Microstructure and strength of microporous MgO refractory aggregates with nano-sized pores

ABSTRACT In this work, microporous MgO refractory aggregates with nano-sized pores were prepared by in situ decomposition method using Mg(OH)2 as raw material. The effects of firing temperatures (1600–1700°C) and compacting pressures (100–200 MPa) on the microstructures and properties of microporous MgO refractory aggregates were thoroughly studied. The results showed that the microporous MgO refractory aggregates contained two types of pore structures, which were intra-particle pores with pore sizes of 180.0–230.0 nm formed by in situ decomposition of Mg(OH)2 and inter-particle pores with pore sizes of 1.5–3.0 μm derived from particle packing between Mg(OH)2 pseudomorph particles, respectively. Besides, the firing temperatures had a great influence on the intra-particle pore size and microcrystallite size. And the compacting pressures not only influenced the intra-particle pore size via packing behaviors but also affected the inner firing behaviors of the pseudomorph particles due to the increase in H2O vapor pressure. Overall, at a compacting pressure of 150 MPa and firing temperature of 1650°C, the sample had the best comprehensive performance with a bulk density of 1.92 g/cm3, a compressive strength of 11.9 MPa, an apparent porosity of 45.0%, a relative aggregate tube strength of 25.2% and a median pore size of 262.3 nm.


Introduction
Magnesia-based refractories are widely used for working linings of high-temperature furnaces including steel, non-ferrous metals and cement due to their excellent slag resistance and mechanical properties [1] [2][3][4][5]. The existing magnesia-based refractories use dense sintered magnesia or fused magnesia with high thermal conductivities as aggregates, resulting in significant loss of working linings which is not conducive to the energy conservation and emission reduction for high-temperature furnaces [6][7][8][9][10][11][12][13]. Several researchers have proposed that it is a crucial way to reduce the thermal conductivity of refractories and solve the above problems via lightweight design of the magnesia-based refractories by using microporous aggregates instead of dense aggregates.
Generally, microporous aggregates are fabricated by crushing and sieving microporous ceramics. The preparation methods of microporous ceramics include foaming method [14], direct foam-gelcasting method [15] and in situ decomposition method [16][17][18], etc. For example, Liu et al. [14] prepared porous magnesium ceramics with the porosities of 51.78-80.70% and the compressive strengths of only 0.74-11.32 MPa using MgO and boric acid as raw materials by foaming method. Qiao and Wen [15] obtained porous magnesium aluminate spinel (MgAl 2 O 4 ) ceramics with high porosities of 75.14-81.84%, pore sizes of  nm and the compressive strengths of only 4.0-14.3 MPa applied MgAl 2 O 4 powder and TiO 2 powder as raw materials via the direct foam-gelcasting method. Yan et al. [17] fabricated porous spinel ceramics with an apparent porosity of 40.0%, a compressive strength of 75.6 MPa and a large average pore size of 2.53 μm using magnesite and bauxite as raw materials by the in situ decomposition method. Comparing the above three methods, it can be seen that the porous ceramics prepared by foaming method and direct foamgelcasting method had higher apparent porosities but lower compressive strengths, while the porous ceramics prepared via the in situ decomposition method obtained moderate porosities, small pore sizes and high compressive strengths. Moreover, the preparation process of the in situ decomposition method is simple and conducive for industrial production.
