The mixed body of clay and flyash containing unburnt carbon was examined at various sintering conditions in order to recycle flyash as environment-affinitive construction materials. Experimental results can be summarized as follows : when large aggregate of 2.54 cm dia. was sintered at a heating rate of $20^{circ}C$/min, heterogeneous phase with bulgings, inner pores, and cracks were observed at the sintering temperature of $1200^{circ}C$. Accordingly, heating patterns for the complete removal of the heterogeneous phase were proposed as countermeasures. The compressive strength of finally obtained aggregate was 670~870kg/$ extrm{cm}^2$, which is over two and a half times stronger than the minimum requirement of 200kg/$ extrm{cm}^2$.