**3.4 Summary**

450 Advances in Crystallization Processes

Fig. 17. Visual observation of normal and carbonated concrete specimens exposed to sodium

In summary, according to the above test results, two main conclusions can be deduced:

brucite, were the determining factors for material damage.

1. Sulfate crystals cannot be identified in the cement paste or concrete partially exposed to Na2SO4 and MgSO4 solutions. The chemical reaction products, ettringite, gypsum and

Fig. 16. ESEM image and EDS analysis of surface of cement paste [53]

sulfate solution [53]

Fig. 18. XRD pattern of mortar [53]

The "salt weathering" on concrete was just received a lot of attention in the recent years. Based on the above analysis of a limited number of research reports, the experimental results already showed convincing appearances that were completely opposite to the basic principles of salt weathering distress on porous materials. On the contrary, the experimental results of long term field tests and indoor tests rather tended to indicate that chemical sulfate attack is the mechanism for the concrete damage.
