Paradoxical drying of a fired-clay brick due to salt crystallization

• Influence of crystallization on the formation of receding drying front.
• Critical moisture content is not an intrinsic property of a porous material.
• Critical moisture content varies depending on the initial drying rate.
• Initial drying rate influences efflorescence type formed at the material surface.

We investigated how salt crystallization inside a porous building material influences the formation of a receding drying front. Nuclear Magnetic Resonance (NMR) is used to measure non-destructively both hydrogen and dissolved sodium ions during drying process. In this study, we focused on the influence of NaCl on the drying behavior of porous media. The results show that salt changes the drying behavior. At low relative humidities (RH~0%), the drying rate of a brick saturated with NaCl solution is much lower than the drying rate of water saturated brick. Moreover, the presence of salt suppresses the development of a receding front. In this case homogenous drying of the material continues till very low saturation values. This is due to salt crystallization near the surface of the brick that causes blockage of the pores. This blockage reduces evaporation rate at the surface and allows maintaining a continuous hydraulic connection between the surface of the porous medium and the liquid present inside the material till low saturation values. In the case of a salt solution saturated brick, increasing relative humidity to 55% and 70% leads to a paradoxical situation where the evaporation rate is greater for 55% and 70% RH than for 0% RH. The paradox is explained by the impact of evaporation rate on the efflorescence microstructure, leading to the formation of a blocking crust for sufficiently high evaporation rates and non-blocking efflorescence for sufficiently low evaporation rates. The fundamental difference between the two types of efflorescence is demonstrated from a simple imbibition experiment. Using a simple continuum scale model of drying, critical moisture content was determined and all the essential features of the experimental results are validated. It is shown that critical moisture content can be very low in the case of fired-clay brick due to the low threshold of the pore space, which is consistent with the relatively large pore size distribution of the fired-clay brick.