**3.3 Our tests [53, 54]**

The starting point of our tests is to find a trace of salt crystals in the concrete as a direct evidence by means of XRD, SEM and EDS [53, 54]. Sulfate crystals can be easily identified in stone [23, 55]. However, in case of concrete elements, it is hard to identify them. Concrete technologists always attribute this to the coring and sawing operations when preparing samples for experimental analysis, as lapping water can readily dissolve salts from original and treated surfaces [2, 3, 4]. However this is not the main cause for the problem. Samples also can be taken in a dry manner to avoid the influence of water. Furthermore, In our study, the tests were designed to avoid the influence of water within the detection process of sulfate crystals.

Cement paste and cement – fly ash paste specimens and normal concrete specimens were partially exposed to Na2SO4 and MgSO4 solution under constant and fluctuating storage conditions respectively. After a period of exposure, the specimens were moved out from the solution and did not touch solution or water any more. The surface of the specimens was cleared by a thin blade and a soft brush. The samples for XRD and SEM were dried in a vacuum container with silica gel.

"Salt Weathering" Distress on Concrete by Sulfates? 447

Cement – fly ash paste specimens (20×20×150mm) were partially exposed to Na2SO4 solution. After 5 months exposure under the constant storage condition (20°C and 60%RH), some cracks were found near the upper edge above solution level of the cement-fly ash paste specimen. Along the crack, small pieces were carefully broken off using a thin blade.

The middle image is the zoomed surface of a small piece with magnification of 25 times. The left side is the outer surface of paste in contact with air, and zone A is the surface of a crack.

B

Two distinct parts can be observed at the right and left hand side of the white line in Zone A. At the right side a large amount of dense granular crystals cover the surface (Fig. 12), while at the left side porous crystals can be found accompanied with white substance (Fig. 13). It can be found that the crystals at left and right sides are both calcite. However, some calcite crystals at left side are peeled off and crystal caves are left. Some crystals are honeycombed with small pores. According to the EDS analysis, Na and S are also present. Obviously, the crystallization of sodium sulfate results in damage of the calcite crystals. At

Fig. 12. ESEM image and EDS analysis of f the granular crystal at left side [54]

**3.3.2 Cement – fly ash paste partially exposed to Na2SO4 solution under a constant** 

A

**storage condition** 

The ESEM image of a small piece is shown in Fig. 11.

Fig. 11. Cracks in cement – fly ash paste [54]

Zone B is a small point in the bulk of paste.

the right side, the calcite crystals show no damage.

#### **3.3.1 Cement paste partially exposed to Na2SO4 and MgSO4 solution under a constant storage condition**

The test of cement paste specimens (20×20×150mm) partially exposed to Na2SO4 and MgSO4 solution under a constant storage condition (20°C and 60%RH) is based on the study performed by Ruiz-Agudo [42] in which limestone specimens were partially submerged in a 19.4 g/100ml sodium sulfate solution and a 33.5 g/100ml magnesium sulfate solution respectively and located in a controlled environment (20°C±2°C, and 45%±5% RH). Results showed that the limestone specimens were severely damaged in both cases. While salt weathering by sodium sulfate consisted of a detachment of successive stone layers, magnesium sulfate induced the formation and propagation of cracks within the bulk stone. Thenardite (Na2SO4) and epsomite (MgSO4·7H2O) crystals were identified by ESEM in the pores of limestone.

Before immersion minor shrinkage cracks were observed in the cement paste specimens. These cracks were focused upon in detail because narrow micro-fissures appear to be important in the decay process due to the effectiveness of crystallization pressure generated by salt growth [37]. So, if crystallization is the mechanism of decay of cement paste, salt crystallization should first occur in the shrinkage cracks and sodium sulfate or magnesium sulfate crystals should be identified in these cracks.

Fig. 10. ESEM and EDS analysis of white substance on the surface of shrinkage crack [54] (a) ESEM image of white substance (b) EDS analysis of prismatic crystal (c) EDS analysis of flocculent crystals

However, based on the micro-analysis results the sulfate crystals were not detected in the atmospheric part of the paste partially exposed to Na2SO4 solution. On the contrary large amounts of ettringite crystals, the main chemical sulfate attack product, were identified as the reason for paste spalling. Another important observation is that a layer of white substance was formed on the surface of shrinkage cracks in the atmospheric part of the paste partially exposed to MgSO4 solution. Two distinct crystals can be distinguished: prismatic crystals surrounded by flocculent crystals. According to the EDS analysis, the prismatic crystal is gypsum and the flocculent crystal is brucite (shown in Fig.10). The products of this white substance in the shrinkage cracks are the same as the products in the interfacial zone of concrete fully immersed in magnesium sulfate solution [56].

