Differential Thermal Analysis (DTA) is a technique used to analyze the physical and chemical changes that occur in a material when it is heated. The goal is to characterize the material by studying the rate of temperature change as the material is heated at a constant rate of heat flow. DTA is based on the principle that when a material is slowly heated, its temperature rises. However, if the material undergoes any endothermic reaction, such as changes in crystalline structure, losing water or CO2, or decomposition, its temperature remains constant. The results of DTA are presented as DTA curves, which represent the difference in temperature between the sample and an inert material. DTA is advantageous over Thermogravimetric Analysis (TGA) because changes that do not involve weight loss can also be detected. By comparing the DTA curve of a sample with that of known compounds, the qualitative composition of the sample can be determined. Additionally, the size of the peak in the DTA curve is directly related to the amount of heat involved in a transition, which allows for the quantitative composition of the sample to be determined.
If a concrete sample is analyzed using DTA, an endothermic peak due to calcium hydroxide is found at 500°C, while the peak due to the conversion of silica occurs at 570°C. When various fire-damaged concrete samples are subjected to DTA, the presence of a peak at 500°C indicates that calcium hydroxide is present, and the sample has not been exposed to a temperature greater than 500°C. If only the peak at 570°C is present, it means that silica conversion did not occur in the sample, and it was not exposed to a temperature greater than 570°C. If no peak is present, it means that the sample was exposed to a temperature exceeding 500°C.
X-ray Diffraction (XRD) is a technique based on the principle that a crystal of a substance has a unique diffraction pattern. When a monochromatic X-ray beam is directed at a crystal, it gets reflected by various crystalline planes, and interference occurs among the reflected beams. This interference results in a diffraction pattern consisting of dark and bright fringes, depending on the phase difference among the interfering beams. A crystal with an unknown composition can be identified by obtaining its diffraction pattern and comparing it with diffraction patterns of already identified crystals. XRD can determine the size of crystal planes, molecular structure, and whether a sample is a single compound or consists of more than one compound. In the case of polymers, the degree of crystallinity can be determined because the non-crystalline portion will scatter the X-ray beam to give a continuous background, while the crystalline portion will give a discontinuous diffraction pattern.
X-ray diffraction can also be used to determine the extent of deterioration in concrete that has been subjected to fire. By using this technique, the temperature to which the damaged concrete has been exposed can be determined.