Understanding refractory api 936 reading iv

$Page 1: Understanding refractory api 936 reading iv$
Understanding REFRACTORY For API936 Personnel Certification ExaminationReading 4- ACI Committee 547 My Pre-exam Self Study Notes26th September 2015

Charlie Chong/ Fion Zhang


Refractory for Steel Making

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Refractory for Petrochem


BODY OF KNOWLEDGE FORAPI936 REFRACTORY PERSONNELCERTIFICATION EXAMINATIONAPI certified 936 refractory personnel must have knowledge of installation, inspection, testing and repair of refractory linings. The API 936 Personnel Certification Examination is designed to identify applicants possessing the required knowledge. The examination consists of 75 multiple-choice questions; and runs for 4 hours; no reference information is permitted on the exam. The examination focuses on the content of API STD 936 and other referenced publications.


REFERENCE PUBLICATIONS:A. API Publications: API Standard 936; 3rd Edition, Nov 2008 - Refractory Installation Quality Control

Guidelines - Inspection and Testing Monolithic Refractory Linings and Materials.

B. ACI (American Concrete Institute) Publications: 547R87 - State of the art report: Refractory Concrete 547.1R89 - State of the art report: Refractory plastic and Ramming Mixes

C. ASTM Publications: C113-02 - Standard Test Method for Reheat Change of Refractory Brick C133-97 - Standard Test Methods for Cold Crushing Strength and Modulus of

Rupture of Refractories C181-09 - Standard Test Method for Workability Index of Fireclay and High

Alumina Plastic Refractories C704-01 - Standard Test Method for Abrasion Resistance of Refractory Materials

at Room Temperatures


Today’s Exam Result Releases (ASNT)3 passes 1 flop. It is bad, not too bad.


too

20150925

Today’s Exam Result Releases (ASNT)3 passes 1 flop. It is bad, not too bad as long you have not give-up!


20150925

Exam Result Releases3 passes 1 flop. It is bad, not too bad.



http://independent.academia.edu/CharlieChong1http://www.yumpu.com/zh/browse/user/charliechonghttp://issuu.com/charlieccchong

Charlie Chong/ Fion Zhang http://greekhouseoffonts.com/



The Magical Book of Refractory


Fion Zhang at Shanghai26th September 2015


Video Time- shotcrete refractory


■ https://www.youtube.com/watch?v=s81LE7XXZ4A&list=PLey7s_Oct4OK9-7tMIx5cp9-RjSdetDTq

Reading IVContentStudy note One: Refractory Concrete: Abstract of State-of-the-Art ReportReported by ACI Committee 547 (haphazard book) Study note Two: Study note Three: Study note Four:



Refractory Concrete: Abstract of State-of-the-Art ReportReported by ACI Committee 547(haphazardly Clauses)


Refractory concretes are currently used in a wide variety of industrialapplications where pyreprocessing and/or thermal containment is required.The service demands of these applications are becoming increasingly severeand this, combined with the constant demand for refractories with enhancedservice life and more efficient means of installation, has resulted in an everexpanding refractory concrete technology. ACI Committee 547 has preparedthis state-of-the-art report in order to 547R-5 meet the need for a betterunderstanding of this relatively new technology.

The report presents background information and perspective on the history and current status of the technology. Composition and proportioning methods are discussed together with a detailed review of the constituent ingredients. Emphasis is placed on proper procedures for the (mixing?) installation, curing, drying, and firing. The physical and engineering roperties of both normal weight and light weight refractory concretes are reported, as are state-of-the-art construction details and repair/maintenance techniques. Also included is an indepth review of a wide variety of applications together with the committee‘s assessment of future needs and developments.


Chapter 1- Introduction1.1 Objective of reportThe objective of this report is to provide a source of information on the manyfacets of refractory concrete technology. The report is intended as a unifiedand objective source of information to aid the engineer or consumer incategorizing and evaluating monolithic refractory concrete technology and themany materials and processes available today. It is not intended to be aspecification or standard, and should not be quoted or used for that purpose.

1.2 Scope of reportRefractory concrete is concrete suitable for use at temperatures up to about 3400⁰F (1870⁰C). It consisted of a graded refractory aggregate bound by a suitable cementing medium. This report is concerned with refractory concrete in which the binding agent is a hydraulic cement, and does not consider concretes which use waterglass (sodium silicate), phosphoric acid, or phosphates as a principal cementing agent. It covers all facets of refractory concrete installation and use, including the properties of individual ingredients and concretes, placing techniques, methods of curing and firing, repair procedures, construction details, and current and future applications.


1.3 NomenclatureThe following definitions are used in this report:ACID REFRACTORIES - Refractories containing a substantial amount ofsilica that may react chemically with basic refractories, basic slags, or basicfluxes at high temperatures.

APPARENT POROSITY (ASTM C20) - The relationship of the volume of theopen pores in a refractory specimen to its exterior volume, expressed as apercentage.

BASIC REFRACTORIES - Refractories whose major constituent is lime,magnesia, or both, and which may react chemically with acid refractories,acid slags, or acid fluxes at high temperatures. (Commercial use of this termalso includes refractories made of chrome ore or combinations of chrome oreand dead burned magnesite).


CALCIUM ALUMINATE CEMENT - The product obtained by pulverizingclinker which consists of hydraulic calcium aluminates formed by fusing orsintering a suitably proportioned mixture of aluminous and calcareousmaterials.

CASTABLE REFRACTORY - A proprietary packaged dry mixture of hydraulic cement and specially selected and proportioned refractory aggregates which, when mixed with water, will produce refractory concrete or mortar.

CERAMIC BOND - The high strength bond which is developed between materials, such as calcium aluminate cement and refractory aggregates, as a result of thermochemical reactions which occur when the materials are subjected to elevated temperature.

EXPLOSIVE SPALLING - A sudden spalling which occurs as the result of a build-up of steam pressure caused by too rapid heating on first firing.

GROG – Burned refractory material, usually calcined clay or crushed brick bats.


HEAT RESISTANT CONCRETE - Any concrete which will not disintegratewhen exposed to constant or cyclical heating at any temperature below whicha ceramic bond is formed.

HIGH ALUMINA CEMENT - See calcium aluminate cement.

NEUTRAL REFRACTORIES - Refractories that are resistant to chemical attack by both acid and basic slags, refractories, or fluxes at high temperatures.

REFRACTORY AGGREGATE - Materials having refractory properties which form a refractory body when bound into a conglomerate mass by a matrix.

REFRACTORY CONCRETE – Concrete which is suitable for use at high temperatures and contains hydraulic cement as the binding agent.


SOFTENING TEMPERATURE - The temperature at which a refractorymaterial begins to undergo permanent deformation under specified conditions.This term is more appropriately applied to glasses than to refractoryconcretes.

THERMAL SHOCK - The exposure of a material or body to a rapid change in temperature which may have a deleterious effect.


1.6 Non-hydraulic refractory.The following discussion, while not pertinent to the main theme of the report,will be of some interest and use to the reader.

