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Several technological problems arise on adding MgO to castable systems, due to its high tendency to react with water and the subsequent great volumetric expansion promoted by this reaction. Nevertheless, very few basic researches have been systematically carried out related to these castables behavior during the hydration and dehydration process. In the current manuscript (Part I), a novel approach based on the combined use of exothermic profiling and in situ time-resolved laboratory X-ray diffraction (LXRD) has been used to evaluate, as a first step, the hydration behavior of two MgO varieties (dead burned and reactive grade) in the absence of calcium aluminate cement (CAC). On the other hand, the dehydration behavior has been followed by conventional LXRD, differential thermal analysis, thermogravimetric, and dilatometric analyses. The results obtained and their implications have been discussed and compared in order to facilitate the understanding of the interactions in CAC-based castables containing magnesia, which is studied in Part II of the paper.