Note: Descriptions are shown in the official language in which they were submitted.
````~ 104774Z
1 dRc KGROUND OF THE INVENTION
This invention relates to a process for increasing the mechanical
strength of porous articles, the process comprising adding sulphur to
the raw material from which the article is produced, drying and heating
the molded and set article and finally cooling the article.
German patent specification No. 549,200 discloses a process of
this kind which includes a transitory or brief heating o~ a molded and
set article of concrete to about 120C. This gives rise to chemical
reactions between the sulphur melted at this temperature and the calcium
hydroxide present in the concrete, whereby calcium polysulphides and
calcium thiosulphate are formed.
For the sake of completeness it is pointed out that from report
No. PB-221,324 "Internally Sealed Concrete" published in March 1973
and distributed by National Technical Information Service (VS Depart-
ment of Commerce) it is known to impregnate road surface layers of con-
crete with plastics material or wax material to make the layers tighter
and hence reduce the risk of frost injuries deriving from water seeping
into the ground. The report describes the addition of the plastics
material to the initial material and explains that in response to heating,
the plastics material melts and flows into the pores of the concrete
which are thus closed. As regards the cavities left by the plastics
material the report emphasizes that after the solidification the walls
of the said cavities are coated with a film of plastics material and
are thus closed to the penetration of water. The additives used entail
a certain reduction of the compression strength of the concrete, but
this is regarded as acceptable in view of the improved tightness.
SVMMARY OF THE INVENTION
According to the present invention there is provided a process for
preparing porous articles having increased mechanical strength, such as
articles made of concrete, comprising the steps of
a) mixing the raw material from which the articles is to be pro-
~ ` - 2 -
.. . .
~4774Z
duced, with particulate sulphur,
b) molding the article trom the mixturO obtaill~d,
c) drying the molded and set article at a temperature which is lower
than the melting point of the sulphur, but so high that free water eva-
porates from the article, and holding the article at the drying tempera-
ture during a period long enough to ensure that at least substantially
the entire amount of free water has been expelled from the article,
d) heating the desiccated article to a tempera ture above the melt-
ing point of sulphur, and
e) cooling the article.
It hss been found that it particular the mechanical strength of
articles produced by the method according to the invention is consider-
ably increased which is believed to result primarily from the fact that
internal pores, cavities and cracks, in particular those having flat
and irregularly formed surfaces with sharp edges and corners, are filled
up by solidified sulphur. As a consequence, the otherwise occurri~g notch
effects, which give rise to high stress concentrations and promote the
propagation of rupture lines and surfaces, are eliminated or at least
substantially reduced. The process may readily be emplied to very large
articles cast or molded in situ, such as bridges, houses, road slabs
etc. made from concrete.
While the process described in German patent specification No.
549 200 is carried out under conditions which permit and promote the
chemical reactions referréd to above, a characteristic feature of the
process according to the present invention is that, contrary to this,
it is attempted to prevent such reactions to the highest possible degree.
Experiments made by the inventor have shown that the reaction products
formed in fact weaken the concrete instead of increasing itsstrength.
The expulsion of virtually all free water, as effected according to the
invention, proceeds far beyond the drying normally performed on concrete
articles which after being set contain rather considerable quantities of
. ~
1~4774Z
free water, dependent on the so-called water-cement number of the raw
materials, which number typically is 0.4 or higher in order to ensure a
desired moldability of the concrete. In the prior art the presence of
free water is a condition for the occurrence of the desired chemical
reactions between sulphur and hydroxyl ions, and tests have confirmed
that the reactions can occur with sulphur in its solid state if only
free water is present. This agrees with the fact that the German patent
specification prescribes a brief heating up to about 120 C at which point
the sulphur just starts to melt.
It is also a condition for the advantages gained by the invention
that the heat treatment is performed in two distinct stages, viz. a dry-
ing stage followed by a sulphur melting state, in contradistinction
to the prio} art according to the specification of German patent No.
549 200 and that the first stage, which is only terminated when a vir-
tually complete desiccation of the article treated has been performed,
is carried out within a relatively low temperature interval in which
the sulphur does not melt. Otherwise there would be the risk that
molten sulphur more or less blocked the pores, cracks and ducts in the
article through which the water was supposed to penetrate to the sur-
face and be removed.
