castable refractory concrete

$: Castable Refractory Concrete$
Castable refractory concretesW. E. Lee, W. Vieira, S. Zhang, K. Ghanbari Ahari, H. Sarpoolaky, and C. Parr

Castable refractories containing calcium aluminatecement (CAC) are used ubiquitously in a range offurnace lining applications in the iron and steel,cement, glass, ceramic, and petrochemicalindustries. This review outlines their developmentfrom conventional high cement materials, throughlow cement and ultra-low cement castables to thepresent materials which may be entirely free ofCAC. Castables are defined in terms of both CaOcontent and installation procedure. Productionroutes, compositions, and microstructuralevolution on hydration, setting, dehydration, andfiring are described for pure CACs and castable 1 Typical powder processed refractoryrefractories. The development of the low cement microstructuresystems is discussed in terms of particle packing,dispersion, and rheology highlighting the influenceof colloidal matrix additions of silica and alumina. calcined bauxite, and sintered MgO while bondingRecent developments including cement free, self-

systems may be based on carbon derived from pyro-flowing, shotcreting, and basic castables arelysed pitches and phenolic resins, mullite and glassdescribed and the potential for carbon-containingfrom decomposed clays, or alumina and calciumsystems evaluated. IMR/368aluminate phases formed from fired hydraulic calciumaluminate cements (CACs). The most significant trend© 2001 IoM Communications Ltd and ASM International.

Professor Lee, Dr Vieira, Dr Zhang, Dr Ahari, and Mr in refractories technology in the last two decades hasSarpoolaky are in the Department of Engineering Materials, been the ever increasing use of monolithics, orUniversity of Sheffield, UK and Mr Parr is with Lafarge

unshaped refractories, which now, in many countries,Aluminates, Paris, France.account for more than 50% of total production.Owing to improved refractories quality their con-sumption has decreased dramatically in the last twodecades while the ratio of monolithics to preshapedAbbreviationsrefractories (bricks) has been steadily increasing.1–6

BFA brown fused aluminaThe reasons for the rapid growth of monolithics,

CAC calcium aluminate cementat the expense of bricks, are their ready availability,

CVC casting vibration castablefaster, easier, and cheaper installation, and fewer

HAC high alumina cementcorrosion-susceptible lining joints.6–11 The term

LCC low cement castablemonolithic usually includes a wide variety of material

SFC free or self-flowing castabletypes and compositions, with various bonding sys-

TA tabular aluminatems, ranging from fluid cement pastes to stiff plastic

ULCC ultra-low cement castablelumps.6,11,12 Monolithic materials were first used as

WFA white fused aluminaa distinct refractory product in 1914, when the first

Chemistry commercial refractory plastic, a simple blend ofA Al2O3 crushed firebrick and fireclay, was produced.5,6,12C CaO From this, monolithics have evolved into a versatile,F Fe2O3 widely used class of refractory materials that offerH H2O performance and cost effectiveness comparable, andM MgO sometimes even superior, to those of shaped refractor-S SiO2 ies. The success of monolithics is due to significantT TiO2 advances in the type and quality of their binders,

aggregates, and additives as well as to innovation intheir design and installation techniques.5,6Introduction The evolution of monolithic refractories over thepast century has been described in a recent review.13Refractories are a group of ceramic materials used in

massive quantities to line vessels in which other A significant advance in monolithics technology wasthe development of refractory concretes or castablesmaterials (such as metals, glass, and cements) are

manufactured at elevated temperatures. They consist based on CACs.6–8,11,14 Castables are complex refrac-tory formulations, requiring high quality, precision-of large sized (up to centimetres) aggregate (filler)

phases held together with finer (sometimes submicro- sized aggregates, modifying fillers, binders, and addi-tives.11,14 Refractory castables are dry granular mater-metres), often porous, binder phases conferring the

microstructure shown schematically in Fig. 1. Typical ials which require water addition. Installation is bycasting or pouring into place, vibration placement,aggregates include fused alumina, tabular alumina,

ISSN 0950–6608 International Materials Reviews 2001 Vol. 46 No. 3 145

146 Lee et al. Castable refractory concretes

trowelling, or projection (spraying or shotcreting). based systems are reviewed. Finally, the possibility ofcarbon-containing castables is discussed.The majority of castables contain a CAC binder,

though a few still use Portland cement.6 While con-ventional castables, which contain the largest amount Historical evolutionof cement, still make up the greatest percentage ofthose produced, use of reduced cement varieties, low The first refractory concrete was made and put to

practical use by Sainte-Claire Deville, in France,cement castables (LCCs) and ultra-low cement cas-tables (ULCCs), has grown significantly over the past sometime before 1856.5,12,13,36–38 He heated mixtures

of alumina and lime and mixed this reaction product10 years.6 This is because the CaO present in thecement leads to deterioration of high temperature with corundum aggregate and water to produce high

temperature crucibles. However, the hydraulic prop-properties.Castables or refractory concretes commonly con- erties of compounds formed by reacting lime with

alumina were known long before the individual cal-tain bonds based on high alumina cement (HAC), areactive phase, or a gel. They may be cast in moulds cium aluminates were isolated in a pure condition

and positively identified.36 The cementitious action ofto form specific products (precast shapes) or cast ‘inplace’, as when forming a lining for a kiln furnace. lime had already been appreciated by the Egyptians

and Romans, who relied to some extent on the ratherDense concretes are prepared using discrete particlesizes, with the largest up to several centimetres in slow action of atmospheric carbon dioxide to carbon-

ate the lime and so develop the strength of theirsize. Mechanical vibration may be used to assist theflow of the concrete or to enable mould filling with a mortars.39 This type of cementitious action, however,

is not entirely satisfactory if air is excluded as, forlower liquid content in the slurry. Some products are,however, cast without vibration, and such concretes example, in underwater construction.

The next notable advance in cement developmentare said to be free or self-flowing.15 Some refractorycompositions may be premixed with water and then stemmed from the work of John Smeaton, around

1756, who recognised that the calcination of certainpumped under pressure to the site of placement,where they are projected or sprayed on to the surface. selected limestones would give powders with

hydraulic setting properties. In the 1840s, followingThis process is called wet gunning or shotcreting, andthe concrete is termed a shotcreting or sprayable the works of L. G. Vicat (1846) in France and Joseph

Aspdin in England, these ‘natural’ cements, as theycastable.16 This is quite different from the earlier drygunning process where the powder and water are were called, were superseded by Portland cement, a

calcium silicate product prepared by calcining to amixed at the nozzle of the device used to placethe slurry. (partially melted) clinker a wet ground mixture of

limestone and clay.12,39Modern castables are used increasingly in almostevery refractory application, such as for the repair of The development of the first CACs stemmed from

the shortcomings of calcium silicate cements exposedstacks17 and lining of iron and slag runners in blastfurnaces,2,3,7,18 torpedo ladle throats and bar- to the action of ground waters containing sulphates.39

In the second half of the 19th century many patentsrels,2,17–19 steel ladles2,3,7,17,18,20–27 and tundishlinings,2,3,7,17,18,28 hearths, soaking pits, and skid rails were granted on methods for making calcium alumin-

ate type cements by combining lime with bauxite.of reheat furnaces,7,17,19 nose ring and discharge areasof rotary cement kilns,17–19,29,30 direct reduction However, it was not until 1918 that the Lafarge

Company in France, based on a patent by Bied inkilns,19 coke oven door plugs,7,31 cyclones and transferlines of fluidised catalytic cracking unit vessels of the 1908, began to sell a CAC, marketing it as a sulphate

resistant product for sea water corrosion resistantpetrochemical industry,6,7,17,19,29 foundry ladles andheat treating furnaces,6,18 aluminium reverberatory concrete.5,37,38 Production was based on the use of

cupolas or small blast furnaces which were top-fedfurnaces and ladles,6,7,16,19,29 boilers and waste incin-erators,6,16,32 repairing of sliding gate plates,33 fabri- with a mixture of high iron bauxite, limestone, and

coke, and from which pig iron and an aluminous slagcation of monolithic porous plugs, seating (well )blocks and powder injection lances,33 desulphurising were tapped separately at the bottom. The slag on

grinding to powder gave high alumina cement.39and argon stirring lances,29 refractory lining of snor-kels in RH degassing vessels33 and, more recently, Despite the early work of Deville, it was not until

the mid-1920s that the high temperature propertiesfabrication of shrouds and submerged entry nozzlesused for the continuous casting of steel.34,35 of CAC were fully appreciated. Before that, calcium

aluminate was often seen as an alternative to PortlandIn this review the important historical develop-ments leading to modern castable systems (see the cement and no mention was made of its potential in

refractories applications. In 1924, the Universal Atlassection ‘Historical evolution’ below) and the methodsof distinguishing the various types (see the section Cement Division of the US Steel Corporation began

manufacturing a CAC for use as a binder in refractory‘Classification’ below) are briefly described. The pro-duction and hydration and dehydration of pure CACs mixes.5,38 In 1929, refractory castables bonded with

CAC were already manufactured industrially in theare considered before examining refractory systemsin which they are bonding phases: conventional cas- USA, while production in Japan commenced in

1939.12,37 During the early days of refractory con-tables, LCCs and ULCCs. The importance of particlepacking, dispersion, and rheology is highlighted as cretes, the main aggregates available for use were

calcined clays and crushed fired refractory bricks.well as the types of submicrometre powder and aggre-gate used. Then the modern developments of cement Tabular alumina, although available in the 1940s, was

not then widely used in monolithic refractories, pre-free, free flowing, shotcreting, and spinel and MgO-

International Materials Reviews 2001 Vol. 46 No. 3

Lee et al. Castable refractory concretes 147

sumably because of its relatively high cost. The con- teristics of shotcreted LCCs were first published in1986, it was not until after the advent of the self-cretes were crudely made and even more crudely

applied. Mixing was commonly done by hand in a flowing technology that wet gunning became a moreeffective installation tool. Pumping self-flow mixesmortar box or wheelbarrow, and casting, slap trowel-

ling, and hand forming were the most common early over significant distances, both horizontally and verti-cally, enabled the technology to evolve to include wetforms of installation, though some gunning was also

done.5 gunning.16Nowadays, refractory concretes are available whichIn the 1950s, following experiments made with

purer raw materials, Alcoa and Lafarge began market- contain low, intermediate, or high purity CACs inlarge (conventional ), small amounts ( low and ultraing high purity CACs specifically developed for the

refractories industry, resulting in a wide range of low), or none at all (non-cement, cement free), com-bined with a wide variety of organic and inorganicHACs produced from mixtures of pure alumina and

limestone and containing small amounts of silica and additives, such as deflocculants, setting retarders andaccelerators, and aggregates such as calcined claysiron oxide.5,38,39 By 1960, castables based on high

purity CAC and tabular alumina aggregates were and bauxites, tabular and sintered aluminas, whiteand brown fused aluminas, sintered and fused mag-common, claiming advantages in the areas of refrac-

toriness, erosion, and abrasion resistance.5,40 nesia, chrome ore, zircon, kyanite, mullite, siliconcarbide, alumina–magnesia spinels, and fused silica.However, these had relatively simple compositions,

consisting of refractory aggregates and cement, the Cement free castables are used in numerous molteniron and steel contact applications, where the elimin-latter added in sufficient amounts to give suitable

room temperature strength.18 The major disadvan- ation of lime in the refractory matrix improves hightemperature behaviour. Such castables use a varietytages of these conventional castables, containing as

