INTRODUCTION

Typically, MgO-CaO refractories are composed of50-80 wt. % of MgO. Different ways have been proposed to produce MgO-CaO refractories [1-5].  Due to exhibiting an excellent chemical resistance against basic environments (as slag and fluxes) at high-temperature, good thermal shock resistance, low vapor pressure, thermodynamic stability in the presence of carbon,  and a suitable abrasion resistance, MgO-CaO refractories are widely used in ferrous, non-ferrous and cement industries. However, in spite of these advantageous properties, the application of MgO-CaO refractory bricks have not been popular due to their tendency to hydration when exposed to the atmosphere [5-10].

Recently Nano-technology was introduced to the MgO-CaO refractories industry, and nowadays it is an important tool included in many research projects [11]. In this study, all the efforts which donned for improvement the hydration resistance of MgO-CaO refractories were investigated and revealed that the use of nanoparticles has been the best results.

Refractories and Magnesia-Doloma Refractories

Refractories are in use since mankind began to develop metallurgical process, being clay the first refractory raw material ever used. This traces refractory development back to years 3500-3000 BC, and at around 1500 BC furnaces made of refractory bricks have started to be developed for the production of metals and glass. Up to the 19th century, refractories were composed of natural ores, such as dolomite stones and clay, because, the temperatures required for ore beneficiation, as well as the aggressiveness of the industrial slags, were not as demanding as those of modern industry. It was at the end of the 18th and the beginning of the 19th century that the foundations of modern metal beneficiation, the development of Portland cement and of modern glass processes started to impose higher requirements on the refractory industry. The new processes demanded higher quality refractory linings, which brought the need to use higher quality raw materials. Silica, zircon sand, chrome ore, magnesite, dolomite, and fireclay started to be used according to the particularities of the process for which the refractory was needed. Schaefer rediscovered monolithic linings at 1914, which were pliable in the beginning, but evolved to cement-bonded powdery concretes in the 1930’s [12]. In the 1960’s, calcium-aluminate cement, more specifically Cement Fondue started to be used for refractory applications, followed by higher-quality 70% and 80% cement at the end of 1970’s and beginning of 1980’s. Concomitantly, the difference between mechanical and corrosion resistance of castables, when compared to bricks, started to be diminished, due to the introduction of super-fine raw materials and dispersing aids to castables, which enabled the reduction of cement and water content, creating a more compact microstructure with enhanced properties. In the beginning of the 1990’s, pumping was adapted from the building to the refractory industry, which enabled very high installation rates, and also reduced the material losses and environmental problems associated to dry gunning [13]. Refractories play an important role in metallurgical, glassmaking and ceramic industries, where they are formed into a variety of shapes to line the interiors of furnaces or kilns or other devices for processing the materials at high temperatures [14-16]. Many of the scientific and technological inventions and developments would not have been possible without refractory materials. Dreaming about producing one kilogram of any metal without the use of refractory is almost quite impossible. The ASTM C71 defines the refractories as “nonmetallic materials having those chemical and physical properties that make them applicable for structures or as components of systems that are exposed to environments above 1000 °F (538°C) [12, 14]. In tune with the changing trends in steelmaking, especially in ladle metallurgy, the high performing shaped refractories are on an increasing demand in recent years. The higher campaign lives and the variety of the newer steelmaking operations are decided by the availability and performance of such shaped refractories with superior high- temperature mechanical strength, erosion and corrosion resistance [17]. Initially, the ladles were used only to transport the steel from steel making unit to casting bay, but nowadays the refining process is also carried out in the same. Thus, steel producers throughout the world have been putting on a continuous effort to improve the ladle life in order to increase the performance of ladles as well as reduces the specific consumption of refractories so as to have a strong grip over the cost and quality of steel and also to increase the ladle availability with lesser number of ladles relining per day. Due to the above-said reasons, there had been a great technological evolution in ladle lining concept such as; Zonal lining concept, which deals with both selections of refractory quality and refractory lining thickness [17-19].The type of refractories to be used is often dictated by the conditions prevailing in the application area. Generally, refractories are classified into two different groups:

(a) Based on raw materials, the refractories are subdivided into three categories such as acidic (zircon, fireclay, and silica), basic (dolomite, magnesite, magnesia-carbon, chrome-magnesite and magnesite-chrome) and neutral (alumina, chromite, silicon carbide, carbon, and mullite).

