Porous hydrate shell

Enamel mineral has many large hydroxyapatite crystals, whereas bone has many small ones with numerous vacancies and substitutions. These differences increase the elasticity of bone compared with enamel and promote its interactions with the surrounding collagen. Recently, a tightly bound hydration shell that fills a porous collagen-apatite junction was discovered around normal bone crystals. The water-filled pores are normally immobile, but repeated stresses cause the water to leak out from between the mineral and collagen. The drying increases mineralization and crystal formation, which may explain the decreased elasticity of bones with age. 

Studies using ion-thinned sections, wet cells and backscattered electron images of polished sections show that a space develops between the shell and the anhydrous material (S40,S41,S68) (Fig. 7.6c). In this respect, the hydration of cement differs from that of C3S, in which the C-S-H grows directly over the C3S surfaces, without any detectable separation (S41). By 12 h, the spaces are up to 0.5 pm wide. They are likely to be filled with a highly concentrated or colloidal solution, and the shells are evidently sufficiently porous at this stage that ions can readily migrate through them (S68). The existence of spaces shows that reaction proceeds by dissolution and precipitation further evidence for this is provided by the fact that the C-S-H also deposits on the surfaces of pfa particles, if these are mixed with the cement (D28). Some other relatively unreactive or inert admixtures behave in the same way. 

A somewhat different mechanism of water flow due to the motion of cations having a hydration layer (the solvation shell) has been postulated. It employs porous/ion exchange membranes whose pore diameters are in the range of 1-5 nm. When such a membrane is placed between two electrodes containing an aqueous salt solution, and electrolysis takes place on the application of... 

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