| MgO-Cr2O3 refractories have been widely used in cement rotary kilns and secondary refining vessels in metallurgical industries for their excellent properties such as high refractoriness, excellent slag corrosion resistance and thermal shock resistance. Whereas, the environment problems which caused by toxic hexavalent chromate have been attracting people to pay much attention on this subject. Many studies have been carried out by developed countries on developing new type environmental friendship chrome-free refractories to substitute MgO-Cr2O3 refractories, and the chrome-free refractories have been used in 60%-70% cement rotary kilns. Study on developing the substitution of MgO-Cr2O3 refractories was carried out early in our country, but this subject progressed at a slower rate. However, the development of the substitution of MgO-Cr2O3 refractories in our country would progress at a rapid rate with the increase in environmental concerns and the establishment of correlative laws by state.MgO-CaO refractories were considered to be one type of chrome-free refractories to substitute MgO-Cr2O3 refractories for their properties of excellent stability at high temperatures, excellent corrosion resistance to alkalinity slag and adherence ability. However, their application has been inhibited for their poor thermal shock resistance and hydration resistance. It has been reported that the addition of ZrO2 can appreciably improve the properties of the MgO-CaO refractories, but the addition of ZrO2 causes the price of product appreciably rising, and then the application of these refractories was inhibited. At present, the content of ZrO2 in MgO-CaO-CaZrO3 refractories in overseas was more than 10%, which was too expensive to our country. Therefore, it was important for all refractory researchers to decrease the amount of ZrO2 for the purpose to improve the properties of the MgO-CaO refractories, resulting to the cost of the refractories appreciably reduced and the application of MgO-CaO-CaZrO3 refractories promoted.At the present time, the research technique on MgO-CaO refractories with ZrO2 addition in our country was focused on the synthesis of MgO-CaO refractories with granule or micro powder ZrO2 addition, the effect of the starting materials and burning system on sintering, microstructure and perforemance of MgO-CaO-CaZrO3 refractories was studied. This study was still at the primary stage with a wide gap between developed countries and us.Recently, the application of nano-technology in refractories was very active, the nano-sized additives could appreciably promote the performance of refractories due to the special characteristic such as size effect. In addition, magnesite, which was used as the main starting material to synthesize MgO-CaO-CaZrO3 refractories, is aboundant in our state. However, the MgO-CaO clinker was always synthesized from high-grade magnesite, it would caused the problem that the utilization ratio of low-grade magnesite and magnesite powder was very low. Due to the manufacturing status of MgO-CaO clinker and the researching status of MgO-CaO-CaZrO3 refractories, raw magnesite and calcium carbonate were used as starting materials to synthesize MgO-CaO clinker, in this paper, the effect of technological conditions on purification and the sintering of MgO-CaO clinker was studied, and then the as-received and synthesized MgO-CaO clinker were used as the main materials while micro-sized and nano-sized ZrO2 as additives to synthesized MgO-CaO-CaZrO3 refractories, the contrast between the different effect and mechanism of micro-sized and nano-sized ZrO2 on the sintering, microstructure and performance of MgO-CaO refractories were studied. The main conclusions were listed as following:(1) High-density and high-purity MgO-CaO clinker was synthesized through hydrous carbonation craft measures and special designment while using the natural low-cost raw materials such as magnesite and calcium carbonate as the main composition. No mill dust was generated during the experiment, and caused little environmental problem. It was an appropriate craft to put fine ore and low-grade magnesite to rational use.(2) The densification of MgO-CaO-CaZrO3 refractories was promoted when the sintering temperature was increased and the soaking time was prolonged. The MgO-CaO-CaZrO3 refractories were dense when sintered at 1600℃×3h. Owing to the formation of small sized CaZrO3, formed by the reaction of CaO and ZrO2, facilitated to sintering, the densification of the MgO-CaO refractories was appreciably promoted with ZrO2 addition, and the addition of nano-sized ZrO2 was more effective.(3) The cold crushing strength, bending strength and the thermal shock resistance of the MgO-CaO refractories with ZrO2 addition were increased due to the toughness and the direct-bonding of the grains were increased. The addition of nano-sized ZrO2 was more effective attributed to its well dispersion.(4) The slaking resistance of the MgO-CaO refractories was appreciably improved by 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.(5) The slag corrosion resistance of the MgO-CaO refractories was enhanced by addition of ZrO2 due to the formation of self-protective coatings on the surface of the refractories and the increase of the viscosity of the liquid phase and thus inhibited further penetration of slag at elevated temperatures. Due to the reaction of CaO and 2CaO·SiO2 to form 3CaO·SiO2 and the increase of direct-bonding of the MgO-CaO refractories with ZrO2 addition, the aherence ability of the MgO-CaO refractories was promoted with ZrO2 addition, and the nano-sized ZrO2 addition was more effective.(6) Since the high purity of the synthesized MgO-CaO clinker, barely low melting point phases existed in the microstructure of the materials, and then the performance of the MgO-CaO refractories with ZrO2 addition was promoted further.(7) The performance of MgO-CaO refractories was appreciably promoted by ZrO2 addition, the effect of nano-sized ZrO2 addition was more effective than the micro-sized one due to the special effect of the nano powder, the performance of MgO-CaO refractories was appreciably promoted by less nano-sized ZrO2 addition.
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