Calorimetry and other methods in the studies of reactive magnesia–hydratable alumina–microsilica hydrating mixtures

Abstract

The reactivities of the binary and ternary mixtures of reactive magnesia (MgO≡M), hydratable alumina (Al2O3≡A) and microsilica (SiO2≡S) micropowders were investigated by calorimetric method and other analytical techniques (X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), differential thermal/thermogravimetric (DTA-TG) analysis and scanning electron microscope (SEM) observations). Effect of water/solid mass ratio and temperature on the hydration behavior of the M–A, M–A–S, M–S, A–M–S and A–S hydratable binders were established. The methodological considerations on both in situ and ex situ mixing techniques have also been taken. The initial mixing peak was due to wetting of the binder particles and initial dissolution reactions. A second, extensive heat peak was associated with the formation of crystalline and noncrystalline hydration products, i.e., brucite Mg(OH)2, magnesium aluminate hydrate (MAH; H≡H2O)-like phase, magnesium silicate hydrate (MSH)-like phase and magnesium aluminum silicate hydrate gels (MASH). Negligible heat was evolved during the hydration reaction of A–S mixture, and the second reaction peak was not observed. Nevertheless, the presence of Al2O3 and SiO2 in other combinations contributes to the consumption of Mg(OH)2 which was formed during the initial stages of hydration and leads to the formation of cementitious products, like M–A–H, M–S–H and M–A–S–H.