CA1038140A

CA1038140A - Constructional units

Info

Publication number: CA1038140A
Application number: CA230,286A
Authority: CA
Inventors: James N. Lowe; Byron C. Grebe
Original assignee: GREBE BYRON
Current assignee: GREBE BYRON
Priority date: 1974-08-07
Filing date: 1975-06-27
Publication date: 1978-09-12
Also published as: AU8376775A; IN143100B; JPS5145118A; FR2281337A1; DE2532423A1; IT1041445B; NL7509452A; GB1466773A; BE832221A; ZA754064B

Abstract

"Constructional Units"

ABSTRACT OF THE DISCLOSURE

A constructional unit is cast from a mixture containing calcined gypsum as a binding agent and water forming a fluid composition by pouring the composition into a mould where it is allowed to set so that it is self supporting and is warmed by the heat of reaction of the gypsum with the water and then the unit so formed is provided with a water resistant surface by applying a moisture curing liquid synthetic resin which swells during curing to the surface of the unit. The resin is drawn into the surface of the unit by capillary action and is there cured by the warmth and moisture remaining in the set composition so that it fills and seals the pores of the surface. Preferably the synthetic resin, which is a polyurethane composition is applied to the surface of the unit before or immediately after it has been removed from its mould and while the unit is at a temperature of from 90°
to 180°F. The unit is then placed in a heat insulating and moisture impervious jacket to control the dissipation of heat and moisture from it and maintain its heat and moisture content for a period of at least two hours during which the resin is cured.

Description

1~)38140 It has previously been proposed to cast building panels or other constructional units from a mixture which contains calcined gypsum, which is gypsum partially dehydrated by means of heat and having the approximate 5. chemical formula CaS04. ~H20, instead of, or as well as, Portland cement to form a binding agent. The calcined gypsum may be autoclaved, which gives high strength or it may be atmospherically calcined. The calcined gypsum content in the mixture has the great advantage that the 10. cast unit gains adequate strength to enable it to be removed from its mould very much more quickly than can - ordinary Portland cement concrete or mortar. Indeed the mould, formwork or other support can be removed within thirty minutes or sometimes even less from the time of 15. casting and it is possible for the cast unit to be handled j -~ -and transported within a matter of a few hours after removal from its support. This makes it possible both to have a rapid turn round in use of the mould or other casting support and also to minimize the area occupied by 20. the cast units while they are gaining strength. Both these factors give rise to an increase in productivity and a consequent decrease in cost.
However, calcined gypsum after setting is soluble to some extent in ~ater and this has made it unsatisfactory 25. to use constructional units containing calcined gypsum for ~ !-~ .
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exterior purposes where they are exposed to the weather or for any otherpurposes where they become damp.
Attempts have been made to waterproof the exterior surfaces of such units, but it has always been thought that it was necessary to provide a ~ -continuous impervious film or coating over the surface which is to be rendered waterproof and the attempts so far made have not been successful.
The present invention provides a method of casting a constructional unit from a mixture containing by-weight, from 10% to 90% calcined gypsum and from 90% to 10% portland cements as binding agent and providing the unit with a water resistant surface, wherein the mixture is mixed with water to form a fluid composition, the composition is poured into a mould, casting form or other casting support and is allowed to set so that it is self-support-ing and is warmed solely by the heat of the reaction of the gypsum with the water, and then, while the set composition is still warm and moist, a liquid synthetic resin which cures in the presence of moisture is applied to the surface of the unit which is to be made water resistant, the resin being drawn into the set composition by capillary action and there being cured so that it fills and seals the pores of a surface layer of the composition.
This invention is based on our discovery that it is not necessary to provide a continuous impervious coating over the surface of the unit in order to make it water resistant but instead water resistance can be pro-vided by making use of the well-known characteristic of cast calcined ': .

