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The lattice parameter and thermal expansion coefficient ͑TEC͒ for the spinel-type Si 3 N 4 phase prepared under high-pressure and high-temperature conditions are determined for 14 KϽTϽ302 K by x-ray diffraction, at a synchrotron-radiation source, and for 0 KϽTϽ1683 K by first-principles calculations. The low-and high-temperature experimental TEC data are in excellent agreement with the calculations. Our results provide the first experimental evidence that the linear expansion coefficient tends to vanish for TϽ100 K. The experimental temperature dependence of the TEC is accurately represented by polynomials over the complete T range.

The effect of gamma radiation in the range of 0-1409 MGy on the structure of a new mineral additive to cement based composites was investigated in the perspective of employing them as radioactive waste protection material. According to the authors knowledge, it is the first paper dealing with observations of the cement matrix, both pure and modified, treated with so giant radiation dose. The absorption of gamma radiation modifies the morphology of the additive grains, causes decomposition of cement hydrates and clinker relicts in cement paste containing the additive at twice higher radiation dose than that inducing the decomposition of the reference pure cement paste and the cement paste containing pozzolane additives.