To fully understand the properties of cement and long-term effects, measurements such as hydration levels, loss on ignition (LOI), material composition (eg calcium carbonate in interground limestone) and the assessment of kinetic parameters of thermal decomposition are key factors.
EDXRF (Energy dispersive X-ray fluorescence) is a long-established and powerful analysis technique for raw materials and finished products as a method for quality control testing in the cement industry.
WD-XRF (wavelength dispersive X-ray fluorescence) and XRD (X-ray diffraction) can also be used, but as with all analytical techniques they can be limited.1 Thermal analysis is generally used as a complementary technique to XRF and XRD. when it comes to fully understanding all of the above parameters.
Image credit: Hitachi High-Tech Analytical Science
What is Thermal Analysis?
Thermal analysis is a group of techniques in which a property of the sample is monitored against time or temperature while the temperature of the sample is programmed in a specified atmosphere. The main parameters can be measured by evaluating the effect of temperature and/or time on cement.
These instruments typically consist of a detection unit (eg furnace with thermocouples and/or balance), a temperature control unit to manage the furnace temperature and a data acquisition unit that has the capacity to record the signals from the sensor and the sample temperature. See the article : Outdoor Tennis Could Be The First Climate Change Loss Of Sport. for evaluation.
While thermal analysis includes an extensive range of techniques, the primary thermal analysis techniques used are Differential Scanning Calorimetry (DSC), Dynamic Mechanical Analysis (DMA), Simultaneous Thermogravimetry Analysis (STA), Thermogravimetry (TG), or Thermomechanical Analysis (TMA). The main techniques typically used for cement analysis are DSC and TG.
DSC and TG Explained
DSC is a technique in which the temperature of the sample unit, consisting of a sample and reference material, is varied in a specific program. The temperature difference between the sample and the reference material is determined as a function of temperature. Figure 1 shows a simple representation of a generic heat flux DSC oven.
By measuring heat flow, this method facilitates the measurement of exothermic and endothermic reactions, including evaporation, which can be used to quantify materials such as dihydrate and hemihydrate gypsum present in cement. On the same subject : Fun in the Sun: Summer Food Security.2
Figure 1. DSC oven. Image credit: Hitachi High-Tech Analytical Science
The TG technique is where the mass of the sample is tracked against time or temperature while the temperature of the sample is programmed in a specific atmosphere. It is often applied to identify evaporation, decomposition, oxidation and other effects of temperature change that result in changes in mass.
Figure 2. STA furnace R = reference and S = sample. Image credit: Hitachi High-Tech Analytical Science
STA (TG/DTA or TG/DSC) is similar to TG, but it also incorporates differential thermal analysis in a single device. This is the most used technique when doing cement analysis, because it offers TG and DTA / DSC results, in addition to the ability to reach high temperatures (> 1000 ̊C). Figure 2 shows an example of a simplified representation of a STA horizontal double beam design.
TG or STA can also be connected to an external detector such as an FTIR, MS or GCMS, which facilitates the identification of the evolved gas throughout the experiment.
DSC and STA in Practice
Step 1: Installation and Calibration
DSCs and STAs are very easy to use and do not require much sample preparation. Read also : Can high-tech clothing fight climate change? The Bay Area startup thinks so. Once installed in a vibration-free area, the system can only be powered by stored gas (nitrogen and/or dry air).
During calibration, the DSCs and STAs must be checked regularly to ensure they are within specifications. If a recalibration is required, it can usually be carried out by the users.
Step 2: Sample Preparation and Analysis
The sample could take the form of a dry powder or a paste of cured cement, with an accurately known weight. It is then placed in a crown, usually made of aluminum or ceramic, before being placed in the sample holder.
Depending on the method, the cross will be either open, semi-hermetic or hermetic. Sample weight can vary between 10 and 100 mg depending on the analysis and the required information. A reference is also introduced, consisting of an empty circle of the same type as the one used for the sample.
Once the position of the sample is determined, the temperature program begins the analysis. The temperature program is either isothermal or increases at a constant rate depending on the required method to be performed.
Depending on the system purchased, it may be possible to insert the samples manually or with the help of an autosampler (which holds up to 50 samples).
Figure 3. Sample positioning in the DSC oven. Image credit: Hitachi High-Tech Analytical Science
Step 3: Results Interpretation
To interpret the results, TG, DTA and DSC results are used.
Figures 4 and 5 show the TG and DTA results from different types of cement. The results were obtained by carefully following ASTM C1872 – 18.
Figure 4 shows the TG (top three curves) and DTG (derivative thermogravimetry) (bottom three curves) results for NBS: SRM633 dried 1 and hydrated 3 in addition to JCM-211M 2. In this case, the X-axis is representative of the sample temperature while the y-axis (left) is the percentage of weight loss (%wt), and the y-axis (right) stands for the derivative of the percentage of weight loss.
All weight loss is due to the different elements within the cement materials and is given in table 1. A complete list of weight loss can be found in a paper published by Collier.1
The difference between samples is seen in the weight loss curves 1 and 2. This determines the difference in the component ratio. The increase of free water and the amount of OH groups can also be seen and quantified in the weight loss curves 1 and 3.
Figure 4. TG (%) and DTG results for 1 NBS (SRM633), 2 JCM-211M, 3 NBS Hydrated. Image credit: Hitachi High-Tech Analytical Science
Table 1. Weight loss during the analysis of cement samples. Source: Hitachi High-Tech Analytical Science
Figure 5 shows DTA curves for each cement sample. An endothermic peak is present in each sample, indicating that there is a relationship between the weight loss and the size of the peak. Since calibration of the DTA signal to quantify heat flow is possible on some instruments, results can be used to quantify components.
Figure 5. DTA results for 1 NBS (SRM633), 2 JCM-211M, 3 NBS Hydrated. Image credit: Hitachi High-Tech Analytical Science
DSC is also a vital technique when it comes to testing specific materials that have an impact on cement properties. Gypsum is one of the main constituents of Portland cement; therefore, it is crucial to understand its properties before and after mixing.
The energy released throughout the milling process could generate a partial dehydration of the gypsum to hemihydrate, which could affect the rate of settling in addition to the long-term properties of the cement.3 Since neither XRD nor XRF have the ability to provide this information offer , DSC could play a central role in cement characterization.
Figure 6. DSC results for dehydrated gypsum. Image credit: Hitachi High-Tech Analytical Science
Figure 7. Dryness equation of gypsum. Image credit: Hitachi High-Tech Analytical Science
Figure 6 shows the DSC result for gypsum. The two peaks (157 ̊C and 202 ̊C) show the dehydration process of gypsum, as shown in Figure 8.
By integrating both peaks, it is clear to see if the stoichiometry of the equation is what is expected and if the milling had any influence on the final product.
Thermal analysis, especially DSC and TG/STA, are both key and complementary techniques to EDXRF, WDXRF and XRD for cement analysis.
They provide valuable information such as hydration levels, loss on ignition (LOI), material composition (eg calcium carbonate in interground limestone) and the kinetic parameters of the thermal decomposition of cement, which other techniques cannot.
They are relatively easy to use, and with the help of an autosampler, the automation of the complete analysis from sample introduction to the export of results is made possible.
Hitachi Thermal Analyzers
Hitachi’s range of thermal analyzers have an unparalleled level of core stability, world-leading sensitivity and advanced capabilities that give users the complete picture of the thermal behavior of cement. The analyzers have been optimized to identify minute changes to provide visibly improved thermal analysis.
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References
This information was provided by Hitachi High-Tech Analytical Science, reviewed and adapted from materials.
For more information on this source, please visit Hitachi High-Tech Analytical Science.
