Thermal expansion of C3S and Mg-doped alite

The lattice thermal expansion coefficients were measured using in situ high temperature x-ray powder diffraction on triclinic tricalcium silicate (C3S) in the temperature range 25-970 degrees C and on monoclinic alite of composition Ca(2.879)Mg(0.112)Al(0.0439)Si(0.971)O5 in the temperature range 30-1150 degrees C. Full profile refinements were performed using the Rietveld method. The structural model of Mumme [1] was adopted for the alite crystal structure, implying a space group Cm and cell constants a=12.18, b=7.04, c=9.25 Angstrom, beta=116.11 degrees at 25 degrees C. All three lattice parameters show similar temperature behaviour and relative thermal expansion coefficients. There is a clear change in slope at about 900 degrees C, which is assumed to be related to the phase change between two monoclinic forms of alite. Refinements of C3S were conducted using the structural model of Golovastikov et al. [2] implying a space group <P(1)over bar> and cell constants a=11.67, b=14.28, c=13.72 Angstrom, alpha=105.3, beta=94.2, gamma=90.0 degrees at 25 degrees C. The C3S lattice parameter b shows a different trend in temperature with respect to a and c. The same behaviour is reproduced by the relative thermal expansion coefficient alpha. Three clear changes in slope are found and regarded as phase transition points between triclinic modifications of C3S. Integrated intensity analysis for selected peaks, together with TG/DTA in the temperature range 25-1150 degrees C, were also used in order to preliminarly characterize the phase transitions.