Formerly, it was generally assumed that no improvement of the
strength properties would be achieved by the addition of sulphur to the
starting materials, since the cracks and pores mentioned above do only
occur in connection with the setting of the article, and it has, thus,
been found that an addition of sulphur to cement - with a view to utiliz-
ing a material obtainable at a low cost as a filler -causes a reduction
of the strength of about 15%.
The invention is based on the recognition, which must be regarded
as novel and surprising on the background of the experience and the
quoted former assumption, that the sulphur particles, which during the
setting of the article are substantially evenly distributed therein,are
1047742
able to migrate from their original positions after being melted during
the heating step and that they are able - as a result of capillary effects
without any outer positive pressure being applied to the article - to
penetrate into and - assuming a proper dosing of the added sulphur -
substantially fill up all cracks and pores in the article which other-
wise reduce its ultimate strength. In this connection the virtually
complete desiccation of the article, which according to the invention
is performed before it is heated to the melting temperature of the
sulphur, is also important in that molten sulphur wets dry concrete, but
does not wet or only with difficulty wets moist concrete, so that the
above-mentioned migration of the sulphur into the internal pores and
cracks of the article is to a high degree promoted by the previous de-
siccation. It is obvious that the migration of the sulphur causes the
formation of new or secondary cavities at the original locations of the
sulphur particles, but as these cavities will have a very regular shape
they do not result in any essential weakening of the material.
It is preferred that the sulphur is added in the form of spherical
or at least substantially spherical particles with a maximum diameter of
about 10 mm. This ensures that the secondary cavities in the impregnated
article also have a spherical or approximately spherical shape and that
they are so relatively small that their influence on the strength pro-
perties is negligible.
- DETAILED DESCRIPTION
An embodiment of the process according to the invention will now
be described in greater detail with reference to the accompanying flow
sheet.
For producing a sulphur-impregnated concrete article, the conventio-
nal raw materials, such as cement, pebbles, gravel and water, are mixed
with sulphur, preferably in the form of spherical particles or grains.
The mixing can be effected in any suitable known manner, including the
so-called dry mixing where all or part of the solid constitùents are
1~47742
mixed with each other before the water is added. The preferred grain
size of the added sulphur depends inter alia on the desired properties
of the finished article and on the grain size of the remaining aggre-
gates. The quantity of sulphur added will normally amount to 8-9% of
the dry weight of the finished concrete article.
The article is produced according to conventional casting or mold-
ing technique and is set in the normal way, which if desired includes
a heating to promote the setting process. After the setting the article
is heated to a temperature below 119C which is the melting point of sul-
phur. The artic1e is kept at said temperature so long that substantially
all free water has evaporated and has been removed from the internal
pores formed in the article during the previous production steps. The
drying may be performed in a single stage at about 100 C or a slightly
higher temperature, but particularly with articles made from cement or
other raw materials containing calcium compounds it may be advantageous
to perform it in two successive stages, for example as a preliminary
drying at about 60 C and a final drying at about 90 C. The prolongation
of the drying time due to the reduced drying temperature is compensated
for by the advantage that the undesirable chemical reactions mentioned
above are strongly restrained at the lower treatment temperatures.
The temperature is now increased to such a value that the sulphur
distributed in the article melts, that is to say a temperature above
119C, but below 155 C, since at essentially higher temperatures long
molecule chains are formed which give the sulphur a syrupy viscosity
and thus counteract the desired migration of the sulphur to -the pores
or cavities mentioned above.
After a suitable time has passed, the sulphur will have filled up
the pores, and subsequently the article is cooled to ambient temperature.
Both the heating and the cooling should be effected comparatively slowly
in order that thermal stresses are as far as possible avoided. The
treatment may be effected at atmospheric pressure. After the cooling
.
.
104774Z
step, the sulphur has solidified in the pores of the article, and even if
the compression strength of the sulphur is lower than that of the con-
crete the net result is an increase in the compression strength of the
article as compared to a corresponding non-impregnated article.
The sulphur used for the impregnation may be ordinary technically
pure sulphur, but there may in advance be added other substances or
compounds which favourably modify the properties of the sulphur, for
- example organic substances with double bonds or polysulphides. These
or other additives may reduce the inflammability of the sulphur and
its vapour pressure, whereby the risk of a sublimation of the sulphur
occurring if the finished article is subjected to elevated temperatures,
is reduced.
