**4.5 Study of the effect of different kinds of sulfates in "salt weathering" on concrete**

As abovementioned, Ruiz-Agudo [42] studied salt weathering distress of limestone specimens submerged in sodium sulfate and magnesium sulfate solutions respectively. The results showed that these two sulfates both severely damaged stone.

In the full immersion attack, because of the simultaneous significant decomposition of the C-S-H gel that accompanies the formation gypsum and ettringite, admittedly, people think the overall corrosive action of magnesium sulfate is greater than that of sodium sulfate [81, 91]

However, during the process of "salt weathering" on concrete, the test results showed the opposite appearance. Nehdi and Hayek [47] observed the appearances of the cement mortar partially exposed to 10% sodium sulfate and magnesium sulfate solution in a RH cycling between 32±3% and >95% condition respectively. The results showed that a large amount of efflorescence covers the surface when mortar is exposed to sodium sulfate solution. On the contrary, the surface of mortar subjected to magnesium sulfate solution is clean. It seems that sodium sulfate performs more corrosive effect on mortar than magnesium sulfate. The tests [53] also showed the same results. The aggregates and cement paste were completely separated in the upper part of concrete after 8 months exposure. However, the samples exposed to magnesium sulfate solution showed little damage. As shown in Fig. 22.

Fig. 22. Visual observation of concrete specimens partially exposed to Na2SO4 and MgSO4 solutions [53]

This appearance may indicate that the concrete damage cannot be explained by salt weathering. First, MgSO4 showed a harmful effect on stone due to salt weathering. As a porous material concrete should also show a similar scaling manner. Secondly, Fig. 23 shows the surface tensions of NaCl, Na2SO4 and MgSO4 [92]. According to Eq. 5 the equilibrium heights of capillary rise of sodium sulfate and magnesium sulfate should be

distribute in the cement paste similar to the alkali activated cement in which Na2SO4 and powders are mixed before adding water, resulting in Na2SO4 homogeneously distributing in

In summary, concerning the role of mineral additions in the sulfate attack on partially exposed concrete, the exposure conditions and the solution transport mechanism are

**4.5 Study of the effect of different kinds of sulfates in "salt weathering" on concrete**  As abovementioned, Ruiz-Agudo [42] studied salt weathering distress of limestone specimens submerged in sodium sulfate and magnesium sulfate solutions respectively. The

In the full immersion attack, because of the simultaneous significant decomposition of the C-S-H gel that accompanies the formation gypsum and ettringite, admittedly, people think the overall corrosive action of magnesium sulfate is greater than that of sodium sulfate [81, 91] However, during the process of "salt weathering" on concrete, the test results showed the opposite appearance. Nehdi and Hayek [47] observed the appearances of the cement mortar partially exposed to 10% sodium sulfate and magnesium sulfate solution in a RH cycling between 32±3% and >95% condition respectively. The results showed that a large amount of efflorescence covers the surface when mortar is exposed to sodium sulfate solution. On the contrary, the surface of mortar subjected to magnesium sulfate solution is clean. It seems that sodium sulfate performs more corrosive effect on mortar than magnesium sulfate. The tests [53] also showed the same results. The aggregates and cement paste were completely separated in the upper part of concrete after 8 months exposure. However, the samples

exposed to magnesium sulfate solution showed little damage. As shown in Fig. 22.

Fig. 22. Visual observation of concrete specimens partially exposed to Na2SO4 and MgSO4

This appearance may indicate that the concrete damage cannot be explained by salt weathering. First, MgSO4 showed a harmful effect on stone due to salt weathering. As a porous material concrete should also show a similar scaling manner. Secondly, Fig. 23 shows the surface tensions of NaCl, Na2SO4 and MgSO4 [92]. According to Eq. 5 the equilibrium heights of capillary rise of sodium sulfate and magnesium sulfate should be

different from the full immersion cases. It needs further research.

results showed that these two sulfates both severely damaged stone.

the cement paste.

solutions [53]

almost the same, showing similar efflorescence zone due to salt weathering. The reason for the opposite appearance of MgSO4 in concrete may be the insoluble brucite due to chemical reaction that blocks the capillary. The role of sulfates in the "salt weathering" on concrete also needs further research.

Fig. 23. Surface tensions of NaCl, Na2SO4 and MgSO4 [92]

#### **4.6 Study of the role of concrete carbonation in "salt weathering" on concrete**

The negative effect of carbonation on corrosion of reinforcing steel in concrete is well known. As to the sulfate attack on concrete, as a result of carbonation, the total porosity would be reduced and the permeability of concrete could be improved [93-94]. So, Gao [95] pointed out that the carbonation layer could mitigate diffusion of sulfate ions to some extent in the full immersion situation.

However, when the concretes are partially exposed to sulfate solutions, the situation may be different. V.T. Ngala [94] studied the effect of carbonation on the ratio of capillary to total porosity of cement paste. The results showed that the capillary pore fraction greatly was improved after carbonation (shown in Fig. 24). This will promote the capillary suction of concrete, forming a more severe sulfate pore solution in the concrete and resulting more severer concrete damage.

Fig. 24. Ratio of capillary to total porosity of non-carbonated and carbonated paste [94]

"Salt Weathering" Distress on Concrete by Sulfates? 459

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From Fig. 24, compared to cement paste, the ratios of capillary pores fraction of cement + 30% FA and cement + 65% BFS were higher than cement paste after carbonation. Some research showed that blended concrete has high carbonation rate [96], high degree of carbonation [97] or large carbonation depth [98] compared to the ordinary cement concrete. The carbonation susceptibility of blended concrete may be another reason for the negative effect of mineral addition on sulfate resistance of partially exposed concrete.

Besides, according to the review of indoor tests of "salt weathering" on concrete, two experimental results were observed showing that sulfate crystallization can be detected in the calcite crystals, the carbonation products of concrete (Fig. 12) and that carbonation could accelerate the concrete damage (Fig. 17). It might be that the efflorescence also occurs after concrete carbonation.

In summary, the effect of carbonation on sulfate resistance of partially exposed concrete is not clear. Further research will contribute to disclose the mechanism of "salt weathering" on concrete.
