# Experimental Methods to Evaluate the Carbonation Degree in Concrete--State of the Art Review
## 2 Qualitative Methods
### Phase Analysis
#### 2.3.2 X-ray Diffraction (XRD)  
The alterations in the microstructure of the sample, comprehensively due to carbonation reaction and the position of the carbonation front, were also assessed using X-ray diffraction. Several studies have investigated the mineralogical identification and quantification of the carbonated sample using X-ray diffraction analysis [28, 54, 69, 70, 71]. In detail, XRD technique could determine the carbonation area through the observation of phase peaks of formation CaCO\({}_{3}\) at all investigated depths [71] and be manifested by the intensity, the disappearance (such as portlandite C-H, ettrinigite AFt, calcium silicate hydrate C-S-H and calcium aluminate hydrate C-A-H) and generation (including calcite, vaterite  
Figure 5: The variation of CH and CC content in non-carbonated (Ref) and carbonated (0.55) concretes. The dashed lines show the carbonation depth measured by indicator [26].  
and aragonite) of the diffraction peaks created by the carbonation reaction, as shown in Figure 6[72].  
Due to XRD data results at different temperature conditions and carbon dioxide concentration, the variations in the phase composition and products of concrete have been clearly observed. For example, at the same temperature and relative humidity, the diffraction peak intensity of calcium carbonate increased with the increase in carbon dioxide concentration. In addition, XRD data results also reflected that the phase composition and diffraction peaks of the sample varied more significantly with the rise of temperature at the same carbon dioxide concentration and relative humidity conditions due to faster decomposition of the sample under higher temperature. Meanwhile, mineralogical changes, such as calcium carbonate polymorphs and physicochemical properties, have been observed other reports [54,69,73]. At the higher level of carbonation duration, CH percentages decreased (from 75% to 55%) with the increase of CaCO\({}_{3}\) percentages (from 3% to 10%) in the accelerated carbonation condition. This decrease is more significant at higher CO\({}_{2}\) concentration, temperature, and relative humidity [74]. This influence is characteristic of the carbonation of CH and CSH, which are consumed to create calcium carbonate. This behavior is totally in agreement with that shown in thermo-gravimetric analysis. Although the X-ray diffraction technique was reported to have a standard plus or minus 5% margin of error [75], this acceptable percentage margin of error could be of less importance when comparing XRD results with data on different methods, interpreted under comparable conditions. [PERSON] et al. [61] compared the values of carbonation depth obtained by means of phenolphthalein and XRD and indicated that all the received results were aligned along the line of equality, despite the fact that the carbonation depth measured through phenolphthalein was usually significantly lower than that observed by means of XRD technique. This phenomenon was indicated due to the limited carbonation period, i.e., _one-week duration_, that supported the elimination of the large difference between the two approaches. Like the TGA technique, although the X-ray diffraction method could give a potential determination of mineralogical components at various profiles of sample sections, investigating carbonation-induced differences concerning chemical compositions, the powdered sample is based on very small areas that have representing difficulty in representing the population in the entire specimen.  
Figure 6: XRD data results of the phase composition at different curing conditions [73].