Castable Refractory Concrete

(1)

W. E. Lee, W. Vieira, S. Zhang, K. Ghanbari Ahari, H. Sarpoolaky, and C. Parr

Castable refractories containing calcium aluminate cement (CAC) are used ubiquitously in a range of furnace lining applications in the iron and steel, cement, glass, ceramic, and petrochemical industries. This review outlines their development from conventional high cement materials, through low cement and ultra-low cement castables to the present materials which may be entirely free of CAC. Castables are defined in terms of both CaO content and installation procedure. Production routes, compositions, and microstructural evolution on hydration, setting, dehydration, and

firing are described for pure CACs and castable 1 Typical powder processed refractory

refractories. The development of the low cement microstructure

systems is discussed in terms of particle packing, dispersion, and rheology highlighting the influence

of colloidal matrix additions of silica and alumina. _{calcined bauxite, and sintered MgO while bonding} Recent developments including cement free,

self-systems may be based on carbon derived from

pyro-flowing, shotcreting, and basic castables are

lysed pitches and phenolic resins, mullite and glass

described and the potential for carbon-containing

from decomposed clays, or alumina and calcium

systems evaluated. IMR/368

Professor Lee, Dr Vieira, Dr Zhang, Dr Ahari, and Mr in refractories technology in the last two decades has Sarpoolaky are in the Department of Engineering Materials, _{been the ever increasing use of monolithics, or} University 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 two decades while the ratio of monolithics to preshaped

Abbreviations

refractories ( bricks) has been steadily increasing.1–6 BFA brown fused alumina

The reasons for the rapid growth of monolithics, CAC calcium aluminate cement

at the expense of bricks, are their ready availability, CVC casting vibration castable

faster, easier, and cheaper installation, and fewer HAC high alumina cement

corrosion-susceptible lining joints.6–11 The term LCC low cement castable

monolithic usually includes a wide variety of material SFC free or self-flowing castable

types and compositions, with various bonding sys-TA tabular alumina _{tems, ranging from fluid cement pastes to stiﬀ plastic} ULCC ultra-low cement castable

lumps.6,11,12 Monolithic materials were first used as WFA white fused alumina

a distinct refractory product in 1914, when the first

Chemistry commercial refractory plastic, a simple blend of

A Al

2O3 crushed firebrick and fireclay, was produced.5,6,12

C CaO From this, monolithics have evolved into a versatile,

F Fe

2O3 widely used class of refractory materials that oﬀer

H H

2O performance and cost eﬀectiveness comparable, and

M MgO sometimes even superior, to those of shaped

refractor-S SiO

2 ies. The success of monolithics is due to significant

T TiO

2 advances in the type and quality of their binders,

aggregates, and additives as well as to innovation in their design and installation techniques.5,6

Introduction

_{The evolution of monolithic refractories over the}

past century has been described in a recent review.13 Refractories are a group of ceramic materials used in

massive quantities to line vessels in which other A significant advance in monolithics technology was the development of refractory concretes or castables materials (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 by casting or pouring into place, vibration placement, aggregates include fused alumina, tabular alumina,

(2)

trowelling, or projection (spraying or shotcreting). based systems are reviewed. Finally, the possibility of carbon-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 evolution

of cement, still make up the greatest percentage of

those 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 product 10 years.6 This is because the CaO present in the

cement 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), a

reactive 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 of to form specific products (precast shapes) or cast ‘in

place’, 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 rather Dense concretes are prepared using discrete particle

sizes, 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 their size. Mechanical vibration may be used to assist the

flow 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, for lower 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 development are said to be free or self-flowing.15 Some refractory

compositions may be premixed with water and then stemmed from the work of John Smeaton, around 1756, who recognised that the calcination of certain pumped 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, following This process is called wet gunning or shotcreting, and

the 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 they castable.16 This is quite diﬀerent from the earlier dry

gunning process where the powder and water are were called, were superseded by Portland cement, a calcium silicate product prepared by calcining to a mixed at the nozzle of the device used to place

the slurry. (partially melted) clinker a wet ground mixture of

limestone and clay.12,39 Modern castables are used increasingly in almost

every refractory application, such as for the repair of The development of the first CACs stemmed from the shortcomings of calcium silicate cements exposed stacks17 and lining of iron and slag runners in blast

furnaces,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 patents rels,2,17–19 steel ladles2,3,7,17,18,20–27 and tundish

linings,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 areas

of 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 in kilns,19 coke oven door plugs,7,31 cyclones and transfer

lines 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 resistant petrochemical industry,6,7,17,19,29 foundry ladles and

heat 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-fed furnaces 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 slag cation 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.39 and 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 properties fabrication of shrouds and submerged entry nozzles

used for the continuous casting of steel.34,35 of CAC were fully appreciated. Before that, calcium aluminate was often seen as an alternative to Portland In 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 Atlas section ‘Historical evolution’ below) and the methods

of 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 the are considered before examining refractory systems

in 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 particle

packing, 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

(3)

MgO-sumably because of its relatively high cost. The con- teristics of shotcreted LCCs were first published in 1986, 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 more eﬀective installation tool. Pumping self-flow mixes mortar 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 wet forms of installation, though some gunning was also

done.5 gunning.16

Nowadays, refractory concretes are available which In the 1950s, following experiments made with

purer raw materials, Alcoa and Lafarge began market- contain low, intermediate, or high purity CACs in large (conventional ), small amounts ( low and ultra ing 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 inorganic HACs produced from mixtures of pure alumina and