(b) Based on the manufacturing process, the refractories are subdivided into two categories such as shaped refractories (available in the form of different brick shapes, and includes the oxide and non-oxide systems) and unshaped refractories (which includes mortars, castables and monolithic) [17, 20].

The MgO-CaO system is remarkable for the high liquidus and solidus temperatures over the complete range 100% MgO- 100% CaO, as the eutectic for the CaO–MgO binary system occurs at 2370 ºC [3, 21]. MgO–CaO bricks are high-value refractories composed of lime (CaO) and periclase (MgO). MgO –CaO refractories have some advantageous and disadvantageous compared to MgO and CaO refractories (Table 1). Typically, these refractories are composed of 50-80 wt. % of MgO. MgO-CaO refractories are considered as one type of chrome-free refractories that are suitable for substituting the MgO-Cr₂O₃.  Different ways have been proposed to produce MgO-CaO refractories. A new approach is using sintered and fused Co-clinker of magnesite and dolomite as a  starting material for the MgO-CaO refractories which would lead to more homogenous products with more desirable properties. Another way is mixing magnesite and dolomite and calcination them at high temperature that let to in–situ MgO-CaO refractory brick [2, 6].These refractory bricks have been playing a crucial role as a refractory material in various industries such as secondary metallurgy (AOD, VOD, etc.), non-ferrous furnaces (copper converter) and cement making (rotary kiln) because of their great advantageous such as high temperature stability, low thermal expansion, excellent thermal shock resistance , outstanding erosion–corrosion performance at high temperatures, wide availability of raw materials, low vapor pressure, and thermodynamic stability in the presence of carbon in a composite oxide/carbon refractory[21, 22]. Furthermore, MgO-CaO refractories are beneficial to removing inclusions from molten steels; thus, they have been considered to be one of the effective refractory types for processing clean steel products [23].In recent years, with the increasing demands of molten steel purity, the awareness of environmental protection and resource shortage grows, MgO-CaO materials have become one of the attractive steelmaking refractories [24].However, in spite of these advantages properties, the application of MgO-CaO refractory bricks has not been popular due to their tendency to hydration when exposed to the atmosphere[1, 25-28].  The CaO and MgO phases react easily with moisture in the atmosphere and formation CaO (OH) ₂ and Mg (OH) ₂ phases (Eq. 1 and 2), the volume expansion of the resultant can cause severe damage to the materials [29, 32- 36].

CaO + H₂O   =   Ca (OH) ₂                           (1) [1, 2 and 5]

MgO+H₂O    = Mg (OH) ₂                             (2) [1, 2 and 5]

