CA2446224A1 - Heat- and fire-resistant moulded part - Google Patents

Abstract

The invention relates to a heat- and fire-resistant moulded part in the form of an artificial round billet for chimney, an insulating panel or a light construction brick with a density between 400 and 1,000 kg/m3. Said moulded part is composed of hydraulically-cemented mixtures comprising, besides water, 10 to 60 weight % high-alumina cement and 10 to 60 weight % xonolite in a mixture which is not yet cemented.

Description

SMB
Heat-Resistant and Refractory Molded Part The present invention relates to a heat-resistant and refractory molded part in the form of an artificial log for fireplaces, a heat-insulating plate or a light-weight constructional brick having a density of from 400 to 1000 kg/m3 consisting of hydraulically cured compositions, and methods for the preparation thereof.
Heat-resistant and refractory molded parts consisting of hydraulically cured compositions are used, inter alia, as artificial logs for fireplaces, but also as heat-insulating plates or light-weight constructional bricks.
Thus, US 4,940,407 describes such artificial logs, which, however, heat up very slowly due to their high specific gravity, store a lot of energy and therefore cool down also very slowly. Moreover, they are very heavy.
US 5,800,875 describes artificial logs which predominantly consist of mineral wool fibers and a high temperature binder. The readily cured form is then suitably painted, again dried and packaged for sale. Due to the use of mineral wool fibers, the temperature resistance is limited. In addition, it is undesirable for safety purposes that fragments of such fibers and pieces of fibers can intrude into the ambient air of inhabited rooms.
US 5,612,266 describes artificial logs which are prepared by casting a mineral foam into molds to be cured therein. These products contain organic components which lead to undesirable smelling when heated.
DE 195 17 267 C1 describes a material having good fire-protection properties contairiing at least 5% by weight of ettringite together with minor amounts of xonotlite and aluminous cement. The ettringite is supposed to decompose upon heating with consumption of energy to thereby improve the fire-protection properties. Such a material is not useful for the present invention.
US 3,042,536 relates to a filling material which contains calcium silicate, for example, in the form of xonotlite, together with asbestos fibers, in addition to aluminous cement. Molded parts made of this material have not been described.
Thus, it has been the first object of the invention to provide artificial logs for fireplaces which have as low as possible a density and therefore take up and release only a relatively small amount of heat energy. Therefore, these products heat up very quickly and also cool down quickly. This results not only in a saving of heating energy, but also in increased safety, since only for a very short time after the switching off of the heating are the artificial logs so hot that they would cause burns when touched or could ignite combustible materials when in contact therewith.
Further, it has been the object of the invention to provide heat-insulating plates or light-weight constructional bricks made of such a material, because these must meet similar demands.
Surprisingly, these objects are achieved particularly well by hydraulically cured compositions containing, in an uncured state, in addition to water, aluminous cement, optionally in admixture with Portland cement, phosphate binders or water glass, fillers and, if desired, fibers, curing accelerators, curing delayers, plasticizing agents and foaming agents, wherein the uncured composition contains from 10 to 60% by weight of aluminous cement and from 10 to 60% by weight of xonotlite.
Typically, the uncured composition contains from 50 to 200 weight parts of aluminous cement and from 10 to 250 weight parts of xonotlite.
As the xonotlite, in practice, synthetic xonotlite is used, which is obtained in the form of felted globules in the most frequently employed process. However, according to the invention, a needle-shaped material may also be employed, which is obtained, for example, in the processing of the spherical felted xonotlite as a by-product or in the recycling of xonotlite waste and dust.
The use of felted balls is particularly preferred. Such xonotlite can be prepared by "heating with stirring" according to US 3,501,324 or GB 1 277 272.
Suitable xonotlites include, in particular, synthetic commercial products, such as Promaxon°, produced and sold by the company Promat. By the addition of major amounts of xonotlite, the density can be reduced, and the thixotropy of the uncured composition can be increased. Undesirable deposition phenomena and thus inhomogeneities are avoided thereby.
The density of the molded parts according to the invention is within a range of from 400 to 1000 kg/m3, preferably within a range of from 400 to less than 700 kg/m3.
Preferably employed fillers include wollastonite, tobermorite, finely divided amorphous silica and/or reactive aluminum oxide.
As light-weight fillers, mainly those selected from the group consisting of pearlites, vermiculites, fly ashes and/or glass beads may be employed.
The molding is effected, for example, by introducing into molds, e.g., by pouring, but may also be effected by extrusion or compression. Thus, artificial logs for fireplaces, but also heat-insulating plates or light-weight constructional bricks may be prepared.
Preferred compositions contain from 20 to 40% by weight of synthetic xonotlites, from 20 to 40% by weight of aluminous cement, from 0 to 50%, preferably 10%, by weight of fine amorphous silica, which both acts as a binder and improves the flow properties of the composition.
In another preferred mixture, the proportions are from 20 to 40% by weight of xonotlite and from 40 to 60% by weight of aluminous cement.

