Wanna do a loft style renovation.

I just want to fill the gaps in the ceiling. Can not be to much work, right?

 

Can I use any kind of cement to fill the gaps? Do the gaps need to be pre treated before applying the cement? I mean will the new to be added cement stick to the old cement?

 

Then paint it with a color leaving textures because I also want indirect (hidden) lighting by mounting some overhanging gypsum to the beams to hide cables and lights.

You mention gypsum so why not use it to line the ceiling.

 

You can of course try to cement render it but thats a skilled job and needs the right mix and thickness otherwise it will fall off before you even finish.

 

In fact its not going to stick unless you bond it to e.m.l first (expanded metal lathing)

Its gonna be messy thats the truth.

 

Just overboard it with "dot and dab" and plasterboard or if you are happier with fixing to timber grounds or a metal frame then do  that.

 

Any ceiling fixer will have those concrete beams boxed out and boarded in minutes so look at going down that route.

I agree I think your best to just cover it with 9mm gypsum board. It's fast, cheap and very easy,  depending on where you are located the cost for installation including painting should run between 200-300 baht per sqm. Just go to a local building materials store and get a Thai person to ask for a per sqm quote. The store will arrange everything. 

I'd put in a suspended ceiling using metal frames connected to the wall where the paint line stops and 60cm x 60cm panels.  No painting required, it will hide the wiring and the lighting is easy to install and can be relocated easy if desired in the future.  I used it in my entire house, 320 square meters, when I built it 18 years ago and it still looks nice even though the overhead lighting has been modified/added many times.