Low temperature exothermic effects on cooling of homoionic clays

Results of differential scanning calorimetry (DSC) experiments on homoionic montmorillonites (Ca(2+), Mg(2+), Na(+), K(+)) and Na-kaolinite are reported. The low temperature exothermic peaks were observed on cooling down to -90 degrees C after the initial peak corresponding to the breakdown of supercooling of water in macro- and mezopores. In some experiments, the samples were then warmed to -10 degrees C or -5 degrees C and cooled again to -90 degrees C. According to expectations, the non-equilibrium exothermic peaks were absent in this case, being replaced by a wide peak analogical to that obtained on warming. However, the low temperature exothermic peaks were still observed, but their fields were characteristically cut, proportionally to the extent of the previous warming. The stochastic deconvolution applied to the endothermic peaks obtained on warming allowed to compare the thermal effects corresponding to the cooling and warming. The results gave evidence that the portion of water, solidification of which can be attributed to the low temperature exothermic peaks, on warming melts in a wide temperature range and a separated melting point does not exist. (C) 2011 Elsevier B.V. All rights reserved.