**3.3.1 Cement paste partially exposed to Na2SO4 and MgSO4 solution under a constant** 

The test of cement paste specimens (20×20×150mm) partially exposed to Na2SO4 and MgSO4 solution under a constant storage condition (20°C and 60%RH) is based on the study performed by Ruiz-Agudo [42] in which limestone specimens were partially submerged in a 19.4 g/100ml sodium sulfate solution and a 33.5 g/100ml magnesium sulfate solution respectively and located in a controlled environment (20°C±2°C, and 45%±5% RH). Results showed that the limestone specimens were severely damaged in both cases. While salt weathering by sodium sulfate consisted of a detachment of successive stone layers, magnesium sulfate induced the formation and propagation of cracks within the bulk stone. Thenardite (Na2SO4) and epsomite (MgSO4·7H2O) crystals were identified by ESEM in the

Before immersion minor shrinkage cracks were observed in the cement paste specimens. These cracks were focused upon in detail because narrow micro-fissures appear to be important in the decay process due to the effectiveness of crystallization pressure generated by salt growth [37]. So, if crystallization is the mechanism of decay of cement paste, salt crystallization should first occur in the shrinkage cracks and sodium sulfate or magnesium

(a) (b) (c)

However, based on the micro-analysis results the sulfate crystals were not detected in the atmospheric part of the paste partially exposed to Na2SO4 solution. On the contrary large amounts of ettringite crystals, the main chemical sulfate attack product, were identified as the reason for paste spalling. Another important observation is that a layer of white substance was formed on the surface of shrinkage cracks in the atmospheric part of the paste partially exposed to MgSO4 solution. Two distinct crystals can be distinguished: prismatic crystals surrounded by flocculent crystals. According to the EDS analysis, the prismatic crystal is gypsum and the flocculent crystal is brucite (shown in Fig.10). The products of this white substance in the shrinkage cracks are the same as the products in the interfacial zone

Fig. 10. ESEM and EDS analysis of white substance on the surface of shrinkage crack [54]

(a) ESEM image of white substance (b) EDS analysis of prismatic crystal

of concrete fully immersed in magnesium sulfate solution [56].

**storage condition** 

pores of limestone.

sulfate crystals should be identified in these cracks.

(c) EDS analysis of flocculent crystals

## **3.3.2 Cement – fly ash paste partially exposed to Na2SO4 solution under a constant storage condition**

Cement – fly ash paste specimens (20×20×150mm) were partially exposed to Na2SO4 solution. After 5 months exposure under the constant storage condition (20°C and 60%RH), some cracks were found near the upper edge above solution level of the cement-fly ash paste specimen. Along the crack, small pieces were carefully broken off using a thin blade. The ESEM image of a small piece is shown in Fig. 11.

Fig. 11. Cracks in cement – fly ash paste [54]

The middle image is the zoomed surface of a small piece with magnification of 25 times. The left side is the outer surface of paste in contact with air, and zone A is the surface of a crack. Zone B is a small point in the bulk of paste.

Two distinct parts can be observed at the right and left hand side of the white line in Zone A. At the right side a large amount of dense granular crystals cover the surface (Fig. 12), while at the left side porous crystals can be found accompanied with white substance (Fig. 13). It can be found that the crystals at left and right sides are both calcite. However, some calcite crystals at left side are peeled off and crystal caves are left. Some crystals are honeycombed with small pores. According to the EDS analysis, Na and S are also present. Obviously, the crystallization of sodium sulfate results in damage of the calcite crystals. At the right side, the calcite crystals show no damage.

Fig. 12. ESEM image and EDS analysis of f the granular crystal at left side [54]

"Salt Weathering" Distress on Concrete by Sulfates? 449

**3.3.3 Cement – fly ash paste partially exposed to Na2SO4 solution under a fluctuating** 

Specimens (10×40×150 mm) were placed in a fluctuating condition (40±2°C and 35±5% RH for 24 hours, 10±1°C and 85±5% RH for 24 hours). After 3 cycles they were broken into several small pieces along some cracks. We checked the products on the surface of a crack.

