Screens? You won't be able to open basement windows at all once you put in glass block. No air flow, and trapped humidity in summer, forcing you to run a dehumidifier if the basement is damp at all.
I have no experience with glass block, but have seen shows on it--using both mortar and silicone. From what I recall, mortar was longer lasting and better in an exterior application.
Personally, I think glass block seriously detracts from the look of an old house.

Unless you're in a desert, opening basement windows in the summer only allows in more humid outside air. (See Read This Before You Move In. from Building Science Corp./US Dept. of Energy Building America.)

Mortar or dry systems are both acceptable. But you must use only the proprietary systems that the manufacturers specify. Pre-fabricated glass block panels mortared or nailed are a simpler way to go.

We're in suburban Chicago and a previous owner replaced our original basement windows with glass block. They are mortared in place. In the middle of each window, 2 blocks were omitted and a vinyl framed window/screen unit is in place in each.

We have those little windows open quite frequently in the summmer and they were a massive blessing when the basement flooded last year and we had to dry the basement out. These screened window units are not removable, nor would we want them to be. However, the screen is removable. The screen is installed with 2 short screws on the outside, the window portion latches on the inside. Once the screen is off, a person cannot fit through - so they are secure.

Although our basement was 3/4 finished at the time of the flood (which was sewage backup), we removed the screens and brought the hose in from outside to do the clean up. The extra ventilation was helpful.

I hate hate hate the look of glass block, but in the 6 yrs we've lived here, they've functioned well. That being said, I'd get rid of them if I could (hubbie disagrees with me) because of the look of them.

hosta
the other issue w/glass block is if you are installing them on a finished part of basement you will block an egress.

Can't imagine not using mortar or something between them, it's a basement,it rains & will come in so will ants & bugs. GF has above ground level(about 2 ft up), glass blocks on 1 side of a bathroom about 4ft wide 6 ft high,was a remodel about 10 yrs ago & is cracked all across the bottom up the outermost side & between some of blocks in middle, rain usually comes from opposite direction so far it is holding inside the house. We are in earthquake country so that may have caused some of it.