Much effort has been made to improve the performance of MgO-CaO bricks. It has been reported that physical properties of MgO-CaO refractory bricks could be improved by using pitch, tar, flake, and vein graphite minerals [22, 26, 68, and 38]. For example, multi-impregnated pitch-bonded dolomite refractory brick for ladle furnace was described by Rabahand Ewais [39]. Brick samples were prepared from a blend of calcined dolomite mineral and coal tar pitch. The blend was hotly mixed and pressed under a compression force up to 151 MPa. Green bricks were baked for 2 h at temperatures up to 1000◦C. Voids in the baked bodies were filled with carbon by multiple impregnations using low softening point coal tar pitch. Each impregnation step (30 min)was followed by calcination at 1000◦C. Brick samples containing8–12 wt.% coal tar pitch binder and pressed under 108–151 MPa acquired quantify crushing strength. However, multi-impregnating favored the mechanical strength of the baked brick samples and improved their hydration resistance (>45 days). Dolomite brick samples containing 10 wt. % coal tar pitch and pressed at 108 MPa gave high hydration resistance (more than 60 days in normal condition) compared to the hydration resistance of the commercial bricks (30 days). The prepared brick samples have acceptable density, chemical stability, outstanding resistance and good mechanical properties that would meet the requirements of ladle furnace (LF) for steelmaking industry. Although the aforementioned method (used of tar, coal, and pitch) have acceptable results, due to released CO and CO₂ gases into the atmosphere it can lead to polluting the air. Also, the hydration resistance of MgO-CaO refractories can be improved by treating in a CO₂atmosphere or by surface phosphate coating which leads to the formation of a dense layer on the surface of CaO and protects Cao grain from hydration [1].For example, Min Chen et al. [40] reported the effect of porosity on carbonation and hydration resistance of CaO materials. Cao pellets with different porosity were carbonated at 700 ◦C in the CO₂ atmosphere. The carbonation rate was controlled by the diffusion of CO₂, regardless of the difference in porosities. For the low-porosity pellet, carbonation reaction only occurred on the surface, with a dense CaCO₃ film thus formed, which combined well with the substrate material; while for the pellet of high-porosity, the carbonation reaction occurred simultaneously both on surface and inside pores, and each CaO grain was surrounded by CaCO₃ film that contained micro fissures. Hydration test results showed that carbonation treatment could effectively improve the hydration resistance of CaO materials regardless of porosity, but the carbonated high-porosity pellet was prone to breakage due to a poor combination between the carbonated CaO grains. Therefore, for the purpose to improve the hydration resistance by carbonation treatment, it is recommended that the CaO materials should be either with less appreciable apparent porosity or with a limited carbonation ratio for the high-porosity CaO material. Also, much effort has been made to improve the performance of MgO-CaO bricks through the addition of different additives [27, 37, and 41], such as V₂O₅ [42],CaF₂ [43], CuO [9], FeTiO₃[44], ZrO₂ [45, 46], Ce₂O[23],NiO [47],BaO [48], Al₂O₃[49], ZrSiO₄ [50], La₂O₃ [51, 52]and Fe₂O₃ [7, 53, 54, 55],  For example;Min Chen et al.[56] investigated the slaking resistant of  CaO aggregate from lightweight CaCO₃ with oxide addition. For this propose, CaO aggregate was sintered from reagent-grade lightweight CaCO₃ powder by the addition of 0–20% (molar ratio) MgO and ZrO₂, respectively. The results showed that the CaO derived from lightweight CaCO₃ was highly sinterable and compact CaO aggregate with relative density above 96% was obtained after sintering at 1400 °C for 2 h, but further increase of compactness was restrained due to the occurrence of abnormal grain growth. The densification of the aggregate was promoted due to the behavior of oxide addition on restraining the grain growth of CaO. With increasing the amount of oxide addition, the microstructure of CaO aggregate underwent a restructuration process. Homogeneous microstructure, with well -growing MgO grains occupying most of the boundary triple points of CaO grain, formed by the addition of 20% MgO. Especially when 20% ZrO₂ was added, a CaZrO₃ layer formed around CaO grains. The slaking resistance of the aggregate was appreciably improved due to the promotion of densification, the formation of CaO solid solution (while MgO added) and the modification of microstructure. In another study [78] the effect of NiO addition on the sintering properties of dolomite clinker was investigated. In this study, nature dolomite was carried out in the presence of NiO by two-step calcination process.  