Floor tiles to the balcony and waiting for Spring

In between Otto's, the German sausage's daily three walks outdoors I've been gluening  the tiles on my house's balcony floors.

Gluening and painting the tiles was no trouble, but I'm not sure how to add the plaster between the tiles. You see, my dear random reader, the tiles are not real ceramic tiles at all. I've cut the pieces from that stuff similar to egg carton. You know that stuff that's found inside nearly any package nowadays. It's great for cutting the way you like with a very lively tile like texture, but I'm not sure how  it's going to react whith wet plaster.  Should I try plastering on a small example piece or just go ahead with my actual work, that's a question! Any opinions out there?? You may have realized I'm not a person who believes in planning ahead!?

 

Otto has taken me for some fantastic walks during these weekends he has stayed with us. The weather has been really sunny so that without sunclasses one could go snowblind... BUT! What the hell, it's mid of March and there's still more snow one can tolerate, nights are really freeeeezing and thoug the days are quite comfortable with a lower temperature, the snow doesn't seem to melt at all! Help! We need some help over here at this northern front! Please send us some warmer weather from over there, where ever you are!! Thank you in advance!