much as 30%* cement, were the high water content of bonding mechanisms. Bond systems used includeclay bonding, gel bonding, hydratable alumina bond-required for placement, which increased the porosity

and lowered the strength of the material, their loss of ing, and phosphate bonding. Other additions to cas-tables include the use of stainless steel fibres andstrength during the dehydration process, and the

sharp drop in strength at high temperatures due to organic bake-out fibres, the first to prevent damagefrom thermal shock, and the second to increasethe fluxing action of CaO.7,18,33,37 Improvements in

this product were largely due to higher purity cements permeability to allow water removal during the dehy-dration period.14,50and aggregates, while the base technology stayed

the same.41In the late 1970s, reduced cement materials based Classificationon the Prost patent of 1969 were manufac-

tured.18,29,37,42–47 These LCCs contained at most Refractory castables are classified by the ASTMaccording to their lime content into conventional2·5% lime, achieved by dramatically reducing the

amount of cement binder, which is partially replaced (CaO>2·5%), low cement (2·5%>CaO>1·0%),ultra-low cement (1·0%>CaO>0·2%), and cementby fine oxide particles, and distributing it evenly

within the mix, with the aid of deflocculants and free (CaO<0·2%).14,30,38 According to this classifi-cation, castables containing up to 1% high aluminasimilar additives. The grain size distribution of the

aggregates was also altered so that the interstices are (80% Al2O3 ) cements may fall into the last category,despite the presence of some cement. These definitionsprogressively filled by smaller grains to obtain maxi-

mum packing density, which also increases the can be misleading, and have led to the introductionof new types of so-called ‘cement free’ castable madeamount of water utilised in flow. Later, ULCCs,

characterised by an even lower lime content (<1·0%) up of a mixture of cement and other bonding agents,such as r-alumina.51,52were developed. Low and ultra-low cement castables

have uniform microstructure with low porosity and Castables can also be classified by installationmethod as casting vibration castables (CVCs) andhigh strength throughout the low and intermediate

temperature range, and a low lime level that improves self-flowing castables (SFCs).31 Shotcreting castablesare SFCs but because not all SFCs are suitable forhigh temperature strength and corrosion resistance.15

In the last two decades they have successfully replaced pumping and wet gunning, particularly those withdilatant behaviour,16 these castables should also bea variety of other monolithics, such as conventional

high cement castables, plastics, ramming and gunning separately classified. Also, it is possible that a vibrat-able castable will become self-flowing if more watermixes, as well as many brick compositions.

Following the success of LCCs and ULCCs and is added to the mix, or vice versa, and therefore, insome cases, the classification of castables with nothe appreciation of the rheological properties of satu-

rated systems, a new family of refractory concretes mention of the amount of water required for instal-lation is meaningless. Attempts have been made towas developed in the mid-1980s, free or self-flowing

castables (SFCs).15,31,48,49 These are LCCs or ULCCs classify castables according to the nature of the disper-sing phase, i.e. inorganic or organic.31 However, thiswith a consistency after mixing that allows them to

flow and degas without application of external is also meaningless, since modern high performancematerials may include a combination of several addi-energy.48 Self-flow technology also paved the way for

a new placement technique, referred to as wet gun- tives, both organic and inorganic in nature.Classification of modern castables is difficult.ning, shotcasting, or shotcreting. Although the charac-However, a proper classification should include asmuch information as possible about the chemical*All percentages are wt-% unless otherwise stated.



nature, rheological behaviour, and installation calcined alumina is often the choice for the highpurity type (HP CAC).characteristics of the castable.

Alternatively, cements or CACs may be classifiedaccording to chemical composition, forming five mainCalcium aluminate cementsgroups (Table 3).12 In Table 3, Group A includes

Despite the steady decrease in the amount of cement fondu and lumnite cements, while Groups B and Cused in modern high performance refractory castables, include the two alumina cements for refractoriesCACs, particularly high alumina cements, continue specified in the Japanese Industrial Standards. Groupto be the most important hydraulically setting agents D includes high purity alumina cements of theused for bonding concretes,53,54 mainly because they 70%Al2O3 class, such as the Denka high aluminadevelop high strength within 6–24 h of placement. cement and Secar 71 (Lafarge). High purity aluminaUsually, high alumina castables require only 24 h to cements with ~80%Al2O3 , such as the Denka superdevelop 70–80% full strength when properly cured, high alumina cement, CA–25 (Alcoa), and Secar 80in contrast to 28 days for normal Portland cement (Lafarge) are typical of Group E. An importantconcretes.53 Calcium cements are usually classified variable in these commercial cements is admixtureaccording to purity and Al2O3 content. In 1972, content, e.g. Secar 71 is additive free while Secar 80Briebach39 classified them in four major groups ( like all 80% alumina cements) contains a cocktail of(Table 1). Only cements in Group 2 were referred to additions which will confer consistent properties onas ‘high alumina cements’, while those in Group 4 the cement but will influence rheology in a castablewere often called ‘white high alumina cements’, system based on this cement.because of their colour. Today, however, any calciumcement from Groups 2, 3, or 4 is often termed CAC Production techniquesor high alumina cement (HAC), or simply calcium Calcium aluminate cements are formed by reactionaluminate (CA).38,53 of lime and alumina either by a sintering or clinker

A more recent classification of HACs38,53 is given process or from fusion.12,38 In the latter limestonein Table 2. Relatively pure limestone (54–55%CaO, and bauxite raw materials are melted at 1450–1550°C41–43% loss on ignition, and less than 2% total in reverbatory furnaces fired by powdered solid fuel.39impurity) is used as the lime source for producing all The molten calcium aluminate is continuously tapped,CA cements. The low purity cements (LP CAC) are cooled, and ground into cement. Other melting pro-manufactured from bauxites containing up to cesses involve the use of vertical shaft and rotary~18%Fe2O3 and 9%SiO2 . Low iron (2–4%Fe2O3 ) kilns, while electric arc resistance furnaces have alsoand low SiO2 (5–7%) bauxites are used to manufac- been used where electrical energy is relatively cheap.38ture the intermediate purity cements (IP CAC), while In this process the proportioned dry raw mix of Bayer

Al2O3 and limestone is either fed as ground or as anagglomerate into a rotary kiln, similar to that used

Table 1 Classification and composition of calcia in the manufacture of Portland cement. The productcontaining cements

is sintered at 1315–1425°C, cooled, and then groundCement group to cement fineness together with any additives. These

include calcined alumina to obtain the desired Al2O31 2 3 4content, gypsum or other materials to control the set,Calcium Calcium aluminate cementsand plasticisers to improve workability.38,53 Thesilicate

White high cement colour ranges from black to white, dependingMineral Portland High alumina ... alumina

on the impurities present and on the iron oxideChemical analysis, wt-% quantity and oxidation state. High purity cements

SiO2

17–26 4–9 4–6 0·1–1·4are white.Al

2O

35–12 35–45 50–65 65–80

On sintering, the raw mix generally transforms intoFe2O

31·7–2·7 10–15 1–3 0·1–1·0

CaO 53–65 36–39 29–40 17–25 higher alumina phases as the material temperatureincreases in the kiln. Both lime/alumina ratio andtemperature determine the amount and type of cal-cium aluminate phases formed during the process.Table 2 Classification and properties of calciumHigh lime calcium aluminates form initially withaluminate cementsferrites and silicates. Uncombined lime and alumina

Typebegin to react with the high lime products and form

Low purity Intermediate purity High purity lower lime or higher alumina compounds, as predictedby the CaO–Al2O3 binary phase diagram (Fig. 2).Chemical analysis, wt-%

SiO2

4.5–9·0 3·5–6·0 0·0–0·3 These reactions continue in the kiln until the mix isAl

2O

339–50 55–66 70–90 completely combined as follows38

Fe2O

37–16 1–3 0·0–0·4

CaO 35–42 26–36 9–28 C+A�C3A�C12A7�CA�CA2�CA6Surface area, m2 g−1

High purity CAC sinters readily, even though veryWagner (ASTM C115) 0·14–0·16 0·16–0·24 0·22–0·30refractory high purity limestone and Bayer calcinedBlaine (ASTM C204) 0·26–0·44 0·32–1·00 0·36–1·50

BET 0·60–1·00 0·80–5·00 0·60–18·00 alumina are used as raw materials in rotary kilnDensity, g cm−3 3·05–3·25 2·95–3·10 3·00–3·30 calcination. A 1360°C eutectic occurring atVicat initial set, h5min 3:00–9:00 3:00–12:00 0:30–6:00 ~50 wt%CaO/Al2O3 (Fig. 2) enhances liquid phase(ASTM C191) sintering of these refractory oxides.



Table 4 Typical mineral constituents of calciumaluminate cements

Cement purityRelativehydration rate Low Intermediate High

Fast C12

A7

C12

A7

C12

A7

Moderate CA CA CASlow CA

2CA

2CA

2C

2S C

2S ...

C4AF C

4A

F. . .

Non-hydrating C2AS C

2AS CA

6CT CT AA A ...

highest strength among the phases listed during therelatively short time available for hydrating refractoryconcretes. It takes some time to start setting, buthardens rapidly after the initial set.

Calcium dialuminate (CA2 ) is the secondary phasein CACs (<25%) and is more refractory than CA buttakes an excessively long time to set though accel-erated at high temperature.38 While hydration of CA

2 Binary CaO–Al2O

3phase diagram; in wt-% is known to be accelerated by the presence of CA2 ,

the opposite does not hold true, and the hydration ofCA2 may actually be hindered by the presence of

The two most critical areas of cement productionCA.55 The strength of CA2 after three days hydration

are development of the clinker phases and the grind-is comparable to that of the pure CA and, unlike in

ing process. Not only are the amount and proportionsCA, it always increases with time.

of clinker phases important, but also their reactivity.C12A7 hydrates rapidly and can be used to control

The crystallinity of a cement phase is important inthe setting rate of CACs when used in small quantities;

controlling cement reactivity. It has been shown,it has a relatively low melting point. C2S and C4AF

for example, that C12A7 , generally considered anare common in Portland cement, but can also occur

extremely reactive phase of the sintered clinker, canin the high silica and iron rich low purity CACs,

be rendered quite unreactive when present in therespectively. C4AF forms hydrates of calcium alumin-

fused form.45 Control of the particle size on grindingate and calcium ferrite or solid solutions of the two

is important, because variations in the particle sizehydrates, and in its setting rate it resembles C12A7distribution can not only affect cement hydration, but(Ref. 12). C2AS (gehlenite) shows little tendency to

also its reactivity with the aggregates in the concrete.hydrate and is an undesirable component of aluminacement which limits refractoriness and hot strength

Phase composition properties.38CA6 is the only non-hydrating phase in the pureTypical phases present in commercial CACs, accord-

ing to their relative reaction rates with H2O (Refs. calcium aluminate system and is often a reactionproduct in alumina castables bonded with high purity38, 53) are listed in Table 4. They form the hydrated

cement phases responsible for developing strength aluminate cement. It is believed that CA6 is mostreadily formed in alumina castables when using CA2after curing the concrete in a humid environment.

Useful properties of these minerals are listed in as a precursor.56 More recently, studies on the prop-erties and microstructure of the CA6 phase haveTable 5.

