There are 2 basic types of cement in Thailand,One is fast drying and idea for concrete,beams ect (ie.....Chaang Deang (Red Elephant), Nok Insee Phet (Diamond Bird) ...),the other dries slower which is good for brick work, render, floor tiles, etc.(ie...Sua Faa (blue tiger), Sua Kiaow (Green Tiger)..)
I used Chaang Deang for all load bearing parts when building, and Sua Kiaow for everything else but note that Sua Faa (Blue tiger) is a special cement to use for rendering and it contains a plasticizer, so there is no need to add any.
I would use a mix of one to one and sieve the sand through some of that blue mesh first so the big grains of sand and any other small debris is removed.
I have never used the pre-mixed stuff so will leave that for others to comment on .