Concrete
Fortress Material
<&recipe>Concrete is a very hard and resistant composite material. It's cheap to craft and makes for an excellent use for the excess <link;slag;slag> produced by making steel.
<&brick>Concrete can be cut and assembled into various bricks and pillars using the §2Stonecutter§r.

Concrete bricks have the same blast resistance as concrete, but can be disassembled faster than their solid counterpart, making them a good structural material.
<&partial>Concrete can be cut into slabs of varying thickness using a §2Stonecutter§r.

You can also use that machine to make slabs and stairs of concrete, as well as concrete tiles which are smoother and more polished.
<&leaded>Leaded concrete is a reinforced material, which is highly resistant to explosions. It may also afford protection against radiation, if that is something you worry about.
<&liquid>Liquid concrete can be produced in a <link;mixer;Mixer> by combining half a bucket of Water with §2Gravel§r, §2Clay§r and §2two pieces of Sand§r. When emptied onto the ground it will spread like water, before solidifying after some time.
<br>Multiple buckets can be used to fill a larger area by placing it at key locations. Note that if it spreads too thin, the liquid will dry into slabs instead of full blocks.
If the concrete dries while a creature is standing in it, they will be trapped and unable to move. You can also fill a <link;chemthrower;Chemical Thrower> with liquid concrete in order to create improvised platforms or immobilize enemies.