The present invention relates to the moulding of shaped articles from cement paste, in some embodiments the articles being useful for the preservation of cooking oil during frying. It also relates, in some embodiments to shaped articles obtainable by the above process.

BACKGROUND TO THE INVENTION WO 2008/015481 (Bratton et al), the disclosure of which is incorporated herein by reference, discloses a method for preserving cooking oil in a fryer during deep fat frying which comprises treating the oil while in situ in said fryer during said deep fat frying with an hydraulically set product consisting of >50wt% of a mixture of milled white OPC clinker and white OPC, optionally silica l-2wt% and/or titania (Ti02)l-2 wt%. The solid material may be derived from OPC 20-35 wt% of (OPC + clinker) and clinker 65-80wt% of (OPC + clinker) e.g. from OPC about 25wt% of (OPC + clinker) and clinker about 75wt% of (OPC + clinker).

WO 2009/019512 (Bratton et al), the disclosure of which is incorporated herein by reference explains that mixtures of cement clinker and cement can give rise to bodies having a surprisingly favorable combination of porosity and mechanical strength. It is also said to be surprising that macroscopic bodies of the hardened paste are effective to remove impurities from oil and in particular that impurities may become trapped within the structure of the body which may be a stand-alone disc or briquette so that contaminant removal in some embodiments is not a mere surface phenomenon.

Commercial prototypes were shown in e.g. Figs 12a-12d of WO 2009/019512. The samples employed were cast in profiled moulds nominally 150mm x 100mm x 20mm with 15 x 20mm holes perforating them to increase the available surface area and promote good oil circulation when located in deep-fat fryers. The perforation feature allowed an increase in available surface area and thus promoted good oil circulation with a path length to their centers of 5-50mm, e.g. 10-30mm and in an embodiment about 25mm. The properties of a typical product were: Mass Volume Surface area Surface area:

Material type

(g) (cm ³ ) (cm ² ) Volume

10mm disk 19.63 19.64 54.98 2.80

20mm disk 39.27 39.27 70.69 1.80

40mm disk 78.54 78.54 102.10 1.30 waffle -

204.00 203.73 483.04 2.37 prototype waffle -

182.00 181.73 464.53 2.56 production

SUMMARY OF THE INVENTION

The present invention is concerned with the production of articles of relatively large mass e.g. 50g in a manner that can be conducted on a large scale, the articles consistently exhibiting a porous microstructure useful for treating cooking oil. In some embodiments the invention is concerned with the production of such articles in open moulds by casting, where a similar microstructure is obtainable both at regions of the article that are in contact with the mould and regions that during casting are open to the atmosphere. Shaped articles may have masses e.g. of ~300g (or 280g), e.g. 500g, some embodiments lkg, e.g. 3kg. Such articles in embodiments may be generally planar with at least one through hole, e.g. 10-40 through holes e.g. 15 through holes. In some embodiments the shaped articles have a mass of 265-295g, nominally 280g, SD of 4.2. In other embodiments e.g. for use in fryers holding up to 3 litres of oil, shaped articles of lesser mass e.g. about 80g may be appropriate.

In one aspect the invention provides a method of moulding a shaped article having a porous structure, comprising:

introducing into a mould a paste comprising (a) clinker, gypsum and lime, (b)

OPC and clinker or (c) OPC, wherein initial set is allowed to take place in a first period at a relatively low temperature with minimal loss of water but sufficient to allow achievement of initial set, and

wherein further hardening to a green state in which the shaped article is removable from the mould is allowed to take place in a second period at a relatively high temperature and at high humidity with minimal or no loss of water.

Embodiments of the invention provide a method of moulding a shaped article having a porous structure, comprising:

introducing into a mould a paste comprising OPC 10-50wt% of solids, OPC clinker 90-50wt% of solids and water and allowing it to harden,

wherein initial set is allowed to take place in a first period at a relatively low temperature with minimal loss of water but sufficient to allow achievement of initial set, and

wherein further hardening to a green state in which the shaped article is removable from the mould is allowed to take place in a second period at a relatively high temperature and at high humidity.

In some embodiments the setting reaction gives rise to porous structures which are permeable to free fatty acids and trans fats in cooking oil and promote reaction between impurities in the oil and the cement. In embodiments porosity extends throughout shaped articles made from cured cement which on immersion in water behave in the manner of a sponge. It is believed (although the working of the invention is not dependent on this belief) that the initial setting step carried out at ambient or slightly elevated temperature and with loss of water controlled by the high humidity environment results in partial curing with preservation of the porous structure that was present in the paste, and that this porous structure is preserved in the subsequent further hardening step where green strength is attained and in subsequent operations.

In a further aspect the invention provides a porous article hydraulically moulded from (a) clinker, gypsum and lime, (b) OPC and clinker and (c) OPC,

said article having a face that was adjacent the mould with a nominally even pore distribution of 25-35% by area with pore sizes 2-10μπι, a mid-depth having < 1- 2μπι pores evenly distributed and representing 20-25% by area and a face furthest from the mould having 2-5 μπι pores representing 20-25%) of the surface by area, and having a water absorption measurable after drying at 120°C of 26-30 wt% and/or an oil absorption measurable at an oil temperture of 36-40°C and on drying at 120°C of 14-20 wt%.

