Concept:
Monolithic refractories are unshaped refractory materials that are installed without being pre-formed into bricks.
Step 1: Castables are supplied as dry refractory mixes.
Step 2: Water is added before installation.
Step 3: The mixture is then cast into place like concrete.
Step 4: Since castables are not pre-shaped bricks, they belong to the class of monolithic refractories.
Therefore, the correct answer is:
\[
\boxed{\text{Monolithic refractory}}
\]