Try HF or ammonium bifluoride in teflon container, You are going to end up with some Zr-Si-O-F glass. Once the glue is made, heat-treat it (perhaps in a controlled water/water-vapor environ (may be alkali enriched) to drive off HF, especially if glass transition of the mix is lower. Zircon has high melting point (~1850 C), so progressive degassing would make it more stiff. I hope you also have checked ZBLAN glasses