The population of relatively small pores at mid-depth in the moulded article is believed to be helpful in reducing or controlling foaming, and it is also believed (although the working of the invention is not dependent on the truth of this theory) that fatty acids become attracted to and held within this mid-depth region and polymerise so that they cannot be returned into the oil. The face that was furthest from the mould may have a population of inwardly extending fissures or channels of size 5-50μπι providing further access to the internal structure of the article and may carry a layer of calcite crystals from free water at the outer face of the article during the curing operation. Embodiments of said article having a 3-point bend strength of 5-9.5 Mpa, a bulk density of 1.6-1.8 and when placed in cooking oil at frying temperature does not give rise to substantial foaming (i.e. do not interfere with the operation of the fryer or with cooking operations).

BRIEF DESCRIPTION OF THE DRAWINGS

How the invention may be put into effect will now be described by way of example only with reference to the accompanying drawings, in which:

Fig. lis a plan view of a mould for moulding articles according to the invention; Fig. 2 is a section on the mould taken along the line A-A of Fig. 1 and Fig. 3 is a trimetric top view of the mould;

Fig 4 is a plan view of the mould place on a stacking frame, Fig 5 is a trimetric top view of the stacking frame and Fig. 6 is a trimetric underneath view of the stacking frame;

Fig. 7 is a side view of a stack of the frames and moulds;

Fig 8 is a part sectional view of two moulds and two stacking frames;

Figs. 9 and 10 are graphs of water and oil uptake against time for various moulded articles;

Figs l la-l lg, 12a-12i and 13a-13i are micrographs of portions of moulded articles showing pore structure; and Fig. 14 shows strength and porosity for a number of samples made as described in Example 1.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Cement and cement paste

Hydraulically settable materials include inorganic materials e.g. hydraulic cement, gypsum hemihydrate, calcium oxide, or mixtures thereof) which develop strength properties and hardness by chemically reacting with water and, in some cases, with carbon dioxide in the air. Examples of known hydraulic cements include the broad family of Portland cements (including ordinary Portland cement without gypsum), high alumina cements, calcium aluminate cements (including such cements without set regulators), silicate cements (including β-dicalcium silicates, tricalcium silicates, and mixtures thereof), magnesium oxychloride cements, geopolymer cements (Pyrament- type cements), macrodefect-free (MDF) cement, densified with small particles (DSP) cement and a-dicalcium silicate which can be made hydraulic under hydrating conditions.

Materials used in embodiments of the invention are hydraulic cements. This means that the materials react with water to form a cementitious reaction product (calcium silicate hydrate (CSH) gel) that acts as "glue" which binds the cement particles together. The most common cement is Portland cement.

Portland cement and Portland cement clinker which may be used herein are made primarily from a calcareous material such as limestone or chalk and from alumina and silica both of which are found in clay or shale. Marl, a mixture of both calcareous and argillaceous materials is also used. The raw materials are ground in a large rotary kiln at a temperature of around 1400°C and the materials partially sinter together into roughly shaped balls usually a few millimeters in size up to a few centimeters. This product is known as clinker and up to now has been used almost exclusively as an intermediate in the production of cement. When it has cooled it is then ground to a fine powder and some gypsum is added to give a final product known as Portland cement.

Particularly suitable filter treatment materials are white ordinary Portland cement (OPC), white OPC cement clinker and combinations thereof. Clinker for forming such cements is kept as low as possible in transition metals e.g. chromium, manganese, iron, copper, vanadium, nickel and titanium and e.g. Cr ₂ 0 ₃ is kept below 0.003% or in some embodiments 0.005, Mn ₂ 0 ₃ is kept below 0.03%, and Fe ₂ 0 ₃ is kept below 0.35% in the clinker or in some embodiments below 0.5%, the iron being reduced to Fe(II) to avoid discoloration of the cement. Limestone used in cement manufacture usually contains 0.3-1% Fe ₂ 0 ₃ , whereas levels below 0.1% are sought in limestone for white OPC manufacture, levels < about 0.3wt% being desirable and BaO levels of < about 0.02-0.03wt% also being desirable since excessive barium can cause cracking Free magnetic iron is preferably present in amounts <0.005wt%, excessive amounts of free magnetic iron in some embodiments causing flaking on the back face of the moulded articles. Apart from the white color which gives rise to products which are aesthetically pleasing and promote food industry and final customer confidence, the low transition metal content helps to minimize leaching of undesirable ionic species into the oil, especially iron and aluminum. Furthermore white OPC and white cement clinker contain relatively few iron and copper sites which can accelerate oxidation processes within the oil. White OPC clinker e.g. from Aalborg (which is 97% ground clinker plus lime) has the following composition with phases represented as Bogue composition:

C ₃ S 65.0%

C ₂ S 21.0%

C ₃ A 5.0%

C ₄ AF 1.0%

CaS0 ₄ 0%

Production of cement from clinker involves grinding and addition of 2-10 wt% CaS0 ₄ . Aalborg white OPC has added calcium sulfate and has the following calculated Bogue composition (corrected to free lime content):

C ₃ S 66.04%

C ₂ S 20.1%

C ₃ A 4.64%

C ₄ AF 1.04%

CaS0 ₄ 3.45% Lime and gypsum in OPC will be varied by manufacturers depending on the available starting materials for cement manufacture in order to give industry standard reactivity. However, contents (wt%) may be as indicated below and the gypsum content being calculated from the S0 ₃ figure

The present articles may be made from a mixture of OPC and OPC clinker, the clinker being the major component. In embodiments the mixture is derived from OPC 15-35 wt% of (OPC + clinker) and clinker 65-85 wt% of (OPC + clinker), e.g. in one embodiment about 25 wt% of (OPC + clinker) and clinker about 75 wt% of (OPC + clinker) and in a further embodiment OPC about 20 wt% of (OPC + clinker) and clinker about 80 wt% of (OPC + clinker). It will be appreciated that the OPC and OPC clinker should be thoroughly mixed as with a mechanical mixer for optimum properties of the moulded article.

