The present invention relates to a plaster mix which may be used in the formation of low density plaster products.

The word 'plaster' is used to denote a solid product based on calcium sulphate dihydrate which has been formed by the hydration of a calcium sulphate having initially a lower state of hydration suc?ι as, for example, calcium sulphate hemihydrate. The words "plaster mix" are used to denote a mixture of ingredients from which plaster may be produced, the mixture comprising either the dry ingredients or the paste, slurry or dispersion which is initially formed when these are mixed with water. Plaster may also be referred to as gypsum. Plaster may be used for a variety of purposes. There is a large scale use of this material for constructional purposes either as an interior housing wall or -ceiling surface or in the form of sheet boarding which may be used in conjunction with a timber framework in the construction of the interior walls and ceilings of buildings.

There is also a widespread use of plaster as an absorbent material for liquid spillages or wastes. Plaster may be used in a suitable particular form as a pet litter or as an absorbent for ^' oil spillages on floors. Other uses for plaster may be as a carrier for active chemicals for agricultural or horticultural purposes, as a soil conditioner or as a modelling material.

It is highly desirable that plaster should have a relatively low density. Low density plasterboard can enable cost savings to be made in its transport and handling. Low density pet litter can be attractive to customers because of its ease of handling and because of its high liquid capacity per unit weight. Low density, however, tends to be accompanied by a reduction in strength which can manifest itself in the partial breakdown of granules to produce fines during transport or use and in increased losses in the transport or use of boarding due to accidental impact damage.

It is known that an increase in the quantity of water present in a plaster mix can cause a reduction in the density of the plaster. United States Patent No. 4163674 relates to the production of liquid-absorbent granules of plaster by mixing calcium sulphate hemihydrate with a controlled quantity of water. The granules had a bulk density of from 26-4 to 51.8 lbs/cu. ft. (0.42 to 0.83 g/cc) which was achieved by mixing plaster and water in an •amount of from about 50 wt % water to 50 wt % plaster to about 15 wt % water to 85 wt % plaster.

United States Patent No. 3616173 relates to the - production of fire resistant wallboard having a gypsum core and to the control of the density of the core with water to be at least about 35 lbs/cu. ft. (0.56 g/cc). The core additionally contains a fine particle size clay such as a kaolin or a bentonite in from 0.5% to 20%, preferably from 2% to 5% which is found to have an effect on the shrink resistance and degree of sag of the plaster and also contains glass fibres and vermiculite.

It is an object of the present invention to enable the production of plaster having a relatively low bulk density but nevertheless having a high resistance to crushing.

The present invention provides a plaster mix capable of setting to yield a plaster having a relatively low bulk density the -mix comprising a calcium sulphate capable of

hydrating in the presence of water to yield calcium sulphate dihydrate, a swelling clay and water and being characterised by:

(a) a content of water in excess by weight of the calcium sulphate capable of hydration

(b) a content of aluminium sulphate

(c) relative proportions of swelling clay and aluminium sulphate selected so as to achieve in the plaster product a bulk density below 0.5 g/cc and preferably below 0.42 g/cc and a crush strength of at least

2.0 kg and preferably of at le-.st 2.5 kg. Methods for the determination of the bulk density and the crush strength of plaster are described below.

The invention further provides a dry plaster mix comprising a calcium sulphate capable of hydrating in the presence of water to yield calcium sulphate dihydrate and a swelling clay and being characterised by relative contents of swelling clay and aluminium sulphate selected to achieve on the addition of water in a quantity in excess by weight of the calcium sulphate capable of hydration a plaster product having a bulk density below 0.5 g/cc preferably below 0.42 g/cc and a crush strength of at l∞ast 2.0 kg and preferably of at least 2.5 kg.

The ingredients used to put the present invention into effect will now be described more particularly. Calcium sulphate is available in several forms capable of hydration to the dihydrate such as the gamma and beta forms of anhydrous calcium sulphate and calcium sulphate hemihydrate. The last compound is preferred since it is available readily as a product which is already adapted to t_..a satisfactory production of plaster products.

The swelling clay which is utilised may be selected from layered aluminium or magnesium silicates, which may be naturally occurring minerals or synthetic products, which are capable of expanding by the adsorption of water betwefr i the layers. A number of layered magnesium or

aluminium silicates have the property of swelling to an appreciable degree notably naturally occurring or synthesised silicates of the smectite ineralogical group which includes the montmorillonites, the hectorites and the sapσnites, amongst other groups of minerals.

Attapulgite and sepiolite minerals are also capable of swelling and may be-utilised while kaolinites generally either do not swell or do so to an insufficient degree to make them utilisable in the practice of this invention.

Preferably the swelling clay mineral is selected from the montmorillonties or is synthetic hectorite. Amongst the montmorillonites sodium montmorillonite, often known as sodium bentonite, has a high degree of swelling and is preferred.

Aluminium sulphate may be provided in the form of anhydrous aluminium sulphate or a hydrate thereof (e.g. .16 H2O) or by an aluminium sulphate containing material such as the alkali metal or ammonium alums or any other material delivering aluminium sulphate to the mix.

It is fundamental to the practice of the invention that a very large quantity of water, relative to the quantity of calcium sulphate, is used. Very suitably -the quantity of water is from greater than 100 g to 150 g per 100 g of the calcium sulphate capable of hydration. The incorporation of such a large quantity of water in the mix gives rise to problems associated with a tendency for a proportion of the water to separate on the surface of the plaster during setting thereby reducing the quantity of water effective to reduce bulk density in the interior of the plaster and adversely affecting the surface quality. Preferably not more than 130 g of water is used per 100 g of the calcium sulphate.

