Hydration Resistance of CaO Material Prepared by Ca(OH)2 Calcination with Chelating Compound

The hydration resistance of CaO materials prepared by Ca(OH)2 calcination with chelating compounds are investigated in this paper. The crystalline phases and microstructure characteristics of sintered specimens were studied by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy and energy dispersive spectrometer (SEM, EDS). The bulk density, apparent porosity, and hydration resistance of samples were also tested. The results showed that chelating compounds improved the hydration resistance of the treated CaO specimens significantly. The surface-pretreated specimens showed an increase in bulk density and a decrease in apparent porosity after heating. The surface pretreatment of the Ti chelating compound promoted the solid phase sintering and grain growth of CaO specimens, which increased the density of the heated CaO sample. The Al chelating compound promoted the liquid-phase sintering of CaO specimens, which led to the grain growth and increased density of the sample. CaO grains were bonded by the formed tricalcium aluminate (C3A) and the apparent porosity of the sample was reduced, reducing the contact area of CaO with water vapor. The Al chelating compound was more effective in improving the hydration resistance of the CaO material in the situation of this study.


Introduction
CaO refractories have excellent properties such as high stability, low vapor pressure at high temperature, good slag resistance, high refractoriness, and excellent action in purifying molten steel [1][2][3]. It does not pollute the molten steel but absorbs non-metallic inclusions such as S, P, Al 2 O 3 in the molten steel [4][5][6]. In addition, it is one of the best materials for preparing crucibles, filters, and nozzles used for metallurgical purposes. However, CaO's easy hydration characteristic is a limiting factor in many applications, as well as in storage and preparation. The research on improving the hydration resistance of CaO refractories has been uninterrupted in the past thirty years, and there are two main methods to enhance the hydration resistance of CaO materials. The first method consists of introducing additives by promoting solid-phase sintering (e.g., ZrO 2 [7][8][9], TiO 2 [10], rare earth oxides La 2 O 3 [11] and CeO 2 [4,12]) or liquid-phase sintering (e.g., Al 2 O 3 [13], Fe 2 O 3 [14,15], CuO [16], V 2 O 5 [17], Cr 2 O 3 [18], etc.) which will respectively produce CaO materials with a denser structure, or form a low-melting-point phase to coat the surface of the CaO grain. Kahrizsangi et al. reported that the addition of nano-TiO 2 to a magnesite-dolomite refractory matrix helped in the densification process by solid-state sintering and increased the hydration resistance [10]. Wei et al. studied the effects of Zr(OH) 4 and Al(OH) 3 on the hydration resistance of CaO granules. The results showed that the hydration resistance of CaO granules was improved significantly [19]. Those methods reduced the contact area of CaO with water vapor and improved the hydration resistance of CaO materials. The formation of vitreous phases was conducive to a better resistance to hydration [20]. The second method Active lime was used to prepare Ca(OH) 2 in this study. The Ca(OH) 2 powder was prepared by hydrating, filtrating, drying, and fine grinding the active lime. The prepared Ca(OH) 2 powder was pressed into a cylindrical sample of Ø36 mm × 20 mm under a pressure of 60 MPa. The apparent porosity of the obtained Ca(OH) 2 sample was 43.2%. The Ca(OH) 2 cylindrical samples were then placed into two prepared chelating compound solutions (ethanol solution of 75 mass% concentration) before being impregnated in vacuum for 10 min. After the samples were dried out, the amount of absorbed Ti chelating compound and Al chelating compound were measured to be 6.92 wt.% and 7.09 wt.%, respectively. After that, the samples were sintered at 1600 • C for 3 h. The composite structure was carried out by infiltration [25]. The experimental process is shown in Figure 1. The sample without pretreatment was named C 0 , samples pretreated with Ti and Al chelating compounds were named C T and C A , respectively. Two measurements were used to evaluate the hydration resistance of CaO materials (the tests of CaO sample weight gain rate were done in a curing chamber and in air, respectively). Thermal analysis (TG-DSC) was used to assist the characterization of hydrated samples. Scanning electron microscopy (SEM, Nova 400 Nano-SEM, FEI Company, Hillsboro, OR, USA) and EDS spectrum analysis (INCA IE 350 Penta FET X-3, Oxford, UK) were used to assist the analysis of the sample microstructure before and after hydration test. The composition of the surface phase of CaO specimens obtained by pretreatment was studied by X-ray diffraction analysis (X'Pert Pro, Philips, Eindhoven, the Netherlands; using Ni-filtered Cu Kα radiation at a temperature of 20 • C) and Materials 2019, 12, 2325 3 of 10 X-ray photoelectron spectroscopy (Thermo Escalab 250XI, Thermo Fisher Scientific, Waltham, MA, USA). The bulk density and the apparent porosity of the specimens were measured using Archimedes' method with kerosene.