Where (a) clinker, gypsum and lime, (b) OPC and clinker or (c) OPC are used these may together comprise 100 wt% of the treatment material (apart from incidental ingredients as aforesaid) or they may comprise >50 wt%, typically >75 wt%, more typically >90 wt% of the treatment material. The further ingredients that may be used in combination with (a) OPC clinker, gypsum and lime, (b) OPC clinker and OPC or (c) OPC or a mixture of any of them may be selected from calcium silicate, magnesium silicate, feldspars (natural) (albite), zeolites (natural & synthetic) (Na & Ca forms), silica (amorphous and crystalline)/sand, wollastonite, calcium hydroxide, alumina (hydrated), aluminium silicates, clays (bentonite, perlite), pillared clays, activated clays/ earths, talcs/kaolinite, other silicate minerals (amphiboles, granite porphyry, rhyolite, agalmatolite, porphyry, attapulgite) etc. A further material that may be used according to the invention as treatment material with and without OPC and clinker is calcium silicate. However, the applicants have tested forms of calcium silicate as well as titanium oxide (see above) as additives, but these failed to provide any across the board advantage from a simple 2- material powder mix. Other incidental ingredients that may be added to (a) clinker, gypsum and lime, (b) OPC and clinker or (c) OPC, include titania (Ti0 ₂ ) typically in an amount of 1-2 wt% to promote whiteness and strength and/or silica typically in an amount of 1-2 wt% to promote strength. It is desirable, however, to select materials that are compatible in particle size to the cementitious materials e.g. clinker and OPC. For example, incorporation of Ti0 ₂ may lead to a significant reduction in effectiveness, probably because pigment grade Ti0 ₂ has a particle size of 0.25 μπι and is effective to at least partly block the internal structure of the material.

Various fillers and aggregates may also be included in the mouldable compositions including sand, clay, silica sand and other inorganic materials. However, the use of fillers of this kind is not preferred.

The cement clinker as supplied is of particle size 2-20mm and is milled to a similar particle size distribution to the cement e.g. to a nominal size of about 14.5 μπι.

It is preferred where possible to start with clinker which has a relatively narrow distribution of particle size because unevenness in particle size distribution is reflected in unevenness in the particle size distribution of the resulting milled product. The closer the particle size distributions are of the OPC and the OPC clinker the less well-packed the resulting particles will be and hence the higher the porosity of the resulting cast or moulded article. PSD's of the cement and clinker may be in the range dlO 2 - 3.5 μπι, d50 12-17μιη and d90 35-100μιη.

Unmilled clinker, being of relatively large particle size is relatively insensitive to moisture and can be stored in air e.g. in sacks or bags, although storage should be under dry conditions. After milling moisture sensitivity is increased, and if the milled clinker absorbs moisture then hydrated phases may start to appear which may be harmful to the properties of the final product. For that reason either the clinker should be used immediately after milling or it should be stored in moisture-impermeable bags or containers e.g. bags lined with polyethylene. Similarly OPC is liable to deteriorate in storage owing to the presence of moisture and should be preserved dry prior to use.

CaS0 ₄ in OPC acts as a retardant and extends setting time, and in the present mixtures its proportion is lower than usual. For that reason pastes made from these clinker-rich mixtures have a relatively short working life after addition of water e.g. about 30 minutes at ambient temperatures. Some extension of working life may be obtained by agitation and by cooling the water used to form the cement paste, by external cooling of the paste during and after addition of water and/ or by adding compatible set retarders.

In a variant use of OPC may be omitted and calcium sulfate and optionally lime may simply be added to clinker e.g. in 25 wt% of the normal amounts. Mixing at the dry powder stage may be facilitated by the fact that the dry powder is in some embodiments added gradually to a vortex of stirred water and formed into paste.

An important feature of compositions for moulding is water content. By definition, water is an essential component of a hydraulically settable material. The hydration reaction between hydraulic binder and water yields reaction products that give the hydraulically settable materials the ability to set up and develop strength. The preferred amount of added water within any given application is primarily dependent upon several variables, e.g. (a) the amount of water required to react with and hydrate the binder, (b) the amount of water required to give the hydraulically settable composition the necessary rheological properties and workability, and (c) the amount of water needed, where porosity is aimed at, to achieve a desired level of porosity. In order for the composition to have adequate workability, water must generally be included in quantities sufficient to wet each of the components and also to at least partially fill the interstices or voids between the particles e.g., of binder and aggregate if present. Furthermore the amount of water should in most cases be sufficient that there are no domains of the moulded product where unreacted cement remains. In some embodiments the amount of water is such that when the paste or other composition has been introduced into the mould, a faint sheen of water is apparent on the upper or exposed surface of the composition, but the amount of water is insufficient that relative movement of the particles is too free leading to a runny mixture that is difficult to control, or that a free-flowing layer of water develops on the composition.. The appropriate solids to water ratio for any composition and end product properties will vary depending on the materials used and the fineness of the particles present, a fine mixture generally requiring a greater relative amount of water, and for each case needs to be determined by experimental trial. Stoichiometric hydration requires a ratio of water to cement of about 25wt% (on the basis that OPC + clinker = 100 wt%), but it is standard practice for workability to add water in excess of that required for hydration e.g. in amounts of 28-42wt% e.g. 32- 37wt%, in some embodiments about 35 wt%. Cement solids have a specific gravity of about 3 so that if a paste is formed with more water than is stoichiometric and no water is lost during the curing process the resulting cured cement article will have a porosity that is significantly greater than would be expected simply on comparison of the weights of the ingredients added. In some embodiments the OPC: clinker ratio may be from 0.176 to 0.667 and the solids:water ratio may be from 0.176 to 0.667.

