Interactions between Cement and Combined Concrete Admixtures. The Influence on Cement Paste Rheology (Interacties tussen cement en gecombineerde hulpstoffen. De invloed op de reologie van cementpasta)

During the last decades, innovative concrete applications have been theresult of technological developments as new processing techniques, the use of concrete admixtures and advanced materials. All these improvements increase the demand for better performing concrete with enhanced workability. However, the lack of reliable engineering tools in traditional concrete production methods has led to some inconsistent control of the workability. Therefore, today s research is increasingly investigating advanced models that capture the flow behavior of fresh concrete as accurately as possible. These models are continuously improving butgenerally keep facing problems which are linked to the agglomeration ofthe cement particles and to the implementation of combined concrete admixtures. The objective of this work is to investigate the rheological mechanism in cement paste for the combined use of superplasticizer, retarder and accelerator. In particular, an effect on the particle agglomeration is aimed for in order to contribute to a more fundamental understanding of the concrete s flow behavior. Concerning the superplasticizer impact on cement paste, thermodynamic modelling and mineralogy studies led to the conclusion that the superplasticizer can change the hydrate morphology to such an extent that the interparticle contact forces are modified. The extra addition of retarder led to a densified polymer layer at the cement grain surface which contributed to the steric stabilization of the cement paste. An additional electrostatic effect and the induced changes in hydrate morphology are also demonstratedto contribute to the low paste viscosity. With the extra addition of a calcium salt accelerator to the cement paste, an interstitial structure was assumed to diminish the rheological effect of the polymer layer. All the developed conceptual mechanisms were implemented in a coherent agglomeration model, based on measurable parameters. In this model, the internal and external hydrates in a cement agglomerate were defined and quantified. Generally, the cumulative amount of hydrates within the agglomerates influenced the agglomerate stability and the amount of external hydrates determined the reagglomeration rate. For the superplasticizer only, a proportional relation was found between the change in agglomeration rate and the external agglomerate connectivity while, for the extra addition of retarder and accelerator, a reverse relation was found. The latter was attributed to a mechanical contribution of the interstitial volume. On the one hand, the retarder creates a source of slowly hydratingnuclei and particles in that volume and, on the other hand, these particles are expected to coagulate due to the accelerator addition. In the second case, the interstitial volume also delivered a contribution to thestress resistance of the cement paste at rest. This fundamental research combines dedicated analytical methods and conceptual models to improve the understanding of particle agglomeration and contributes to a more extensive insight into the concrete rheology.Contents Acknowledgement ........................................................................................ iii Abstract ......................................................................................................... v Samenvatting ................................................................................................ vii Nomenclature ................................................................................................ ix Contents ........................................................................................................ xi Chapter 1 - General introduction .................................................................. 1 1.1 Subject of the research............................................................................ 1 1.2 Research strategy .................................................................................... 5 1.3 Outline ..................................................................................................... 6 Chapter 2 - State of the art............................................................................ 9 2.1 Introduction to concrete rheology .......................................................... 9 2.2 Cement and admixtures ........................................................................ 13 2.2.1 Cement ........................................................................................... 13 Hydration of clinker minerals .............................................................. 14 Hydration of Portland cement ............................................................. 19 Rheologically important ions and phases ............................................ 22 2.2.2 Admixtures ..................................................................................... 25 Superplasticizer ................................................................................... 26 Retarder ............................................................................................... 28 xii _________________________________________________________ Accelerator .......................................................................................... 31 2.2.3 Summary ......................................................................................... 33 2.3 Cement paste rheology ......................................................................... 34 2.3.1 Forces within the cement/water/SP system .................................. 35 Forces in-between particles ................................................................ 35 Suspension related forces ................................................................... 38 2.3.2 Rheological parameters .................................................................. 40 Cement particle size distribution ......................................................... 41 Adsorption ........................................................................................... 43 2.3.3 Combined admixtures .................................................................... 50 2.3.4 Incompatibility ................................................................................ 52 2.4 Modelling cement paste rheology ......................................................... 55 Agglomeration kinetics ........................................................................ 58 Agglomerate connectivity .................................................................... 