Interactions in Calcium Aluminate Cement (CAC)-Based Castables Containing Magnesia—Part II: Hydration–Dehydration Behavior of CAC and their Mixtures with Dead-Burned and Reactive-Grade MgO
A priori 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. This is detrimental to the volume stability and accordingly, the strength of the self-forming spinel-containing refractory castables. Nevertheless, very few basic researches have been systematically performed related to these castables behavior during the hydration and the dehydration process. In the current work, the hydration of a neat paste of a commercial ∼70 wt% Al2O3 calcium aluminate cement (CAC) with and without two varieties of MgO additions (dead burned and reactive grade) was continuously followed at 24°±2°C by monitoring the exothermic heat evolution and the changes in pH. Additionally, in situ time-resolved laboratory X-ray diffraction (in situ-LXRD) was continuously registered from 0 to 62 h during the dissolution of the anhydrous pastes and subsequent precipitation of hydrates. Further, the evolution of hydration and dehydration was studied from room temperature up to 1200°C by dilatometry, thermogravimetry, and differential thermal analyses to establish volumetric changes and reaction process, respectively. Also, isothermal treatments were carried out at selected temperatures, ranging from room temperature to 1200°C, and subsequently analyzed by conventional LXRD. Finally, the change of the modulus of rupture and the Young's modulus at room temperature, of the neat paste of the CAC with and without dead-burned MgO, was measured and analyzed as a function of temperature, employing treated samples between 110° and 1400°C.
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