GB2359549A

GB2359549A - Refractory cementitious mixtures

Info

Publication number: GB2359549A
Application number: GB0109337A
Authority: GB
Inventors: Richard C First; Frank T Gay; Jeffrey T Champa
Original assignee: MBT Holding AG
Current assignee: MBT Holding AG
Priority date: 1999-03-03
Filing date: 2000-02-28
Publication date: 2001-08-29
Also published as: GB0109337D0

Abstract

A refractory cementitious mixture, particularly suitable for application by spraying comprises a refractory cement, an organic foaming agent, and aggregate. When applied to a substrate and fired, it gives a dense, pore-free coating. A method of spraying the mixture, with added water on to a substrate through a nozzle, into which are introduced compressed air and sufficient non-accelerating agent to cause substantially instantaneous stiffening of the mixture on the substrate, is described.

Description

REFRACTORY MIXTURES WITH INCREASED FLOWABILITY FIELD OF THE INVENTION The present invention is directed to foamable refractory mixtures. More particularly, the present invention is directed to refractory mixtures that are foamed in situ to have increased flowability for a spray process application, and that have the foam removed from the final composition after spraying.

BACKGROUND OF THE INVENTION In the process of spraying concrete by the wet spraying process, a cementitious mixture of pumpable consistency is conveyed by pumping or by pneumatically conveying the mix to a nozzle at the application point, through tubing or hose lines. At the nozzle; compressed air is introduced; this breaks up the compact concrete stream, which is then forced from the nozzle and which can be sprayed on to a substrate. Rapid setting or hardening of this sprayed cementitious mixture may be achieved by the introduction into the concrete stream of a hardening activator which can be added by means of special dosing devices or by including it in the compressed air. This method is described in USPN 5,628,940. However, it is still desired that a concrete mixture have increased flowability over long distances.

Typically, high pressures (about 4,000 psi (28.17 MPa) or greater) are needed to convey a cementitious concrete mix for a shotcrete application either by pump or pneumatically. It is desired to reduce the pressure required to reduce equipment and operating costs, and to provide for ease of use.

For refractory concretes in particular, conveying the cementitious mixture is more difficult. Refractory concretes used in the refractory industry as the primary binder for monolithic refractory linings are typically based on alumina-rich calcium aluminate cements. These cements have a different stoicluometry than the calcium aluminate component present in portland cements. The structure of the hardened refractory concrete is crystalline in nature, while the hardened portland cement concrete structure is gelatinous in nature. In addition, unused, dry refractory concrete mixtures have aging problems which are not experienced by portland cement concrete mixtures. Particularly, over the course of several months of storage, flowability of the refractory concrete mixture is decreased, and even accelerated mixture set times are extended.

One method for increasing the flowability of cementitious mixtures is to optimize the particle size distribution of the aggregates in the cementitious mixture. However, in refractory cementitious mixtures based on alumina and spinel, the mixture exhibits dilatancy, even with well-engineered particle sizes.

Another method known in the art to increase the flowability of cementitious mixtures is to add foam to the mixture to decrease the density. Typically, a foam is generated separately from the cement mixture and is then mixed with the cementitious mixture. In USPN 5,393,341, the foaming of a concrete mixture is advanced to a quasi one step operation. This operation does not require pre-manufacture of the foam, but does require the foaming agent be added separately to the cementitious mixture through a mixing chamber. This requires that the foaming agent be supplied separately from the cementitious mixture, and requires the additional step, at the job site, of mixing the foaming agent into the cementitious mixture.

The art does not disclose a refractory cementitious mixture for use in shotcreting operations, with a foaming agent present therein that is foamed in situ to increase the flowability of the refractory mixture and wherein the refractory mixture is then defoamed before coating a substrate with a cementitious coating comprising the mixture. There is an art-recognized need for improvement in the flowability of refractory concrete mixtures.

For pumping applications for refractories, foaming agents have not been previously utilized in refractory mixtures because it has been desired to eliminate as much air from the mixture as possible, so that the sprayed refractory mass is not porous and is equivalent in density to cast refractory units. Also, foaming agents are organic, and when the furnace is heated initially, any foaming agents that are present are "burned out", which could potentially lead to undesirable porosity in the final, fired article, if the refractory unit were foamed. Porous coatings tend to give ingress to the furnace wall, by heat and reactants.

