Preparation and Use of High Porosity Silicocalcareous Mass

In current industrial practice, known porous silicocalcareous masses are made from a thorough mixture of lime, ground silica or ground fossil silica and asbestos in a suspension water. The resulting paste, introduced in the containers, molds or bottles, is then subjected to heat in an autoclave to initiate and complete the lime-silica reaction which results in producing the silicocalcareous material; this material is then transferred to a drying oven to remove the water and create the desired porosity.

Asbestos is added to the aqueous composition of silica and lime for effecting its function as a reinforcing fiber and suspension agent. However, recent constraints relating to health and safety connected with the handling of asbestos have led to reconsideration of the use of this natural fiber the characteristic properties of which in making porous silicocalcareous masses, function to reinforce the crystalline structure and to aid in maintaining the suspension of the solid products, namely silica and lime, in a large amount of water.

It is known that the stabilization of the suspension can be markedly improved by the use of suspension agents. The effect of adding substantial amounts, up to 10%, of organic suspension agents is known from U.S. Pat. No. 3,406,030. Certain cellulose derivatives such as alkylhydroxyalkyl-celluloses, in particular methyl- and ethylhydroxyethyl-cellulose, make it possible to obtain an excellent stabilization of the suspension. The amount to be introduced varies with the desired degree of stabilization and the nature of the suspension agent.

Compensation for discontinuing use of asbestos by the addition of organic suspension agents in very large amounts has been considered. It has been shown that the addition of an organic suspension agent, in a large amount, can appreciably correct the defect of decanting the solid products during making of the silica/lime and water paste. However, because of the destruction of this type of organic compound during firing of the paste, a decrease in the mechanical properties of the dried high-porosity silicocalcareous masses has been noted. The brittleness, fragility and friability of the resulting porous materials are such that they render them unsuitable for the applications mentioned above.

It has therefore been sought to develop a thixotropy phenomenon shortly after the filling of the bottle or mold, which would make it possible to live a bottle or mold with a liquid mixture wherein the setting of the paste is then achieved before putting the bottle or mold containers in the autoclave.

It has been found that the addition of amorphous ultrafine synthetic silica, having a high specific surface of 150 to 300 m.sup.2 per gram and thus a high absorption capacity, as a total or partial replacement of the ground silica, makes it possible very advantageously to obtain high-porosity homogeneous silicocalcareous masses that exhibit satisfactory mechanical properties.

The aqueous paste, based on mixtures of divided silica and lime, contains at least 50% of ultrafine synthetic silica, and this amorphous ultrafine silica is very advantageously used in an amount between 70 and 100% in relation to the total amount of the silica in the silicocalcareous mass.

The total amount of silica used is slightly greater than that of the lime, in a proportion of 10 to 30%, and the water is used in an amount at least three times greater than that of the solids, preferably in a ratio of 3 to 4.6:1.

The particles of amorphous ultrafine synthetic silica have a granulometry on the order of several microns. These elementary particles can be associated in three-dimensional chains thus forming an aggregate type structure, and by drying the aggregates are associated into agglomerates. These particle arrangements in aggregates and agglomerates create a macroporous structure which exhibits a high absorption capacity. This absorption capacity can be expressed by the take-up of liquid in milliliters per 100 g of product, e.g. DOP or oil take-ups. The DOP take-up is equal to the amount of dioctylphthalate (DOP) in milliliters necessary to start making 100 g of the product into a paste. The products having an absorption capacity at least equal to 150 ml. and preferably between 170 and 350 ml., are particularly suited for use in this process.

These amorphous ultrafine synthetic silicas, known as silica pigments, are in the form of light fine powders having an apparent specific mass on the order of 0.25 to 0.35 kg/dm.sup.3 of absorbing amounts, up to twice their weight, of water. These ultrafine silicas, for example, are made by the pyrogenation of silicon tetrachloride or by chemical precipitation of a soluble silicate in an aqueous medium.

The amorphous ultrafine synthetic silicas containing a slight molar proportion of water in their molecular constitution, which can be a molar fraction of up to about 1/10, i.e. 10%, and silicas hydrated with a slight amount of water as part of their constitution and containing a slight molar fraction of aluminum oxide and sodium oxide, are very advantageous constituents in making the above-mentioned pastes which are useful for making a silicocalcareous porous mass, such as described above. There are two absorbing amorphous synthetic silicas corresponding to the formulas of 10 SiO.sub.2, 1 H.sub.2 O and 14 SiO.sub.2, 1 Al.sub.2 O.sub.3, 1 Na.sub.3 O, 3 H.sub.2 O, which are particularly suited to making silicocalcareous porous masses, the second of these comprising about 10% aluminum oxide and about 8% sodium oxide.

These amorphous synthetic silicas can be used either alone or in mixture.