The results showed that the doping of NiO to natural dolomite changed the lattice constants of CaO and MgO and made the MgO lattice distortion happen, which consequentially reduced the activation energy of the grain growth and promoted the sintering of the dolomite. Without additive the bulk density and the apparent porosity of dolomite clinker after the sintering at 1600°C were 3.30 g/cm³ and 3.4%, the crystal size of MgO only was 3.26 μm. But when the addition of NiO accounted to 0.75%, the bulk density and the apparent porosity of dolomite clinker after the sintering at 1600°C were 3.33 g/cm³ and 2.7%, respectively. At the same time, the crystal size of MgO reached to 3.54 μm [47]. Zhang Han et al. [43] studied the effect of CaF₂ on the sintering properties of MgO-CaO materials. The results show that with increasing the addition of CaF₂, the bulk density of the samples increased, while the apparent porosity decreased and the densification of MgO-CaO materials promoted. When the amount of CaF₂ exceeded 2wt. % the densification degree of samples decreased. The nature of CaF₂promoting densification of MgO-CaO materials could be concluded as follows: due to its thermal defects, F-entered into the octahedral voids that existed in CaF₂crystal structures and produced F-vacancies with positive charge, then combined with O₂-vacancies by electrostatic attraction during the migration process, which increased the diffusion speed of O₂-and enhanced the diffusion of MgO, then promoted the growth of periclase grains.  In another study, A. Ghosh et al. [73] investigated   the densification and properties of lime with V₂O₅ additions.  For this propose, sintering of lime was carried out in the presence of V₂O₅ by a single firing process. A pure limestone was crushed, mixed with 1, 2 and 4 wt. % V₂O₅, pelletized and fired between 1550 and 1650 ^oC. The sintered lime was evaluated by bulk density, apparent porosity, microstructure, hydration resistance and hot modulus of rupture (HMOR) at 1300 ^oC. Incorporation of V₂O₅ forms liquid phase with lime at elevated temperature and influences the densification process by liquid phase sintering. As a result bulk density of sinters improved and they become more hydration resistant due to the larger grain size of the lime phase. The hot strength increased up to a certain temperature followed by deterioration because of the pressure of higher amount of liquid phase. Sintering behavior and hydration resistance of reactive dolomite was studied by Ghosh and Tripathi [24]. The hydroxide derived from dolomite was developed through pre-calcination of dolomite followed by its hydration. For hydroxide development, after pre-calcination, one sample was air quenched and the other powder was a furnace cooled before hydration. The air quenched samples showed better densification than that of the furnace cooling process at the same temperature. Fe₂O₃addition enhances sintering by liquid formation at higher temperature. The grain size of doloma with Fe₂O₃addition is bigger than that without additive. Hydration resistance was related to densification and grain size of sintered dolomite.  H. A. Yeprem investigated the effect of iron oxide addition on the hydration resistance and bulk density of doloma. At his study, pure (with no additives) and mill scale (98.66 wt. % Fe₂O₃ content) added (up to 1.5 wt. %) natural dolomite of Selcuklu-Konya-Turkey fired at 1600–1700 ◦C for 2–6 h using the one-stage process. According to the results of experiments with 15 sintered samples, sintering temperature, soaking time and increase of the mill scale amount were found to increase the bulk density and thus decrease the observed apparent porosity. In hydration resistance tests, it seemed that the same characteristics also increased the resistance. Furthermore, EDX analysis of the dolomas that were sintered at three different temperatures each with 0.5 wt. % mill scale additions and also at 1700 ^oC/2 h with 0–1 wt. % mill scale additions were performed. Quantities of Fe^{2+, 3+} inside the periclase (MgO) were examined [55, 57]. A.G.M. Othman et al. [37] studied The Hydration-resistant lime refractories with addition ilmenite raw materials. For this reason, the ferri-ilmenite ore existing at Abu Ghalaga, Eastern Desert was added as a dopant material in amounts of 