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Heat-resisting and refractory concretes
Heat-resisting and refractory concretes Ron Montgomery
The definition of refractory concrete will vary depending on the specific reference. However, a good general definition is: Concrete which is suitable for use at high temperatures composed of hydraulic cement (calcium aluminate cement) as the binding agent. Combined with heat resistant, refractory aggregates and or fillers. The boundary between heat-resistant and refractory concrete is somewhat arbitrary, but is probably about 1000°C although some definitions of refractory concrete start at 1500°C. However, there is, in fact, a more or less continuous spectrum of high-temperature resistant concretes, extending from about 300-400°C (the limit of concretes bound with Portland cements) to 2000°C or more, using high range calcium aluminate cements (CAC) containing 80 per cent alumina.
Although they were not developed specifically for high-temperature use, the heat-resistant and refractory properties of CACs quickly became apparent. Early applications began
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Heat-resisting and refractory concretes from about the 1930s and significant growth of these applications took place from the 1950s onward. Ordinary Portland cement (OPC) has limited use at high temperatures for a number of reasons. At temperatures of ~500°C the hydrated lime or Portlandite (Ca(OH)2), which forms a significant proportion of hydrated Portland cement, will dehydrate to form quicklime (CaO) (Robson). This is a reversible reaction: Ca(OH)2 ¢:~ CaO + H20 Exposure to moisture (atmospheric moisture is sufficient) leads to rehydration of the quicklime which is an expansive reaction and will disrupt the concrete (Figure 4.1). Thus any application where there is cycling between high temperatures and ambient temperatures is excluded. Furthermore, Portland cement is high in lime and silica, which form low melting point compounds at the service temperature of the refractory concretes.
Figure 4.1 Portland cement and calcium aluminate cement concrete cylinders after 8 cycles of 6 hours at 500°C, followed by 24 hours in humid conditions.
Hydrated lime is not formed during the hydration of CAC. Thus it is not subject to the disruption caused by the rehydration of quicklime. The hydrates that are formed do dehydrate progressively as the temperature increases to about 300°C and above. However, the compounds formed are themselves stable and at even higher temperatures (> 1000°C) begin to react with the refractory aggregates (see section 4.4) to form new stable phases.
It is often said that the refractoriness (i.e. the softening point or service temperature limit) of CAC is governed by the alumina content. The higher the alumina, the more refractory
Heat resisting and refractory concretes
the cement. Whilst this is true in the first instance, refractoriness is also dependent on the presence or not of compounds that form low melting point eutectics - CaO, SIO2, Fe203. Lower grade grey CAC (-39 per cent A1203) contains relatively high amounts of these compounds- about 36 per cent CaO, 5 per cent SiO2 and 16 per cent Fe203. (Table 4.1) (Hewlett, pp. 709-778).
Table 4.1 Typical analyses of calcium aluminate cements compared to Portland cement Colour range
Grey Portland
Grey CAC 'low' range
Buff CAC 'mid' range
White CAC 'high' range
White CAC 'high' range
A1203 CaO SiO2 Fe203 TiO2 MgO
4-6 63-67 19-23 2-3.5 < 0.5 --1
36--42 36-40 3-8 12-20 -2 --1
48-60 36-40 3-8 1-3 -3 --0.1
65-75 25-30 < 0.5 < 0.5 -
80-82 15-20 < 0.2 < 0.2 -
As the alumina (A1203) content increases, in the higher grades of CAC, then the relative amounts of compounds that form low melting point eutectics decrease, leading to higher refractoriness (Figure 4.2) (Lafarge Aluminates Ltd). Ultimately these compounds are present in only trace quantities (< 1 per cent of total) and high-range CAC (-70 per cent A1203) is composed of almost pure calcium aluminates.
1900
Refractoriness (°C) 1800
1700 1600 1500 1400 1300 1200 AI203% CaO% Fe203% Si02%
S
f
Chemical composition 39 36 17 5
52 36 2 5
70 28 < 0.5 < 0.5
80 18 < 0.2 < 0.2
Figure 4.2 Relationshipbetween refractorinessand chemical compositionof calcium aluminate cements.
In order to extend the refractoriness further, even more free alumina is added to 70 per cent A1203 CAC to increase the alumina content to -80 per cent. This, with very few exceptions, is the current upper limit of the alumina content of modern CACs.
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Heat-resisting and refractory concretes
The aggregates used in heat-resisting and refractory concretes are generally not those used in conventional concrete. There is, if anything, a much wider range of these specialist aggregates available, most of them being synthetic or derived from heat treating naturally occurring materials. Much as with the cements, the temperature resistance of these aggregates depends on their nature and chemistry (Figure 4.3) and many of the same regulations regarding alumina content etc. hold true. However, the applications of heat-resisting and refractory concretes are extremely diverse and resistance to high temperature is not the only factor that needs to be taken into account. Such properties as thermal insulation, abrasion resistance; resistance to molten metals and slags, expansion/contraction etc. need to be considered. The final properties of the concrete depend very much on the correct combination of cement and aggregate and, as will be seen later, reactive fillers such as finely ground aluminas and fume silica. Heat-resisting and refractory aggregates lightweight aggregates? 350°C
Siliceous sand
500°C
Limestone
900°C
Basalt,granite, whinstone
1100°C
Synthetic (manufactured?) calcium aluminate
1400°C
42-44% AI203 firebrick
1550°C
Sillimanite, gibbsite
1800°C
Whitefused alumina
2000°C
Tabular alumina
Figure 4.3 Service temperature limits of heat resisting and refractory aggregates, when combined with the appropriate calcium aluminate cement.
Table 4.2 gives a non-comprehensive list of typical aggregates used in heat-resistant and refractory concretes together with an indication of their properties when combined with the appropriate CAC (Robson; Hewlett; Lafarge Aluminates Ltd).
........
