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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, selfsystems may be based on carbon derived from pyroflowing, 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 aluminate phases formed from fired hydraulic calcium aluminate cements (CACs). The most significant trend

reactant contact the refractories' composition, physical texture, and product transport nature of the bond Penetration and dissolution mechanisms are phase (which is the first to be penetrated by liquid), reviewed for predominantly single phase oxide, two phase oxide, and oxide-carbon composite and the characteristics of melt and reaction products refractories by liquid silicates. Theoretical models affect the rate of the reaction. Consequently, to underof these processes, as well as static (sessile drop, stand the corrosion mechanisms, refractories microdipping, and crucible) and dynamic (rotating finger structures (in particular, composition and texture of and rotary slag) experimental tests, along with grain and bond phases, the linkage of grain to bond, their practical limitations are considered. Direct and internal or residual stress), melt properties (congruent or homogeneous) attack involves (especially composition and viscosity as a function of dissolution of a solid into a liquid with no temperature), and their wetting and interaction at intermediate solid phase leading to active high temperatures should be examined together. corrosion. Indirect (incongruent or heterogeneous) Almost all liquid metals do not wet ceramics while attack leads to formation of one or more new solid phases at the original solid/liquid interface. This almost all slags, glasses, and fluxes do, so attack at may lead to passive corrosion. Examples of direct the slag line, where the less dense (typically silicate) and indirect attack in a range of refractory-liquid liquid floats on the denser metal liquid, is frequently systems are described highlighting the critical a problem. Carbon based ceramics are not wetted by influence of the composition and hence viscosity silicate liquids, behaviour which has led to their of the local liquid adjacent to the solid. Penetration extensive use in iron-and steelmaking refractories.and corrosion can be controlled either through the Wetting behaviour gives an indication of penetration local liquid composition via the refractory or the (simple permeation via open porosity without chemibulk liquid or by microstructural control of the cal reaction) but not attack or corrosion (penetration refractory by, for example, internal generation of and chemical reaction). This review begins by examindense layers or external deposition/generation of passive coatings. IMR/325 ing simple theoretical treatments of these phenomena. Much research has examined refractories in different

b-TCP, anorthite, and a-quartz (SiO 2) in a calcium aluminosilicate glassy matrix.5,12,13 Iqbal et al.5 used scanning electron microscopy (SEM) and TEM to show that commercial bone china microstructures consist of lath-like

saturate the clay with CaO, with the result that little of the b-TCP needs to break down and form low melting phases. Commercial bone china microstructures are far fromThe b-TCP crystals, which now act as a filler, confer equilibrium consisting of distinct regions of lath-like refractoriness on the body. The proportions of clay, flux, (0•4-10 mm) anorthite and spheroidal (1-3 mm) and bone may be varied from the typical 25 : 25 : 50 ratio b-tricalcium phosphate (b-T CP) nodules embedded in depending on the properties desired and other components a heterogeneous composition glass along with isolated may be substituted. For example, ball clay and gum arabic irregular (∏30 mm) a-quartz crystals. T he composcan be included in the body recipe to increase green strength ition and morphology of the phases formed on firing and bentonite clay (predominantly montmorillonite) can be suggest that anorthite crystallised in clay relicts by the added as a plasticiser to increase workability and green reaction of metakaolin with CaO, b-T CP crystallised strength.3 f rom decomposition of bone ash, and that the liquidIn the UK bone china is manufactured using the following formed on vitrification has variable composition general processing route. Hard raw materials are crushed depending on the composition of adjacent phases. P 2 O 5 by jaw and roller crushers, weighed, wet ball milled, sieved, was never detected in the glass suggesting that any and passed over magnets to remove metal particles. Clays P 2 O 5 that dissolves in the liquid on firing is transient. (with a particle size of ~0•2-6 mm) are blunged. Bone ash T he cracks sometimes observed in and around b-T CP is milled to a median size of ~3-5 mm and fluxes to clusters arise from thermal expansion mismatch ~10 mm. Weighed proportions of clay, flux, and filler are between b-T CP and the surrounding glass and then mixed in mixing arks, again sieved, and passed over anorthite. BCT /381 magnets and stored. De-watering may be carried out by filter pressing or spray drying. After filter pressing, the T he authors are in the Department of Engineering bodies are aged and passed through a pug mill to de-air Materials, University of SheYeld, Mappin Street, the body and homogenise the water content. Shape forming SheYeld S1 3JD, UK. Manuscript