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

purity CAC and tabular alumina aggregates were and bauxites, tabular and sintered aluminas, white and 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, silicon carbide, 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 molten iron 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 high temperature behaviour. Such castables use a variety tages of these conventional castables, containing as

much as 30%* cement, were the high water content of bonding mechanisms. Bond systems used include clay 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 and strength during the dehydration process, and the

sharp drop in strength at high temperatures due to organic bake-out fibres, the first to prevent damage from thermal shock, and the second to increase the 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.14,50

and aggregates, while the base technology stayed the same.41

In the late 1970s, reduced cement materials based

Classification

on the Prost patent of 1969 were

manufac-tured.18,29,37,42–47 These LCCs contained at most Refractory castables are classified by the ASTM according to their lime content into conventional 2·5% 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 cement by 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 alumina similar additives. The grain size distribution of the

aggregates was also altered so that the interstices are (80% Al

2O3) cements may fall into the last category, despite the presence of some cement. These definitions progressively filled by smaller grains to obtain

maxi-mum packing density, which also increases the can be misleading, and have led to the introduction of new types of so-called ‘cement free’ castable made amount 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 asr-alumina.51,52

were developed. Low and ultra-low cement castables

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

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

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

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

Following the success of LCCs and ULCCs and is added to the mix, or vice versa, and therefore, in some cases, the classification of castables with no the 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 to was 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, this with a consistency after mixing that allows them to

flow and degas without application of external is also meaningless, since modern high performance materials 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 as much information as possible about the chemical *All percentages are wt-% unless otherwise stated.

(4)

nature, rheological behaviour, and installation calcined alumina is often the choice for the high purity type (HP CAC).

characteristics of the castable.

Alternatively, cements or CACs may be classified according to chemical composition, forming five main

Calcium aluminate cements

_{groups (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 C used in modern high performance refractory castables, include the two alumina cements for refractories CACs, particularly high alumina cements, continue specified in the Japanese Industrial Standards. Group to be the most important hydraulically setting agents D includes high purity alumina cements of the used for bonding concretes,53,54 mainly because they 70%Al

2O3 class, such as the Denka high alumina develop high strength within 6–24 h of placement. cement and Secar 71 (Lafarge). High purity alumina Usually, high alumina castables require only 24 h to cements with~80%Al

2O3, such as the Denka super develop 70–80% full strength when properly cured, high alumina cement, CA–25 (Alcoa), and Secar 80 in contrast to 28 days for normal Portland cement (Lafarge) are typical of Group E. An important concretes.53 Calcium cements are usually classified variable in these commercial cements is admixture according to purity and Al

2O3 content. In 1972, content, e.g. Secar 71 is additive free while Secar 80 Briebach39 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 on as ‘high alumina cements’, while those in Group 4 the cement but will influence rheology in a castable were often called ‘white high alumina cements’, system based on this cement.

because of their colour. Today, however, any calcium

cement from Groups 2, 3, or 4 is often termed CAC Production techniques

or high alumina cement (HAC), or simply calcium _{Calcium aluminate cements are formed by reaction} aluminate (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 limestone} in Table 2. Relatively pure limestone (54–55%CaO, _{and bauxite raw materials are melted at 1450–1550}_°C 41–43% loss on ignition, and less than 2% total _{in reverbatory furnaces fired by powdered solid fuel.39} impurity) 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%Fe₂O

3 and 9%SiO2. Low iron (2–4%Fe2O3) kilns, while electric arc resistance furnaces have also and low SiO

2(5–7%) bauxites are used to manufac- been used where electrical energy is relatively cheap.38 ture the intermediate purity cements (IP CAC), while _{In this process the proportioned dry raw mix of Bayer}

Al

2O3and limestone is either fed as ground or as an agglomerate into a rotary kiln, similar to that used Table 1 Classification and composition of calcia _{in the manufacture of Portland cement. The product}

containing cements

is sintered at 1315–1425°C, cooled, and then ground

Cement group _{to cement fineness together with any additives. These}

include calcined alumina to obtain the desired Al 2O3

1 2 3 4

content, gypsum or other materials to control the set,

Calcium Calcium aluminate cements

and plasticisers to improve workability.38,53 The

silicate

White high _{cement colour ranges from black to white, depending} Mineral Portland High alumina ... alumina

on the impurities present and on the iron oxide

Chemical analysis, wt-% _{quantity and oxidation state. High purity cements} SiO2 17–26 4–9 4–6 0·1–1·4

are white.

Al2O3 5–12 35–45 50–65 65–80

On sintering, the raw mix generally transforms into

Fe2O3 1·7–2·7 10–15 1–3 0·1–1·0

CaO 53–65 36–39 29–40 17–25 higher alumina phases as the material temperature

increases in the kiln. Both lime/alumina ratio and temperature determine the amount and type of cal-cium aluminate phases formed during the process. Table 2 Classification and properties of calcium

High lime calcium aluminates form initially with aluminate cements

ferrites and silicates. Uncombined lime and alumina

Type

begin to react with the high lime products and form

Low purity Intermediate purity High purity _{lower lime or higher alumina compounds, as predicted}

by the CaO–Al

2O3 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 is} Al2O3 39–50 55–66 70–90 _{completely combined as follows38}

Fe2O3 7–16 1–3 0·0–0·4

CaO 35–42 26–36 9–28 C+AC

3AC12A7CACA2CA6

Surface area, m_{2 g−1}

High purity CAC sinters readily, even though very

Wagner (ASTM C115) 0·14–0·16 0·16–0·24 0·22–0·30

refractory high purity limestone and Bayer calcined

Blaine (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 kiln Density, g cm₋₃ 3·05–3·25 2·95–3·10 3·00–3·30 calcination. A 1360°C eutectic occurring at Vicat initial set, h5min 3:00–9:00 3:00–12:00 0:30–6:00 ~50 wt%CaO/Al

2O3 (Fig. 2) enhances liquid phase

(ASTM C191)

(5)

Table 4 Typical mineral constituents of calcium aluminate cements

Cement purity Relative

hydration rate Low Intermediate High

Fast _C12A7 _C12A7 _C12A7

Moderate CA CA CA

Slow _CA2 _CA2 _CA2

C2S C2S ...

