In this work, we investigated the drying process of a hardened gypsum plaster model (GP) that showed sodium sulfate efflorescence after subsequent exposure to water. Visible efflorescence could be prevented by adding small amounts of calcium formate (CF). Prism samples were prepared to investigate this observation, with an emphasis on chemical processes in the pore water. In the pure plaster, both sodium and sulfur accumulate at the surface of the prism, which leads to sodium sulfate precipitation. In the sample containing CF, calcium slightly accumulates at the surface, which leads to lower sulfur concentrations, because the porewater is in equilibrium with gypsum. Thermodynamic calculations show that higher sodium concentrations are then necessary to reach sodium sulfate supersaturation than without CF. These concentrations are not exceeded during early stages of the drying process, which ultimately leads to the sodium sulfate precipitation inside the prism. Therefore, sodium sulfate efflorescence can be hindered on gypsum plaster by the addition of CF.