0.5, 1.0, 2.0, and 3.0%. Densification parameters and hydration resistance of the fired grains were investigated. The densest hydration resistant grains were selected to assess their refractory quality by determining load-bearing capacity and thermal shock resistance. These results were interpreted in the light of phase composition and microstructure of the fired grains. It is concluded that dense and hydration resistant lime grains can be processed by doping the pure limestone powder with 2.0–3.0 wt. % of ferri-ilmenite before firing up to 1550C. Such level of ilmenite content has contributed in the densification of lime particles in the solid state and also by limited amount of the developed liquid phase. Hence, direct-bonded lime network is formed with partial interruption by a platey calcium–alumino–ferrite–titanate phase, which crystallized on cooling from the liquid phase at the grain boundaries of the lime–lime network. This improves the bulk density of fired grains to about 3.2–3.3 g/cm³ and its rate of hydration to 4.15–3.80 g/h without significant deterioration of its load-bearing capacity and thermal shock resistance.  A. Ghosh et al. [9] studied the effect of CuO addition on the sintering of lime. The result showed that Hydration resistance was measured at 50^oC in 95% relative humidity through the weight gain after 3 h. Addition of CuO up to 2 wt.% improved the hydration resistance, but it was not significantly high in comparison to that of 1 wt.%CuO. The use of a higher level of CuO in lime did not show any further improvement in hydration resistance. The CaO forms a low melting compound (2CaO.CuO) with CuO which helps liquid phase sintering of lime. When the liquid content increased in the sintered lime grain growth takes place simultaneously along with pore growth. L. Liu et al. [48] reported the effect of BaO addition on densification and mechanical properties of Al₂O₃-MgO-CaO refractories. Results indicated that the formation of calcium hex aluminate (CaO. 6Al₂O₃, or CA₆) grains with a high aspect ratio in the alumina-rich zone depressed the densification of the sample without BaO addition, resulting in a higher apparent porosity of 21.2%. When 6 wt. % BaO was added, a new phase of Ba₂Mg₆Al₂₈O₅₀ (BAM) with a lower aspect ratio was formed and the densification of the sample with an apparent porosity of 5.52% was promoted. In addition, mechanical performance was significantly improved due to an increase in compactness and modification of the microstructure. The cold compressive strength increased from 348 MPa to569 MPa and the flexural strength increased from 178 MPa to 243 MPa by addition of 6 wt. % BaO. Meanwhile, the breadth of the widest crack after the thermal shock test decreased from 7 µm to 1 µm in the refractory. A. Miskufova et al. [49] reported the properties of CaO sintered with addition of active alumina. They evaluated the influence of active gama alumina addition on green and sintered CaO material properties, microstructures and mineralogical phase formation. Experimental results have shown the possibility to prepare more stable CaO with excellent properties by energy saving one-stage burning process of natural ground limestone with small addition of γ-Al₂O₃ (1 wt. %) at up to 1550°C for two hours. The additive caused increasing of the sintered density but especially significant decreasing of apparent porosity of CaO. X-ray diffraction and energy dispersive X-ray fluorescence analysis confirmed mainly the presence of 3CaO∙Al₂O₃ on the grain boundaries. Formation of other phases during sintering, more specifically 12CaO∙7Al₂O₃ and CaO∙6Al₂O₃ with lower tendency to hydration was also proved. In another research, CH. Hee Chao at al. [58] studied the effect of Al₂O₃, MgO and SiO₂ on sintering and hydration resistance of CaO ceramics. CaO ceramics were prepared by conventional sintering process and their hydration behaviors were evaluated by measuring weight increment on saturated water vapor pressure at ambient temperature. CaCO₃ and limestone were used as CaO source materials and Al₂O₃, MgO and SiO₂ were added as sintering agents. Al₂O₃ was as liquid phase sintering agent to increase densification and grain growth rates, whereas MgO and SiO₂, densification and grain growth inhibitors. Regardless of composition, all of the prepared CaO ceramics showed the improved hydration resistance as bulk density increased. Therefore, to decrease contact area between CaO and water vapor by increasing bulk density with the Al₂O₃ sintering additive was effective for the improvement of CaO hydration resistance.