Materials 2019, 12, x FOR PEER REVIEW 3 of 10 250XI, Thermo Fisher Scientific, Waltham, MA, USA). The bulk density and the apparent porosity of the specimens were measured using Archimedes' method with kerosene.

Hydration Resistance of CaO Samples
For measurement 1, specimens were placed in a chamber under a constant temperature of 50 °C and humidity of 90% for 10 h. In measurement 2, the specimens were placed in the air for 35 days. During this time, the temperature varied from 7.8 °C to 24.4 °C, and the relative humidity varied from 28% to 96%. The hydration resistance of the specimens was characterized by the weight gain rate, as in the following Equation (1): where M1 is the weight of the sample before hydration, and M2 is the weight of the sample after hydration.
(і) The results obtained under controlled conditions are shown in Figure 2. It could be seen from the results that the average hydration weight gain rate of sample C0 was 0.55%. However, the hydration weight gain rate of samples CT and CA decreased to 0.15% and 0.11%, respectively. (іі) The hydration weight gain rates of the samples placed in the air are shown in Figure 3. The hydration weight gain rate of sample C0 increased significantly during testing and gained more than 0.8% weight in 35 days. However, the weight gain rate of sample CT was about 0.3%. In particular, the weight gain rate of sample CA was about 0.02% after 35 days. The results showed that both

Hydration Resistance of CaO Samples
For measurement 1, specimens were placed in a chamber under a constant temperature of 50 • C and humidity of 90% for 10 h. In measurement 2, the specimens were placed in the air for 35 days. During this time, the temperature varied from 7.8 • C to 24.4 • C, and the relative humidity varied from 28% to 96%. The hydration resistance of the specimens was characterized by the weight gain rate, as in the following Equation (1): where M 1 is the weight of the sample before hydration, and M 2 is the weight of the sample after hydration. (i) The results obtained under controlled conditions are shown in Figure 2. It could be seen from the results that the average hydration weight gain rate of sample C 0 was 0.55%. However, the hydration weight gain rate of samples C T and C A decreased to 0.15% and 0.11%, respectively.

Hydration Resistance of CaO Samples
For measurement 1, specimens were placed in a chamber under a constant temperature of 50 °C and humidity of 90% for 10 h. In measurement 2, the specimens were placed in the air for 35 days. During this time, the temperature varied from 7.8 °C to 24.4 °C, and the relative humidity varied from 28% to 96%. The hydration resistance of the specimens was characterized by the weight gain rate, as in the following Equation (1): where M1 is the weight of the sample before hydration, and M2 is the weight of the sample after hydration.
(і) The results obtained under controlled conditions are shown in Figure 2. It could be seen from the results that the average hydration weight gain rate of sample C0 was 0.55%. However, the hydration weight gain rate of samples CT and CA decreased to 0.15% and 0.11%, respectively. (іі) The hydration weight gain rates of the samples placed in the air are shown in Figure 3. The hydration weight gain rate of sample C0 increased significantly during testing and gained more than 0.8% weight in 35 days. However, the weight gain rate of sample CT was about 0.3%. In particular, the weight gain rate of sample CA was about 0.02% after 35 days. The results showed that both (ii) The hydration weight gain rates of the samples placed in the air are shown in Figure 3. The hydration weight gain rate of sample C 0 increased significantly during testing and gained more than 0.8% weight in 35 days. However, the weight gain rate of sample C T was about 0.3%. In particular,   Figure 4 shows the TG results presenting mass losses of hydrated samples after being tested in measurement 1. The hydrated sample was uniformly ground and then subjected to thermal analysis under N2 atmosphere. The lesser mass losses in the TG result indicated the lower weight gain rates of samples CA and CT in the hydration test. It can be seen from Figure 4 that the mass loss of sample C0 was the largest and that of sample CA was the smallest, meaning that the Al chelating compound enhanced the hydration resistance of CaO samples more effectively.  Figure 5 shows the effect of surface pretreatment on the apparent porosity and bulk density of sintered CaO samples. The bulk density of sample CT increased from 2.99 g/cm 3 to 3.07 g/cm 3 , and the apparent porosity decreased from 4.12% to 1.33% when compared with sample C0. At the same time, the bulk density of sample CA increased to 3.13 g/cm 3 and the apparent porosity of sample CA decreased to 1.04% when compared with sample C0.  Figure 4 shows the TG results presenting mass losses of hydrated samples after being tested in measurement 1. The hydrated sample was uniformly ground and then subjected to thermal analysis under N 2 atmosphere. The lesser mass losses in the TG result indicated the lower weight gain rates of samples C A and C T in the hydration test. It can be seen from Figure 4 that the mass loss of sample C 0 was the largest and that of sample C A was the smallest, meaning that the Al chelating compound enhanced the hydration resistance of CaO samples more effectively.   