**2. Salt weathering distress on porous materials**

#### **2.1 Salt crystallization in pore**

The work of Carl W. Correns on crystallization pressure is undoubtedly a milestone in the field of durability of porous materials [10], and the equation (Eq. (1)) exhibited in his paper written in 1949 for crystallization is broadly used and quoted.

$$P = \frac{RT}{v} \ln(\frac{C}{Cs}) \tag{1}$$

Where *R* is the ideal gas constant, *T* is the absolute temperature, *v* is the molar volume, *C* is the concentration of solution, and *Cs* is the concentration of saturated solution. *C/Cs* is the supersaturation.

The above equation indicates that supersaturation is the key factor for crystallization. The supersaturation should be maintained during the process of salt crystallization. The crystal will grow until the supersaturation is consumed. He also pointed out that a thin layer/film of aqueous solution always remains between the crystal and the internal solid walls of the porous network. The thin layer allows the solute to diffuse from the pore solution to the crystal surface that is growing against the pore wall. If this thin layer did not exist, the crystal would go into contact with the pore wall, the growth would stop and no

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.

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

The work of Carl W. Correns on crystallization pressure is undoubtedly a milestone in the field of durability of porous materials [10], and the equation (Eq. (1)) exhibited in his paper

ln( ) *RT C <sup>P</sup>*

Where *R* is the ideal gas constant, *T* is the absolute temperature, *v* is the molar volume, *C* is the concentration of solution, and *Cs* is the concentration of saturated solution. *C/Cs* is the

The above equation indicates that supersaturation is the key factor for crystallization. The supersaturation should be maintained during the process of salt crystallization. The crystal will grow until the supersaturation is consumed. He also pointed out that a thin layer/film of aqueous solution always remains between the crystal and the internal solid walls of the porous network. The thin layer allows the solute to diffuse from the pore solution to the crystal surface that is growing against the pore wall. If this thin layer did not exist, the crystal would go into contact with the pore wall, the growth would stop and no

*v Cs* <sup>=</sup> (1)

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

**2. Salt weathering distress on porous materials** 

written in 1949 for crystallization is broadly used and quoted.

presented in the third part.

supersaturation.

**2.1 Salt crystallization in pore** 

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 impact on the crystallization stress [14].

On the other hand, for a crystal, when the equilibrium is established between solution and crystal, the solubility product will satisfy:

$$
\gamma\_{cl}\kappa\_{cl} = \frac{RT}{\upsilon} \ln(\frac{\text{C}}{\text{Cs}}) \tag{2}
$$

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

$$
\sigma\_W = \chi\_{cl} (\mathbf{x}\_{cl}^C - \mathbf{x}\_{cl}^E) \tag{3}
$$

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

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

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].

"Salt Weathering" Distress on Concrete by Sulfates? 435

If the water is containing salts, these salts cannot be carried by the vapour and therefore build up at this position. This concentration effect causes back-diffusion of salt away from the wet-dry interface. If the salts concentration near the wet-dry interface ever exceeds the solubility of the salt compounds present, precipitation is likely to occur [31-34]. The absorption-diffusion relationship can be described by the definition of the Peclet number [34]:

*m c*

*<sup>D</sup>* <sup>≡</sup> <sup>θ</sup> (6)

*hL Pe*

Where, *h* (m3 m-2 s-1) is the drying rate, *L* (m) the length of the sample, and *θm* (m3 m-3) the maximum fluid content by capillary saturation. *Dc* (m2 s-1) is the diffusion coefficient of the ions in the moisture in the porous medium. For Pe<<1 diffusion dominates and the ionprofiles will be uniform, whereas for Pe>>1 absorption dominates and ions will be

Y. T. Puyate et al discussed the chloride transport due to wick action in the concrete in detail [31-34]. One vital conclusion is that it was the vapour pressure of the solution and the relative humidity of air which control the position of the wet-dry interface [33]. The position of drywet interface locates in the inner of the concrete faced to a low relative humidity situation (0%) (Pe>>1), and a sharp peak of chloride concentration exceeding the saturation value occurs [34]. In contrast, in a high relative humidity condition (78%) (Pe<<1), the location of the interface is close to the concrete surface [31, 32]. Therefore, high evaporation can induce

Nuclear magnetic resonance (NMR) is used to study the crystallization of sodium chloride due to wick action. Measuring the moisture and ion profile in a fired-clay brick cylinder (Ø

According to this diagram (Fig.3), when Pe<<1, i.e. very slow drying or high relative humidity, the ion profile remains homogeneous and for some time no crystallization will occur. The average NaCl concentration slowly increases until the complete sample has reached saturation, forming a high concentration pore solution zone. When Pe>>1, i.e. very fast drying or low relative humidity, ions are directly advected with the moisture to the top of the sample and a saturation peak will build up with a very small width. If the rate of

20×45mm), an efflorescence pathway diagram is plotted [35, 36] as shown in Fig. 3.

accumulated at the drying surface.

severe crystallization distress.