However, there are still two problems of the microporous MgO refractory aggregates prepared by the in situ decomposition method [19][20][21]. First, the available high-grade magnesite raw materials are becoming increasingly depleted and unsustainable. With the continuous exploitation of magnesite resources, resulting in excessive consumption of high-grade minerals and an increasingly apparent resource crisis, and it is difficult to meet the sustainable development of magnesia-based refractories. Second, all the nano-pores produced by the decomposition of magnesite disappear, and the pore size is large. Nano-sized pores are benefit for the heat insulation effect, slag resistance and mechanical properties of the microporous MgO refractory aggregates. First, under the condition of the same porosity, the smaller the porosity with the larger pore area, and the smaller area occupied by the solid phase. As a result, the smaller the heat transfer through the solid phase, which can improve the heat insulation effect of the materials. Second, materials containing nano-sized intracrystalline pores can cause the molten slag permeate into the pores preferentially. Due to the existence of nano-sized pores, an isolation layer can be formed quickly, which improves the slag resistance of the microporous MgO refractory aggregates. For instance, Fu et al. [22] studied that the required dissolution depth for achieving a slag supersaturation is 19.14 μm without nano-sized intracrystalline pores. For comparison, only a dissolution depth of 0.0085 μm is necessary to approach the slag supersaturation for materials containing nano-sized intracrystalline pores. Thirdly, the mechanical properties can be improved through the decrease in the possibility of stress concentration due to the small pore size and the uniform the distribution. For example, Yan et al. [17] significantly increased the compressive strength from 7.3 to 15.2 MPa by reducing the median pore size of microporous spinel aggregates from 9.5 to 6.8 µm.
Magnesite has many impurities such as CaO, SiO 2 and Fe 2 O 3 , and it is easy to form more liquid phases under high temperatures. According to sintering theory, the densification in the intermediate stage of sintering depends on mass transfer through the liquid, while the rates of substance diffusion through the liquid are higher than through the solid. There were two types of pores with different structures in the MgO aggregates prepared by magnesite, which were intraparticle pores formed by in situ decomposition of magnesite and inter-particle pores derived from particle packing between MgO particles, respectively. During the sintering, the liquid phase was formed in the MgO microcrystallite at high temperatures. Wetting, penetration and segregation of the liquid phase can make the MgO particles fragmented. The intra-particle pores were discharged outward as the particle rearrangement and grain coalescing and growing, forming the inter-particle pores which were extremely difficult to be eliminated [23,24].
Salt Lake in Qinghai Province, China, is rich in magnesium resources. Only in Qinghai Tsarhan Salt Lake area, the annual by-product of bischofite produced owing to the production of potash fertilizer is nearly 5 million tons, and the accumulation of bischofite reaches billions of tons so far. At present, based on the Salt Lake bischofite, Mg(OH) 2 has been successfully extracted by ammonia method and other processes, and the MgO content of its calcined powder reaches more than 99 wt%. However, its application in the field of refractory is still mainly used for the synthesis of traditional dense sintered magnesia and fused magnesia, which has no obvious cost-performance advantage over magnesia raw materials produced from magnesite, leading it hard to promote its application [25][26][27][28][29][30][31][32].
Combining the above problems in preparing microporous MgO refractory aggregates and the current technological status of Mg(OH) 2 from Qinghai Salt Lake, the in situ decomposition method using Mg(OH) 2 from Qinghai Salt Lake as raw material is expected to expand the utilization of magnesia resources from Qinghai Salt Lake in China and achieve the sustainable development of magnesia-based refractories.
Firing temperature is a very common and important factor in the study of refractories. Besides, green density is also an important factor during sintering stage in the study of refractories. Higher green densities give more initial particle contacts smaller initial pores, with less densification needed to obtain a final density. All else being constant the sintered density increases with the green density. In part, this is because the higher green density inhibits grain boundary breakaway from the pores. For instance, compaction pressure is applied on powder to control the density of the materials with the structures we expect before sintering [23]. Therefore, microporous MgO refractory aggregates with nanosized pores were prepared by using Mg(OH) 2 powder from Qinghai Salt Lake of China as raw material and the in situ decomposition method in this work. The effects of compacting pressures and firing temperatures on the microstructure and strength of the microporous MgO refractory aggregates were thoroughly investigated.

Preparation of the samples
Mg(OH) 2 powder (d 50 = 40.8 μm, Qinghai Western Magnesium Co., Ltd., China) made from bischofite in Qinghai Salt Lake of China was employed as raw material. The chemical composition of Mg(OH) 2 is listed in Table 1. The MgO content was up to 99.53 wt% after removing the ignition loss. Firstly, 5 wt% water was added into Mg(OH) 2 powder during mixing in a mixer. After obtaining a homogeneous mixture, the blends were pressed to cylindrical samples (36 mm in height and 36 mm in diameter) at 100, 150 and 200 MPa, respectively. The green samples were dried at 110°C for 24 h and heated for 3 h in an electric furnace at 1600°C, 1650°C and 1700°C, respectively. The detailed heating rate is shown as follows: the temperature was raised to 400°C at the rate of 120°C/h and kept for 1 h, and then the temperature was raised to 1600°C, 1650°C and 1700°C at the rate of 200°C/h and kept for 3 h, respectively. According to the compacting pressure and firing temperature, the samples were named 1600-100, 1600-150, 1600-200, 1650-100, 1650-150, 1650-200, 1700-100, 1700-150 and 1700-200, respectively.

