**3.2.1.1 Visual observation of concrete cylinders**

According to the photographs of visual observation at the end of exposure, we can find that:

1. When concrete cylinders were exposed to Na2SO4 solution, as abovementioned the concrete cylinders exposed to high relative humidity condition were deteriorated more severely than low relative humidity condition (as shown in Fig. 7).

Condition 1 Condition 2

Fig. 7. Visual observation of concrete deterioration. 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; Condition 2: steady at 20°C and 82% relative humidity from 28 to 530 days (Phase I ), and then from 530 to 1132 days [2]

Correspondingly, Fig. 8 shows the visual observation of concrete exposed to Na2CO3 and NaCl solutions under Condition 1 and Condition 2.

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

According to the photographs of visual observation at the end of exposure, we can find that: 1. When concrete cylinders were exposed to Na2SO4 solution, as abovementioned the concrete cylinders exposed to high relative humidity condition were deteriorated more

Condition 1 Condition 2

Correspondingly, Fig. 8 shows the visual observation of concrete exposed to Na2CO3 and

Condition 1 Condition 2

(Na2CO3)

Fig. 7. Visual observation of concrete deterioration. 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; Condition 2: steady at 20°C and 82% relative humidity from 28 to 530

severely than low relative humidity condition (as shown in Fig. 7).

principles.

**3.2.1.1 Visual observation of concrete cylinders** 

days (Phase I ), and then from 530 to 1132 days [2]

NaCl solutions under Condition 1 and Condition 2.

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 of evaporation of solution.

## **3.2.1.2 Mass of scaling materials**

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 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:


"Salt Weathering" Distress on Concrete by Sulfates? 445

the reason why the position where Na2O content is highest corresponds with most severe deterioration of concrete exposed to Na2CO3 and NaCl solutions. However the situation is opposite in the case of Na2SO4, the position, where SO3 content is highest, locates on the top

Elevationn on cylinder, mm

Na2CO3, NaCl Na2SO4

In summary, based on the above analysis of indoor tests, two conclusions can be deduced: 1. Concrete partially exposed to Na2SO4 is susceptive to being more severely deteriorated under high RH environment than low RH environment. This appearance is in conflict

SO3 distribution Deteriorated concrete

Na2O distribution Deteriorated concrete

2. The most severe deterioration does not occur in the portion of concrete containing the highest sulfates content. This is also in conflict with the basic principles of salt

The starting point of our tests is to find a trace of salt crystals in the concrete as a direct evidence by means of XRD, SEM and EDS [53, 54]. Sulfate crystals can be easily identified in stone [23, 55]. However, in case of concrete elements, it is hard to identify them. Concrete technologists always attribute this to the coring and sawing operations when preparing samples for experimental analysis, as lapping water can readily dissolve salts from original and treated surfaces [2, 3, 4]. However this is not the main cause for the problem. Samples also can be taken in a dry manner to avoid the influence of water. Furthermore, In our study, the tests were designed to avoid the influence of water within the detection process of sulfate

Cement paste and cement – fly ash paste specimens and normal concrete specimens were partially exposed to Na2SO4 and MgSO4 solution under constant and fluctuating storage conditions respectively. After a period of exposure, the specimens were moved out from the solution and did not touch solution or water any more. The surface of the specimens was cleared by a thin blade and a soft brush. The samples for XRD and SEM were dried in a

portion of cylinder that shows little or no deterioration ( as shown the black line ).

Fig. 9. Schematic of salt distribution in the concrete cylinders

Elevationn on cylinder, mm

with the basic principles of salt weathering.

weathering.

Deteriorated section

**3.3 Our tests [53, 54]**

crystals.

vacuum container with silica gel.

Corresponding to Phase I, this also means that high RH is in favor of the deterioration effect of Na2SO4 on concrete.

