Research Progress of Additives to Improve Hydration Resistance of Magnesia-calcium Materials

Magnesia-calcium materials have stable hot performance,good resistance to the erosion and corrosion of liquid steel and steel slag,and a special role in purifying liquid steel,so they are widely used in iron and steel industry.However,hydration of magnesia-calcium materials seriously restricts their use,so researches have been done to improve their hydration resistance,obtaining a series of achievements.In this paper,the improvements on the hydration resistance of magnesia-calcium materials by additives in recent 20 years were presented,and their mechanisms were summarized.

Effect of WO3 on Hydration Resistance of CaO Aggregate Prepared by Thermal Decomposition

The application of CaO refractories was limited due to its poor hydration resistance.Introducing appropriate additive is an effective way to improve its hydration resistance.Additive WO3 was introduced in the preparation of CaO aggregates using calcination method in this study.The introduction of WO3 decreased the apparent porosity of CaO aggregates and promoted the grain growth of CaO by the formation of liquid at high temperatures.The average grain size of the specimens with 0.4 mass%WO3 and treated at 1550℃was increased to 51.6μm comparing to 33.5μm that without WO3 additive.As a result,the hydration resistance of CaO aggregate was significantly improved.Besides,WO3 sublimate easily,therefore,less WO3 was remained after the heat treatment.The further increase of WO3 addition had little effect on the hydration resistance of CaO specimens.

Effect of CeO2 and Calcination Temperature on Hydration Resistance of CaO Aggregates

Active lime was used to produce Ca（OH）2fine powder.And then Ca（OH）2fine powder with analytically pure CeO2fine powder（0,0.2%,0.4%,0.6%,0.8%and1.0%by mass,respectively）were mixed via wet mixing.After drying and ball milling,the specimens were pressed and fired at 1 450℃,1 500℃,1 550℃,1 600℃and1 650℃,respectively.The hydration resistance of the specimens was evaluated by measuring the mass gain after soaking the specimens in a chamber kept at 20℃and90%relative humidity.The influences of CeO2and calcination temperature on the hydration resistance of CaO aggregates were investigated by determination of bulk density,apparent porosity,and XRD,SEM and EDS analysis.The results show that：（1）the sintering of CaO aggregates is promoted by CeO2addition and the hydration resistance of CaO aggregates is improved;（2）CeO2mainly locates in CaO grain boundary,some CeO2is solved in the CaO grains which improves CaO sintering;（3）the appropriate calcination temperature is 1 550℃.When the calcination temperature is below 1 550℃,the specimens are sintered inadequately.The density and hydration resistance of CaO aggregates are nearly stable when the calcination temperature is above 1 550℃.

Modification of Hydration Resistance of SrO with ZrO_(2) Addition

Considering the high melting point and high stability of SrO,it has potential applications in the field of the preparation of titanium alloys.To improve the hydration resistance,SrO samples with different ZrO_(2)additions were fabricated after firing at 1750℃for 6 h.The phase,the microstructure,and the hydration resistance of the samples were researched.The results show that the sample with 33 mol%ZrO_(2)addition possesses good hydration resistance with the mass gain after hydration for 13 days of less than 0.42 mass%.The SrO crucible with 33 mol%ZrO_(2)addition does not react with TiNi alloy melt during melting,which provides a support for searching new refractories with good hydration resistance for induction melting titanium alloys.

Application and Hydration Resistance of SiC Refractories for Water-cooled Wall of Waste Incinerators

According to the dual needs of heat transfer and thermal insulation in incinerators,different parts of the incinerators are designed with refractories of different SiC contents.Water vapor is mainly concentrated in the zones of feeding,drying and combustion,and the main damage factors of SiC refractories include high temperature and hydration.The results of hydration resistance of typical SiC-containing refractories show that the higher the SiC content,the worse the hydration resistance.Si_(3)N_(4)-SiC refractories after oxidation treatment have good hydration corrosion resistance,which are appropriate for water-cooled wall of waste incinerators.

Compositions and Properties of Hydration-Resistant Coating of Magnesia-Calcia Materials

The complex coating on the MgO CaO clinker was prepared by dipping method and adding the second mineral to restrain the formation and expansion of the cracks. Meanwhile, the microstructure and the morphology of the coating and hydration resistance of the MgO CaO clinker were also investigated. The result shows that the coating consists mainly of Ca 2P 2O 7, MgO and CaCO 3, its thickness ranges from 5μm to 20μm, and the coating is dense and even, no micro cracks are observed. Also, the coating adheres closely to the matrix. The hydration resistance of the MgO CaO clinker after treatment is improved greatly, and the castables prepared with the treated MgO CaO clinker have better physical properties than MgO castable.

Effect of La_2O_3 Addition on Hydration Properties of CaO Clinker

To obtain CaO clinker with high density, low po- rosity, and good hydration resistance, the effect of the La2 03 content on physical properties, such as sintering property and hydration resistance of CaO clinker was re- searched in this paper. Apparent porosity and bulk density were measured by Archimedes principle with kerosene as the medium. Two methods were used to test hydration resistance of CaO clinker. One was carried out in a hu- midity cabinet at 20℃ with 90% of relative humidity. Another was the boiling method. SEM and EDS charac- terization analyses of CaO clinker were pe.rformed to observe the microstructural information and the hydration mechanism was put forward. The results show that： （1） the specimen with 0.79 mass% of La2O3 addition has lower apparent porosity, and higher bulk density; （2） the hydration mass gain of CaO clinker with 0.79 mass% La2O3 addition is lower, which is less than 1% after 20 days ; （3） La2O3 mainly distributes in the grain boundary, attd a small amount of La203 dissolves in grains. The average grain size of CaO specimens with 0.79 mass% La2O3 addition sintered at 1 450℃, 1 550℃, and 1 650℃, is 44. 5 μm, 48. 6 μm, and 63. 5 μm , respectively; （4） solid solution is formed due to the similar ionic radius of La^3＋ and Ca^2＋, which promotes the sintering of CaO clinker and improves the hy- dration resistance simultaneously.