Monocalcium aluminate (CaO.Al2O3 or CA) is the revealed its great potential as a strong thermal shockresistant, refractory material38,57,58 and its importantmost important component of CACs because it gener-

ally occurs in large amounts (40–70%), has a rela- role in the bonding of corundum and spinelaggregates.40tively high melting point (1600°C), and develops the

Table 3 Chemical composition (wt-%) of commercial alumina cements

Group

A B C D E

Composition 1 2 3 4 5 6 7 8 9 10

SiO2

3·05 8·29 4·72 4·59 2·52 3·41 0·19 0·20 0·08 0·20TiO

21·95 2·08 2·16 2·09 3·23 2·49 Tr. Tr. Tr. Tr.

Al2O

342·11 41·34 47·55 52·28 56·69 56·21 73·36 71·07 80·22 79·53

Fe2O

315·55 11·32 9·52 5·34 0·89 1·88 0·35 0·09 0·15 0·22

CaO 37·46 35·24 34·82 35·27 35·75 35·39 24·46 27·70 17·63 17·12MgO 0·65 1·17 1·19 0·35 0·43 0·53 0·30 0·33 0·35 0·44Na

2O 0·08 0·10 0·06 0·10 0·08 0·08 0·18 0·21 0·66 0·51

K2O 0·05 0·09 0·02 0·05 0·11 0·04 0·04 0·02 Tr. 0·07

Loss on ignition −0·21 0·80 0·35 −0·05 0·07 0·15 0·43 0·17 1·06 1·44



Generally, the characteristics of CACs are associ-ated with the amount of alumina, lime, and impuritiespresent in the products. Increased alumina contentwill confer higher refractoriness, while a high limecontent in the cement increases cured strength. Ironimpurities lower the carbon monoxide resistant athigh temperatures, and siliceous compounds reduceresistance to hydrogen atmospheres under similarconditions.38

HydrationThe hydration mechanisms of CACs have been stud-ied extensively (e.g. Refs. 8, 59–65). However, system-atic structural studies of cements are difficult. Startingmaterials and hydration products are typically multi-phase systems, which are, because of the occurrenceof metastable phases, often compositionally variable,sensitive to experimental conditions, such as temper-ature, duration, and intensity of mixing, and typicallyonly partially crystallised. For these reasons, identifi-cation and quantification of all of the chemical phasespresent in cements is not always possible.66

Figure 3 shows reaction schemes for hydration ofCA, CA2 , and C12A7 . When any cement is mixedwith water the hydraulic minerals begin to dissolvequickly forming a saturated solution of ions. In CACsCa2+ and Al(OH)−4 ions form. Nucleation ofhydration products and their subsequent crystalgrowth produces an interlocked network that gives‘setting’ and then strength.65 Rates of hydration area strong function of the starting CaO/Al2O3 (C/A)ratio and temperature. The hydration of CA occursthrough initial dissolution and subsequent precipi-tation of CAH10 and C2AH8 from the supersaturatedsolution. An induction or incubation period occursbefore this precipitation and is a reflection of thenucleation barrier.67

Hydration starts with contact of cement with water.Initial attack of the anhydrous phase particle surfacesproduces a layer of hydrated calcium aluminate anda layer of apparently amorphous aluminium hydrox-ide. In the induction period, the hydration rate isextremely low, and the thickness of the hydrated

(a)

(b)

(c)

surface layer grows slowly. When the hydration layer3 Reaction schemes for a CA, b CA

2, and c C

12A

7reaches a critical thickness, the stress caused by(after Ref. 63)intruded water molecules ruptures the layer and the

induction period terminates (the reaction as a wholeaccelerates) with the formation of crystalline nuclei CAH10 can be detected after 6–24 h. The presence of

C2AH8 can be detected after 24 h.that grow by a dissolution–crystallisation mechanismto produce the metastable hexagonal hydrate CAH10 . The hydration of calcium aluminates has been

extensively studied using a variety of methods, includ-Depending on the initial crystallinity of the aluminate,

Table 5 Properties of CAC mineral constituents

Chemical composition, wt-% Cold crushing Setting time,Density, strength, h5min Crystal

Mineral C A F S Tm

, °C g cm−3 MPa Initial–final system

C 99·8 ... . . . . . . 2570 3·32 ... . . . CubicC

12A

748·6 51·4 ... . . . 1415–1495 2·69 15 0:05–0:07 Cubic

CA 35·4 64·6 ... . . . 1600 2·98 60 7:00–8:00 MonoclinicCA

221·7 78·3 ... . . . 1750–1765 2·91 25 18:00–20:00 Monoclinic

C2S 65·1 ... . . . 34·9 2066 3·27 ... . . . Monoclinic

C4AF 46·2 20·9 32·9 ... 1415 3·77 ... . . . Orthorhombic

C2AS 40·9 37·2 ... 21·9 1590 3·04 ... . . . Tetragonal

CA6

8·4 91·6 ... . . . 1830 3·38 ... . . . Hexagonala-A ... 99·8 ... . . . 2051 3·98 ... . . . Rhombohedral



%C

ON

VE

RS

ION

TEMPERATURE, °C

C2AH8+AH3

C3AH6+2AH3

5 Temperature dependence of calcium aluminatehydrates formation (after Ref. 65)

4 Typical curve for evolution of heat in calciumof the cement increases rapidly. The evolution of heataluminate cementand the development of strength in CACs are relatedto each other, both stemming from the hydration

ing solution chemistry,68–70 X-ray diffraction,71,72 reactions.12calorimetry,62,73–75 1H NMR (nuclear magnetic reson- Recent NMR studies indicate formation of anance) relaxation rates, and continuous wave 27Al intermediate product, which could be forming directlyNMR spectroscopy.61,66,75 from the surface hydrated layer or, alternatively, by

The hydration of cement is an exothermic process, dissolution and reprecipitation. It has been suggestedand the heat of hydration evolved can be easily that the end of the incubation period and the start ofdetected by means of isothermal conduction calor- the main reaction period, which is followed by massiveimetry (ICC). Adiabatic calorimetric studies of cal- precipitation, are actually due to the formation ofcium aluminate cements with water show two this material.61exothermic peaks, as illustrated in Fig. 4.76 The first Depending on the water/cement ratio, curing con-occurs immediately on contact with water, due to the ditions, and the presence of impurities, differentheat of wetting and the rapid dissolution of cement hydrated products may form during the process. Into form a solution saturated in lime and alumina.77 particular, the hydration process varies with curingThe first peak is also believed to be associated with temperature8,12 and the use of additives, such asthe formation of the first reaction layer on the grains setting retarders and accelerators.66 In castables, theof the hydraulically active substances.62 The dormant hydration of cement can be additionally altered by(induction or incubation) period then follows, during the presence of impurities in the aggregates such aswhich hydrate nuclei form and develop. It has been Na2O (Ref. 74) and microsilica additions.45,59,64,79observed that from 25 to 70°C the length of the The stable hydration products formed from the initialinduction period decreases with increasing temper- sintered mineralogical phases and their crystallineature.61 Once critical nuclei have been formed, mass- structures generally develop within 3–6 months underive bulk precipitation of hydrates occurs, giving rise ambient conditions, or within the first 24 h if curingto a second major exothermic peak, and initiating the is performed at higher temperature. The followinghardening process. reactions take place when CAC and water are com-

Evolved heat of hydration differs widely depending bined8,12,38,65on the type of cement and its mineral constituents.As indicated by Table 6, most of the hydration reac-tions in CACs end in a short period of time and theamount of heat liberated by these cements over oneday approximately equals that liberated by Portland

C12A7CA

CA2

+H

CA<21°C

CA21–35°C

CA>35°C

CAH10 (metastable hexagonal )

AH3 (gel )

C2AH8 (metastable hexagonal )

C3AH6 (stable cubic)

AH3 (crystal )

(1)

(2)

(3)

cement over 28 days.12 The evolution of heat duringhydration is greatly affected by temperature, and theamount of heat evolved increases with the ambienttemperature. The time needed to reach the maximumrate of heat evolution, after water addition, is also As the temperature rises above 21°C, the metastableaffected by temperature, and so is the setting time. hydrates CAH10 and C2AH8 change into the moreThe longest time required is usually at about 30°C. stable compounds, C3AH6 and AH3 .8,12 The tran-The factors influencing the evolution of heat and sitional crystalline change from one calcium alumin-setting time of CACs, particularly around 30°C, are ate hydrate to another is commonly referred to asnot yet fully understood.78 After setting, the strength ‘conversion’. Figure 5 summarises the changes in prin-

cipal hydration product with curing temperature.The morphologies of the main CAC hydrates ( listedTable 6 Heats of hydration of Portland and CA

cements, cal g−1 in Table 7) vary extensively though in general C3AH6forms as cuboids, C2AH8 as platelets, CAH10 asTime, daysneedles or hexagonal prisms, and gibbsite as tablets

Cement 1 3 7 28or needles. Figure 6 shows the C2AH8 and C3AH6CA cement 77–93 78–94 78–95 ... morphologies developed in the fine matrix CAC of

Portland cement 23–46 42–65 47–75 66–94 an LCC system.80 Direct formation of the denser



Formation of stratlingite avoids the conversion reac-tion and may, therefore, lead to superior low andintermediate temperature strength and stability.Furthermore, the presence of other forms of aluminaalso influences hydration and rheological properties.83The surface area, Na2O content, and releasability ofthe aluminas are particularly important and can havean effect on the high temperature microstructuraldevelopment, see the section ‘Calcined alumina’below.

AdditivesChemical additives or admixtures are used to controlthe reactivity of the cement. These are ground andblended with the cement during the manufacturingprocess to control or modify the properties of thebinder.38 The use of such admixtures in castables isconsidered by many45 unacceptable because mostformulations incorporate deflocculants and buffers tocontrol the rheology of the product and the additionof admixtures to the cement can alter the propertiesof the castable by interacting with these additives.

Besides those additives used for dispersion, see thesection ‘Submicrometre (superfine) matrix additions’below, the most common additions to calcium alum-a 18 h, C

2AH

8hexagonal plates; b 11 d, C

3AH

6deltoid icositetrahedra

inate containing castables control the setting charac-6 Hydrate microstructure in CAC with water/teristics of the cement. They are the so-called setcement ratio 2 at ambient temperature;80

retarders and accelerators. Among the retarders, themarker 10 mmfollowing are significant: citric84–86 and phosphoric8acids, diluted acetic acid,38 boric acid,60,87 borax,8,38

stable hydrates (AH3 and C3AH6 ) at higher curingsodium citrates60,88 and gluconates,60 hydroxy-

temperatures produces greater porosity and largercarboxylic acid salts,8,38 saccharides,8 magnesium and

pore sizes than when the metastable and less densebarium hydroxides,38 sodium chlorides and sul-

hydrates (CAH10 and C2AH8 ) are formed at lowerphates,38 starch, sugar, sea water, and many other

temperatures. While this gives lower green strengths65acids and acidic compounds.38 The most commonly

it also gives coarse, permeable hydrate phases so thatcited set accelerators are: lithium salts and carbon-

vapour species can escape on drying. The imper-ates,38,88 calcium hydroxide,38,60 Portland

meable AH3 gel phase developed at low curing tem-cement,38,51 slaked lime,66 hydratable aluminas,51

peratures increases the danger of explosive spallingsodium and potassium carbonates,38 sodium sili-

on drying due to steam build up. For this reason,cates,38 and many other alkalis and alkaline com-

refractory CAC castables are generally cured abovepounds.38 It has been suggested88 that the combined

27°C. Best practice for safe heating and maximumuse of set retarders and accelerators is more effective

strength development is to cure for at least 24 h atand allows better control of the hydration process.