C4AF C4AF ...

Non-hydrating _C2AS _C2AS _CA6

CT CT A

A A ...

highest strength among the phases listed during the relatively short time available for hydrating refractory concretes. It takes some time to start setting, but hardens rapidly after the initial set.

Calcium dialuminate (CA

2) is the secondary phase in CACs (<25%) and is more refractory than CA but takes an excessively long time to set though accel-erated at high temperature.38 While hydration of CA 2 _{Binary CaO–Al2O3 phase diagram; in wt-%} is known to be accelerated by the presence of CA

2, the opposite does not hold true, and the hydration of CA

2 may actually be hindered by the presence of The two most critical areas of cement production

CA.55 The strength of CA

2after 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 proportions

CA, it always increases with time. of clinker phases important, but also their reactivity.

C

12A7hydrates rapidly and can be used to control The crystallinity of a cement phase is important in

the setting rate of CACs when used in small quantities; controlling cement reactivity. It has been shown,

it has a relatively low melting point. C

2S and C4AF for example, that C

12A7, generally considered an are common in Portland cement, but can also occur extremely reactive phase of the sintered clinker, can

in the high silica and iron rich low purity CACs, be rendered quite unreactive when present in the

respectively. C

4AF forms hydrates of calcium alumin-fused form.45 Control of the particle size on grinding

ate and calcium ferrite or solid solutions of the two is important, because variations in the particle size

hydrates, and in its setting rate it resembles C 12A7 distribution can not only aﬀect cement hydration, but

(Ref. 12). C

2AS (gehlenite) shows little tendency to also its reactivity with the aggregates in the concrete.

hydrate and is an undesirable component of alumina cement which limits refractoriness and hot strength

Phase composition _{properties.38}

CA

6 is the only non-hydrating phase in the pure Typical phases present in commercial CACs,

accord-ing to their relative reaction rates with H

2O (Refs. calcium aluminate system and is often a reaction_{product in alumina castables bonded with high purity} 38, 53) are listed in Table 4. They form the hydrated

cement phases responsible for developing strength aluminate cement. It is believed that CA

6 is most readily formed in alumina castables when using CA

2 after 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 CA

6 phase have Table 5.

Monocalcium aluminate (CaO.Al

2O3or CA) is the revealed its great potential as a strong thermal shock_{resistant, refractory material38,57,58 and its important} most important component of CACs because it

gener-ally occurs in large amounts (40–70%), has a rela- role in the bonding of corundum and spinel aggregates.40

tively 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 0·20 TiO2 1·95 2·08 2·16 2·09 3·23 2·49 Tr. Tr. Tr. Tr. Al2O3 42·11 41·34 47·55 52·28 56·69 56·21 73·36 71·07 80·22 79·53 Fe2O3 15·55 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 17·12 MgO 0·65 1·17 1·19 0·35 0·43 0·53 0·30 0·33 0·35 0·44 Na2O 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

(6)

Generally, the characteristics of CACs are associ-ated with the amount of alumina, lime, and impurities present in the products. Increased alumina content will confer higher refractoriness, while a high lime content in the cement increases cured strength. Iron impurities lower the carbon monoxide resistant at high temperatures, and siliceous compounds reduce resistance to hydrogen atmospheres under similar conditions.38

Hydration

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

Figure 3 shows reaction schemes for hydration of CA, CA

2, and C12A7. When any cement is mixed with water the hydraulic minerals begin to dissolve quickly forming a saturated solution of ions. In CACs Ca2+ and Al(OH)−₄ ions form. Nucleation of hydration products and their subsequent crystal growth produces an interlocked network that gives ‘setting’ and then strength.65 Rates of hydration are a strong function of the starting CaO_{/Al2O3 (C/A)} ratio and temperature. The hydration of CA occurs through initial dissolution and subsequent precipi-tation of CAH

10and C2AH8from the supersaturated solution. An induction or incubation period occurs before this precipitation and is a reflection of the nucleation barrier.67

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

(a)

(b)

(c)

surface layer grows slowly. When the hydration layer

3 _{Reaction schemes for a CA, b CA2, and c C12A7} reaches 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 whole

accelerates) with the formation of crystalline nuclei CAH

10can be detected after 6–24 h. The presence of C

2AH8can be detected after 24 h. that grow by a dissolution–crystallisation mechanism

to produce the metastable hexagonal hydrate CAH

10. 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 ... ... Cubic C12A7 48·6 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 Monoclinic CA2 21·7 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 ... ... Hexagonal a-A ... 99·8 ... ... 2051 3·98 ... ... Rhombohedral

(7)