It is desirable to use demineralised water for hydration and for subsequent process tasks e.g. washing as described below.

The hydraulic reaction of cement powder with water is complex. The component oxides shown in the table above combine to from four main compounds. These are

C ₃ S, Tricalcium silicate 3CaO.Si0 ₂

C ₂ S, Dicalcium silicate 2CaO.Si02

C ₃ A, Tricalcium aluminate 3CaO.Al ₂ 03

C4AF, Tetracalcium aluminoferrite 4CaO.Al ₂ 03.Fe ₂ 03 These compounds react with water to form calcium hydroxide and hydration products generally known as gel. One relatively fast reaction which causes setting and strength development is the reaction of tricalcium silicate which is the major and characteristic mineral in Portland cement with water to give the so-called C-S-H phase of cement according to the equation:

2Ca ₃ Si0 ₅ + 6H ₂ 0→ 3Ca0.2Si0 ₂ .3H ₂ 0 + 3Ca(OH) ₂ .

A further reaction which gives rise to "late" strength in cement is the reaction of dicalcium silicate with water also to form the C-S-H phase of cement:

2Ca ₂ Si0 ₄ + 4H ₂ 0→ 3Ca0.2Si0 ₂ .3H ₂ 0 + Ca(OH) ₂ .

Moulds

In embodiments of the present process the mould is of a deformable non- metallic material of low thermal conductivity and removal of the article is by deformation of the mould. The mould may be open-topped and may exhibit no resistance to release of a moulded article from the mould through its open face once the article has been released by mould deformation e.g. by manual pressure or by the application of a hand or other tool.

Conventional high volume cementitious paste casting moulds are manufactured from alloy materials that are rigid and require complex and expensive ejector mechanisms or dismantleable mould pieces. Owing to the length of time required for curing cement, if continuous manufacturing is to be performed then a large number of moulds is necessary, often three times the number of articles to be moulded per cycle. In order to ensure clean ejection, alloy moulds often require PTFE or similar coatings that abrade with use. Such moulds require off-site recoating and refurbishment which either necessitates yet further moulds to be manufactured or process closedown whilst refurbishment is carried out. If the shape of the intended moulded article has to be changed then a correspondingly large number of moulds have to be changed or replaced. The high capital cost and on-going maintenance and management of change associated with alloy moulds restricts the use of cementitious materials and increases the product cost to the customer.

Embodiments of the present process employ a flexible mould that is manufactured from inexpensive plastics materials by injection moulding or vacuum forming, the moulds being inexpensive to manufacture and therefore needing only a limited mould lifespan. This has the advantages of quick time to market, low unit cost, simple change management through natural wastage, easier cleaning and lower impact on the product cost, enabling products to manufactured at a lower price and in greater variety. Many of the mould plastics can be reground and recycled to manufacture further moulds once the moulds have worn out and been replaced or used for other plastic moulded products.

Low temperature, e.g. 0-120°C, casting and curing processes may use low temperature-rated plastics. A suitable low cost low surface-energy material is LDPE which may be injection moulded at very low cost and with low wall thicknesses, e.g. 0.5-3mm. The resulting mould may exhibit sufficient rigidity to accurately form a desired casting without distortion, but also a measure of flexibility that allows simple manual or mechanical de-moulding and ejection of the component on deliberate distortion of the mould. Mechanical pusher rods, stripper plates and other expensive components normally associated with alloy mould or other rigid mould designs are not needed. Other materials that could be used include generic polypropylene. Generic plastics with similar properties may be substituted specialist plastics such as DuPont Hytrel, Zytel and similar nylon based materials depending on the size and process requirements of the mould. Modifications to cast component feature detail may be easily effected by alteration to the injection mould tooling with inserts or direct machining of feature alterations in the mould and then reproduced in high volume by standard injection moulding processes.

If a mould is to be discarded after a single use a preferred alternative manufacturing method for the mould is thermoforming, specifically vacuum forming. In this case the vacuum formed mould may be reproduced at exceedingly low cost with a very thin wall and stripped away like consumer "blister pack" packaging which may be discarded and preferably recycled for re-use.

An embodiment of the mould for a 3 x 5 aperture generally rectangular briquette has a base flange 12 for distributing the weight of cement paste or slurry and for supporting sidewalls 10. A top surface 3 of the sidewalls provides a reference surface for the mould cavity 17 Underside support feet 14 prevent distortion - location and use depend on mould support requirements for any specific shaped cast component. Mould cavity 17 is provided for casting into and for providing provide the component external profile, Upstand features 18 create depth detail and any required apertures. Tooling holes 15 provide positioning registration for regular arraying of the moulds in a horizontal matrix on a baseboard with cooperating location pegs for mould-filling (not shown) and for vertical registration with other cooperating system components if stacked vertically. Stiffening ribs 16 are optionally provided to facilitate ejection of an injection moulded mould component when manufactured with thin a wall section that is liable to flex during ejection if ejector pins are provided in the corners of the component. To create through-apertures within the moulded part the upstand features 18 are preferably coplanar or above mould top face 13 to which face the mould is normally filled and in many embodiments wiped to create a flat cast component surface. All Sidewalls 10 and the internal cavity 17 and internal upstand features 18 are drafted towards the top face 13 to facilitate ejection off the injection mould or thermoform tooling as well as to facilitate demoulding of the component cast within this mould. Moulds may be designed to be filled fully to the open "top" surface and then screeded to create a flat cast product face. Alternatively the mould may be deeper than the intended final product and may be filled to a predetermined lower volume by dispensing an exact volume or to an exact level required for the product to create a desired cast product thickness. The second method has the advantages that it is simpler, there is no screeding step and consequential waste of material, no screed mechanism need be provided, and the operations of mould and process line and filling station cleaning are reduced. For removal of the article the mould may be inverted and may be removed manually or by an ejection tool that may e.g. push down on what are then downwardly-facing regions of the mould defining through-holes in the article.