1.6.1 Refractory brick - High quality brick, known as firebrick, with uniquechemical and physical properties is obtained by blending different types ofclay and other ingredients and by varying both the method of processing andthe burning temperatures. In addition to the many varieties of fireclay brick,have been developed:■ high alumina, ■ insulating, ■ silica, ■ fused aggregate, and■ basic firebrick


Refractory brick remains a major construction material for applications in which heat containment and control is necessary and in many instances, is the only satisfactory solution to a specific problem. Brick has a number of disadvantages when compared to monolithic refractories. Thesedisadvantages include: multiple joints, complicated anchoring, higher placement costs, more difficult repair procedures, the need to maintain expensive inventories of special or scarce items, a certain inflexibility in structural design, and higher fuel requirements during manufacture.

Keywords:Brick refractoryMonolithic refractory


1.6.2 Plastics and ramming mixes - Plastic refractories and ramming mixesare refractories which are tamped or rammed in place and are used formonolithic construction, for repair purposes, and for molding special shapes.These materials find extensive use in industry. They usually employ a clay,alumina, magnesite, chrome, silicon carbide, or graphite base, and areblended with a binder.

Heat setting mixes are likely to contain fireclay or phosphoric acid H3PO4 as a binder.

Air or cold-setting mixes generally contain fireclay and sodium silicateNa2O as the binder.

Compared to ramming mixes, plastic refractories have higher moisture contents and therefore, higher plasticity.


Plastics are generally placed without use of forms. With the exception ofsome specialized tabular alumina castables, plastics have a somewhat higherservice limit than castable refractories. Their main disadvantages are greatershrinkage and crack development. Except for phosphate bonded materialscured above 600⁰F (315⁰C), plastics generally have lower cold and hotstrengths than refractory concretes. In addition, plastics tend to have arelatively low strength zone on the cool side of the lining. Ramming mixesusually have higher density and less shrinkage than plastic refractories. Withtheir low water content, they must be forced into place and require strongwell-braced forms. Some of the dryer medium grind ramming mixes aresuitable for gunning, and are used for patching and maintenance materials.


1.6.4 Gunning mixes other than refractory concretesAs used in this section, the term “gunning mixes” does not refer to refractoryconcrete and should not be confused with gunned refractory materials whichproduce refractory concrete.

Gunning mixes are mixtures of non-hydraulic setting ingredients which are installed hot or cold, usually by the shotcrete method. Gunning mixes generally have low rebound loss, are predominately used for patching or resurfacing brick or other refractories, have a strong internal bond, and exhibit excellent adhesion or bond to the existing refractory lining. They find extensive use in basic oxygen, electric arc and open hearth furnaces, among other applications.


Chapter 2 - Criteria for refractory concreteselection2.1 IntroductionRefractory concrete is usually made with high alumina cement. It is notgenerally used as a structural material and its primary purpose is as aprotective lining for steel, concrete or brick structures. Some of the destructive forces that refractory concretes withstand are abrasion, erosion.

Physical considered a consumable material requiring replace, abuse, hightemperatures, thermal shock, hot and molten metals, clinker, slag, alkalies,mild acid or replacement after an appropriate service life. acid fumes, expansion, contraction, carbon monoxide,and flame impingement.

Refractory concretes are categorized as either normal weight or lightweight. The former are also referred to as “heavy refractory concretes” and the latter are often called “insulating refractory concretes.” Table 2.la shows the characteristics of a typical range of normal weight refractory concretes; Table 2.lb shows the characteristics of lightweight refractory concretes.


2.2 Castables and field mixesRefractory concretes are usually prepared at the job site from materialssupplied to the user in either of two ways: (1) prepackaged so-called“refractory castables;” (2) field mixes.

Refractory castables are plant packaged mixes composed of ingredients that are weighed, blended and usually bagged in convenient sizes for shippingand handling. They require only mixing with water on the job to produce refractory concrete.

Field mixes are made from material components which are proportioned and mixed on the site just prior to the addition of water.


2.5 Load bearing considerationsMost application designs of refractory concrete consider that there is athermal gradient through the material with heat conducted from the hot face tothe cold face.

A cross section of the refractory will usually have a layer at the hot face that has a ceramic bond, an intermediate section with a weaker combination of ceramic and a partial hydraulic bond, and a cold face section that retains most of its hydraulic bond.

hot face that has a ceramic bond

an intermediate section with a weakercombination of ceramic and a partial hydraulic bond

cold face section that retains most of its hydraulic bond


Refractory concrete linings in this type of situation are usually well anchored and self-supporting. Castables containing high proportions of coarse aggregates produce refractory concrete with good load bearing characteristics. Certain types of refractory concrete tend to have low strengths in the intermediate temperature zones [1500⁰F -2250⁰F (820⁰C -1230⁰C)] and should not be subjected to excessive mechanical abuse or dead load. Generally, lightweight concretes designed for insulating purposes should not be subjected to impact, heavy loads, abrasion, erosion or other physical abuse. Normally, both the strength and the resistance to destructive forces decline as the bulk density of the refractory concrete decreases. There are a number of special refractory castables available which have better than average load-bearing capabilities and withstand abrasion much better than the standard types.


2.7 Corrosion influencesHigh temperature in combination with a corrosive environment can have aserious deleterious effect on both the concrete and the backup steel structure.Alkalies can effect the service life of refractory. Generally, the higher density,higher purity refracconcretes. The furnace charge can give off both alkalies(K2O) and the fuel sulfur compounds (SO2) as refractory concretes have better corrosion resistance than pores. These can penetrate into the pores of the refractory concrete and react; their reaction products the lower density, lower purity types. cool, solidify, and expand, sometimes causing the hot face of the refractory to peel or shear away.

In certain applications, the refractory concrete is subjected to highly reducing conditions. Low-iron refractory concretes should be used for this type ofapplication.


2.10 Abrasion and erosion resistanceAbrasion and erosion begin with the wearing away of the weakest matrixconstituent, binder, leaving the coarse or hard aggregate to eventually fallaway.

A hard aggregate, a high modulus of rupture MOR, and high compressivestrength at the hot face are necessary for good abrasion and erosionresistance in refractory concretes.


Chapter 3 - Constituent ingredients3.2 BindersThe binders principally used in refractory concretes are calcium aluminatecements. However, ASTM type portland cements can be used in somerefractory applications up to an approximate maximum of 2000⁰F (1090⁰C)with selected aggregates, if special precautions are taken to ensure a soundrefractory concrete. Cyclic heating and cooling tends to disrupt portlandcement concretes and adding a fine siliceous material to react with thecalcium hydroxide, formed during hydration, is helpful in alleviating theproblem. Calcium aluminate (high alumina) cements are commerciallyavailable hydraulic binders. They are specifically designed for use in monolithic refractory concrete construction.


They are generally classified under three basic categories: ■ Low Purity, ■ Intermediate Purity, and ■ High Purity.

This is a relative classification scheme and is based primarily on the total ironcontent of the cement. Binder selection is primarily based on the servicetemperature desired for the refractory concrete. Maximum servicetemperatures are extended with increasing Alumina, Al2O3 and decreasing iron contents (forming low temperature eutectic?) . Lower iron content binders are also beneficial in reducing carbon monoxide (CO) disintegration of concrete (Section 2.7).

Comments: Lower Fe content■ Reduce forming low temperature iron compound which go into liquid■ Reduce CO disintegration (how?)