It is therefore an object of the invention to provide for increased flowability of a refractory cementitious mixture in a spray process application.

It is another object of the invention to provide a refractory mixture for a spray process application with an organic foaming agent present therein, that does not contribute undesired porosity to the resulting sprayed coating.

SAY OF THE INVENTION The present invention therefore provides a refractory cementitious mixture comprising a refractory cement, an organic foaming agent, and aggregate.

The present invention also provides a method for spraying a substrate with a refractory cementitious mixture comprising: providing a refractory cementitious mixture comprising a refractory cement, an organic foaming agent, aggregate, and water; foaming the refractory cementitious mixture; conveying the mixture to a spray nozzle; introducing compressed air and an amount of a non accelerating stiffening agent to the spray nozzle sufficient to provide for substantially instantaneous stiffening of said refractory cementitious mixture on said substrate; and, spraying said refractory cementitious mixture on to said substrate, wherein said mixture substantially instantaneously stiffens upon contact with said substrate, and hydrates to form a substantially defoamed, non-porous refractory coating.

DETAILED DESCRIPTION OF THE INVENTION The refractory cements which can be used with the present invention include, but are not limited to, calcium aluminate cement, colloidal silica, silicon oxide, hydratable alumina, hydratable aluminum oxide, magnesia, and mixtures thereof. Refractory cement mixtures conventionally comprise about 10 to about 40 weight percent fines including about 0 to about 30 weight percent calcium aluminate cement, 0 to about 10 weight percent fumed or micro silica and about 0 to about 20 weight percent calcined alumina, with the remainder of the mix being a graded aggregate, comprising of, but not limited to, calcined flint clays, bauxites or tabular alumina. The mix may also contain up to about 5 weight percent synthetic fibers.

The calcium alunvnate phases which constitute the bulk proportion of high alumina, calcium aluminate cement are primarily CA, CA2. Other alumina-containing phases in high alumina calcium aluminate cement are C4AF, C4A3 S (sulfate) and C2AS, all of which are crystalline. The aluminate phase assemblage in high alumina calcium aluminate cement differs significantly from that of the primary calcium- and aluminum containing phase in Portland cement which is <B>CA</B> with C4AF also present in the majority of Portland cements. Upon intermixing the cement with water, various phases begin to hydrate and form hydration reaction products. The ultimate hydration product in the high alumina, calcium aluminate cement system is a crystalline product, whereas, in Portland cement, the primary hydration product is an amorphous gel. Abbreviations have the industry accepted meaning as set forth in Table A below.

Table A Chemical Abbreviation Chemical Equivalent CA CaO-Al2O3 CA2 CaO-2A1203 C4AF 4CaO.A1203.Fe2O3 C4A3 S 4Ca0-3A1203-S03 C2AS 2Ca0-A1203-SiO2 C3A 3Ca0-AI203 An example of a high alumina content, calcium aluminate refractory concrete mixture is set forth below in Table 1, listed in weight percentages.

Table 1 Calcined flint clay (0.25 - 0.33 inch (0.6-0.84 cm), nominally) 15% Calcined flint clay (0.083 - 0.2 inch (0.21-0.51 cm), nominally) 20% Calcined flint clay (less than 0.083 inch (0.21 cm), nominally) 25% Bauxite (less than 0.02 inch (0.51 cm), nominally) 25% Calcined alumina (0.3 pM, nominally) 5% Microsilica (containing about 85% SiO2) 5% Calcium aluminate cement (containing > 90% A1203) 5% The refractory mixture may additionally comprise water. The water is present in an amount from about 4% to about 8% based on the dry weight of the cement.

The rheological properties of the refractory cementitious mixture of the present invention are different from air-entrained mixes of the prior art which contain up to 10% by volume air. The foaming agent of the present invention gives a reduction in the dilatancy that permits pumpability with about 25-35% by volume air.

