The reaction mechanism that takes place in ZrSiO4–Mg Ca(CO3)2 mixtures was studied in air up to 1300°C by collecting neutron diffraction patterns during the heating ramp. Neutron diffraction intensities were used to monitor and establish the mechanism of reaction that occurs in successive stages. (a) MgCa(CO3)2 decomposition yielding MgO and CaCO3; (b) CaCO3 decomposition; (c) reactions between CaO, MgO, and ZrSiO4 involving the formation of phases such as: tetragonal-ZrO2, α-Ca2SiO4, and Ca3MgSi2O8, some of them acting as transitory phases; and (d) formation of CaZrO3. The results obtained by this technique agree with data obtained by differential thermal analysis and thermogravimetry. The final product has a porous structure, due to the release of CO2, with a very narrow pore size distribution (≈1 μm). This open-porosity can be controlled by tailoring the reaction sintering process.