**2.3 Crystallization of Na2SO4 and MgSO4**

Sodium sulfate is known to be a salt that causes the worst crystallization decay on porous materials and has become widely used in accelerated durability testing [38]. However, the sodium sulfate system is complicated, because under different conditions (temperature and relative humidity), it will form two stable phases (thenardite, Na2SO4 and mirabilite, Na2SO4·10H2O) or one metastable phase (heptahydrate, Na2SO4·7H2O) [12, 13] [39]. The metastable phase (Na2SO4·7H2O) is formed during the rehydration of the anhydrous sodium sulfate phase (Na2SO4) to the nucleation of mirabilite. Prior to mirabilite [12] [39], the crystallization pressure exerted by heptahydrate does not cause damage under the condition of the cooling experiments [36], and it can not be observed in building stone [13].

Regarding the damage caused by the crystallization of sodium sulfate, there are two views. One school thinks that the crystallization of thenardite is more destructive [40], because the crystallization of thenardite can generate higher pressure than mirabilite at the same supersaturation [41]. However, more and more experimental results support another school that the dissolution of thenardite producing a solution highly supersaturated with respect to mirabilite will cause the precipitation of mirabilite and result in the damage of porous materials [13] [38]. I.e. the transformation between thenardite and mirabilite can generate severe large crystallization pressure, resulting in porous materials damage.

The only naturally occurring members of the MgSO4·nH2O series on Earth are epsomite (MgSO4·7H2O, 51 wt% water), hexahydrite (MgSO4·6H2O, 47 wt% water) and kieserite (MgSO4·H2O, 13 wt% water). In aqueous systems, epsomite is stable at T below 48.4oC, hexahydrite is stable in the T range 48.4–68 oC, and kieserite is stable at T > 68 oC [42]. Thus, at the normal temperature, the crystallization of epsomite (MgSO4·7H2O,) is the distress reason.

#### **2.4 Summary**

In summary, according to above review, the following basic principles of salt weathering on porous materials can be concluded:


"Salt Weathering" Distress on Concrete by Sulfates? 439

2. As to the role of mineral additions, with the increase of dosage of fly ash and natural pozzolan, the concrete cylinders showed worse visual observation. This may be explained by the fact that salt crystallization in smaller pores can form higher crystallization stress due to the refinement of mineral additions. On the other hand, if we compare concrete H3 and H5, H4 and H6, we can find that with the same dosage the natural pozzolan (H5 and H6) played a more negative role in concrete damage than fly ash (H3 and H4). However the compressive strength of reference cylinders of H5 and H6 were higher than H3 and H4 respectively. The tests of PCA also showed similar

3. Comparing the normal concrete and blended concrete, Irassar et al [46] attributed an increase in capillary suction height caused by the pore size refinement of mineral addition to the more severe deterioration of blended concrete. However, this is in conflict with the following observations. Following the explanation based on the height of capillary sorption, concrete with a low W/C ratio should be more susceptible to salt weathering than with high W/C ratio. In the paper by Nehdi and Hayek [47], we can find that the sorption height of mortar with W/C of 0.45 is higher than with W/C of 0.6. If the above explanation is right, the mortar with low W/C (0.45) should be more susceptible to damage than the mortar with high W/C (0.6). Obviously, this conclusion

results. This appearance cannot be explained by the salt weathering.

Fig. 5. Evolution of visual rating at the atmosphere part of concrete [46]

Besides, there are also some field cases, in which a wide variety of efflorescence salts (sodium sulfate, sodium chloride and magnesium sulfate) were routinely observed on the evaporating surface of the foundation concrete in South California [48, 49] However, the damage was not caused by salt crystallization. Much of the cement paste had lost its integrity, mainly as a result of the removal of portlandite, de-calcification of the calcium silicate phases and the ultimate replacement of calcium with magnesium in many of the cementitious compounds. Many reaction products, in particular magnesium silicate hydrate, brucite, Friedel's salt (3CaO·Al2O3·CaCl2·10H2O), sodium carbonate and thaumasite were

Another interesting case [50] concerns the slabs of Yongan Dam, which is in Keshi, Xinjiang, P.R. China, where the land is arid and rich in various kinds of salts, especially sulfate salts. The slabs were constructed in August – October 2003. The air temperature fluctuates significantly: the highest temperature is up to 37 oC and at night it is only 15-20 oC. Within

is opposite to the result of field tests.

found at some depth within the concrete.
