Cement-superplasticizer interaction link between macroscopic phenomena and microstructural data of the early cement hydration

Superplasticizers are commonly used for industrial high performance concrete applications such as self compacting concrete, high strength con crete and prefabricated concrete elements. They are added in order to control the workability of concretes, mortars and cement suspensions. Enhanced workability properties are as sociated with undesired delay of hardening. This study aims to investigate the influence of a p olycarboxylate-ether based superplasticizer (PCE) with different architectures on the rheological properties of cement suspensions. Thereby, a special emphasize is used o n the investigation of the microstructures in the cement suspension. The thesis contains three main foci: i) Investigation of various PCE architectures on the w orkability properties of cement suspensions. ii) Application of a non-destructive sample preparation echnique for (cement) suspensions, qualitative and quantitative microscop ic characterization of particulate structures in cement suspensions. iii) Experiments on the interaction of pure mineral phas es of cementitious materials with PCEs with different architecture. The study on the PCE influence on plain cement susp ensions shows that in terms of workability, the charge density of the PCE is of ma jor importance. In general, higher charged PCE has a stronger adsorption ability and results i n enhanced workability properties. However, the stronger the PCE-cement interaction, t he s ronger is the setting and hardening of cement suspensions delayed. This delay is an und esire side effect, which can induce serious problems on a construction site. Thus, for an optimization of the PCE architecture, both effects, fluidization and retardation have to be taken into account. Enhanced workability and associated rheological pro pe ties are closely related to the microstructural evolution of cement suspensions. Ho wever, due to the lack of suitable sample preparation, microscopic investigations on long ran ge particle structures were not possible up to date. Therefore, in this study a major aim has b een the modification of cryo sample preparation from life sciences to cement suspension s. High-pressure freezing as used in this study enables the preservation of long range partic le arrangements in cement suspensions. Additionally, the FIB nanotomography (focused ion b eam) has been modified for cryoexperiments. The successfully implemented cryo-fixa tion and cryo-FIB enables qualitative descriptions of microstructural phenomena and their quantification. Cryo-FIB cross sections illustrate the microstructu ral difference of a non-dispersed and a dispersed cement suspension containing PCE for the first time. Furthermore, this study demonstrates that in plain cement suspensions the h ydrate phases tend to deposit on clinker surfaces, whereas in presence of PCE the hydrate pa r icles remain dispersed in the pore solution. Additionally, high resolution cryo-SEM im ages enables to investigate microstructural features below 100 nm such as e.g. early C-S-H crystals and diffuse structures such as C-S-H gels and PCE networks. However, the cement-water system is very complex. F or the development of new generations of superplasticizers, it is important t o understand the mechanism of interaction between superplasticizer molecules and the major mi neral components in cement suspensions. Therefore, tricalciumsilicate, calcium silicate hyd rate and ettringite has been synthesized and their PCE adsorption ability, rheological behaviour and change of zeta potentials in presence of PCE has been recorded in an synthetic pore solut ion environment (as preset in real cement pore solutions). This study demonstrates that the e tringite is the major mineral phase which strongly controls the overall surface properties an d ssociated particle interactions and thus also the workability of cement suspensions. The pro duction of ettringite during early hydration is governed by the tricalciumaluminate co ntent and the availability of sulfate. This thesis shows the importance of the link betwee n macroscopic data and the microstructural development of cement suspensions i n order to get a more complete picture about the mechanisms of PCE-cement interaction. Fur thermore, the thesis points out the importance of high-pressure freezing and the power of cryo-microscopy concerning qualitative and quantitative descriptions of partic ulate structures. The latter is just about to commence.