Preparation of Al-AlN Bonded Corundum Based Refractories by in-situ Reaction

The Al-AlN bonded corundum based refractories was synthesized with the starting materials of fused alumina and metallic aluminium in nitrogen atmosphere through in-situ reaction. The study indicated that with the addition of 14% metallic aluminium （13% of Al powder and 1% of Al fiber） , the material nitridized at 1100℃ for 3h has excellent physical properties. It is found that a massive quantity of alN in the forms of both whiskers and hexagon pellet particles is formed, and a large quantity of Al remained in the matrix. This multiple bonding system resulted in the excellent mechanical properties of the material. It is also found that the hydration tendency of the prepared material is inhibited, and the material exhibited excellent thermal shock resistance. The nature of protective oxidation of the bonding system is characterized. Thus, this material may become a new candidate of carbon-free sliding plate material for continuous casting.

Mullitization of Andalusite and Influence of Aggregates on Properties of Aluminosilicate Refractories Based on Andalusite Matrix

The mullitization of andalusite from Xinjiang, Chi- na and influence of aggregates on properties of alumino- silicate refractories were studied by XRD, SEM, EDS, DSC. It was found that mullitization was obvious at 1 400 ℃, but considerable at 1 500 ℃. The refractories with corundum aggregates had the highest RUL and the lowest creep rate, but refractories with bauxite aggregates had the lowest RUL and the highest creep rate.

Influences of C/S Mass Ratio and Kieserite Impregnation on Performance of 95-Grade Magnesia Bricks

The irfluences of C/S mass ratio and kieserite im-pregnation on performance of 95-grade magnesia bricks prepared using sintered magnesia DBM95 as main start-ing material, silica fi, me and limestone as additives were researched. The cold and hot physical properties were tested according to Chinese standards on refractories. The hydration resistance was determined in a high pres-sure autoclave with 0. 55 MPa of water vapor pressure for different holding times of 0. 5 h, 3 h, and 5 h. The tri-al results show （ 1 ） adding silica fume to decrease C/S ratio from 0. 84 to 0. 33 improves sintering property; addin,g limestone to increase C/S ratio J？om 0. 84 to 1.95 causes more pores and negatively influences physi-cal properties： BD decreases, AP increases, MOR and CCS decrease generally, but HMOR increases obviously; （2） for non-impregnated specimens, adjusting C/S mass ratio can not improve hydration resistance; （3） the impregnation with kieserite solution can improve hydra-tion resistance of magnesia bricks significantly.

Selective sintering of magnesia-calcia materials by utilizing hot spots during induction sintering process

Magnesia-calcia refractories are widely used in the production process of clean steel due to their excellent high-tem-perature stability,slag resistance and ability to purify molten steel.However,there are still problems such as difficult sintering and easy hydration.Magnesia-calcia materials with various calcium oxide contents were prepared by using induction sintering,and the sintering property combined with the hydration resistance of the materials was investigated.The experimental results showed that the magnesia-calcia materials prepared under induction field had higher density,microhardness and hydration resistance.In particular,the relative density of induction sintered magnesia-calcia materials with 50 mo1%CaO was greater than 98%,and the average grain size of CaO was 4.56μm,which was much larger than that of traditional sintered materials.In order to clarify the densification and microstructure evolution mechanism of the magnesia-calcia materials,the changes in temperature and magnetic field throughout the sintering process were analyzed by using finite element simulation.The results showed that the larger heating rate and higher sintering temperature under the induction sintering mode were beneficial to the rapid densification.In addition,the hot spots generated within the material due to the difference in high-temperature electric conductivity between MgO and CaO were the critical factor to realize selective sintering in MgO-CaO system,which provides a novel pathway to solve the problem of difficult sintering and control the microstructure of high-temperature composite material used in the field of high-purity steel smelting.

Effect of TiO_(2) addition on microstructures and properties of Mgo-Cao refractory aggregates

MgO-CaO(40 wt.%CaO)refractory aggregates were prepared using the calcined dolomite and light-burned magnesia fine powder as raw materials and TiO_(2) as additive.The effect of TiO_(2) on their phase composition,microstructures and properties was investigated by X-ray diffraction and scanning electron microscopy.The properties such as bulk density,apparent porosity,relative aggregate tube strength and hydration resistance were also investigated.The results showed that the CaTiO_(3) generated by the reaction between CaO and TiO_(2) was distributed around the CaO grain boundaries and intermittently distributed with MgO,which formed an isolation layer around CaO and greatly improved the hydration resistance.Meanwhile,the introduction of TiO_(2) promoted sintering and increased the grain size,further improving the strengths and hydration resistance of the materials.In addition,the most significant enhancement in the hydration resistance and strengths of the samples was achieved when 1.0-2.0 wt.%TiO_(2) was added.In this case,the relative strength of aggregate increased from 33.3% to 37.3%-43.1%,and the mass gain after the hydration test decreased from 3.13% to 1.26%-1.45%.