The evaporation of an aqueous NaCl solution in a hydrophobic two-dimensional model porous medium was studied experimentally. The process is characterized by the formation of efflorescence at the porous medium surface. It is shown that the onset of efflorescence formation and its growth depends on the initial solution concentration. The experiments show that the invasion patterns are little affected by the presence of salt, but that the efflorescence contributes to gradually increase of the evaporation rate as the initial concentration in salt is increased. Then, a second phase was identified where the efflorescence dries out and drying becomes very slow. Results suggest that there exists an initial concentration marking the transition between the regime with high impact of efflorescence and the regime with low impact of efflorescence.