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Concretes for use up to about ~ 1000°C are considered to be 'heat resistant'. However, as stated earlier, this temperature is not an immutable boundary but a convenient marker. Many heat-resistant concretes are used at temperatures well below 1000°C and sometimes above this temperature. The important thing is that the choice of cement and aggregate are appropriate to the application. It is likely that concrete technologists or engineers will come across the need to use heat-resisting, rather than true refractory concretes, at some time, as their applications are
Heat resisting and refractory concretes Table 4.2 Properties of heat-resisting and refractory concretes 1 Type of aggregate Insulating concretes
Type of CAC Colour
% A1203
Approximate service temperature limit2 (°C)
Approximate fired densit~¢ 3 (kg/m ~)
Indicative thermal conductivity 4 (W/m.°K)
Vermiculite Pedite Diatomite Pumice Expanded Clay Sintered PFA Expanded Chamotte
Grey Grey Grey Grey Grey Grey Grey Buff White White
39 39 39 39 39 39 39 50 70 80
900 900 900 900 1100 1100 1200 1350 1700 1800
550 600 850 1200 1450 1500 1500 1500 1500 1300
0.17 0.18 0.25 0.50 0.35-0.58 0.46--0.58 0.45-0.60 0.65-1.00
Grey Grey Grey Grey Grey Grey Grey
39 39 39 39 39 39 39
350 500 800 900 1000 1100 1200
2200 2200 1920 2400 2500 2600 1920
1.4 1.1 1.4 1.2-1.5 1.1
Grey
39
1200
2650
-
Grey Grey
39 39
1300 1300
1900 2050
0.9-1.1 0.9-1.1
Buff White White Grey Buff White Grey Buff White Grey Buff White White White White White White
50 70 80 39 50 70 39 50 70 39 50 70 80 70 80 70 80
1450 1500 1520 1350 1450 1600 1400 1500 1700 1400 1550 1650 1750 1800 1900 1900 2000
2030 2000 2100 2160 2160 2160 2560 2560 2560 3000 3000 3000 3050 3000 3050 2850 2900
0.7-1.0 0.5-0.7 0.8
Bubble Alumina Heat-resistant concretes
Siliceous Sand Limestone Crushed House Brick Basalt, Granite Emery, Alag "v~ Firebrick (35% A1203) Olivine Dense refractory concretes
Firebrick (44% A1203) Chamotte (42-44% A1203)
Silliminite, Gibbsite
Calcined Bauxite Brown Fused Alumina
White Fused Alumina Tabular Alumina
1.5
1.7 1.5-1.8 1.5-1.8 1.2-1.7 2.30 1.4-1.8 1.9-2.3
1 Robson (pp. 196, 197, 216); Lafarge Aluminates Ltd. 2 Maximum hot face temperatures, assuming a thermal gradient through the mass of the concrete. 3 Density after firing to 1000°C unless service temperature limit is lower. 4 Coefficients of thermal conductivity vary considerably depending on the temperature at which they are measured. The figures given are indicative values for 500-1000°C.
o f t e n m o r e c l o s e l y l i n k e d to c o n v e n t i o n a l c o n c r e t e a p p l i c a t i o n s . T h e s e s p e c i a l i s t c o n c r e t e s are o f t e n still m i x e d a n d p l a c e d o n - s i t e , as r e a d y m i x e d c o n c r e t e s c o n t a i n i n g C A C s are n o t g e n e r a l l y a v a i l a b l e . It is i m p o r t a n t to r e m e m b e r , h o w e v e r , that m o s t o f the f a c t o r s g o v e r n i n g
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Heat-resisting and refractory concretes conventional concretes will apply to heat-resistant concrete. Questions such as the relationships between shrinkage, strength, abrasion resistance, durability etc. and the water/cement ratio, compaction and, to a lesser extent, cement content will all hold true. However, there are other important parameters that must also be taken into account if the concrete is to perform correctly. Normally grey CAC (39 per cent alumina) will have sufficient temperature resistance for most heat-resisting applications up to 1000°C. Indeed in the case of some aggregates, it will be the properties of the aggregate, rather than the cement, that will limit the service temperature of the concrete, as these may be lower than the CAC itself, e.g. vermiculite, perlite, basalt and some heat-treated aggregates are themselves less refractory than grey CAC. In refractory concretes (see section 4.11) the reverse is often the case, and the higher refractoriness of the aggregate will extend the temperature range of a given CAC.
The thermal insulating properties of these concretes are primarily associated with their density and the major factor affecting density would be the density of the aggregate. Lightweight aggregates vary in density from the extremely light, (e.g. Perlite; bulk density 100-110 kg/m 3) to moderately light (e.g. Sintered PFA or expanded clay; bulk density 600-800 kg/m3). The insulating properties (i.e. the thermal conductivity) of concretes made with these aggregates will be in the range 0.15-0.5 W/m.°K (Figure 4.4) (Robson). 0.6 Thermal conductivity W/m °K at ~550°C
Expanded clay~ /
0.5
7
HT insulating brick I J l ' 0.4
Pumice 41~~
0.3 Diatomite~ Perlite 0.2
f
Vermiculite 0.1
Fired density (kg/m 3) 0
400
I
I
600
800
I
1000
/
I
1200
1400
1600
Figure 4.4 Relationship between fired density and thermal conductivity of heat-resisting insulating concretes (Robson, p. 217).
Heat resisting and refractory concretes This type of concrete would often be used in conjunction with a dense refractory concrete in heat-retaining vessels. The dense or 'hot face' concrete would then be on the inside of the vessel, with the insulating concrete behind. In this way the thermal gradients and heat losses can be closely controlled. Since CAC would be the cement binder for both of these concretes, they can be cast monolithically. Even when the hot face concrete requires a higher range CAC (e.g. 50-70 per cent alumina) due to the higher temperature inside the vessel in question, this does not pose a problem, as high- and low-range CACs are compatible with each other. As with heat-resisting concrete, grey CAC (39 per cent A1203) would normally be sufficiently heat resisting for insulating concrete, since the service temperatures of these materials rarely exceed 1000°C. There are, however, exceptions and very high temperature insulating concretes can be made with specialist aggregates such as bubble alumina or micro-porous calcium hexa-aluminate (CaO.6A1203). In this case the high-purity 70 per cent or 80 per cent A1203 CAC would be necessary, as service temperatures would be higher than the range of grey CAC.
The service conditions of heat-resistant and refractory concretes often also require a degree of abrasion resistance, due to direct physical contact (e.g. wheeled or tracked vehicles, billets of hot metal etc.) or erosion due to gas-borne particulates. Many dense, high-temperature concretes, although they may not be designed specifically for abrasion, have a reasonable degree of abrasion resistance. There are, however, some notable applications, where the severe conditions require that the concrete must withstand heat, thermal shock and heavy abrasion (e.g. floors in foundries and hot metal treatment or handling areas, fire training areas and constructions). For these applications there is a particularly suitable type of synthetic aggregate, which is made by crushing and grading calcium aluminate clinker (Lafarge Aluminates Ltd). This aggregate has similar chemistry and mineralogy to grey CAC. In concrete, this aggregate bonds hydraulically with the CAC matrix, increasing both the strength and resistance to abrasion and erosion. Furthermore, the hardness of the aggregate is high, and so wear rates are very low. Such concrete is often used where there is a multiplicity of constraints - heat, abrasion, thermal shock (high and low temperatures) and chemical attack.