It can be found that a large amount of needle-like crystals grow on the surface like a hedgehog. Some pores are filled with a cluster of the needle-like crystals. Based on the EDS

As we know, in concrete the weak interfacial transition zone (ITZ) plays a particularly important and even determining role in the main characteristic of concrete. A number of full immersion tests already showed that concrete deterioration occurred first in the ITZ by sulfate attack [56-60]. In this test, the concrete was made with just cement and aggregate to emphasize the role of ITZ in concrete deterioration. The concrete specimens (10×40×150 mm) were partially exposed to Na2SO4 and MgSO4 for 8 months. The results showed that: (1) the harmful effect of MgSO4 is much weaker than Na2SO4. This appearance also cannot be explained by the mechanism of salt weathering. This will be discussed in detail in section 4; (2) in the case of Na2SO4, damage also initiated in the ITZ. A large amount of gypsum crystals were formed on the surface of cement paste of ITZ in the upper part of concrete

Besides, the effect of carbonation on the salt weathering on concrete was studied. Before exposure a group of concrete cylinders were placed in an accelerated carbonation chamber with 10% CO2 concentration at 20 ± 2 oC and 60% ± 5% RH for 14 days. Then, these cylinders were partially exposed to Na2SO4 solution. After 8 months exposure, the carbonated

During the process of cleaning the surface of cylinders, a lot of small mortar pieces could be easily brushed off. According to the XRD analysis (Fig.18) Na2SO4 crystals and CaCO3 crystals were present in the mortar. This appearance also means that salt crystallization can

cylinders were deteriorated more severely than normal concrete (shown in Fig. 17).

**3.3.4 Normal concrete partially exposed to Na2SO4 and MgSO4 solution under a** 

**condition** 

Fig. 15 shows the SEM image and EDS analysis.

**constant storage condition** 

above solution (shown in Fig. 16).

occur in the CaCO3 crystals.

Fig. 15. SEM image and EDS analysis of the surface of a crack [54]

analysis and combining the XRD analysis, the needles are ettringite.

Fig. 13. ESEM image and EDS analysis off the granular crystal at right side [54]

According to the above observation, two conclusions can be drawn:


As we know, the crack formation is attributed to some expansive products present in the paste. When a small piece was broken off along the crack, the inner zone B on the surface of piece was a weak part in the paste and the source of crack initiation. The analysis of the products in this zone can disclose the real reason for the crack formation. Fig. 14 shows the ESEM image of the surface of Zone B.

Fig. 14. ESEM and EDS analysis of white square [54]

In Fig. 14, a large amount of short needle crystals are found in this zone. According to the EDS analysis, there are Ca, Al, Si, S, Na, and O elements. Combining the XRD analysis, thenardite was not identified and the crystals were ettringite.

Fig. 13. ESEM image and EDS analysis off the granular crystal at right side [54]

1. If salt crystallization is causing crack formation, the salt crystals should be identified at the right side in Fig. 11 to form sub-efflorescence instead of in the area in contact with

As we know, the crack formation is attributed to some expansive products present in the paste. When a small piece was broken off along the crack, the inner zone B on the surface of piece was a weak part in the paste and the source of crack initiation. The analysis of the products in this zone can disclose the real reason for the crack formation. Fig. 14 shows the

In Fig. 14, a large amount of short needle crystals are found in this zone. According to the EDS analysis, there are Ca, Al, Si, S, Na, and O elements. Combining the XRD analysis,

According to the above observation, two conclusions can be drawn:

2. Salt crystallization can occur in the calcite crystals.

Fig. 14. ESEM and EDS analysis of white square [54]

thenardite was not identified and the crystals were ettringite.

ESEM image of the surface of Zone B.

air.

### **3.3.3 Cement – fly ash paste partially exposed to Na2SO4 solution under a fluctuating condition**

Specimens (10×40×150 mm) were placed in a fluctuating condition (40±2°C and 35±5% RH for 24 hours, 10±1°C and 85±5% RH for 24 hours). After 3 cycles they were broken into several small pieces along some cracks. We checked the products on the surface of a crack. Fig. 15 shows the SEM image and EDS analysis.

