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120 t,min

Fig. 1.6. Temperature curves, through supercooling (rsup) and initial crystallization (fcr) (solid line), and warming (dotted line), of sand (1) and clay (2).

ning of crystallization (t'c") has been reached by the system? Let us consider broadly the process of the gradual freezing of bound water as heat removal continues, to explain the nature of this phase transition over a range of negative temperature.

The fact that the soil has reached the temperature of the beginning of crystallization (t = t1c"<0°C) and that the first holocrystalline ice nuclei have been formed means that a new complex physico-chemical system with such interfaces as mineral surface-unfrozen (bound) water-ice has been formed. The mutual effect of the surface forces of mineral particles and ice will be minimal in this case, as the curve for the unfrozen water bond energy due to the mineral surface with respect to the thickness of the liquid-like film h, (Em(t) =f{\/h"), and the curve for ice (Eis(t) =f[\/h")) cross each other at a not very high energy level (Fig. 1.7, section I-I). Section I-I represents in this case a conventional surface dividing the unfrozen water layer into two parts. One, the left part, situated between this section and the mineral surface (MS) will be affected mainly by the surface forces of the mineral substrate. The other (right part), which is smaller in thickness, is the part of the water layer between the section I-I and the ice surface (IS) and is confined mainly by the surface forces of the ice substrate. It will have practically the same texture as the ice. In other words, this part of the unfrozen water represents a quasiliquid film on the ice surface, the texture of which has already undergone modification. The surface forces of the mineral as well as of the ice substrate in this case cause distortion of the hydrogen bonds in adjacent layers of the unfrozen water modifying the water texture towards the texture of the crystalline lattices of the substrate. Such a distortion is transmitted to the t

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