The foaming agents of the present invention are organic. Foaming agents which can be used with the present invention include alkanolamides, alkanolamines, alkylaryl sulfonates, polyethylene oxide-polypropylene oxide block copolymers, alkylphenol ethoxylates, carboxylates of fatty acids, ethoxylates of fatty acids, sulfonates of fatty acids, sulfates of fatty acids; fluorocarbon containing surfactants, olefin sulfonates, olefin sulfates, and mixtures thereof. A preferred foaming agent is an alpha-olefin sulfonate, which is sold under the trademark RHEOCELL RHEOFILLTM from Master Builders, Inc., Cleveland, Ohio.

Alkanolamide foaming agents according to the present invention include, but are not limited to, those having from about 12 to about 20 carbon atoms.

Alkylaryl sulfonate foaming agents according to the present invention include, but are not limited to, those having one aryl group and having alkyl groups with about 12 to about 20 carbon atoms. Polyethylene oxide-polypropylene oxide block copolymer foaming agents according to the present invention include, but are not limited to, those having about 10 to about 20 units of each block.

Alkylphenol ethoxylate foaming agents according to the present invention include, but are not limited to, those having an alkyl group of about 12 to about 20 carbon atoms.

Carboxylates of fatty acid foaming agents according to the present invention include, but are not limited to, those in which the fatty acid moiety has about 12 to about 20 carbon atoms.

Ethoxylates of fatty acid foaming agents according to the present invention include, but are not limited to, those in which the number of ethoxylate groups is about 10 to about 20 and the fatty acid moiety has about 12 to 20 carbon atoms.

Alkanolamine foaming agents according to the present invention include, but are not limited to, those having from about 12 to about 20 carbon atoms.

Sulfonates of fatty acid foaming agents according to the present invention include, but are not limited to, those in which the fatty acid moiety has about 12 to about 20 carbon atoms.

Sulfates of fatty acid foaming agents according to the present invention include, but are not limited to, those in which the fatty acid moiety has about 12 to about 20 carbon atoms.

Fluorocarbon-containing surfactant foaming agents according to the present invention include, but are not limited to, those having about 12 to about 20 carbon atoms and one or more CH2 moieties are replaced by CF2 moieties. Olefin sulfonate foaming agents according to the present invention include, but are not limited to, those having about 12 to about 20 carbon atoms.

Olefin sulfate foaming agents according to the present invention include, but are not limited to, those having about 12 to about 20 carbon atoms.

The foaming agents can be present in the refractory mixture from about 0.02 to about<B>0.1%,</B> preferably from about 0.02 to 0.06%, based on the total weight of the refractory mixture. The optimum quantity of the foaming agent to be used will depend upon the aggregate gradation of the refractory concrete mixture. The more gap-graded the aggregate gradation, the less dose of foaming agent will be required for a desired foam content.

The foam is generated in the refractory cementitious mixture by the mixing action. As the components of the cementitious mixture are mixed, the foam is generated.

The approximate amount of foam or porosity created in the refractory cementitious mixture depends upon several factors, some of which include the gradation of the aggregate particles, the percentage of fine particles (greater than 140 mesh (ASTM E-11)) in the mixture, the water content of the mixture, the type of mixer used for mixing, the amount of mix time, and the ambient temperature. When using the preferred alpha-olefin sulfonate foaming agent of the present invention, the typical porosity (foam) content generated by the addition of the specified or nominal dose is from about 25 to about 35% by volume.

Normally, for the finished refractory cementitious articles or coatings, it is desired to have limited and controlled porosity. The present invention, which features increased purnpability of the refractory cementitious mixture and increased volume of compressed air at the nozzle, increases the velocity of the sprayed refractory cementitious mixture onto the substrate to be coated, such that the foam is broken and the resultant porosity of the mixture is reduced upon impact of the material with the substrate surface. The resulting refractory coating is dense, and is able to protect the substrate, such as a furnace wall, from the ingress of heat and reactants.

The stiffening agent promotes stiffening of the refractory cementitious mixture upon application to the substrate to prevent the mixture from slumping. The stiffening agents are non-accelerating with respect to the cement. Stiffening agents that can be used with the present invention include pre-gelatinized starches, cellulose ethers, polyethylene oxides, alginates, carrageenans, polyvinyl alcohol, synthetic polyelectrolytes, natural gums, and mixtures thereof. A preferred stiffening agent is a proprietary mixture of cellulosic ethers sold under the Trademark PS-1151 by Master Builders, Inc., Cleveland, Ohio.