% CONVERSION TEMPERATURE, °C C2AH8+AH3 C3AH6+ 2AH3

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

4 Typical curve for evolution of heat in calcium

of the cement increases rapidly. The evolution of heat aluminate cement

and the development of strength in CACs are related to each other, both stemming from the hydration ing solution chemistry,68–70 X-ray diﬀraction,71,72 _reactions.12

calorimetry,62,73–75 1H NMR (nuclear magnetic reson- _{Recent NMR studies indicate formation of an} ance) relaxation rates, and continuous wave 27Al _{intermediate product, which could be forming directly} NMR 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 suggested} and the heat of hydration evolved can be easily _{that the end of the incubation period and the start of} detected by means of isothermal conduction calor- _{the main reaction period, which is followed by massive} imetry (ICC). Adiabatic calorimetric studies of cal- _{precipitation, are actually due to the formation of} cium aluminate cements with water show two _{this material.61}

exothermic 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, diﬀerent} heat of wetting and the rapid dissolution of cement _{hydrated products may form during the process. In} to form a solution saturated in lime and alumina.77 _{particular, the hydration process varies with curing} The first peak is also believed to be associated with _{temperature8,12 and the use of additives, such as} the formation of the first reaction layer on the grains _{setting retarders and accelerators.66 In castables, the} of 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 as} which hydrate nuclei form and develop. It has been _Na

2O (Ref. 74) and microsilica additions.45,59,64,79 observed that from 25 to 70°C the length of the _{The stable hydration products formed from the initial} induction period decreases with increasing temper- _{sintered mineralogical phases and their crystalline} ature.61 Once critical nuclei have been formed, mass- _{structures generally develop within 3–6 months under} ive bulk precipitation of hydrates occurs, giving rise _{ambient conditions, or within the first 24 h if curing} to a second major exothermic peak, and initiating the _{is performed at higher temperature. The following} hardening process. _{reactions take place when CAC and water are}

com-Evolved heat of hydration diﬀers widely depending _{bined8,12,38,65} on 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 the amount of heat liberated by these cements over one day approximately equals that liberated by Portland

C 12A7 CA CA 2 +H CA <21°C CA 21–35°C CA >35°C CAH 10(metastable hexagonal ) AH 3 (gel ) C 2AH8(metastable hexagonal ) C 3AH6(stable cubic) AH 3 (crystal ) (1) (2) (3) cement over 28 days.12 The evolution of heat during

hydration is greatly aﬀected by temperature, and the amount of heat evolved increases with the ambient temperature. The time needed to reach the maximum

rate of heat evolution, after water addition, is also As the temperature rises above 21°C, the metastable aﬀected by temperature, and so is the setting time. hydrates CAH

10 and C2AH8 change into the more The longest time required is usually at about 30°C. stable compounds, C

3AH6 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 as not 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 ( listed Table 6 Heats of hydration of Portland and CA

cements, cal g−1 in Table 7) vary extensively though in general C 3AH6 forms as cuboids, C

2AH8 as platelets, CAH10 as

Time, days

needles or hexagonal prisms, and gibbsite as tablets

Cement 1 3 7 28

or needles. Figure 6 shows the C

2AH8 and C3AH6

CA 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}

(8)

Formation of stratlingite avoids the conversion reac-tion and may, therefore, lead to superior low and intermediate temperature strength and stability. Furthermore, the presence of other forms of alumina also influences hydration and rheological properties.83 The surface area, Na

2O content, and releasability of the aluminas are particularly important and can have an eﬀect on the high temperature microstructural development, see the section ‘Calcined alumina’ below.

Additives

Chemical additives or admixtures are used to control the reactivity of the cement. These are ground and blended with the cement during the manufacturing process to control or modify the properties of the binder.38 The use of such admixtures in castables is considered by many45 unacceptable because most formulations incorporate deflocculants and buﬀers to control the rheology of the product and the addition of admixtures to the cement can alter the properties of the castable by interacting with these additives.

Besides those additives used for dispersion, see the section ‘Submicrometre (superfine) matrix additions’ below, the most common additions to calcium

alum-a 18 h, C2AH8 hexalum-agonalum-al plalum-ates; b 11 d, C3AH6 deltoid icositetralum-ahedralum-a

inate containing castables control the setting charac-6 Hydrate microstructure in CAC with water/

teristics of the cement. They are the so-called set cement ratio 2 at ambient temperature;80

retarders and accelerators. Among the retarders, the marker 10mm

following are significant: citric84–86 and phosphoric8 acids, diluted acetic acid,38 boric acid,60,87 borax,8,38 stable hydrates (AH

3 and C3AH6) at higher curing sodium citrates60,88 and gluconates,60 hydroxy-temperatures produces greater porosity and larger

carboxylic acid salts,8,38 saccharides,8 magnesium and pore sizes than when the metastable and less dense _{barium hydroxides,38 sodium chlorides and} sul-hydrates (CAH

10 and C2AH8) are formed at lower phates,38 starch, sugar, sea water, and many other temperatures. While this gives lower green strengths65 _{acids and acidic compounds.38 The most commonly} it also gives coarse, permeable hydrate phases so that

cited set accelerators are: lithium salts and carbon-vapour species can escape on drying. The

imper-ates,38,88 calcium hydroxide,38,60 Portland meable AH

3gel phase developed at low curing tem- cement,38,51 slaked lime,66 hydratable aluminas,51 peratures increases the danger of explosive spalling

sodium and potassium carbonates,38 sodium sili-on drying due to steam build up. For this reassili-on, _{cates,38 and many other alkalis and alkaline} com-refractory CAC castables are generally cured above

pounds.38 It has been suggested88 that the combined 27°C. Best practice for safe heating and maximum _{use of set retarders and accelerators is more eﬀective} strength development is to cure for at least 24 h at

and allows better control of the hydration process. 30–38°C covered in an impermeable membrane to

The 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 to

that retarders influence the kinetics of hydration by 30–38°C.81 The increase in porosity and its eﬀect on _{slowing down the dissolution of the anhydrous cement} strength are also heavily influenced by original total

particles.89 It is believed that accelerators influence water/cement ratio.

mainly the dormant period of hydration. Lithium The presence of other phases such as microsilica

salts are mostly used as accelerators, and it is believed or reactive magnesia along with CAC in castables is _{that lithium ions interact with Al(OH)−}

4 to precipitate known to influence the hydration mechanism.64,82

insoluble lithium aluminate hydrate, thereby increas-Pantjadarma64 determined that stratlingite (gehlenite _{ing the Ca2+/Al(OH)−}

4 ratio in the solution. Rapid hydrate, C

2ASH8) formed with microsilica present hydrate formation is then promoted, and the setting having the same platey morphology as C

2AH8 sug- time is consequently reduced.89 gesting it formed by reaction of silica and C

2AH8.

