![]() In comparison with IR spectroscopy, almost no preparation is required, and thus the negative effects induced by sample preparation could be minimized 23. Overall, Raman spectroscopy has demonstrated its potential to study both hydration and deterioration of cement-based materials in the literature. In addition, Raman spectroscopy was also extensively applied to characterize of the deterioration products of cement-based materials under carbonation, sulfate, and alkali–silicate reactions attacks 22. Raman spectroscopy has attracted increasing attention since it was first employed to characterize the cement-based materials in 1976 19, which was followed by several systematical work on the spectrum of the main compounds of the cement 20, 21. Raman spectroscopy, a scattering based technology with high chemical fingerprint sensitivity, is a complementary technique to IR spectroscopy 18. These two techniques are discussed below. Apart from the above techniques, infrared (IR) spectroscopy 8 and Raman spectroscopy have also been used to investigate the carbonation of cement paste, benefiting from their capability to follow identify the carbonated products in accordance with the characteristic peak of the C–O stretching bond. As it is almost impractical to obtain powder at different depth at a microscale, high resolution spatial analysis on the carbonation depth is thereby rarely achieved, which is sometimes of significant importance. ![]() ![]() However, both XRD and TG are traditional bulk analysis techniques 17, and powder sample should be prepared prior to each test. Both XRD and TG could achieve quantitively analysis and have provided valuable information to assess the carbonation of the concrete 10, 16. In principle, XRD is capable to investigate the crystal structure of carbonated products and thus determine the carbonation degree and carbonation front, and TG is dependent on the different decomposition temperature of Ca(OH) 2 (420–600 ☌) 14 and CaCO 3 (440–800 ☌) 15. In addition, XRD and TG are also widely employed to investigate the carbonation depth by characterizing the hydrated and carbonated products 9. Though phenolphthalein indicator is capable to give a continuous picture of the carbonated and uncarbonated zone, it can only identify the degree of carbonation above/below 50% (i.e., the pH is higher/lower than 9) 7 and the less carbonated location is impossible to be identified. Spraying the phenolphthalein indicator on the carbonated cement is one conventional method to assess the carbonation depth 8 and it is recognized as the most convenient method to capture the color change boundary between the carbonated and uncarbonated concrete where the pH is around 9 13. To explore the carbonation evolution, several techniques have been well documented by former researchers, such as phenolphthalein indicator 7, 8, X-ray diffraction (XRD) 9, thermogravimetric (TG) analysis 10, 11, and Infrared spectroscopy (IR spectroscopy) 8, 12. Thereby, development of a quantitative analysis technique is attracting increasing attentions. To assess the above effects, it is of great significance to precisely follow the carbonation evolution so that the carbonation induced corrosion and alteration of the mechanical property could be followed precisely. ![]() On the other hand, the precipitation of CaCO 3 may form a denser structure than uncarbonated cement 6. The consumption of the hydroxides would certainly reduce the pH and potentially initiate corrosion of the steel reinforcement 4, 5. Essentially, the carbonation begins with the ingress of the CO 2 into the pore solution to form carbonic acid which mainly reacts with Ca(OH) 2 (Reaction 1) and calcium-bearing phases (such as C–S–H (Reaction 2)) to form CaCO 3, resulting in the neutralization of the alkaline environment 3. These properties would, however, be affected by the carbonation, which is one of the major degradation issues for cementitious material 2. Hydration of the Portland cement mainly produces portlandite (Ca(OH) 2) and other calcium-bearing phases like C–S–H 1, which contributes to the development of the mechanical properties and alkalinity of the material. Portland cement is one of the most common construction materials around the world, benefiting from its excellent mechanical property and relatively well durability. ![]() As one of the major degradation types, carbonation of cement paste has attracted extensive attentions with attempts to understand the underlying mechanism. ![]()
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