Application of Nanotechnology in magnesia doloma refractories

 Nano-technology is mainly defined by size and comprises the visualization, characterizations, production and manipulation of structures which are smaller than 100 nm [59, 60]. The structures the dimensions of which range from 100 nm down to approx. 0.1 nm exhibit special mechanical, optical, electrical, and magnetic properties which can differ substantially from the properties of the same materials at larger dimensions.  Therefore, nanotechnology is a very active research field and has applications in a number of areas. Currently, significant attention has been paid to the application of nanotechnology in the development of refractories products [61-63]. Nanotechnology has been introduced to refractories. It has been reported that the performance of the refractories was appreciably improved for the good dispersion of nano-sized particles in the microstructure and reaction activity. Several efforts have been made by various researchers to improve the properties of refractories (bricks and castable) by using Nanoparticles (Table 2). The application of nanotechnology is aimed at obtaining the following unique properties of brick and castable refractories: ultra-high compressive strength, relatively high tensile strength and ductility, more efficient cement hydration, increased aggregate-paste bond strength, high corrosion resistance, control of cracks and self-healing. In the case of refractory materials, the same properties as well as, high resistance to thermal shock, abrasion, and chemical corrosion must be obtained [62, 106-108]. The first papers on nanotechnology in refractories causing a big interest appeared in UNITECR (The Unified International Technical Conference on Refractories) in 2003.The researchers of these as well as later published papers try to modify the matrix (binding phase) of advanced refractory materials with nano-sized additives[59, 107, 109, 110]. Recently Nano-technology was introduced to the Magnesai-Doloma refractories, and nowadays it is an important tool included in many research projects. Several research groups have been working on the addition of different types of additives in Magnesai-Doloma refractories, and some of them have focused their investigations on the use of Nano-oxides, due to the reported benefits of adding these particles to ceramic bodies. In their research work, Salman Ghasemi-Kahrizsangie et al. [7] studied the densification and properties of Fe₂O₃ nanoparticles added CaO refractories. For this propose, up to 8wt. % of Nano-iron oxide was added to CaO refractory matrix. As a result, it was found that the presence of Nano-iron oxide in the CaO refractory matrix induced 2CaO.Fe₂O₃ (C₂F), CaO.Fe₂O₃ (CF) and 3CaO.Al₂O₃ (C₃A) phase’s formation, which improved the sintering process. Nano-iron oxide also influenced the bonding structure through a direct bonding enhancement. On the Other hand, the presence of Nano-iron oxide was resulting in improved properties of CaO refractory matrix refractories such as bulk density, hydration resistance (Fig.1) and cold crushing strength. The maximum flexural strength at 1200°C is achieved by the samples containing 4wt. % nano-Fe₂O₃. Also, they reported the effect of nano-TiO₂ additions on the densification and properties of the magnesite-dolomite ceramic composite. Nano-titania, up to 8 wt. %, was added to Magnesite-Dolomite refractory matrix. As a result, it was found that the presence of Nano-TiO₂ in the Magnesite-Dolomite matrix induced titanates formation (Mg₂TiO₄ and CaTiO₃), which improved the sintering process. Nano-titania influenced the bonding structure through a direct bonding enhancement. In general the addition of 6 wt. % of Nano-TiO₂ contributed to reaching a maximum increment in physical and mechanical properties. Also, the hydration resistance increase with addition Nano-TiO₂ up to 8 wt. %( Fig. 2) [2]. Another interesting report comes from Min Chen et al [28], who studied different sizes of zirconia (micro–nano-powders) added to MgO–CaO refractories sintered at 1600 °C. The results showed that the densification of the MgO–CaO refractories were appreciably promoted when a small amount of ZrO₂ was added owing to the formation of small size CaZrO₃ facilitated to sintering, and the densification was promoted further with increasing the amount of ZrO₂ due to the volume expansion caused by the reaction of the added ZrO₂ and CaO to form CaZrO₃ in the refractories, and the addition of nano-sized ZrO₂ was more effective. The thermal shock resistance of the MgO–CaO refractories was improved by modification of the microstructure due to the formed CaZrO₃ particles that predominately located on the grain boundaries and triple points in the whole microstructure, and the addition of nano-sized ZrO₂ was more effective attributed to its good dispersion and the critical addition amount was effectively decreased to 6%. The slaking resistance of the MgO–CaO refractories was appreciably improved by the addition of ZrO₂ due to its effect on decreasing the amount of free CaO in the refractories; promotion of densification as well as modification of microstructure, the nano-sized ZrO₂ addition was more effective due to its higher activity (Fig. 3). The slag corrosion resistance of the MgO–CaO refractories was enhanced by the addition of ZrO₂ due to the increase of the viscosity of the liquid phase and thus inhibited further penetration of slag at elevated temperatures. Also, the use of ZrO₂ nanoparticles on the densification and properties of ZrO₂ magnesia – doloma refractories was investigated by Salman ghasemi-kahrizsangi et al. [21].  In their work, the effect of nano and micro ZrO₂ addition on the densification and hydration resistance of MgO-CaO refractories was investigated. 0,2,4,6 and 8 wt. % ZrO₂ was added to MgO-CaO refractories that contain 35 wt. % CaO. Results show that with the addition of ZrO₂ the bulk density and hydration resistance of the samples increased (Fig. 4) while apparent porosity decreased. Also, the hydration resistance of the samples was appreciably improved by the addition of ZrO₂ due to its effect on decreasing the amount of free CaO in the refractories, promotion of densification as well as modification of the microstructure. Also, it revealed that the nano ZrO₂ addition was more effective than micro ZrO₂ due to its higher activity.  In another study, MgAl₂O₄ nanoparticles were added to MgO–CaO refractory ceramic composites in the range of 0–8 wt. %. Refractory specimens were obtained by sintering at 1650◦C for 3 h in an electric furnace. Results show that with additions of MgAl₂O₄ nanoparticles the bulk density of the samples increased.  But the apparent porosity and cold crushing strength decreased and increased, respectively with addition MgAl₂O₄ nanoparticles up to 6 wt. % and for further MgAl₂O₄ nanoparticles, due to the thermal expansion mismatch, the results is reversed. Also, the hydration resistance of the samples was appreciably improved by the addition of MgAl₂O₄ nanoparticles due to its effect on decreasing the amount of free CaO in the refractory composite and promotion of densification by creating a dense microstructure (Fig. 5) [111].

