In the previous study, we suggested a new concept named as GRSS(Gel-Route Stabilization/Solidification) and investigated a material system applicable to volatile radioactive waste, a reaction condition and chemical limitations. From these results, this paper investigated the effect of some additives, which have been considered as the major component of ceramic waste forms(apatite and sodium zirconium phosphate), on the thermal stability and chemical durability. The Si-P system with Ca at appropriate composition shows good thermal stability but low chemical durability while the Si-P system with Zr shows relatively low thermal stability but relatively high chemical durability.