What is observed in silicate brick masonry with prolonged heating up to 500°C?

Solution:

Silicate brick is characterized by poor heat resistance. When heated to 500°C, it undergoes significant structural changes.

1. Color and porosity change: The brick turns red and becomes more porous.
2. Material destruction: The internal structure of the brick is disrupted, leading to a decrease in its strength and durability.
3. Slight melting: While not complete melting, the surface can become glassy or slightly deformed.
4. The option "slight melting of the brick, which however does not change its basic protective qualities" is incorrect because the protective qualities are significantly affected by the changes. The option "delamination of the brick with cracks" is a consequence of the structural changes, but the primary observation is the change in color and porosity and destruction.

Answer: change in color and porosity of the brick.