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gypsum compositions of providing "suction" by capillary - action whilst the composition is still moist to draw the sealing resin into the surface layer of the composition and thus make the surface layer of the 5. composition itself water resistant instead of adding a water resistant coating to it.
We have found that a liquid moisture curing polyurethane resin composition is particularly satisfactory for use in the method in accordance with 10. the invention, but any moisture curing resin which swells on curing will effect the necessary surface layer seal and in particular there are a series of synthetic resins based on soya bean oil which are satisfactory for the i purpose.
15. Preferably the liquid synthetic resin is applied to the surface of the cast composition before or directly after the unit has been removed from its mould or other casting support, and while the composition of which the unit is formed is still being heated by the exother~ic 20. reaction of the calcined gypsum with the water with which it is mixed. The heat still being generated in the unit then assists in bringing about a rapid cure of the resin so that in effect the resin which impregnates the surface layer of the composition is to some extent baked as it 25. cures by the heat in the composition.

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1~38140 The method in accordance with the present invention can particularly advantageously be used in con~uction with the method described in our co-pending Application of even date herewith in which the liquid composition is heated before it is poured into the mould or other casting support and is then subsequently further heated by the heat of the exothermic reaction 50 that it reaches a temperature of from 90 to 180F.
In the method described in our co-pending Application, Number 230,411, the binding agent comprises a mixture of from 10% to 90% calcined gypsum and from 90% to 10% Portland cement. In order to cure the Portland cement and give a much higher eventual compressive strength than can be obtained by using calcined gypsum alone as the binding agent, the cast composition, after remoYal from the mould at a temperature of from 90 to 180F has the dissipation of its heat and moisture controlled so that the temperature and moisture content are maintained for a period of at least two and preferably three hours to cause the cement to be cured. During this curing time, the syn-thetic resin applied to the surface of the composition is also fully cured so that at the end of the curing period the surface of the unit already has its water resistance.
At the end of the period for curing the Portland cement, the unit is dried either by causing air to flow over its surface or by applying a vacuum to its surface, but the sealing of part of the surface of the unit which has been made water resistant means that the moisture in the composition can only be removed from such remaining surface 1 -5. area as there may be. This has the added advantage that it facilitates the maintenance of the moisture content of the composition during the curing of the cement and also it enables the drying of the composition to be accurately controlled. For instance, if an L-shaped unit is cast 3 10. having one flange twice as thick as the other flange, one face of the thinner flange may be made water resistant by the method in accordance with the invention so that during subsequent drying water is removed from the composition through both faces of the thicker flange but through only ~ `
15. one face of the thinner flange. This means that the rate of moisture removal is made substantially uniform throughout both flanges of the unit. This reduces internal differential stresses and thus avoids cracking of the unit which may ~ -otherwise occur due to uneven drying out.
20. An example of a method in accordance with the invention will now be described with reference to the accompanying diagrammatic drawings, in which:-Figure 1 is a layout plan of a plant for making building panels; ar~d, 25. Figures 2(a) to 2(c) are side views to a larger scale of parts of the plant illustrating the sequence of i , ~38~40 operations in the manufacture of the building panel.
The plant comprises a mixer l of the type having a mixing chamber containing two contra-rotating paddles and having a feed hopper 2 at its top for the supply of the 5. materials to be mixed together. A water supply pipe 3 discharges into the feed hopper 2 and is provided with a heater 4 for heating the water before it is discharged into the mixer. The mixer l has a flexible discharge pipe 5 fitted with a shut-off cock 6.
lO. Adjacent the mixer l is a pivotally mounted moulding table 7 which is arranged to be moved from a horizontal position shown in Figure 2(a) to an upright position shown in Figure 2(b). The moulding table 7 contains a mould 8 for forming a panel 9 with five parallel stiffening ribs 15. 10. ' On the side of the moulding table 7 remote from the ~i mixer l is a jacket structure ll comprising a number of heat insulating and moisture impervious walls 12 with t compartments 13 between them. The compartments 13 are 20. closed by end walls which are similar to the walls 12 and they are also closed at the top by removable covers 14.
All of the walls of the jacket structure 11 and also the covers 14 comprise a core of foamed polystyrene to provide the heat insulation with cover sheets of polyethylene to ~l 25. make them moisture-resistant. The walls 12 also comprise i .

