Abstractthe rock-type and lime (CaO) derived from limestone have been studied by X-ray diffraction. The compositions studied covered the range 10 - 90% mixture of porcellanite and limestone batched in powdered form. After firing for 2h at 1150°C and 1200°C, three reaction products were detected: 3CaO.2SiO2, 3CaO.Al2O3.2SiO2 and 2CaO.Al2O3.SiO2 in compositions within the ranges 50 - 90%, 30 - 70% and 10 - 70% porcellanite respectively. Further, within the range of composition where each reaction product formed, yield tended to increase through a maximum then to decrease with increasing porcellanite content. However, compared with firing at 1200°C, the yield in reaction products was low after firing at 1150°C. In addition, on firing at 1150°C unreacted free lime (derived from decomposition of the limestone) was retained in all the compositions studied. Nevertheless, on firing at 1200°C free lime was detected only in compositions containing 50% and less porcellanite. These observations suggest that: (i) the thermal energy associated with firing at 1150°C is too low to sustain the reactions, once initiated, and (ii) on firing at 1200°C the retention of unreacted free lime in compositions containing 50% and less porcellanite is a consequence of body composition rather than firing temperature. Finally, scanning electron microscopy suggests that at 1200°C the reaction product 3CaO.2SiO2 is in molten physical state.