3.3 AggregatesThe maximum service temperatures of selected aggregates mixed with appropriate calcium aluminate cements are listed in Table 3.3a. Thesemaximum temperatures are based on optimum conditions of binder andaggregate. Thermal properties of aggregates, such as volume change(expansion, shrinkage or crystalline inversion) and decomposition, can affectthese maximum temperatures, along with the chemical composition of bothaggregate and binder and the reactivity between these mix constituents.

Comment:inversion [1]: A change in crystal form without change in chemicalcomposition; as for example, the change from low quartz to high-quartz, orthe change from quartz to cristobalite.


Temperature stability of the aggregate determines the maximum serviceconditions below approximately 2400⁰F (1320⁰C). Therefore, any type ofcalcium aluminate cement can be used at these temperatures. For conditionsabove 2400⁰F (1320⁰C), binder purity also becomes a design factor.Generally, the low purity binder can be used with proper aggregates up to2700⁰F (1480⁰C), intermediate purity to 3000⁰F (1650⁰C) and high purity to3400⁰F (1870⁰C). Aggregate gradation is an important consideration indesigning refractory concrete. Table 3.3b provides suggested guidelines fornominal maximum size and grading of refractory aggregates.


For refractory mix designs a 1:3 or 1:4 by bulk volume dry basis cement:aggregate mix is generally used to satisfy typical applications. In certaincases the ratio may change from as low as 1:2 to as high as 1:6, with thelatter being used for lightweight concretes. Within the range of normal usage,increasing the cement content will provide higher strength development.However, increased cement content may also result in increased shrinkage.A higher aggregate content will increase insulating or refractory properties,depending on the type of aggregate selected for the mix. Combinations ofvarious aggregates can be made to secure the desirable properties of each.

3.3.1 Lightweight aggregates - Perlite, expanded shale, expanded fireclay,and bubble alumina are the more commonly used lightweight aggregate forcommercial insulating concretes.


TABLE 3.3a- Maximum service temperature of selected aggregates mixedwith calcium aluminate cements under optimum conditions


TABLE 3.3b- Aggregate grading


3.4 Effects of extraneous materialsExtraneous materials commonly associated with portland cements, either asadmixtures or as contaminants from equipment or surrounding conditions,may behave differently when used with calcium aluminate cement mixes.Many castables contain proprietary additions which may be adverselyaffected by field admixtures.

admixtures


Chapter 4 - Composition and proportioning4.1 IntroductionIn designing mixes, refractory concretes are not only defined by density butalso by operating temperature. Refractory concretes fall into three subclassesbased on service temperature ranges.

The first sub class is “ceramically- bonded concrete,” defined as concrete in which the cement binder and the fine aggregate particles react thermochemically to form a bond. This bond is referred to as the ceramic bond and may occur at temperatures as low as 1650⁰F (900⁰C).

The second subclass is “heat resistant concrete,” defined as concrete in which the cement has dehydrated but has not formed a ceramic bond.

The third category is concrete which still has some hydraulic bond when heated but performs satisfactorily under cyclic conditions.


4.3 Field mixes4.3.1 Ceramically bonded concrete - The ceramic bond can be formed attemperatures as low as 1650⁰F (900⁰C). To aid formation of the ceramic bond,concretes operating above this temperature should have 10-15 percent of theaggregate passing a No. 100 sieve. Most field insulating concretes are madewith presoaked aggregate. Since the specified proportions are based on drymaterials, the actual batch mixes may require correction.


4.3.2 Heat resistant concrete - This concrete is generally used in the range930⁰F (500⁰C) to 1650⁰F (900⁰C). Many coarse aggregates are unsuitable foruse as refractory aggregates because they contain quartz, which has a large volume change at 1065 ⁰F (575⁰C) .

4.4 Water contentA majority of the aggregates used in refractory and heat resistant concreteshave high water absorbency. For this reason specific water/cement ratios aregenerally not used in developing mix designs. Instead, water requirementsare arrived at by periodically conducting a “ball-in-hand” test (ASTM C860).This test is illustrated in Fig. 4.4. The correct water content is that which willprovide a placeable, rather than a pourable, mix. When using well-soakedaggregates, it may be necessary to add little or no water at the mixer. It issometimes found that a mixture which appears fairly stiff when dischargedfrom the mixer will yield excess water as the concrete is placed.



Chapter 5 - Installation5.1 IntroductionRegardless of the quality of the refractory cement, aggregate, and/or castable,and regardless of the research devoted to the selection of correct materialsfor a specific application, maximum service life will not be obtained unless therefractory concrete is installed properly. The most frequently used methods ofinstalling refractory concretes are casting and shotcreting.

5.2 Casting5.2.1 Mixing - Proper mixing of castables is of primary importance. Careshould be taken to avoid mixing previously hydrated material into freshrefractory concrete. Mixers, tools and transporting equipment used previouslywith portland or other type cement concretes must be cleaned prior to mixing.Remains of lime, plaster, or portland cement will induce flash set and willlower refractoriness.


Generally, paddle mixers are used for small to medium size jobs involvingcalcium aluminate cement concretes. In a paddle mixer, normal weightrefractory concretes should be mixed for about 2 to 4 min. Refractoryconcretes of less than 60 lbs/cu ft (960 kg/m3) density should be mixed nolonger than necessary to insure thorough wetting. This precaution isnecessary because the lightweight aggregate may break-up during the mixingaction and reduce the effectiveness of the concrete as a heat insulator.Refractory concretes in the 75 to 90 lb/cu ft (1200-1400 kg/m3) range shouldbe mixed for approximately 2 to 5 min. Because working time may be short,all castables should be cast immediately after mixing.


5.2.3 Mixing and curing temperature - Mixing and curing temperature can affect the type of hydrates formed in set concrete. A castable develops its hydraulic bond because of chemical reactions between the calcium aluminate cement and water. To get the maximum benefits from these chemical reactions, it is preferable to form the stable C3AH6 during the initial curing period. The relative amount of C3AH6 formed versus metastable CAH10 and C2AH8 can be directly related to the temperature at which the chemical reactions take place.

Note: C = CaO, A = Al2O3, H = H2O. = H2O.


Recent work illustrates the significant impact of mixing and curingtemperatures on strength properties. Fig. 5.2.3 shows the flexural strength of a tabular alumina, high purity cement castable plotted as a function of mixing and curing temperatures. It can be seen that the strength developed after mixing and curing at 85⁰F (30⁰C) and drying at 230⁰F (110⁰C) is nearly twice that of the concrete mixed and cured at 60⁰F (15⁰C) and dried at 230⁰F (110⁰C).

Explosive spalling of high purity cement concretes can occur when casting and curing temperatures below 70⁰F (21⁰C) are used. Thus, a refractory concrete containing a high purity cement should be cast or cured above 70⁰F (21⁰C). This spalling phenomenon is less likely to occur with low or intermediate purity cement binders.


Explosive spalling of high purity cement concretes can occur when casting and curing temperatures below 70⁰F (21⁰C) are used. Thus, a refractory concrete containing a high purity cement should be cast or cured above 70⁰F (21⁰C). This spalling phenomenon is less likely to occur with low or intermediate purity cement binders.

21⁰C


Fig. 5.2.3 - Flexural strength of tabular alumina, high purity cement castable (ASTM C268)


Fig. 5.2.3 - Flexural strength of tabular alumina (MOF?), high purity cement castable (ASTM C268)


5.2.4 Transporting - Other than shotcreting and pumping, the techniques for transporting refractory concretes are similar to those used for portland cementconcrete. Some calcium aluminate cement binders have a shorter placing time available.


