Paradoxical drying due to NaCl cystallization and influence of inhibitor

We investigated how salt crystallization inside a porous building material influences the formation of a receding dryingfront. Initially, the drying behavior of fired-clay brick samples vacuum saturated with water and salt solution (3m NaCl) was studied. The samples were dried at 0% RH and 1 l min-1 air flow rate. Fig. 1a and 1b show the measured moisture profiles during drying of water and salt saturated samples, respectively.

Fig 1 The measured moisture profiles for (a) water saturated (b) 3m NaCl saturated fired-clay brick plotted as a function of position. The profiles are given for every 0.45 h and 2.26 h for water and salt saturated bricks. The samples were dried using dry air with a flow of 1 l min-1 and 0% relative humidity. The drying surface is at 0 mm. The vertical arrow shows the homogenous drying of the sample (externally limited) and the horizontal arrow shows the penetration of the receding drying front (internally limited).

In case of water saturated samples the first few profiles are almost horizontal, representing the first (externally limited) drying stage (shown by a vertical arrow in fig. 1a). Afterwards, a drying front develops which recedes below the sample surface (shown by a horizontal arrow in fig. 1a). This represents the second (internally limited) drying stage. The addition of salt changes the drying behavior of the fired-clay brick (fig. 1b). Two effects were seen. Firstly, the presence of NaCl reduces the drying rate compared to the drying rate of water saturated fired-clay brick. Complete drying of water saturated fired-clay brick took about one day, in comparison to more than one week for NaCl saturated fired-clay brick of the same dimensions and at the same drying conditions. Secondly, the receding drying front vanishes and homogenous drying of the material continued till low saturation values. Hence, drying stage-1 prolongs and homogeneous drying is maintained till low saturation values.

To investigate this, additional drying experiments were performed on samples saturated with salt solution at 55% and 70% relative humidity. The samples were vacuum saturated with 3m NaCl solution and dried inside NMR at room temperature and 1 l min-1 air flow rate. The results are shown in fig. 2 rate of volume change (dV/dt) is plotted as a function of moisture content.

For the brick dried at 0% RH a continuous decrease of the flux with decreasing moisture content is seen. Thus, there is no constant rate period in this case. However, at high humidities initially a constant drying rate is maintained (stage-1) and later a falling drying rate period is observed (stage-2). This leads to a paradoxical drying situation since the evaporation rate is greater for 55% RH and 70% RH than for 0% RH. Thus, in the presence of NaCl a receding front develops again at high humidities. At the end of the experiment, the efflorescence formed on the surface of the fired-clay brick was collected and weighed. At 0% relative humidity 6% -7% of the NaCl crystallized as efflorescence. This efflorescence had the form of a very thin crust on the outer surface of the fired-clay brick and was strongly adhered to the substrate. It was not easy to remove the efflorescence from the substrate. On the other hand, at 55% and 70% RH, a significant amount of NaCl crystallized as efflorescence. About 48% and 40% of the salt crystallized as efflorescence at 55% and 70% RH, respectively. The type of efflorescence formed at high humidities was rather fragile and was easy to remove from the substrate by rubbing. Pictures of the efflorescence formed on the surface of the materials are shown in fig. 2.5. The efflorescence is clearly quite different at 0% RH compared to the efflorescence at 55% and 70% RH. Tis suggests distinguishing two types of efflorescence, referred to as “patchy” and “crusty” and that can be referred to as well as “non-blocking” and “blocking”. The efflorescence obtained at 0% RH is blocking whereas the efflorescence at 55% and 70% RH is non-blocking.

Effect of inhibitor
In the absence of inhibitor after approx. 15 hours, the saturation concentration was achieved in the top few mm of the sample . This causes a dramatic drop in the drying rate for salt saturated brick. Since, most of the salt crystallized as sub-florescence, it causes a more severe blockage of the pores near the drying surface. However, in the presence of inhibitor the crystal morphology changes from cubic to dendritic. The salt solution creeps along the branches of the dendrites and transports more and more dissolved salt ions towards the drying surface causing the efflorescence observed at the end of drying experiment. Pictures of the materials with efflorescence are shown in fig. 4. Approx. 26% and 69% of the salt crystallized as efflorescence in the presence of 0.001 m and 0.01 m inhibitor respectively. Because of the formation of efflorescence in the presence of inhibitor the average salt ion concentration inside the brick remained below saturation. Therefore, the system remained open and less blockage occurred compared to the salt saturated system without inhibitor. As a consequence of this no dramatic drop in drying rate was seen and the paradoxical disappears.

In case of water saturated fired-clay brick two drying stages were observed, i.e., a continuous drying rate period followed by a receding drying front period. These results are in accordance with the standard drying behavior of water saturated porous media. However, at 0% RH, NaCl suppresses the formation of a drying front. This is due to the extremely low drying rate, which is mainly caused by pore blockage near the drying surface. For NaCl salt saturated fired-clay brick the evaporation rate is higher at high relative humidities and salt ions crystallize as efflorescence on the surface of the brick. Because of the higher evaporation rate the water transport cannot be maintained. This leads to the penetration of front at high humidities, also in the presence of salt. Hence, drying with salt leads to a paradoxical situation in which increasing the relative humidity in the external air and thus reducing the external evaporation demand can increase the evaporation rate. Sufficiently high evaporation rates lead to the formation of blocking efflorescence (crust) whereas lower rates can lead to non-blocking efflorescence. Addition of inhibitor was found to be useful at low humidity conditions. At low humidity, due to the crystal morphology in the presence of inhibitor salt crystallizes as nondestructive efflorescence.