Wollastonite is added in amounts of from 0 to 40%, preferably from 20 to 30%, by weight, more preferably 30% by weight. Wollastonite improves the heat resistance and resistance to thermal shocks, which often led to cracking and premature destruction in known products of the prior art. From 0 to 5%, preferably 2%, by weight of phosphates also increases the temperature resistance; for example, suitable materials include sodium polyphosphate glasses as sold by the company Chemische Fabrik Budenheim under the designation of Budit 8 H. Further, the mixtures may contain usual curing accelerators, curing delayers, plasticizing agents and foaming agents. If desired, the material may be mechanically rein-forced by from 0 to 5%, preferably from 2 to 3%, by weight of fibers, wherein so low amounts of even organic fibers do not result in offensive smells or other undesirable properties. Suitable fibers include, for example, MMMF (man-made mineral fibers), such as glass fibers. Suitable but less preferred fibers are cellulose fibers, organic fibers, such as PVA or PP.
Usual curing accelerators, curing delayers, plasticizing agents and foaming agents may be added, especially to facilitate the preparation process. These additives include calcium hydroxide, aqueous sodium hydroxide, sodium carbonate, calcium carbonate, lithium carbonate, borax, citric acid, aluminum hydroxide. Calcium carbonate is preferred, acting as both a curing accelerator and a suitable filler.
The molded parts according to the invention are preferably free of asbestos fibers and material which decompose upon the action of heat. In contrast to DE
195 17 267 C1, for example, the material according to the invention contains substantially no ettringite, at least it contains less than 5% by weight. The material is preferably free of organic fibers etc. The molded parts according to the invention are intended for repeated heating and cooling. Therefore, it is not reasonable to add components which will decompose upon the first heating.
The method according to the invention for the preparation of these molded parts is effected, in particular, by mixing with water said mixture of aluminous cement, optionally in admixture with Portland cement, phosphate binders or water glass, fillers and, if desired, fibers, curing accelerators, curing delayers, plasticizing agents and foaming agents, and molding and curing the composition, wherein the uncured composition should contain from 10 to 60% by weight of aluminous cement and from 10 to 60% by weight of xonotlite.
Especially when artificial logs for fireplaces are prepared, these are colored with refractory pigments either already in the composition, or else later as ready-cured logs.
An examination of the products prepared according to the invention has shown that they have a very good structural and dimensional stability and are highly resistant towards thermal shocks. They have a high mechanical strength and yet a density of only from 400 to 1000 kg/m3. This has the result that little energy is stored since the system has a low specific heat. This again results in quick heating and quick cooling. Thus, the products can be touched without danger relatively soon after turning off the heating.
Especially when heat-insulating plates or light-weight constructional bricks are prepared from the material, these have good insulation properties, wherein the so-called lambda values can be adjusted by varying the ratio of cement to xonotlite.
These products too do not contain any hazardous fibers or produce undesirable smells upon heating due to the decomposition of organic components. Both these heat-insulating plates and light-weight constructional bricks can be employed for different purposes, for example, for heat accumulating furnaces or for heat insulation in the industry in the production of aluminum, ceramics and cement.
Figure 1 shows the structure of xonotlite in the form of needles.
Figure 2 shows xonotlite in the form of felted globules.
Examples The invention is further illustrated by the following Examples.

