Direct-bonded doloma

V. DOLOMA-BASED REFRACTORY PRODUCTS A. Direct-Bonded Doloma Bricks (Fired)... 

Table 6 Chemical and Physical Properties for Various Direct-Bonded Doloma Brick...

It is the lime phase of the doloma grain that provides the direct bond and is responsible for the chemical behavior of the brick. The microstructure of a fired doloma brick shows a high degree of direct bond, as Figure 3 shows. 

Figure 3 Microstructure of fired doloma showing direct bonding between doloma grains.

In the past there has been some reluctance to using direct-bonded bricks for steel-making applications because of their susceptibility to thermal shock. Significant improvement in the thermal shock resistance of the brick was accomplished through the addition of small amounts of zirconia. The enhancement is attributed to microcracking caused by the expansile formation of calcium zirco-nate as a result of reaction between the lime in the doloma and the zirconia. The microcracking is clearly illustrated in the microstructure shown in Figure 4. 

Further improvements to the slag resistance of both direct-bonded and carbon-bonded doloma bricks have been made through the addition of high-purity magnesia. Doloma bricks that have been enriched with MgO are typically used in areas of ladles and AOD vessels where higher slag resistance is required. The chemical and physical properties of the direct-bonded bricks used in steel making are shown in Table 6. 

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