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7. Thermodynamic and poromechanic crystallization pressure of sodium sulfate heptahydrate 75 saturation of the solution was found; the growth rate being in the range between 1 and 6 × 10 −6 m/s [59]. The influence of nucleation sites on the crystallization dynamics of sodium sulfate heptahydrate was studied by adding different amounts of chemically neutral silica and calcite powders into a solution. The crystallization rate of heptahydrate was found to be enhanced by an increase of nucleation surface [60]. Sodium sulfate heptahydrate was also reported to form during cooling of porous building materials saturated with a sodium sulfate solution [77]. In the present chapter the development of stress during the crystallization of sodium sulfate salts is studied by cooling of a solution in porous materials. To this end a nondestructive measurement of the concentration of the pore solution in a sample by Nuclear Magnetic Resonance (NMR) is combined with an optical measurement of the dilation of the sample during crystallization. First, the theory of cooling-induced crystallization in a sample and the expansion of the sample resulting from crystallization pressure is discussed. In particular, the concept of thermodynamic and poromechanic crystallization pressure and the relation between these two is considered. Next, the experimental setup and the selected materials will be described. Finally the results of the experiments will be discussed. 7.2 Crystallization pressure Let us consider a sample of a porous material initially saturated with a sodium sulfate solution which is unsaturated with respect to thenardite (point A in Figs. 7.1 and 7.2). If one cools down the sample, the pore solution will first supersaturate with respect to mirabilite and later on also with respect to heptahydrate [59, 60] (curve AB in Fig. 7.1). Since there is no crystallization, the sample will change length only as a result of the thermal dilatation, as shown schematically in figure 7.2. In that case, the length of the sample is given by: l = l 0 − ∫ T T 0 αdT , (7.1) where l and l 0 are the actual and initial length of the sample, respectively, T and T 0 the actual and initial temperature, and α the linear thermal-expansion coefficient of the material. Hence, the linear deformation or strain of the sample, dε, is proportional to the temperature change dT : ( ) l − l0 dε = d = αdT . (7.2) l 0 As soon as the concentration of the solution in the sample reaches the supersolubility of heptahydrate, the crystallization will start in the pores (point B in Figs. 7.1 and 7.2). If the temperature is kept constant, isothermal crystallization will occur [60]. Heptahydrate crystals will be formed in the porous material until the concentration of the pore solution reaches the heptahydrate solubility at the given temperature (point C in Fig. 7.1) and no supersaturation is present anymore. As soon as the crystallization process starts, the crystals will start to fill up the pores of the material. A short time later, the crystals will completely fill up a pore and a crystallization pressure will start to develop. The exerted
7. Thermodynamic and poromechanic crystallization pressure of sodium sulfate heptahydrate 76 crystallization pressure will lead to an expansion of the sample. If, after the isothermal crystallization process has stopped, the solution is cooled down further, the sample will change its length because of a combination of thermal contraction and crystallization pressure. Depending on whether the thermal dilatation or the crystallization pressure is dominant, the sample will either shrink (curve CE in Fig. 7.2) or expand (curve CD in Fig. 7.2). If the sample is cooled further, the region of so-called cryohydrates ”h + ice” (see Fig. 7.1) will be reached, where heptahydrate can coexist with ice. Any additional cooling will initiate, at a certain moment, the exothermal transformation from heptahydrate into decahydrate (mirabilite) and a crystallization pressure resulting from decahydrate will develop [77]. Stage I Stage II Stage III cooling isothermal crystallization non-isothermal crystallization Stage IV transformation to decahydrate Expansion, temperature, crystal volume temperature A dilatation B viscoelastic relaxation delay C pressure dominant D cooling dominant E F decahydrate crystals volume heptahydrate transformation Time Fig. 7.2: Schematic representation of the expansion, temperature of and volume of crystals in a sample saturated with a sodium sulfate solution during a cooling. Four experimental phases can be distinguished: Stage I - cooling of a sample; Stage II - isothermal crystallization of heptahydrate; Stage III - non-isothermal crystallization of heptahydrate, Stage IV - transformation from heptahydrate to dehydrate.
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Sodium sulfate heptahydrate in weat
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To my family
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CONTENTS 1. Introduction . . . . .
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1. INTRODUCTION 1.1 Introduction Sa
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1. Introduction 11 This phenomenon
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1. Introduction 13 1 2 initial mass
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1. Introduction 15 However, our pre
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2. SINGLE CRYSTAL GROWTH OF SODIUM
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2. Single crystal growth of sodium
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Page 26 and 27: 2. Single crystal growth of sodium
Page 32 and 33: 3. CRYSTALLIZATION UNDER WETTING/NO
Page 34 and 35: 3. Crystallization under wetting/no
Page 44 and 45: 4. NUCLEATION ON MINERAL SUBSTRATES
Page 46 and 47: 4. Nucleation on mineral substrates
Page 52 and 53: 5. SODIUM SULFATE HEPTAHYDRATE FORM
Page 54 and 55: 5. Sodium sulfate heptahydrate form
Page 62 and 63: 6. CRYSTALLIZATION OF SODIUM SULFAT
Page 64 and 65: 6. Crystallization of sodium sulfat
Page 72 and 73: M 6. Crystallization of sodium sulf
Page 74 and 75: 7. THERMODYNAMIC AND POROMECHANIC C
Page 78 and 79: 7. Thermodynamic and poromechanic c
Page 92 and 93: 8. NMR COMBINED WITH ACCELERATED WE
Page 94 and 95: 8. NMR combined with accelerated we
Page 96 and 97: ∆ 8. NMR combined with accelerate
Page 106 and 107: 9. CONCLUSIONS AND OUTLOOK This cha
Page 108 and 109: 9. Conclusions and Outlook 107 Soft
Page 110 and 111: APPENDIX: CALCULATION OF SALT CRYST
Page 112 and 113: Appendix: Calculation of salt cryst
Page 114 and 115: Bibliography 113 [15] L.M. Luquer,
Page 116 and 117: Bibliography 115 [47] D. Rosenblatt
Page 118 and 119: Bibliography 117 [78] H.P. Huinink,
Page 120 and 121: LIST OF PUBLICATIONS Published: •
Page 122 and 123: ACKNOWLEDGMENT This thesis would no
Page 124: CURRICULUM VITAE Tamerlan Saidov wa
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