Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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30377
2813
This invention relates to a process for the manufacture of
shaped products by compression of a wet mixture containing calcium sulphate
dihydrate (CaS04.2I120), cement and sand.
It is known that shaped products can be made by compressing a
5 wet mixture composed of CaS04.2H20 and additives. The CaS04.2H20 may be
natural gypsum or, and more particularly, gypsum obtained in the chemical
industry, e.g. in the preparation of phosphoric acid. The additives may
be calcined gypsum (CaS04.~H20), anhydrite (CaS04), lime, cement and/or
sand.
The manufacture of shaped products by compression of such mix-
tures has been attended by one or more disadvantages, for instance as
follows:
1) Such mixtures generally require the use of high moulding pressures.
Thus in most instances very high pressures of the order of 400 kg/cm
is required to produce a shaped product that has a crushing strength
of 150 kg/cm , as is required in building elements used for load-
bearing structures. The presses normally used operate below such a
high pressure.
2) The shaped products in most instances have a low water resistance, and
effloresence in service unless special additiyes are incorporated.
3) If use is made of wet gypsum from the chemical industry, the presence
of large amounts of water increases processing costs due to the need
of an expensive drying-out operation or the incorporation of a large
proportion of calcined gypsum or dry anhydrite and lime.
4) The mixtures may be difficult to process, e.g. they may stick to the
ram of the press used in the moulding operation.
The invention is directed to a combination of components of cal-
cium sulphate-containing mixtures whereby such disadvantages are mitigated,
whereby moulded products of good crushing strength e.g. from 135 to
1~5 kg/cm , preferably about 150 kg/cm , are obtained.
The invention provides a process for the manufacture
of shaped products, comprising compressing a mixture containing
CaSO4.2H20, cement and sand, wherein
~1) the cement content of the said mixture is from
8 to 15% by weight based on the combined dry weight of the
CaS04.2H20 and sand,
(2) the water content of the said mixture is from
5 to 25% by weight based on the total composition,
(3) the sand comprises inorganic filler particles
having a grain size of up to about 4 mm,
(4) the pressure (P in bars) used and the said content
S of the said mixture expressed as the wt.% of SiO2 based on the
CaS04.2H20 content of the said mixture is according to the
formula
P = 0.06 (60-S)2 + 90 to 0.12 (60-S)2 + 90, and
(5) the shaped products are allowed to condition for
at least one day after compression, in a relative humidity from
80 to 100%, that shaped products having a crushing strength
from 135 to 165 kg/cm2 are obtained.
The calcium sulphate dihydrate used in the mixtures
in the process of the invention may be obtained from any source.
When the moulded products obtained by the process
of the invention are allowed to condition over a period of time
of at least two days, the moulded articles develop a crushing
strength between 135 and 165 kg/cm2.
By virtue of the relationship between the moulding
pressure and the sand content, the pressure used can be kept
comparatively low and the sand content be made relatively high.
The latter possibility has additional advantages, viz.:
(1) a relatively high sand content allows the use of
a wet gypsum such as that obtained from the chemical industry;
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~2) a higher sand content contributes to the ease
with which the mixture is processed, and
~3) a higher sand content results in lower radio-
activity of the shaped products if these are made with gypsum
from the industrial phosphoric acid preparation.
The sand content is from 25 to 40% wt. which according
to the invention corresponds to a moulding pressure of about
150 kg/cm2. Such a pressure allows the use of presses of the
type normally used. Preferably coarse sand is used having a
grain size of up to about 4 mm the sand preferably having not
more than 10% by weight of particles smaller than 250 microns.
By sand is meant any kind of inorganic filler particle having
said size and also includes porous particles for example ob-
tained by expanding clay, shale or mica.
The cement content used according to the invention
is in fact according to practical limits. A proportion of
cement above the range specified is too expensive, and a content
below the said range results in a final product of insufficient
strength.
Furthermore if the cement content is too high,
ettringite may be formed, which is undesirable in that it
causes cracking of the shaped products. For this reason
Port]and cement is preferably used, particularly Portland
cement B, to ensure sufficiently rapid setting. A cement con-
tent of 10 to 13% wt. is particularly useful.
The shaped products have a higher water-resistance
than shaped gypsum products made from the hemihydrate by the
pouring method. In an atmosphere with 100% relative humidity,
no decrease of the crushing strengh is noticeable. By condition-
ing the shaped products at high relative humidity for some time,
troublesome efflorescenceis completely avoided.
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The water present in the mixture used in the process
of the invention may derive from the water of hydration of the
calcium sulphate, e.g. from the filtered, centrifuged or air- -
dried gypsum obtained in the chemical industry and/or may be
derived from the moisture content of wet sand. The minimum
water content is that which just ensures binding of the cement
present, the maximum being that which just does not prevent the
mixture being compressed.
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The invcntion is now hereinafter particularly described in the
Examples and in the accompanying drawing, which is a graph showing the rela-
tion between the pressure exerted and the crushing strength of shaped products
measuring 20 x 10 x 5 cm. Use has been made of CaS04.2H20 obtained in the
preparation of phosphoric acid. The water content of the mixture was 16% wt.,
this water being contained in the sand component which is according to
Dutch Standard NEN 3072. In all instances the proportion of Portland-cement B
used in the mixture was 12.5% wt. based on the dry gypsum used and sand.The
abscissa indicates the pressure in kg/cm and the ordinate indicates the
crushing strength in kg/cm of the shaped products. The crushing strength is
determined on usual way by pressing the final shaped products between two flat
plates until fracture. Curves 1, 2 and 3 indicate the relation between pres-
sure and crushing strength, for the following compositions:
Curve 1: no sand admixed;
Curve 2: 30% wt. sand based on dry CaS04.2H20, and
Curve 3: 40% wt. sand based on dry CaS04.2H20.
Curve 1 shows that a crushing strength of 150 kg/cm is ob*ained
only by use of high pressure ( >400 kg/cm 3. When 30 and 40% wt. sand is
added (based on dry gypsum) this crushing strength value is obtained with
pressures of 150 and 125 kg/cm respectively, curves 2 and 3). By the use of
the higher sand content (curve 3) at higher pressures crushing strengths can
be obtained that are considerably higher than 150 kg/cm . A pressure of
400 kg/cm leads to a crushing strength of 300 kg/cm . This crushing strength
value is not normally required in moulded elements for use in load-bearing
structures. The graph also shows that by the use of a sand content of 30% wt.
based on dry gypsum (curve 2), the result aimed at is obtained with the use
of pressures normally exerted by pressures in use in sand-lime bricks production,
which are of the order of 150 kg/cm . The use of add-tives in the mixture,
e.g. plasticizers, or bf much fine sand, can result in further deviations from
the formula given, such deviations being of the order of plus or minus 10%,
such deviations being within the scope of the invention.
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The invention also provides shaped products formed by the novel
process hereinbefore set Porth having a crushing strength of more than
oo ~;g/cm2,
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