Capillary Absorption Of Cementitious Materials And Its Quantitative Relationship With Pore Structure

As a prominent problem of concrete structure in real project,durability is not only related to the construction of economic environment and the efficient use of energy resources,but also has an important impact on the safe operation of human society and the sustainable development of economy.The existence and transportation of water is a key factor and necessary condition that lead to the durability problems of cementbased structure.On the one hand,water is a necessary carrier for harmful ions to enter the concrete structure.On the other hand,the existence of water in concrete is closely related to other deterioration behaviors of concrete structure,including some chemical damage processes,like carbonation,and physics damage process,like freeze-thaw damage.Capillary absorption,infiltration and diffusion are the main ways of water transmission.Because most of the projects are in unsaturated state,capillary absorption is the main way for moisture to transport in cement-based materials.Therefore,a better understanding of capillary absorption process has great significance for the durable research of cement-based materials.For porous materials,pore structure plays a critical role in the process of capillary absorption.So,this paper establishes an inversed model based on classical capillary absorption theory,Lucas-Washburn equation.The features of capillary absorption of cement paste are measured by X-ray tomography(XCT),using to calculate the pore structure of cement paste samples via the proposed inverse model.(1)This thesis expounds the theory of capillary absorption of porous materials and summaries the state of art and testing methods of capillary absorption.It also reviews the application of XCT technology in cementitious materials.(2)The capillary absorption process of cement paste sample is tracked continuously and non-destructively by XCT technology.The images and evolution process of capillary absorption were obtained.(3)The position of capillary absorption front was determined through average grave value method.The capillary absorption law of cement paste samples with different water to cement ratio was analyzed and the relationship between the average grave value and the mass of absorbed water is established.With this relationship,the average grave value curve is converted to the curve of water spatial distribution in the sample.The accuracy of capillary absorption features measured by XCT technology is verified by traditional spilt test and titration test respectively,which demonstrates the reliability and applicability of using XCT method to investigate the capillary water absorption phenomenon of cement-based materials.(4)Based on the information of water absorption front and the spatial distribution water in the sample measured by XCT technology,the pore structure of the sample is derived via the inverse model.The pore structure obtained from the proposed model is compared with the results obtained from the mercury intrusion porosimetry(MIP)to verify the validity of the model.