Cellulose ether stiffening agents according to the present invention include, but . are not limited to, hydroxy ethyl cellulose.

Polyethylene oxide stiffening agents according to the present invention include, but are not limited to, those with a weight-average molecular weight greater than about 100,000.

Polyvinyl alcohol stiffening agents according to the present invention include, but are not limited to, those with a weight-average molecular weight greater than about 1,000.

Synthetic polyelectrolyte stiffening agents according to the present invention include, but are not limited to, polyacrylamides with a weight-average molecular weight greater than about 10,000, polyvinyl sulfonates with a weight-average molecular weight greater than about 1,000, carboxy vinyl polymers with a weight-average molecular weight greater than about 1,000, and mixtures thereof.

Natural gum stiffening agents according to the present invention include, but are not limited to, guar gum, welan gum, and mixtures thereof.

The quantity of stiffening agents which are used in the process of the present invention as a percentage by weight of the refractory cementitious mixture depends upon factors with which a person skilled in the art is familiar. These factors include: 1.) The particular stiffening agent used; 2.) The types of admixtures used in the refractory concrete mix; and 3.) The temperature of the refractory concrete mix. In the present invention, stiffening agents can be added in a preferred range of from about 0.5 to about 0.7% based on the total weight of the refractory cementitious mixture.

The stiffening agent stiffens the refractory cementitious mixture on the target substrate by retaining the water which is present at the pore walls after the pores collapse upon impact. The stiffening mechanism does not significantly affect the normal setting behavior of the refractory cementitious mixture.

The conveying of the mixture can be accomplished by pumping or by pneumatic conveyance.

In a preferred method, the foamed refractory cementitious mixture is pumped through a hose, at the end of which is attached a nozzle which brings together the following: the hose containing the pumped cementitious mixture, a hose that delivers a known dose of stiffening agent, and one or two hoses which deliver a high volume of compressed air. It is preferred that a high volume of compressed air be delivered at the nozzle.

In the process of the present invention, the abovementioned mixtures do not adversely affect the finished properties of the refractory cementitious mixture as compared to refractory cementitious mixtures without the added foaming and/or stiffening agents when the refractory cementitious mixture is applied as described herein. According to the invention, the material should not be pumped any faster than needed so that the compressed air can break up the compact refractory cementitious stream and also provide the necessary velocity to remove the porosity of the refractory cementitious mixture upon impact with the substrate by breaking the foam This will result in the desired density and porosity in the finished product. After the refractory mixture is applied to the substrate, such as a furnace wall, by the method of the present invention, the mixture is fired. The organic materials in the mixture, the foaming agent and stiffening agent, are "burned out" upon firing and are thus no longer present. The resulting refractory cementitious coating does not develop porosity or glassy phases that are deleterious to the integrity of the coating. In contrast, inorganic foaming agents, when fired with the refractory concrete, remain in the article, and they affect the properties of the fired refractory article, for example, by forming glassy phases which compromise the integrity of the coating at operating temperatures. Thus, in a process according to the present invention, the high temperature performance properties of the fired refractory article are not significantly affected. Because the refractory mixture at this point is not foamed, the burning out of the organic materials does not significantly create porosity in the final fired article.

In one embodiment, the novel refractory mixture is adapted for use in a shotcrete process, for spraying the refractory mixture on to a substrate. Although this specification describes shotcrete applications, the present invention is applicable to any application in which a refractory mixture is to be conveyed to an application point and sprayed on a substrate.

The invention will now be further illustrated by the following non-limiting examples. EXAMPLES As used herein, the following terms have the given definitions. PLC is defined as percent length of change. MOR is defined as the Modulus of Rupture. HMOR is defined as hot modulus of rupture.

For mixtures that are sprayed by a shotcrete process, the mixtures are pumped through an Allentown MR-450 pump, from Allentown Pump and Gun, a division of Master Builders, Inc. The hose is 1.5 inches (3.81 cm) in diameter. Compressed air is added at the nozzle. Testing is done to determine the effects of the foaming agent and stiffening agent in the present invention. The results are listed in Table 2A. All mixes are based on a refractory cementitious mixture prepared with a proprietary tubular alumina/spinel refractory concrete mixture, sold under the tradename of SFL-224 from Alcoa Industrial Chemicals, Bauxite, Arkansas. Each mix contains 300 pounds (136.1 kg) of refractory cement. The percent water is reported based on the amount of water in the total weight of cement and water. The foaming agent used in the example Mixes with a foaming agent present is an alpha-olefin sulfonate, which is sold under the trademark RHEOCELL RHEOFILLTM by Master Builders, Inc., Cleveland, Ohio.

