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This thesis deals with the rheology of cement-based suspensions with a particular focus on the effect of the polycarboxylate ether (PCE) superplasticizer structure on the thixotropic behaviour. Thixotropy is critical to the properties of the fresh and workable state of cementitious materials, as it governs the processing behaviour of cement-based suspensions and influences their subsequent stages up to the final hardened state. With modern challenges in concrete technology, such as reducing the environmental impact of cement using fine supplementary cementitious materials and introducing advanced technology such as 3D concrete printing, this property has become increasingly important. To investigate the effect of PCE, seven non-commercial PCE superplasticizers, each of which differs in their anionic charge ratio and side chain length, were used. The experiments were conducted in an application-oriented concept of constant workability, i.e. slump flow, by adjusting the individual PCE dosage. The first experimental focus is on investigating the effect of PCE side chain length and anionic charge ratio on thixotropy in ordinary Portland cement (OPC) pastes. A more pronounced thixotropy is found with shorter PCE side chains and higher anionic charge ratios. This phenomenon is correlated with faster early hydration kinetics and more ettringite precipitation, which increases the specific surface area. Moreover, the effect of PCE is investigated under different boundary conditions, including the timing of PCE addition, w/b-ratios, and the comparison of OPC with low-clinker cement (LCC). The results highlight that the PCE addition mode has a significant effect on thixotropy. Direct addition often increases thixotropy, especially at high PCE charge densities, due to amplifying early hydration kinetics. Conversely, delayed addition results in less significant changes. With respect to the w/b-ratio, it is evident that the differences in thixotropy between the polymers are more pronounced at lower ratios. When comparing OPC and LCC, the same trends and correlations of the effect of the polymer structure on thixotropy can be seen regardless of the type of cement. This work presents an analytical model to identify the underlying physicochemical modifications induced by the PCE polymer structure, examining the role of PCE in both colloidal and contact interactions and their consequences on thixotropy. This part of the thesis highlights the key role of initial hydration kinetics and ettringite formation in the context of adsorption by modifying the surface area as well as increasing particle contacts due to additional small particles. Moreover, the findings are validated at the mortar level and compared with the results at the paste level. This comparison is important because the presence of aggregates in the mortar introduces different boundary conditions due to the non-deformable nature of the aggregates. The latter leads to increased shear concentration in the paste, affecting rheology and hydration kinetics. The results of the experiments suggest that the thixotropy in both the mortar and the paste is influenced similarly by the structure of the PCE and its mode of addition modifying colloidal and contact interparticle interactions.