Also, Up to 3wt. % of Cr₂O₃ nanoparticles were added to MgO-CaO refractory matrix. As a result, it was found that the presence of Cr₂O₃ nanoparticles in the MgO-CaO refractory matrix induced CaCr₂O₄ and MgCr₂O₄ phases formation, which improved the sintering process. Cr₂O₃ nanoparticlesalso influenced the bonding structure through a direct bonding enhancement. On the other hand, the presence of Cr₂O₃ nanoparticles resulted in improvement properties of MgO-CaO refractory matrix such as bulk density, hydration resistance (Fig. 6), and cold crushing strength. The optimum properties have been achieved by the samples containing 1.5wt. % Cr₂O₃ nanoparticles [112].

CONCLUSION

In this review paper, we mentioned all efforts done to improve the performance of MgO-CaO refractories and it was found that the use of Nano-additives has the best results compared to microparticles. Studies show that in the recent years, researchers strongly have been using Nano-additives and have achieved satisfactory results. The results show that in general the use of Nano-additives to improve the properties of magnetite-dolomite refractories through the following ways:

- Promotes densification of MgO-CaO by forming solid-solution and by creating cation or anion vacancies (solid –state sintering mechanism).

- Or by liquid phase sintering mechanism.

Generally, the improvement hydration resistance trend of nano additives is MgAl₂O₄< Fe₂O₃<TiO₂<Cr₂O₃<ZrO₂. Also, the use of nano-additives compared with other additives (micro) with smaller amounts, have better results. Which leads to cost savings, and subjected to the attention of refractories producers and consumers.

CONFLICT OF INTEREST

The authors declare that there are no conflicts of interest regarding the publication of this manuscript.

If you have any problem in viewing images please download the PDF version.