Fig. 15. SEM image and EDS analysis of the surface of a crack [54]

It can be found that a large amount of needle-like crystals grow on the surface like a hedgehog. Some pores are filled with a cluster of the needle-like crystals. Based on the EDS analysis and combining the XRD analysis, the needles are ettringite.

## **3.3.4 Normal concrete partially exposed to Na2SO4 and MgSO4 solution under a constant storage condition**

As we know, in concrete the weak interfacial transition zone (ITZ) plays a particularly important and even determining role in the main characteristic of concrete. A number of full immersion tests already showed that concrete deterioration occurred first in the ITZ by sulfate attack [56-60]. In this test, the concrete was made with just cement and aggregate to emphasize the role of ITZ in concrete deterioration. The concrete specimens (10×40×150 mm) were partially exposed to Na2SO4 and MgSO4 for 8 months. The results showed that: (1) the harmful effect of MgSO4 is much weaker than Na2SO4. This appearance also cannot be explained by the mechanism of salt weathering. This will be discussed in detail in section 4; (2) in the case of Na2SO4, damage also initiated in the ITZ. A large amount of gypsum crystals were formed on the surface of cement paste of ITZ in the upper part of concrete above solution (shown in Fig. 16).

Besides, the effect of carbonation on the salt weathering on concrete was studied. Before exposure a group of concrete cylinders were placed in an accelerated carbonation chamber with 10% CO2 concentration at 20 ± 2 oC and 60% ± 5% RH for 14 days. Then, these cylinders were partially exposed to Na2SO4 solution. After 8 months exposure, the carbonated cylinders were deteriorated more severely than normal concrete (shown in Fig. 17).

During the process of cleaning the surface of cylinders, a lot of small mortar pieces could be easily brushed off. According to the XRD analysis (Fig.18) Na2SO4 crystals and CaCO3 crystals were present in the mortar. This appearance also means that salt crystallization can occur in the CaCO3 crystals.

"Salt Weathering" Distress on Concrete by Sulfates? 451

2. Salt crystallization can occur in the calcite crystals, the carbonated products of concrete.

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

According to the limited literature review, the conclusion can be made that the so-called "salt weathering" on concrete in effect is rather chemical sulfate attack. In order to systematically disclose the principles of this appearance, some issues should be further

Sulfates likely do not crystallize in a cement paste because in the highly alkaline pore solution other less soluble salts, e.g. ettringite, or gypsum, are preferably precipitated according to chemical equilibria theory. Salt crystallization in porous materials is difficult because salt crystallization occurrence has to reach and even exceed a thresholdsupersaturation. However, the chemical reactions in pore solution can occur regardless of the sulfate concentration and decrease the possibility of physical attack due to consuming sulfates and decreasing the sulfate concentration of pore solution, moreover, high concentration solution will increase the rate of chemical reaction. This will make it is very

In Fig. 9, the SO3 distribution showed some powerful evidence that the sulfates were consumed, resulting in the severest concrete damage. I.e. if there was no chemical reaction and if it were salt weathering causing concrete damage, the ion distribution curves of Na2SO4 should show similar features to Na2O distribution of Na2CO3 and NaCl. Certainly, this explanation is not convincing enough to disclose the mechanism. Further studies may be performed through thermodynamic calculation to check the negative effect of chemical

**4.2 Study of the concentration of solution on the formation of pore solution zone in** 

In the previous tests, an opposite appearance to salt weathering was that the concrete was susceptive to be damaged under a higher relative humidity condition. Combining the role of relative humidity in wick action and chemical sulfate attack, it can be explained that a wider sulfate pore solution can be formed in the upper part of concrete in contact with moist air and chemical sulfate attack occurring in the pore solution zone resulted in concrete damage.

**4.1 Study of the reason why salt crystallization cannot occur in concrete**  The reason why salt cannot occur in concrete may be explained as follows:

sulfate attack is the mechanism for the concrete damage.

difficult that the pore solution reaches supersaturation.

reaction on the supersaturation formation.

**3.4 Summary** 

**4. Further research** 

studied.

**concrete** 

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

Fig. 17. Visual observation of normal and carbonated concrete specimens exposed to sodium sulfate solution [53]

Fig. 18. XRD pattern of mortar [53]

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

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 brucite, were the determining factors for material damage.

2. Salt crystallization can occur in the calcite crystals, the carbonated products of concrete.
