Quantitative characterization of three-dimensional pore structure in hardened cement paste

Significance 

According to statistics, concrete, a cement-based material, is the second most consumed substance globally -behind water. Most cement-based materials are comprised of liquid, solid and gaseous phases whose properties are strongly correlated with the performance of modern concrete. Various properties, such as porosity, directly affect the mechanical performance as well as transport properties of concrete, and are therefore vital. Consequently, characterization of concrete is crucial as it helps assess the performance of concrete. So far, several characterization techniques have been devised, among which mercury intrusion porosimetry is the most widely applied. Nonetheless, this technique has some inherent drawbacks that limit its effective use. Another suitable method is the X-ray computed tomography (CT), which still does not provide satiable results. Particularly, the resolution of the ordinary X-ray CT and micro X-ray CT used are not enough for micro pore scale research. Therefore, it is imperative that techniques that can offer high contrast as well as high precision to image 3D pore structures in cement-based materials be developed.

Recently, a team of Southeast University scientists’ researchers: Dr. Rusheng Qian, Prof. Yunsheng Zhang (Corresponding author), Dr. Guojian Liu, Dr. Wei She in collaboration with Dr. Cheng Liu at University College London and Dr. Lin Yang at Zhengzhou University conducted a study focusing on applying a novel contrast enhanced X-ray microtomography technique, for the first time ever, to quantitatively characterize 3D pore structure in cement pastes. Additionally, they advanced a new threshold value segmentation method for pore space, based on region of interest combined with volume compensation factor. Their work is currently published in the research journal, Materials Characterization.

The authors adopted a centrifuge device with 4000 RPM that could facilitate the intrusion of a low-melting point (65℃) metal alloy into the pore space of hardened cement paste with cavity formation shaped by centrifuge tube. X-ray microtomography was combined with metal centrifugation porosimetry (XCT-MCP) so as to quantitatively investigate 3D pore structure. The low-melting-point metal alloy was melted and introduced into pore space in pastes with water cement ratio of 0.5 and 1.0 at a temperature of 65 °C. Lastly, the researchers quantitatively observe and analyzed the actual 3D pore structure by X-ray microtomography after the molten metal alloy had been consolidated. Detailed procedure was shown in Fig. 1.

It was observed that the porosity determined by XCT-MCP was close to MIP for the samples examined and the relative errors in the range of 5.27–10.05% were found in the values given by XCT-MCP and mercury intrusion porosimetry (MIP). Generally, the contrast between pore space and solid phase in X-ray microtomography imaging was improved due to the intrusion of the low melting-point metal alloy.

In summary, the study presented the application of microtomography combined with metal centrifugation porosimetry using a novel metal alloy, to quantitatively investigate 3D pore structure in hardened cement paste. It was established that the proposed XCT-MCP method is a powerful and reliable technique for studying the 3D pore structures in cementitious materials, and it could be speculated that future experiments could be carried out on the other materials.

Quantitative characterization of three-dimensional pore structure in hardened cement paste using X-ray microtomography combined with centrifuge driven metal alloy intrusion - Advances in Engineering
Fig. 1 The method flow chart

About the author

Zhang Yunsheng is TC2 Chair of Asian Concrete Federation, members of American Concrete Society (ACI) and RELM and a professor in Structural Engineering from Southeast University (SEU) Nanjing, P.R.China.

His research areas include High performance and ultra high performance cement based composites, Durability and service life prediction of concrete subjected to the action of combined several deterioration factors, Utilization of industrial residues in concrete, etc. Successfully secured at least 20 projects from China National Natural Science Fund project and projects from Ministry of Education and Ministry of Science and Technology of China in recent 10 years, Prof. Zhang had published over 250 papers published in international and national journals and been awarded First grade for the science and technology progress in China building material of and Second grade for the science and technology progress from China government.

Reference

Rusheng Qian, Yunsheng Zhang*, Cheng Liu, Lin Yang, Guojian Liu, Wei She. Quantitative characterization of three-dimensional pore structure in hardened cement paste using X-ray microtomography combined with centrifuge driven metal alloy intrusion. Materials Characterization volume 145 (2018) page 277–283.

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