Dehydration and firing

Table 7 Properties of CA cement hydrates

Strength loss is known to occur in hardened calcium

Chemical composition, wt-%

aluminate pastes as the metastable hexagonal CAH 10

Crystal Density,

Hydrate CaO _Al2O3 _H2O system g cm−3 _{phase dehydrates through the hexagonal C}

2AH8 transition phase into the stable cubic C

3AH6.12,38,51,90

CAH10 16·6 30·1 53·5 Hexagonal 1·72

C2AH8 31·3 28·4 40·3 Hexagonal 1·95 This loss of strength can best be appreciated by C3AH6 44·4 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 CAH 10 is

(9)

allowed to form during low temperature curing, the coarser pores in the plates and cuboids which sub-sequently collapsed. Sintering led to ceramic bonding metastable 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 with

time and/or temperature. The gross restructuring

_{Conventional castables}

from H

2O loss leading to pore formation and nominal _{Conventional castables consist of graded refractory} 50% volume shrinkage on conversion of CAH

10 to _{aggregates bonded with the aluminous hydraulic} the denser C

3AH6anda-AH3 are disruptive to a rigid _{cements described in the section above. The properties} structure and account for the observed loss in

of these concretes depend largely on the choice of mechanical strength.

refractory aggregate and hydraulic cement.19 They As the temperature is increased, the dehydration

contain ~15–30%CAC,18,29,33,37 this amount being process continues, until all phases lose their water of

necessary to achieve satisfactory strength at low and crystallisation. Dehydration temperatures may be

intermediate temperatures though it makes the mater-measured by thermal analysis techniques, such as

ial thirsty. The 8–15% water generally added during diﬀerential thermal analysis (DTA), derivative

therm-processing is mainly used to develop the hydraulic ogravimetry (DTG), and diﬀerential scanning

calor-cement bond (6–10%) and to make the concrete flow imetry (DSC). The dehydration is also a complex and

(2–6%), allowing its proper installation. However, a not fully understood process. CAH

10 loses part of its _{relatively large amount of water (0–5%) is often taken} water of crystallisation at temperatures lower than

up by the porosity of the aggregates and does not indicated by thermal analysis (50–70°C). Part of this

contribute to the hydraulic bond. water of crystallisation is even lost at low humidity

at room temperature and, since CAH

7 is sometimes formed, it is believed that CAH

10 may have three Microstructural development

water molecules that are easily dehydrated. Such _{All the reactions on hydration/dehydration/firing} unstable hydrates are expressed as CAH

x (x<10). described for CACs in the section ‘Dehydration and C

3AH6, in its turn, often shows a stepwise dehy- firing’ above, occur in refractories in which they dration process, usually indicated in the DTA curve _{are the main component of the bond system but} by two endothermic peaks, one around 300°C and

complicated by the presence of the various the other close to 500°C. It has been claimed that the _{additives/admixtures in the bond system. These may} first 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 is} 1·5H

2O phase (C3AH1·5) might represent some inter- the presence of the aggregates which increase water mediate stage or compound had been suggested.92 _{requirements and may react with the bond system at} However, more recent studies indicated that the

higher temperature. C

3AH1·5is merely a metastable structural relict which All CAC hydrates in the bond phase decompose to constitutes a step in the dehydration of C

3AH6 at the calcium aluminates (C

12A7, CA, and CA2) and low partial pressures of water.93 AH₃gel and gibbsite

eventually, if enough free alumina is present, CA 6 usually 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 CACs C

12A7 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 C

12A7. At material which is extremely fine and active.36 Lime _{1000–1200°C CA reacts with alumina to form coarse} and alumina reappear and recombine in a way similar _{and globular CA}

2 (Fig. 7b) while at >1300°C CA2 to that of the original raw materials in the kiln.38,94 _{reacts with alumina to form hexagonal platelets of} Table 9 depicts the mineralogical changes in HACs _CA

6(Fig. 7c–f ). This morphology is believed to assist as the temperature increases up to 1500°C.12 The

physical interlinking of the microstructure improving microstructural changes associated with these reac- _{high temperature strength.}

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

Disadvantages

drying at 110°C, CAH₁₀ and C

2AH8 either

dehy-drated to amorphous products or, if water was pre- These high cement castables have three major disadvantages.7,18,19,29

sent, converted to C

3AH6and gibbsite. Firing above

300°C dehydrated the pastes completely and reduced First, because they need so much water they are usually porous and open textured, which greatly their strength due to increased porosity and pore

growth. 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 is morphologically 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 dried at 110°C is generally about 9–17%,33 but can be as Table 8 Dehydration temperature of CA cement

low as 8%.18 Although some of this porosity is due hydrates

to entrapped air bubbles, most of it is caused by the

Hydrate _CAH10 _C2AH8 _C3AH6 _AH3

excess water added on mixing. On heating, the

Dehydrating temperature,_{°C 100–130 170–195 300–360 210–300}

(10)

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 of aggregate 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 temperatures new porosity depends on the amount of chemically

bonded water and is therefore dependent on cement (often quoted to be between 538 and 982°C7), when the hydraulic bond has already broken down, due to type 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%Al2O3 CAC 80%Al2O3 CAC