The quantity of aluminium sulphate is of considerable importance and is to be selected carefully to achieve a suitable product. As increasing quantities of aluminium sulphate are used the strength of the plaster product is found to increase until, at a relatively high level, the

strength of the plaster begins again to decrease. With an increasing quantity of aluminium sulphate the bulk density of the plaster is found also to increase and soon reaches an unacceptable level. It has been found by scanning electron microscopy that as increasing quantities of aluminium sulphate are used the crystal shape of the plaster changes to give an increasing cross-sectional dimension relative to length and, also, a reduced proportion of small size crystals resulting in an increased average crystal length to above 3 microns.

If the quantity of swelling clay which is present in the mix is increased this is found to have an adverse effect on the strength of the plaster but is found also to cause a reduction in the bulk density of the plaster. By suitably adjusting the relative quantities of swelling clay and aluminum sulphate in the light of this new information a plaster having a bulk density of below 0.42 g/cc and a crush strength of at least 2.5 may be produced. Preferably the quantity of aluminium sulphate is selected within the range of 5 g to 12 g per 100 g of calcium sulphate capable of hydration and the quantity of swelling _^ clay is selected within the range of 0.2 g to below 2 -g per 100 g of said calcium sulphate where the clay used is sodium bentonite. More preferred proportions are 8 to 12 g of aluminium sulphate and 0.5 to 1.5 g sodium bentonite on the same basis. The control of the quantity of swelling clay is within narrow limits. Where clays other than sodium bentonite are used, for example, clays which show a lesser degree of layer separation such as calcium and magnesium montmorillonite, attapulgite or saponite, the quantity thereof which is required may have to be determined on a functional basis having regard to the properties of the plaster required. It is envisaged that such clays be present in less than 5 g/100 g calcium sulphate. Besides its effect in modifying the effect of the aluminium sulphate the use of a swelling clay enables

the large quantity of water which is required to be used without surface separation during setting.

Depending on the end use envisaged for the plaster product the plaster mix may have incorporated in it various other ingredients and the plaster may be processed in various ways during or after formation. The setting rate may be adjusted by the addition of accelerators such as potassium salts for example the sulphate, or by seeding with calcium sulphate dihydrate or by the addition of retarders such as polyphosphates, for example sodium tripolyphosphate, or acetic acid. If a foamed product is required this may be achieved by adding a carbonate such as calcium carbonate which reacts appropriately with the aluminium sulphate. If a litter product is to be produced the plaster may be formed in the form of sheets, allowed to set for a time and then broken into pieces of the approximate size required or alternatively crushed finely and reformed by agglomeration or pelleting processes to the size required.

It is a feature of this invention that the litter so produced has excellent clumping properties. The term 'clumping' refers to a tendency of a litter to form a-firm self-adherent mass or clump at a point of wetting. As a result of this, when a cat urinates on the litter, the urine is localised in a clump which may be removed bodily using a scoop or other suitable implement, leaving the remainder of the litter uncontaminated. Preferably the clump is relatively deep, rather than being flat, for easy removal, is firm enough to retain its integrity fully during removal and is large enough to absorb all of the urine. It has been found, according to the invention that while the product generally has very good clumping ability the degree of this ability is related to the particle size of the product which is preferably from 0.25 to 1.5 mm and particularly preferably from about 0.5 to about 1 mm as isolated using sieves having openings corresponding to these sizes. In a trial of a product fraction having

sizes from 0.5 to 1.0 mm it was found that an addition of 25 ml of water to a bulk of the material a clump having a weight of 67.8 g formed which was extremely firm and could be removed without any problems of shedding peripheral material. Material of this size range produced according to the present invention may have a bulk density of from about 0.3 to 0.5 g/cc. The low density litter produced according to this invention, as well as having excellent clumping ability is relatively dust free and has a low tread-out tendency.

If a plaster board product is to be produced the plaster mix may have incorporated into it suitable fibrous materials, for example glass fibre and fillers or other additives known in that art.

The plaster product of the invention was subjected to the following tests to determine its suitability.

The granules were weighed into a 250 ml measuring cylinder up to the 250 ml mark (w) . Using an Engelsmann jolting volumeter JEL-ST-2 the sample was given six raps.

•The volume of granules (v) was noted. w Bulk density = - gms/cc v

Using a Stevens CR Analyser and plotter a number of individual granules were crushed and an average recorded.

CR Analyser settings:

Cycle = Return

Load = Peak

Speed = 10 mm/minute

Distance = 1 mm

The crushing strength of each granule was determined as the first peak on each plot. The result was indicated in kg.

The invention will now be more parti larly described with reference to the following Examples in ^' which a number of plaster mixes were prepared some being within (Ex 6, 7) and some outside (Ex 1-5) the scope of this invention.

The procedure for preparing the mixes having the compositions indicated in the following Table was:

The sodium bentonite, if used, was added to the plaster and blended to give a homogenous powder. The water and the aluminium sulphate were mixed until all the aluminium sulphate was dissolved and a homogenous solution formed.

The solution of aluminium sulphate was mixed at ~1000 rpm while the powder was added to the solution over as short a period of time as possible without forming lumps.

The resulting slurry was quickly poured onto trays and allowed to set without disturbance over approximately 3 minutes. The trays were then dried at 110°C over a period of several hours until the water content reached approximately 10% wt. The dried plaster slabs were granulated and sieved to between 1 and 5 mm size. The finished product was placed in sealed plastic bags to prevent moisture absorption.

The composition of the mixes and the test results are summarised in the following Table.

Ex Calcium Sodium Water Aluminium Test Results No. Sulphate Bentonite Sulphate Crush Bulk Hemihydrate Strength Density g g g g kg g/cc