First period - up to initial set

As is well known after initial mixing of the cement there is a period of dormancy during which C3S and C2S slowly dissolve but super saturation with Ca ²⁺ has not been reached and the cement is workable. Supersaturation with Ca ²⁺ triggers formation of crystals of calcium silicate hydrate (CSH) and calcium hydroxide (CH) that eventually mesh together and cause the paste to stiffen. For casting the present articles water loss during the dormant period which normally is substantial is reduced (but not entirely eliminated) so that the microstructure at the time of casting is preserved so far as possible and shrinkage of the paste which promotes water expulsion is avoided. An extended period before initial set also allows equilibration between material adjacent the mould surface and material adjacent the free surface so that differential water loss which could give rise to cracking of the moulded product is avoided. For that purpose initial set is allowed to take place in a first period at a relatively low temperature e.g. ambient temperature ~(20°C) with minimal loss of water but sufficient to allow achievement of initial set. Standing time during this initial period may be e.g. from 12 to 36 hours and the mould will not be agitated or disturbed in this period so as not to interfere with development of the initial set. In other embodiments the standing time may be 12-36 hours at 28-32°C, e.g. 30°C.

In an embodiment the following procedure may be used. 80 parts by weight of ground clinker and 20 parts by weight of OPC are dry mixed e.g. in a drum mixer and 35 parts by weight of demineralised water is added. This amount of water is a general guideline and the weight of water will vary according to solubility of the dry materials which in turn may vary depending on the particle size. A test volume measurement for the amount of water required to achieve a correctly hydrated slurry or paste must be performed. Pre-mix powder slowly to a vortex of water in a mechanical mixer and mixing is continued until the slurry is of an even consistency. The amount of water should be such that the paste or slurry is not too runny but when placed in the mould a water sheen is apparent on the free surface of the mix.

When mixing is complete, the individual moulds may be filled as follows. Moulds are placed on a vibrating table in array of e.g. four moulds. Using a jug ~0.5kg of slurry or paste is removed at each time and each mould is slowly filled, the nominal final dried product mass in an embodiment being ~330g. The moulds in the array may be filled to -75% full each (~225cc) and then topped up with the balance of ~75cc while the vibration is applied. When the four moulds in the array have been filled, vibration may be continued for a short period to allow any remaining air to escape. The moulds are allowed to stand for a relatively short period e.g. -two hours to permit final escape of any trapped air and placed in an environment where water loss is minimised and then covered with a sheet of water-impermeable material such as polyethylene which touches the tops of the moulds and is taped down so that it retards but does not entirely prevent escape of moisture. In an alternative procedure the moulds are simply tapped with a hammer to assist escape of air, there being little or no applied vibration. Again moulds deeper than the depth of the intended product may be employed as mentioned above.

Second hardening period - achievement of green strength

After initial set has been achieved further hardening to a green state in which the shaped article is removable from the mould is allowed to take place in a second period at a relatively high temperature and at high humidity. During this period hardening to the green state takes place with as little loss as possible of the porosity that has been preserved in the article at the end of the first period. Water loss in the second period is therefore also minimised to minimise loss of pore size. The pores which are present at the end of this period are retained during subsequent processing steps. The second hardening period may in some embodiments be for -2-6 hours at ~30-60°C and at -70- 100%RH, e.g. for ~4 hours at about 40°C and at ~95%RH. A material in its green state indicates that it has cured sufficiently for form stability but has yet to achieve its final strength.

To facilitate curing within the mould to achieve adequate robustness to allow demoulding of the component without breakage or distortion it is necessary to hydrate the curing cement product through provision of an ambient high relative humidity. That can be achieved by placement in a closed chamber with a controlled source of moisture to provide the necessary RH and appropriate temperature to accelerate cure to achieve the desired product parameters. When employing a closed chamber with a controlled RH, to simplify handling, a stacking system is envisaged that provides an interlocking framework permitting stable multiple stacking of moulds yet allowing access of essential moisture for the cement hydration process and egress of any process gases (normally C02) to prevent mould distortion through development of over-pressures between stacked moulds.

There is provided a rectangular plastic frame that sits on the flanged base of a mould as shown in Figs 1-3 providing an interlock feature to provide keying of the stacked moulds, a separator feature that fixes the vertical distance apart to facilitate a body of high RH air above the mould face and vent slot features in the sidewalls to provide access to high RH air and for the removal of reaction gases, typically C0 ₂ .

With reference to Figs 4-8, a mould 14 as shown in Figs 1-3 is filled and laid down on a flat surface on mould flanges 20. Main frame 21 is laid on mould 24 such that the inner profile 23 of frame 21 cooperates with the upstanding body features of mould 24 to locate and orient the frame in the horizontal plane. A bottom land 29 of the frame rests on top of the first mould flange 20 and traps it in place, this effect being increased as every further mould and frame is added to the stack.