Even if the advantages of the invention are first and foremost of
importance in articles which are impregnated with sulphur throughout
their entire volume, the process may also be used in cases where it
is principally desired to improve the properties of the surface layers
of the finished article, for example where it is desired to increase
the resistance to wear and the tightness of the surface. In this case,
there may be non-molten grains or particles of sulphur in the finished
article, but as the compression strength of the sulphur is only about
15% lower than the compression strength of normal concrete, the unmolten
particles do not give rise to any essential weakening of the material.
Example I
For evaluating the advantages obtainable by the invention, tests
were performed on circular concrete cylinders having a diameter of 10 cm
and a length of 20 cm. The water-cement number of the cylinders was
0.4 and their cement content was 370 kg/m . Cylinders with 100 and 200
kg sulphur per m3, respectively, and reference cylinders with no sulphur
content were cast. The sulphur was added in the form of spheres or globu-
les having diameters in the intervals 0.5 to 1 mm and 1 to 2 mm, respec-
tively. The cast cylinders were dried at 60 C during four days and then
1~)4774Z
at 95 C during three days. Next they were heated to 150C during three
hours. After cooling and solidification of the sulphur, relevant phy-
sical parameters for the various cylinders including the reference
cylinders with no sulphur addition were measured. The tests were per-
formed on cylinders which between the casting and the heat treatment
had been stored in the laboratory in periods varying between 3 and 28
days.
The measurements showed that the compression strength of the
sulphur-impregnated cylinders had been increased 47-78% on the average
as compared to the non-impregnated cylinders and that the increase in
strength was highest in the specimens having the highest sulphur con-
tent and when the diameters of the sulphur particles were within the
interval 1-2 mm.The measurements furthermore showed that the stiffness
(Young's modulus) of the material was increased essentially and that
lS there was an approximate proportionality between load and deformationup to higher loads than in the case of non-impregnated specimens.
Neasurements of transverse rupture strength (modulus of rupture) were
- also performed by bending prismatic specimens of corresponding composi-
tions and having the dimensions 4 x 4 x 16 cm. In this case a maximum
increase in the strength of about 80% was measured at a sulphur con-
tent of 200 kg/m . The cement content in the prismatic specimens was
slightly higher than in the cylindrical specimens, viz. about 500 kg/cm3.
Besides the strength properties mentioned above, the tensile
strength, the cleaving strength, the watertightness and the resistance
against attacks by acids and salts have turned out to be improved by
the addition of sulphur to the specimens.
Example II
Test cylinders having a diameter of 4.5 cm and a length of 9 cm
were cast from a mixture of 660 g fired gypsum (semihydrate: CaSo4,
1/2 H20), 330 g water and 132 g sulphur in the form of spheres having-
- diameters between 0 and 2 mm. After one day the cylinders were removed
.
. ~ .
:
1~47742
from the moulds and were dried at 105C for 24 hours. Heating the
cylinders to 140C during 4 1/2 hours and a subsequent cooling resulted
in an increase in the compression strength of between 75 and 95% as
compared to specimens in which the sulphur had not been caused to melt
and thus impregnate the specimens. It was also found that the impregnated
gypsum specimens could hold nails and screws and that they could stand
dropping 1 m onto a floor without cracking. These properties are of a
considerable value in practice since they increase the possible scope
of application of gypsum in the production of articles such as wall and
ceiling plates for use in the building industry.
Example III
Moist clay is mixed with about 20% sulphur in the form of spheres
having diameters of from 0-2 mm and the mixture is molded into test
bodies which are dried at about 105 for 18 hours. Next, a heating is
lS performed to a temperature of between 120 and 155 C, for example 140during about 3 hours. This causes the sulphur to melt and impregnate
the clay, and after cooling the solidified sulphur acts as a primary
binding agent in the ceramic article produced. By the use of this
technique it is possible to dispense with the highly energy-consuming
and time-consuming firing at high temperatures which is traditionally
used for binding the clay particles together by sintering. The articles
produced resist temperatures up to about 100 C, and consequently the
technique can be used for the production of earthenware and pottery
of many kinds including pipes, flower pots, vases and other household
implements. It may also be applicable, e.g. in developing countries, in
the production of building blocks.
