Abstract It is important to know which parameters are effective for the manufacture of expanded clay aggregate and to determine how effective these parameters are on expansion. In this study, expansion tests were conducted by using samples from three separate fields to determine how different parameters influence manufacturing. Clay grain size, pellet size, temperature in the furnace and firing time were selected in the expansion tests as parameters. Their effect on production was evidenced separately. It was found that decreasing the clay size has a positive effect on expansion while a decrease in pellet size has a negative effect on it. Also, temperature in the furnace and the time period during which the clay remains in the furnace are critical values for the resultant aggregate quality. The conditions for optimum manufacturing were explained in detail.