| 1. |
- Li, Yongqiang, 1992, et al.
(author)
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Assessment of compositional changes of carbonated cement pastes subjected to high temperatures using in-situ Raman mapping and XPS
- 2022
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In: Journal of Building Engineering. - : Elsevier BV. - 2352-7102. ; 45
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Journal article (peer-reviewed)abstract
- This paper presents a new method for assessing the compositional changes of carbonated cement pastes subjected to high temperatures. In this new method, in-situ Raman mapping combined with X-ray photoelectron spectroscopy (XPS) was used to monitor the phase transformation in carbonated cement pastes subjected to various high temperatures from 30 to 950 degrees C. Two kinds of carbonated areas, i.e., vaterite dominated and calcite dominated, were found in the in-situ Raman measurements. With the elevation in temperature, most of the vaterite was converted to calcite at 500 degrees C and completely decomposed at 600 degrees C, while the decomposition of calcite started at 600 degrees C and finished at 720 degrees C. Meanwhile, the depolymerization of the calcium modified silica gel to the silicate phases with a lower degree of polymerization was initiated at 500 degrees C, which led to the crystallization of beta-C2S at 600 degrees C. The generation of beta-C2S was found to increase with the elevation in temperature and became the dominant phase at 950 degrees C. In conclusion, the high temperature could affect the stability of carbonated cement pastes at 500 degrees C and above. The in-situ Raman mapping measurement has provided an extraordinary view of the spatial distribution of interesting phases subjected to high temperatures in a nondestructive way, which should be more consistent with the true condition in the material.
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| 2. |
- Li, Yongqiang, et al.
(author)
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Chemical and mineralogical characteristics of carbonated and uncarbonated cement pastes subjected to high temperatures
- 2021
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In: Composites Part B: Engineering. - : Elsevier BV. - 1359-8368. ; 216
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Journal article (peer-reviewed)abstract
- The fire-resistance of carbonated concrete under high temperatures is significant due to its direct exposure during an accidental fire. To evaluate the carbonation effect on fire-resistance of concrete, the chemical and mineralogical changes of uncarbonated and carbonated cement pastes subjected to high temperatures were thoroughly investigated in this research by employing micro-measurement methods including thermal-gravimetric analysis (TGA), X-ray diffraction (XRD) and Si nuclear magnetic resonance ( Si NMR). Uncarbonated cement paste results showed the decomposition of portlandite at 400 °C with the formation of lime, whilst the depolymerization of C–S–H happened simultaneously to generate monomeric silicon tetrahedron. Above 720 °C, all the C–S–H depolymerized to crystalline C S. Carbonated cement pastes on the other hand showed that amorphous calcium carbonate and part of vaterite decomposed between the range of 400–600 °C, while the rest of the vaterite and calcite were decomposed at 600–720 °C. The individual content of calcium carbonate polymorph could not be obtained using a TGA curve. Besides, the calcium-modified silicate gel was significantly decomposed at 500 °C and completely depolymerized to crystalline C S at 950 °C. In summary, carbonated pastes show better resistance to high temperatures with its heat absorption capacity 3.3 times as high as the uncarbonated sample, which delays the temperature development in the inner layer. Therefore, a reasonable carbonation process could help to improve the fire resistance of concrete to some extent.
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| 3. |
- Liu, W., et al.
