On the Interpretation of Low Temperature Calorimetry Data

The effect of selected factors and phenomena on Low Temperature Calorimetry (LTC) results has been investigated, in order to determine the possibilities and limitations of using LTC for characterisation of the porosity of cement-based materials. LTC was carried out on a model material with mono-sized pores of approximately 14 nm saturated with either distilled water or a sodium chloride solution, as well as on water, the salt solution, and an artificial pore solution, alone. It was found that supercooling is unavoidable during the liquid-solid phase transition, and that even at low temperature gradients equilibrium cannot be obtained with the sample size used (1 g liquid). Correcting for non-equilibrium between reference block and sample, the Gibbs-Thomson equation seems applicable for estimation of pore sizes of the investigated size. The estimate may be further improved by taking the effect of ions on freezing point depression into account. It is proposed that the connectivity of pores, e.g. in cement-based materials, may be characterised based on cooling curves, whereas the pore size distribution may be characterised based on the heating curve. Cooling should be undertaken at a high rate to limit transport of liquid, whereas heating should be undertaken at a low rate to limit the effect of non-equilibrium.