5.3 ShotcretingShotcreting of refractory concrete is particularly effective where, (1) forms areimpractical, (2) access is difficult, (3) thin layers and/or variable thicknessesare required, or (4) normal casting techniques cannot be employed.

Note: from the aforementioned, it sounds that casting is the preferred method?

5.3.1 Equipment - There are two basic types of shotcrete methods: dry-mix and wet-mix.

■ The drymix method conveys the aggregate and binder pneumatically to the nozzle in an essentially dry state where water is added in a spray.

■ The wet-mix method conveys the aggregate, binder and a predeterminedamount of water, either pneumatically or under pressure, to the nozzle where compressed air is used to increase the velocity of impact.

The dry method, though it produces greater rebound, is the most suitable andrecommended technique for shotcreting refractory concrete. An exception isthe recommended use of a wet-mix gun for hot patching.


The dry method, though it produces greater rebound, is the most suitable andrecommended technique for shotcreting refractory concrete. An exception isthe recommended use of a wet-mix gun for hot patching.


5.3.2 Installation - To ensure a uniform covering free of laminations and withminimum rebound, the nozzleman should move the nozzle in a small circularorbit and where possible, maintain the flow from a 3- 4 ft (0.9-1.2 m) distanceat right angles to the receiving surface. The shotcrete should be left in itsasplaced state. If for some reason scraping or finishing is required, theabsolute minimum should be done so as to avoid breaking the bond orcreating surface cracks. Shotcreting of refractory concretes can increase thein-place density and result in other changes in the physical properties. Thiseffect is more pronounced in lower density castables, and must be taken intoaccount when specifying thicknesses and material quantities for insulatingapplications. The user should be aware that certain aspects of portlandcement concrete shotcrete practice do not apply to refractory shotcrete.


5.4 Pumping and extrudingCertain refractory concretes can be installed with positive displacementpumps in conjunction with rigid or flexible pipelines. The design of the mix iscritical, and special attention must be given to the absorptive characteristicsand sizing of the aggregate. Some applicators use the term “extruding” todescribethe conveying and placing of refractory concrete at velocities that arevery low or close to zero on exit from the pipeline. When extruding, mixing ofthe refractory castable and water can be done internally or externallydepending on type of extruding device.


5.5 Pneumatic gun castingPneumatic gun casting, or gun casting, is a relatively new technique forcasting concrete and is finding increased uses for refractory concrete.Conventional dry shotcrete equipment and procedures are utilized with theexception that an energy reducing device is attached to the nozzle body inplace of the standard shotcrete nozzle tip.

5.8 FinishingSurface finishing or rubbing of refractory concretes should be kept at a minimum. Use of a steel trowel should be avoided, and the final surface can be lightly screeded to grade but should not be worked in any manner.


Shotcreting


Shotcreting


Chapter 6 - Curing, drying, firing6.1 IntroductionRefractory concrete should be properly cured for at least the first 24 hr.Following this curing it should be dried at 220⁰F (105⁰C), and then heatedslowly until the combined water has been removed before heating at a morerapid rate.

6.2 Bond mechanismsCalcium aluminate cements have anhydrous mineral phases which react with water to form alumina gel and crystalline compounds which function as abinder for the concrete. The hydration of these cements (Fig. 6.2) is exothermic. The rate of the chemical reaction is relatively fast. For all practicalpurposes, calcium aluminate concretes will develop full strength within 24 hr of mixing. The total drying shrinkage of calcium aluminate cement concretes in air, is comparable to that of portland cement concrete. In order to provide for complete hydration, and to control drying shrinkage, special attention must be given to the curing of refractory concretes.


Fig. 6.2 - Hydration reaction products of calcium aluminates


The Wiki: Phase diagram of calcium aluminates present in the anhydrous calcium aluminate cement before hydration.

https://en.wikipedia.org/wiki/Calcium_aluminate_cements

2570


6.3 CuringThe temperature of hardening calcium cement rises rapidly. If the exposedsurfaces are not kept damp, the cement on the surface may dry out before itcan be properly hydrated. The application of curing water prevents thesurface from becoming dry and furnishes water for hydration. In addition, theevaporation has a cooling effect which helps to dissipate the heat of hydration.

Conversion of the high alumina cement hydrates, which occurs if the cementis allowed to develop excessive heat, does not present the same problem inrefractory concretes that it does in high alumina cement concretes used forstructural purposes. It has been shown that if refractory concrete is fullyconverted by allowing it to harden in hot water and then heated to 2500⁰F(1370⁰C), the fired strength is equal to that obtained for well cured concrete.When possible, however, refractory concrete should be kept cool byappropriate curing under 210⁰F (99⁰C) for two reasons:


The entire refractory concrete structure does not usually reach the maximum service temperature, and the higher cold strengths obtained by good curing may be useful in the cooler portions of the refractory.

If the temperature within the concrete reaches a high level during hardening, the thermal stresses produced during cooling may be sufficient to cause cracking.


Curing should start as soon as the surface is firm. Under normal atmospherictemperatures, this will occur within 4 to 10 hr after mixing the concrete. Theconcrete should be kept moist for 24 hr by covering with wet burlap, by finespraying or by using a curing membrane. Alternate wetting and drying can bedetrimental to the cure of the concrete. When using a curing membrane, thecompound should contain a resin and not a wax base, and should be appliedto the surface as soon as possible after placing and screeding. The reason fordiscouraging the use of wax is that a hot surface will melt the wax, causing itto be absorbed into the concrete, breaking the membrane.


6.4 DryingThe large amount of free water in the refractory concrete necessitates adrying period before exposure to operating temperatures. Otherwise, theformation of steam may lead to explosive spalling during firing.


6.5 FiringFollowing drying of the refractory concrete, the first heat-up should be at areasonably slow rate. A typical firing schedule, for a 9 in. (22.9 cm) thick lining,consists of applying a slow heat by gradually bringing the temperature up to220⁰F (105⁰C), and holding for at least 6 hr. The temperature is then raised ata rate of 50-100⁰F (10⁰C - 40⁰C) per hr up to 1000⁰F (540⁰C) and again held for at least 6 hr. The first hold is to allow remaining free water to evaporate,and the second hold is to eliminate the combined water without danger of spalling.

Beyond 1900⁰F (540⁰C), the temperature of the refractory concrete can beraised more rapidly. Calcining of the green concrete into a refractory structurewill take place between 1600⁰F (820⁰C) and 2500⁰F (1370⁰C). Wall thicknessand mix variations may require somewhat different rates of heating, but thehold temperatures should remain at least 6 hr. If steam is observed duringheat-up, the temperature should be held until steam is no longer visible.


typical firing schedule, for a 9 in. (22.9 cm) thick lining, consists of applying a slow heat by gradually bringingthe temperature up to 220⁰F (105⁰C), and holding for at least 6 hr. The temperature is then raised at a rate of 50-100⁰F (10 - 40⁰C) per hr up to 1000⁰F (540⁰C) and again held for at least 6 hr. The first hold is to allow remaining free water to evaporate, and the second hold is to eliminate the combined water without danger of spalling. Beyond 1900⁰F (540⁰C), the temperature of the refractory concrete can be raised more rapidly. Calcining of the green concrete into a refractory structure will take place between 1600⁰F (820⁰C) and 2500⁰F (1370⁰C)..

