Such a process is known from the Dutch patent application No. 8801023.

The aforementioned patent application describes a process for coating a surface with a layer containing a stone pattern involving the use of a mortar that contains as binders 4-10 parts of a synthetic resin dispersion in water and 0-5 parts of cement per 100 parts of sand and/or gravel.

A problem with the known process is that the creation of the stone pattern does not proceed as desired, especially if the mortar does not contain any cement at all. Usually a stone pattern is created by cutting out joints, for example in the form of a brick masonry pattern, in a layer of the mortar, not yet set, that has been applied to a surface. However, when the joints are being cut, parts of the remaining layer regularly break away, especially at the points where two

joints intersect. This leads to the formation of an unacceptable, irregular pattern. It is often even necessary to coat the surface with a new layer of mortar and to create a new stone pattern in it.

The invention aims to provide a process that does not present the aforementioned drawback.

This aim is surprisingly achieved because in the process according to the invention use is made of a mortar that contains 0.01-5 parts by weight of thickening agent and parts by weight of thixotropic agent per 100 parts by weight of sand and/or gravel.

With the process according to the invention the stone pattern can be created without any, or at least with practically no parts breaking away from the layer.

A major further advantage is that the mortar needs to contain a smaller amount of pigments compared with the known mortar in order to obtain a comparable colour effect Yet another advantage is that it is possible for the mortar not to contain any cement at all while the above-mentioned advantages are still achieved, while in the case of the known mortar the absence of the cement results in less good results. The advantage of the absence of the cement in the mortar is that the mortar can be marketed ready for use, as a single- component system, while for the known mortar the cement is to be admixed shortly before the mortar is used.

Admixing of the cement generally takes place on the building site, by builders who do this inaccurately, so

that quality differences arise between different mortar batches.

Further, the mortar can very suitably be applied in different layer thicknesses to all kinds of substrates, with the mortar having good spreading properties and directly adhering well to the substrate while it has not yet set, so that the mortar can, for example, suitably be applied to vertical surfaces.

Spraying is a suitable mortar application method Yet an other advantage of the process according to the invention is that, if a brick masonry pattern is created, the appearance of the ultimate layer very closely approximates that of brick masonry.

Mortars that are suitable for being used in the process according to the invention can, for example, contain a water-soluble polymer as the thickening agent.

Examples of such water-soluble polymers are polyvinyl alcohol, polyvinyl sulphonate, polyacrylic acid, polymethylacrylic acid, partly hydrolyzed polyvinyl acetate, cellulose and modified cellulose.

Preferably, a modified cellulose is used.

Examples of suitable modified celluloses are methyl hydroxyethyl cellulose, hydroxyethyl cellulose and sodium carboxymethyl cellulose.

Preferably, the mortar contains 0.1-4 parts by weight of thickening agent per 100 parts by weight of sand and/or gravel. Even more preferably, the mortar contains 1-3 parts by weight of thickening agent per 100 parts by weight of sand and/or gravel.

Mortars that are suitable for use in the process according to the invention may, for example,

contain kieselguhr, precipitated and ground silicon dioxide, fumed silicagel and the like as the thixotropic agent.

Preferably, use is made of kieselguhr.

Kieselguhr can for example be recovered from diatomaceous earth and is commercially available.

Preferably, the mortar contains 0.01-0.8 parts by weight of thixotropic agent per 100 parts by weight of sand and/or gravel. Even more preferably, the mortar contains 0.1-0.5 parts by weight of thixotropic agent per 100 parts by weight of sand and/or gravel.

Mortars that are suitable for use in the process according to the invention may, for example, contain 4-20 parts by weight of binder per 100 parts by weight of sand and/or grinder. The mortar at least contains a synthetic resin as binder. Although the mortar may contain cement, for example up to 4 parts of cement per 100 parts by weight of sand and/or gravel, it is preferred to leave the cement out of the mortar altogether, for the above-mentioned advantages are best achieved and the mortar can be marketed as a single- component system.

Preferably, the mortar contains 4-12 parts by weight of synthetic resin per 100 parts by weight of sand and/or gravel and no cement. More preferably, the mortar contains 5.5-9 parts by weight of synthetic resin per 100 parts by weight of sand and/or gravel and no cement.

The process according to the invention can be used to coat surfaces of various substrates with layers with various stone patterns. It is for example

well possible to coat inside walls, outside walls, partitions, etc. Examples of suitable substrates are stone, concrete, wood, fibreboard, polystyrene foam and other stiff insulation materials and plaster.

It is also well possible to coat not only substrates with only a smooth surface, but also substrates with complex surfaces, such as surfaces curved once or twice, multi-surfaces, such as, for example, stepped surfaces, etc.

The surface to be coated can be cleaned, for example with a degreasing agent and/or water, before the coating process.

