Casting support under fireplace

[Michaeltm]

Hello,

I am renovating a room with a low (about 150cm) basement underneath. The joists and part of the sill have rotted. One of the beams goes under the fireplace and now needs to be removed.
A carpenter came over and advised me to build a support under the fireplace now that the beam needs to be removed.
He suggested that I build a U-wall of Lecablock, the open side facing the foundation, and then fill this with stone and concrete/cement.
For the Lecablocks, I should use KC mortar, but I don't know which type of block to use. I didn't know there were different models.
As for the filling material, I don't know what type of stone to use and whether I should use concrete or cement, and which type in that case?
Once the space between the Lecablocks and the basement wall is filled, he suggested continuing the last two to three decimeters with bricks.

So:

Which type of Lecablock?
Which type of stone?
Which type of concrete/cement?

Thanks in advance for all the answers.

/Michael