1~38140 steel stanchions at intervals to enable them to support vertical loads and the covers 14 incorporate steel cross- I-beams which rest on the stanchions.
A fan 15 supplies air to a manifold 16 which has a 5. branch leading to the bottom of each of the compartments within the jacket structure ll.
A gantry crane 17 runs on rails 18 and spans the area in which the moulding table 7 and the jacket structure 11 are situated for handling the panels 9.
- 10. The panel 9 is made from a composition of the following constituents:-Autoclaved calcined gypsum 50 lb.
Finely-ground sulphate-resisting cement 50 lb.
15. Fluidising agent (Melment or Gum Arabic) 0.3 lb.
Sisal Fibre 0.5 lb.
Foaming agent (sodium lauryl sulphonate) 0.5 lb.

Retarding agent (sodium citrate) 0.05 lb.
Wax resin (in a water emulsion)l lb.
20. Water 38 lb.

The composition is mixed as follows:-Operation Time ElaPsed (a) 90% of water ladded to mixer l.

(b) Sodium citrate 25. dissolved in water in mixer.

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Operati ~ Q ~ ~ 4 Time Elapsed (c) Mixer operated at 40 RPM - I
90% of dry materials of the ~ -composition set out above added Datum time D
(d) Mix for one minute 1 Min.
5. (e) Foaming agent and further 10% of water added to mixer.
(f) Mixer speed increased to 55 RPM:
mix for ~0 seconds. 1~ Min.
(g) Final 10% of dry materials added 10. to mixer. Mix for three minutes. 4~ Min.
(h) Mix discharged into mould through pipe 5. 5 Min.
(i) Temperature of cast materials - starts to rise. 45 Min.
15. (j) Mould table tipped to upright position and panel removed from mould. 46 Min.

When the panel is to be load bearing, it is preferably reinforced and for this purpose a steel reinforcement cage 19 20. is fixed in a conventional manner in the mould 8 before the mixed composition is supplied to the mould from the discharge pipe 5. The reinforcement cage 19 incorporates a lifting eye 20, or alternatively when no reinforcing cage is required, a .
lifting eye 20 alone is cast into the panel 9.
25. After the mould table 7 has been tipped into its upright position, the panel 9 is lifted from the mould by the crane 17 to which a cover 14 of the jacket structure 11 is attached. I

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1038~40 The mould table 7 is then at once lowered to its original horizontal position ready for casting the next ¦ .
- panel and the panel 9 which has just been cast is rested upon the floor of the plant and is held suspended by the ¦;
5. crane in a vertical position. I' The face of the panel which is to form an external ¦ -face of a building structure, which in this case is the flat left-hand side face as shown in Figure 2(b) of the ¦ . .-drawings is now sprayed with a moisture curing liquid ¦ -10. polyurethane resin, although this may alternatively be done while the unit is still in the mould after the cast composition has set. The liquid polyurethane resin is .
drawn by capillary action into the hot moist composition ~ :
forming the panel and it is found that absorption takes 15. place to a depth of about 6 mm. Sprayijng continues until absorption ceases, and it has been found that an application of approximately 1 litre per square metre of panel.surface .
is typically required, but in many cases less complete impregnation calling for an application of 0.1 litre per 20. sq. metre may be all that is required.
Immediately spraying with liquid polyurethane resin has been completed, the panel is transported into one of the compartments of the jacket structure 11 and the compartment is at once closed by the cover sections 14.
25. The continuing exothermic reaction.of the water with the mixture of calcined gypsum and cement causes the temperature - 10 - .
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lQ38140 of the panel 9 to rise and the raised temperature is maintained by the heat insulating properties of the walls 12. Further, the relative humidity of the atmosphere within the compartment rapidly rises to 100% and curing 5. of the Portland cement in the composition then takes place in these hot moist conditions. The temperature of the panel may rise to from 140 to 180F and is usually about 160F. Curing may continue for from 3 to 5 hours or in some cases longer and the hot moist conditions are also 10. particularly favourable for rapid curing of the liquid resin contained in the pores of the surface layer of the flat face of the panel. As the resin is cured, it swells and accordingly the face of the panel is rendered completely water resistant. .
15. When curing of both the cement and the polyurethane resin is complete, the fan 15 is started and air is blown ; through the compartment containing the panel 9 and this together with the heat remaining in the panel evaporates the moisture from the panel and reduces its moisture 20. content to from 2 to 10% by weight. After an air drying : period of from 2 to 3 hours, the panel is removed from the ~acket structure 11 and at once has sufficient strength to enable it to be handled and transported. As an alternative to air drying, the panel may be vacuum dried by the

