Silica brick thermal expansion

Thermal Expansion and Thermal Shock Resistance In many situations it is difficult to directly substitute silica brick for red shale or fireclay. Changes in design might be needed to avoid subjecting the silica brick to destructive tensile or shear stresses during operation because of expansion differences. The thermal expansion of the specialty type silica product (Type 2) is much less than that of acid brick. The vitreous silica material which contains some crystalline Si02 (Type 1) has a thermal expansion that closely matches that of acid brick at temperatures less than 800°F. Above that temperature, the expansion is much less. 

In olivenes, FeO is an impurity. This impurity tends to reduce the refractoriness of forsterite bricks. The iron content is adjusted to get a refractoriness of 1750°C. The RUL value minimum is 1550°C. Thermal expansion is low, which results in a moderately good spalling resistance. Fosterite is resistant to both acid and basic slags. Its resistance to acid slag is not as much as that of silica, and its resistance to basic slag is not as much as that of chrome-magnesite. 

Pure cordierite is an expensive material, and it is known that less pure materials containing some cordierite also exhibit low thermal expansion coefficients. For this reason, semicordierite materials are frequently used as kiln furniture and car deck block in ceramic kilns. In some cases, alumina-silica brick compositions are also used for the same product apphcations. Properties of some products used in kiln furniture/deck block applications are shown in Table 7. The maximum use temperature of these products is typically 1200°C or higher. 

Figure 25 describes the thermal expansion of the silica KD brick-only and the silica KD brick/mortar composite samples. The mortar-only sample was also tested. The brick-only and the brick/mortar composite samples are similar except for temperatures above about 1100°F. The mortar softens considerably, as reflected in the mortar-only sample. Due to the confinement of the mortar within the mortar joint in the brick/mortar composite sample, the mortar maintains reasonable strength. This confirms that the true compressive stress-strain behavior of mortar must be tested with the mortar contained in the mortar joint (2). Containment of the mortar is an important and necessary parameter in testing the thermomechanical behavior of mortar. 

Figure 26 Comparison of coefficient of thermal expansion of silica KD composite, mortar-only, and brick-only.

Figures 33 and 34 show the temperature dependent thermal expansion of the silica KN brick-only, mortar-only, and brick/mortar composite samples. The interpretive results are quite similar to the interpretive results of the silica KD tests. These results also show that the mortar-only (unconfined mortar) tests do not reflect the true confined thermomechanical behavior of mortar in mortar joints. Strength patterns as a function of temperature are also similar to the silica KD brick.

Expansion allowance is often used to reduce the expansion stress in the refractory lining. Expansion allowance is often used in lining systems made of magnesia and silica brick because of their high coefficient of thermal expansion. Approximate methods are used in which a percent of the full expansion displacement is taken for the amount of expansion allowance. Materials such as cardboard and plastic inserts are placed in brick joints that burn out at low temperatures. Compressible blanket of board materials are also used. Typically, low-density (10 to 25 pcf) blankets will compress to about 10 to 20% of their original unloaded thickness. Higher-density (50 to 60 pcf) board materials will only compress to about 80 to 90% of their original unloaded thickness. 

---