At times there is interest in controlling the thioxotropy phenomenon in order to work with pastes that are easy to handle and do not prematurely set during lining of the containers. The setting of the paste can be slowed down by the use of amorphous ultrafine synthetic silicas having a somewhat slighter absorption capacity. The use of amorphous ultrafine silicas having an absorbing power between 150 ml. and 200 ml. makes it possible to obtain pastes that are easy to handle and to transfer while being maintained in the liquid state for several hours. Ultrafine silicas containing slight molar fractions of aluminum oxide and sodium oxide, each fraction, for example, being on the order of 1/20, make it possible to achieve excellent results in this regard.

Comparison tests made with amorphous ultrafine silica corresponding to the formula 10 SiO.sub.2, 1 H.sub.2 O and of ultrafine amorphous synthetic silica corresponding to the formula 14 SiO.sub.2, 1 Al.sub.2 O.sub.3, 1 Na.sub.2 O, 3 H.sub.2 O made evident the advantage of these mixtures. A reference viscosity was taken as a standard and was followed by noting the viscosity, which developed, of three pastes made with a =100% amorphous ultrafine silica, 10 SiO.sub.2, 1 H.sub.2 O, take-up DOP 340, b=50% silica 10 SiO.sub.2, 1 H.sub.2 O, DOP 340 and 50% silica 14 SiO.sub.2, 1 Al.sub.2 O.sub.3, 1 Na.sub.2 O, 3 H.sub.2 O, DOP 180, and c=100% 14 SiO.sub.2, 1 Al.sub.2 O.sub.3, 1 Na.sub.2 O, 3 H.sub.2 O, DOP 180. In regard to paste a, the reference viscosity was reached after 60 minutes, with regard to paste b, this same viscosity appeared after 3 hours, and in the case of paste c, this viscosity was reached only after 6 hours of setting of the paste at ambient temperature. These tests clearly showed the role of the absorbing power of the silica.

When a particularly effective high-porosity silicocalcareous material is desired, that has a high mechanical strength and is not friable when broken, it has been found advantageous to add a minor proportion of nonreactive synthetic fibers, either of inorganic or organic origin, having a good mechanical strength. The presence of these chemical fibers in the silicocalcareous mass improves the tenacity of the porous material, thus making the product less fragile to impacts. These synthetic fibers are introduced into the porous material in a proportion of 3 to 15%, preferably 4 to 6%, in relation to the amount of solids in the material. Inorganic synthetic fibers, such as alumina silicate can be used leading to interesting results in this process.

Synthetic chemical fibers of the layered polymer type such as carbon or carbonaceous fibers, or of the scale polymer type such as aromatic polyamide fibers, alone or in mixture, are advantageous reinforcement fibers for use in the process of this invention.

The paste is prepared by the addition of different constituents in water, namely lime, ground silica (if any), amorphous synthetic ultrafine silica and synthetic fiber (if any). The mixture is stirred for 5 to 20 minutes until a thoroughly homogeneous mixture is obtained, then the resulting paste is introduced into bottles, containers or molds, under a partial vacuum. The bottles are then provided with a porous plug and put into an autoclave for firing for 17 to 24 hours under saturated steam pressure of about 10 bars, and are then subjected to a gradual drying for several days at a maximum temperature of 330.degree. C.

In a first stage of use, the quicklime can be slacked by a thorough mixing of the latter with a portion of water.

The present process makes it possible to obtain silicocalcareous masses wherein the porosity can exceed 90%.

These porous materials are particularly suitable for packing lining containers and bottles which are intended to store dissolved gases, and particularly acetylene. The nature of these materials makes them particularly useful in the field of thermal insulation and also in the building construction, particularly for use as sound insulation.

Examples that illustrate the invention in a nonlimiting way are set forth below:

There are thoroughly mixed, in a mixer, 48.7 kg of quicklime in 200 liters of water, while stirring for 10 to 15 minutes, to effect complete slaking of the lime, then there are added, in sequential order, 186 liters of water, 15 kg of ground silica (100.mu.) and 41 kg of amorphous ultrafine synthetic silica, corresponding to the formula 10 SiO.sub.2, 1 H.sub.2 O, the elementary particles of which are arranged in aggregates and agglomerates which create a macroporous structure absorbing DOP 340 (ml per 100%). Stirring is maintained for 10 to 15 minutes until a homogeneous liquid paste is obtained which is quickly introduced, under a partial vacuum of about 600 millibars, into bottles for use in storing acetylene in the dissolved state.

The filled bottles are then provided with a porous plug before being put into the autoclave. The autoclave firing period is 20 hours under a saturated steam pressure of 10 bars, then progressive drying is performed for 4 days at a maximum temperature of 330.degree. C.

