Magnesium oxide (MgO) or magnesia is one of the most important raw materials in the refractory industry. Because of its high refractoriness (melting point of 2800 °C) and corrosion resistance, the presence of this oxide in refractory compositions promotes an increase in the performance of pre-shaped linings for steel production [C.M. Peret, J.A. Gregolin, L.I.L Faria, V.C. Pandolfelli, Patent generation and the technological development of refractories and steelmaking, Refract. Appl. News, submitted for publication; M. Rigaud, Z. Ningsheng, Major trends in refractories industry at the beginning of the 21st century, China's Refract. 11 (2) (2002) 3–8; W.E. Lee, R.E. Moore, Evolution of in-situ refractories in the 20th century, J. Am. Ceram. Soc. 81 (6) (1998) 1385–1410; A. Nishikawa, Technology of monolithic refractories, Tokyo: Technical report No. 33-7, PLIBRICO Japan Co. Ltd, 1984, pp. 98–101]. Nevertheless, in refractory castables formulations, magnesia additions are restricted to coarse particles (usually above 50 μm) and contents of up to 10 wt.% due to their high tendency to react with water and the following great volumetric expansion promoted by this reaction [G.K. Layden, G.W. Brindley, Kinetics of vapor-phase hydration of magnesium oxide, J. Am. Ceram. Soc. 46 (11) (1963) 518–522; A. Kitamura, K. Onizuka, K. Tanaka, Hydration characteristics of magnesia, Taikabutsu Overseas 16 (3) (1995) 3–11; A. Yoschida, T. Nemoto, A. Kaneyasu, Evaluation method for hydration resistance of magnesia fine powder and effect of B 2O 3 content in magnesia raw materials, in: Proceedings of UNITECR’ 2003, 2003, pp. 21–30; R.A. Wogelius, K. Refson, D.G. Fraser, G.W. Grime, J.P. Goff, Periclase surface hydroxylation during dissolution, Geochim. Cosmochim. Acta 59 (9) (1995) 1875–1881; A. Kaneyasu, S. Yamamoto, A, Yoshida, Magnesia raw materials with improved hydration resistance, Taikabutsu Overseas 17 (2) (1996) 21–26; P. Brandão, G.E. Gonçalves, A.K. Duarte, Mechanisms of hydration/carbonation of basic refractories—Part I, Refract. Appl. News 3(2) (1998) 6–9; P. Brandão, G.E. Gonçalves, A.K. Duarte, Mechanisms of hydration/carbonation of basic refractories—Part II: investigation of the kinetics of formation of brucite in fired basic bricks, Refract. Appl. News 3 (2) (1998) 9–11; P. Brandão, G.E. Gonçalves, A.K. Duarte, Mechanisms of hydration/carbonation of basic refractories—Part III, Refract. Appl. News 8 (6) (1998) 23–26; S. Chatterji, Mechanism of expansion of concrete due to the presence of dead-burnt CaO and MgO, Cem. Concr. Res. 25 (1) (1995) 51–56] . Despite the great number of studies describing the behavior of powdered magnesia or high-magnesia bricks, not much research has systematically been done related to its hydration behavior in castables [A. Yoschida, T. Nemoto, A. Kaneyasu, Evaluation method for hydration resistance of magnesia fine powder and effect of B 2O 3 content in magnesia raw materials, in: Proceedings of UNITECR’ 2003, 2003, pp. 21–30; S. Chatterji, Mechanism of expansion of concrete due to the presence of dead-burnt CaO and MgO, Cem. Concr. Res. 25 (1) (1995) 51–56; T.A. Bier, C. Parr, C. Revais, H. Fryda, Chemical interactions in calcium aluminate cement based castables containing magnesia, in: Proceedings of UNITECR’ 1997, 1997, pp. 15–21; D. Jeong, H. Kim, Y. Kim, S. Lee, Development and application of basic dam block for Tundish, J. Tech. Assoc. Refract. Japan 23 (1) (2003) 4–10; K.G. Ahari, J.H. Sharp, W.E. Lee, Hydration of refractory oxides in castable bond systems—I: alumina, magnesia and alumina–magnesia mixtures, J. Eur. Ceram. Soc. 22 (2002) 495–503; K.G. Ahari, J.H. Sharp, W.E. Lee, Hydration of refractory oxides in castable bond systems—II: alumina–silica and magnesia–silica mixtures, J. Eur. Ceram. Soc. 23 (2003) 3071–3077; Y. Koga, M. Sato, K. Sekeguchi, S. Iwamoto, Effects of alumina cement grade and additives on alumina–magnesia castable containing aluminum lactate, Taikabutsu Overseas 18 (1) (1997) 43–47] . In this paper, aspects of magnesia hydration were briefly reviewed. Novel insights concerning magnesia hydration products generated under different curing conditions were attained adapting techniques already used in castables study.