(author)
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Changes in chemical phases and microscopic characteristics of fly ash blended cement pastes in different CO 2 concentrations
- 2020
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In: Construction and Building Materials. - : Elsevier BV. - 0950-0618. ; 257
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Journal article (peer-reviewed)abstract
- The effects of CO2 concentration on changes in chemical phases and microscopic characteristics for fly ash (FA) blended cement pastes were investigated in this study. Several microscopic test methods, including X-ray diffraction (XRD), thermo-gravimetric analysis (TGA), 29Si nuclear magnetic resonance (29Si NMR) and scanning electron microscope (SEM), were used to characterize the chemical compositions and microscopic features. The XRD results showed that the precipitation of allotropic calcium carbonate (CC̅) includes calcite (c), aragonite (a) and vaterite (v). The ratio of c/(a + v) was around 0.6 under 3% and 20% CO2, while more percentage of calcite was generated under 100% CO2 (c/(a + v) = 0.79). The precipitation of more calcite than vaterite and aragonite happened with the CO2 concentration elevated to 100%. TGA analysis indicated that the total content of CC̅ was similar under all accelerated conditions and higher than that under natural carbonation. Additionally, in the 29Si NMR spectra, more C-S-H (about 70%) was decalcified after accelerated carbonation compared with natural carbonation (54.1%). The decalcification degree was the same for 3% and 20% CO2 and showed the highest value under 100% CO2. The microstructure changes characterized by SEM observation exhibited denser microstructure after carbonation with the formation of CC̅ but no apparent difference was observed with different CO2 concentrations based on the SEM pictures. Compared with the carbonation of ordinary Portland cement (OPC) paste, the carbonation of FA blended cement paste was more inclined to precipitate as calcite than vaterite and aragonite and caused a lower decalcification degree of C-S-H. Overall, similar to OPC paste, the carbonation results obtained in natural and accelerated conditions for FA blended cement pastes were different and the conditions between 3% and 20% CO2 were similar while 100% CO2 showed different results.
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| 4. |
- Hong, Shuxian, et al.
(author)
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Determination of impressed current efficiency during accelerated corrosion of reinforcement
- 2020
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In: Cement and Concrete Composites. - : Elsevier BV. - 0958-9465. ; 108
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Journal article (peer-reviewed)abstract
- Impressed current technique is a widely adopted method for accelerating corrosion of steel reinforcement. The mass loss of steel can be theoretically estimated by Faraday's law with the assumption that all of the impressed current participated in the corrosion reaction. In the present study, accelerated corrosion process of reinforcing steel with impressed current was non-destructively traced by X-ray micro-computed tomography (μCT) with high accuracy. The mass loss of steel at different accelerated corrosion periods was analyzed and the efficiency of impressed current was calculated. It was found that the mass loss of steel did not always follow Faraday's law, and impressed current efficiencies varied with different corrosion periods. In the early corrosion periods, the water electrolysis occurred and lowered the current efficiency due to the high anode potential. While in the late corrosion periods, the natural corrosion caused the current efficiency to exceed 100%.
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| 5. |
- Zhang, Jianchao, et al.
(author)
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Investigating the influence of fly ash on the hydration behavior of cement using an electrochemical method
- 2019
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In: Construction and Building Materials. - : Elsevier BV. - 0950-0618. ; 222, s. 41-48
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Journal article (peer-reviewed)abstract
- © 2019 Elsevier Ltd Fly ash is an industrial by-product that is widely used in the cement industry. Traditional methods used to investigate the influence of fly ash on the hydration behavior of cement are destructive and cannot accomplish a continuous tracing of the hydration process. In this study, the influence of fly ash incorporation on the hydration process of cement is evaluated using measured electrochemical data obtained from the electrochemical impedance spectroscopy approach. A novel equivalent circuit model that considers the electrochemical characteristics of cement during the hydration process is proposed to investigate the electrochemical property of the blended cement. The resistance parameter of the model, Rct1, was found to have a positive correlation with hydration degree and compressive strength during hydration. With this parameter, the method can be used to non-destructively trace and characterize the effects of fly ash incorporation on the cement hydration process.
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| 6. |
- Zhang, Jianchao, et al.
(author)
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Water distribution modelling of capillary absorption in cementitious materials
- 2019
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In: Construction and Building Materials. - : Elsevier BV. - 0950-0618. ; 216, s. 468-475
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Journal article (peer-reviewed)abstract
- In this paper, a novel capillary absorption model is proposed to investigate the capillary absorption process of cementitious materials. This model mainly consider the influence of tortuosity of the capillary pores of cement paste, which is the key factor affecting the capillary absorption process. The proposed model is verified by the capillary absorption process of paste samples with different water/cement ratios. The absorption height and water content profile of testing samples are measured by X-ray computed tomography. The results show that the proposed capillary absorption model that takes the tortuosity of capillary pores into consideration is suitable for investigating the capillary absorption process of water in cementitious materials.
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