After drying, the porous masses, which are contained in the bottles, are homogeneous and exhibit a longitudinal shrinking of less than 1 mm, a crushing strength of 15 to 20 kg/cm.sup.2 and a porosity of 90%.+-.1%.

Simultaneously, a mixture is made of 423 liters of water, 50 kg of quicklime, 57.5 kg of ultrafine synthetic silica, as in example 1, and 7 kg of synthetic alumina silicate fibers are added, while stirring is maintained for 20 minutes, before the mixture is transferred into bottles. After firing and drying, the mass under the same conditions as described above, the bottles each contain a homogeneous mass having 89.+-.1% porosity and 10 to 20 kg/cm.sup.2 mechanical strength.

There are mixed, in a first stage, 250 liters of water and 50 kg of quicklime, for 15 minutes, to effect slaking of the lime (final temperature 56.degree. C..+-.10.degree. C.). After this, the mixture is transferred into a mixer, and there are added, in sequential order, 148 liters of water, 8 kg of groung silica (100.mu.) and 49.5 kg of amorphous ultrafine silica 10 SiO.sub.2, 1 H.sub.2 O, having an absorbing macroporous structure, and an absorption capacity in ml per 100 g: take-up DOP=280 after 5 minutes of mixing a homogeneous paste is obtained, into which 5.5 kg of carbonaceous fibers are introduced. After firing and drying of the mixture, under the conditions indicated above, the porous material exhibits the following physical characteristics: porosity 90%.+-.1% and crushing strength of 20 to 35 kg/cm.sup.2.

Slaking of 50 kg of quicklime in 300 liters of water is effected, while vigorous stirring is maintained for 10 minutes; then 205 liters of water and 59 kg of amorphous ultrafine synthetic silica are introduced, while stirring as in example 3 and the stirring is maintained for 20 minutes. There are then added 4 kg of perfectly defibered carbonaceous fibers to the mixture and the stirring is continued for 10 minutes after introduction of the fibers. The suspension which is introduced into the containers is subjected to a firing under 11 bars of saturated steam, for 20 hours and then is progressively dried at atmospheric pressure at a maximum temperature of 330.degree. C. until it achieves constant weight. The silicocalcareous material, thus prepared, has the following physical characteristics: a porosity of 92%.+-.1%, density of 0.210.+-.0.05, and crushing strength of 15 to 20 kg/cm.sup.2.

In a first stage, 250 liters of water and 50 kg of quicklime are mixed for 15 minutes to effect complete slaking of the lime. Then, the mixture is transferred into a mixer, and there are added, in sequential order, 148 liters of water, 28 kg of amorphous ultrafine synthetic silica, 10 SiO.sub.2, 1 H.sub.2 O, of an absorbing macroporous structure, having an absorption capacity in ml per 100 g: DOP take-up=280, and 29.5 of amorphous ultrafine synthetic silica, corresponding to the formula 14 SiO.sub.2, 1 Al.sub.2 O.sub.3, 1 Na.sub.2 O, 3 H.sub.2 O, and having a specific surface of 180 m.sup.2 /g, with an absorbing macroporous structure, with an absorption capacity in ml per 100 g: DOP take-up=180. A homogeneous paste is obtained, after 5 minutes of mixing, into which 5.5 kg of carbonaceous fibers are introduced.

After firing and drying, under the conditions indicated above, the porous material exhibits the following physical characteristics: porosity 90%.+-.1% and crushing strength of 20 to 35 kg/cm.sup.2.

Slaking of 50 kg of quicklime in 300 liters of water is effected, while vigorous stirring is maintained for 10 minutes, then there are introduced, while stirring, 205 liters of water and 59 kg of amorphous ultrafine synthetic silica, corresponding to the formula 14 SiO.sub.2, 1 Al.sub.2 O.sub.3, 1 Na.sub.2 O, 3 H.sub.2 O and having a specific surface of 180 m.sup.2 /g, of an absorbing macroporous structure with an absorption capacity in ml per 100 g: DOP take-up=180; stirring is maintained for 20 minutes.

There are then added 4 kg of perfectly defibered carbonaceous fibers, and stirring is continued for 10 minutes after introduction of the fibers into the mixture. The suspension, which is introduced into containers, is subjected to firing under 11 bars of saturated steam, for 20 hours, and then progressively dried at atmosphereic pressure at a maximum temperature of 330.degree. C. until it achieves a constant weight.

The silicocalcareous material, thus prepared, has the following physical characteristics: a porosity of 92%.+-.1, density of 0.210.+-.0.05, and a crushing strength of 15 to 20 kg/cm.sup.2.

It will be obvious to those skilled in the art that various changes may be made without departing from the scope of the invention and the invention is not to be considered limited to what is described in the specification.