Why do silica bricks expand on heating?

Silica bricks are made from high-purity silica sand and undergo a manufacturing process that involves heating the raw material to a high temperature and then cooling it rapidly. Due to their high purity, silica bricks have a low coefficient of thermal expansion, which means they do not expand or contract significantly with changes in temperature.

However, under certain conditions, such as exposure to high temperatures and rapid temperature changes, silica bricks can undergo a phenomenon known as cristobalite transformation. Cristobalite is a mineral form of silica that has a different crystal structure than the alpha-quartz structure found in silica bricks.

When silica bricks undergo rapid heating or cooling, the internal stresses caused by the sudden temperature change can cause the silica to transform into cristobalite. This transformation causes the volume of the brick to increase, leading to expansion.

The expansion of silica bricks due to cristobalite transformation can lead to cracking and damage to the brick structure, which can compromise its overall performance and durability. To prevent this, special care must be taken during the design and construction of high-temperature systems using silica bricks, including selecting appropriate materials, controlling firing conditions, and minimizing rapid temperature changes.