30–38°C covered in an impermeable membrane toThe reaction mechanisms of set retarders and accel-

maintain a humid environment for hydration, fol-erators are still unclear,8 but it is generally agreed

lowed by a 24 h air cure with the surface exposed tothat retarders influence the kinetics of hydration by

30–38°C.81 The increase in porosity and its effect onslowing down the dissolution of the anhydrous cement

strength are also heavily influenced by original totalparticles.89 It is believed that accelerators influence

water/cement ratio.mainly the dormant period of hydration. Lithium

The presence of other phases such as microsilicasalts are mostly used as accelerators, and it is believed

or reactive magnesia along with CAC in castables isthat lithium ions interact with Al(OH)−4 to precipitate

known to influence the hydration mechanism.64,82insoluble lithium aluminate hydrate, thereby increas-

Pantjadarma64 determined that stratlingite (gehleniteing the Ca2+/Al(OH)−4 ratio in the solution. Rapid

hydrate, C2ASH8 ) formed with microsilica presenthydrate formation is then promoted, and the setting

having the same platey morphology as C2AH8 sug-time is consequently reduced.89

gesting it formed by reaction of silica and C2AH8 .

Dehydration and firingTable 7 Properties of CA cement hydrates

Strength loss is known to occur in hardened calciumChemical composition, wt-% aluminate pastes as the metastable hexagonal CAH10Crystal Density,

Hydrate CaO Al2O

3H

2O system g cm−3 phase dehydrates through the hexagonal C2AH8

transition phase into the stable cubic C3AH6 .12,38,51,90CAH10

16·6 30·1 53·5 Hexagonal 1·72C

2AH

831·3 28·4 40·3 Hexagonal 1·95 This loss of strength can best be appreciated by

C3AH

644·4 27·0 28·6 Cubic 2·52 considering the morphological and volume changes

AH3

. . . 65·4 34·6 Hexagonal 2·42 accompanying this conversion. When CAH10 is



allowed to form during low temperature curing, the coarser pores in the plates and cuboids which sub-sequently collapsed. Sintering led to ceramic bondingmetastable hexagonal prisms (density 1·72 g cm−3)

and gel (2·42 g cm−3) solidify and eventually will and improved strengths.convert to the stable cubic (2·52 g cm−3) type withtime and/or temperature. The gross restructuring Conventional castablesfrom H2O loss leading to pore formation and nominal

Conventional castables consist of graded refractory50% volume shrinkage on conversion of CAH10 toaggregates bonded with the aluminous hydraulicthe denser C3AH6 and a-AH3 are disruptive to a rigidcements described in the section above. The propertiesstructure and account for the observed loss inof these concretes depend largely on the choice ofmechanical strength.refractory aggregate and hydraulic cement.19 TheyAs the temperature is increased, the dehydrationcontain ~15–30%CAC,18,29,33,37 this amount beingprocess continues, until all phases lose their water ofnecessary to achieve satisfactory strength at low andcrystallisation. Dehydration temperatures may beintermediate temperatures though it makes the mater-measured by thermal analysis techniques, such asial thirsty. The 8–15% water generally added duringdifferential thermal analysis (DTA), derivative therm-processing is mainly used to develop the hydraulicogravimetry (DTG), and differential scanning calor-cement bond (6–10%) and to make the concrete flowimetry (DSC). The dehydration is also a complex and(2–6%), allowing its proper installation. However, anot fully understood process. CAH10 loses part of itsrelatively large amount of water (0–5%) is often takenwater of crystallisation at temperatures lower thanup by the porosity of the aggregates and does notindicated by thermal analysis (50–70°C). Part of thiscontribute to the hydraulic bond.water of crystallisation is even lost at low humidity

at room temperature and, since CAH7 is sometimesformed, it is believed that CAH10 may have three Microstructural developmentwater molecules that are easily dehydrated. Such

All the reactions on hydration/dehydration/firingunstable hydrates are expressed as CAHx (x<10).

described for CACs in the section ‘Dehydration andC3AH6 , in its turn, often shows a stepwise dehy-

firing’ above, occur in refractories in which theydration process, usually indicated in the DTA curve

are the main component of the bond system butby two endothermic peaks, one around 300°C and

complicated by the presence of the variousthe other close to 500°C. It has been claimed that the

additives/admixtures in the bond system. These mayfirst 4·5 molecules of water are lost in one step, and

react with the cement and effect its hydration, setting,the next 1·5 in another.91 The possibility that the

and firing behaviour. An additional complication is1·5H2O phase (C3AH1·5 ) might represent some inter-

the presence of the aggregates which increase watermediate stage or compound had been suggested.92

requirements and may react with the bond system atHowever, more recent studies indicated that the

higher temperature.C3AH1·5 is merely a metastable structural relict which

All CAC hydrates in the bond phase decompose toconstitutes a step in the dehydration of C3AH6 at

the calcium aluminates (C12A7 , CA, and CA2 ) andlow partial pressures of water.93 AH3 gel and gibbsite

eventually, if enough free alumina is present, CA6usually dehydrate between 210 and 300°C (Table 8)(Fig. 7). At room temperature alumina gel, hydrated

but they may otherwise convert to boehmite (AH),calcium aluminates, and free alumina coexist.96 On

which only dehydrates at ~530–550°C.heating to 200°C dehydration occurs and by 400°C

The compounds formed during hydration of CACsC12A7 starts to form from amorphous dehydrated

dehydrate up to~550°C.12 The process of hydration,calcium aluminates. At 900°C elongated CA forms

followed by dehydration, creates the anhydrous(Fig. 7a) from reaction of alumina and C12A7 . At

material which is extremely fine and active.36 Lime1000–1200°C CA reacts with alumina to form coarse

and alumina reappear and recombine in a way similarand globular CA2 (Fig. 7b) while at >1300°C CA2to that of the original raw materials in the kiln.38,94reacts with alumina to form hexagonal platelets of

Table 9 depicts the mineralogical changes in HACsCA6 (Fig. 7c–f ). This morphology is believed to assist

as the temperature increases up to 1500°C.12 Thephysical interlinking of the microstructure improving

microstructural changes associated with these reac-high temperature strength.

tions and the morphologies of the phases formed werestudied in detail by Parker.95 She determined that on

Disadvantagesdrying at 110°C, CAH10 and C2AH8 either dehy-drated to amorphous products or, if water was pre- These high cement castables have three major

disadvantages.7,18,19,29sent, converted to C3AH6 and gibbsite. Firing above300°C dehydrated the pastes completely and reduced First, because they need so much water they are

usually porous and open textured, which greatlytheir strength due to increased porosity and poregrowth. Pastes fired to 900°C and dried pastes were reduces the strength. The low porosity and per-

meability of castables at temperatures below 21°C ismorphologically similar while above 900°C CA crys-tallised and sintering began to occur leading to ascribed to the alumina gel formed upon curing.53

The open porosity of a conventional castable driedat 110°C is generally about 9–17%,33 but can be asTable 8 Dehydration temperature of CA cementlow as 8%.18 Although some of this porosity is duehydratesto entrapped air bubbles, most of it is caused by theHydrate CAH

10C

2AH

8C

3AH

6AH

3 excess water added on mixing. On heating, theDehydrating temperature, °C 100–130 170–195 300–360 210–300 hydraulic bond is first modified, as conversion takes



a 900°C; b 1200°C; c, d, e 1300°C; f 1400°C

7 Microstructural evolution on firing conventional castable matrix96

place, and then destroyed by the dehydration process. varies from 22 to 26%, depending on the type ofaggregate used.During this textural modification, the pore size distri-

bution changes and porosity grows significantly. The Second, conventional castables show a character-istic drop in strength at intermediate temperaturesnew porosity depends on the amount of chemically

bonded water and is therefore dependent on cement (often quoted to be between 538 and 982°C7 ), whenthe hydraulic bond has already broken down, due totype and content. The final open porosity of conven-

tional refractory concretes fired at 1000°C generally the dehydration process, but the still sluggish sintering

Table 9 Mineralogical changes of dehydrated high alumina cement on heating

70%Al2O

3CAC 80%Al

2O

3CACTemperature,

°C C12

A7

CA CA2

CA6

A C12

A7

CA CA2

CA6

A

500 X ... . . . . . . X ... . . . . . . . . . X600 X ... . . . . . . X ... . . . . . . . . . X700 X ... . . . . . . X X ... . . . . . . X800 X ... . . . . . . X X X ... . . . X900 X X X ... X X X X ... X

1000 X X X ... X ... X X ... X1100 ... X X ... X ... X X ... X1200 ... X X ... . . . . . . X X ... X1300 ... X X ... . . . . . . X X ... X1400 ... X X ... . . . . . . . . . X X X1500 ... X X ... . . . . . . . . . X X X



has not yet allowed the development of a ceramic Low cement castables and ultra-lowbond. The exact temperature range when strength cement castablesdeteriorates is not absolute, but may depend on

First attempts to improve the performance ofvarious factors, such as the type and proportion ofhydraulic castables by reducing water and cementhydrates, the curing temperature, and heating sched-content were largely unsuccessful, since the mechan-ule. C3AH6 becomes thermodynamically unstableical resistance was insufficient for compositions withabove ~292°C but its dehydration is a stepwiseless than 10% cement.33 Refractory castables with noprocess. In practice, higher temperatures may actuallymore than 5–8% cement, characterised by excellentbe necessary for the complete dehydration of thecold and hot strengths, were first mentioned in ahydrates, since heating increases water vapour press-French patent granted in 1969 to Prost and Pauillac.46ure, particularly inside closed pores, further delayingReduction of the cement content without anythe dehydration process. Although some hydratesreduction in strength was accomplished by thedehydrate rapidly others, such as micas, clays, andaddition of ~2·5–4% fine (<50 mm, but ideally lessC4A3H3 , may be heated for days or weeks at temper-than 1 mm) clay minerals and 0·01–0·30% defloccu-atures several hundred degrees above their equi-lants (such as alkali metal phosphates and carbon-librium stability limit without much effect.93 Thickates). The objective was to reduce the amount of(tens of centimetres) linings may develop hydrother-water by promoting a homogeneous distribution ofmal pressures that can form C4A3H3 and boehmitethe cement so that the hydraulic bond could be fully(AH).97 Although decomposed hydrates may start toutilised. Despite their lower porosity and better cor-react with alumina aggregates at temperatures as lowrosion resistance, compared with conventional cas-as 900°C, significant sintering of calcium aluminatetables, the first generation of LCCs was too sensitivecrystallites themselves and with neighbouring aluminato rapid heating, mainly because the chemicallycrystals only occurs at temperatures close to 1100°C.94bonded water was released in a much narrowerEarly attempts to accelerate the formation of a cer-temperature range.29,33,37 This led to explosive spal-amic bond generally consisted in making additions ofling since the outer layers closed off and internalvarious fluxes, most of which did little good andwater pressure built up. Further improvements led tousually reduced the maximum temperature of use.36the development of concretes characterised by aValues for the cold crushing strength of conventionalpseudozeolithic bond, which releases the chemicallyrefractory castables fired at 1000°C usually vary frombonded water slowly between 150 and 450°C, rather10 to 30 MPa, averaging ~60% of the strength afterthan within a narrow temperature range.29,33,99 Thisdrying.19minimised the problems associated with explosionsFinally, the high lime content of these castablesduring heating but, because LCCs and ULCCs arefavours formation of a fluid vitreous phase at highdense materials with low permeability, baking out istemperature via the eutectic liquid in the CaO–Al2O3– always difficult, especially in thick installations.14SiO2 (CAS) ternary system which may encourage

