Investigation On Mass Transport And Transfer Processes On The Fractured Surface Of Cement-based Materials

Cement-based materials are most used building and construction materials. Due to external loading or internal processes, cracking can develop easily in the materials. The existence of cracks has important influence on the durability and performance of engineering structures. Cracks change substantially the permeability of material, and the fracture surfaces of cracks have important specific surface and chemical reactivity, thus can lead to a series of physical and chemical processes that deteriorate the stability of materials and accelerates the degradation processes of materials and structures. The fundamental question here is the transport and mass exchange of fluids in fractures of cement-based materials. It involves fluid mechanics, surface behavior and physico-chemical change of cement-based materials.This thesis studies the physical and chemical processes between the flowing fluid and the fracture surfaces and the influence of these processes on the topography of fracture surfaces. The physical processes include the flow behavior of liquids in fractures with rough surfaces, the leaching effect of flowing water as well as the surface adsorption of ions in flowing solution. The chemical process includes the secondary hydration of cement grains exposed on the fracture surfaces. This thesis firstly chooses several geometry parameters to characterize the surface roughness of fracture surfaces of cement-based materials; then a self-designed permeation device is employed to study the flow behaviors of liquids and the physico-chemical processes and disk specimens of several cement-based materials are used. Lastly, a mass transport and exchange model is established and the process-relevant parameters are determined with the obtained experimental data. Special attention is given to the surface roughness change by the above processes.From the obtained results in this thesis, it is observed that the liquid flow between rough fracture surfaces of cement-based materials shows strong time dependence and liquid flow decreases substantially with time. It is proved that this time dependence is a purely mechnical effect of fracture surface roughness and the flow behavior does not observe Darcy’s law. For surface leaching process, it is observed that the surface leaching by flowing water has much higher leaching rate than the surface leaching process in static water. The reason lies in the fact that flowing water takes away the dissolved calcium ions from dissolution sites constantly and thus accelerates the surface dissolution process. Moreover, the surface leaching decreases the surface roughness of mortar surfaces but increases the surface roughness of cement paste. For surface adsorption process, it is observed that the ion adsorption capacity of fracture surface is deceased by surface flow and surface adorption achieves its maximum capacity in static ion solution. The explanation is that the flow decreases the thickness of ion adsorption layer at surface thus the adsorption quantity of ions is decreased. For surface secondary hydration, the chemical process changes substantially the permability of the single crack as well as the surface roughness.