True refractory concretes, such as are used in heat containment, metallurgy, ceramics and cement industries, are usually highly specialist and are prepared pre-packed ready for site mixing and installation. The technology involved with these 'castable' refractories is more akin to refractory technology than concrete technology and these are usually manufactured and installed by specialist companies. There are today a number of categories of refractory castable, and these tend to be classified by their cement content or their method of installation. These are Conventional Castables (CC), Low Cement Castables (LCC), Ultra Low Cement Castables (ULCC),
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Heat-resisting and refractory concretes Self-Flowing Castables (SFC) (the last can be low or medium cement content) and Gunning Castables. Conventional castables have cement contents of about 15-25 per cent by weight and are therefore similar in this respect to normal concretes. The particular combination of CAC and refractory aggregate more or less governs the upper service temperature limit of the concrete (see Table 4.2). This type of refractory concrete was the only type of hydraulically bound castable in use from the 1930s until about the 1980s and still represents a significant percentage of the market. The high-temperature performance of conventional castables is, to some extent, limited by their chemistry and in particular the lime (CaO) content (see section 4.3 above). Thus a conventional castable containing 15 per cent of a high-range CAC (70-80 per cent A1203) with an aggregate composed entirely of alumina (e.g. tabular alumina, a refractory aggregate made from pure sintered alumina and very widely used in high-temperature refractory concretes) would still contain 2.5-4.5 per cent CaO, dependent on the cement used. This limits the upper service temperature at which such concretes can be used. With the availability of sub-micron-sized fillers such as Fume Silica (FS), the development of LCCs and ULCCs having lower cement contents, and therefore lower lime contents, became possible (see section 4.9).
The lime content of CAC was one of the factors governing the high-temperature performance of refractory concretes. An 80 per cent alumina CAC contains about 18 per cent lime (CaO) in the form of calcium aluminates. Cements having higher alumina contents (i.e. 90 per cent) and therefore lower lime contents were tried, but never really gained commercial success. Thus, the way to reduce the lime content of the refractory concrete (and hence improve its high-temperature performance) was to reduce the cement content. Low-cement castables were developed during the 1970s (Clavaud et al., 1985) using particle-packing theories that had been developed in the 1920s and 1930s. However, these theoretical Particle Size Distributions (PSD) were not possible in practice without aggregates (or more correctly fillers) having particle sizes of 0.1 to 1 microns. Fume silica began to be available at around this time, initially as a by-product of the ferro-silicon industry, and then as a specifically manufactured product in its own right (Clavaud et al., 1985; Elkem website; Hewlett, pp. 676-708). This material, together with micronized aluminas, allowed the PSD of the concretes to follow the theoretical curves down to --0.1 micron, which allowed cement contents to be decreased to 5-8 per cent. These concretes became known as Low-Cement Castables (LCC). Further development in this technology, improvement of dispersion by additives etc. allowed further reductions in cement contents down to -2 per cent. Hence the term Ultra-Low-Cement Castables (ULCC). The great advantage of LCCs and ULCCs is that their mechanical performance at high temperature was greatly superior to that of conventional castables (Clavaud et al., 1985). This allowed refractory concretes to be used in applications which were hitherto the domain of pre-fired refractory bricks.
Heat resisting and refractory concretes
Self-flow Castables (SFC) are the refractory equivalent of self-compacting concrete. The installation of LC and ULC castables was sometimes difficult, particularly if the cast was large and complex in shape and often not in ideal site conditions. The continued development of flow-enhancing additives, together with close attention to the PSD of the whole concrete, not just the submillimetre sizes, has allowed the development of concretes that flow into place, without vibration being necessary to compact or de-air the concrete. The development of SFCs did ease these problems, but these have not become as widespread in use as the LC and ULC castables.
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4.11.1 Placing and compaction As with conventional concretes, the placing and compaction of heat-resisting and refractory concretes is extremely important. The methods used are identical to those used with conventional concrete, thus no specialist equipment or skills are necessary. Similarly materials for moulds and shuttering are standard, but it may be necessary to pay more attention to dimensions if interlocking precast sections are involved. With careful attention to the mix design of the mortar or concrete, application by gunning is commonly undertaken. This work is normally conducted by specialist contractors for areas where access is difficult for regular casting and may also be carried out at elevated temperatures.
4.11.2 Curing When using any CAC-based concretes proper curing is of the utmost importance. Poorly cured concrete will lead to dusty and friable surfaces and possible failures in service. The objective of curing is to maintain the moisture in the concrete so that proper hydration takes place. Again the methods used are similar to conventional concrete, but due to the rapid hardening and high heat evolution of CAC concretes, it is important to start curing 3-4 hours after placing and continue until at least 24 hours.
4.11.3 Drying and firing After curing there is still a considerable amount of water left in the 'green' (unfired) concrete. This water must be allowed to escape as the concrete is heated for the first time if spalling is to be prevented. Drying-out time is extremely important before the concrete goes into service for the first time. Natural drying out or forced drying at up to ~ 100°C, to eliminate as much of the free water as possible, is ideally advised before exposure to higher temperatures. After drying, as the concrete is heated from 100°C to about 350°C, the water of
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Heat-resisting and refractory concretes hydration is driven from the concrete. This is the water combined within the hydrated cement and will not be eliminated by drying out at 100°C. Heating schedules during first firing are very important. The exact details of the schedules often vary with application, thickness, type of concrete etc. However, a good rule of thumb for conventional castables is no more than 50°C per hour up to 500°C, with a hold at this temperature for about 6 hours. Then continue, perhaps at a slightly higher rate, up to the service temperature (private communications). For particularly thick sections (> 100 mm) a hold at different temperatures may be advisable. If reasonable drying-out and first firing schedules are not possible, or indeed if the sections are very large (>500 mm) it is prudent to incorporate some artificial means of escape for the water. An addition of a small amount of more porous aggregate or the use of organic fibres (e.g. polypropylene) will form 'natural' paths for the water vapour to escape. After the first firing to the full service temperature, no further special heating schedules should be necessary, unless the concrete is allowed to become saturated again, due to prolonged exposure to water (e.g. external storage over a winter period). Intermittent wetting should not be problematical e.g. fire training areas, where heat-resisting CACbased concretes are known to give satisfactory service.
4.11.4 Reinforcement ............. :~:.~:~:~
~:~,~,~,~,,:~
~:~:~::~
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~:~ ~:~,~:,~: ~:~::~, ~,~, ~
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Steel reinforcement in concrete intended for use at high temperatures must be carefully considered. At temperatures that are very moderate with respect to heat-resisting and refractory concretes, ~300°C, the difference of the thermal expansion of the steel and the concrete, becomes such that the normal bond between them is reduced or lost. At higher temperatures, particularly with heavy-gauge steel, this may also lead to cracking or spalling. Above 400°C, the tensile properties of ordinary steels begin to decrease rapidly as the temperature increases and the advantages gained from its presence are progressively lost (Robson). Thus any substantial reinforcement that may be necessary, in floor slabs for example, should be as far away as possible from the hot face and the temperature at the level of the steel should not exceed 300°C. Lighter gauge meshes may be employed, but these should never be above the mid-point in the cast. If necessary, steel fibres may be used in heavy industrial areas. These may be of normal mild steel but stainless steel fibres have more tolerance to high temperatures.