Fig. 3. Efflorescence pathway diagram [35]

$$\sigma\_{\rm{VV}} = \frac{\gamma\_{\rm{CL}}}{r\_s} - \frac{R\_g T}{V\_m} \ln(\frac{\rm{C}}{\rm{C}s}) \tag{4}$$

Where, *rs* is the radius of the small pore entrance.

The duration and intensity of the transient crystallization pressure depend on three factors [14]: (1) the rate of supply of solute; (2) the rate of growth of crystal; (3) the rate of diffusion of solute to macro-pores. High evaporation can result in high supersaturation, and increase the growth of crystal and result in a high transient stress [14] [20], leading to severe damage by salt crystallization.

The supersaturation can be produced by cooling, evaporation and drying and wetting cycle. If the temperature dependence of the solubility of a salt is high, a drop of temperature can result in supersaturation. Supersaturation caused by evaporation always occurs when one face of the porous material is in contact with the solution and the other face is exposed to relatively dry conditions, i.e., the salt weathering process.

As to the relationship between strength and durability of porous materials, it always shows positive correlation [21-23]: porous materials with higher strength can suffer stronger salt crystallization distress.

#### **2.2 Characteristics of salt weathering distress**

In the process of salt weathering, efflorescence and sub-efflorescence will occur. Efflorescence always occurs on the surface of the material, and shows little or no damage. On the contrary, sub-efflorescence forms under the material surface and results in significant damage [24-26]. Some interesting studies showed that addition of ferrocianides ([Fe(CN)6]4-) can promote NaCl efflorescence growth as opposed to sub-efflorescence growth in porous stones, and minimize salt damage [27, 28].

Wick action is the transport of water (and any species it may contain) through a concrete (porous material) element face in contact with water to a drying face with less than 100% relative humidity of air [29]. The mechanism involves capillary sorption and evaporation.

During the process of wick action, if there is no evaporation, the solution level can increase through capillary rise in the concrete according to Eq. 5: [30]

$$h = \frac{2\chi\_{\cup\nu} \cdot \cos\Theta}{r \text{g\circledast\,}} \tag{5}$$

where *h* is the height of capillary rise, *γLV* is the liquid/vapor interfacial energy, *θ* is the contact angle, *r* is the pore radius, g is the gravitational acceleration, and *ρ* is the density of the solution. In the case of water in concrete cos*θ*≈1, *γLV* is ~ 400 mJ·m-2, and *r* is the typically 10~100nm. Therefore, *h* is about 1-10m. However, the pores will easily lose water due to evaporation. The pores of 10 um will start to empty when the relative humidity is lower than 95%. So, when the relative humidity is lower, *h* will decrease. After some time, a state of equilibrium (wet-drying interface) may be reached. Then the rate of water entering the concrete by capillary sorption matches the rate of water leaving the opposite face of the concrete element by water vapour diffusion.

434 Advances in Crystallization Processes

*s m*

The duration and intensity of the transient crystallization pressure depend on three factors [14]: (1) the rate of supply of solute; (2) the rate of growth of crystal; (3) the rate of diffusion of solute to macro-pores. High evaporation can result in high supersaturation, and increase the growth of crystal and result in a high transient stress [14] [20], leading to severe damage by salt

The supersaturation can be produced by cooling, evaporation and drying and wetting cycle. If the temperature dependence of the solubility of a salt is high, a drop of temperature can result in supersaturation. Supersaturation caused by evaporation always occurs when one face of the porous material is in contact with the solution and the other face is exposed to

As to the relationship between strength and durability of porous materials, it always shows positive correlation [21-23]: porous materials with higher strength can suffer stronger salt

In the process of salt weathering, efflorescence and sub-efflorescence will occur. Efflorescence always occurs on the surface of the material, and shows little or no damage. On the contrary, sub-efflorescence forms under the material surface and results in significant damage [24-26]. Some interesting studies showed that addition of ferrocianides ([Fe(CN)6]4-) can promote NaCl efflorescence growth as opposed to sub-efflorescence

Wick action is the transport of water (and any species it may contain) through a concrete (porous material) element face in contact with water to a drying face with less than 100% relative humidity of air [29]. The mechanism involves capillary sorption and evaporation.

