Crystallization in pores

This review discusses the thermodynamics of crystallization within porous materials and the factors that influence stress development and cracking. The maximum driving force for crystallization is related to the supersaturation for crystals growing in solution, and to the undercooling for crystals growing from a melt. However, the stresses generated on the pore walls depend on other factors, including the pore size, the energy (gamma(cs)) of the interface between the pore wall and the crystal, and (for acicular crystals) the yield stress or buckling strength of the crystal. The fact that growing crystals push particles over large distances indicates that gamma(cs) is often large. If gamma(cs) were small, crystals would tend to nucleate on pore walls rather than pushing them away, and the crystals would propagate through the pore network without resistance. Even when the crystallization pressure is large, the stress existing in a single pore cannot cause failure because it acts on too small a volume. For fracture to occur, the crystals must propagate through a region of the network large enough that the stress field can interact with the large flaws that control the strength. In concrete, growth on this scale requires that the driving force be sufficient to permit the crystals to pass through pores as small as the breakthrough radius (which is the size of the entry into the percolating network of larger pores that controls the permeability of the body). (C) 1999 Elsevier Science Ltd. All rights reserved.