Frame 21 has a bottom corner relief notch 27 located in each of the four underside corners to provide clearance to stiffening mould ribs 21 located in the corner flanges of the mould 24. Frame 21 provides four upstand pegs 22, one in each corner, that register and cooperate with mould corner holes 22, while the mould flange 20 is supported on upstand support lands 24 distributed around the frame 21 top surface. Vent Slots 25 are provided around frame 21 to permit a free flow of air - in this application often with an introduced level of high relative humidity e.g. >70%, in some embodiments >95% RH. Upstand pegs 22 project above the upstand support lands 24 and they are taller than the thickness of mould flange 20 to provide registration for the next Frame 21 and mould 24 to be located on top of the current stack. Peg receiving holes 23 in the bottom corners of the upper frame 21 cooperate with the upstand pegs 22 of the lower frame 21 to provide good registration and stability to the stack.

Flexible plastic moulds often take a set or display some small corner flange distortion and for this reason top corner relief notches 26 are provided in each corner to allow for some tilting of the mould flanges 20 in the corners without affecting stack stability. Bottom mass relief slots 28 of the general form shown provide means of reducing the overall mass of usually rigid plastic employed in manufacture of the frame 21. It will appreciated that the load of each mould and the composition with which it is filled is transferred to its frame, and the loads are then transferred from one frame to another so that the load path does not pass through the individual moulds. Third period

After the second hardening period the article may be removed from the mould and allowed to harden for a third hardening period at a higher temperature than for the first period and while maintaining its water content.

In one embodiment the article is returned to a humidifi cation chamber and further hardened in a high humidity atmosphere at an elevated temperature, again with minimal or no loss of water. The third hardening period may be for 2-6 hours at 30- 60°C and at 70-100%RH, e.g. for about 4 hours at about 40°C and at about 95%RH. After the third hardening period the article may be water-soaked for 0.5 to 5 hours e.g. at ambient temperatures for about 2 hours to remove any loose material and surface fines and to cure any unreacted material.

In another embodiment the third hardening period and soaking may be combined. After the second hardening period the article may be removed from the mould and may be subjected for a third period to combined hardening and soaking in water, the water being at a higher temperature than that of the first period. For example, in some embodiments the soaking in water may be at 30-60°C for 2-6 hours e.g. at about 40°C for about 4 hrs. However, the temperature and time conditions specified above may be more than needed and in some embodiments the water soak can be at ambient temperatures for 0.1-5 hours e.g. 0.25-lhour.

In either case the material is converted from a green state to a further hardened state, and the further hardening is with both the mould regions and the previously open regions subject to the same hardening environment, so that all surfaces of the final article tend towards similar porosity and the resulting moulded article is of improved performance and lifetime.

Final treatment

In some embodiments, drying may be at 70-120°C at 0.01-lbar for 2-12 hrs e.g. at 120°C for 4 hrs at 1 bar. For example, the cured article may be placed in racks in an oven at 120°C (RH nominally ambient) for e.g. ~ 4 hrs, raising the temperature from room temperature at a rate of 10°C every 5 minutes. Drying weight loss may be 20-33 wt% under the above specified conditions of 4 hours at 120°C. The product is then cooled and packed e.g. in cardboard boxes with plastics sheet separators between successive layers of product.

Porosity can be estimated by standing a cement article in water until the article is saturated with water, drying it in an oven e.g. at about 100°C - 120°C so that free water (i.e. water which has not become combined as water of crystallization in the cement) is driven off, comparing the water- saturated and dry weights and adjusting for the density of the cement. Estimated in this way, porosities of >10%, e.g. 30-50% are desirable e.g. -34—45%, increase in porosity significantly beyond about 50% in some embodiments giving rise to articles of reduced mechanical strength, the porosity being due to physical voids within the article.

In embodiments, the articles have a water absorption of 25-45 wt%, measurable as 26-30 wt% after drying at 120°C and 30-42 wt% after drying at 220°C, and an oil absorption in the case of an article that has been dried at 120°C as used in manufacture of 14-20 wt% at an oil temperature of 36-40°C. It may have a strength of 5-9.5 MPa and a bulk density of 1.6 to 1.8. Its top surface (i.e. the surface that was nearest the mould) may a nominally even pore distribution of 25 to 35% by area with pore sizes to be between 2mm- 10mm and at least 15% by SEM inspection. Its mid-depth region may have <l-2mm pores evenly distributed and represent 20-25%> by area with at least 15% by SEM inspection and with an even homogeneous distribution of small well bound solids regions. Its lower face (i.e. the surface that was furthest from the mould) may have 2-5 μιη pores & 5-50μιη fissures covering 20-25% of the surface by area, with at least 15%) by SEM inspection and with an even carpet of features having the appearance of coral/polyps/plates across whole surface plus a small amount of debris.

The invention will now be further described in the following examples.

EXAMPLE 1

Test pastes were made from white OPC clinker, white OPC and demineralised water in the proportions indicated below. Particle size data for the starting materials were as indicated below:

Each paste was slightly over-filled into a mould for a rectangular block formed with a 3 x 5 pattern of holes. The mould was of LDPE of hardness 70 Shore D and wall thickness 1.6 mm. It exhibited sufficient flexibility to permit manual de-moulding. Trapping one narrow end (140mm wide) of a 189mm long mould and applying a torque of 0.7Nm about the long axis at the opposite end twists mould to produce a 12° angular twist. The mould before filling was mounted on a vibration table and after filling was vibrated for a few minutes to allow air to come to the surface and bubbles to break, after which surplus paste was removed. The paste was then cured in situ at ambient temperatures and at 95% RH for 24 hours, further cured to a green state for 4 hrs at 95% RH, released from the mould, yet further cured for 4 hrs at 95% RH and water soaked for 2 hours in demineralised water. It was then dried overnight at 1 10°C, ramping the temperature from ambient at 10°C/5minutes to permit most of the water to be removed in a controlled fashion without breakdown of the product structure. . (P2) (P4) (P7)

