Microstructure and phase evolution of alumina–spinel self-flowing refractory castables containing nano-alumina particles Sasan Otroj * , Arash Daghighi Faculty of Engineering, Shahrekord University, Shahrekord, Iran Received 23 July 2010; received in revised form 29 September 2010; accepted 6 November 2010 Available online 2 December 2010 Abstract The microstructure and phase composition of alumina–spinel self-flowing refractory castables added with nano-alumina particles at different temperatures are investigated. The physical and mechanical properties of these refractory castables are studied. The results show that the addition of nano-alumina has a great effect on the physical and mechanical properties of these refractory castables. With the increase of nano-alumina content in the castable composition, the mechanical strength is considerably increased at various temperatures. It is shown that nano-alumina particles can affect formed phases after firing. The platy crystals of CA 6 are detected inside the grain boundaries of tabular alumina and spinel grains in samples fired at 1500 8C. CA 6 phase can be formed at lower temperatures (1300 8C) with the addition of nano-alumina particles. As a result of using nanometer-sized alumina particles with high surface area, the solid phase sintering of the nano-sized particles and CA 6 formation can occur at lower temperatures. # 2010 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: B. Microstructure; C. Mechanical properties; D. Al 2 O 3 ; D. Spinel; E. Refractories 1. Introduction The increasing application and demand for refractory castables as an alternative to conventional brick encourages researchers and producers to investigate their special char- acteristics. Physical, chemical and mechanical properties of refractory castables at high temperatures, as well as their processing, are the focus of such investigations [1]. In steel industry, there is a trend to increase the use of high- alumina refractory castables containing spinel MgAl 2 O 4 , because of their higher refractoriness and of their better corrosion resistance than Al 2 O 3 castables [2]. Both Al 2 O 3 – spinel and Al 2 O 3 –MgO castables are widely used as steel ladle linings below the slag line because of increasing labor costs and the severe secondary steelmaking environment in the ladle. In Al 2 O 3 –spinel castables, spinel are added to the mixture as a grain phase, while in Al 2 O 3 –MgO castables MgO reacts with Al 2 O 3 to form in situ spinel during service. Al 2 O 3 –MgO castables are replacing Al 2 O 3 –spinel castables because of their superior slag resistant properties and lower costs. However, Al 2 O 3 –spinel castables are still commonly used in the non-impact pad area of the steel ladle bottom [3–8]. The performance of Al 2 O 3 –MgO and Al 2 O 3 –spinel castables are investigated with profound interest in the past few years. The important criteria leading to improvements of these refractory castables are resistance towards high basic corrosive slags, molten metal and slag penetration, spalling and hot strength [1,8–13]. Generally, refractory castables can be considered as composites, with the bonding phase being the matrix and aggregates being reinforcement particles [2,14]. Successful performance of refractory castables containing fine (1–100 mm) and superfine (<1 mm) powders during the lining application and high- temperature services is attributed to the ability of these powders to fill the voids between castable aggregates (>100 mm) [15]. However, the most extensively used fine and superfine particles in Al 2 O 3 –spinel refractory castables are calcined and reactive alumina [8,16]. Refractory researchers recently started to engineer the quality of advanced castables by laying down nano-sized materials in the composition. Very high surface energy and rapid diffusion paths usually make the nano-particles far more reactive in the refractories, which frequently encounter the aggressive environment, particularly in the steel industries. www.elsevier.com/locate/ceramint Available online at www.sciencedirect.com Ceramics International 37 (2011) 1003–1009 * Corresponding author. Tel.: +98 381 4424438; fax: +98 381 4424438. E-mail address: sasan.otroj@gmail.com (S. Otroj). 0272-8842/$36.00 # 2010 Elsevier Ltd and Techna Group S.r.l. All rights reserved. doi:10.1016/j.ceramint.2010.11.013