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Structural and compositional transitions of Datong coal ash and its CaCO3 additional effects were carefully examined at a temperature range of 300 to 1 600 ℃ by using XRD and solid state NMR.The quantitative estimation of amorphous structures of ashes can be successfully obtained through the analyses of solid state NMR spectra.Viscosity of molten ash and its changes with CaCO3 addition were also evaluated up to 1 700 ℃ by using a rotary type viscometer.Glasses with poor crystalline and amorphous phase were continuously formed through the euteetic reaction of silica above fusing temperature (FT＞1 500 ℃) that caused broadening and shift of 29Si and 27Al peaks in NMR results.With the additional amount of CaCO3 increasing,the peaks shifted to downfield obviously; the fraction of Si(OAl)0(OSi)4 decreased,while that of Si(OAl)1(OSi)1 at -84.3×10-6 increased apparently.These transitions indicated the destruction of large alumina-silicate framework into small segments by the addition of Ca ion.