I'm fixing up my workshop in the basement. The floor is a concrete
slab, and I plan to cover it with vinyl/ rubber tiles. Moisture is not
a problem.

There is a gap between the slab and the cinder block walls. This gap
is only about 1/8" wide at two of the walls, but along the long wall
the gap widens in a few places to about 1". Along the edge of the slab
there is insulation, some kind of Styrofoam. The top of this
insulation has deteriorated somewhat over the years (the house is 20+
years old), but looks reasonable. In most spots, the gap is only about
1/4" deep (some loose insulation chips, small "rubble" is visible),
but for about 3-4 feet the gap is much deeper, about 1 - 1 1/2".
Again, there is some "rubble" visible in the gap. Using a flashlight,
I can see a space between the insulation and the foundation wall, but
I can't see how far down this space goes, for obvious reasons.

I was thinking of sealing the gap (the top section that is) before
laying the tiles (now or never...).
Cement is the obvious choice, but I've heard of this thing called
"temperature" and "expansion". That's what I get for watching all
those DIY shows. Ignorance is bliss......

So, I thought about cutting a strip of thin board (1/8"), place this
in the gap against the wall, and then fill the gap with cement,
effectively using the board as a form. That should leave enough room
for expansion.
What material would work best? Vinyl concrete patcher? Some sort of
epoxy? Ordinary cement?

Thanks.