Two other French patents in 1976 and 197747crystal formation (e.g. mullite or spinel, see the sec-further reduced the cement content of the castablestions ‘Microstructural evolution on drying and firing’to less than 3%, again by using dispersing additives,and ‘Spinel and magnesia based castables’ below) butsuch as sodium tripolyphosphate (0·01–0·05%), andoften will remain as a glass (Fig. 7e) or low meltingfine particles. Part of the cement was replaced by fineanorthite and gehlenite on cooling which degradesparticles ranging from 10 to 1 mm, but the decisiverefractoriness and corrosion resistance.96,98 Thestep was the use of submicrometre particles rangingvolume of viscous phase in a refractory castable forfrom 0·1 to 0·01 mm, which could be easily disperseda given temperature and refractory aggregate isin water without forming a sol or a gel. The idea wasmainly determined by the impurity content of theto reduce the water requirement by eliminating thebinding phase, i.e. by the composition and amount ofintergranular voids, which are often filled with excesscement used. Even with a high purity CAC containingwater during the castable placement. This was70–80%Al2O3 , it is impossible to reduce the CaOaccomplished by carefully grading the particle sizecontent of conventional castables to less than 3%,distribution, so that interstices were progressivelywhich is still a high amount, particularly if silicafilled by smaller particles to obtain the maximumcontaining aggregates are used. Further reduction ispacking density.11,33,99 Water requirements wereonly possible by reducing the cement content.further reduced by proper selection of deflocculantsUnlike fired refractory bricks, whose final prop-and water reducing agents, which prevented coagu-erties are largely fixed before reaching the user, alation of the fine powders and improved dispersion.refractory concrete has properties which evolve andThe reduction in the water content required foralter for a considerable time after it has been put intovibration casting and maximising of the packinguse. In the case of refractory concrete it is the behav-density means these materials have high density, lowiour at the service temperature which is more import-porosity, and good mechanical and abrasion resist-ant than the unfired strength.36 By the end of theance.11,99–101 Low and ultra-low cement castables1960s, there was little doubt about what should beusually require 3–7% water for placement, dependingdone to improve the performance of refractory cas-on the grade.14,33,99tables. Reducing the amount of cement without spoil-

To appreciate fully modern refractory castables theing other properties of the material proved difficultinter relation between particle packing, dispersionand challenging, but after several attempts it finallytechnology, and rheology is critical. Understandingled to the development of a new range of products:

the low and ultra-low cement castables. the relation between the first two of these gave rise



particular type of particle, often in small amounts,usually between 0·05 and 0·5%.

Refractory castables consist mainly of fine powders,aggregate, and water, and it is generally agreed thatthe workability of the material is governed by theflow properties of the fine powders.107 Therefore,the study of abnormal flow properties in a finepowder–water system, such as slurries and pastes, isimportant. Rheology is the science of flow anddeformation of materials.102,108,109 In a suspension ofcompletely dispersed particles, the shear resistancedepends primarily on the viscosity of the liquid andthe interparticle forces. In such a system, the effectivestress, or interparticle stress, is independent of press-ure imposed in processing. However, in a morecrowded slurry, as is the case with castables, shearmay be momentarily blocked by neighbouringparticles. The resistance to shear flow is dependenton particle translation away from the plane of shear,which is time dependent. The shear resistance will bevery shear rate dependent. It also depends on mechan-ical interactions between particles. In a nearly close-

8 Relations between particle packing, dispersion, packed system, initial flow produces significant con-and rheology

tact stress between particles. Volume dilation of thesystem must occur to accommodate shear flow. Inthese systems the effective stress and the shear resist-

to the new range of LCCs and ULCCs, while incorpo- ance is not independent of the confining pressure.102In castables, the situation is further complicated byrating the third further improved the overall under-the presence of aggregates, whose elastic and plasticstanding of the technology and allowed thecharacteristics, and importantly segregation tendency,development of SFCs (Fig. 8).play an important role in the flow properties of thematerial.109 Studart et al.110 obtained a rheologicalmap by considering viscosity, yield stress, andParticle packing, dispersion, and rheologyabsorbed energy to obtain cement free, self-flowingThe main idea behind LCCs and ULCCs is to reduce(see ‘Non-cement or cement free castables’ and ‘Freethe water requirement for placement while main-or self-flowing castables’ below) HACs. Both particletaining strength. A major breakthrough in the devel-size distribution and matrix rheology are importantopment of this technology was the realisation thatwhen manufacturing such castables.this could be accomplished by improving the packing

Refractory castables are subjected to a wide rangedensity of the material. More efficient particle packingof shear rates during processing and installation,

reduces the maximum size of the interstices betweenvarying from the very low rates required for gravity

particles. For a size distribution which packs morelevelling (10−1 s−1) to the very high shear rates

efficiently, less of the liquid is segregated in largecharacteristic of spraying (105 s−1), through the inter-

interstices and more of it is effectively mobilised inmediate rates usually used in pumping and mixing

flow. The packing density of about 62% for a monos-operations (1–103 s−1). Above a particular shear rate,

ize system can be increased above 75% by adding a the hindered rotation and particle interference mayspecific proportion of a finer size that packs efficiently cause the appearance of shear thickening or dilatantin the interstices among the coarse fraction.102 behaviour. Low cement castables characterised by

However, the idea of reducing the water require- dilatant behaviour are subjected to high resistance toment for placement by simply improving the packing flow at high shear rates during the mixing process,density of the castable would not have been successful and therefore proper mixing is only possible withwithout the proper use of additives to allow adequate excess water, which is undesirable. The absence ofdispersion of the submicrometre powders. A range of shear thickening is also important in the pumping ofchemical additives are used for this purpose includ- castables. Increasing the solids loading (solid concen-ing sodium carbonates,8,89,102 sodium silicates8,102 tration) in the system generally decreases the shearand borates,102 sodium pyrophosphates,8,102 rate at which dilatant behaviour begins. Dilatancyhexametaphosphates44,60,85,103,104 and tripoly- may be reduced by the use of fine particles, such asphosphates,85,103,105,106 ammonium102,104 and microsilica, or any ultrafine material exhibiting sig-sodium102,105,106 polyacrylates, sodium sulphon- nificant pseudoplastic behaviour at high shear rate,ates,102,106 sodium citrates38,102 and gluconates,38 and including ultrafine alumina (<0·5 mm). Dispersion ofmany others offered commercially under proprietary agglomerates, by the use of deflocculants, or modifi-names such as Darvan 7S (organic polyacrylate poly- cation of the particle size distribution to producemer) and Castament FS10 (polyglycol based poly- additional fines may also extend the range of shearmer).89 These deflocculants are used separately or in rate before shear thickening is observed in the cast-

able. As for the fines, it is generally believed that acombination with each being used to deflocculate a



narrower particle size distribution increases dilatant microsilica has a relatively low bulk density(0·2–0·45 g cm−3) and is readily dispersed in refrac-behaviour. Finally, it has been shown that particles

of a more irregular shape not only cause an increase tory concretes improving casting properties. Densifiedmicrosilica contains loosely bonded secondaryin viscosity but also a severe dilatant behaviour,109

which is understandable since anisometric particles agglomerates which increase the bulk density(0·5–0·6 g cm−3) and improve the handling character-produce a larger effective hydrodynamic volume

during flow.102 istics of the material, but it requires a high intensitywet mixing to assure complete dispersion. There isno difference in chemical composition betweenundensified and densified microsilica, provided theSubmicrometre (superfine) matrix additionsgrade is the same. The main physical and chemicalAccording to the French patents,46,47 the main roleproperties of microsilicas commonly used in refractor-of the submicrometre powder additions is to act as aies applications are listed in Table 10.filler, exactly filling the void spaces between the larger

The primary function of microsilica in refractoryparticles, so that the densest possible packing iscastables is to act as a filler. Once properly dispersed,achieved. Therefore, any refractory material could bemicrosilica fills the voids between the coarser particles,used, as long as it was solid and did not react withreleasing the entrapped water and increasing thewater.110 Submicrometre powders commonly used inpacking density. In this sense, microsilica has tra-the early years of this technology included alumina,ditionally been found to be more effective than finesilica, chromium oxide, zirconia, titanium oxide, sili-calcined aluminas, though this situation appears tocon carbide, clay minerals, and even carbon.14,33 Frombe changing as new much finer superground reactivethese, two relatively new refractory raw materialsand dispersing aluminas become available.114,115 It ishave come into significant use in both LCCs andclaimed that use of microsilica reduces the openULCCs: superfine silica powder and reactive alumina.porosity from about 20–30% to 8–16% after firing

Colloidal silica at 1000°C, and that this reduces the characteristicSilica fume, also referred to as fumed silica, volatilised drop in strength at intermediate temperatures oftensilica,79 silica flour, or white carbon,8 is a byproduct observed in conventional castables.116,117of silicon metal production, with the quality variations However, acting as a filler is by no means the onlyinherent to any byproduct. Only in the last two effect of microsilica in refractory castables. Studiesdecades has this material been supplied at a consistent with microsilica containing cement pastes have shownquality level. Many of the early field problems with that microsilica reacts with the calcium aluminateLCCs, such as erratic setting behaviour and low phases in the cement and water to form zeolithicstrength, are directly traceable to variable quality CASH phases.116 This complex pseudozeolithic bond,silica fume. The material’s purity and pH have been just like the zeolites themselves, is characterised by ashown to have drastic effects on LCCs, with impurit- weak water binding potential and, consequently, theies, such as iron oxide and alkalis, reducing the chemically bonded water is not released abruptly instrength and increasing the viscosity, respectively, and a narrow temperature interval, but rather progress-a lower pH increasing the setting time.14,111 ively over a wider temperature range. Because of this,

The term microsilica was later introduced by Elkem it has been argued that castables with a ‘pseudozeo-A/S for the material obtained after cleaning, classify- lithic’ bond are not prone to explosive spalling duringing, and homogenising the silica rich fume released drying out.117 However, the mechanism by whichduring production of ferrosilicon and silicon metal in microsilica reacts with CACs is not yet fullyelectric arc furnaces.79,112 Microsilica is an amorphous understood.8silicon dioxide consisting of submicrometre spherical In microsilica containing castables, the physicalprimary particles with an average diameter of and chemical characteristics of microsilica can drasti-~0·15 mm. These spheres are the building units ofprimary agglomerates which consist of a few spheres Table 10 Typical properties of microsilicabonded together by material bridges. Thus, the effec-

Mineral 960 971 983tive size distribution becomes rather wide in theTypical chemical composition,submicrometre range. This wide particle size distri-wt-%bution is believed to bring beneficial effects, as it

SiO2

97·00 96·00 97·50 98·30increases the packing efficiency and enhances theAl

2O

30·40 0·40 0·40 0·20

workability of the concrete.112 CaO 0·15 0·20 0·20 0·20High purity microsilica from the production of MgO 0·30 0·30 0·10 0·07

Fe2O

30·10 0·10 0·10 0·05metallic silicon is usually preferred for refractories

Na2O 0·10 0·10 0·10 0·04applications.43,44 In refractory castables, use of high

K2O 0·20 0·40 0·30 0·25

quality, high purity microsilica is almost mandatory, P2O

5. . . 0·10 0·10 0·06

since low purity microsilica reduces flow and increases SO3

. . . 0·10 0·10 0·01C ... 1·80 0·50 0·40water requirement for placement.113 The material isLoss on ignition (750°C) ... 2·00 0·60 0·60highly reactive in cementitious and ceramic bond

Median particle size, mm 0·15 ... . . . . . .systems, leading to improved ceramic bonding (form-Surface area, m2 g−1 18–28 22 20 ...