During the process of wick action, if there is no evaporation, the solution level can increase

*rg* <sup>γ</sup> ⋅ θ <sup>=</sup> <sup>ρ</sup>

where *h* is the height of capillary rise, *γLV* is the liquid/vapor interfacial energy, *θ* is the contact angle, *r* is the pore radius, g is the gravitational acceleration, and *ρ* is the density of the solution. In the case of water in concrete cos*θ*≈1, *γLV* is ~ 400 mJ·m-2, and *r* is the typically 10~100nm. Therefore, *h* is about 1-10m. However, the pores will easily lose water due to evaporation. The pores of 10 um will start to empty when the relative humidity is lower than 95%. So, when the relative humidity is lower, *h* will decrease. After some time, a state of equilibrium (wet-drying interface) may be reached. Then the rate of water entering the concrete by capillary sorption matches the rate of water leaving the opposite face of the

(5)

*W*

Where, *rs* is the radius of the small pore entrance.

relatively dry conditions, i.e., the salt weathering process.

growth in porous stones, and minimize salt damage [27, 28].

through capillary rise in the concrete according to Eq. 5: [30]

2 cos *LV <sup>h</sup>*

concrete element by water vapour diffusion.

**2.2 Characteristics of salt weathering distress** 

crystallization.

crystallization distress.

ln( ) *CL <sup>g</sup>*

*R T C r V Cs*

<sup>γ</sup> σ= − (4)

If the water is containing salts, these salts cannot be carried by the vapour and therefore build up at this position. This concentration effect causes back-diffusion of salt away from the wet-dry interface. If the salts concentration near the wet-dry interface ever exceeds the solubility of the salt compounds present, precipitation is likely to occur [31-34]. The absorption-diffusion relationship can be described by the definition of the Peclet number [34]:

$$Pe \equiv \frac{hL}{\Theta\_m D\_c} \tag{6}$$

Where, *h* (m3 m-2 s-1) is the drying rate, *L* (m) the length of the sample, and *θm* (m3 m-3) the maximum fluid content by capillary saturation. *Dc* (m2 s-1) is the diffusion coefficient of the ions in the moisture in the porous medium. For Pe<<1 diffusion dominates and the ionprofiles will be uniform, whereas for Pe>>1 absorption dominates and ions will be accumulated at the drying surface.

Y. T. Puyate et al discussed the chloride transport due to wick action in the concrete in detail [31-34]. One vital conclusion is that it was the vapour pressure of the solution and the relative humidity of air which control the position of the wet-dry interface [33]. The position of drywet interface locates in the inner of the concrete faced to a low relative humidity situation (0%) (Pe>>1), and a sharp peak of chloride concentration exceeding the saturation value occurs [34]. In contrast, in a high relative humidity condition (78%) (Pe<<1), the location of the interface is close to the concrete surface [31, 32]. Therefore, high evaporation can induce severe crystallization distress.

Nuclear magnetic resonance (NMR) is used to study the crystallization of sodium chloride due to wick action. Measuring the moisture and ion profile in a fired-clay brick cylinder (Ø 20×45mm), an efflorescence pathway diagram is plotted [35, 36] as shown in Fig. 3.

Fig. 3. Efflorescence pathway diagram [35]

According to this diagram (Fig.3), when Pe<<1, i.e. very slow drying or high relative humidity, the ion profile remains homogeneous and for some time no crystallization will occur. The average NaCl concentration slowly increases until the complete sample has reached saturation, forming a high concentration pore solution zone. When Pe>>1, i.e. very fast drying or low relative humidity, ions are directly advected with the moisture to the top of the sample and a saturation peak will build up with a very small width. If the rate of

"Salt Weathering" Distress on Concrete by Sulfates? 437

Under a low relative humidity condition, due to the high evaporation rate, the position of the wet-dry interface will move to the inner part of the element and closer to the bottom of the element, where a saturation peak will build up with a very small width, forming supersaturation, and resulting in sub-efflorescence and more severe deterioration. The breadth of efflorescence zone decreases and the average concentration of the pore solution will remain constant at nearly the initial concentration of exposure solution (as shown in

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

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

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

In summary, according to above review, the following basic principles of salt weathering on

1. Supersaturation is the key factor for salt crystallization. During the process of salt

2. High evaporation results in the formation of strong sub-efflorescence, causing severe deterioration. Low evaporation results in weak crystallization distress but causes the formation of a pore solution zone with high concentration in the part of porous

weathering the supersaturation must be maintained at a high level.

of the cooling experiments [36], and it can not be observed in building stone [13].

Fig. 4 (c)).

materials damage.

reason.

