ⁰ C, because it then leads to the formation of example 2
Anhydrite could come in, which can decrease the is

The strength and frost resistance values of the natural plaster of paris is crushed in a jaw crusher

nits would lead. After that, some of it is put in a ball mill up to

After the heat treatment, the product is applied to a specific surface area of 1700 cm ² / g and in any known manner, e.g. B. by air cooling with other part in a hammer crusher up to a Fraktels blower or by natural convection to 40 20 tion of 1.0 to 10.0 mm crushed
The product is then cooled to 50 ° C. 40% by weight of gypsum granulate from the frac moistening is then wetted to saturation, ie with a water ion of 1.0 to 10.0 mm with 60% by weight of gypsum powder from the spemenge which is mixed for the crystallization of the gypsum-specific surface of 170 cm ² / g and the hemihydrate is necessary to form the double hydrate. Based on the prepared mixture up to a water / natural gypsum based on the weight of the dried product is 2s weight ratio of 0.06 moistened. The mixture this amount approx. 18 wt .-%. The humidification can be molded in a press at a specific pressure of by immersing the product in water, water- 17 MPa. The manufactured product is placed in an atomizer or by saturation of the product with a drying cabinet in which the tempe-water vapor is brought about. temperature at a rate of 2 ° C / min up to

As a result of the water saturation, the ironworks goes up to 30 180 ° C. At this temperature, the Hemihydrate CaSO ₄ ■ 0.5 H ₂ O held back in the natural gypsum certificate for 2 hours. Then one lets Double hydrate CaSO ₄ · 2 H2O over, whereby it under it in the course of 30 minutes up to a temperature of Recrystallized to form a solid structure. Cool down to 43 ° C. Thereafter, the products are made with or without special- Then the product is sprayed from

their measures for this purpose dried, whereby they a 35 moisture on its surfaces moistened and im

Gain ultimate compressive strength of 10 to 30 MPa. Drying cabinet at a temperature of 50 ° C in the If necessary, the finished product can be sanded and dried slightly over the course of 3 hours. Finally will

or any other surface treatment - it is ground on the front with a grinding wheel-

treatment. fen.

It is possible to add a dye, z. B. 28 MPa, the frost resistance 50 cycles, the water a mineral dye, add with the water, what absorption capacity 10%.
a coloring of the finished products enables and
does not affect the manufacturing process. Example 3

The proposed method enables the 45

Position of a large variety of plaster of paris molded products- natural gypsum is crushed in a jaw crusher, sen with improved physical-mechanical properties- Then a part of it is shanked in a ball mill and expanded application possibilities in construction- a specific surface area of 2000 cm ² / g and the being. Because the other part of the gypsum stone is six times higher than the usual values, the other part of the gypsum stone is crushed to a fraction of 1 to 7 mm in a hammer crusher.
Products can be used for the external cladding of buildings and structures. 40% by weight of the fraction 1 to 7 mm is then used. In the case of a 60% by weight fraction with the specific upper shaping by pressing, the area of 2000 cmVg is also mixed. The prepared Mi option of providing the visible surface of the products with relief is provided until a water / plaster geometry is achieved. 55 weight ratio of 0.1 moistened. The mixture

Below are a few more specific examples being used in a press at a specific pressure of

Practical applications listed: 20 MPa molded. The finished product is in one

Drying cabinet housed and the temperature of Example I of room temperature at a rate of

60 3 degrees / min up to a temperature of 18O ⁰ C increased.

Natural gypsum is put in a jaw crusher and then for a dehydration of 9% by volume of the natural gypsum

in a ball mill up to a specific surface double hydrate in the molded product, this becomes 1

before the particles of 1000cm ^J / g are crushed. Maintained at said temperature for one hour.

The moisture content of the powder is 3%. The powder The product is then stored at room temperature

is moistened up to a water / Nalurgips-Ge 65 course of 30 minutes up to a temperature of 40 ° C

weight ratio of 0.07 and the mixture cooled in a hy-.

draulic press at a specific pressure. The cooled product is produced by atomization

of 15 MPa. The molded product will dry out from moisture on its surface

moistened and lightly dried in a drying cabinet at a temperature of 50 ° C over the course of 3 hours. The compressive strength of the finished product is 24 MPa _{1 and} the frost resistance is 45 cycles.

Example 4

Natural gypsum is processed in a jaw crusher and then in a ball mill to a specific surface area Crushed by lOOcnWg, moistened until a water / plaster weight ratio of 0.05 is reached and molded in presses at a specific pressure of 28 MPa. The molded product is rolled into one Housed drying cabinet, in which the temperature at a rate of 2 degrees / min to one is Temperature of 140 ° C is increased. The holding time at this temperature is 1.5 hours, during which time it leads to a dehydration of 95% by volume of the natural gypsum double hydrate comes.

The product is heated under natural conditions to a temperature of Cooled 50 ° C. Moistening is done by atomizing moisture onto the surface of the product. The subsequent drying of the product takes place in the drying cabinet at one temperature of 50 ° C for 3 hours.

The compressive strength of the finished product is 14 M Pa _{1 and} the frost resistance is 40 cycles.

Example 5

Natural gypsum with a natural moisture of 5% is broken up to a specific surface of 800 cm / g and shaped in presses. The pressing takes place at a specific pressure of 30 MPa. The molded products are heated up to a temperature of 160 ^° C. at a rate of 2 degrees / min. The product is kept at the maximum temperature for 2 hours, with dehydration of 92 vol% of the natural gypsum double hydrate. Under natural conditions, the product is heated to 40 ° C in 30 minutes cooled down.

The wetting of the product was carried out by immersion in water to saturation. the Compressive strength of the finished product is 15 MPa, the frost resistance 45 cycles.

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Claims (5)

Patent claims: 1. A method for producing low-pore plaster of paris blow dryer products, in particular cladding panels, by shaping comminuted natural gypsum double hydrate CaSO ₄ ■ 2H ₂ O mixed with water and subsequent heat treatment and solidification of the shaped product, characterized in that the shaped products are one per se known heat treatment in an air atmosphere at a temperature of 100 to 200 ⁰ C and the temperature rise up to the value mentioned is carried out at a rate of 2 to 3 degrees / min, after which the heat-treated products are cooled to a temperature of not more than 50 ° and through Moisturizing to be solidified. 2. A method for producing low-pore plaster of paris molded products according to claim 1, characterized in that characterized in that the gypsum products to 90 to 95Vol .-% of the natural gypsum double hydrate of the formed Products become dehydrated. 3. A method for producing low-pore molded plaster products according to one of the claims I or 2, characterized in that when the products are formed from crushed natural gypsum double hydrate the water / natural gypsum double hydrate weight ratio is 0.05 to 0.1. 4. A method for producing low-pore molded plaster products according to one of the claims 1 to 3, characterized in that 40 wt .-% of the natural gypsum double hydrate with a fraction from 1 to 10 mm and 60% by weight with a fraction with a specific surface area of the particles of 1700 to 1800 cmVg can be used. 5. A method for producing low-pore molded plaster products according to one of the claims 1 to 4, characterized in that the shape of the products by pressing at a specific pressure of at least 6 MPa is carried out.