Temperature,

°C _C12A7 CA _CA2 _CA6 A _C12A7 CA _CA2 _CA6 A

500 X ... ... ... X ... ... ... ... X 600 X ... ... ... X ... ... ... ... X 700 X ... ... ... X X ... ... ... X 800 X ... ... ... X X X ... ... X 900 X X X ... X X X X ... X 1000 X X X ... X ... X X ... X 1100 ... X X ... X ... X X ... X 1200 ... X X ... ... ... X X ... X 1300 ... X X ... ... ... X X ... X 1400 ... X X ... ... ... ... X X X 1500 ... X X ... ... ... ... X X X

(11)

has not yet allowed the development of a ceramic

_{Low cement castables and ultra-low}

bond. The exact temperature range when strength

_{cement castables}

deteriorates is not absolute, but may depend on

First attempts to improve the performance of various factors, such as the type and proportion of

hydraulic castables by reducing water and cement hydrates, the curing temperature, and heating

sched-content were largely unsuccessful, since the mechan-ule. C

3AH6 becomes thermodynamically unstable _{ical resistance was insuﬃcient for compositions with} above ~292°C but its dehydration is a stepwise

less than 10% cement.33 Refractory castables with no process. In practice, higher temperatures may actually

more than 5–8% cement, characterised by excellent be necessary for the complete dehydration of the

cold and hot strengths, were first mentioned in a hydrates, since heating increases water vapour

press-French patent granted in 1969 to Prost and Pauillac.46 ure, particularly inside closed pores, further delaying

Reduction of the cement content without any the dehydration process. Although some hydrates

reduction in strength was accomplished by the dehydrate rapidly others, such as micas, clays, and

addition of ~2·5–4% fine (<50 mm, but ideally less C

4A3H3, 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 Thick

ates). The objective was to reduce the amount of (tens of centimetres) linings may develop

hydrother-water by promoting a homogeneous distribution of mal pressures that can form C

4A3H3 and boehmite _{the cement so that the hydraulic bond could be fully} (AH).97 Although decomposed hydrates may start to

utilised. Despite their lower porosity and better cor-react with alumina aggregates at temperatures as low

rosion resistance, compared with conventional cas-as 900°C, significant sintering of calcium aluminate

tables, the first generation of LCCs was too sensitive crystallites themselves and with neighbouring alumina

to rapid heating, mainly because the chemically crystals only occurs at temperatures close to 1100°C.94

bonded water was released in a much narrower Early 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 of

ling since the outer layers closed oﬀ and internal various fluxes, most of which did little good and

water pressure built up. Further improvements led to usually reduced the maximum temperature of use.36

the development of concretes characterised by a Values for the cold crushing strength of conventional

pseudozeolithic bond, which releases the chemically refractory castables fired at 1000°C usually vary from

bonded water slowly between 150 and 450°C, rather 10 to 30 MPa, averaging~60% of the strength after

than within a narrow temperature range.29,33,99 This drying.19

minimised the problems associated with explosions Finally, the high lime content of these castables

during heating but, because LCCs and ULCCs are favours formation of a fluid vitreous phase at high

dense materials with low permeability, baking out is temperature via the eutectic liquid in the CaO–Al

2O3– _{always di}_{ﬃcult, especially in thick installations.14} SiO

2 (CAS) ternary system which may encourage _{Two other French patents in 1976 and 197747} crystal formation (e.g. mullite or spinel, see the

sec-further reduced the cement content of the castables tions ‘Microstructural evolution on drying and firing’

to less than 3%, again by using dispersing additives, and ‘Spinel and magnesia based castables’ below) but

such as sodium tripolyphosphate (0·01–0·05%), and often will remain as a glass (Fig. 7e) or low melting

fine particles. Part of the cement was replaced by fine anorthite and gehlenite on cooling which degrades

particles ranging from 10 to 1mm, but the decisive refractoriness and corrosion resistance.96,98 The

step was the use of submicrometre particles ranging volume of viscous phase in a refractory castable for

from 0·1 to 0·01mm, which could be easily dispersed a given temperature and refractory aggregate is

in water without forming a sol or a gel. The idea was mainly determined by the impurity content of the

to reduce the water requirement by eliminating the binding phase, i.e. by the composition and amount of

intergranular voids, which are often filled with excess cement used. Even with a high purity CAC containing

water during the castable placement. This was 70–80%Al

2O3, it is impossible to reduce the CaO _{accomplished by carefully grading the particle size} content of conventional castables to less than 3%,

distribution, so that interstices were progressively which is still a high amount, particularly if silica

filled by smaller particles to obtain the maximum containing aggregates are used. Further reduction is

packing density.11,33,99 Water requirements were only possible by reducing the cement content.

further reduced by proper selection of deflocculants Unlike fired refractory bricks, whose final

prop-and water reducing agents, which prevented coagu-erties are largely fixed before reaching the user, a

lation of the fine powders and improved dispersion. refractory concrete has properties which evolve and

The reduction in the water content required for alter for a considerable time after it has been put into

vibration casting and maximising of the packing use. In the case of refractory concrete it is the

behav-density means these materials have high behav-density, low iour 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 the

ance.11,99–101 Low and ultra-low cement castables 1960s, there was little doubt about what should be

usually require 3–7% water for placement, depending done to improve the performance of refractory

cas-on the grade.14,33,99 tables. Reducing the amount of cement without

spoil-To appreciate fully modern refractory castables the ing other properties of the material proved diﬃcult

inter relation between particle packing, dispersion and challenging, but after several attempts it finally

technology, and rheology is critical. Understanding led to the development of a new range of products:

(12)

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 that the workability of the material is governed by the flow properties of the fine powders.107 Therefore, the study of abnormal flow properties in a fine powder–water system, such as slurries and pastes, is important. Rheology is the science of flow and deformation of materials.102,108,109 In a suspension of completely dispersed particles, the shear resistance depends primarily on the viscosity of the liquid and the interparticle forces. In such a system, the eﬀective stress, or interparticle stress, is independent of press-ure imposed in processing. However, in a more crowded slurry, as is the case with castables, shear may be momentarily blocked by neighbouring particles. The resistance to shear flow is dependent on particle translation away from the plane of shear, which is time dependent. The shear resistance will be very 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 the system must occur to accommodate shear flow. In these 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.102 In castables, the situation is further complicated by rating the third further improved the overall

under-the presence of aggregates, whose elastic and plastic standing of the technology and allowed the

characteristics, and importantly segregation tendency, development of SFCs (Fig. 8).

play an important role in the flow properties of the material.109 Studart et al.110 obtained a rheological map by considering viscosity, yield stress, and

Particle packing, dispersion, and rheology

absorbed energy to obtain cement free, self-flowing The main idea behind LCCs and ULCCs is to reduce

(see ‘Non-cement or cement free castables’ and ‘Free the water requirement for placement while

main-or self-flowing castables’ below) HACs. Both particle taining strength. A major breakthrough in the

devel-size distribution and matrix rheology are important opment of this technology was the realisation that

when manufacturing such castables. this could be accomplished by improving the packing

Refractory castables are subjected to a wide range density of the material. More eﬃcient particle packing

of shear rates during processing and installation, reduces the maximum size of the interstices between

varying from the very low rates required for gravity particles. For a size distribution which packs more _{levelling (10−1 s−1) to the very high shear rates} efficiently, less of the liquid is segregated in large _{characteristic of spraying (105 s−1), through the} inter-interstices and more of it is effectively mobilised in _{mediate 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 may} specific proportion of a finer size that packs efficiently _{cause the appearance of shear thickening or dilatant} in 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 to} ment 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 with} without the proper use of additives to allow adequate _{excess water, which is undesirable. The absence of} dispersion of the submicrometre powders. A range of _{shear thickening is also important in the pumping of} chemical 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 shear} and borates,102 sodium pyrophosphates,8,102 _{rate at which dilatant behaviour begins. Dilatancy} hexametaphosphates44,60,85,103,104 and tripoly- _{may be reduced by the use of fine particles, such as} phosphates,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 of} many 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 produce mer) and Castament FS10 (polyglycol based poly- additional fines may also extend the range of shear mer).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 a combination with each being used to deflocculate a

(13)

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. Densified microsilica contains loosely bonded secondary in 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 intensity wet mixing to assure complete dispersion. There is no diﬀerence in chemical composition between undensified and densified microsilica, provided the

Submicrometre (superfine) matrix additions

grade is the same. The main physical and chemical According to the French patents,46,47 the main role

properties of microsilicas commonly used in refractor-of the submicrometre powder additions is to act as a

ies applications are listed in Table 10. filler, exactly filling the void spaces between the larger

The primary function of microsilica in refractory particles, so that the densest possible packing is

castables is to act as a filler. Once properly dispersed, achieved. Therefore, any refractory material could be

microsilica fills the voids between the coarser particles, used, as long as it was solid and did not react with

releasing the entrapped water and increasing the water.110 Submicrometre powders commonly used in

packing density. In this sense, microsilica has tra-the early years of this technology included alumina,

ditionally been found to be more eﬀective than fine silica, chromium oxide, zirconia, titanium oxide,

sili-calcined aluminas, though this situation appears to con carbide, clay minerals, and even carbon.14,33 From

be changing as new much finer superground reactive these, two relatively new refractory raw materials

and dispersing aluminas become available.114,115 It is have come into significant use in both LCCs and

claimed that use of microsilica reduces the open ULCCs: 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 characteristic Silica fume, also referred to as fumed silica, volatilised drop in strength at intermediate temperatures often silica,79 silica flour, or white carbon,8 is a byproduct observed in conventional castables.116,117

of silicon metal production, with the quality variations However, acting as a filler is by no means the only inherent to any byproduct. Only in the last two _{eﬀect of microsilica in refractory castables. Studies} decades has this material been supplied at a consistent with microsilica containing cement pastes have shown quality level. Many of the early field problems with that microsilica reacts with the calcium aluminate LCCs, such as erratic setting behaviour and low phases in the cement and water to form zeolithic strength, 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 a shown to have drastic eﬀects on LCCs, with impurit- weak water binding potential and, consequently, the ies, such as iron oxide and alkalis, reducing the chemically bonded water is not released abruptly in strength and increasing the viscosity, respectively, and a narrow temperature interval, but rather progress-a lower pH increprogress-asing 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 during ing, and homogenising the silica rich fume released drying out.117 However, the mechanism by which during production of ferrosilicon and silicon metal in microsilica reacts with CACs is not yet fully electric arc furnaces.79,112 Microsilica is an amorphous understood.8

silicon dioxide consisting of submicrometre spherical In microsilica containing castables, the physical primary particles with an average diameter of and chemical characteristics of microsilica can drasti-~0·15 mm. These spheres are the building units of

primary agglomerates which consist of a few spheres

Table 10 Typical properties of microsilica bonded together by material bridges. Thus, the

eﬀec-Mineral 960 971 983

tive size distribution becomes rather wide in the

Typical 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 98·30

increases the packing eﬃciency and enhances the

Al2O3 0·40 0·40 0·40 0·20

workability of the concrete.112 CaO 0·15 0·20 0·20 0·20

High purity microsilica from the production of MgO 0·30 0·30 0·10 0·07

Fe2O3 0·10 0·10 0·10 0·05

metallic silicon is usually preferred for refractories

Na2O 0·10 0·10 0·10 0·04

applications.43,44 In refractory castables, use of high

K2O 0·20 0·40 0·30 0·25

quality, high purity microsilica is almost mandatory,

P2O5 ... 0·10 0·10 0·06

since low purity microsilica reduces flow and increases _SO3 ... 0·10 0·10 0·01

C ... 1·80 0·50 0·40

water requirement for placement.113 The material is

Loss on ignition (750_°C) ... 2·00 0·60 0·60

highly reactive in cementitious and ceramic bond

Median particle size,mm 0·15 ... ... ...

