as shown in fig2 the preferred waterproofing material ( 10 ) of the invention is a laminate consisting of a core layer ( 11 ) containing montmorillonite . the core layer ( 11 ) is united with a support sheet ( 12 ) and is desirably but not essentially overlaid by a cover sheet ( 13 ). the essentials of the material ( 10 ) of the invention will probably be best apparent from a detailed description of the way it is made and the apparatus which is used to make it . referring , therefore , to fig1 and 3 , it will be seen that preferred apparatus of the invention comprises one or more silos ( 14 ) containing particulate sodium bentonite clay linked by respective feeder pipes ( 15 ) to a high speed , high shear mixer ( 16 ). supply means for liquid components , namely water into which other constituents , such as alcohol , polyacrylates , carboxymethyl cellulose , have been dissolved , takes the form of tanks or nozzles ( 17 ) leading into the mixer ( 16 ). a mixing paddle ( 18 ) is shown diagrammatically in fig3 . the mixer ( 16 ) is connected by a duct ( 19 ) to an extruder ( 20 ) which includes a vacuum chamber ( 22 ). within the extruder , there is a cavity for receiving a deformable mass , which reduces in dimension from the end adjacent the inlet duct ( 19 ) to the outlet at die ( 24 ). the aforesaid cavity is divided into two compartments by a perforated partition ( not shown ). a conventional worm or screw extends the length of the cavity , including through the partition and serves to convey the kneaded mixture received the mixer ( 16 ) through to the die ( 24 ). as the material is forced through the apertures of the partition it is further mixed and compressed . the tapering shape of the cavity contributes to compression of the material , which is then subject to suction ( vacuum ) as it approaches the die ( 24 ). this removes any air trapped or potentially trapped within the mixture so as to avoid any gap or weakness in the eventual waterproof sheet . the die ( 24 ) is formed with a circular aperture and fitted with a conical or bell shaped insert so that the material is extruded in annular form , as shown in fig1 . this believed to be the most effective way of obtaining an even pressure across a distance of anything from 1 to 4 m ( approximately 3 . 25 to 13 . 0 feet ) and obtaining a pipe having a diameter of between 30 and 170 cm and a thickness between 4 and 6 mm . the resulting pipe of extruded material is immediately slit at the top , as indicated in fig1 by positioning a knife or blade at that position , adjacent the die ( 24 ). the material ( 30 ) is then supported upon rollers as it is gradually uncurled and lowered onto a roller conveyor ( 26 ). support and cover sheets ( 32 , 34 ) of geotextile material ( woven or non - woven ) are brought into engagement with the waterproofing sheet ( 30 ) as indicated in fig3 . their adhesion to the waterproofing material ( 30 ) may be enhanced by passing between one or more pressure nips ( not shown ), as this will more reliably embed the fabric of the sheets ( 32 ) and / or ( 34 ) into the deformable waterproofing layer ( 30 ). the final laminate is formed into a roll ( 36 ) ready for storage or transportation . as shown in fig3 a , the particles of partially hydrated bentonite clay and to come into alignment as the degassed homogenised material is forced through the die ( 24 ). the support sheet ( 32 ) is preferably a sheet of woven material which is relatively loose weave , being quite porous in a direction transverse to its plane . it can be made of any geotexile material which is suitable for disposal within the ground for long periods . typical materials for weaving or forming the fabric of the sheet ( 32 ) can be polypropylene , polyesters including nylon , and many other plastics materials alone or in blends . the material should be sufficiently strong to support the composite laminate to be formed and can be similar to many of the facing sheets used in relation to the prior known materials discussed in the introduction hereto . polypropylene and cotton mixers can also be used . a typical support and / or cover sheet can be of a print weave and of a weight from 30 to 2000 g per square meter . the particulate montmorillonite can be supplied to the hopper ( 14 ) from a mill or like supply and in the preferred embodiment is of 200 mesh . finer mesh can be sued although great advantages are not obtained . meshes up to 50 mesh can be used , but at sizes greater than 100 mesh , union between the montmorillonite particles is less effective . the process which takes place in the mixer ( 16 ) can be either a continuous or a bath process . within the mixer ( 16 ) a measured quantity ( i . e . weight ) or montmorillonite is mixed with a measured quantity of liquid from the tanks ( 17 ) to produce a shapable mass . in making a typical product in accordance with the invention 5 kilogrammes of montmorillonite were mixed with 0 . 446 kilogrammes of sodium carboxymethyl cellulose ( cmc ), 2 . 5 liters of methanol and 1 . 