OPC 18.9% 25.0% 12.6% 17.0% 14.8% 20.0%

Clinker 56.7% 75.0% 61.5% 83.0% 59.3% 80.0%

Water 24.5% 32.4% 25.9% 35.0% 25.9% 35.0%

Total 100.0% 100.0% 100.0%

Porosity of the product was determined by water absorption and mechanical strength was determined by an Instron or MORI 3 -point bend test. Results were as indicated below:

From the standpoint of processability, the P2 mixture was considered to have too high a water content for its constituent materials and the paste was runny. The P4 material was too dry, the top surface of the paste in the mould had a matt appearance leading to undesirably low surface porosity. The P7 paste was firm but on insertion into the mould exhibited surface sheen indicating the presence of water throughout the paste up to the top surface, and likely optimum surface porosity to both sides of the molded article.

Strength and porosity of a number of samples made by the above method is illustrated in Fig. 14. Example 2

Effect of age upon material properties

The effects of moisture e.g. as a result of prolonged storage in containers that are imperfectly watertight are shown by the following X-ray diffraction measurements on samples.

(a) Relatively new OPC clinker after milling exhibited the following phases with measured percentages:

C3 S (monoclinic) 71.35%

C2S (β) 25.09

C3 A (Na, orthorhombic) 0.99

C3A (Na, cubic) 2.60

(b) An older sample of milled OPC clinker exhibited the following phases with measured percentages:

C3 S (monoclinic) 56.41%

C3S (triclinic) 15.57

C2S (β) 23.54

C3 A (Na, orthorhombic) 0.33

C3A (Na, cubic) 2.81

Portlandite 1.35

(c) A sample of relatively new OPC exhibited the following phases:

C3 S (monoclinic) 74.29%

C2S (β) 18.92

C2S (y) 1.58

C3 A (Na, cubic) 3.36

Anhydrite 0.80

Bassanite 1.06

(d) An older OPC sample exhibited the following phases:

C3 S (monoclinic) 62.53%

C3S (triclinic) 5.13

C2S $) 22.31

C2 A (Na, orthorhombic) 0.35 C3A (Na, cubic) 3.37

Anhydrite 6.31

Bassanite 1.06

(e) A moulded product according to the invention exhibited in its upper region (nearer the mould) the following phases:

C3 S (monoclinic) 40.83%

C2S (β) 18.69

C2S (γ) 6.27

C3A (Na, orthorhombic) 3.33

Calcite 26.55

Portlandite 4.34

Crystallinity 72.0

(f) A moulded product according to the invention exhibited in its mid-depth the following phases:

C3 S (monoclinic) 37.93%

C2S (β) 24.79

C2S (γ) 2.27

C3 A (Na, orthorhombic) 3.98

Calcite 4.53

Portlandite 26.50

Crystallinity 69.0

(g) A moulded product according to the invention exhibited in its lower region (furthest from the mould) the following phases:

C3 S (monoclinic) 38.85%

C2S (β) 13.03

C2S (γ) 2.83

C3 A (Na, orthorhombic) 5.76

Calcite 13.72

Portlandite 25.82

Crystallinity 72.0

Deterioration in older samples of clinker or OPC is evidenced by reduction in monoclinic C3S content, phase changes and appearance of calcite or portlandite. Such deterioration, if allowed to proceed to far, gives rise to products which when placed in oil for frying give rise to foaming.

Example 3

Liquid uptake tests

Simple water and oil uptake tests can indicate filter structure and can be used for product screening.

Total "short-term" - typically 20-30mins - uptake (wt%) gives a measure of overall porosity due to larger pores accessible at atmospheric pressure - osmosis/wicking action may, over longer test periods give a higher porosity figure as the smaller pores eventually fill with the designated liquid. Taken with the total liquid uptake wt % the rate of uptake is an indicator of surface pore size and filter inner body general pore distribution %. Although these simple atmospheric pressure uptake tests are not absolute, the curves so generated may be used to screen known unacceptable product attributes without employing complex or expensive equipment and methods.

In tests whose results are shown in Figs 9 and 10 below four filters were evaluated, designated as follows:

Filter 1: This filter was from a batch that "foamed" in use with cooking oil and thus had unwanted internal chemistry, believed to be the result of due to ageing source clinker that may have pre-reacted prior to use in manufacture.

Filter 2: Known "good performer"

Filter 3: Made under recent pilot production conditions

Filter 4: Made under previous pilot production conditions, known "bad performer" with e.g. small pores at surface/low porosity.

A water absorption graph appears as Fig. 9.

As cooking oil viscosity varies significantly with temperature, for the oil uptake test sunflower oil was used at about 36°C to 43°C. In production a specific brand and type of oil will be nominated and maintained at a similar controlled temperature. In the oil uptake graph of Fig. 10 a "final" value is shown after about 2 hours immersion that occurs through wicking or osmosis. It approximates the BS standard vacuum porosity test process results where fuller pore filling is achieved. For the purpose of a quick production line test the approximately 20-30 min values provide adequate discrimination. Results were as follows:

Filter 1: Good wt% water absorption but medium to poor oil uptake indicating reasonable overall porosity, but too small average surface pore sizes - in the region of 2μιη diameter or less.

Filter 2: High and rapid overall wt% water absorption, indicating good overall porosity. High (-20%) oil uptake and rapid initial oil uptake rate indicating good (IRO 2μιη-10μιη or larger) surface pore size and distribution.