62 Chapter 3 - Materials and methods ............................................................. 67 3.1 Material properties and mixing procedures .......................................... 67 3.1.1 Cement ....................................................................................... 67 3.1.2 Admixtures.................................................................................. 71 3.1.3 Mixing procedure ....................................................................... 72 3.2 Analytical techniques ............................................................................. 73 3.2.1 Rheometry .................................................................................. 73 3.2.2 Cement physico-chemistry ......................................................... 78 Chapter summary .................................................................................... 81 Chapter 4 - The influence of superplasticizer on cement paste rheology... 83 4.1 Experimental observations of the superplasticizer influence ............... 83 4.1.1 Hydration kinetics ........................................................................... 84 4.1.2 Structure development .................................................................. 86 4.1.3 Flow behavior ................................................................................. 87 _________________________________________________________ Contents xiii 4.2 At the origin of different rheological performance induced by superplasticizer ........................................................................................... 89 Thermodynamics and mineralogy ....................................................... 91 Sensitivity factors of cement ............................................................... 98 Summary............................................................................................ 100 4.3 Relating rheology and hydration kinetics through an agglomeration model ......................................................................................................... 102 Anti-thixotropic histogram ................................................................ 102 Internal and external hydrates concept ............................................ 105 Quantifying agglomeration parameters ............................................ 108 Summary............................................................................................ 118 Chapter summary ...................................................................................... 118 Chapter 5 - The influence of retarder on the plasticized cement paste rheology ..................................................................................................... 121 5.1 Experimental observations of the retarder influence ......................... 121 5.1.1 Hydration kinetics ......................................................................... 122 5.1.2 Structure development ................................................................ 123 5.1.3 Flow behavior ............................................................................... 124 5.2 At the origin of the retarder influence ................................................ 126 5.2.1 Paste viscosity .............................................................................. 127 5.2.2 Hydration rate .............................................................................. 129 5.2.3 Interparticle forces ....................................................................... 131 5.2.4 Contact forces .............................................................................. 138 Summary ............................................................................................... 143 5.3 The influence of the retarder in the agglomeration model ................ 144 5.3.1 Agglomeration parameters .......................................................... 145 5.3.2 Micromechanical mechanism....................................................... 147 5.3.3 Agglomeration diagram ................................................................ 151 xiv _________________________________________________________ Summary ................................................................................................ 153 Chapter summary ...................................................................................... 153 Chapter 6 - The influence of accelerator on the retarded plasticized cement paste rheology ........................................................................................... 155 6.1 Experiments ......................................................................................... 155 6.1.1 Hydration kinetics ......................................................................... 156 6.1.2 Structure development ................................................................ 157 6.1.3 Flow behavior ............................................................................... 158 6.2 At the origin of the accelerator influence on retarded paste ............. 160 Summary ................................................................................................ 164 6.3 The influence of the accelerator in the agglomeration model ............ 165 6.3.1 Agglomeration parameters .......................................................... 165 6.3.2 Micromechanical mechanism ....................................................... 167 6.3.3 Summarized agglomeration diagram ........................................... 168 Chapter summary ...................................................................................... 170 Chapter 7 - Conclusions ............................................................................. 173 7.1 Identified mechanisms ........................................................................ 173 7.2 Conceptual modelling .......................................................................... 174 7.3 Original contributions .......................................................................... 176 7.4 Outlook ................................................................................................ 177 Appendix A................................................................................................. 181 Appendix B ................................................................................................. 183 References ................................................................................................. 185 List of publications ..................................................................................... 201 Articles in internationally reviewed academic journals ............................ 201 Papers at international scientific conferences .......................................... 201nrpages: 218status: publishe