Why can underfired bricks have poor resistance to frost?

Underfired bricks exhibit poor resistance to frost because they lack the necessary density and structural integrity achieved through proper firing in a kiln.

When bricks are not fired at the appropriate temperatures—typically ranging from 870° to 1,100° C—they remain porous. This high porosity allows water to penetrate the brick's interior. In freezing conditions, this trapped water expands as it turns into ice, creating internal pressure that can cause the brick to crack, spall, or disintegrate. Proper firing ensures the material undergoes the necessary sintering and densification, which reduces porosity and enhances the durability required to withstand environmental stressors like frost.1