8 liters of water . both the cmc and the methanol make the mixed and kneaded product more flexible and extrudable . although the above particular mixtures have proved suitable many variations can be made . methanol alone or water alone can be used , but neither of these is satisfactory . the material desirably contains a bulking agent , an anti - fungicidal preserving agent , to prevent growth of mould in or on the material and desirably a lubricant to assist in the extrusion process and provide a degree of flexibility to the plastic mass . cmc is a very desirable substance in that it provides all these properties . it has anti - fungicidal properties , it is a lubricant and it makes the product more flexible . it also has the great advantage that upon contact by water , in use , it dissolves . those areas of the outer surface of the material when first contacted by water have the cmc dissolved out of them leaving micro pores into which more water can penetrate , wash out more cmc and cause rapid expansion of the adjacent montmorillonite . this greatly increases the rate of water transfer into the material . a bulking agent which dissolves in water and aids water ingress to the montmorillonite is very desirable . instead of being provided by a single material these properties can be provided by other materials . although many synthetic materials do have these properties , they tend to be expensive and simple plat extracts which are much cheaper than desired . as a bulking agent / lubricant guargum can be used or starch . in connection with these two materials a separate preservative such as any conventional anti - fungicidal or anti - microbial agent would have to be used . any convenient liquid alcohol can be used having from 1 to 12 carbon atoms . above c12 alcohols tend to be too viscose for use but below that number any convenient alcohol can be used . it is expected , however , that methyl alcohol will be used because of its cheapness and easy availability . the cmc can be in the form of sodium carboxy methyl cellulose or any other convenient compound thereof . protection against bacterial attack is important because the bacterial reactions can produce hydro carbons which react with the sodium ions in the clay . this can reduce the swellability of the clay . the montmorillonite used is desirably sodium montmorillonite but calcium montmorillonite or treated calcium montmorillonite and other smectites can also be used . if reinforcement is required within the montmorillonite layer in order that it can be laid on steep slopes without loss of function it can be desirable to incorporate a reinforcing layer within the plastic mass , as shown in fig4 and 5 . this can be done by embedding a reinforcing layer ( 40 ) into the mass ( 11 ) as it is being extruded , the result being shown in fig4 . in this respect , it must be mentioned that the plastic mass of waterproofing material can be extruded as a sheet , rather than a pipe which is slit to form a sheet , within the scope of the invention , although extrusion as a sheet does not produce such a good evenly spread layer when such a wide sheet is required . the reinforcing layer can be made in the form of a core ( 42 ) having bristles ( 44 ) or comparable formations extending outwards , as shown in fig5 which , with the cored ( 42 ) disposed centrally in the body of montmorillonite ( 11 ), extend to the surface thereof and contact and possibly project through the surface layers ( 12 , 13 ). the material of the reinforcement and the surface layers can be made such that the exposed bristles can be heat sealed to contact and be secured to the outer layers . it is envisaged that it would be possible for the montmorillonite mass to be extruded or formed into a pair of sheets and the reinforcement fed between them and to have its bristles projecting through each of the two part layers of the montmorillonite core and project to the other surfaces thereof and be united with the support / cover sheets . the substance which convert the powdered montmorillonite into a plastic fluent mass may need some degree of treatment , for example by evaporation , drying or partial chemical change to ensure that the final material cannot deform further in use or in storage . this can be effected by means of a treatment facility ( not shown ) which the finished laminate is passed through . when the liquid in the mixture is essentially water or an evaporable liquid the treatment facility will be in the form of an oven and it may reduce the solvent water convent of the montmorillonite containing layer from 20 % down to 5 % or less . after leaving the treatment facility the laminate can be allowed to cool and then be fed to a store roll in conventional manner . a knife or the like can be provided for cutting the laminate as it leaves the oven when the roll is full . as previously mentioned , the laminate may be passed between one or more pairs of rollers to cause the support / cover sheets ( 32 , 34 ) to be partially embedded in surface zones of the plastic mass of material forming the core ( 11 ) whilst the core is in a plastic state . there is no need for any adhesive , which is an expensive and unreliable component . if the core is treated , either by evaporation