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Thermodynamic Modelling of Cao- Al2o3-Sio2-H2O-Based Cements Rupert Jacob Myers

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Thermodynamic Modelling of Cao- Al2o3-Sio2-H2O-Based Cements Rupert Jacob Myers Thermodynamic Modelling of CaO- Al2O3-SiO2-H2O-Based Cements Rupert Jacob Myers Supervisors: Prof. John L. Provis Dr. Susan A. Bernal Dr. Barbara Lothenbach A thesis submitted in total fulfilment of the requirements for the degree of Doctor of Philosophy Department of Materials Science & Engineering The University of Sheffield May 2015 Abstract Most concrete is produced using calcium (alkali) aluminosilicate hydrate (C-(N-)A- S-H)-based cement. However, the chemistry of this phase in many cement-based materials is still not fully understood. This thesis presents a structural and thermodynamic investigation of C-(N-)A-S-H and C-(N-)A-S-H-based cements to provide insight into the chemistry of these materials. A mixed cross-linked and non-cross-linked tobermorite-like structural model for C- (N)-A-S-H is developed (the CSTM), which more appropriately describes the spectroscopic information available for this phase. Application of the CSTM to a Na2SiO3-activated slag cement cured for 56 and 180 days indicates the presence of a poorly-crystalline zeolite-like phase. The role of Al in cross-linking of C-(N-)A-S-H is also studied, which provides a more advanced description of the chemistry and structure of C-(N-)A-S-H than previously reported. A thermodynamic model for C-(N-)A-S-H (CNASH_ss) is derived, which greatly advances the utility of thermodynamic modelling of C-(N-)A-S-H-based cements by explicitly defining Al and alkali uptake in this phase. The chemistry of alkali- activated slag (AAS)-based cements is simulated using CNASH_ss and an ideal solid solution thermodynamic model for MgAl-OH-LDH that is also developed in the thesis. This analysis provides a good description of Na2SiO3-activated slag cement chemistry and accurately predicts chemical shrinkage in this material. Phase diagrams for NaOH, Na2SiO3, Na2Si2O5 and Na2CO3-activated slag-based cements are also simulated. These results can be used to design the chemistry of AAS-based materials. III IV Abstract A detailed analysis of C-(N-)A-S-H solubility is presented, for Ca, Al, Si and alkali concentrations most relevant to C-(N-)A-S-H-based cements and at temperatures of 7-80°C. Solubility products for alkali-free C-(N-)A-S-H change slightly between 7°C and 80°C and as a function of Al/Si ratio. However, less soluble C-(N-)A-S-H is formed at higher Ca and alkali content. These results are important for understanding the stability of C-(N-)A-S-H in the majority of cement-based materials used worldwide. Table of Contents Abstract ...................................................................................................................... III Table of Contents ........................................................................................................ V Preface ........................................................................................................................ XI Declaration .............................................................................................................. XIII Acknowledgements .................................................................................................. XV List of Tables and Figures ...................................................................................... XVII List of Tables ...................................................................................................... XVII List of Figures ................................................................................................... XXIV Nomenclature ........................................................................................................ XLIII 1 Introduction ........................................................................................................... 1 2 Literature Review ................................................................................................ 11 2.1 Cement Chemistry ....................................................................................... 11 2.1.1 Portland Cement-Based Materials ........................................................ 11 2.1.2 Alkali-Activated Slag (AAS) Cement .................................................. 12 2.1.3 Calcium (Alkali) Aluminosilicate Hydrate (C-(N-)A-S-H) ................. 15 2.1.4 Structural Models for C-(N-)A-S-H ..................................................... 23 2.1.5 Secondary and Minor Cement Hydrate Phases .................................... 29 2.2 Thermodynamic Modelling ......................................................................... 33 2.2.1 Applications in Cementitious Systems ................................................. 33 2.2.2 General Concepts ................................................................................. 34 2.2.3 Solid Solutions ..................................................................................... 37 V VI Table of Contents 2.2.4 Thermodynamic Models for C-A-S-H ................................................. 39 2.2.5 Thermodynamic Models for C-S-H ..................................................... 39 2.2.6 Thermodynamic Data for Secondary and Minor Cement Phases ........ 46 2.3 Conclusions ................................................................................................. 47 3 Materials and Methods ........................................................................................ 49 3.1 Introduction ................................................................................................. 49 3.2 Materials ...................................................................................................... 49 3.2.1 Alkali-Activated Slag (AAS) Cement.................................................. 49 3.2.2 Laboratory-Synthesised C-(N-)A-S-H ................................................. 50 3.3 Experimental Techniques ............................................................................ 51 3.3.1 X-ray Diffraction (XRD) and Rietveld Analysis ................................. 51 3.3.2 Scanning Electron Microscopy (SEM) ................................................ 53 3.3.3 29Si Magic Angle Spinning Nuclear Magnetic Resonance (MAS NMR) .............................................................................................................. 54 3.3.4 27Al MAS NMR ................................................................................... 57 3.3.5 Thermogravimetric Analysis (TGA) .................................................... 61 3.3.6 Ion Chromatography (IC) and pH Analysis ......................................... 61 3.4 Thermodynamic Modelling ......................................................................... 62 3.4.1 Modelling Method ............................................................................... 62 4 Cross-Linked Substituted Tobermorite Model (CSTM) ..................................... 69 4.1 Introduction ................................................................................................. 69 4.2 Derivation of a Generalised Structural Model for C-(N-)A-S-H ............... 71 4.2.2 Non-Cross-Linked Tobermorite-Like Structural Model ...................... 71 4.2.1 Cross-Linked Tobermorite-Like Structural Model .............................. 73 4.2.3 ‘Cross-Linked Substituted Tobermorite Model’ (CSTM) .................. 76 4.3 Application of the CSTM ............................................................................ 80 Table of Contents VII 4.3.1 Characterisation of an Alkali-Activated Slag (AAS) Cement ............. 80 4.3.2 An Additional Aluminosilicate Reaction Product? .............................. 89 4.4 Conclusions ................................................................................................. 96 5 Nanostructural Analysis of Na2SiO3-Activated Slag Cement ............................. 99 5.1 Introduction ................................................................................................. 99 5.2 Experimental ............................................................................................. 100 5.3 Results and Discussion .............................................................................. 101 5.3.1 X-ray Diffraction ................................................................................ 101 29 5.3.2 Si MAS NMR .................................................................................. 102 27 5.3.3 Al MAS NMR ................................................................................. 112 5.3.4 Characterisation of the C-(N-)A-S-H Gel .......................................... 122 5.3.5 Perspectives ....................................................................................... 126 5.4 Conclusions ............................................................................................... 127 6 Thermodynamic Model for C-(N-)A-S-H: Derivation and Validation ............ 129 6.1 Introduction ............................................................................................... 129 6.2 Sublattice Solid Solution Model for C-(N-)A-S-H ................................... 130 6.2.1 Sublattice Solid Solution Definition .................................................. 130 6.2.2 End-Member Selection ....................................................................... 137 6.2.3 Thermodynamic Basis of the Sublattice Solid Solution Model ......... 142 6.3 Modelling Method ..................................................................................... 146 6.3.1 Modelling System Definition ............................................................
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