Radioactive borate waste containing a high concentration of boron (B) is problematic to be solidified using

cement because soluble borate such as boric acid hinders the hydration reaction. In this study, borate waste was

used as a raw material for metakaolin-based geopolymer according to the characteristic that B replaces a part of

Si. Geopolymers using KOH alkaline activator (K-geopolymers) showed higher compressive strength than geopolymers

using NaOH alkaline activator (Na-geopolymer). In addition, the compressive strength increased

proportionally to the Si/(Al+B) ratio regardless of the alkaline cation species. These variations in compressive

strength might be due to the viscosity of the geopolymer mixture, atomic size of alkaline cations, and the increase

in Si content. The characteristic analyses (XRD, FT-IR, and solid state 11B MAS NMR) indicated that B was

incorporated into the geopolymer structure. Thus, the K-geopolymer has a dense and homogeneous microstructure.

In a semi-dynamic leaching test, less B leached from the geopolymers compared to the cement waste

form. Consequently, borate waste can be solidified using metakaolin-based geopolymer, and the use of a KOH

alkaline activator is advantageous in terms of mechanical property and structural durability.