**3.2.1 H. Haynes tests [2, 9]**

440 Advances in Crystallization Processes

this temperature range, the transformation between thenardite and mirabilite can occur. However, it was found that the slabs became gray and mushy throughout the thickness

In summary, according to the above analysis the appearances of long term field tests did not show convincing evidences to support "salt weathering" causing the deterioration of

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

 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]

where they were in contact with groundwater due to thaumasite sulfate attack.

concrete partially exposed to sodium sulfate environment.

**3.2 Indoor tests** 

section 2.2.

H. Haynes and his coworkers performed very important and systemical tests about the salt weathering distress on concrete. In the two papers [2, 9], different ambient conditions were created within storage cabinets whose temperature and relative humidity were controlled. The concrete cylinders (Ø76 × 145mm) were partially exposed to 5% Na2SO4, NaCO3 and NaCl solutions. A partial submergence condition was achieved by wetting the specimen to a height of 25 mm. At the height of 50mm, a plastic cover to the container functioned as a quasi-vapor retarder to minimize evaporation. The plastic cover did not touch the cylinder. Hence, within the region of 25 to 50mm the cylinder was exposed to a moist environment. Above 50mm (2 in.), the concrete was exposed to ambient environmental conditions. In the test program the author said that "the sulfate solution and tap water were replaced on a monthly basis; however, replacements for evaporation loss were provided at 2-week intervals. Much of the solution evaporated in the 40 °C and 31% relative humidity environment where, in general, at the end of 2 weeks, minor amounts of solution remained; and at times, no solution remained".

The tests were divided into two Phases for 3.1 years. The performance of concrete cylinders under five storage conditions was studied in detail, the exposures were:

Condition 1: steady at 20°C and 54% relative humidity from 28 to 530 days (Phase I), and then 20°C and 32% relative humidity from 530 to 1132 days (Phase II),

Condition 2: steady at 20°C and 82% relative humidity from 28 to 530 days (Phase I ), and then from 530 to 1132 days (Phase II),

Condition 3: 40°C and 74% relative humidity from 28 to 406 days (Phase I), and then 40°C and 31% relative humidity from 406 to 1132 days (Phase II),

Condition 4: 2-week cycles between 20°C and 54% relative humidity and 20°C and 82% relative humidity from 28 to 530 days (Phase I), and then 2-week cycles between 20°C and 31% relative humidity and 20°C and 82% relative humidity from 530 to 1132 days(Phase II),

Condition 5: exposed to 2-week cycles between 20°C and 82% relative humidity and 40°C and 74% relative humidity from 28 to 406 days(Phase I), and then 2-week cycles between 20°C and 82% relative humidity and 40°C and 31% relative humidity from 406 to 560 (847) days (Phase II).

The effects of Na2SO4, Na2CO3 and NaCl were compared. The visual observation was photographed, the average mass of scaling materials was collected, the species of concrete were identified by petrographic analysis, and chemical analysis was employed to study the ions distribution. According to the experimental results, they concluded that salt weathering

"Salt Weathering" Distress on Concrete by Sulfates? 443

Condition 1 Condition 2 Fig. 8. Visual observation of concrete deterioration exposed to Na2CO3 and NaCl solutions [9]

2. Under Condition 3 (40°C /74%RH, 40°C /31%), concrete showed the most significant scaling in the case of NaCl, on the contrary, the concrete showed least scaling in the cases of Na2CO3 and Na2SO4 solutions. These appearances are contradictory because the concretes should show similar scaling manners due to the salt weathering in case of Na2CO3 and NaCl solutions. If the mechanism of concrete damage is also salt weathering in case of Na2SO4 solution, the concrete should also show similar scaling manners in NaCl solutions. If the mechanism of concrete damage is the chemical sulfate attack, the scaling manners of concrete cylinders by Na2CO3 and Na2SO4 solutions should show big difference. The possible reason of the above contradictory appearances may be the fast evaporation rate of Na2CO3 and Na2SO4 solutions. At high ambient temperature, Na2CO3 and Na2SO4 solutions would dry up soon, but some NaCl solution would remain. The tests under Condition 3 should be further studied to avoid the effect

During Phase I from 28 days to 406 or 530 days: the worst damage occurred under Condition 5 (cycle 20°C/82% RH and 40°C/70% RH) in the case of Na2SO4 solution (the mass of scaled material was about 16g). However, in the case of Na2CO3 solution the worst damage appeared under Condition 1 (20°C/54% RH) (the mass of scaled material was just

1. In the case of Na2SO4 solution the mass of scaled material under Condition 1(20°C/32% RH) was less than Condition 2(20°C/82% RH). The opposite appearance was observed in case of Na2CO3: the mass of scaled material was about 22g under Condition 1(20°C/32% RH) and about 1g under Condition 2 (20°C/82% RH). Similar appearance

2. The mass of scaled material of concrete under Condition 1 (20°C/32% RH) was less than Condition 4(cycle 20°C/82% RH and 20°C/31% RH) in case of Na2SO4. However, the mass of scaled material of concrete under Condition 1 (20°C/32% RH) was almost the same as Condition 4(cycle 20°C/82% RH and 20°C/31% RH) in the case of Na2CO3.

about 2.8g). This means that high RH can accelerate concrete damage by Na2SO4.

During Phase II from 406 or 530 days to the end of tests:

also observed in the case of NaCl solution.

of evaporation of solution. **3.2.1.2 Mass of scaling materials**  (NaCl)

plays the predominant role in concrete damage. However, there are a few questionable points about the relationship between the evidences and the conclusion based on the basic principles.
