**1. Introduction**

430 Advances in Crystallization Processes

Zheng, M.P.; Xiang, J.; Wei, X.J.; Zheng, Y. (1989). *Saline lakes on the Qinghai-Xizang (Tibet) Plateau*, Beijing Science and Technology Press, ISBN 7-5304-0519-5, Beijing Zheng, X. Y.; Tang, Y.; Xu, C.; Li, B.X.; Zhang, B.Z.; Yu, S.S. (1988). *Tibet saline lake*; Chinese

Science Press, ISBN 7-03-000333-0, Beijing

Salt weathering, also called salt crystallization or physical salt attack, is defined as the basic degradation mechanism that a porous material, such as stone and masonry, undergoes at and near the Earth's surface [1]. The parts of porous materials in contact with relatively dry air near the Earth's surface will be severely deteriorated but the parts buried in salts environment look sound.

Generally, the idea of sulfate attack on concrete means that a complex physiochemical process including several harmful productions formation through chemical reaction, such as ettringite and gypsum, following the crystal growth of these productions in cracks or pores resulting in concrete damage. However, another concept was given more and more attention that "salt weathering/physical salt attack" on concrete partially exposed to environment specially containing Na2SO4 or MgSO4. ACI (American Concrete Institute) created a new subcommittee, ACI 201-E (Salt Weathering/Physical Salt Attack) in 2009. In 2011, an ballot was performed to discuss if it is necessary to separate the "physical salt attack" from chapter 6 "sulfate attack" as chapter 8 for ACI 201.2R. There were also more and more reports discussing this topic [2-9]. It seems that this topic will be high interest and relevance for the concrete community.

Certainly, concrete is also a kind of porous material. When partially exposed to an environment containing salts (especially sodium sulfate), such as in the case of a foundation, dam, column, flatwork and tunnel, a large amount of efflorescence will appear on the surface of the concrete accompanied with a similar scaling manner as salt weathering distress on masonry, showing a freezing-and-thawing-like deterioration on the surface of concrete [2] (Fig. 1). Therefore, concrete technologists logically and involuntarily define this phenomenon as salt weathering distress on concrete or physical attack on concrete.

Apparently, it seems reasonable to attribute salt weathering to the decay of concrete partially exposed to sulfate environment. Concrete technologists subjectively accepted that

"Salt Weathering" Distress on Concrete by Sulfates? 433

crystallization pressure would form [11]. Diffusion through this thin layer will equalize the concentration at the tip of the crystal and in the gap between the side of the crystal and the pore wall [12] [13]. The concentration and mobility of ions within this gap have a profound

On the other hand, for a crystal, when the equilibrium is established between solution and

ln( ) *cl cl RT C*

Where, *γcl* is the crystal /liquid interfacial energy; *κcl* is the surface curvature of crystal. Eq. (2) means two facts: a smaller spherical crystal is in equilibrium with a higher concentration than a larger flat crystal (equilibrium growth). The larger crystal (a relatively flat crystal) will grow and consume the supersaturation. Consequently, the smaller crystal will dissolve and the liberated solution will diffuse to the larger crystal (non-equilibrium growth) [14].

For equilibrium growth, a confined crystal can only exert stress if it is in contact with a pore solution that is supersaturated with respect to the unloaded face of the crystal [15]. The stress

Where, *<sup>E</sup>* κ*cl* is the curvature of the pore entrance (labeled point E), and *<sup>C</sup>*κ*cl* is the curvature

Because *<sup>E</sup>* κ*cl* is less convex (positive) than *<sup>C</sup>*κ*cl* , the compressive strength is negative, but it creates a tensile stress in the hoop direction around the pore. This tensile stress is the destructive "crystallization pressure" A high equilibrium crystallization pressure requires a confined crystal in a pore of any geometry with a very small pore entrance [16]. Therefore, the stones with a bimodal pore size distribution are extremely susceptible to salt attack [17-19].

For non-equilibrium growth, all of the crystals in internal pores of a matrix with a distribution of pore sizes are unstable with respect to macroscopic crystals that nucleate in large voids. During the drying (evaporation) or in the presence of a sharp temperature gradient, the smaller crystals will dissolve and feed the growth of the larger one, reaching another equilibrium. During this equilibrium, a high transient stress can be produced (Eq. (4)) [14].

*v Cs* γκ = (2)

( ) *C E* σ =γ κ −κ *W cl cl cl* (3)

impact on the crystallization stress [14].

can be obtained by Eq. (3) [11]:

of other internal points (labeled point C) (Fig.2).

Fig. 2. Schematic of crystal of salt growing in a pore [14]

crystal, the solubility product will satisfy:

salt weathering or salt crystallization cannot be avoided in concrete, because concrete is also a kind of porous material similar to stone. However, in effect, some field and indoor research results of "salt weathering" distress on concrete have shown a number of appearances opposite to the basic principles of salt weathering on porous materials. Therefore, it is necessary and imperative to present this problem to avoid further confusion.

Fig. 1. Deterioration of railway tunnel (Southwestern Region, China)

This review paper includes three parts. First, the basic principles of salt weathering on porous materials are reviewed. Second, some field and indoor tests of "salt weathering" on concrete by sulfates are presented. Some appearances, which were generated by "salt weathering" on concrete but were opposite to the basic principles of salt weathering on porous materials, are analyzed in detail. Several points that need further study are presented in the third part.
