CA1047742A - Process for increasing the mechanical strength of porous articles - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A process for making articles having increased mechan-ical strength properties, such as bridges, buildings, roads, etc. of concrete, in which the raw material, from which the article is molded, is mixed with particulate sulphur. After molding and setting, the article is heated to a temperature below the melting point of sulphur and held at that tempera-ture until substantially all free water has been removed from the article. Following the desiccation the article is heated to a higher temperature at which the sulphur melts and migrates into internal cavities in the article. Finally, the article is cooled so that the sulphur solidifies and fills up said internal cavities. The process may also be used for making articles of clay at temperatures below the normally employed firing temperature.

Description

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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 -.. . .

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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 . ~

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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 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 .
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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 .
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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.

Claims (5)

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

1. A process for preparing porous articles having increased mechanical strength, such as articles made of con-crete, comprising the steps of:
a) mixing the raw material from which the article is to be produced, with particulate sulphur, b) molding the article from the mixture obtained, 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 evaporates from the article, and holding the article at the drying temperature 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 temperature between 119°C and 155°C, and e) cooling the article.

2. A process as claimed in claim 1, wherein the sulphur is added to the raw material in the form of spherical or sub-stantially spherical particles having a maximum diameter of about 10 mm.

3. A process as claimed in claim 2, wherein the diameter of the sulphur particles is at least 1 mm.

4. A process as claimed in claim 1, wherein the drying, heating and cooling steps are carried out at atmospheric pressure.

5. A process as claimed in claim 1, wherein the drying step c) is carried out as a preliminary drying at a relatively low temperature followed by a final drying at a higher tempera-ture, preferably between 90°C and 100°C.