| Superplasticizer has become one of the key components in modern concrete. Polymer latex is often used as cement mortars modifier to improve properties such as flexural strength, water proofing and so on. Cement asphalt mortar consisting of Portland cement and asphalt as co-binders, serves as a vibration-absorbing layer in the slab track system of railroad structures. Superplasticizers, polymer latexes and asphalt emulsions are regarded as chemical admixtures in this thesis. Their addition has substantially changed the workability of cement-based materials. However, fundamental research on their effects and working mechanisms largely lags behind industry development and engineering application, which has become the neck bottle for further development of new chemical admixtures and cement-based composites with higher performance.Rheological properties are major elements of the workability of cement-based materials and closely related to the microstructure. This thesis aims at revealing the correlation of rheological properties with microstructure of cement-based materials. Superplasticizers, polyacrylate latexes and asphalt emulsions, which differ in molecular/particle size from nanometer to micron, were employed to study their effects on the rheological properties of fresh cement pastes(fcps) and working mechanism. Fluidity, yield stress and plastic viscosity of fcps at various superplasticizer to cement ratio(Sp/C), water to cement ratio(W/C), temperature(T) and elapsed time(t) were measured. The viscoelastic properties were also estimated by a microrheometer. Optical microscope was used to directly observe the microstructure of fcps and the correlation of microstructure with fluidity of fcps was established. Polymers with different molecular structures were chosen to investigate their adsorption by total organic carbon, zeta potential tests and their retardation effects by isothermal calorimetry, in order to disclose their working mechanisms. Based on the microstructure and the abovementioned mechanism, a conceptial model was proposed. Relative hydration degree was introduced to indicate the evolution of rheological properties and two models were developed. Rheological equations including Sp/C, W/C, T and t were also deduced. The effects of chemical admixtures on the pore structure and impermeability of hardended cement paste were elucidated.Results indicate that chemical admixtures change the rheological behaviors through the disassembly of flocculated cement grains by their adsorption. The addition of latexes and asphalt emulsions could increase the volume fraction of solid phase and further affect the rheological properties. As a reactive system, fcp undergoes a continuous change on the microstructure due to hydration and consequently the rheological properties continuously develop over time. The changes of rheological properties with T and t originate from the development of hydration degree. Moreover, the stability of latex and asphalt emulsion also results in the changes of rheological properties.The main contributions of this thesis are summarized as follows. 1) The effects of W/C, Sp/C, T and t on the rheological behaviors of fcps were investigated and were formulated into two rheological equations. 2) The microstructure of fcps was analyzed quantitatively, and the correlation of microstructure with rheological properties was established. 3) The adsorption behaviors of polycarboxylate polymers with different functional groups and their effects on cement hydration were elucidated. 4) The different working mechanism of NSF from that of PCE superplasticizer was revealed. 5) Relative hydration degree was introduced to indicate the coupled effects of the temperaure and time on rheological properties. 6) A conceptual microstructure-rheology model was developed. All of these new findings may lend theoretical supports to the development of new chemical admixtures and new cement-based composites with higher performance. |
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