Figure 4 shows the TG results presenting mass losses of hydrated samples after being tested in measurement 1. The hydrated sample was uniformly ground and then subjected to thermal analysis under N2 atmosphere. The lesser mass losses in the TG result indicated the lower weight gain rates of samples CA and CT in the hydration test. It can be seen from Figure 4 that the mass loss of sample C0 was the largest and that of sample CA was the smallest, meaning that the Al chelating compound enhanced the hydration resistance of CaO samples more effectively.  Figure 5 shows the effect of surface pretreatment on the apparent porosity and bulk density of sintered CaO samples. The bulk density of sample CT increased from 2.99 g/cm 3 to 3.07 g/cm 3 , and the apparent porosity decreased from 4.12% to 1.33% when compared with sample C0. At the same time, the bulk density of sample CA increased to 3.13 g/cm 3 and the apparent porosity of sample CA decreased to 1.04% when compared with sample C0.  Figure 5 shows the effect of surface pretreatment on the apparent porosity and bulk density of sintered CaO samples. The bulk density of sample C T increased from 2.99 g/cm 3 to 3.07 g/cm 3 , and the apparent porosity decreased from 4.12% to 1.33% when compared with sample C 0 . At the same time, the bulk density of sample C A increased to 3.13 g/cm 3 and the apparent porosity of sample C A decreased to 1.04% when compared with sample C 0 . The densification of the samples after surface pretreatment was improved, which reduced the amounts of open pores and cracks on the surface of CaO material. Al2O3 was a decomposition product of the Al chelating compound at high temperature, which acted as a liquid-phase sintering aid to promote the grain growth of CaO, making the structure denser [26]. TiO2 was a decomposition product of the Ti chelating compound; it can also promote solid-phase sintering of CaO because Ti 4+ replaces Ca 2+ to form a solid solution at high temperature and produces Ca vacancy [VCa''] (Equation (2)) [27]. The diffusion and migration of atoms are the basic factors for sintering. The generation of vacancies provides a diffusion source of particles. This can improve the self-diffusion coefficient and the vacancy diffusion coefficient. This is beneficial to the diffusion and migration of the particles, and promotes sintering [28]. Figure 6 shows the XRD patterns on the surface of the samples sintered at 1600 °C for 3 h. The figure indicates that Ca3Ti2O7 was formed on the surface of the sample CT, and it promoted the sintering density of the CaO sample. A small amount of Ca3Al2O6 was formed on the surface of the sample CA, which had a positive effect on preventing the surface of the CaO sample from contacting with water vapor. Ca3Ti2O7 and Ca3Al2O6 phases were found in the CaO-TiO2 and CaO-Al2O3 phase diagrams (Figure 7). The surface treatment of CaO material slowed down the hydration process in two aspects. The first was in the formation of a small amount of water-resistant compounds on the surface of CaO which filled the pores on the surface, reducing the contact area between CaO grains and water vapor. The second was in the promotion of the sintering degree of the surface portion of the CaO sample, which reduced the porosity and the probability of defect formation.  The densification of the samples after surface pretreatment was improved, which reduced the amounts of open pores and cracks on the surface of CaO material. Al 2 O 3 was a decomposition product of the Al chelating compound at high temperature, which acted as a liquid-phase sintering aid to promote the grain growth of CaO, making the structure denser [26]. TiO 2 was a decomposition product of the Ti chelating compound; it can also promote solid-phase sintering of CaO because Ti 4+ replaces Ca 2+ to form a solid solution at high temperature and produces Ca vacancy [VCa"] (Equation (2)) [27]. The diffusion and migration of atoms are the basic factors for sintering. The generation of vacancies provides a diffusion source of particles. This can improve the self-diffusion coefficient and the vacancy diffusion coefficient. This is beneficial to the diffusion and migration of the particles, and promotes sintering [28].  (Figure 7). The surface treatment of CaO material slowed down the hydration process in two aspects. The first was in the formation of a small amount of water-resistant compounds on the surface of CaO which filled the pores on the surface, reducing the contact area between CaO grains and water vapor. The second was in the promotion of the sintering degree of the surface portion of the CaO sample, which reduced the porosity and the probability of defect formation. The densification of the samples after surface pretreatment was improved, which reduced the amounts of open pores and cracks on the surface of CaO material. Al2O3 was a decomposition product of the Al chelating compound at high temperature, which acted as a liquid-phase sintering aid to promote the grain growth of CaO, making the structure denser [26]. TiO2 was a decomposition product of the Ti chelating compound; it can also promote solid-phase sintering of CaO because Ti 4+ replaces Ca 2+ to form a solid solution at high temperature and produces Ca vacancy [VCa''] (Equation (2)) [27]. The diffusion and migration of atoms are the basic factors for sintering. The generation of vacancies provides a diffusion source of particles. This can improve the self-diffusion coefficient and the vacancy diffusion coefficient. This is beneficial to the diffusion and migration of the particles, and promotes sintering [28].