**2.4 Summary** 

porous materials can be concluded:

materials in contact with air.

**2.3 Crystallization of Na2SO4 and MgSO4** 

crystallization is high enough, i.e. if there are enough nucleation sites at the top to form crystals, the average NaCl concentration of the solution in the sample itself will remain constant at nearly the initial concentration.

The mechanism of efflorescence is the crystals growing at a free surface: the crystals in the pores cannot be stable and will dissolve and diffuse towards the atmosphere (an infinite pore) (Eq.2). Because the crystals are in contact with the solution only in their bases, they cannot grow laterally but form long needles like whisker [14]. This is the reason why efflorescence is un-harmful for the porous materials. Sub-efflorescence precipitates when the evaporative flux is greater than the capillary flux in the porous materials where the solution is supplied by the capillary suction and evaporation [37].

In summary, the efflorescence and sub-efflorescence of salt weathering distress on the porous material can be schematically shown in Fig. 4.

Fig. 4. Schematic of salt weathering distress on porous material. (a) no evaporation condition, (b) low evaporation condition, (c) high evaporation condition

As we know, the capillary absorption just occurs in the interconnected pores between air and water. When a porous element is partially subjected to the sulfate solution under no evaporation condition, a pore solution zone will be formed as shown in Fig. 4(a). The solution cannot rise from the solution surface to the top of the element by capillary absorption due to few or no interconnected pores from the bottom to the top in a relatively long distance. The interconnected pores can form from the solution surface to the side surface of the element in a relatively short distance, resulting in the generation of capillary absorption.

Under a low evaporation condition, the wet-dry interface can occur in the tip of the pore solution zone, where the rate of evaporation is fast compared with the rate of solution rise, because solution rises into the bulk at a rate that decreases with height. At the same time the sulfate concentration of pore solution will slowly increase until the complete sample has reached saturation, forming a high concentration pore solution zone, where the efflorescence occurs. Near the solution a liquid film occurs on the surface of the element, where the rate of rise if fast compared with the evaporation and the sulfate concentration is close to the exposure solution [20]. In this case the deterioration due to salt crystallization is minor (as shown in Fig. 4 (b)).

crystallization is high enough, i.e. if there are enough nucleation sites at the top to form crystals, the average NaCl concentration of the solution in the sample itself will remain

The mechanism of efflorescence is the crystals growing at a free surface: the crystals in the pores cannot be stable and will dissolve and diffuse towards the atmosphere (an infinite pore) (Eq.2). Because the crystals are in contact with the solution only in their bases, they cannot grow laterally but form long needles like whisker [14]. This is the reason why efflorescence is un-harmful for the porous materials. Sub-efflorescence precipitates when the evaporative flux is greater than the capillary flux in the porous materials where the solution

In summary, the efflorescence and sub-efflorescence of salt weathering distress on the

(a) (b) (c)

(a) no evaporation condition, (b) low evaporation condition, (c) high evaporation condition

As we know, the capillary absorption just occurs in the interconnected pores between air and water. When a porous element is partially subjected to the sulfate solution under no evaporation condition, a pore solution zone will be formed as shown in Fig. 4(a). The solution cannot rise from the solution surface to the top of the element by capillary absorption due to few or no interconnected pores from the bottom to the top in a relatively long distance. The interconnected pores can form from the solution surface to the side surface of the element in a

Under a low evaporation condition, the wet-dry interface can occur in the tip of the pore solution zone, where the rate of evaporation is fast compared with the rate of solution rise, because solution rises into the bulk at a rate that decreases with height. At the same time the sulfate concentration of pore solution will slowly increase until the complete sample has reached saturation, forming a high concentration pore solution zone, where the efflorescence occurs. Near the solution a liquid film occurs on the surface of the element, where the rate of rise if fast compared with the evaporation and the sulfate concentration is close to the exposure solution [20]. In this case the deterioration due to salt crystallization is

constant at nearly the initial concentration.

is supplied by the capillary suction and evaporation [37].

porous material can be schematically shown in Fig. 4.

Fig. 4. Schematic of salt weathering distress on porous material.

relatively short distance, resulting in the generation of capillary absorption.

minor (as shown in Fig. 4 (b)).

Under a low relative humidity condition, due to the high evaporation rate, the position of the wet-dry interface will move to the inner part of the element and closer to the bottom of the element, where a saturation peak will build up with a very small width, forming supersaturation, and resulting in sub-efflorescence and more severe deterioration. The breadth of efflorescence zone decreases and the average concentration of the pore solution will remain constant at nearly the initial concentration of exposure solution (as shown in Fig. 4 (c)).