105⁰C@6hrs

540⁰C@6hrs

820⁰C ~ 1370⁰C@6hrs

dT/dt = 10⁰C - 40⁰C/hr

Tem

pera

ture

Time


Chapter 7 - Properties of Normal WeightRefractory Concretes7.1 IntroductionThere are various physical properties and tests which are standard in therefractory industry and these are usually provided in the materialspecifications. Table 2.1a is an example of typical data for normal weightrefractory concrete.


7.2 Maximum service temperatureThe recommended maximum service temperature will normally assume thatthe castable will be used in a clean, oxidizing atmosphere, such as is presentwhen firing with natural gas. The maximum service temperature is usuallydetermined as the point above which excessive shrinkage will take place. It isabout 150-200⁰F (70-90⁰C) below the actual softening point of the concrete. Iffuel has solid impurities, such as in coals or heavy fuel oils, or if the solids ordust in the process contact the refractory, the maximum permissible servicetemperature will usually be considerably reduced. Solid impurities can reactwith the concrete and produce compounds of lower melting point which meltand run. This is generally referred to as slagging. The lower softening pointthus represents a limit for the operating temperature. Slag forming reactionsusually do not occur below about 2500⁰F (1320⁰C) except in the presence ofalkalies where reactions can occur in the 1900-2000⁰F (1040-1090⁰C) range.A reducing atmosphere can lower the melting point and hence the maximumoperating temperature by 100-200⁰F (40-90⁰C) if sufficient quantities of ironcompounds are present in the refractory.


7.4 Shrinkage and expansionIn discussing shrinkage and expansion of a refractory concrete, it is importantto define the distinction between:■ the independent effects of permanent shrinkage or ■ expansion and reversible thermal expansion.

Permanent change is determined by measuring a specimen at room temperature, heating it to a specified temperature, cooling to roomtemperature, and remeasuring it. The difference between the two measurements is the permanent change, which occurs during the first heating cycle. Subsequent heating to the same or lower temperature will have little or no additional effect on the permanent change. Heating to a higher temperature may cause some additional permanent change.

Reversible thermal expansion of a specimen which has been previously stabilized against further permanent change, is the dimensional change as a specimen is heated. Upon cooling, the specimen contracts to its original size.At any given temperature, the net dimensional change of a refractory concrete is the sum of the reversible expansion and the permanent shrinkage corresponding to the highest temperature to which the castable has been heated.


At any given temperature, the net dimensional change of a refractory concrete is the sum of the reversible expansion and the permanent shrinkage corresponding to the highest temperature to which the castable has been heated.

New higher temperature

Previous higher temperaturereversible expansion

permanent shrinkagecorresponded to +∆T

∆T


7.4.1 Permanent shrinkage and expansion - The initial heating of a refractoryconcrete usually causes shrinkage. At higher temperatures permanentexpansion can occur. This effect, which varies with the maximum temperatureattained, must be considered with reversible thermal expansion whencalculating the net expansion (or shrinkage) at service temperature. TheASTM rating of castables is based on no more than 1.5 percent permanentlinear shrinkage occurring at prescribed temperatures (ASTM C64 and C401).Most normal weight refractory concretes will have less than 0.5 percentpermanent linear shrinkage after firing at 2000⁰F (1090⁰C). The permanentchange appears as cracks after the first firing. These cracks will generally beabout 2-3 ft (0.6-0.9 m) on centers, and may vary, depending on the concretethickness and the anchor spacing. Usually, the width of the cracks at roomtemperature is partly dependent on the permanent shrinkage. Normally, thecracks will be tightly closed at operating temperatures. Such cracking, whichmay start during drying, is to be expected and will not adversely affect theservice performance of the refractory.


7.4.2 Reversible thermal expansion - The reversible thermal expansion ofmost refractory concretes is approximately 3 x 10-6 in./in./⁰F (5 x 10-6

cm/cm/⁰C However, the expansion coefficient may be as high as 4 x 10-6

in./in./⁰F (7 x 10-6 cm/cm/⁰C) for high alumina concretes and to 5 x 10-6

in./in./⁰F (9 x 10-6 cm/cm/⁰C) for chrome castables.

Reversible thermal expansion:■ most refractory concretes: 3 x 10-6 in./in./⁰F (5 x 10-6 cm/cm/⁰C■ high alumina concretes: 4 x 10-6 in./in./⁰F (7 x 10-6 cm/cm/⁰C) ■ chrome castables: 5 x 10-6 in./in./⁰F (9 x 10-6 cm/cm/⁰C)

Fig. 7.4.2 shows typical length changes due to permanent shrinkage and reversible expansion.


Fig. 7.4.2 - Net thermal expansion of a typical refractory concrete


7.5 Strength7.5.1 Modulus of rupture - Modulus of rupture is measured by means of aflexure test and is considered as a measure of tensile strength (ASTM C268).The extreme fiber tensile strength calculated from this test will be 50 to 100percent higher than the tensile strength derived from a straight pull test.Typical modulus of rupture values are 300 to 1500 psi (2.07-10.4 MPa).Shotcreting can increase modulus of rupture values by up to 50 percent. Fig.7.5 shows typical trends of modulus of rupture strength versus temperature.

7.5.2 Cold compressive strength (crushing) – The test is ordinarily run on 9 x4½ x 2½ in. (22.9 x 11.4 x 6.4 cm) specimens 9” straights in brickterminology with pressure applied to the smallest. surface (ASTM C133).Failure in this test is generally due to shear. Crushing strengths vary from1000 to 8000 psi (6.9 to 55.2 MPa). Typically, compressive strengths arethree to four times (3 ~ 4 X) greater than modulus of rupture values.

4½2½

9


Fig. 7.5 - Effect of temperature on modulus of rupture


7.6 Thermal conductivityFor normal weight refractory concretes, thermal conductivity tends to varywith density. Typical values (k factors) range from about 5 Btu-in./sq ft -hr-⁰F(72 W-cm/m2-⁰C) for 120 pcf (1920 kg/m3) material to about 10 Btu-in./sq ft–hr-⁰F (144 W-cm/m2-⁰C) for 160 pcf (2560 kg/m3) material. There is usually anincrease in thermal conductivity with temperature.

120 pcf: 5 Btu-in./sq ft -hr-⁰F160 pcf: 10 Btu-in./sq ft–hr-⁰F

7.10 Specific heatThe specific heat of a refractory concrete increases with temperature from about 0.20 Btu/lb/⁰F (837 J/ kg-⁰C) at 100⁰F (40⁰C) to about 0.29 Btu/lb/⁰F (1210 J/ kg-⁰C) at 2500⁰F (1370⁰C). This can vary plus or minus 0.025 units, depending on the aggregate.

at 100⁰F: 0.20 Btu/lb/⁰Fat 2500⁰F: 0.29 Btu/lb/⁰F


Chapter 8 - Properties of lightweight refractory concretes8.1 IntroductionRefractory concretes are widely used as insulating materials. They have awide range of densities (20 to 100 pcf (320 to 1600 kg/m3) and can beformulated to have high maximum service temperatures and relatively highstrengths. This often allows the use of these materials as single component,exposed service linings. Table 2.1b shows physical property values for typicallightweight refractory concretes.


