The invention relates to an aqueous mortar on the basis of aluminous cement for casting refractory objects.

Mortars of this type are known. They are applied for the manufacture of for instance refractory tiles and for lining furnaces or ovens, since mortars on the basis of aluminous cement retain their strength permanently at higher temperatures.

A mortar of the type stated in the preamble, which comprises wollastonite, is known from FR 2900653.

The mortar according to the present invention is distinguished from the known mortar in that the mortar comprises calcium silicate in powder form and quartz flour.

Refractory objects can likewise be realized with the mortar according to the invention, but they can be cast in random, complex shapes, wherein the end product is heat-resistant up to 1250 degrees Celsius. They further have an attractive, smooth surface and can be finished mechanically if desired. Calcium silicate contributes toward the fire resistance of the mortar. Calcium silicate also imparts resistance to great temperature changes to the mortar. This property is also referred to in the field as resistance to thermoshock. Quartz flour likewise contributes toward the fire resistance of the mortar, but also makes the mortar wear-resistant and impact-resistant. Wollastonite also contributes toward the impact resistance of the mortar.

A favourable realization of the inventive mortar, wherein the end product is characterized by a slight shrinkage, has the feature that the mortar comprises wollastonite in needle form and wollastonite in powder form. For this purpose the mortar preferably comprises 38.5-42.5% wollastonite in needle form and 4-6% wollastonite in powder form.

In a preferred embodiment the mortar according to the invention comprises 19%-21 % calcium silicate. The choice for this percentage of calcium silicate optimizes the properties of fire resistance and resistance to thermoshock.

According to a further preferred embodiment, the mortar according to the invention comprises 14%-16% quartz flour. The choice for this percentage of quartz flour optimizes the properties of fire resistance, wear resistance and impact resistance of the mortar according to the invention.

A further favourable realization has the feature that the mortar also comprises emulsifying agents and a defoamer. More particularly a first emulsifier and a second emulsifier are preferably applied, both on the basis of per se known modified polyacrylates dissolved in water. The second emulsifier can here be give a high acidity so that it also functions as retardant. The processing time of the mortar can then be chosen in simple manner by selecting a suitable emulsifier mixture, wherein the total quantity must amount to about 1 % of the weight of the aluminous cement.

A further favourable realization has the feature that the mortar also comprises lithium carbonate, which can bring about curing at relatively low temperatures, this increasing the strength of the end product.

A further favourable realization has the feature that the mortar also comprises a thickening agent. The mortar preferably comprises 0.3-0.7% fumed silica as thickening agent, whereby more particularly the thermoshock properties of the end product improve greatly and the end product retains its shape better at high temperatures.

A further favourable embodiment in which the mechanical properties are improved still further has the feature that 0.5-1.5% glass fibre particles are also added to the mortar, for instance in the form of loose fibres with a length of 1-3 centimetres.

A further favourable realization has the feature that 10-20% plastic filler in powder form is also added to the mortar. When the end product is heated to 200 degrees Celsius, the plastic then melts and attaches to the surrounding solid substance. The end product hereby becomes stronger and highly corrosion- resistant, and retains its refractory properties to a significant extent. As plastic can be used for instance a mixture of polyester and epoxy in powder form as utilized in the powder-coating industry. Waste originating from the powder-coating industry can more particularly be utilized as plastic filler for the mortar.

A further favourable realization has the feature that 10-20% glass powder is also added to the mortar. When the end product is heated to 900-1000 degrees Celsius, the glass then melts and attaches to the surrounding solid substance. The end product hereby forms a very good insulator which is particularly suitable for manufacturing insulating parts of switches and insulators, and more particularly high-voltage switches and high-voltage insulators and sheathing. Because the material is highly refractory it will remain usable, even if an incidental flashover occurs over the surface. As glass powder can be used for instance the waste produced in the industrial processing of glass, particularly during grinding, milling and drilling of glass. The invention also relates to a method for manufacturing an aqueous mortar as specified in the foregoing paragraphs, wherein aluminous cement, wollastonite, calcium silicate, quartz flour and fumed silica are dry mixed in powder form, wherein the emulsifiers and the defoamer are then added and mixing takes place again, after which water is added while mixing continues until a uniform, pourable mass is obtained, after which lithium carbonate is added and further mixing takes place.

The invention also relates to a method for manufacturing refractory objects, wherein the objects are manufactured in moulds in a casting process using a mortar manufactured according to the method specified in the foregoing paragraph. After curing, the objects are here preferably removed from the moulds and packed in a foil. The objects packed in foil are then preferably cured for 8 to 12 hours at a temperature of 60 to 70 degrees Celsius, after which the product is finished.

A further favourable realization of the inventive method, wherein the objects can fully develop their heat-resistant properties, has the feature that the objects are then dried without foil for 8 to 12 hours at a temperature of 90 to 100 degrees Celsius.