ing, e.g. mullite and forsterite) at reduced firing tem- pH 6–8 6·5 6·0 5·3peratures both in high alumina and magnesia Bulk density, g cm−3

Undensified ... 0·25 0·40 0·40based products. Microsilica is usually available inDensified ... 0·60 0·55 ...undensified (U) and densified (D) forms. Undensified



cally change the properties of the system. Because of increases with the cement content, the high cementits high specific surface area (20 m2 g−1), microsilica castables usually show lower hot strength.120 Thein some cases makes up more than 50% of the total subsequent drop in strength observed at 1500°C,particle surface area of the system, and its surface despite the precipitation of mullite, was attributed tocharacteristics and impurity content significantly the presence of impurities, particularly alkalis fromaffect the casting and setting properties of the castable. the alumina, and further studies were carried out to

When microsilica is added to a refractory contain- confirm that reducing the alkalis in the system coulding CAC, this causes modifications to the original improve the strength.44 However, despite the overallcementitious bond phase. First, it has been shown improvement in hot strength, both at 1400 andthat hydration of cement is hindered or diluted by 1500°C, this did not change the general trend, sincemicrosilica, and therefore full conversion does not the strength also dropped at 1500°C for the low alkalioccur.59,79 Second, it has also been suggested that compositions.microsilica delays the setting of the castable by seques- Based on a series of studies on the use of microsilicatering the multivalent cations Ca2+ and Al3+. Esanu in high alumina LCCs and ULCCs,44,117,119,120 it waset al.118 demonstrated that fumed silica participates suggested that to increase the castables hot strengthactively in hydration by interacting with additive both the impurities and the cement content shouldcontaining water and releasing silicic acid. be lowered to a minimum, while the amount ofAdditionally, the reactive silica may react with C2AH8 microsilica should be increased.119 While this is trueat slightly elevated temperatures (40–60°C) to form to a certain extent, since increasing the amount ofC2ASH8 (see the section ‘Hydration’ above). When silica with respect to that of the cement actuallyheated above 210°C, the chemically bonded water is pushes the composition of the matrix towards thereleased, and C2ASH8 dehydrates and becomes silica corner in the C–A–S alkalis quaternary system,amorphous. From ~1000°C upwards silicate liquid and therefore away from any eutectic liquid in themay form and gehlenite (C2AS) and anorthite (CAS2 ) system, it could be argued that this could also becrystallise.118,119 At these temperatures and up to achieved by reducing the amount of microsilica with~1200°C, microsilica containing castables exhibit respect to that of the cement. Of course, this wouldsuperior properties compared with microsilica free also require improvements in the properties of thecastables, including a reduction in porosity of about castables at the lower temperature range, where the5–8% and a large increase in cold crushing strength, addition of microsilica boosts the strength andpresumably due to the high reactivity of microsilica.117 reduces the porosity. This, fortunately, has been madeHowever, as the temperature is further increased possible by the use of special superground reactiveabove 1200°C, the hot strength deteriorates, aluminas, which improve packing and speed up thedepending on the amounts of cement and microsilica, development of a ceramic bonding phase at muchdue to eutectic liquid formation in the C–A–S ternary.

lower temperatures (see the section ‘Calcined alumina’However, it has been observed that, in high alumina,

below).114,115microsilica containing LCCs and ULCCs, the hot

The benefits of adding microsilica to bauxite basedstrength sometimes increases significantly above

LCCs and ULCCs are clear since these materials1300°C, reaching its maximum around 1400°C, only

already contain silica and therefore the eutectics into drop again at 1500°C.44 The increase in hot

the ternary C–A–S system cannot be avoided, otherstrength is believed to be caused by the growth of

than by reducing the amount of cement.43,121elongated needle-shaped mullite crystals from theHowever, as far as high alumina, LCCs and ULCCsliquid phase, which interlock the structure improvingare concerned, the use of microsilica, despite itsthe bond. As soon as the castable is heated to 1400°C,acknowledged beneficial effects to the rheology andthe bond phase of the castable reacts and produces apacking of the system, is detrimental to the hotviscous liquid formed mainly from cement and micro-strength at temperatures around and above 1500°C,silica. Within a few hours, the liquid starts to dissolvedue to the formation of liquid phase. One advocatedalumina, which goes into solution with silica andsolution to this problem is the reduction in thelime; the liquid becomes saturated, and mullite pre-amount of cement, in some cases down to levels ascipitates. As mullite is formed, the composition of thelow as 0·5%. But so far this has only proven feasibleliquid changes, and its amount decreases. The finalwith the simultaneous addition of cement (0·5%) andcompositions of the glassy phase are either close tohydraulic alumina (0·5%), so that both the settinganorthite or comparatively richer in silica. Furthertime and flow could be properly adjusted.33,52 In thismullite precipitation is hindered, either by longcase, the bond is predominantly interlinked mullitediffusion paths or because the glassy phase reaches aneedles, and it has been shown that for higher contentsstable composition.44,119of microsilica (10%) the drop in hot strength atStudies involving addition of microsilica to high1500°C may be significantly minimised, and the hotalumina LCCs and ULCCs indicate that there is amodulus of rupture in some cases approaches that ofminimum amount of microsilica required for mullitemicrosilica free HACs.122 However, though this issueprecipitation, which depends on the amount ofhas not yet been properly addressed, the mullite bond,cement. It has been suggested that for castables withas it is in bricks, is often more prone to corrosion by1·5% cement (0·27%CaO), strengthening due to themetal and steelmaking slags than the high aluminaformation of mullite is only observed for microsilicabond. Therefore, for applications where corrosioncontents above 6%. For castables with 7·5% cement,resistance is the ultimate goal, such high microsilicaat least 10% microsilica is necessary, but since there

is always some liquid present in the castable, and this ULCCs are not appropriate.



Calcined alumina crystal size increases, green density increases. Also, asthe crystal size distribution is broadened, green den-Calcined aluminas can be divided into three main

categories based on both sodium oxide content and sity increases. High green density minimises shrinkageduring sintering. Coarser crystalline aluminas exhibittotal impurities.123 The first category includes normal

calcined alumina with a soda content greater than minimum shrinkage at 1500°C, while finer aluminasare in the final stages of sintering at this temperature.0·1%, usually between 0·18 and 0·55%, and

~99·0–99·5%Al2O3 . The second category contains In low moisture LCCs fine aluminas that exhibit highreactivity but low water demand are particularlycalcined aluminas, sometimes referred to as ‘low soda’,

with an alumina content of about 99·7% and less useful. These are usually low soda aluminas thatexhibit high green densities.123than 0·1% soda. The third category includes high

purity calcined aluminas with at least 99·9%Al2O3 . A range of calcined aluminas are available, includ-ing the most recent type referred to as ‘dispersingAlmost all calcined aluminas, including those for

refractory applications, are produced by heat treat- aluminas’. These are characterised by a bimodalparticle size distribution, which improves packing,ment of gibbsite from the Bayer process.

After calcined alumina aggregates are produced, and are believed to contain organic ingredients thatallow better dispersion and control of the cementthey can be further processed, by milling to separate

them into their individual crystals. The time and setting.114,115 Some of the most commonly cited com-mercial calcined aluminas101,123 are listed in Table 11intensity of this step determines the particle size

distribution of the finished product. Normal calcined which gives their main properties and characteristics.The use of fine reactive aluminas results in LCCsaluminas, also called milled aluminas,101 are usually

ground below 44 mm (95–99%), but these agglomer- with excellent hot properties and very low mixingwater requirements for placement. Unfortunately,ated aluminas have not been completely separated

into their ultimate crystals. In applications where a SFCs formulated without microsilica can becomedilatant, making them unsuitable for pumping.114more completely ground alumina is required, the

superground aluminas, also referred to as thermallyreactive aluminas, or simply reactive aluminas, should

Aggregatesbe used. These are finely ground calcined aluminaswhose relatively high surface area fine crystals exhibit Low and ultra-low cement castables are basically a

mix of two main components: the refractory aggregatehigher densification and reaction rates when com-pacted and sintered into ceramic products. Sintering and the binding system.106 The fine fraction below

45 mm usually represents the bond system, whichtemperatures required to densify completely ceramicsmade from fine superground aluminas are usually consists of the hydraulic binder, fine and superfine

ceramic powders, and admixtures of deflocculants,200°C lower than those made from regular ground,coarser aluminas. These reactive aluminas are not, water reducing agents, set retarders, and accelerators.

This fraction will become the matrix of the solidhowever, capable of hydraulic activity to form chemi-cal bonds at ambient temperatures. Hydratable alum- concrete after setting and will give rise to a ceramic

bonding phase on firing, which will bind together theinas (such as r-aluminas) which are capable of formingroom temperature chemical bonds in cement free refractory aggregates (see Fig. 1). Owing to its mul-

tiple roles in controlling the flow behaviour andcastables are considered in the section ‘Non-cementor cement free castables’ below. setting time of the castable, as well as the strength

and properties of the binding ceramic matrix, the fineIn refractory applications, both normal and lowsoda aluminas are mainly used. Normal soda alum- portion of these materials has been widely investi-

gated. However, the refractory aggregate, which is theinas have traditionally been recommended for alum-ina enrichment of refractories where a small amount fraction above 45 mm, is also extremely important in

this partnership, not only with respect to the particleof soda can be tolerated.101 However, in moderncastables technology, not only has the soda content size distribution of the mix, but also regarding the

overall chemical composition of the material. Theof the aluminas come under closer scrutiny, becauseit can greatly affect the dispersion of alumina, increas- aggregate system normally comprises 60–85% of the

castable mix, and its chemical composition and physi-ing the viscosity,111 and the hydration of cement,shortening the induction period,74 but also the physical characteristics significantly affect the final prop-

erties of the castable, particularly thermal shock andcal characteristics such as crystal shape and size,particle size distribution, specific surface area, a-Al2O3 corrosion resistance.

Practically any natural or synthetic refractory oxidecontent, compacting behaviour, and sintering reactiv-ity. The particle size distribution of the alumina plays that is normally used for refractory bricks can be

used as aggregate in LCCs and ULCCs. However,an important role in filling the interstices between thelarger aggregates to increase packing density, and it alumina, fused or sintered, is the most common

aggregate used due to its high strength, relatively lowhas been found that combining several kinds ofultrafine alumina with different particle size distri- thermal expansion coefficient, and good resistance to

chemical attack, though it may be dissolved at highbutions may further contribute to reduce the waterrequirement for placement.23 The use of calcined temperatures in molten alkalis and calcium containing

silicates. Besides, in CA cement systems, alumina is aaluminas with multimodal particle size distributionsmay have similar effects on packing.114 In general, compatible oxide, which does not change the refrac-

toriness of the binary CaO–Al2O3 system (Fig. 2),the finer the alumina the greater its reactivity, whichimproves sintering at lower temperatures speeding up and this also accounts for its widespread use in

cement containing castables.early development of a ceramic bond. Usually, as the



Table 11 Typical properties of reactive calcined aluminas43,115

Type of alumina

16SG 152SG A17 1000SG 3000FL RA7 RA10 RA15

Composition, wt-%Al

2O

343,115 99·7 99·7 99·7 99·7 99·7 ... . . . . . .

SiO2

0·03 0·04 0·03 0·02 0·02 ... . . . . . .Fe

2O

30·01 0·04 0·01 0·02 0·02 ... . . . . . .

Na2O 0·08 0·08 0·08 0·08 0·08 ... . . . . . .

