Effects of Microsilica Content on Microstructure and Strength of Lightweight Castable Refractories Containing Porous Corundum-Spinel Aggregate

High-strength, lightweight castable refractories based on spinel were prepared by introducing porous corundum-spinel aggregate and appropriate microsilica. The effects of microsilica content on microstructure and properties of lightweight castable refractories were investigated using SEM, XRD, mercury porosimetry measurements and FactSage thermochemical software. It’s found that microsilica was almost dissolved into a liquid phase at high temperature, which promoted liquid sintering. With increasing microsilica content, the matrices became denser and average pore size of matrices increased, then apparent porosity decreased, crushing strength increased and content of liquid phase in castables at 1600°C increased. The appropriate microsilica content is 0.65 to 1.95 wt%, which reaches to a compromise among apparent porosity (33-38%), crushing strength (82-125MPa) and liquid content (1.80~4.97 wt%).


Introduction
Dense alumina-magnesia (or alumina-spinel) castable refractories have been widely used as working linings of ladles due to their excellent properties and easy installation [1][2][3][4][5][6].The high thermal conductivity of the castable causes decreasing of temperature of molten steel in the ladles and increasing of the temperature of the steel shell of ladles, which leads ladle misshapening and results in the waste of energy [6][7].On the other hand, in order to prepare dense aggregates, the raw materials should be sintered or electrofused at a high temperature, resulting in high energy consumption and environment pollution [7].
Some researches paid attention to insulating of industrial furnaces, but the use of traditional lightweight refractories was limited by their chemical compositions and physical properties, such as alumina silicate lightweight refractories or low-strength refractories [8][9][10][11][12].In order to save energy and resolve some problems resulting from the high thermal conductivity and heavy weight of refractories used in ladles, introducing corundum-spinel aggregates with high strength and low density into castables may be an alternative way.
In previous work, we have prepared porous corundum-spinel ceramics [13].These porous ceramics have a micro-pore size and high strength.If the porous ceramics were introduced into castables, the higher water demand for placement and the expansion from the reaction between MgO and Al 2 O 3 would deteriorate the strength.Microsilica is extensively used in castables because it can reduce water demand for placement, improve matrix sintering [14][15][16].In this work, lightweight castable refractories with high strength were fabricated by introducing porous corundum-spinel aggregates and appropriate microsilica, and the effects of microsilica content on the microstructure and properties were investigated.

Experimental
Five lightweight corundum-spinel castables containing different SiO 2 content were prepared using porous corundum-spinel as the aggregate (65 wt%) and sintered spinel powder (10 wt%), magnesia powder (6.5 wt%), α-Al 2 O 3 micropowder (6 wt%), white corundum powder and microsilica as the matrix, fixed total percentage (12.5 wt%) of white corundum powder and microsilica.The microsilica content in five castables was 0, 0.65 wt%, 1.95 wt%, 2.60 wt%, 3.90 wt% respectively.The water content (11.8 wt%) was similar for all batches of castables.Microstructure of the porous aggregate is shown in Fig. 1.The chemical composition and physical properties of the porous aggregate is given in Tab.I.The samples with length of 160mm, height of 40mm and width of 40mm were cast.The samples were cured for 24h at room temperature, dried at 110ºC for 24h, and heated at 1100ºC and 1600ºC for 3h, then furnace-cooled.

Apparent porosity, bulk density and crushing strength
Fig. 2 (a and b) gives the relationship of apparent porosity and bulk density with microsilica content and heating temperature.At 1600ºC, with increasing microsilica content, the apparent porosity decreases gradually but the apparent porosities are more than 33% when the microsilica content is less than 1.95 wt%, with further increase of the microsilica content to 3.90 wt%, the apparent porosity is less than 30%, which is associated with a higher extent of sintering.The bulk density increases with increasing microsilica content, but the bulk density has a maximum value of 2.59 g/cm 3 .Fig. 2 Variation of (a) apparent porosity and (b) bulk density with microsilica content and heating temperature Fig. 3 gives the relationship of crushing with microsilica content and heating temperature.It can be seen, after heating at 1100ºC and 1600ºC, with increasing microsilica content, the crushing strength increases obviously.

Fig. 3 Variation of crushing strength with microsilica content and heating temperature
Microsilica has a beneficial effect on the sintering of the castables, but over much microsilica decreases the apparent porosity which is unfavorable for lightweight refractories.When the microsilica content is 0.65⊥1.95wt%, a compromise is reached between apparent porosity (33⊥38 %) and crushing strength (82⊥125 MPa).

Phase identification
XRD patterns of castables sintered at 1600°C for 3h were prepared to identify the formed phases (Fig. 4).Spinel and corundum are the phases in five castables, and no crystals containing the Si element were detected, indicating that almost all of the silica had dissolved in the liquid phase.

Microstructure and pore size distribution
Fig. 5 shows the microstructures of matrices in castables sintered at 1600°C for 3h.It can be seen that the matrix in the castable containing 1.95 wt% microsilica is denser than that without microsilica.Fig. 5 Microstructures of matrices in castable containing 0 and 1.95 wt% microsilica sintered at 1600°C Fig. 6 Pore size distribution of matrices in castables containing different microsilica content sintered at 1600°C Fig. 6 gives the pore size distribution of matrices in five castables sintered at 1600°C for 3h.The pore size distributions are mono-peak mode, and the pore size is less than 10 µm.With increasing microsilica content, the curves shift towards the right and the peak value decreases which indicates that the average pore size increases and porosity decreases with increasing microsilica content.

Discussion
Microsilica affects strongly the apparent porosity and crushing strength, which mainly resulted from the influence of microsilica content on their matrices, because these castables have the same aggregates.When microsilica was introduced into an alumina-spinel castable, on the one hand, it could promote sintering and inhibit the expansion of the formation of spinel, on the other hand, too many liquid phases would appear when too much microsilica was added, which would deteriorate the properties of castables at high temperature.
The species of matrices in castables containing different microsilica content as a function of temperature, calculated by FactSage thermochemical software, are shown in Fig.
When increasing the sintering temperature to 1600°C, the isothermal phases in the matrices are spinel and liquid phase.With increasing microsilica content, the content of spinel decreases and the content of liquid phase increases, which is why the matrix becomes denser and the strength increases with increasing microsilica content.

Fig. 8 Content of liquid phase in castables at 1600°C
The content of the liquid phase in castables at 1600°C calculated from the data in Fig. 7 is shown in Fig. 8.When the microsilica content is less than 1.95 wt%, the content of liquid phase is lower than 5 wt%, which would not have a strong effect on properties of castables at elevated temperature.Thus, the most appropriate content of microsilica is 0.65 ~ 1.95 wt%.

Conclusions
Microsilica was almost dissolved into liquid phase at high temperature, which promoted liquid sintering.With increasing microsilica content, the matrices became denser and average pore size of matrices increased, then apparent porosity decreased, crushing strength increased and content of liquid phase in castables at 1600°C increased.The appropriate microsilica content is 0.65 to 1.95 wt%, which making a compromise among apparent porosity (33 38%), crushing strength (82 125MPa) and liquid content (1.80~4.97wt%).

Fig. 4
Fig. 4 XRD analysis of castables containing different content of microsilica after sintering at 1600°C