8.4 Shrinkage and expansionThe reversible thermal expansion of lightweight concretes will vary from 2.5 x 10-6 to 3.5 x 10-6 in./in./⁰F (4.5 x l0-6cm/cm/⁰C) Because of compensating permanent shrinkage, the thermal expansion of lightweightrefractory concrete is normally insignificant and is usually ignored in the design of lightweight refractory concrete systems.

8.5 StrengthStrengths of lightweight refractory concrete are measured by both a modulus of rupture and a crushing test.8.5.1 Modulus of rupture - Typical values range from approximately 50 (0.3 MPa) to 400 psi (2.8 MPa).


8.6 Thermal conductivityThermal conductivity is one of the most important physical properties of alightweight refractory concrete and is controlled primarily by the density of theconcrete. For hydraulically bonded, alumina-silica concretes, a usablecorrelation exists between concrete density [after drying at 230⁰F (110⁰C)]and the thermal conductivity (k factor). Typically, the thermal conductivity forinsulating concretes ranges from 1 to 4 Btu-in./sq ft-hr-⁰F (0.1 to 0.6 W/M2-C).

8.6.2 Cold compressive strength (crushing) – Cold crushing strengths varyfrom 200-500 psi (1.4-3.5 MPa) for lightweight refractory concretes withdensities up to 50 pcf (800 kg/m3). For materials having densities in the 75-100 pcf (1200-1600 kg/m3) range, the cold crushing strength varies from 1000- 2500 psi (6.9-17-3 MPa). Table 8.5.1 shows the difference between the cold and hot modulus of rupture for a typical 2800⁰F (1540⁰C) lightweightrefractory concrete.


TABLE 8.5.1 - Hot and cold modulus of rupture of a 2800 ⁰F (1538 ⁰C) lightweight refractory concrete containing expanded fireclay aggregate


TABLE 8.5.1 - Hot and cold modulus of rupture of a 2800 ⁰F (1538 ⁰C) lightweight refractory concrete containing expanded fireclay aggregate


8.10 Specific HeatThe specific heat of a lightweight refractory concrete is approximately thesame as that of normal weight concrete. The range is from 0.2 Btu/lb/⁰F (837J/kg-⁰Cl at 100⁰F (40⁰C) to approximately 0.3 Btu/lb/⁰F (1255 J/kg-⁰C) at2500⁰F (1370⁰C).


Chapter 9 - Construction details9.1 IntroductionConstruction details are an important ingredient in the successful applicationof refractory concrete. Proper design details and careful implementation areessential, and parameters such as support structure integrity, forms, anchors,and construction joints have a major influence on the overall quality andperformance of refractory concrete installations.

9.2 Support structureNormally, refractory concrete is permanently supported by a back-upstructure. The support material is usually bolted or welded steel which, priorto installation of the refractory concrete, should be checked to ensure thatthere is no warpage and that all joints are structurally sound and tight.

9.3 FormsBoth metal and wood forms are used for refractory concrete.


9.4 AnchorsAn anchor is a device used to hold refractory concrete in a stable positionwhile counteracting the effects of dead loads, thermal stressing and cycling,and mechanical vibration. Anchors and anchoring systems are not designedto function as reinforcement. Anchors are produced as alloy steel rods orcastings, and prefired refractory ceramic shapes. The requirements of aparticular installation will determine the type and positioning of anchors.Typical factors to be considered are: unit size, wall thickness, number ofrefractory concrete components, area of application, and service temperature.


9.4.1 Metal anchorsThe most frequently used metal anchors are V-clips,studs, and castings. However, in special applications, welded wire fabric, hexsteel and chain link fencing are used. Generally, metal anchors are extendedfrom the cold face for 2/3 to 3/4 of the lining thickness and are staggered toavoid formation of planes of weakness. Metal V-clips, stud anchors andcastings are available in carbon steel, Type 304 stainless alloy, Type 310stainless alloy, and other suitable alloys. The choice of material depends onthe temperature to which the anchors will be exposed.

Carbon steel can be used for anchor temperatures of up to 1000⁰F (540⁰C).

Type 304 stainless is suitable for anchor temperatures of up to 1800⁰F (980⁰C) and

Type 310 stainless is adequate up to 2000⁰F (1095⁰C). Depending on the grade of alloy,

alloy steel castings can sustain a maximum temperature of between 1500⁰F (815⁰C) and 2000⁰F (1095⁰C).

ceramic anchors are available with maximum service temperature ratings of up to 3200⁰F (1760⁰C). (see next clause)


9.4.2 Pre-fired refractory anchors (ceramic anchors) –The principal use of ceramic anchors is to anchor refractory plastic, rather than refractory concrete. However, ceramic anchors are used in areas whererefractory concrete is subjected to high service temperature. In addition, they are sometimes used as a substitute for metal anchors where concrete thicknesses are 9 in. (230 mm), or greater. Ceramic anchors usually are composed of refractory aggregates, clays, and binders. They are mechanically pressed into shapes which provide for attachment to either the wall or roof and are ribbed to aid in securing the refractory concrete. Ceramic anchors are pre-fired at elevated temperature to provide a strong, dense structure. Depending on the composition, service conditions, and other factors, ceramic anchors are available with maximum service temperature ratings of up to 3200⁰F (1760⁰C). Ceramic anchors are attached to structural wall or roof supports by bolts and/or metal support castings. In order to minimize the tendency of the refractory concrete to sheet spall, the hot face of the ceramic anchor should extend to the hot face of the refractory concrete.


9.4.6.1 Thin single component linings.Plain metal chain link fencing is often used to anchor single componentlinings, less than 2 in. (50 mm) thick, composed of lightweight or medium weight refractory concrete and exposed to low to moderate mechanicalstresses and/or service temperatures.

9.4.5.2 Single component linings up to 9 in. (230 mm) thick.Normally, single component linings 2 in. (50 mm) to 9 in. (230 mm) thick, composed entirely of lightweight, medium weight or normal weight refractoryconcrete, and exposed to moderate stresses and service temperatures use metal anchors.

9.4.5.3 Single component linings greater than 9 in. (230 mm) thick.Normal weight refractory concrete linings, greater than 9 in. (230 mm) thick, utilize either ceramic or metal anchors. The type of anchor chosen will depend on the operating parameters.


9.4.5.4 Roofs.Two types of anchor systems, internal and external, are used for single component roofs. The choice depends on roof thickness and on constructionand design preferences.

9.4.5.5 Multicomponent linings.Multicomponent linings of 9 in. (230 mm) or less in thickness which aresubjected to moderate service temperatures and mechanical stresses shouldemploy metal anchors. Multicomponent linings of 9 in. (230 mm) or greaterthickness, composed of a combination of lightweight or medium weightrefractory concrete as back-up in conjunction with a normal weight refractoryconcrete, can use a combination of ceramic and metal anchors. Withmulticomponent shotcrete linings, the backup component is applied directly tothe shell and provisions must be made either to protect the anchor (metal orceramic) from rebound build-up, or to clean the anchor after placing of theback-up layer. Rebound build-up can destroy the grip between the heavyweight refractory concrete and the ceramic anchor.