2~ application to that face which is not impregnated with resin of a vacuum pad or sheet. This sucks the water out ' - 11- ' ., ,: ' ..

~Q3~140 of the panel very rapidly and drying then only takes a few minutes.
Where the faces of the panel other than that or ~ -those which have been made water-resistant by impregnation 5. with polyurethane or some other moisture curing synthetic resin are to be painted or have other finishes applied to them, they are preferably first partially sealed by applying to them an acrylic latex emulsion after the panel has been dried. The acrylic latex emulsion unlike the moisture 10. curing synthetic resin which swells during curing, does not fully seal the pores in the surface layer to which it is applied, but allows the surface to "breathe" so that the moisture content of the panel can adjust itself through this face and no water vapour pressure is built up in the ., 15. pores of the panel as would occur if aIl of its faces were made water-resistant.
~hen some of the faces of the panel are to be coated with acrylic latex emulsion, the panel is preferably vacuum dried and when it is dried in this way, it is still 20. hot at the end of the drying operation and this heat can be used to effect rapid drying of the acrylic latex emulsion.
Either the composition from which the unit is cast or the polyurethane or other resin with which its surface is impregnalted, or both, may be coloured for 25. decorative purposes.
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Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method of casting a constructional unit from a mixture containing by weight, from 10% to 90% calcined gypsum and from 90% to 10% portland cement as a binding agent and providing the unit with a water resistant surface, wherein the mixture is mixed with water to form a fluid composition, the composition is poured into a mould, casting form or other casting support and is allowed to set so that it is self-supporting and is warmed solely by the heat of the reaction of the gypsum with the water, and then, while the set composition is still warm and moist, a liquid synthetic resin which cures in the presence of moisture is applied to the surface of the unit which is to be made water resistant, the resin being drawn into the set composition by capillary action and there being cured so that it fills and seals the pores of a surface layer of the composition.

2. A method according to claim 1, in which the liquid synthetic resin is a polyurethane resin composition.

3. A method according to claim 1, in which the liquid synthetic resin is applied to the surface of the cast composition while the composition is still rising in temperature from the heat of the exothermic reaction of the calcined gypsum with the water.

4. A method according to claim 3, in which the liquid synthetic resin is applied to the surface of the cast composition before or directly after the unit has been removed from its mould or other casting support and the dissi-pation of the heat and moisture of the composition is then restricted for a period of at least two hours and the synthetic resin is cured during this time.

5. A method according to claim 4, in which the dissipation of the heat and moisture in the composition is controlled by enclosing the unit in a heat insulating and moisture impervious jacket.

6. A method according to claim 4, in which an acrylic latex emulsion is applied, after the unit has been dried to a part at least of the surface of the unit which has not been rendered moisture resistant by the application of the moisture curing synthetic resin which cures in the presence of moisture.

7. A method according to claim 6, in which the unit is dried by the application of a vacuum to it directly after the temperature and moisture content has been maintained for the period of at least two hours and the acrylic latex emulsion is applied to the unit directly after drying and while the unit is still hot.