Testing and characterization methods
The chemical composition of the raw material was determined by inductively coupled plasma-atomic emission spectrometry (ICP-AES, IRIS Advantage Radial, Thermo Scientific Inc., USA) according to the Chinese standard of GB/T 6900-2006. The particle sizes were measured by the laser particle size analyzer (Mastersizer 2000, Malvern Instruments Ltd., UK). The microstructure was analyzed by scanning electron microscopy (SEM, Nova 400, FEI Company). The microcrystallite sizes were measured by Nano Measurer 1.2.0 (Jie Xu, Shanghai key laboratory of molecular catalysis and innovative materials, China). The pore size distribution was assessed by mercury intrusion porosimetry (AutoPore IV 9500, Micromeritics Instrument Corporation, USA). The linear shrinkage was the ratio of the difference in height of the sample before and after firing to the height of the sample before firing. The apparent porosities and bulk densities were measured by Archimedes' principle with kerosene as the immersion medium. The compressive strengths were tested by a mechanical testing machine (YAW-1000D, Jinan Hengsi Shengda Instrument Co., Ltd., China) and the loading rate was 1.0 MPa/s. The relative aggregate tube strengths were determined by using a procedure described previously [9,33,34]. For this purpose, aggregates with a size of 5-4 mm were first sieved and then filled into a tube with a diameter and height of 36 mm. Then, a pressure of 20 MPa was applied for 10 s. Finally, the mass ratio (m1/m0 × 100%) of aggregates larger than a 3-mm sieve (m1) relative to the initial aggregate mass (m0) was calculated.

Microstructures
The microstructures of the samples compacted at 150 MPa and fired at different temperatures are shown in Figure 1. It can be clearly seen that all the samples were composed of Mg(OH) 2 pseudomorph particles which were formed by the decomposition of Mg(OH) 2 . The samples had the two types of pore structures [35,36], which were intra-particle pores with nano pore sizes formed by in situ decomposition of Mg(OH) 2 and inter-particle pores with micron pore sizes derived from particle packing between Mg(OH) 2 pseudomorph particles, respectively. When the firing temperature was 1600°C, the MgO microcrystallite median size was 1.34 μm, and the number of neck connections between the MgO microcrystallites was few. With the firing temperature raising to 1650°C and 1700°C, the MgO microcrystallite median size increased to 2.30 and 3.15 μm, respectively. At the same time, the number of neck connection between them increased gradually. The nano-sized pores merged and grew obviously at 1700°C, and the degree of densification inside the Mg(OH) 2 pseudomorph particles enhanced.
The microstructures of the samples compacted under different pressures and fired at 1650°C are given in Figure 2. When the compacting pressure was 100 MPa, the MgO microcrystallite median size was 2.82 μm and the intra-particle pore size was relatively large. With the compacting pressure increasing to 150 and 200 MPa, the MgO microcrystallite median size decreased to 2.30 and 1.87 μm, respectively. The interparticle pore size progressively decreased and the neck connection between Mg(OH) 2 pseudomorph particles increased.