systems, leading to improved ceramic bonding (form- _{Surface area, m}_{2 g−1} _18–28 ₂₂ ₂₀ _... ing, e.g. mullite and forsterite) at reduced firing tem- pH 6–8 6·5 6·0 5·3

peratures both in high alumina and magnesia Bulk density, g cm₋₃

Undensified ... 0·25 0·40 0·40

based products. Microsilica is usually available in

Densified ... 0·60 0·55 ...

(14)

cally change the properties of the system. Because of increases with the cement content, the high cement its high specific surface area (20 m2 g−1), microsilica castables usually show lower hot strength.120 The in 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 to characteristics and impurity content significantly the presence of impurities, particularly alkalis from aﬀect 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 could ing CAC, this causes modifications to the original improve the strength.44 However, despite the overall cementitious bond phase. First, it has been shown improvement in hot strength, both at 1400 and that hydration of cement is hindered or diluted by 1500°C, this did not change the general trend, since microsilica, and therefore full conversion does not the strength also dropped at 1500°C for the low alkali occur.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 microsilica tering the multivalent cations Ca2+ and Al3+. Esanu in high alumina LCCs and ULCCs,44,117,119,120 it was et al.118 demonstrated that fumed silica participates _{suggested that to increase the castables hot strength} actively in hydration by interacting with additive _{both the impurities and the cement content should} containing water and releasing silicic acid. _{be lowered to a minimum, while the amount of} Additionally, the reactive silica may react with C

2AH8 microsilica should be increased.119 While this is true at slightly elevated temperatures (40–60°C) to form _{to a certain extent, since increasing the amount of} C

2ASH8 (see the section ‘Hydration’ above). When silica with respect to that of the cement actually heated above 210°C, the chemically bonded water is _{pushes the composition of the matrix towards the} released, and C

2ASH8 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 the} may form and gehlenite (C

2AS) and anorthite (CAS2) system, it could be argued that this could also be crystallise.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 would} superior properties compared with microsilica free _{also require improvements in the properties of the} castables, including a reduction in porosity of about _{castables at the lower temperature range, where the} 5–8% and a large increase in cold crushing strength, _{addition of microsilica boosts the strength and} presumably due to the high reactivity of microsilica.117 _{reduces the porosity. This, fortunately, has been made} However, as the temperature is further increased _{possible by the use of special superground reactive} above 1200°C, the hot strength deteriorates, _{aluminas, which improve packing and speed up the} depending on the amounts of cement and microsilica, _{development of a ceramic bonding phase at much} due 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).114,115}

microsilica containing LCCs and ULCCs, the hot

The benefits of adding microsilica to bauxite based strength sometimes increases significantly above

LCCs and ULCCs are clear since these materials 1300°C, reaching its maximum around 1400°C, only

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

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

than by reducing the amount of cement.43,121 elongated needle-shaped mullite crystals from the

However, as far as high alumina, LCCs and ULCCs liquid phase, which interlock the structure improving

are concerned, the use of microsilica, despite its the bond. As soon as the castable is heated to 1400°C,

acknowledged beneficial eﬀects to the rheology and the bond phase of the castable reacts and produces a

packing of the system, is detrimental to the hot viscous liquid formed mainly from cement and

micro-strength at temperatures around and above 1500°C, silica. Within a few hours, the liquid starts to dissolve

due to the formation of liquid phase. One advocated alumina, which goes into solution with silica and

solution to this problem is the reduction in the lime; the liquid becomes saturated, and mullite

pre-amount of cement, in some cases down to levels as cipitates. As mullite is formed, the composition of the

low as 0·5%. But so far this has only proven feasible liquid changes, and its amount decreases. The final

with the simultaneous addition of cement (0·5%) and compositions of the glassy phase are either close to

hydraulic alumina (0·5%), so that both the setting anorthite or comparatively richer in silica. Further

time and flow could be properly adjusted.33,52 In this mullite precipitation is hindered, either by long

case, the bond is predominantly interlinked mullite diﬀusion paths or because the glassy phase reaches a

needles, and it has been shown that for higher contents stable composition.44,119

of microsilica (10%) the drop in hot strength at Studies involving addition of microsilica to high

1500°C may be significantly minimised, and the hot alumina LCCs and ULCCs indicate that there is a

modulus of rupture in some cases approaches that of minimum amount of microsilica required for mullite

microsilica free HACs.122 However, though this issue precipitation, which depends on the amount of

has not yet been properly addressed, the mullite bond, cement. It has been suggested that for castables with

as it is in bricks, is often more prone to corrosion by 1·5% cement (0·27%CaO), strengthening due to the

metal and steelmaking slags than the high alumina formation of mullite is only observed for microsilica

bond. Therefore, for applications where corrosion contents above 6%. For castables with 7·5% cement,

resistance is the ultimate goal, such high microsilica at least 10% microsilica is necessary, but since there