Phase Analysis of CaO Samples
TiO2 CaO Ti ▪▪ Ca + V"Ca + 2OO (2) Figure 6 shows the XRD patterns on the surface of the samples sintered at 1600 °C for 3 h. The figure indicates that Ca3Ti2O7 was formed on the surface of the sample CT, and it promoted the sintering density of the CaO sample. A small amount of Ca3Al2O6 was formed on the surface of the sample CA, which had a positive effect on preventing the surface of the CaO sample from contacting with water vapor. Ca3Ti2O7 and Ca3Al2O6 phases were found in the CaO-TiO2 and CaO-Al2O3 phase diagrams (Figure 7). The surface treatment of CaO material slowed down the hydration process in two aspects. The first was in the formation of a small amount of water-resistant compounds on the surface of CaO which filled the pores on the surface, reducing the contact area between CaO grains and water vapor. The second was in the promotion of the sintering degree of the surface portion of the CaO sample, which reduced the porosity and the probability of defect formation.             Figure 10 shows images of the samples' surface microstructure after having been heated at 1600 °C for 3 h. Table 3 shows the energy-dispersive spectrometry (EDS) results of points A, B, and C in Figure 10. The average gap sizes of C0, CT, and CA were respectively 4.62, 2.31, and 1.44 μm. It can be seen that the gaps between the CaO grain boundaries in sample CT and CA were smaller than those in sample C0. Sample CA had the most compact structure. Ca3Ti2O7 was formed at the grain surface and the grain boundary, filling pores and enhancing the degree of crystal bonding. For sample CA, the surface grain size development was significantly greater than that of sample C0, which increased the grain size of CaO from 50 to 70-100 μm. Ca3Al2O6 was formed on the grain boundary and filled triple points, which increased the density of the crystal. The formation of a liquid phase at high temperatures facilitated grain growth because of the faster migration rate of atoms in the liquid phase. Although Ca3Al2O6 was also hydratable, it converted some free CaO to Ca3Al2O6, and the hydration resistance was better than that of CaO-covered defects sites [30]. Chelating compounds promoted the bonding of CaO grains on the surface and formed a protective coating to reduce the contact area between CaO and water vapor, thus enhancing the hydration resistance of the CaO sample.   Figure 10 shows images of the samples' surface microstructure after having been heated at 1600 • C for 3 h. Table 3 shows the energy-dispersive spectrometry (EDS) results of points A, B, and C in Figure 10. The average gap sizes of C 0 , C T , and C A were respectively 4.62, 2.31, and 1.44 µm. It can be seen that the gaps between the CaO grain boundaries in sample C T and C A were smaller than those in sample C 0 . Sample C A had the most compact structure. Ca 3 Ti 2 O 7 was formed at the grain surface and the grain boundary, filling pores and enhancing the degree of crystal bonding. For sample C A , the surface grain size development was significantly greater than that of sample C 0 , which increased the grain size of CaO from 50 to 70-100 µm. Ca 3 Al 2 O 6 was formed on the grain boundary and filled triple points, which increased the density of the crystal. The formation of a liquid phase at high temperatures facilitated grain growth because of the faster migration rate of atoms in the liquid phase. Although Ca 3 Al 2 O 6 was also hydratable, it converted some free CaO to Ca 3 Al 2 O 6 , and the hydration resistance was better than that of CaO-covered defects sites [30]. Chelating compounds promoted the bonding of CaO grains on the surface and formed a protective coating to reduce the contact area between CaO and water vapor, thus enhancing the hydration resistance of the CaO sample.  Figure 10 shows images of the samples' surface microstructure after having been heated at 1600 °C for 3 h. Table 3 shows the energy-dispersive spectrometry (EDS) results of points A, B, and C in Figure 10. The average gap sizes of C0, CT, and CA were respectively 4.62, 2.31, and 1.44 μm. It can be seen that the gaps between the CaO grain boundaries in sample CT and CA were smaller than those in sample C0. Sample CA had the most compact structure. Ca3Ti2O7 was formed at the grain surface and the grain boundary, filling pores and enhancing the degree of crystal bonding. For sample CA, the surface grain size development was significantly greater than that of sample C0, which increased