Mortars based on geopolymers have been proposed for the conditioning of magnesium alloy scraps. The qualification of the immobilization process requires the confirmation that, during the setting of the geopolymer matrix, the temperature in the waste container would not exceed a threshold where adversarial consequences could be observed, such as increased scrap corrosion (accompanied with hydro genproduction) or structural matrix disorders. An experimental campaign has culminated in 2020 with the pouring of a large geopolymer mortar quantity (2.7 m3) in an industrial size cubic container, and the monitoring of the ensuing temperature transient inside the mortar and on the package walls. We present a numerical thermal model based on finite elements, we justify the hypothesis and adopted material parameters, and we compare the obtained numerical simulation with the experimental result. The comparison between the measured temperatures and the calculated ones is rather acceptable. The agreement regarding the maximum temperature inside the mortar is especially satisfactory. Sensibility analyses have shown that the temperature peak is not dependent on the external cooling fluxes ( irradiance and atmospheric convection), but only on the mortar thermal properties.