Pore characteristics
The pore size distribution and cumulative pore size distribution curves of the samples fired at different temperatures are plotted in Figure 3. The pore size distributions of all the samples were bimodal. The small pore peak 1 was between 220.0 and 230.0 nm, corresponding to the intra-particle pores in the Mg(OH) 2 pseudomorph particles in Figure 1. The big pore peak 2 is located at 2.0-3.0 μm, corresponding to the inter-particle pores between the Mg(OH) 2 pseudomorph particles in Figure 1. According to Figure 1, when the firing temperature increased from 1600 to 1650°C, the peak value of the peak 1 increased significantly and the peak 2 shifted to the left. It indicates that the intra-particle pore size hardly changed, but the inter-particle pore size decreased sharply. When  the firing temperature reached 1700°C, the peak 1 shifted to the right and the peak value decreased, while the peak 2 shifted to the right and the peak value decreased apparently. It illustrates that nanosized pores merged and grew obviously in the Mg(OH) 2 pseudomorph particles at 1700°C, and the degree of densification increased, while the interparticle pore size also became larger. The pore size distributions as a function of the cumulative volume are given in Figure 3. The median pore sizes of the samples 1600-150, 1650-150 and 1700-150 were 279.3, 262.3 and 278.7 nm, respectively.
The pore size distributions and cumulative pore size distributions curves of the samples compacted at different pressures are drawn in Figure 4. The pore size distributions of the samples showed a bimodal distribution, with the small pore peak 1 located between 180.0-230.0 nm, corresponding to the intra-particle pores in Figure 2, and the large pore peak 2 located between 1.5-3.0 μm, corresponding to the inter-particle pores in Figure 2. When the compacting pressure increased from 100 to 150 MPa, the peak value of peak 1 increased significantly and shifted to the right, and the peak value of peak 2 decreased and shifted to the left. Combined with Figure 2, it can be observed that the inter-particle pore size was markedly reduced. With the compacting pressure increasing from 150 to 200 MPa, the peak value of peak 1 decreased and shifted to the left, and the peak value of peak 2 dropped and distinctly shifted to the left. It can be concluded that the intra-particle pore size decreased, and the inter-particle pore size also evidently decreased. The median pore sizes of the samples 1650-100, 1650-150 and 1650-200 were 306.9, 262.3 and 240.5 nm, respectively.

Physical properties
The linear shrinkage of the samples is displayed in Figure 5. When the compacting pressure was the  same, the linear shrinkage of the sample gradually increased with the firing temperature rising. For example, when the compacting pressure was 150 MPa and firing temperature increased from 1600°C to 1700°C, the linear shrinkage increased from 11.5% to 13.5%. When the firing temperature was the same, the linear shrinkage steadily dropped as the compacting pressure became higher. For instance, the linear shrinkage was reduced from 14.4% to 10.8% when the compacting pressure increased from 100 to 200 MPa at 1650°C.
The apparent porosities and bulk densities are shown in Figure 6. As the firing temperature increased at the same compacting pressure, there is no doubt that the apparent porosity dropped and the bulk density increased obviously. For example, when the compacting pressure was 150 MPa and the firing temperature increased from 1600 to 1700°C, the apparent porosity decreased from 46.5% to 41.0%, and the bulk density increased from 1.88 to 2.08 g/ cm 3 . Similarly, raising the compacting pressure also led to an increase in the densification of the samples.
The compressive strengths and the relative aggregate tube strengths are presented in Figure 7. At the same compacting pressure, the compressive strength gradually enhanced with the increase of firing temperature. For instance, when the firing temperature increased from 1600°C to 1700°C at the compacting pressure of 150 MPa, the compressive strength was enhanced from 10.6 to 16.1 MPa. When the compacting pressure was 200 MPa, the compressive strength could not be measured due to the cracking of the samples. Thus, the relative aggregate tube strengths of the samples were added as the comparison. The results reveal that increasing the compacting pressure could also improve the mechanical properties of the samples.

Discussion
The presented results indicate that microporous MgO refractory aggregates containing nano-sized pores were successfully prepared by the in situ decomposition method using Salt Lake Mg(OH) 2 powder as raw material. Since the composition of raw material, particle size distribution and heating rate of each group of the samples were the same, the evolution of microstructure and strength of the microporous MgO refractory aggregates will be analyzed from the two perspectives of compacting pressure and firing temperature in the following. The schematic diagram of the pore-forming mechanism of the samples in terms of different compacting pressures and firing temperatures is given in Figure 8.