9.5 Reinforcement and metal embedmentThe use of steel as a reinforcement should be avoided. In general, the metalwill cause cracking due to the differential expansion, caused by temperatureor oxidation, between the metal and concrete. For the same reason heavymetal objects such as bolts, pipes, etc. should never be embedded inrefractory concrete.


9.6 JointsIn cast installations, construction joints occur at the junction of walls and roofsor where large placements are broken into separate sections. Cold joints ofthis type will not bond and should be avoided where it is necessary to containliquid or gases. It is often necessary to include a provision for expansion.Expansion joints can be formed by inserting materials such as wood,cardboard, expanded polystyrene or ceramic fiber in the appropriate location.Shotcrete installations require construction joints at transitions betweenmaterials, or when application must be curtailed due to shift changes ormaterial supply. In these cases, the in situ refractory concrete should betrimmed back to produce a clean edge perpendicular to the shell. (not cascaded!) Expansion compensating materials are not generally inserted intothis type of joint. If a joint edge is allowed to stand for a prolonged period of time (more than 4 hr), it should be thoroughly moistened before any new material is applied.


Chapter 10 - Repair10.1 IntroductionRepair of refractory concrete should be considered only when economicsdictate that cost and downtime do not justify complete replacement. Beforeundertaking a repair, an effort should be made to determine the cause of theprevious failure. If possible, the design and/or construction details should bemodified to reduce the possibility of a recurrence of failure and to prolongservice life between repairs. Hot repair techniques are valuable for minimizingdowntime and for extending an operating run until a scheduled shutdown. Hotrepairs are especially suitable for temporary repairs of localized failures andhot spots.


Refractory Repair

http://www.rebl-refractories.com/refractory-issues/


Hot repairs are especially suitable for temporary repairs of localized failures and hot spots.(hot welding repair)

http://www.fosbel.com/Industries/Glass/Ceramic_Welding.aspx


Hot repairs are especially suitable for temporary repairs of localized failures and hot spots.(hot welding repair)

http://www.fosbel.com/Industries/Glass/Ceramic_Welding.aspx


10.2 Failure mechanismsSome of the phenomena that can cause failure are:

(1) Thermal stress and thermal shock; (2) Exposure to excessive temperatures; (3) Mechanical loading;(4) Erosion and abrasion: (5) Corrosive environments;(6) Anchorage failures and (7) Operational problems or upsets.


10.3 Surface preparationWhen the installation to be repaired is made of mortar or concrete, it isimportant to prepare the surface of the old material so that a mechanical bondwill be formed between it and the new refractory concrete. No significantchemical bond will be formed, and adhesion of the repair material mustdepend primarily on the mechanical bond. Preparation of the surface requiresremoval of all deteriorated or spalled materials and roughening of theexposed sound surface of the old concrete. In all cases, the chipping of oldmaterial must leave a flat base, and square shoulders approximatelyperpendicular to the hot face, completely around the perimeter of the repairsection. If this is done properly, there is no need to chamfer the edges orprovide fillets to walls and floors.

flat base

square shoulders approximatelyperpendicular to the hot facehot face


Once initial removal of loose concrete has been completed, the old refractory should be sounded with bars or hammers to make certain only sound material remains. Areas that were not chipped should be thoroughly sandblasted to remove any traces of soot, grease, oil or other substances that could interfere with the bond. Excess sand and loose debris must then be blown from the surface with compressed air. Particular care must be taken to remove any debris from around the anchors.


10.4 Anchoring and bondingIf possible, patches should be anchored with a minimum of two anchorswhich should be solidly attached to the shell. In cases where this isimpossible, anchors should be solidly embedded in the old refractory.Ceramic anchors should extend to the hot face of the new refractory concrete.Otherwise, sheet spalling may occur. If metal anchors are used, they shouldbe brought as close as possible to the hot face (3/4 or 2/3?) . The distance will depend on the metallurgy of the anchors and the thermal conductivity ofthe concrete. Where anchors are not practical, or repairs are shallow, mechanical bonding will be aided by cutting chases or keyways in a waffle pattern across the entire surface of the repair section and by slightly undercutting the existing refractory. In certain limited applications, where other means are not available, the bond may be improved by precoating the surface to be repaired with a bonding agent. When repairing refractory concrete with a similar cast-in-place material pre-wetting is required, and use of a neat calcium aluminate cement slurry may improve bonding.


Where anchors are not practical, or repairs are shallow, mechanical bondingwill be aided by cutting chases or keyways in a waffle pattern across theentire surface of the repair section and by slightly undercutting the existingrefractory.


Where anchors are not practical, or repairs are shallow, mechanical bondingwill be aided by cutting chases or keyways in a waffle pattern across theentire surface of the repair section and by slightly undercutting the existingrefractory.


Anchoring and bonding Minimum 2 anchor bonded to cold face or embedded in underlying

concrete Where anchors are not practical, or repairs are shallow, mechanical

bonding will be aided by cutting chases or keyways in a waffle pattern across the entire surface of the repair section and by slightly undercuttingthe existing refractory.

where other means are not available, the bond may be improved by precoating the surface to be repaired with a bonding agent.


10.5 Repair materialsA wide range of repair products is available for repairing refractory concrete.However, it is usually best to use a material similar to that being repaired.Refractory concrete is frequently used as a repair material and performssatisfactorily in many situations. Among the other available repair materialsare the following:

1. Air setting mortars;2. Phosphate-bonded and clay-based heat-setting mortars;3. Steel-fiber reinforced refractory concrete; (Steel-fiber reinforced refractoryconcrete will generally exhibit superior resistance to cracking and abrasion.However, the fibers will not perform well if the temperatures to which they areexposed induce oxidation. If the conditions are such that the fiber-reinforcedsystem is above the oxidizing, but below the melting temperature of theparticular fibers being used, it is possible that they may still be utilized,depending on the temperature gradient through the concrete, the furnaceatmosphere, the permeability of the concrete, the severity and frequency oftemperature cycles, the exposure time at maximum temperature, and themechanical loading.)


4. Plastic refractories and ramming mixes; and5. Hot repair materials. Some of the repair materials used for hot patching contain calcium aluminate cement as the principal binder, however, most donot. The latter utilize non-hydraulic and chemical binders (see Section 1.6.4). Since these materials are intended for temporary repairs, they may not haveservice life or properties equivalent to those in the original lining.While field mixes can be used for hot gunning, most applications use proprietary (prepackaged) materials which are specially designed for specific conditions of installation. Some manufacturers have designed special spray or gunning equipment and maintenance programs to install their hot repair materials on a planned basis.


10.6 Repair techniques10.6.2 Refractory concrete - When a refractory concrete is selected to effectrepairs, the type of placement procedure must insure that the full thickness ofthe repair section is installed in as short a time as possible, preferably in asingle lift. When refractory concrete is placed by the shotcrete method, certainprecautions must be followed. The area being repaired must be delineated in advance so that the concrete can be shot to the full section depth or thickness before any layer develops an initial set.

It is important that the refractory concrete be cured properly during the 24-hrperiod following placement (see Section 6.3). After the concrete has beenmoist-cured for 24 hr, drying and firing can be initiated (see Sections 6.4 and6.5). Speeding up the moist-curing, drying and firing can result in a markedreduction in the physical properties and life of the repair.