A favourable alternative realization of the inventive method, wherein the objects can fully develop their heat-resistant properties and wherein substantially all the chemically bonded water is removed from the products, has the feature that the objects are then placed without foil in a ceramic oven, in which the temperature is allowed to rise to at least 800 degrees Celsius, and preferably to 900 degrees Celsius, over a period of time of at least 20 hours, after which the oven is allowed to cool slowly.

A further favourable alternative realization of the inventive method, wherein glass powder is added to the mortar so that the objects can be sintered, whereby they are suitable as insulator for high-voltage applications, has the feature that the objects are then placed without foil in a ceramic oven, in which the temperature is allowed to rise to at least 900 degrees Celsius, and preferably to 1000 degrees Celsius, over a period of time of at least 20 hours, after which the oven is allowed to cool slowly.

The invention will now be further elucidated on the basis of three examples.

As first example, different types of two-dimensionaily curved, heat-resistant tiles are cast with which the inner side of an oven can be covered. 38.4 kg aluminous cement is mixed here with 40.5 kg wollastonite in needle form, 5 kg wollastonite in powder form, 5 kg calcium silicate, 10 kg quartz flour and 0.5 kg fumed silica. Mixing takes place with a planet mixer at moderate speed. Care must be taken that the aluminous cement comprises a maximum of 31 % calcium oxide. The calcium silicate is added in the form of a fine powder and can be obtained by crushing waste released in the production and/or processing of Monolite panels as known in the field. The fumed silica can be obtained under the brand name Cab-O- Sil. Once the powders have been sufficiently mixed, a first emulsifier, a second emulsifier and a defoamer are added, after which brief mixing takes place and after which dosed water is added until a uniform mass is obtained. 0.1 kg lithium carbonate is then added, after which brief mixing takes place. The mixture is then poured into silicone moulds arranged on a vibrating table. After the pouring the pouring sides are covered with a plastic foil.

Once the cast products have cured, they can be removed from the moulds, after which they are wrapped in plastic foil in order to prevent drying out. If desired, they can be laid on shaping means which support their shape. They can now be hardened through in a conditioned space for 12 hours at a temperature of 65 degrees Celsius. The foils are then removed and drying takes place for 10 hours at a temperature of 95 degrees Celsius. Finally, the tiles undergo a heat treatment in an oven for 20 hours, wherein the temperature rises gradually to 900 degrees Celsius. After cooling, the product is finished. The tiles can then withstand a temperature of 1250 degrees Celsius, they have a specific weight of 1560 kg/m ³ and a heat conduction of 0.66 W/m.k. Their hardness is 60 shore D and the compression strength is 19 N/mm ² . When heated to 750 degrees Celsius, the expansion amounts to 5 mm per metre, which means that the tiles can be placed substantially without joints.

As second example, different types of street furniture are cast which must be attractive, strong, corrosion-resistant and vandal-proof. For this purpose the same mortar can be manufactured as in the first example, after which 20% plastic in the form of polyester and/or epoxy powder and 1% loose glass fibres with a length of several centimetres are mixed through the mortar. The thus obtained mixture is then poured into steel moulds which are provided beforehand with release agent and, if desired, with glass fibre mats as reinforcement and which are disposed on a vibrating table. After the pouring the pouring sides are covered with plastic foil.

Once the cast parts have cured, they can be removed from the moulds, after which they are wrapped in plastic foil in order to prevent drying out. If desired, they can be laid on shaping means which support their shape. They can now be hardened through in a conditioned space for 12 hours at a temperature of 65 degrees Celsius. The foils are then removed and drying takes place for 10 hours at a temperature of 95 degrees Celsius. Finally, the cast products undergo a heat treatment in an oven for 10 hours, wherein the temperature rises to 200 degrees Celsius. After cooling, the products are finished. If desired, they can be sawn, drilled or milled in a finishing process. It is also possible to co-mould instance mounting brackets and to add dyes to the mortar.

As third example, insulating bodies of high-voltage switches are cast which must have a very high insulating value and which must be able to withstand a flashover. The same mortar can be manufactured for this purpose as in the first examples, after which 10-20% glass powder is mixed through the mortar. The thus obtained mixture is then poured into steel moulds which are provided beforehand with release agents and disposed on a vibrating table. After the pouring, the pouring sides are covered with plastic foil.

Once the cast products have cured, they can then be removed from the moulds, after which they are wrapped in plastic foil in order to prevent drying out. If desired, they can be laid on shaping means which support their shape. They can now be hardened through in a conditioned space for 12 hours at a temperature of 65 degrees Celsius. The foils are then removed and drying takes place for 10 hours at a temperature of 95 degrees Celsius. Finally, the cast products undergo a curing and sintering treatment in an oven for 20 hours, wherein the temperature rises to a maximum of 1000 degrees Celsius. After cooling, the products are finished.