An experimental investigation of hydration mechanism of the binary cementitious pastes containing MgO and Al2O3 micro-powders

Abstract

The reactivity of the binary mixtures of reactive magnesia and hydratable alumina micro-powders with various compositions was investigated by calorimetric method, X-ray diffraction, simultaneous DSC–TGA with thermal evolved gas analysis–mass spectrometry and scanning electron microscope observations. The effect of the MgO/Al2O3 molar ratio (2:1, 1:1 and 1:2) and temperature on the hydration behavior of pastes has been explored using the TAM Air isothermal calorimetry system at 25 and 50 °C. Some structural aspects of a M–A–H (M≡MgO, A≡Al2O3, H≡H2O) binding phase formed upon the hydration of MgO–Al2O3 micro-powders at different curing time have been explored. The main hydrate considered to be important in this system is hydrotalcite Mg6Al2CO3(OH)16·4H2O formed via the isomorphous substitution of Mg2+ ions by trivalent Al3+ one in the brucite-like sheet. This was expressed as a systematic shift of the peak at 2-theta value corresponding to the (003) reflection. Since both hydrotalcite and brucite are identified in the hydrating samples, the thermal stability of hydroxyls in these hydration products can be placed in the decreasing order as follows: (Mg2Al)–OH-hydrotalcite group > (Mg3)–OH-brucite group > (Mg3)–OH-hydrotalcite group.