4.11.5 Shrinkage and thermal expansion .......... ~ ~ ~ ~
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::~ ~ ~, ~,~:~ ~:~ ~ ~, ~
~
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It is not unusual or abnormal for refractory concretes to exhibit cracks after the first firing. These are due to dehydration shrinkage and possibly ceramic reactions between the cement and aggregates at high temperatures. In normal service these cracks will close when the concrete is reheated to its service temperature. After the first firing, the thermal expansion is reversible on cooling and reheating (Figure 4.5) (Robson). These cracks should not cause a problem unless detritus is allowed to accumulate in the cracks between firings. Subsequent firings could then increase the width of the crack.
Heat resisting and refractory concretes 0.4 Linear change (%) 0.2 Initial heating J
Temperature (°C) I
} '
-0.2
100
,
200
i
i
300
400
i
500
i
600
|
i~ ' ~
700..-" ~'800
9'00
1CO0
-~
-0.4
-0.6
Figure 4.5 Linear movement during first firing and subsequent heating and cooling cycles (Robson, p. 214).
4.11.6 Strength after firing The strength development of conventional castables before firing is similar to that of normal CAC concrete. Hardening begins soon after setting (3-4 hours) and about 90 per cent of the full green strength is developed in 24 hours. During the initial drying and firing cycle, strength changes take place that are associated with the loss of free and combined water and at higher temperatures, the reaction of the CAC with the aggregates used. Figure 4.6 (Lafarge Aluminates Ltd; American Concrete Institute) shows the typical strength development of both conventional and low cement castables during the initial firing. Typically a conventional castable loses some strength up to about 500°C, due to the loss of the hydraulic bond in the cement. On further heating there is little change until the point is reached when a ceramic bond begins to form between the cement and aggregate phases. This occurs at 900-1200°C depending on the type of aggregate and cement used (see sections 4.3 and 4.4). In effect the formation of the ceramic bond occurs as the concrete reaches its softening point, thus the strength when tested at the working temperature decreases progressively as the temperature is raised. However, if the concrete is cooled and tested cold, the strength is increased due to the formation of the ceramic bond (see Figure 4.6) (Lafarge Aluminates Ltd; American Concrete Institute). A Low-Cement Castable exhibits higher strengths when tested both hot and cold and shows higher 'refractoriness' due to the lower content of liquid forming phases (see section 4.3). LCCs have particularly good performance at working temperatures, which is one of the reasons for their widespread use in applications such as the iron and steel industries.
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Heat-resisting and refractory concretes 20 CC tested cold . . . . . . . CC tested hot 15
~
LCC tested cold
:E°"
.......... LCC tested hot
2 10-
~
o.
~
. ~ . .
~
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%
ement castable
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0
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.................................
Conventionalcastable
............. -°.
0
0
|
|
200
400
!
!
600 800 Temperature (°C)
i
i
|
1000
1200
1400
Figure 4.6 Changes in the modulus of rupture (flexural strength) of typical conventional and low-cement castables when tested hot and cold, after drying and firing to temperatures from 300°C to 1400°£ (American Concrete Institute; Mathieu).
The applications of true refractory concretes such as used in the iron and steel and nonferrous metals industries, where process temperatures reach 1800°C or more, are beyond the scope of this text. The examples given below are more of an indication of where heatresisting concretes may be encountered by the concrete technologist or engineer.
4.12.1 Domestic flues, fireplaces and chimneys The linings of domestic flues are usually composed of precast, heat-resistant concrete, made with grey CAC and a kiln burnt aggregate, e.g. sintered PFA. Such concretes will resist temperatures up to 1000-1100°C although, in normal use, the flue temperature may only reach a few hundred degrees Centigrade. In a chimney fire, however, the temperature could reach 900°C or more. The elements that comprise the fire hearth itself may be subject to temperatures of up to 1200°C and these are often cast with a low range refractory concrete e.g. grey CAC with firebrick or chamotte. See Table 4.2. The chimneys leading from many industrial processes or incinerator chimneys are often cast with or lined with concretes based on CAC. This can be for heat-resistant purposes, as much as for the risk of chemical attack, due to acidic or other aggressive agents in the flue gases.
4.12.2 Foundry floors .................~........................................................................................................................................... ................................................................................................................................................................ ~................................................................................................... .................................................................................................................................................... ::.:~:~.:~:.~.:................................................................................................................................
~ ..................... ~................................
................ ..............................
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Any industrial area that may be subjected to molten metal spillage requires a concrete that will resist thermal shocks as well as sustained high temperatures. This sort of area is
Heat resisting and refractory concretes also required to resist impact and abrasion. Often, heavy-grade industrial use such as this requires not only the use of CAC but also hard, heat- and abrasion-resistant aggregates. Some basalts and certain granites can withstand this level of treatment, but the exceptional qualities of CAC combined with a synthetic calcium aluminate aggregate (Lafarge Aluminates Ltd) have proven to be particularly suitable in these types of application.
4.12.3 Fire training areas This is a particularly demanding application for any concrete. Fire training areas are exposed to regular thermal cycling, both when fires are set and when they are extinguished. The material being burnt may be of many different types including hydrocarbons, wood and paper, furniture, tyres etc. These burning materials, besides creating intense heat, may produce chemicals which will attack the surrounding concrete. The two most common forms of fire training area are either large fiat surfaces (usually outside, but occasionally inside buildings) and full-scale rooms or even two-storey buildings, including staircases, where firefighting crews can train or try new firefighting equipment. If built with conventional concrete, these structures would quickly become unserviceable. Again the particular properties of concretes based on CAC are required, possibly with synthetic calcium aluminate aggregate (Lafarge Aluminates Ltd).
American Concrete Institute Report 547R-79-Refractory Concrete. Clavaud, B., Kiehl, J.P. and Radal, J.P. (1985) A new generation of low-cement castables. Lafarge Refractories, Venissieux, Fr. Adv. Ceram., 13 (New Dev. Monolithic Refract.), 274-284. Elkem website: www.refractories.elkem c o m - click on 'Elkem Microsilica'. Hewlett, P. (ed.), Lea's Chemistry of Cement and Concrete (4th edn). Arnold, London, pp. 709-778. Lafarge Aluminates Ltd. Technical literature. Mathieu, A. Aluminous cement with high alumina content and chemical binders. Robson, T.D. High Alumina Cements and Concretes. John Wiley, New York.
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The definition of refractory concrete will vary depending on the specific reference. However, a good general definition is: Concrete which is suitable for use at high temperatures composed of hydraulic cement (calcium aluminate cement) as the binding agent. Combined with heat resistant, refractory aggregates and or fillers. The boundary between heat-resistant and refractory concrete is somewhat arbitrary, but is probably about 1000°C although some definitions of refractory concrete start at 1500°C. However, there is, in fact, a more or less continuous spectrum of high-temperature resistant concretes, extending from about 300-400°C (the limit of concretes bound with Portland cements) to 2000°C or more, using high range calcium aluminate cements (CAC) containing 80 per cent alumina.
Although they were not developed specifically for high-temperature use, the heat-resistant and refractory properties of CACs quickly became apparent. Early applications began
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Heat-resisting and refractory concretes from about the 1930s and significant growth of these applications took place from the 1950s onward. Ordinary Portland cement (OPC) has limited use at high temperatures for a number of reasons. At temperatures of ~500°C the hydrated lime or Portlandite (Ca(OH)2), which forms a significant proportion of hydrated Portland cement, will dehydrate to form quicklime (CaO) (Robson). This is a reversible reaction: Ca(OH)2 ¢:~ CaO + H20 Exposure to moisture (atmospheric moisture is sufficient) leads to rehydration of the quicklime which is an expansive reaction and will disrupt the concrete (Figure 4.1). Thus any application where there is cycling between high temperatures and ambient temperatures is excluded. Furthermore, Portland cement is high in lime and silica, which form low melting point compounds at the service temperature of the refractory concretes.
Figure 4.1 Portland cement and calcium aluminate cement concrete cylinders after 8 cycles of 6 hours at 500°C, followed by 24 hours in humid conditions.
Hydrated lime is not formed during the hydration of CAC. Thus it is not subject to the disruption caused by the rehydration of quicklime. The hydrates that are formed do dehydrate progressively as the temperature increases to about 300°C and above. However, the compounds formed are themselves stable and at even higher temperatures (> 1000°C) begin to react with the refractory aggregates (see section 4.4) to form new stable phases.
It is often said that the refractoriness (i.e. the softening point or service temperature limit) of CAC is governed by the alumina content. The higher the alumina, the more refractory
Heat resisting and refractory concretes
the cement. Whilst this is true in the first instance, refractoriness is also dependent on the presence or not of compounds that form low melting point eutectics - CaO, SIO2, Fe203. Lower grade grey CAC (-39 per cent A1203) contains relatively high amounts of these compounds- about 36 per cent CaO, 5 per cent SiO2 and 16 per cent Fe203. (Table 4.1) (Hewlett, pp. 709-778).
Table 4.1 Typical analyses of calcium aluminate cements compared to Portland cement Colour range
Grey Portland
Grey CAC 'low' range
Buff CAC 'mid' range
White CAC 'high' range
White CAC 'high' range
A1203 CaO SiO2 Fe203 TiO2 MgO
4-6 63-67 19-23 2-3.5 < 0.5 --1
36--42 36-40 3-8 12-20 -2 --1
48-60 36-40 3-8 1-3 -3 --0.1
65-75 25-30 < 0.5 < 0.5 -
80-82 15-20 < 0.2 < 0.2 -
As the alumina (A1203) content increases, in the higher grades of CAC, then the relative amounts of compounds that form low melting point eutectics decrease, leading to higher refractoriness (Figure 4.2) (Lafarge Aluminates Ltd). Ultimately these compounds are present in only trace quantities (< 1 per cent of total) and high-range CAC (-70 per cent A1203) is composed of almost pure calcium aluminates.
1900
Refractoriness (°C) 1800
1700 1600 1500 1400 1300 1200 AI203% CaO% Fe203% Si02%
S
f
Chemical composition 39 36 17 5
52 36 2 5
70 28 < 0.5 < 0.5
80 18 < 0.2 < 0.2
Figure 4.2 Relationshipbetween refractorinessand chemical compositionof calcium aluminate cements.
In order to extend the refractoriness further, even more free alumina is added to 70 per cent A1203 CAC to increase the alumina content to -80 per cent. This, with very few exceptions, is the current upper limit of the alumina content of modern CACs.
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Heat-resisting and refractory concretes
The aggregates used in heat-resisting and refractory concretes are generally not those used in conventional concrete. There is, if anything, a much wider range of these specialist aggregates available, most of them being synthetic or derived from heat treating naturally occurring materials. Much as with the cements, the temperature resistance of these aggregates depends on their nature and chemistry (Figure 4.3) and many of the same regulations regarding alumina content etc. hold true. However, the applications of heat-resisting and refractory concretes are extremely diverse and resistance to high temperature is not the only factor that needs to be taken into account. Such properties as thermal insulation, abrasion resistance; resistance to molten metals and slags, expansion/contraction etc. need to be considered. The final properties of the concrete depend very much on the correct combination of cement and aggregate and, as will be seen later, reactive fillers such as finely ground aluminas and fume silica. Heat-resisting and refractory aggregates lightweight aggregates? 350°C
Siliceous sand
500°C
Limestone
900°C
Basalt,granite, whinstone
1100°C
Synthetic (manufactured?) calcium aluminate
1400°C
42-44% AI203 firebrick
1550°C
Sillimanite, gibbsite
1800°C
Whitefused alumina
2000°C
Tabular alumina
Figure 4.3 Service temperature limits of heat resisting and refractory aggregates, when combined with the appropriate calcium aluminate cement.
Table 4.2 gives a non-comprehensive list of typical aggregates used in heat-resistant and refractory concretes together with an indication of their properties when combined with the appropriate CAC (Robson; Hewlett; Lafarge Aluminates Ltd).
........
..........
i
.....
Concretes for use up to about ~ 1000°C are considered to be 'heat resistant'. However, as stated earlier, this temperature is not an immutable boundary but a convenient marker. Many heat-resistant concretes are used at temperatures well below 1000°C and sometimes above this temperature. The important thing is that the choice of cement and aggregate are appropriate to the application. It is likely that concrete technologists or engineers will come across the need to use heat-resisting, rather than true refractory concretes, at some time, as their applications are
Heat resisting and refractory concretes Table 4.2 Properties of heat-resisting and refractory concretes 1 Type of aggregate Insulating concretes
Type of CAC Colour
% A1203
Approximate service temperature limit2 (°C)
Approximate fired densit~¢ 3 (kg/m ~)
Indicative thermal conductivity 4 (W/m.°K)
Vermiculite Pedite Diatomite Pumice Expanded Clay Sintered PFA Expanded Chamotte
Grey Grey Grey Grey Grey Grey Grey Buff White White
39 39 39 39 39 39 39 50 70 80
900 900 900 900 1100 1100 1200 1350 1700 1800
550 600 850 1200 1450 1500 1500 1500 1500 1300
0.17 0.18 0.25 0.50 0.35-0.58 0.46--0.58 0.45-0.60 0.65-1.00
Grey Grey Grey Grey Grey Grey Grey
39 39 39 39 39 39 39
350 500 800 900 1000 1100 1200
2200 2200 1920 2400 2500 2600 1920
1.4 1.1 1.4 1.2-1.5 1.1
Grey
39
1200
2650
-
Grey Grey
39 39
1300 1300
1900 2050
0.9-1.1 0.9-1.1
Buff White White Grey Buff White Grey Buff White Grey Buff White White White White White White
50 70 80 39 50 70 39 50 70 39 50 70 80 70 80 70 80
1450 1500 1520 1350 1450 1600 1400 1500 1700 1400 1550 1650 1750 1800 1900 1900 2000
2030 2000 2100 2160 2160 2160 2560 2560 2560 3000 3000 3000 3050 3000 3050 2850 2900
0.7-1.0 0.5-0.7 0.8
Bubble Alumina Heat-resistant concretes
Siliceous Sand Limestone Crushed House Brick Basalt, Granite Emery, Alag "v~ Firebrick (35% A1203) Olivine Dense refractory concretes
Firebrick (44% A1203) Chamotte (42-44% A1203)
Silliminite, Gibbsite
Calcined Bauxite Brown Fused Alumina
White Fused Alumina Tabular Alumina
1.5
1.7 1.5-1.8 1.5-1.8 1.2-1.7 2.30 1.4-1.8 1.9-2.3
1 Robson (pp. 196, 197, 216); Lafarge Aluminates Ltd. 2 Maximum hot face temperatures, assuming a thermal gradient through the mass of the concrete. 3 Density after firing to 1000°C unless service temperature limit is lower. 4 Coefficients of thermal conductivity vary considerably depending on the temperature at which they are measured. The figures given are indicative values for 500-1000°C.
o f t e n m o r e c l o s e l y l i n k e d to c o n v e n t i o n a l c o n c r e t e a p p l i c a t i o n s . T h e s e s p e c i a l i s t c o n c r e t e s are o f t e n still m i x e d a n d p l a c e d o n - s i t e , as r e a d y m i x e d c o n c r e t e s c o n t a i n i n g C A C s are n o t g e n e r a l l y a v a i l a b l e . It is i m p o r t a n t to r e m e m b e r , h o w e v e r , that m o s t o f the f a c t o r s g o v e r n i n g
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Heat-resisting and refractory concretes conventional concretes will apply to heat-resistant concrete. Questions such as the relationships between shrinkage, strength, abrasion resistance, durability etc. and the water/cement ratio, compaction and, to a lesser extent, cement content will all hold true. However, there are other important parameters that must also be taken into account if the concrete is to perform correctly. Normally grey CAC (39 per cent alumina) will have sufficient temperature resistance for most heat-resisting applications up to 1000°C. Indeed in the case of some aggregates, it will be the properties of the aggregate, rather than the cement, that will limit the service temperature of the concrete, as these may be lower than the CAC itself, e.g. vermiculite, perlite, basalt and some heat-treated aggregates are themselves less refractory than grey CAC. In refractory concretes (see section 4.11) the reverse is often the case, and the higher refractoriness of the aggregate will extend the temperature range of a given CAC.
The thermal insulating properties of these concretes are primarily associated with their density and the major factor affecting density would be the density of the aggregate. Lightweight aggregates vary in density from the extremely light, (e.g. Perlite; bulk density 100-110 kg/m 3) to moderately light (e.g. Sintered PFA or expanded clay; bulk density 600-800 kg/m3). The insulating properties (i.e. the thermal conductivity) of concretes made with these aggregates will be in the range 0.15-0.5 W/m.°K (Figure 4.4) (Robson). 0.6 Thermal conductivity W/m °K at ~550°C
Expanded clay~ /
0.5
7
HT insulating brick I J l ' 0.4
Pumice 41~~
0.3 Diatomite~ Perlite 0.2
f
Vermiculite 0.1
Fired density (kg/m 3) 0
400
I
I
600
800
I
1000
/
I
1200
1400
1600
Figure 4.4 Relationship between fired density and thermal conductivity of heat-resisting insulating concretes (Robson, p. 217).
Heat resisting and refractory concretes This type of concrete would often be used in conjunction with a dense refractory concrete in heat-retaining vessels. The dense or 'hot face' concrete would then be on the inside of the vessel, with the insulating concrete behind. In this way the thermal gradients and heat losses can be closely controlled. Since CAC would be the cement binder for both of these concretes, they can be cast monolithically. Even when the hot face concrete requires a higher range CAC (e.g. 50-70 per cent alumina) due to the higher temperature inside the vessel in question, this does not pose a problem, as high- and low-range CACs are compatible with each other. As with heat-resisting concrete, grey CAC (39 per cent A1203) would normally be sufficiently heat resisting for insulating concrete, since the service temperatures of these materials rarely exceed 1000°C. There are, however, exceptions and very high temperature insulating concretes can be made with specialist aggregates such as bubble alumina or micro-porous calcium hexa-aluminate (CaO.6A1203). In this case the high-purity 70 per cent or 80 per cent A1203 CAC would be necessary, as service temperatures would be higher than the range of grey CAC.
The service conditions of heat-resistant and refractory concretes often also require a degree of abrasion resistance, due to direct physical contact (e.g. wheeled or tracked vehicles, billets of hot metal etc.) or erosion due to gas-borne particulates. Many dense, high-temperature concretes, although they may not be designed specifically for abrasion, have a reasonable degree of abrasion resistance. There are, however, some notable applications, where the severe conditions require that the concrete must withstand heat, thermal shock and heavy abrasion (e.g. floors in foundries and hot metal treatment or handling areas, fire training areas and constructions). For these applications there is a particularly suitable type of synthetic aggregate, which is made by crushing and grading calcium aluminate clinker (Lafarge Aluminates Ltd). This aggregate has similar chemistry and mineralogy to grey CAC. In concrete, this aggregate bonds hydraulically with the CAC matrix, increasing both the strength and resistance to abrasion and erosion. Furthermore, the hardness of the aggregate is high, and so wear rates are very low. Such concrete is often used where there is a multiplicity of constraints - heat, abrasion, thermal shock (high and low temperatures) and chemical attack.
True refractory concretes, such as are used in heat containment, metallurgy, ceramics and cement industries, are usually highly specialist and are prepared pre-packed ready for site mixing and installation. The technology involved with these 'castable' refractories is more akin to refractory technology than concrete technology and these are usually manufactured and installed by specialist companies. There are today a number of categories of refractory castable, and these tend to be classified by their cement content or their method of installation. These are Conventional Castables (CC), Low Cement Castables (LCC), Ultra Low Cement Castables (ULCC),
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Heat-resisting and refractory concretes Self-Flowing Castables (SFC) (the last can be low or medium cement content) and Gunning Castables. Conventional castables have cement contents of about 15-25 per cent by weight and are therefore similar in this respect to normal concretes. The particular combination of CAC and refractory aggregate more or less governs the upper service temperature limit of the concrete (see Table 4.2). This type of refractory concrete was the only type of hydraulically bound castable in use from the 1930s until about the 1980s and still represents a significant percentage of the market. The high-temperature performance of conventional castables is, to some extent, limited by their chemistry and in particular the lime (CaO) content (see section 4.3 above). Thus a conventional castable containing 15 per cent of a high-range CAC (70-80 per cent A1203) with an aggregate composed entirely of alumina (e.g. tabular alumina, a refractory aggregate made from pure sintered alumina and very widely used in high-temperature refractory concretes) would still contain 2.5-4.5 per cent CaO, dependent on the cement used. This limits the upper service temperature at which such concretes can be used. With the availability of sub-micron-sized fillers such as Fume Silica (FS), the development of LCCs and ULCCs having lower cement contents, and therefore lower lime contents, became possible (see section 4.9).
The lime content of CAC was one of the factors governing the high-temperature performance of refractory concretes. An 80 per cent alumina CAC contains about 18 per cent lime (CaO) in the form of calcium aluminates. Cements having higher alumina contents (i.e. 90 per cent) and therefore lower lime contents were tried, but never really gained commercial success. Thus, the way to reduce the lime content of the refractory concrete (and hence improve its high-temperature performance) was to reduce the cement content. Low-cement castables were developed during the 1970s (Clavaud et al., 1985) using particle-packing theories that had been developed in the 1920s and 1930s. However, these theoretical Particle Size Distributions (PSD) were not possible in practice without aggregates (or more correctly fillers) having particle sizes of 0.1 to 1 microns. Fume silica began to be available at around this time, initially as a by-product of the ferro-silicon industry, and then as a specifically manufactured product in its own right (Clavaud et al., 1985; Elkem website; Hewlett, pp. 676-708). This material, together with micronized aluminas, allowed the PSD of the concretes to follow the theoretical curves down to --0.1 micron, which allowed cement contents to be decreased to 5-8 per cent. These concretes became known as Low-Cement Castables (LCC). Further development in this technology, improvement of dispersion by additives etc. allowed further reductions in cement contents down to -2 per cent. Hence the term Ultra-Low-Cement Castables (ULCC). The great advantage of LCCs and ULCCs is that their mechanical performance at high temperature was greatly superior to that of conventional castables (Clavaud et al., 1985). This allowed refractory concretes to be used in applications which were hitherto the domain of pre-fired refractory bricks.
Heat resisting and refractory concretes
Self-flow Castables (SFC) are the refractory equivalent of self-compacting concrete. The installation of LC and ULC castables was sometimes difficult, particularly if the cast was large and complex in shape and often not in ideal site conditions. The continued development of flow-enhancing additives, together with close attention to the PSD of the whole concrete, not just the submillimetre sizes, has allowed the development of concretes that flow into place, without vibration being necessary to compact or de-air the concrete. The development of SFCs did ease these problems, but these have not become as widespread in use as the LC and ULC castables.
...................
55
4.11.1 Placing and compaction As with conventional concretes, the placing and compaction of heat-resisting and refractory concretes is extremely important. The methods used are identical to those used with conventional concrete, thus no specialist equipment or skills are necessary. Similarly materials for moulds and shuttering are standard, but it may be necessary to pay more attention to dimensions if interlocking precast sections are involved. With careful attention to the mix design of the mortar or concrete, application by gunning is commonly undertaken. This work is normally conducted by specialist contractors for areas where access is difficult for regular casting and may also be carried out at elevated temperatures.
4.11.2 Curing When using any CAC-based concretes proper curing is of the utmost importance. Poorly cured concrete will lead to dusty and friable surfaces and possible failures in service. The objective of curing is to maintain the moisture in the concrete so that proper hydration takes place. Again the methods used are similar to conventional concrete, but due to the rapid hardening and high heat evolution of CAC concretes, it is important to start curing 3-4 hours after placing and continue until at least 24 hours.
4.11.3 Drying and firing After curing there is still a considerable amount of water left in the 'green' (unfired) concrete. This water must be allowed to escape as the concrete is heated for the first time if spalling is to be prevented. Drying-out time is extremely important before the concrete goes into service for the first time. Natural drying out or forced drying at up to ~ 100°C, to eliminate as much of the free water as possible, is ideally advised before exposure to higher temperatures. After drying, as the concrete is heated from 100°C to about 350°C, the water of
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Heat-resisting and refractory concretes hydration is driven from the concrete. This is the water combined within the hydrated cement and will not be eliminated by drying out at 100°C. Heating schedules during first firing are very important. The exact details of the schedules often vary with application, thickness, type of concrete etc. However, a good rule of thumb for conventional castables is no more than 50°C per hour up to 500°C, with a hold at this temperature for about 6 hours. Then continue, perhaps at a slightly higher rate, up to the service temperature (private communications). For particularly thick sections (> 100 mm) a hold at different temperatures may be advisable. If reasonable drying-out and first firing schedules are not possible, or indeed if the sections are very large (>500 mm) it is prudent to incorporate some artificial means of escape for the water. An addition of a small amount of more porous aggregate or the use of organic fibres (e.g. polypropylene) will form 'natural' paths for the water vapour to escape. After the first firing to the full service temperature, no further special heating schedules should be necessary, unless the concrete is allowed to become saturated again, due to prolonged exposure to water (e.g. external storage over a winter period). Intermittent wetting should not be problematical e.g. fire training areas, where heat-resisting CACbased concretes are known to give satisfactory service.
4.11.4 Reinforcement ............. :~:.~:~:~
~:~,~,~,~,,:~
~:~:~::~
~ ~:~
~:~ ~:~,~:,~: ~:~::~, ~,~, ~
~ ~ ~,~:~,~:~:
~ ~, ~,:~: ~ ~ ~:~ ,,~:~ :::::::::::::::::::::::::::::::::::::::::::::::
:::::::::::::::::::::::::
......................... :::::::::::::::::::::::::::::::::::::::::::::::
.................. ===================== ::::::::::::::::::::::::
:::::::::.:::
:::::::~,:: ::::::::::::::::::::::
::~:::::: :::: :::: ::::::::: ::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::
.................. ::::::::::::::::::::::::::::::::::::::::::
............
Steel reinforcement in concrete intended for use at high temperatures must be carefully considered. At temperatures that are very moderate with respect to heat-resisting and refractory concretes, ~300°C, the difference of the thermal expansion of the steel and the concrete, becomes such that the normal bond between them is reduced or lost. At higher temperatures, particularly with heavy-gauge steel, this may also lead to cracking or spalling. Above 400°C, the tensile properties of ordinary steels begin to decrease rapidly as the temperature increases and the advantages gained from its presence are progressively lost (Robson). Thus any substantial reinforcement that may be necessary, in floor slabs for example, should be as far away as possible from the hot face and the temperature at the level of the steel should not exceed 300°C. Lighter gauge meshes may be employed, but these should never be above the mid-point in the cast. If necessary, steel fibres may be used in heavy industrial areas. These may be of normal mild steel but stainless steel fibres have more tolerance to high temperatures.