First, the schematic diagram of the effect of compacting pressure on the particle packing state in the green samples is shown in Figure 8(a). When the compacting pressure was low, the particles packed relatively loosely and some pores exist between the particles. When the compacting pressure was higher, the contact between the particles was tighter. The   Secondly, the TG and DSC analysis of Mg(OH) 2 is shown in Figure 9. When the green samples were heated up to 300°C, the Mg(OH) 2 particle surface started to decompose. The porous layer containing nanopores and MgO microcrystallites were formed on the particle surface, as shown in Figure 8(b). When the compacting pressure was low, the water vapor of the decomposition products of Mg(OH) 2 could escape along the inter-particle pores due to the large interparticle distance. When Mg(OH) 2 was completely decomposed, the structure was formed by the packing of Mg(OH) 2 pseudomorph particles. As the compacting pressure became higher, the particles tended to pack closely and the large pores between the particles were blocked, causing it difficult for the water vapor of decomposition products of Mg(OH) 2 to escape. Since the Mg(OH) 2 particle size almost did not change during the process from room temperature to 1000°C, as presented in Figure 10, which showed that the median particle sizes of Mg(OH) 2 at room temperature and at 1000°C was 41.8 and 41.5 μm, respectively, the large interparticle pores were always blocked at high compacting pressure. In the range of 300-1000°C, the water vapor pressure increased continuously with the firing temperature rising. When the water vapor pressure was higher than the strength of Mg(OH) 2 pseudomorph particles, it could produce microcracks inside the pseudomorph particles, as shown in Figure 8(b), and increase their pore volume. When the water vapor pressure was higher than the bonding strength between Mg(OH) 2 pseudomorph particles, cracks tended to form in the samples. This was also demonstrated by the fact that all three groups of samples were cracked, and their compressive strengths could not be measured after firing at 1600-1700°C with the compacting pressure of 200 MPa. Therefore,   compared to the samples with the low compacting pressure, the samples with higher compacting pressure should exhibit a larger internal pore volume after 1000°C treatment, leading to the difference in the internal structure of Mg(OH) 2 pseudomorph particles during that period and could change their later sintering behavior. Thirdly, as the firing temperature rose to 1600°C, the median particle size of Mg(OH) 2 after firing was 37.0 μm, as presented in Figure 10, which was significantly reduced to 10.8% compared with that after firing at 1000°C, and the particles shrank obviously, as given in Figure 8(c), which indicates that the degree of sintering apparently increased when the firing temperature raised above 1600°C.
Take the compacting pressure of 150 MPa as an example, when the firing temperature rose from 1600°C to 1650°C, the linear shrinkage of the samples increased from 11.5% to 12.2% with a bit change of the small pore size within the Mg(OH) 2 pseudomorph particles exhibited in Figures 1 and 3, which reduced the volume and size of large pores between the pseudomorph particles and led to an increase in the proportion of small pores within the pseudomorph particles. Therefore, a quite increase of the small pore peak (Peak 1 in Figure 3) and a significant leftward shift of the large pore peak (Peak 2 in Figure 3) appeared. The significant reduction of the large pore size between the pseudomorph particles and the small pore size inside them did not change much, which resulted in a slight decrease in the median pore size of the samples after firing at 1650°C. When the firing temperature reached 1700°C, the sintering densification of the pseudomorph particles enhanced, and the MgO microcrystallites grew considerably, while the small pores within the pseudomorph particles merged, as shown in Figure 1. At the same time, the large pore size and pore volume between the pseudomorph particles due to the increase of the linear shrinkage of 13.5% resulting in a significant rightward shift of the small pore peak, and a decrease in the peak value (Peak 1 in Figure 3), as well as a distinct decrease in the peak value of the large pore peak (Peak 2 in Figure 3). Compared with the samples sintered at 1650°C, the large pores between the pseudomorph particles did not change a lot, but the small pores inside them combined and grew, causing an increase in the median pore size of the samples after firing at 1700°C. The reason for the rightward shift of Peak 2 was probably due to the increase of inter-particle pore size caused by the shrinkage of the pseudomorph particles after sintering, which was larger than the decrease of interparticle pore size caused by the shrinkage of the samples after firing, increasing the inter-particle pore size between the pseudomorph particles. The increase of firing temperature led to a decrease in pore volume, an increase in the densification of the pseudomorph particles, an increase in the MgO microcrystallite size, and an improvement in the neck connection between the MgO microcrystallites, which should be the main reason for the strength enhancement of the samples.