Some of the mixtures are sprayed by a shotcrete process, while others are cast as bars for comparison. Mixtures 4A, 5A, and 6A are both cast and shot. For the cast Mix 4A, a total of five bars are cast, three fired at 1000 C and two fired at 1500 C.

Mixes 3A, 4A, and 5A have a dispersing agent added to the cementitious mixture. The dispersing agent is beta-naphthalene sulfonic acid formaldehyde condensate (BNS). Additionally, a stiffening agent is added at the nozzle for the mixes that are sprayed. Mix 1 A uses an aluminum salt sold under the Trademark RA-160 by Master Builders, Inc., Cleveland, Ohio. Mix 6A uses sodium silicate sold under the Trademark RA-430 by Master Builders, Inc., Cleveland, Ohio. Mixes 2A-5A use a proprietary mixture of cellulosic ethers, sold under the Trademark PS-1151 by Master Builders, Inc., Cleveland, Ohio.

The amount of foaming agent added does not reduce the pump pressure. The foaming agent in Mixes lA-6A is present in an amount below the effective dosage.

Mix 6A does not have any foaming agent or stiffening agent present in the cast bars and represents the standard against which the sprayed mixes are to be compared.

A second comparative study is conducted using refractory cementitious compositions based on a proprietary tubular alumina/spinel refractory concrete mixture, sold under the tradename of SFL-224 from Alcoa Industrial Chemicals, Bauxite, Arkansas. The results are shown in Table 2B. Each mix contains 300 pounds (l36.1 kg) of refractory cement. The percent water is based on the amount of water in the total weight of cement and water. The foaming agent used in mixes with a foaming agent present is an alpha-olefin sulfonate, which is sold under the trademark RHEOCELL RHEOFILLTM from Master Builders, Inc., Cleveland, Ohio.

Samples of the mixes are both cast and sprayed. The remainder of the mixes have a dispersing agent added to the cementitious mixture, namely BNS. Additionally, a stiffening agent is added at the nozzle at a rate of 1.3 pounds (0.59 kg) per minute for the mixes that were sprayed. The stiffening agent is a proprietary mixture of cellulosic ethers, sold under the Trademark PS-1151 by Master Builders, Inc., Cleveland, Ohio.

The percent water in the mixtures in this study is increased as compared to the first study to determine the effect of increased water in the mixture on pump pressure. The increased water does not reduce the pumping pressure for mixes without a foaming agent, as compared to the results for Mix 1B (shot).

As shown by Mix 2B(shot), using an effective dosage of 0.03% of the foaming agent gives a reduction in pump pressure from 4000psi (28.17 MPa) down to about 2500psi (17.60 MPa).

A third comparative study is conducted with refractory cementitious mixtures prepared with 300 pounds (136.1 kg) of a proprietary tubular alumina/spinel refractory concrete mixture, sold under the tradename of SFL-224 from Alcoa Industrial Chemicals, with 5% water and foaming agent, RHEOCELL RBEOFILLTM, at a dose of 0.018% (24.2 g1136.1 kg). The mixture is pumped at a pressure of about 4000psi (28.17 MPa). Another 0.018% dose of RHEOCELL RHEOFILLTM is added to the mixture and is pumped. The pressure is reduced to about 2800 to 3000 psi (19.72-21-13 MPa).

Next, batches of refractory mixtures are prepared and test panels are sprayed. The results are listed in Table 2C. The refractory mixture used contains SFL-224 refractory cement for all panels, except Panel 5 which uses a similar refractory mixture to that of SFL-224, but which contains a slightly different aggregate gradation (the aggregate was less gap-graded). The pump used is an Allentown MR-450 pump. The pump has a 24" (61 cm) piston and is operated at about 13.5 strokes/minute. The material is pumped through a 1.5" (3.81 cm) hose to a spray nozzle where compressed air and a stiffening agent are added. The stiffening agent is a proprietary mixture of cellulosic ethers, sold under the Trademark PS-1151 by Master Builders, Inc., Cleveland, Ohio.