4.11.5 Shrinkage and thermal expansion .......... ~ ~ ~ ~
~
::~ ~ ~, ~,~:~ ~:~ ~ ~, ~
~
~ , ~ , ~ , ~
~ : ~ , ~ : ~ , , ~
~:~,~,,~,~,~:,
~ , ~ , :
:,::, ::::~ ~:~:.::.::
:~:~::~:~,:~::::~::::~::~:::
:::::::,:: ~:::~::::::,~, ~::::~:::~::::~
~ ~,:::~ :::~:,~ :~ ~:~ :::~ ::~ :~: :~:~::~,~::,~
~:~:::,:~:.,~ ~ ~:~::~::::~:
:,~:~:~:~::~:~::::,:,~::::~:::~:::::.,
::~:~ :~,:::,,,,:::, ~:~:~::~:::::,:, :~:~::~: ~::~:~:~: ................ :::~ :::::::::::::::::::::::::::::::::::
~::~:::~:
~
~:::~ :~::: :,:~ ~::~:~:::.::
~ ..............
It is not unusual or abnormal for refractory concretes to exhibit cracks after the first firing. These are due to dehydration shrinkage and possibly ceramic reactions between the cement and aggregates at high temperatures. In normal service these cracks will close when the concrete is reheated to its service temperature. After the first firing, the thermal expansion is reversible on cooling and reheating (Figure 4.5) (Robson). These cracks should not cause a problem unless detritus is allowed to accumulate in the cracks between firings. Subsequent firings could then increase the width of the crack.
Heat resisting and refractory concretes 0.4 Linear change (%) 0.2 Initial heating J
Temperature (°C) I
} '
-0.2
100
,
200
i
i
300
400
i
500
i
600
|
i~ ' ~
700..-" ~'800
9'00
1CO0
-~
-0.4
-0.6
Figure 4.5 Linear movement during first firing and subsequent heating and cooling cycles (Robson, p. 214).
4.11.6 Strength after firing The strength development of conventional castables before firing is similar to that of normal CAC concrete. Hardening begins soon after setting (3-4 hours) and about 90 per cent of the full green strength is developed in 24 hours. During the initial drying and firing cycle, strength changes take place that are associated with the loss of free and combined water and at higher temperatures, the reaction of the CAC with the aggregates used. Figure 4.6 (Lafarge Aluminates Ltd; American Concrete Institute) shows the typical strength development of both conventional and low cement castables during the initial firing. Typically a conventional castable loses some strength up to about 500°C, due to the loss of the hydraulic bond in the cement. On further heating there is little change until the point is reached when a ceramic bond begins to form between the cement and aggregate phases. This occurs at 900-1200°C depending on the type of aggregate and cement used (see sections 4.3 and 4.4). In effect the formation of the ceramic bond occurs as the concrete reaches its softening point, thus the strength when tested at the working temperature decreases progressively as the temperature is raised. However, if the concrete is cooled and tested cold, the strength is increased due to the formation of the ceramic bond (see Figure 4.6) (Lafarge Aluminates Ltd; American Concrete Institute). A Low-Cement Castable exhibits higher strengths when tested both hot and cold and shows higher 'refractoriness' due to the lower content of liquid forming phases (see section 4.3). LCCs have particularly good performance at working temperatures, which is one of the reasons for their widespread use in applications such as the iron and steel industries.
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Heat-resisting and refractory concretes 20 CC tested cold . . . . . . . CC tested hot 15
~
LCC tested cold
:E°"
.......... LCC tested hot
2 10-
~
o.
~
. ~ . .
~
~~~~~
%
ement castable
\~
.......
\.~
0
"0 0
~ 5
.................................
Conventionalcastable
............. -°.
0
0
|
|
200
400
!
!
600 800 Temperature (°C)
i
i
|
1000
1200
1400
Figure 4.6 Changes in the modulus of rupture (flexural strength) of typical conventional and low-cement castables when tested hot and cold, after drying and firing to temperatures from 300°C to 1400°£ (American Concrete Institute; Mathieu).
The applications of true refractory concretes such as used in the iron and steel and nonferrous metals industries, where process temperatures reach 1800°C or more, are beyond the scope of this text. The examples given below are more of an indication of where heatresisting concretes may be encountered by the concrete technologist or engineer.
4.12.1 Domestic flues, fireplaces and chimneys The linings of domestic flues are usually composed of precast, heat-resistant concrete, made with grey CAC and a kiln burnt aggregate, e.g. sintered PFA. Such concretes will resist temperatures up to 1000-1100°C although, in normal use, the flue temperature may only reach a few hundred degrees Centigrade. In a chimney fire, however, the temperature could reach 900°C or more. The elements that comprise the fire hearth itself may be subject to temperatures of up to 1200°C and these are often cast with a low range refractory concrete e.g. grey CAC with firebrick or chamotte. See Table 4.2. The chimneys leading from many industrial processes or incinerator chimneys are often cast with or lined with concretes based on CAC. This can be for heat-resistant purposes, as much as for the risk of chemical attack, due to acidic or other aggressive agents in the flue gases.
4.12.2 Foundry floors .................~........................................................................................................................................... ................................................................................................................................................................ ~................................................................................................... .................................................................................................................................................... ::.:~:~.:~:.~.:................................................................................................................................
~ ..................... ~................................
................ ..............................
....................................................
Any industrial area that may be subjected to molten metal spillage requires a concrete that will resist thermal shocks as well as sustained high temperatures. This sort of area is
Heat resisting and refractory concretes also required to resist impact and abrasion. Often, heavy-grade industrial use such as this requires not only the use of CAC but also hard, heat- and abrasion-resistant aggregates. Some basalts and certain granites can withstand this level of treatment, but the exceptional qualities of CAC combined with a synthetic calcium aluminate aggregate (Lafarge Aluminates Ltd) have proven to be particularly suitable in these types of application.
4.12.3 Fire training areas This is a particularly demanding application for any concrete. Fire training areas are exposed to regular thermal cycling, both when fires are set and when they are extinguished. The material being burnt may be of many different types including hydrocarbons, wood and paper, furniture, tyres etc. These burning materials, besides creating intense heat, may produce chemicals which will attack the surrounding concrete. The two most common forms of fire training area are either large fiat surfaces (usually outside, but occasionally inside buildings) and full-scale rooms or even two-storey buildings, including staircases, where firefighting crews can train or try new firefighting equipment. If built with conventional concrete, these structures would quickly become unserviceable. Again the particular properties of concretes based on CAC are required, possibly with synthetic calcium aluminate aggregate (Lafarge Aluminates Ltd).
American Concrete Institute Report 547R-79-Refractory Concrete. Clavaud, B., Kiehl, J.P. and Radal, J.P. (1985) A new generation of low-cement castables. Lafarge Refractories, Venissieux, Fr. Adv. Ceram., 13 (New Dev. Monolithic Refract.), 274-284. Elkem website: www.refractories.elkem c o m - click on 'Elkem Microsilica'. Hewlett, P. (ed.), Lea's Chemistry of Cement and Concrete (4th edn). Arnold, London, pp. 709-778. Lafarge Aluminates Ltd. Technical literature. Mathieu, A. Aluminous cement with high alumina content and chemical binders. Robson, T.D. High Alumina Cements and Concretes. John Wiley, New York.
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