At the same firing temperature, taking the firing temperature of 1650°C as an example, when the compacting pressure rose from 100 to 150 MPa, the water vapor pressure generated by the decomposition of Mg(OH) 2 was too large, resulting in microcracks inside the pseudomorph particles, which increased the initial porosity and pore size of Mg(OH) 2 pseudomorph particles, and reduced the number of contact points around the particles and decreased the mass transfer path. These decreased the rates of microcrystallite growing and pore merging. The increase of the pore size within the pseudomorph particles (i.e. the rightward shift of the small pore peak in Figure 4) may be caused by the excessive microcracking. On the other hand, the peak value of small pore peak (Peak 1 in Figure 4) shifted upward sharply and the peak value of large pore peak (Peak 2 in Figure 4) significantly shifted to the left and dropped due to the reduction in the volume and size of large pores between the particles. When the compacting pressure increased to 200 MPa, the samples cracked probably because the water vapor pressure exceeded the inter-particle bonding strength of Mg(OH) 2 pseudomorph particles, and the reduced water vapor pressure after releasing water vapor had little effect on the intra-particle pore size, so the intraparticle pore size did not differ much from that of the samples at the compacting pressure of 100 MPa, and the small pore peak (Peak 1 of Figure 4) shifted to the left. Meanwhile, the volume and size of large pores between the pseudomorph particles were further reduced, which promoted the formation of neck connection, as shown in Figures 2 and 8(c), increased the degree of sintering, and reduced the apparent porosity, as displayed in Figure 6, leading to the reduction of both large and small pore volumes. Therefore, the peak value of small pore peak (Peak 1 of Figure 4) dropped, and the peak value of large pore peak (Peak 2 of Figure 4) shifted significantly to the left and decreased. The increase in compacting pressure caused the reduction of inter-particle pore volume and size and the promotion of neck connection, which may be the main reason for the enhancement in strength of the samples.

Conclusions
In the present work, microporous MgO refractory aggregates containing nano-sized pores were successfully prepared by in situ decomposition method using Salt Lake Mg(OH) 2 powder as raw material. It was found that the compacting pressure and firing temperature had great influences on the microstructures and strengths of microporous MgO refractory aggregates: (1) Microporous MgO refractory aggregates were composed of Mg(OH) 2 pseudomorph particles derived from the decomposition of Mg(OH) 2 which caused the microporous MgO refractory aggregates contain two kinds of pores. One located inside the Mg(OH) 2 pseudomorph particles, called intra-particle pores with a pore size of about 180.0-230.0 nm. The other one located between the Mg(OH) 2 pseudomorph particles produced from the packing of Mg(OH) 2 pseudomorph particles which called inter-particle pores with a pore size of about 1.5-3.0 μm. (2) At a certain compacting pressure, with the rise of firing temperature, the MgO microcrystallite median size increased from 1.34 to 3.15 μm, and the neck connection between microcrystallites also improved gradually, which enhanced the compressive strength of the samples from 10.6 to 16.1 MPa. When the firing temperature was constant, with the compacting pressure rising, on the one hand, the increase in water vapor pressure generated by the decomposition of Mg(OH) 2 produced microcracks, which reduced the mass transfer and decreased the rates of microcrystallite growing and pore merging. The MgO microcrystallite median size was reduced from 2.82 to 1.87 μm. On the other hand, it promoted the tight particle packing between the pseudomorph particles, reducing the size and volume of the pores between them while improving their neck connection, which enhanced the relative aggregate tube strength of the samples from 25.2% to 30.7%. (3) When the compacting pressure was 150 MPa and the firing temperature was 1650°C, the samples showed the best overall performances with a bulk density of 1.92 g/cm 3 , a high compressive strength of 11.9 MPa, an apparent porosity of 45.0%, a relative aggregate tube strength of 25.2% and a median pore size of only 262.3 nm.