The selection of the incorrect value of contact angles is one of the important error sources in calculating the pore size in mercury porosimetry studies. However, from literature, it is difficult to find out an appropriate technique, which can be used to measure both advancing and receding contact angles. In this paper, a new method is proposed to measure the advancing and receding contact angles between mercury and cement-based materials. This method is demonstrated with a measurement on Portland cement paste with the water-to-cement ratio of 0.3. This method can give very consistent results with the standard deviation lower than 1 %. The measured advancing and receding contact angles range from 131.7° to 138.6° and from 123.3° to 128.2°, respectively, as curing age increases from 3 days to 28 days.
For accurately predicting the service life and evaluating the durability of reinforced concrete structure exposed to chloride environments, it is highly desirable to determine the chloride diffusivity in cement paste. Because of continuous cement hydration and chloride binding during the process of chloride diffusion, chloride diffusivity varies with time. In this paper, a computational approacht for predicting the time-depending chloride diffusivity in cement paste is presented. HYMOSTRUC3D, a computer-based cement hydration model, is applied to generate the 3D microstructure of cement paste. Both of the cement hydration process and chloride binding are taken into account in the simulation of microstructure. Finite element method is applied to simulate the diffusion of chloride ions through the microstructure of cement paste and estimate the chloride diffusivity based on Fick's law. A series of statistical analysis are carried out to determine the representatieve elementary volume (REV) of cement paste. The dependences of chloride diffusivity on time and w/c ratio are investigated. Finally, the simulations are validated with the experimentally measured values obtained from the literature. The comparison indicates that the simulated values and measured vlaues are of the same order of magnitude. Moreover, the trend (shape) of simulated relationship (chloride diffusivity vs time, chloride diffusivity vs w/c ratio) fits very well with the experiments.
X-ray computed microtomography (micro-CT) was utilized to obtain three- dimensional (3D) images of the hydrating of
cement paste with water-to-cement (w/c) ratio at different ages, i.e., 1, 3, 7 and 28 days. On the basis of the gray level
histogram of microtomography images, the phase threshold values for each specimen were determined. The component
phases of the cement paste such as pores, hydration products and unhydrated cement particles were identified from
each other. The volume fraction of pore and degree of hydration of the specimens at different curing ages were
estimated from the image analysis and compared with the experimental data measured by mercury intrusion
porosimetry (MIP) tests and loss-on-ignition (LOI) tests respectively. Furthermore, the degree of pore connectivity was
analyzed based on cluster-labelling technique. Finally, transport properties, e.g. diffusivity of tritiated water of each
specimen was estimated and compared with the experimental results obtained from the literature. The results derived
from image analysis show a reasonably agreement with the measured results, which indicates that micro-CT is a
reliable and suitable technique to characterize the microstructure evolution of hydrating cement paste. The obtained
microstructure can be used to estimate the transport properties of cement-based materials.
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