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

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Titanium carbide (TiC)‐coated graphite for refractory castable applications was prepared via reacting titanium metal powder (∼45 μm) and natural graphite flakes (100–300 μm) for 5 h at 950°C in a KCl‐based salt in Ar atmosphere. TiC coatings formed on graphite flakes were uniform, crack‐free, and composed of nanosized particles (20–40 nm). The “template‐growth” mechanism played a dominant role in the coating formation process.

Typical applications of phase diagrams to specific of refractories corrosion has been highlighted,3-7 and problems of refractories corrosion are highlighted.a few cases such as attack of Al 2 O 3 -SiO 2 refractoriesSaturation solubilities of refractories components in molten slags can be estimated using existing by alkali oxides well studied,4,6,7 their use is far from phase diagrams. These can then be used to predict ubiquitous.the corrosion behaviour, and qualitatively compareThis paper seeks to emphasise applications of phase the corrosion resistance, of a refractory in different diagrams to specific refractories corrosion problems slags or different refractories in the same slag. using typical examples. Previously reported experi-If the slag is not saturated with refractories mental results which were not well explained have components, using relevant phase diagrams, the been re-analysed and earlier discrepancies clarified conditions under which solid reaction product using the relevant phase diagrams. phases form at refractories/slag interfaces can be predicted, which assists understanding of direct and indirect dissolution. By checking the Significance of saturation solubility of compatibility between impurities or additives and refractories oxide phases in liquid slag refractories components at high temperatures, their influence on corrosion resistance can be Dissolution of solid oxides in liquid slag is governed predicted, aiding their selection. Phase diagrams by: (a) chemical reaction (or solution) at the also reveal that atmosphere affects refractories refractories/slag interface or ( b) transport (or corrosion resistance by altering the valence of diffusion) of reacting species.8 In the latter case, some components (in particular, iron oxide) in the dissolution rate can be expressed in terms of the refractories and/or slags. The compatibility Nernst equation between refractories and slags indicates the corrosion resistance of different refractories, J=D(C s −C m )/d . . . . . . . . . . (1) assisting refractories selection for specific applications. Finally, phase diagrams can be used where D is the diffusion coefficient (m2 s−1), C m and to assist design of refractories composition. C s are, respectively, concentration and saturation Repeated experimental corrosion test results show solubility of refractory in the slag (g m−3), and d is good agreement with phase diagram predictions. the effective boundary layer thickness (m). Therefore, use of the information which is Increasing D or decreasing d (i.e. increasing available in existing phase diagrams can reduce D/d) increases dissolution rate J. Besides this, it is the need for expensive and time consuming clear from equation (1) that the value of C s −C m experiments to evaluate high temperature strongly influences dissolution rate. If the slag is corrosion of refractories (and other ceramics).