Phase assemblage in ettringite-forming cement pastes: A X-ray diffraction and thermal analysis characterization

Abstract

The study attempts to describe the evolution of the solid phase composition with ongoing hydration in three different calcium aluminate rich cement paste mixtures by means of XRD and thermal analysis. The phase assemblage was followed quantitatively at discrete ages of 1, 7, 28, 56 and 90 days. Ettringite was the main crystalline hydration product. Quantification of amorphous fractions using the external standard method was performed and relatively high amounts of amorphous fractions were reported in all the cases.Thermal analysis revealed that the X-ray amorphous hydrate fraction was mainly composed of monosulphate, AH3 and C-S-H. The presence of strätlingite, was not clearly manifested in any of the DTG curves. A mass balance calculation based on stoichiometric reactions was performed in order to estimate the amounts of monosulphate, AH3, and C-S-H. The quantities of the amorphous portions obtained from QXRD were observed to be higher as those estimated from mass balance calculations. Additional calculations from oxide balance suggested that besides AH3 monosulphate and C-S-H, an X-ray amorphous AFm or/and C-S-H type like phase might form in the early age of hydration.During Rietveld refinement, the impact of the number of Chebyshev background polynomials in the determination of amorphous content was investigated.