The surface to be coated can be pretreated in the manner known to a person skilled in the art, depending on the nature of the substrate. The surface of a porous substrate can for example be impregnated with a synthetic resin to reduce the surface's absorption capacity. The surface can also be pretreated with an adhesive. It is also possible that the surface is not pretreated at all.

The mortar that is used in the process according to the invention contains sand and/or gravel.

The amounts and the types of sand and gravel chosen depend on for example the stone pattern that is to be obtained. It is possible to use mixtures of sand with different particle sizes. It is for example possible to use a mixture of masonry sand and very fine quartz sand, which has a favourable effect on the mortar's workability. The employed sand preferably has a particle size of up to 3 mm. The particle size of the employed gravel is preferably less than 10 mm, even more

preferably less than 5 mm.

The amount of water employed can, if necessary, be adapted just before the mortar is applied, so that the best possible workability of the mortar is obtained. If the mortar is too dry, it is not well possible to distribute the mortar across the surface in a homogeneous layer. If the mortar is too wet, there will be the problem of the mortar sagging, especially on vertical surfaces.

The binder, being at least a synthetic resin dispersion, is preferably in the form of an emulsion in water, so that the synthetic resin can be easily mixed with the mortar's other components using the customary processes and equipment for the preparation of mortars.

The emulsion preferably contains 40-60 parts by weight of the synthetic resin and 60-40 parts by weight of water.

Good results are achieved if an acrylate polymer is used as the synthetic resin.

Preferably, the mortar contains one or more pigments, for example cement pigments. It is possible that the ultimate layer with the stone pattern has virtually any desired colour, so that it is possible to imitate all kinds of stone surfaces as faithfully as possible.

The mortar is usually applied in a layer thickness of 5-10 mm, preferably with the aid of the known techniques for applying decorative plaster.

Immediately after the layer has been applied, or after a short waiting time, the stone pattern is created in the layer of mortar which has not

yet set. This is usually done by cutting joints into the layer to create a stone pattern, for example a pattern of brick masonry, sandlime bricks or natural stones, depending on the employed mortar and the shape and size of the joints. Preferably, the joints have a depth that approximately equals half the thickness of the layer of mortar in which the stone pattern is created.

Preferably, the cut joints are subsequently filled with a joint mortar. This way a very faithful imitation of a stone brickwork surface can be obtained.

The joint mortar may have a composition known for joint mortar. The joint mortar may also have the composition of the mortar used for the layer containing the stone pattern, but the joint mortar then preferably has a different colour.

The invention also relates to a mortar as used in the process according to the invention.

Example I In a concrete mixer a mortar was prepared using 15 kg of masonry sand with a particle diameter of up to 3 mm, 2.5 kg of acrylate synthetic resin emulsion containing 50 wt. % synthetic resin, 150 g of hydroxyethyl cellulose, 50 g of kieselguhr, 125 g of mixing oil and such an amount of water that a mortar that spread well was obtained.

The mortar thus obtained was applied to a 20-mm-thick chipboard in a layer thickness of 10 mm with the aid of a spatula.

Next, grooves imitating the shape of joints were cut into the layer, in a brick masonry pattern. A

regular pattern was obtained, and after the layer had set the grooves could be filled with a customary joint mortar.

Comparative experiment A In the same way as in Example I a mortar was prepared that contained 15 kg of the masonry sand, 2.5 kg of the synthetic resin emulsion, 125 g of mixing oil and water. In the same way as in Example I a layer was applied and a stone pattern was cut into it.

Approximately 7% of the bricks had an irregular shape because part of the layer of mortar had broken away at one of the corners when the joint was cut.

Comparative experiment B In the same way as in Example I a mortar was prepared that contained 15 kg masonry sand, 2.5 kg of the synthetic resin emulsion, 0.3 kg of cement, 125 g of mixing oil and water. In the same way as in Example I a layer was applied and a stone pattern was cut into it.

Approximately 4% of the bricks had an irregular shape because part of the layer of mortar had broken away at one of the corners when the joint was cut Example II Similar to Example I, but this time the mortar also contained 100 g of a red cement pigment.

After the mortar had been applied and had set, a layer having a deep red colour was obtained.

Comparative experiment C Similar to comparative experiment B, but this time the mortar also contained 100 g of the red cement pigment used in Example II. After the mortar had been applied and had set, a layer having a pale red colour was obtained.

Comparative experiment D Similar to comparative experiment B, but this time 250 g of the red pigment was used rather than 100 g. After the mortar had been applied and had set, a layer having a deep red colour, comparable to the layer of Example II, was obtained.

Comparison of the results of Example II and the comparative experiments C and D shows that, if use is made of the process according to the invention, the mortar can contain less pigment to obtain a result comparable to the known process.