Additionally, a refractory cementitious mixture comprising SFL-224 refractory cement with 4.8% water and no foaming agent or stiffening agent is cast as a comparison. The IHMOR for the casting was 2,897 psi (20.40 NtPa).

For Panel 1, a 0.03% dosage of foaming agent, RHEOCELL RHEOFILLTM, is used with PS-1151 stiffening agent added at the nozzle. Panel 2 repeats the same mixture as Panel 1; however, the pump is operated 1.5 times more slowly. Panel 3 reduces the amount of water in the mixture to 4.7% to determine the effect of reduced water. Panel 4 is the same as Panel 3, except that the level of foaming agent RHEOCELL RHEOFILLTM is increased from 0.03% to 0.045%. Panel 5 uses a refractory concrete mixture similar in chemical composition to that of SFL-224, as described above; however, the aggregate gradation is changed, and there is added 5% water and 0.03% RHEOCELL RHEOFILLTM. Panel 6 uses SFL-224 refractory cement with 4.7% water and 0.03% RHEOCELL RBEOFILLTM.

As shown by the results in Mix 2B (shot) in Table 2B and panel 1 in Table 2C, the pump pressure is reduced by using the foaming agents and methods of the present invention. The reduced pump pressure demonstrates that the mixtures had increased flowability.

We have found that the use of the refractory cementitious mixture of the present invention, containing an organic foaming agent, can be sprayed by the disclosed shotcrete process to form a dense, non-porous, refractory cementitious coating on a substrate. The foamed mixture has increased flowability, and is easily pumped and sprayed. The impact of the sprayed mixture against the substrate, with added sufficient compressed air, breaks the foamed concrete stream so as to expel the air upon impact with the substrate, and to result in a non-porous coating.

It should be appreciated that the present invention is not limited to the specific embodiments described above, but includes variations, modifications and equivalent embodiments defined by the following claims.

Claims

CLAIMS 1. A refractory mixture comprising a refractory cement, an organic foaming agent, and aggregate.
2. A refractory mixture according to claim 1, wherein the refractory cement is selected from the group consisting of calcium aluminate cement, colloidal silica, silicon oxide, hydratable alumina, hydratable aluminum oxide, magnesia, and mixtures thereof.
3. A refractory mixture according to claim 1 or claim 2, wherein the foaming agent is selected from the group consisting of alkanolamides, alkanolamines, alkylaryl sulfonates, polyethylene oxide-polypropylene oxide block copolymers, alkylphenol ethoxylates, carboxylates of fatty acids, ethoxylates of fatty acids, fluorocarbon containing surfactants, olefin sulfonates, olefin sulfates and mixtures thereof.
4. A refractory mixture according to any one of claims 1-3, wherein the mixture comprises a stiffening agent selected from the group consisting of pre gelatinized starches, cellulose ethers, polyethylene oxides, alginates, carrageenans, polyvinyl alcohol, synthetic polyelectrolytes, natural gums, and mixtures thereof.
5. A refractory mixture according to claim 4, wherein the stiffening agent is a mixture of cellulosic ethers.
6. A refractory mixture according to any one of claims 1-5, wherein the mixture further comprises an additive selected from the group consisting of dispersants, water reducing agents, set retarders, set accelerators, pigments, and mixtures thereof.
7. A method for spraying a substrate with a refractory mixture according to any one of claims 4-6; comprising: a. providing a cementitious mixture comprising a cement, an organic foaming agent, aggregate, and water; b. foaming the cementitious mixture; c. conveying the mixture to a spray nozzle; d. introducing compressed air and an amount of a non-accelerating stiffening agent to the spray nozzle sufficient to provide for substantially instantaneous stiffening of said refractory mixture on said substrate; and e. spraying said refractory mixture onto said substrate, wherein said mixture substantially instantaneously stiffens upon contact with said substrate, and hydrates to form a substantially defoamed, non-porous refractory coating.
8. A method according to claim 7, wherein conveying is one of pumping and pneumatic flowing.