Introduction:
Gypsum dehydration is a commonplace industrial process involving complex phase transformations. Using the DVS Vacuum, we investigated the influence of relative humidity on gypsum dehydration between 25 °C and 60 °C. The mass loss observed by DVS indicated the formation of dehydrated phases, confirmed through XRD Rietveld refinement. SEM observations provided insights into the vapor transport mechanism during degradation, highlighting the importance of water activity in gypsum dehydration.
Gypsum-based binder technology has been utilized for at least 11,000 years, spanning various applications from building materials to ceramics, medical, and food industries. The mineral gypsum, CaSO4∙2H2O, serves as the raw material for these compounds. To produce the starting materials or binders, gypsum is usually thermally converted into bassanite (plaster of Paris), CaSO4∙0.5H2O, or the water-soluble anhydrite, CaSO4∙nH2O, n < 0.05. The thermal behavior of these hydrate phases in the CaSO4 - H2O system has been the subject of scientific activities for thousands of years. Understanding the mechanisms of these processes is crucial for improving manufacturing processes, ensuring the stability and durability of gypsum-based materials, and optimizing economic efficiency by controlling humidity during the drying process.
Results and Discussion
Gypsum's utility dates back over 11,000 years, from building materials to ceramics, medical uses, and food processing. In modern applications, gypsum (CaSO₄∙2H₂O) is typically converted to bassanite (CaSO₄∙0.5H₂O) or anhydrite (CaSO₄) for various industrial purposes. This research focuses on the effects of controlled humidity and temperature on gypsum drying and the corresponding phase transitions.
DVS Drying:
The weight loss over time at various temperature and relative humidity values is depicted in Figure 1(View Fig in Full Application Note, Download here). From the weight loss curves obtained by DVS, it is evident that humidity has an important influence on the dehydration process. The time-to-weight equilibrium shortens drastically with relative humidity. In addition, the influence of temperature also enhances the dehydration kinetics of gypsum.
Unsurprisingly, the weight loss is faster at lower ambient relative humidity and at higher temperatures
The initial mineral phase can be assumed to be gypsum CaSO4∙2H2O, which can transform during the treatment into bassanite (hemihydrate) with different amounts of water, per unit formula, CaSO4∙1.8 H2O and CaSO4∙0.6 H2O, and finally to a nearly dry water-soluble γ-anhydrite CaSO4∙nH2O, n < 0.05 then to a fully dry insoluble β-anhydrite CaSO4.
The mass loss observed in DVS is close to, but not exactly the stoichiometric value of 20.9% that corresponds to the departure of both coordinated water molecules from the crystal structure, which would result in a full dehydration and a β-anhydrite phase. Instead, mass loss is consistent with a stoichiometric ratio of CaSO4∙0.15 H2O. At a higher water vapor activity of 5% RH, dehydration was only achieved at temperatures over 40 °C while at lower temperatures full equilibrium was not achieved and correspondingly less mass was lost.
Phase Analysis:
More in-depth phase analysis can be performed through XRD. The Rietveld refinement of the diffractograms using the above-mentioned phases yielded reliable results.
The dominating mineral phase after activation was γ-CaSO4, followed by the bassanite containing 1.8 water molecules per unit. The water-insoluble β-anhydrite was not detected in any of the resulting phases. Gypsum, CaSO4∙2 H2O, was not found in most of the end products, except in those runs where the equilibrium of the mass was not achieved due to time limitation.
On average, the weight loss, as determined by the DVS, could be recalculated by the mineralogical composition of the end products expressed as the mass ratio by XRD to DVS within 100 ± 6%.
Morphology Analysis:
The original, untreated gypsum sample contains large crystals with smooth faces corresponding to the crystal system of gypsum (Figure 4a). The large flat faces can be attributed to the (010) face, whereas the edge consists of (011), (120), or (111) faces. Heating at 25 °C under the three applied relative humidity values resulted in more corrugated surfaces of the (010) faces. The SEM examinations reveal that although the particles retain their original shape after the reaction, they are interspersed with a network of cracks. In the enlarged sections of the samples after exposure at 0% RH, 2.5% RH, and 5% RH, it can be seen that layer packages detach from the surface.
It is remarkable that the original particle shape with the habitus which is typical for the starting material gypsum is retained, although - as shown in the XRD analysis - complex phase transformations take place. The surfaces of the (010) faces of particles are crisscrossed by a network of cracks. The appearance of the cracks can be explained by the transformation of gypsum into phases with a semi-hydrate structure. These phases have a lower molar volume (29% less).
Kinetic Analysis:
The kinetic analysis of the gypsum dehydration process revealed that the rate of mass loss over time was influenced by temperature and relative humidity. By plotting the rate of mass loss in an Arrhenius plot, the activation energy (EA) was calculated for different humidity levels: 49.5 kJ/mol at 0% RH, 59.5 kJ/mol at 2.5% RH, and 68.9 kJ/mol at 5.0% RH. Higher relative humidity required more thermal energy to induce water release and structural transformation, as detected by XRD. The study identified the dehydration process at the pore walls, water release into the pore volume, and subsequent transport through the pore volume as rate-determining steps. Understanding these kinetic parameters and the impact of relative humidity can help optimize the dehydration process, making it more efficient and cost-effective for industrial applications.
Potential Benefits for Real-World/Industrial Applications:
- Improved Manufacturing: Better control over dehydration processes can enhance the quality and consistency of gypsum-based products.
- Cost Efficiency: Optimizing humidity and temperature conditions can reduce energy consumption and operational costs.
- Material Stability: Understanding phase transformations ensures the production of more stable and durable materials.
- Versatility: Enhanced knowledge of gypsum dehydration can lead to innovations in various industries, including construction, ceramics, and healthcare.
Conclusion:
With the help of the DVS instrument, it was possible to investigate the influence of temperature and relative humidity on the dehydration kinetics of gypsum at temperatures between 25 °C and 60 °C. The instrument showed great stability over 6 days. The mass loss observed by DVS could be explained by the formation of newly generated minerals as determined through XRD Rietveld refinement.
Through subsequent observation of microstructure by SEM, the sample morphology evolution and insights into the vapor transport mechanism during degradation could be ascertained.
These findings underscore the importance of controlling environmental conditions during the dehydration process to optimize the quality and efficiency of gypsum-based products. Understanding the detailed mechanisms of phase transformations and morphological changes can lead to improved manufacturing processes, enhanced material stability, and cost savings in industrial applications.