Method for forming a final product from alkali carbonate or alkaline earth carbonate, referred to as carbonate, and another species, wherein the carbonate comprises calcium carbonate such that the intermediate species comprises calcium oxide and the other species comprises clay, the method comprising the following steps: b) heat treatment of the carbonate, in a calcination chamber, which is intended to calcinate said carbonate into alkali oxide or alkaline earth oxide, referred to as the intermediate species, and carbon dioxide; c) a step of extracting the carbon dioxide produced during step b); d) a final step which comprises mixing the intermediate species and the other species to form the final product; the method being characterized in that the heat treatment is preceded by a step a) of pumping the gaseous atmosphere into the calcination chamber so as to maintain the gas pressure in said calcination chamber at a pressure of less than 1 atmosphere, advantageously of less than 0.1 atmosphere, more advantageously of less than 0.05 atmosphere, and in that the energy required to carry out step b) is supplied by a radiative source through a window in the calcination chamber, wherein step a) is continued after the start of step b) and for as long as the temperature in the calcination chamber remains below a predetermined temperature, said predetermined temperature being between 200°C and 600°C, more advantageously said predetermined temperature being between 300°C and 500°C, wherein step a) ends as soon as the temperature in the calcination chamber exceeds the predetermined temperature. b) heat treatment of the carbonate, in a calcination chamber, which is intended to calcinate said carbonate into alkali oxide or alkaline earth oxide, referred to as the intermediate species, and carbon dioxide; c) a step of extracting the carbon dioxide produced during step b); d) a final step which comprises mixing the intermediate species and the other species to form the final product; the method being characterized in that the heat treatment is preceded by a step a) of pumping the gaseous atmosphere into the calcination chamber so as to maintain the gas pressure in said calcination chamber at a pressure of less than 1 atmosphere, advantageously of less than 0.1 atmosphere, more advantageously of less than 0.05 atmosphere, and in that the energy required to carry out step b) is supplied by a radiative source through a window in the calcination chamber, wherein step a) is continued after the start of step b) and for as long as the temperature in the calcination chamber remains below a predetermined temperature, said predetermined temperature being between 200°C and 600°C, more advantageously said predetermined temperature being between 300°C and 500°C, wherein step a) ends as soon as the temperature in the calcination chamber exceeds the predetermined temperature. the method being characterized in that the heat treatment is preceded by a step a) of pumping the gaseous atmosphere into the calcination chamber so as to maintain the gas pressure in said calcination chamber at a pressure of less than 1 atmosphere, advantageously of less than 0.1 atmosphere, more advantageously of less than 0.05 atmosphere, and in that the energy required to carry out step b) is supplied by a radiative source through a window in the calcination chamber, wherein step a) is continued after the start of step b) and for as long as the temperature in the calcination chamber remains below a predetermined temperature, said predetermined temperature being between 200°C and 600°C, more advantageously said predetermined temperature being between 300°C and 500°C, wherein step a) ends as soon as the temperature in the calcination chamber exceeds the predetermined temperature.