Example 1 21.8% by weight of xonotlite powder, 18.2% by weight of wollastonite and 56.6%
by weight of aluminous cement (Secar 71) were mixed in a dry state.
In addition, a mixture of water, plasticizing agent (0.7% by weight) and glass fibers (2% by weight) was prepared and added to the mixture of aluminous cement and xonotlite to give a water-to-solids ratio of 1.18. The mixture was homogenized. Li2C03 (0.7% by weight) was added and mixed for one minute, whereupon the product was filled into a mold.
The xonotlite employed was prepared by cutting/milling a xonotlite product which had been prepared by casting and hydrothermal treatment. Such a xonotlite is in the form of needles (cf. Figure 1). The resistance towards repeated temperature variations was tested by quickly cooling a sample after heating for five minutes at 1000 °C. The product exhibits satisfactory properties.
Example 2 30% by weight of xonotlite, 10% by weight of tobermorite powder and 56.6% by weight of aluminous cement (Secar 71) were mixed in a dry state. The xonotlite employed had been prepared by "heating with stirring". As explained in the description, this yields felted globules. The corresponding structure is represented in Figure 2. The further processing was as in Example 1, but using a water-to-solids ratio of 1.8.
The molded part was also tested according to the testing method as described in Example 1. The products did not exhibit any cracks upon heating.
Example 3 A mixture of 40% by weight of globular xonotlite and 56.6% by weight of alumi-nous cement (Secar 71) was mixed. The further processing and testing was as in _ 7 _ Examples 1 and z, also with a water-to-solids ratio of 1.8. The product did not exhibit any cracks upon heating.

Claims (8)

CLAIMS:

1. A heat-resistant and refractory molded part in the form of an artificial log for fireplaces, a heat-insulating plate or a light-weight constructional brick hav-ing a density of from 400 to 1000 kg/m3 consisting of hydraulically cured compositions, wherein the uncured composition contains from 10 to 60% by weight of aluminous cement and from 10 to 60% by weight of xonotlite in addition to water.

2. The molded part according to claim 1, characterized in that said composition contains from 20 to 40% by weight of aluminous cement and from 20 to 40% by weight of synthetic xonotlite.

3. The molded part according to claim 1 or 2, characterized by containing wollastonite, tobermorite, finely divided amorphous silica and/or reactive aluminum oxide as fillers.

4. The molded part according to any of claims 1 to 3, characterized by containing light-weight fillers selected from the group consisting of pearlites, vermiculites, fly ashes and/or glass beads.

5. A method for the preparation of a heat-resistant and refractory molded part according to any of claims 1 to 4, characterized in that a mixture of alumi-nous cement, optionally in admixture with Portland cement, phosphate binders or water glass, fillers and, if desired, fibers, curing accelerators, cur-ing delayers, plasticizing agents and foaming agents, is mixed with water, the composition is molded and cured, wherein the uncured composition con-tains from 10 to 60% by weight of aluminous cement and from 10 to 60%
by weight of xonotlite.

6. The method according to claim 5 for the preparation of an artificial log for fireplaces, characterized in that the uncured composition is placed into a mold which corresponds to the shape of natural logs.

7. The method according to claim 6, characterized in that either the composi-tion is colored with refractory pigments, or else the ready-cured log is col-ored later externally with refractory pigment paints.

8. Use of an hydraulically cured composition containing, in an uncured state, in addition to water, aluminous cement, optionally in admixture with Portland cement, phosphate binders or water glass, fillers and, if desired, fibers, cur-ing accelerators, curing delayers, plasticizing agents and foaming agents, wherein the uncured composition contains from 10 to 60% by weight of aluminous cement and from 10 to 60% by weight of xonotlite, for the preparation of a molded part according to any of claims 1 to 4.