Crystal size, mm 0·4 1·5 3·25 0·4 3·25 ... . . . . . .Particle size, mm 0·5 1·5 3·0 0·5 3·0 1·1 2·1 2·7Surface Area, m2 g−1 10 3 3 10 3 3 2·2 3·7Green density, g cm−3 2·15 2·25 2·50 2·15 2·50 ... . . . . . .Fired density, g cm−3 3·91 3·84 3·75 3·91 3·75 3·50 3·40 3·60Compaction pressure, MPa 345 345 345 345 345 ... . . . . . .

SinteringTemperature, °C 1540 1620 1750 1540 1750 1650 1650 1650Time, h 1 1 1 1 1 ... . . . . . .

White fused alumina (WFA) is produced by fusion sion, and rheology’ above). This dense microstructure,of calcined alumina in an electric arc resistance fur- which accounts for the superior qualities of thesenace, and its microstructure is usually characterised materials, also poses the biggest challenge. Dryingby the presence of sharp and fractured corundum and baking out of LCCs and ULCCs is often difficult,grains. The main impurities present in white fused particularly in thick installations, because of the lowaluminas are sodium and potassium, usually as gas permeability caused by the low porosity of thesea-alumina, iron oxide, titanium oxide, and calcium materials.8,14,33,37 Major problems are the occurrenceoxide. Brown fused alumina (BFA) is produced by of cracks and the risk of explosive spalling,103 believedfusion of bauxite, and its main impurities are iron to be due to the internal vapour pressure, whichand titanium oxides, in amounts often greater than builds up within the cast structure, and whose magni-those found in white fused alumina. tude ultimately exceeds the material strength.8 It is

Sintered aluminas are also prepared from calcined also believed that the internal vapour pressurealumina produced by the Bayer process, so soda depends largely on the curing temperature and mater-exists in small amounts as the major impurity. ial properties, such as the strength and permeability.Sintered aluminas having flat table-like a-Al2O3 crys- Balancing between the vapour pressure and thetals, usually averaging 50 mm or greater, are termed strength undoubtedly determines whether an‘tabular’ alumina (TA). Because of the grain boundary explosion will occur or not during the drying stage.porosity entrapped during rapid sintering, tabular But for this, it is essential to improve the permeabilityalumina has a particle bulk density in the range of these highly dense materials, preferably without3·40–3·65 g cm−3, with characteristic closed spherical compromising the strength. Organic bake out fibresporosity typical of a fully sintered ceramic with sec- are used to reduce the possibility of explosive spallingondary recrystallisation. The excellent thermal shock and they usually allow increased heatingcharacteristic of tabular alumina is attributed in part rates.14,47,99,124 They have their origins in a patent byto the closed spherical pores, which apparently act as Hoganas in 1974, but refinement of this technologycrack arresters. Open porosity is characteristically to make use of small polypropylene fibres occurredlow (<5%).99 only in the 1980s.14 This technology is based on the

addition of small amounts (not more than 0·5% andusually between 0·05 and 0·2%) of organic fibresMicrostructural evolution on drying and firingwhich shrink and melt during the early stages ofLow and ultra-low cement castables are carefullycastable bake out, thus increasing the permeabilitygraded for maximum packing density. Besides theirand creating channels for water removal during thecharacteristic low cement content, they contain super-critical dehydration period, and enabling stressfine or colloidal particles, such as silica fume andrelease.alumina, deflocculants, and setting control additives,

Stainless steel fibres are also believed to increasethat help to reduce further the water requirement forthe spalling resistance of castables at high temper-placement and improve the ceramic bond. The com-ature, preventing catastrophic failure of the instal-positions of some LCCs and ULCCs are given inlation. They are usually added in amounts varyingTable 12.between 1 and 4% by weight. Their function is toDuring drying (dehydration) all of the reactionshold the castable together when cracking occurs, thusdescribed in the sections ‘Dehydration and firing’ andincreasing service life.14‘Microstructural development’ above, occur in the

Metal powders, such as aluminium, have also beenpresence of aggregate phases and various additiveswidely used to increase castable resistance to explosivemade to aid installation. A significant characteristicspalling. The release of H2 during the reaction of theof LCCs and ULCCs is the very low water additionmetal with water is believed to generate gas pipes inrequired for mixing and placement, which accountsthe dense castable structure, thus increasing the per-for the low porosity of the cast structure and improves

the strength (see the section ‘Particle packing, disper- meability.99 It has also been suggested that the heat



Table 12 Composition (wt-%) of low and ultra-low cement castables

Ref. 44 105 105 101 80 104 83 83 83

Spinel .. . . . . . . . . . . . . . . . . . . . . . . 23Fused alumina ... . . . 71·5 84·0 71·0 ... . . . . . . . . .Tabular alumina ... . . . . . . . . . . . . 78 80 80 60Calcined bauxite 82·5 68·0 ... . . . . . . . . . . . . . . . . . .Kyanite .. . 10 ... . . . . . . . . . . . . . . . . . .Microsilica 6 6 10 8 5 ... 5 .. . . . .Calcined alumina ... . . . . . . 7 9·5 ... . . . . . . . . .Reactive alumina 10 10 17 ... 8·5 17 10 15 11Dispersing alumina ... . . . . . . . . . . . . 1 .. . . . . . . .Hydraulic alumina ... . . . . . . 0·5 ... . . . . . . . . . . . .CA cement 1·3 6 1·5 0·5 6 5 5 5 6SHMP 0·18 0·20 ... . . . . . . . . . . . . . . . . . .Polyacrylate (Darvan) .. . . . . 0·05 0·05 0·05 ... 0·05 ... . . .Polyglycol (Castament) .. . . . . . . . . . . . . . . . . . . . 0·5 0·05Sodium carbonate ... . . . . . . . . . . . . . . . 0·005 0·05 0·005Citric acid ... . . . . . . . . . 0·05 ... 0·015 0·15 0·012Water, wt-% 5 5 4 4·1 4·1 4·0 5·0 5·0 5·0

released during the exothermic reaction of aluminium Propertieswith water is effective in decreasing the setting time The main technical advantages of LCCs and ULCCsand strengthening the material.103 An additional are their excellent physical properties, such as highproblem is control of the CAC reaction which changes density, low porosity, high cold and hot strengths,the pH as the CAC goes into solution, effecting the and high abrasion and corrosion resistance.14,33 Thealuminium reaction, setting, and hydrogen generation. porosity and mechanical strength of refractory cas-Whatever the case, this technique requires that pre- tables vary with the temperature. A conventionalcautions be taken to vent the H2 properly to prevent castable has an open porosity of about 9–17% afterformation of explosive atmospheres. drying and~20–30% after heat treatment at 1000°C.

In general, on firing LCCs and ULCCs the reac- Most LCCs have an open porosity not higher thantions described for pure CAC (see the section 10% and 16% after drying and firing, respectively.‘Dehydration and firing’ above) and for conventional Low cement castables usually possess a much finercastables (see the section ‘Microstructural develop- pore size distribution than conventional high cementment’ above) occur. However, the matrix system in castables, which further contributes to increase theLCCs and ULCCs contain additional fine additions corrosion resistance, by hindering the penetration ofof silica and alumina (see the sections ‘Colloidal silica’ metals and slags.30 The cold strength of LCCs risesand ‘Calcined alumina’ above) which will react with steadily with temperature and is often higher thanthe lower CA cement content present compared with that of a conventional castable of similar compositiona conventional castable. The C–A–S and C–M–A–S at all temperatures after dehydration.33 Low cementphase diagrams indicate the phases expected at equi- castables usually have higher modulus of rupture andlibrium for the local matrix composition.125,126 If the lower creep values than conventional high cementbond does not include fine silica low cement, compos- castables.8 All these characteristics provide superioritions are in the alumina phase field while at the high performance over conventional high cement castablescement levels of a conventional castable the matrix and other monolithic materials, such as rammingquickly moves into the CA6 region. As silica is added mixes, plastics, and gunning refractories. However,(in LCCs and ULCCs) the matrix moves back to the high cement castables are more robust with regardalumina field remaining there even with up to 20% to installation procedures and ease of placement,37,40silica. Matrix compositions are designed to avoid the while LCCs and ULCCs, are more sensitive to instal-low melting compositions (anorthite and gehlenite) lation parameters and variations in water additionaccomplished by minimising the matrix silica at high and casting conditions, such as the ambient temper-cement levels or limiting the matrix CaO to <8% in ature, may affect pot life, hardening time, and curingsilica containing LCCs and ULCCs.125 strength.8,37,40

Aggregate phases may have a significant effect onmicrostructural evolution at high temperature (above1000°C). Sarpoolaky et al.127 examined LCCs with Recent developmentsdifferent aggregates (TA, WFA, BFA, and spinel ) but

Non-cement or cement free castablesa similar CAC/fine alumina bond. On firing at 1000°Cthe bond was similar in all four samples consisting Cement free castables are used in numerous molten

iron and steel contact applications, where the elimin-predominantly of alumina, CA, and CA2 . However,after firing at 1600°C platey CA6 and alumina was ation of lime in the refractory matrix increases use

temperature. They usually do not possess the superiorpresent in the bonds of the TA, WFA, and spinelgrain samples but the BFA sample bond contained physical and mechanical properties of LCCs and

ULCCs, but do possess better corrosion resistance tocalcium titanates, more rounded CA6 , and evidenceof a large volume of liquid being formed leading to metals and slags.14 Cement free castables use a variety

of bonding systems, including clay minerals, silica gels,increased density. This had important effects on thecorrosion mechanisms in these refractories. hydratable aluminas, and phosphates.37,51,52,128–130



Among the different binding systems for non- lants.9,17,31,48,131 The basic idea was to reduce thepoints of contact between the aggregates, which slowcement castables, hydratable aluminas have attracted

great attention. The first reported applications came the flow, by increasing the volume of the matrix, yetkeeping its density high.48 With water additions infrom Japan in the early 1980s.37 Hydratable aluminas

are amorphous mesophase transition aluminas (e.g. the range 4·5–8·0%, depending on the product type,these castables self-flow under their own weight, easilyc, r, x), which, similar to cement, harden hydraulically.

They are produced by the rapid dehydration of filling intricate forms and shapes and, because of theircohesive consistency, they can be installed withoutgibbsite (Al(OH)3 ). When in contact with water,

c-alumina rehydrates, forming pseudoboehmite separation of the fine material or the fluid phase fromthe aggregate.(AH1–2 ) and bayerite (AH3 ). During heating, the

hydrated phases lose their chemically bound water, Early low cement, free flow castables had goodflow characteristics, but strengths were significantlygiving rise to the stable form of alumina (a-Al2O3 ),

which at higher temperatures will help to develop a below those of vibrated LCCs. Other free flow ver-sions had high cement levels, and were simplystrong ceramic bond.37,128 The hardening reaction of

the transition alumina can be accelerated by the improved flow, conventional castables. Modern self-levelling castables utilise a sizing and a cement levelinclusion of alkali metal salts or delayed by the use

of carboxylic acids.128 similar to those of LCCs and ULCCs, but manyseparate features of these products have beenWhen used as a substitute for the cement, transition

aluminas are usually combined with microsilica to optimised to generate self-flow behaviour at low waterlevels. The self-flow technology can be applied to apromote the formation of mullite at high temperatures

and improve hot strength.37,51,52 However, it has been wide variety of compositions, ranging from fumedsilica and bauxite based products to high aluminafound that small additions of CAC may still be

necessary, usually around 0·5%, to control the setting and silicon carbide based castables.15 However,despite the significant improvements in the tech-time of the castable.51,52 This is because a castable

with a low hydraulic alumina content will not set, nology, it should be noted that if normal LCCs canbe easily installed at low water additions usingand if more hydraulic alumina is added to make it

set, good flow is only possible with an excess of water, vibration, then their physical properties are generallysuperior to those of free flow castables.17which increases the porosity.