the grain size of CaO from 50 to 70-100 μm. Ca3Al2O6 was formed on the grain boundary and filled triple points, which increased the density of the crystal. The formation of a liquid phase at high temperatures facilitated grain growth because of the faster migration rate of atoms in the liquid phase. Although Ca3Al2O6 was also hydratable, it converted some free CaO to Ca3Al2O6, and the hydration resistance was better than that of CaO-covered defects sites [30]. Chelating compounds promoted the bonding of CaO grains on the surface and formed a protective coating to reduce the contact area between CaO and water vapor, thus enhancing the hydration resistance of the CaO sample.    Figure 11 shows the surface microstructure of the samples after being placed in air for 7 days. The dark parts in Figure 11 are unhydrated CaO particles, and the light parts covering the black CaO grains are Ca(OH) 2 particles. Figure 11a shows the hydration of the sample C 0 , and there were even cracks due to hydration. It can be seen from Figure 11c that the CaO in the C A sample showed little hydration. This also confirmed that the hydration resistance of the CaO sample pretreated by the Al chelating compound was the best of all the tested samples. Ti --12.12 Figure 11 shows the surface microstructure of the samples after being placed in air for 7 days. The dark parts in Figure 11 are unhydrated CaO particles, and the light parts covering the black CaO grains are Ca(OH)2 particles. Figure 11a shows the hydration of the sample C0, and there were even cracks due to hydration. It can be seen from Figure 11c that the CaO in the CA sample showed little hydration. This also confirmed that the hydration resistance of the CaO sample pretreated by the Al chelating compound was the best of all the tested samples.  Figure 12 shows cross-sectional SEM images of CaO samples before and after the hydration test. It shows that Ca(OH)2 layers were formed on the surface of the CaO samples after the hydration. The Ca(OH)2 layer formed in Figure 12d (sample CA) was the thinnest, indicating the lowest reaction degree of CaO and water vapor. Therefore, the hydration resistance of the CaO sample pretreated by Al chelating compound was greatly improved.

Conclusions
Both Ti and Al chelating compounds enhanced the hydration resistance of the CaO material significantly. At the same time, both promoted the surface sintering of CaO and reduced the contact area of CaO with water vapor, making the surface structure much denser. Chelating compounds decomposed at high temperature, and the decomposition products reacted with CaO to form  Table 3. EDS analyses of points A, B, and C in Figure 10.  Figure 12 shows cross-sectional SEM images of CaO samples before and after the hydration test. It shows that Ca(OH) 2 layers were formed on the surface of the CaO samples after the hydration. The Ca(OH) 2 layer formed in Figure 12d (sample C A ) was the thinnest, indicating the lowest reaction degree of CaO and water vapor. Therefore, the hydration resistance of the CaO sample pretreated by Al chelating compound was greatly improved. Ti --12.12 Figure 11 shows the surface microstructure of the samples after being placed in air for 7 days. The dark parts in Figure 11 are unhydrated CaO particles, and the light parts covering the black CaO grains are Ca(OH)2 particles. Figure 11a shows the hydration of the sample C0, and there were even cracks due to hydration. It can be seen from Figure 11c that the CaO in the CA sample showed little hydration. This also confirmed that the hydration resistance of the CaO sample pretreated by the Al chelating compound was the best of all the tested samples.  Figure 12 shows cross-sectional SEM images of CaO samples before and after the hydration test. It shows that Ca(OH)2 layers were formed on the surface of the CaO samples after the hydration. The Ca(OH)2 layer formed in Figure 12d (sample CA) was the thinnest, indicating the lowest reaction degree of CaO and water vapor. Therefore, the hydration resistance of the CaO sample pretreated by Al chelating compound was greatly improved.

Conclusions
Both Ti and Al chelating compounds enhanced the hydration resistance of the CaO material significantly. At the same time, both promoted the surface sintering of CaO and reduced the contact area of CaO with water vapor, making the surface structure much denser. Chelating compounds decomposed at high temperature, and the decomposition products reacted with CaO to form