Filter 3: An initial pilot line filter exhibited water wt% not as high as the previous laboratory made sample, indicating lower overall porosity and was considered a threshold reject in terms of porosity. It was not tested with oil.

Filter 4: Low overall water absorption and similarly low oil uptake, also low oil uptake rate. These results indicate low overall porosity and small diameter/low distribution of pores at the surface.

Example 4

Electron micrographs

Electron micrographs were taken of filters corresponding to Filters 1, 2 and 4 of Example 3.

The micrographs corresponding to Filter 1 (foaming) are shown in Figs 11 a- 11 g, with the micrograph in each group increasing in magnification from top to bottom of the page. Figs 11a and l ib show the top (nearest the mould) with an average pore population of 15% and an average pore size of 3μπι. Coarse uneven surface is shown with a large range of particle and pore sizes, many large holes (40 to ΙΟΟμπι) with loose particle within them (14.5μπι). Bar length (indicating scale) is 200μπι in Fig. 11a and 50μπι in Fig. l ib. Figs l lc-e show the sample at mid depth with a pore population of 10%), pore sizes also about 3μπι and many large holes (40 to ΙΟΟμπι) with loose particle within them (14.5μπι). In Figs 1 If and l lg show the bottom of the sample (furthest from the mould) which has a rough dense back, very few pores, no coral or polyps present, and an appearance more like normal cement set. Pore surface population was 5%> and average pore size was 3 μπι. Bar lengths are 200μπι in Fig 11c and l id and 50μπι in Fig. l ie and 200μπι and 50μπι in Figs 1 If and 1 lg. Micrographs corresponding to Filter 2 (good performance) appear in Figs. 12a- 12i. In Figs 12a, 12d and 12g, bar lengths (appearing at the bottom of the micrographs) were ΙΟΟμιη, in Figs 12b, 12c, 12e and 12h they are ΙΟμιη, and in Fig 12i the bar length is Ιμπι. The top had a pore surface population of 28% with an average pore size of 2.5- ΙΟμιη. There were zones of pores (70%) and zone islands of solid material (30%) and a lot of debris and loose material. The mid-depth region had a pore surface population Of 25%) and average pore size 2-5μιη with an even homogeneous distribution of small well bound solids regions and an even distribution of small to medium sized pores with no nodules or plates. The bottom face had a pore surface population of 23% with average pore size 2-5μιη and fissures of size 5-50μπι. There was an even beautiful crystalline carpet across whole surface plus a small amount of debris.

Micrographs corresponding to Filter 4 appear in Figs 13a-13i. Scales were 300 μπι in Figs 13a, 13b, 5 μπι in 13c, 100, 10 and 2 μπι in Figs 13d, e and f and 300, 100 and ΙΟμπι in Figs 13g, 13h and 13i. At the top of the filter the pore surface population was 8%), pore sizes being about 2μπι with a very even distribution of pores & particles - fine crystalline appearance and a few large flakes 3 to 5μπι. At mid depth pore surface population was 4% and pore sizes <2μπι with very fine pores and particles - crystalline appearance, some large flakes >5μπι. At the bottom the pore surface population was 2% with average pore size 2-5 μπι and with smooth areas interspersed with pores, very regular crystalline appearance.

Example 5

Preparation with de-moulding followed by immediate water soak

Clinker and OPC both stored in a dry state without the possibility of moisture ingress and having following measured PSDs (all in μηι) were employed and were formed into a pre-mix powder ising a dry powder mixer:

Source dlO d50 d90

OPC: 2.08 8.72 36.32

New clinker: 2-3 13-15 40-60 Pastes were made up by mixing clinker and OPC pre-mixes and demineralised water in the proportions indicated below, the water being placed in a mechanical mixer and pre-mix powder being added slowly to the vortex of the water, mixing being continued until an even slurry was obtained. :

Each paste was filled into a mould as described in Example 1, but the mould was not mounted on a vibration table but instead the filled mould was tapped with a flat-edged piece of wood or metal to remove air bubbles with a minimum of agitation so as to minimise separation of particles within the paste. The filled moulds were then maintained as close as possible at 100% RH in a humidity chamber having a floor temperature of 28°C and a top temperature of 32°C, the moulds being located in a mid- height region where the temperature was about 30°C, and were allowed to stand for 24 hours to achieve initial set, the slightly elevated temperature being selected for ease of control of temperature and humidity and also to slightly speed setting. After initial set, the moulds were placed in a humidity chamber under 95-100%) RH and at 40°C for four hours to complete in-moulds curing and achieve green strength.

De-moulding was by inverting the mould with the article present in it and depressing the upstand features 18 e.g. using push rods or a release tool to break the adhesion between these features and the adjoining surfaces of the moulded article, after which the sidewalls 10 may be flexed if necessary to break the adhesion between them and the moulded article, which could then be removed, optionally with slight finger pressure on the mould. Where a release tool is employed, it may advantageously operate on only some of the upstand features 18, e.g. the two outer rows of 5 upstand features but not those of the central row. The de-moulded products were then placed on production racks and washed/soaked in demineralised water at ambient temperature for >15 minutes to promote curing of unreacted cement and to remove loose material. It is believed that the wash/soak step immediately following de-moulding was possible at least partly as a result of the slightly elevated temperature in the initial humidity chamber as compared to Example 1. The washed products were then placed in an oven, heated to 120°C at a rate of 10°C every 5 minutes and dried at 120°C for 4 hours. The dried articles were allowed to cool in a dry environment and packaged in moisture-=resistant material immediately after drying.

The used moulds were cleaned in a sonic bath filled with deionised water and/or citric acid solution for up to 2 hours, washed with deionised water and dried ready for re-use.