Like all binding systems, hydratable aluminas also Self-flowing refractory castables also have theirlimitations, in particular, control of the setting time,have their own problems and disadvantages,37,128

such as the high risk of explosive spalling at low that should be long enough to allow the natural degasof the castable and yet short enough to preventtemperatures, usually around 200–300°C, because of

the impervious structure. Besides, curing of a-alumina sedimentation or segregation.48 Other drawbacksrelate to the strong influence of temperature on flowcontaining, cement free castables must commence at

temperatures above 18°C, or strength is not fully behaviour and setting time, with temperatures above30°C being particularly harmful and detrimental todeveloped. Finally, they are expensive.the installation quality;31 differences in plant sites andapplications, which require different but controlled

Free or self-flowing castables placement and demoulding times; and dilatant flowbehaviour, which complicates handling and dosing.In many cases, castables are selected as the preferred

lining materials just because bricking may be difficult Variations in the cement reactivity, which influencedplacement and setting time substantially in first gener-or even impossible. Low and ultra-low cement cast-

ables are often the first technical choice, but whether ation self-flow systems, have now been addressed bythe cement producers who now supply a consistentthese materials can be satisfactorily installed is not

always immediately apparent. Among the many fac- product. Finally, the water demand of self-levellingsystems is still considerably higher than that oftors that affect the quality of the installation, and of

course the performance of the lining, are the following: vibration castables and, as much as the thixotropicLCCs and ULCCs, overwatering significantly affectsinefficient mixing, excess water additions, difficult

placement, and inexperienced personnel.17 Flow of the mechanical strength and corrosion resistance ofthese materials.115LCCs and ULCCs is usually promoted by internal

or external vibration, since these materials are thixo- Studart and co-workers recently outlined develop-ment of castables which are both cement free andtropic in nature, but this alone is not always enough

to assure complete filling and good consolidation of self-flowing achieved by close control of matrixparticle size distribution and rheology.110,132–134all parts of the lining.

In an attempt to overcome the problems of instal-lation associated with the vibration placement, a new

Shotcreting self-flowing castablesclass of high performance LCCs and ULCCs, charac-terised by a consistency after mixing that allows the Shotcreting, wet gunning, or shotcasting, is an instal-

lation technique by which a refractory compositionmaterial to flow and degas without the application ofexternal energy, was developed in the mid-1980s. is first wet mixed and then transported through pipe

or hose to the site of placement, where it is sprayedThese materials were termed ‘self-flowing castables’(SFCs), and were developed by simply modifying the on to the surface where it remains with no

rebound.16,135–138 The Portland concrete industry hasrheology of the LCC and ULCC systems, mainly bychanging the shape and size distribution of the aggre- been wet shotcreting since the early 1950s, while the

first results on wet shotcreting of refractory castablesgates and by the judicious choice of defloccu-



were published in 1963.139 However, it was not until environmentally friendly, free of particles to preventclogging of the needle valve, and of low viscosity, so1991 that equipment and material improvementsthat it can be easily mixed with the castable.enabled the technique to become an effective instal-Chemically, the accelerator must thicken or stiffenlation tool. The refinements of the technologythe material quickly. However, it is important thatstemmed largely from the increasing environmentalthe material remains in a plastic state for a reasonablerestrictions on dust and the desire to improve furnacelength of time to allow trimming and finishing of theavailability, while reducing the time required forcovered area. Accelerators or set activators that causeconstructing form work.16,135rapid setting or hardening of the castable are notThe development of LCCs and ULCCs withacceptable, because they lead to laminations andimproved flow characteristics was a major step in theusually clog up the nozzle.16 Accelerators used indevelopment of the technology, since it permitted theSFCs include aluminium sulphate,137 polyaluminiumuse of pumpcasting techniques similar to those usedsilicate sulphate (PASS),137 sodium silicates,137 andin the Portland cement industry, and opened the dooralkali or alkaline earth carbonates, silicates, andfor the use of sprayable low cement refractories.135nitrates.16Simultaneously, a concentrated effort was made by

pump manufacturers to design pumps that wouldprovide the necessary pressure to transport heavier,

Spinel and magnesia based castablesmore viscous refractory materials through the rela-tively small hose diameters needed for proper The increasing importance of steel ladles in modernmobility.16 steelmaking technology, and the ever increasing con-

A key feature of wet shotcreting SFCs is that wet cern with steel cleanliness and the efficiency of steelmixing is an independent operation, and therefore the desulphurisation achieved in secondary steelmaking,mixing process can be optimised for each particular has led to the development of alumina–spinel andproduct and the necessary installation conditions alumina–magnesia castable compositions. First,before the material is fed to the pump. Most common alumina–spinel materials, made up of a mixture ofmixers used in the refractories industry can be used WFA or tabular alumina and MgAl2O4 spinel aggre-successfully for shotcreting, as long as the output of gates, superseded high alumina castables, due to theirwet mixed material matches the pumping rate, which better corrosion resistance, and became widely usedis usually between 5 and 9 t h−1. The pump used to line the walls, bottom, and impact pad of ladles.21,24successfully on most jobs has dual reciprocating However, these castables were still susceptible to somehydraulically powered 75 mm dia. material cylinders, structural spalling, as the slag-penetrated layer tendedwhose pistons are capable of pressurising the refrac- to peel off in service.24 Recently, self-forming, or intory castable to above 14 MPa. The nozzle at the situ alumina–magnesia spinel castables became avail-discharge end of the hose is similar to most nozzles able made up of mixtures of aggregates and fineused for dry gunning, but includes several minor reactive magnesia and alumina powders. Reaction ofimportant changes. Air and a set activator, or acceler- the fine fractions leads to in situ matrix spinelator, are introduced through enlarged injection holes, formation.and this breaks the material into smaller particles Reactive magnesias include dead burned, sea waterand mixes it with the accelerator, while providing the derived MgO or lightly calcined Mg(OH)2 . Reactiveenergy for the spraying action as the material exits MgOs hydrate to brucite (Mg(OH)2) exothermicallythe nozzle and is projected on to the surface to be evolving large amounts of heat and with an associatedcovered.16,135 The reaction of the set activator with 2·2 vol.-% expansion which may lead to explosivethe low moisture castable stiffens the material on the spalling during installation so that too fine a MgOwall, allowing close control of lining thickness. powder cannot be used. Alternatively, the MgO can

Most self-flow shotcreting mixes are based on low be coated to prevent early uncontrolled hydration.140moisture, low cement dense castables and conven- Use of already hydrated brucite as the starting materialtional lightweight products, though the technique is overcomes this problem141 and provides a reactivebecoming more attractive to non-cement systems. MgO on firing which readily reacts to form spinel.However, regardless of the binding system, any self- Bier et al.140,142 examined the significant influence offlowing mix used for wet shotcreting must have fine MgO on the rheology and hydration of CACparticular characteristics. First, the top-size aggregate based castables. The presence of magnesia reducesmust be properly chosen to prevent nozzle plugging both the fluidity and working time of magnesia con-and reduce rebound. Second, the SFC must show taining CAC based castables, with the fineness of theminimum dilatancy, good cohesiveness, and low tend- magnesia powder being particularly important.142 Theency to segregation. Besides, after experiencing press- use of admixture combinations are necessary to masterures and movements over significant distances during the rheological properties of these castables, and theypumping, the material must remain fluid enough to usually consist of deflocculants, such as polyacrylatesbreak up inside the nozzle so that the spraying action (Darvan) and sodium phosphates, and set retarders,is effective. Finally, the set activator, or accelerator, such as citric acid and Na2CO3 .85,115,142 Hydration ofmust fully penetrate and activate the castable to hydratable alumina–reactive MgO mixtures also leadsprovide sufficient and rapid stiffening when the mater- to large heat evolution via formation of hydrotalciteial is sprayed on the walls. type phases (Mg6Al2CO3(OH)16 .4H2O).82 A deleteri-

The accelerator is actually a liquid system contain- ous volume increase is also associated with hydrotalciteformation.ing a soluble set activator. Ideally, it should be



match for the magnesia–carbon bricks, usually usedas the standard lining in the slag zone of ladles.22,147Conventional basic castables are mostly used forrepairs, and are rarely considered as premium qualityproducts, because of their poor physical and thermo-mechanical properties, and hydration tendency. It hasbeen suggested147 that the hydraulic CAC bond isnot suitable for magnesia based materials, and that anew bonding system is necessary. Recently, a cementfree bonding system for basic castables has beenreported using amorphous silica gel bonds derivedfrom fumed silica.147 At high temperature, reaction ofthe silica with MgO leads to development of a forster-ite (M2S) bond.148

Future developments9 Dark spinel shells on alumina particles in matrixof ULCC146 Oxide based refractory castables are used extensively

in refractories applications such as blast furnaces,torpedo ladles, tundishes, and cement rotary kilns.However, the current Al2O3 based castables suffer,A significant volume expansion occurs when veryfor example, from poor corrosion and spalling resist-fine powders react to form spinel which is greaterance especially in a basic slag environment.than the 2·45% predicted from molar volumes.143 TheUnfortunately, the newly developed basic (MgO andvolume expansion from direct spinel formationMgO–CaO) castables also have poor spalling andtogether with the calcium aluminates (CA6 at highslag penetration resistance.120 One way to overcometemperature) and alumina provides a dense matrixthese problems is to introduce carbon (as graphitesystem.20,21,24 However, this depends on several fac-flakes) to current oxide castable systems because oftors, such as the amounts and particle size distri-its non-wettability by molten metals and slags andbutions of the powders, the presence of impurities,high thermal conductivity. However, this causes newand the amount of liquid phase formed at highproblems. First, because graphite is not wetted bytemperatures.24,144 If these are not properly con-water, adding even a small amount (5%) requirestrolled, the volume change associated with spinelalso adding considerable amounts of water (>18%)formation may cause an overall bulk volume expan-to obtain reasonable flow behaviour. This results insion of the lining, rather than a densification of thehigh levels of porosity and lower mechanical strengthmicrostructure. Fuhrer et al.98 determined that spineland corrosion resistance which counteracts the meritsformation occurs between 1200 and 1400°C with aof graphite addition. Second, problems associatedtuber-like morphology interlinked with CMAS phaseswith high porosity, such as graphite oxidation andand CA6 platelets derived from the CAC. In theselow mechanical strength, become more serious thanCMAS systems, the presence of silica is detrimentalin denser, pressed carbon containing bricks so thatto the refractoriness145 but it has been found that, ifmore effective additives (antioxidants) are needed.added in small amounts, microsilica accelerates theAlthough aluminium and its alloys are very effectivespinel formation.24 It has also been shown that silicaantioxidants and most commonly used in carboncontributes to improving the thermomechanical prop-containing bricks, they cannot be used directly inerties of the castable, increasing its resistance to crackcastable systems because of their hydration tendency.propagation.21 Finally, it has been reported thatA major research effort is underway to solve thesemicrosilica helps to prevent the hydration of magnesiaproblems and develop carbon containing castables.149(acting as an anti-slaking agent), therefore improving

the service life of the castable.26,142 The matrix micro-structure of a commercial in situ spinel ULCC reveals

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