That looks like an epoxy glue. Epoxies are probably the 'best' glue to use when inserting tubes in pen blanks because they cure fairly quickly (the nominal curing time is usually stated on the package - typically, 5 minutes or 15 minutes, but it's prudent to give it an hour or two to gain full strength), and the final bond is very strong without being brittle. The downside of using epoxy is that it has to be mixed, and that is messy, and you almost always mix more than you need which means that the process wastes some glue.
Some pen turners prefer thick cyanoacrylate (CA) glue because it doesn't require mixing. The curing time for CA is also fairly fast - with thick CA, you need to allow an hour or so depending on temperature and humidity. There is very little waste, but the glue is more expensive. However, cured CA is brittle and the joint can fracture. Also, CA joints have a limited life expectancy although that's probably not an issue with pens.
My preference is for polyurethane glue. I don't know what brands you will find in Belgium - here in the US, the most popular brand is 'Gorilla Glue'. Like CA, it doesn't require mixing so it is faster than epoxy and there is little waste. And like epoxy, the joint is resilient rather than brittle. But the curing time is longer - I typically let glues pen blanks cure overnight. Polyurethane cures on exposure to moisture, so I run a little water into the hole in the blank before apply glue to the tube and insert it into the hole. It foams as it cures, and the foaming action can push the tube out of the blank so you need to apply some kind of clamp to keep the tube in place while it cures. Rubber bands or masking tape works fine, or you can use actual clamps. You will need to trim the excess foam away from the end of the blank - and inevitably, some will find its way inside the tube and need to be removed.
While there is little waste when actually using polyurethane glue, I find that one of the challenges with it is that it has a limited shelf life - because it cures on exposure to moisture, humid air in the shop can accelerate curing in the bottle. I opt to buy smaller bottles so that if a bottle does go bad, I don't waste as much glue. I also store polyurethane glue bottles upside down so that if air does get into the bottle, the premature curing takes place at the bottom of the bottle, and the glue at the top of the bottle remains useable for a while longer.
PVA (eg titebond) is not an ideal glue when inserting tubes because it won't bond with brass. However, if you have nothing else available, you might be able to use if if you are careful to scuff the tubes a lot, use a lot of glue, and then allow it to cure thoroughly before turning the pen. The tube won't actually be glued into the blank, but the glue will cure in the tiny scratches that you create when scuffing the tube, and may be strong enough to hold everything together mechanically while you turn the pen.