### **3.2.1.3 Petrographic analysis**

According to petrographic analysis, abundant gypsum deposits were detected in large and small cracks, microcracks and voids near the surface of concrete. However, the authors presented two points to show that gypsum cannot result in concrete damage:


#### **3.2.1.4 Chemical analysis**

Several cylinders were cut vertically to obtain a 25 mm thick midsection slice. This slice was then cut vertically into two 17 mm exterior sections and one 34 mm interior section. Starting at the bottom, the vertical sections were cut horizontally into six pieces 25 mm each. These pieces were crushed and pulverized to minus No. 50 mesh. The SO3 contents and Na2O contents were determined. The distributions of SO3 contents and Na2O contents are schematically shown in Fig. 9.

The salt distribution in the concrete cylinder provides a powerful evidence supporting that salt weathering is not the major mechanism causing concrete damage.

Based on the above review on the basic principles of salt weathering, supersaturation is the key factor for the salt crystallization. The salt crystals will deposit from the solution during the process of salt crystallization, however, the salt concentration of pore solution must be maintained high for the formation of supersaturation during the whole process. I.e. the salt contents should be highest where salt crystallization distress occurs in the concrete. This is

According to petrographic analysis, abundant gypsum deposits were detected in large and small cracks, microcracks and voids near the surface of concrete. However, the authors

1. "Although trace amounts of gypsum were found near the outer surfaces, gypsum formation is a one-time occurrence, whereas crystallization of mirabilite and thenardite occurred repeatedly due to the biweekly cyclic changes in environmental conditions. Hence, the cycles of mirabilite and thenardite crystallization appear to be responsible for any significant expansion force". It is not clear why the authors thought that "gypsum formation is a one-time occurrence". According to above review, the solution can be drawn into the concrete continuously during wick action. In the presence of sulfate, gypsum crystals can continuously grow. Moreover, the authors pointed out that the pH value of the pore solution in the concrete should have been reduced due to carbonation, whereas for gypsum formation, the pH value of solution should be less

2. "Despite the extensive alteration of the microstructure and the formation of gypsum, the concrete below the solution line was mostly intact with no mass loss, whereas there was substantial mass loss at the surface of the cylinder above the solution line. If gypsum did not cause scaling below the solution line, there is little reason to suspect that gypsum would cause scaling above the solution line. This indicated that salt crystallization alone, or in conjunction with gypsum, caused the scaling above the solution line. As salt crystallization by itself is known to damage rocks, the presence of gypsum is not necessary". This point seems reasonable, however, the authors did not give the quantitative analysis of gypsum. Because according to abovementioned wick action, a much higher concentration pore solution will be formed in the cylinder above the solution than under the solution, resulting in more severe sulfate attack and

Several cylinders were cut vertically to obtain a 25 mm thick midsection slice. This slice was then cut vertically into two 17 mm exterior sections and one 34 mm interior section. Starting at the bottom, the vertical sections were cut horizontally into six pieces 25 mm each. These pieces were crushed and pulverized to minus No. 50 mesh. The SO3 contents and Na2O contents were determined. The distributions of SO3 contents and Na2O contents are

The salt distribution in the concrete cylinder provides a powerful evidence supporting that

Based on the above review on the basic principles of salt weathering, supersaturation is the key factor for the salt crystallization. The salt crystals will deposit from the solution during the process of salt crystallization, however, the salt concentration of pore solution must be maintained high for the formation of supersaturation during the whole process. I.e. the salt contents should be highest where salt crystallization distress occurs in the concrete. This is

salt weathering is not the major mechanism causing concrete damage.

presented two points to show that gypsum cannot result in concrete damage:

effect of Na2SO4 on concrete.

**3.2.1.3 Petrographic analysis** 

than 11.9 [52].

forming more gypsum.

schematically shown in Fig. 9.

**3.2.1.4 Chemical analysis** 

Corresponding to Phase I, this also means that high RH is in favor of the deterioration

the reason why the position where Na2O content is highest corresponds with most severe deterioration of concrete exposed to Na2CO3 and NaCl solutions. However the situation is opposite in the case of Na2SO4, the position, where SO3 content is highest, locates on the top portion of cylinder that shows little or no deterioration ( as shown the black line ).

Fig. 9. Schematic of salt distribution in the concrete cylinders

In summary, based on the above analysis of indoor tests, two conclusions can be deduced:

