**3.2 Indoor tests**

Rodriguez-Navarro and Doehne [26] studied the effect of evaporation on salt weathering distress on stone. After 30 days of exposure to a saturated sodium sulfate solution at constant 20 oC, larger amounts of efflorescence and lower weight losses were observed when the crystallization took place at a relative humidity of 60% instead of at 30% RH. H. Haynes and his coworkers [9] carried out some tests partially exposing the same concrete cylinders to 5% NaCl and Na2CO3 solutions. Severe damage was observed for concrete cylinders, which were placed in constant environment at 20 oC and 54% relative humidity from day 28 to day 530, and then at 20 oC and 32% RH from day 530 to day 1132. The specimens kept in a constant environment at 20 oC and 82% relative humidity from day 28 to day 1132 looked sound. These observations can be explained by the fact that a low relative humidity results in more evaporation, leading to sub-efflorescence [20] that forms deep in the material and results in significant damage [24,25] as explained in the section 2.2.

 However, contradictory observations can be found with respect to concrete exposed to sodium sulfate solution [51]. Two concrete specimens with the same mixture proportions were partially immersed in 10% Na2SO4, solution. One specimen was placed at 80%±5% RH and the other was placed at 30% ±5%. After 75 days of exposure to a constant temperature of 25 oC, the specimens at 80% RH showed signs of deterioration first over a very large area, starting from above the solution level. On the contrary, at 30% RH, the zone of deterioration was narrower and was situated at a certain distance above the solution level. In this case, the first sign of deterioration was a crack, not spalling (shown in Fig. 6).

Fig. 6. Concrete cylinders exposed to sodium sulfate solution for 75 days [51]

Similar results were also observed in the tests performed by H. Haynes and his coworkers [2]. Narrower spalling zone was found in case of concrete specimens exposed to constant environment at 20 oC and 54% relative humidity from day 28 to day 530, and then at 20 oC and 32% RH from day 530 to day 1132. Extensive spalling zone was found under constant environment at 20 oC and 82% relative humidity from day 28 to day 1132. These tests will be discussed in detail as follow.
