Contents lists available at ScienceDirect Journal of the European Ceramic Society journal homepage: www.elsevier.com/locate/jeurceramsoc Original Article The main role of silica—Based cement free binders on the microstructural evolution and mechanical behaviour of high alumina castables Olga Burgos-Montes a , Margarita Álvarez a , Antonio H. de Aza b , Pilar Pena b , Carmen Baudín b, ⁎ a REFRACTARIOS ALFRAN S.A., 41500, Alcalá de Guadaíra, Sevilla, Spain b Instituto de Cerámica y Vidrio, ICV-CSIC, 28049, Madrid, Spain ARTICLE INFO Keywords: High alumina no cement castables High temperature Mechanical properties Microstructure Phase equilibrium diagrams ABSTRACT One of the major innovations for monolithic refractories in the past decade has been the development of cement- free binders, no-cement castables (NCCs). NCCs with colloidal silica suspensions as binders are extended at industrial scale. Microsilica-based powder binders are valuable alternative to colloidal silica to avoid installation and transport difficulties. The success of NCCs depends on the high temperature microstructure developed. Phase equilibrium diagrams are a powerful tool for the understanding of such development. This work deals with the study of alumina castables fabricated using both silica gel technologies with the aim of establishing the main factors that determine the high temperature (1300–1400 °C, 30–300 min) micro- structure and how it affects the mechanical behaviour. Deformation and fracture are determined by the for- mation of mullite in the matrix. Reaction kinetics is highly dependent of temperature, time and the alumina - silica relative amount at local level, determined by the composition of the binder. 1. Introduction Along the last years, many investigations have been addressed to the development of calcium free binders for monolithic refractories (see Refs. [1–3] for review). The use of colloidal silica gel in no-cement castables (NCCs) has been widely investigated and is extended at the industrial scale. Despite the benefits of castables containing this binder, low and fast dry-out, a maximum thermal shock resistance and high mechanical properties, some well known disadvantages (storage and transportation, frost sensitivity of the suspension, etc.) are also high- lighted in many researches [4–9]. Different alternative solutions to overcome the sensibility of the liquid to freezing conditions have been developed such as the development of frost resistant colloidal silica binders [9]. In this framework, the use of microsilica powder-based binders has also been proposed as a valuable alternative to colloidal silica. The easier storage and transportation of powders as compared to those of suspensions together with the avoidance of the possibility of freezing make powders a desirable solution. Several laboratory papers [4–8] as well as preliminary plant trials seem to support the viability of this solution. The performance of monolithic refractories is close linked to the microstructural development that defines the behaviour of the material at high temperatures. In this sense, the formation of mullite in the matrix at the use temperature is fundamental for the performance of no- cement alumina castables [10]. Mullite is a desirable phase to be formed in refactories because its low thermal expansion and con- ductivity, creep resistance and high temperature stability [11]. How- ever, the presence of small amounts of impurities leads to liquid for- mation in mullite materials at relatively low temperatures, which depend on the specific impurity. Fracture behaviour at high tempera- ture and creep resistance of mullite materials are determined by the nature and distribution of these liquids as well as the size and shape of the mullite grains [12–14]. Therefore, the performance of no-cement castables (NCC) will depend on the specific microstructure developed by mullite in the matrix. In this sense, the use of silica-based powder binders containing alumina in their composition, as it has been recently proposed [4,5,7] appears as an attractive solution to induce mullite formation by reaction between the constituents of the binders. How- ever, in order to optimise the behaviour of the materials, the actual microstructural development should be understood. In this context, phase equilibrium diagrams are a powerful tool for the understanding of the behaviour of refractories during service [15,16]. Their use allows inferring, not only the equilibrium microstructure corresponding to a particular composition, temperature and pressure, but also the path through which the material would move into the final state. Based on these aspects, this work deals with the study of the https://doi.org/10.1016/j.jeurceramsoc.2018.04.048 Received 31 January 2018; Received in revised form 13 April 2018; Accepted 21 April 2018 ⁎ Corresponding author. E-mail address: cbaudin@icv.csic.es (C. Baudín). Journal of the European Ceramic Society xxx (xxxx) xxx–xxx 0955-2219/ © 2018 Published by Elsevier Ltd. Please cite this article as: Burgos-Montes, O., Journal of the European Ceramic Society (2018), https://doi.org/10.1016/j.jeurceramsoc.2018.04.048