Silica bricks with identical chemical composition can have differing mineralogical compositions which can cause quite different behavior during use. Therefore, it is not always sufficient to evaluate silica bricks only by their chemical composition. It is essential to also consider the degree of transformation and the thermal expansion behavior of the bricks.
Silica brick contains cristobalite, tridymite and some residual quartz. The crystal phases each have a high and low temperature forms which can transform reversibly. The crystal structure of the individual SiO2 crystal phase can differ widely. This is of great importance during heating and cooling because of the change in the volume.
Quartz requires the smallest volume and the quartz glass the largest. During firing above approximately 900℃, quartz transforms into the other modifications and melt completely at 1725℃. It shows such a transformation at 573℃, tridymite at 117℃, and cristobalite between 225℃ and 270℃. The thermal expansion of cristobalite is considerably greater than that of the tridymite.
Because well transformed silica bricks contain little or no residual quartz, their behavior under the influence of temperature is largely determined by the ratio of cristobalite to tridymite. During heating up, silica bricks expand rapidly with the total reversible expansion being completed at around 800℃. Therefore they are insensitive to the temperature fluctuations above 800℃, but very susceptible below this temperature because of the sudden volume expansion. For this reason, sufficient time must be allowed for heating furnaces up to about 800℃.
During slow cooling , reversible volume decreases take place which are a result of the spontaneous transformation of the crystal structure from the high to the low temperature modification. The reversible and irreversible volume effects can cause considerable stress within the refractory brick structure.
During the firing process, the lime reacts with the quartzite components to form wollastonite. The matrix also contains very small quantities of calcium ferrite, hematite, magnetite, calcium olivine and hedenbergite, which are formed from impurities. These crystalline phases are the reason for the discoloration and spot formation on the silica bricks.
The degree of transformation of the bricks can be determined easily and accurately by the density of the residual quartz content. The density of a silica brick is lowest when the degree of transformation is farthest advanced.
The appearance of the bricks also indicates to the degree of transformation. The reversible thermal expansion also depends on the mineral composition. Tridymite and cristobalite do not expand linearly during heating but exhibit sudden changes in length both during heating and during cooling.