Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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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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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
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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
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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 .
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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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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
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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
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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
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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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