10.6.3 Plastic and ramming mixes - A refractory mortar coating may be usedto improve bonding when repairing refractory concrete with a plastic orramming mix. In order to achieve high density and prevent laminations, it isrecommended that plastic refractories be installed by the pneumatic rammingmethod using a steel wedge-type head. The basic pattern of ramming shouldbe to build up layers of plastic on top of the backing wall. The plastic is placedin strips and laid at right angles to the forms. It is important to angle thepneumatic rammer so that the strips are driven against the form, andsideways against the previously installed material. The repaired area shouldbe trimmed to a rough surface for more uniform drying. Moisture escapeholes should be made by inserting a 1/8 in. (3 mm) diameter pointed rod,approximately two-thirds of the depth of the material, on approximately 6 in.(150 mm) centers. In order to prevent formation of an outer skin, which canseal in moisture, a short period of forced drying of air-setting plasticrefractories is desirable.


Excessive temperature or direct flame impingement, which will seal the surface and prevent escape of moisture, must be avoided. The following heat-curing procedure has been found to give good results with plastic and ramming mixes:

Remove all free moisture at a temperature of not over 250⁰F (120⁰C). Following removal of free and absorbed moisture, raise the temperature at arate of 75-100⁰F (42-56⁰C) /hr until the desired operating temperature is reached. If steam is observed during heat-up, hold the present temperature until it stops. Whenever possible, repairs using plastic mixes should be carried out immediately prior to heat-up. A properly burned-in plastic will exhibit less cracking than a plastic exposed to lengthy air drying.


Remove all free moisture at a temperature of not over 250⁰F (120⁰C). Following removal of free and absorbed moisture, raise the temperature at arate of 75⁰F -100⁰F (42-56⁰C) /hr until the desired operating temperature is reached.

120⁰C

Operating Temperature

dT/dt = 42⁰C - 56⁰C/hrTem

pera

ture

Time


typical firing schedule, for a 9 in. (22.9 cm) thick lining, consists of applying a slow heat by gradually bringingthe temperature up to 220⁰F (105⁰C), and holding for at least 6 hr. The temperature is then raised at a rate of 50-100⁰F (10 - 40⁰C) per hr up to 1000⁰F (540⁰C) and again held for at least 6 hr. The first hold is to allow remaining free water to evaporate, and the second hold is to eliminate the combined water without danger of spalling. Beyond 1900⁰F (540⁰C), the temperature of the refractory concrete can be raised more rapidly. Calcining of the green concrete into a refractory structure will take place between 1600⁰F (820⁰C) and 2500⁰F (1370⁰C)..

105⁰C@6hrs

540⁰C@6hrs

820⁰C ~ 1370⁰C@6hrs

dT/dt = 10⁰C - 40⁰C/hr

Tem

pera

ture

Time


10.6.4 Steel-fiber reinforced refractory concrete10.6.4.1 Cast-in-place mixes.A problem with steel fibers is their tendency to “ball-up”. Clusters of fibers can be broken up by hand feeding or shaking of the sieve before addition to the concrete mix. In some cases, vibration will tighten up the fiber clusters and it is not a recommended method of fiber dispersal. The addition of steel fibers tends to reduce the workability of the mix. Normally, this can be overcome by internal or external vibration. Use of additional water is not recommended since this will degrade cured strength and increase the porosity.

10.6.4.2 Shotcrete mixes. Steel fiber reinforced refractory concretes can be shot into place by either the wet or dry process. Fiber lengths approaching the internal diameter of the material hose or nozzle can be shot successfully. Because rebound of the fibers can be dangerous, the nozzleman and support crew should wear protective clothing when dry shooting with steel fibers.


Steel Fiber Reinforced Concrete


Steel Fiber Reinforced Concrete

http://www.xpandrally.com/images-fibers-for-concrete.html


10.6.5 Hot repair proceduresHot repair procedures are based on standard shotcreting technology.However, because of the high temperatures, certain differences are necessary. Compared to normal shotcreting, the high temperatures require aspecially designed nozzle and an excessive amount of water in the mix in order to insure proper delivery, impingement, compaction, and material retention. Hot shotcreting requires that the nozzleman and a helper stand outside the furnace and manually or mechanically manipulate an extended nozzle or “lance” within the furnace. Special ports or openings must be provided in the furnace for proper access. The length, size, and design of the nozzle depends on the furnace configuration, temperature, and type ofapplication. In general, the best bonds are achieved when the vessel interior is a red or orange color (1500-1700⁰F (815-925 ⁰C). The refractory concrete repair must be allowed to heat-cure prior to placing the unit back in service. The length of time to accomplish this, although usually brief, will depend on the temperature at the time of repair, the type of material used for the repair, and the thickness of the installed material.


Chapter 11- Applications11.1 IntroductionRefractory concretes are currently used in a wide variety of industrialapplications where pyroprocessing or thermal containment is required.Because there are literally hundreds of refractory concretes available, it isimpossible to discuss every composition and its application. Accordingly, onlythe more important applications, where refractory concretes have been usedsuccessfully, are reviewed. Included in the review are the following industries:


a) Iron and steel(b)lNon-ferrous metal(c)lPetrochemical(d)lCeramic processing(e)lGlass(f) Steam power generation(g) Aerospace(h)lNuclear(i) Gas production(j) MHD power generation(k) Lightweight aggregate(l) Incinerator(m) Cement and lime


Chapter 12 -New development and future use of refractory concrete

12.1 IntroductionTraditionally, developments in the refractories industry have been closelyrelated to the process industries, the primary customers for the product. Inrecent years, there have been marked changes in the production and use ofrefractories. A number of factors have contributed to these changes including:

(a)development of new and improved industrial processes,(b)demand for higher temperatures and increased production rates

associated with the above developments,(c) improvement in the quality of refractory products and increased use of

different types of refractories, especially the monolithic castables and,(d) increased technical knowledge of the service behavior of refractory

materials.


With these technological advancements, investigations into the use ofrefractory concretes for special applications is increasing. Typical of thesenew and proposed applications are incinerators, coal gasification plants,chemical process plants, steel plants, and foundries.


12.2 New developments12.2.1 Steel fibers - The following potential advantages are offered by theuse of steel-fiberreinforcement in monolithic construction:

(a) improved flexural strength at ambient and elevated temperatures,(b) improved thermal and mechanical stress resistance,(c) improved thermal shock resistance,(d) improved spall resistance, and(e) improved load-carrying ability.

However, degradation of the steel fibers at high temperature can occur underservice conditions and, therefore, limit the full potential of these materials.Note: See References 197 through 205.

12.2.2 Shotcrete - The use of shotcrete for new refractory construction and forrepairs is a rapidly growing field and successful results have been achieved inmany applications.

12.2.3 Precast shapes - Increasingly, precast shapes are being used for special conditions and this trend will continue.


12.3 Research requirementsUnfortunately, selection and use of refractory concretes is still considered anart and, with a few exceptions, the properties of refractory concretes are notutilized in rational design schemes. In many instances, the wrong propertiesare being measured or the available data are not being used correctly. Futureresearch efforts should be directed towards obtaining a better understandingof the behavior of refractory concretes under service conditions. Increasedemphasis will be placed on elevated temperature properties and how they areinfluenced by such factors as proportioning, grading and compo sition.

Areas of needed research include the following:(a) Dimensional stability(b) Chemical attack(c) Mechanical properties(d) Property measurements and tests(e) Process conditions(f) Rational design procedures



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