or chemically so as to cause the core to harden , there is a physical locking of the surface portions of the core ( 11 ) with portions of the fabrics ( 32 , 34 ) physically uniting them to the surface without the need for adhesive . firstly , because a good portion of the sheets ( 32 , 34 ) are embedded within the material of the core , only a small portion of the body of the fabric is exposed above the surface . thus , in use that fabric surface will be in contact either with anchoring overburden ( at least 150 mm of overlying material is recommended to protect such layers ) or the underlying earth . the overburden or the earth penetrates the fabric quite easily ( it is a very open fabric and after there is intimate contact between the overburden and the underlying earth ). this again has two important consequences . firstly , once the support layer ( 32 ) ( which will normally be in contact with underlying earth ) is intimately connected by the earth ground water enters contacts the montmorillonite and causes swelling which creates a seal . it is a further advantage that because of the intimate contact of the underlying soil or the overburden with the montmorillonite through the support and cover sheets ( 32 , 34 ) there is no possibility that either the cover sheet ( 34 ) or the support sheet ( 32 ) can allow any venting of gas laterally through the fabric . the second advantage of this is illustrated in fig6 and 7 . in fig6 a first piece ( 47 ) of the laminate material of the invention is shown overlapping a lower piece ( 48 ), both lying on the ground ( 49 ). the overlap cover sheet ( 50 ) of the second sheet ( 48 ) is in contact with the support sheet ( 51 ) on the piece ( 47 ). as illustrated in fig7 the sheets ( 50 ) and ( 51 ) are in intimate contact and they are significantly penetrated by montmorillonite from the respective cores of the two laminate sheets . upon entry of water in the direction of arrow ( 52 ) or ( 53 ) the montmorillonite in one or each of the cores can swell and expand into the unfilled portions of the fabric ( 50 , 51 ) and forming effectively a continuous layer of expanded montmorillonite uniting the two cores and providing a completely water tight seal . the invention is not limited to the precise details of the foregoing and variations can be made thereto . as well as montmorillonite , saponite and other smectites can be used . in carrying out a preferred process a batch of about 60 kilogrammes was prepared , the figures given in the following being percentage by weight for the various components . firstly , 25 % water was added to a mixer , followed by 16 % sodium polyacrylate . to these was added 5 % methyl alcohol . when these three had been mixed half of the total bentonite load of 63 % was added . once the mixture had become smooth 1 % carboxy methyl cellulose ( cmc ) and a small ( about 0 . 1 %) of sodium hexametaphosphate was added . both these materials were added slowly and after they had been added the mixture was stirred for some while . thereafter the other half of the bentonite was added , the mixture kneaded for a short time and then passed to an extrusion machine wherein it was first driven towards a perforated plate whence it emerged in vermicelli - like form into a vacuum chamber . in the vacuum chamber air and any other gases such as reaction products and probably some evaporated alcohol are extracted . the percentages of the various materials used can be varied as follows : the function of the pressure during extrusion is to increase the density of the product by eliminating voids which might otherwise form within a less than coherent mass . this , together with the vacuum step which has removed air has the effect of compressing the material to a high density . this moves the molecules slightly closer together during extrusion thus increasing the rate of reaction and encouraging the formation of a complex from the intercalated polyacrylate . desirably the density is greater than 1 , 000 kilogrammes per meter 3 ( 62 . 43 lb / ft 3 ) and a preferred density is over 1 , 300 kilogrammes per meter 3 . the process described above produces a sheet material which can be used to form an ideal barrier against aggressive ionised fluids . such ionised fluid will usually be leachates from plants or sites or may be atmospheric water or ground water contacting the capping of a landfill site . it has been found that fertiliser and other materials which may be applied to foliage above a landfill site forms a highly ionised material as aggressive as any leachate and which can seriously damage conventional bentonite liners . the material referred to above has the acrylate or other liner so securely attached to the bentonite interlayers that the cation exchange capacity ( cec ) of the material is nil or very low . this means that there is no possibility of the smectite turning to a calcium form which will not reswell after drying out . further , as the liner is preferably a plastics material the inherently stable nature of the polymeric plastics material makes the possibility of it being attached by leachate or strong solutions quite remote .