**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:

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 difficult that the pore solution reaches supersaturation.

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 reaction on the supersaturation formation.

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

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.

"Salt Weathering" Distress on Concrete by Sulfates? 453

The main reaction products are gypsum, ettringite, thaumasite, brucite and silica gel. Gypsum and ettringite are the common products of sulfate attack. Brucite and silica gel are

However, the product formation depends on the exposure conditions, such as sulfate

Concerning gypsum, Bellmann et al have discussed the influence of sulfate concentration and pH value of solution on the gypsum formation in detail [52]. They indicate that portlandite will react to gypsum at a minimal sulfate concentration of approximately 1400 mg/l (pH=12.45). With rising pH, higher concentrations of sulfate ions are needed for the reaction to proceed. Between pH values of 12.45 and 12.7, the sulfate concentration slowly increases, whereas it rises dramatically from that level on. In solutions in which sodium ions are the counterpart of the hydroxide ions, the precipitation of gypsum can take place until pH values of approximately **12.9.** Beyond that mark, a further increase of the sulfate

Concerning ettringite, ettringite is not stable in an environment with pH value below 11.5-

Concerning thaumasite, a number of experimental studies show that a high pH value (above 10.5) is in favor of the thaumasite formation [65-67]. If the pH value drops below 10.5 and even further towards 7, thaumasite is unstable, calcite and another calcium-bearing phase will be generated in the field cases [68, 69]. Thaumasite formation needs a relatively cold condition

In summary, the sulfate concentration, pH value and temperature control the reaction

Normally, in the full immersion test, 5% sulfate solutions stored at 23.0 ± 2.0°C are used in laboratories [71]. Compared to ground water in the field, a 5% sulfate solution used in the tests is much more concentrated [72]. Thus, the concrete immersed in the 5% sulfate solution can be regarded as an accelerated test. However, a high sulfate contents pore solution (higher than 5% and 10%) can be formed in the concrete in contact with air (Fig. 19) [61]. Due to concrete carbonation the pH value of pore solution in the concrete will decrease. The ambient temperature during the process of salt weathering in the field is always fluctuating. The exposure conditions of "salt weathering" on concrete are different from the full immersion tests. According to the XRD analysis, the results of long term field tests and indoor tests indicated that gypsum likely was the main reaction products and responsible for the concrete damage [2, 45, 46, 53]. Certainly, the mechanism of chemical sulfate attack

**4.4 Study of the role of mineral additions in "Salt weathering" on concrete** 

showed that the mineral additions accelerated the concrete decay.

An important result of long term field tests is the negative role of mineral additions in the concrete sulfate resistance. Normally, the indoor tests [74-80] always showed that the mineral additions can improve the sulfate resistance of cementitious materials based on the full immersion in 5% sulfate solutions stored at 23.0 ± 2.0°C. However, the long term field tests

found in case of magnesium sulfate. Thaumasite is formed when CO32- is presented.

content and pH value of sulfate environment, temperature and relative humidity.

concentration is unable to lead to the formation of gypsum [52].

should be further and systematically studied.

(below 15oC) [70].

products.

12.0. At this low pH range, ettringite decomposes and forms gypsum [63, 64].

In our previous study [61], the pore solution expression test method was used to squeeze the pore solution in the cement paste. Cement paste samples were partially exposed to 10% Na2SO4 solution under the constant storage condition (20°C and 60% RH). The sulfate concentrations of pore solution in the lower part under solution (labeled L), film zone (labeled M) and efflorescence zone (labeled U) (shown in Fig. 4) were measured respectively. Fig. 19 gives the results.

Fig. 19. SO42- concentration of different parts of the cement paste partially exposed to the 10% Na2SO4 solution [61]

The results confirm that a pore solution zone can be formed in the efflorescence zone in the concrete, and the sulfate concentration was much higher than the lower part under solution and even the exposure solution (10% by mass). The strong chemical reactions occurring in this high concentration pore solution cause severe concrete decay. This also confirms the wick action theory.

Certainly, the ambient temperature and relative humidity of environment are always fluctuating. The boundary, the sulfate concentration and the formation time of pore solution zone were controlled by the evaporation rate due to the interactive effect of temperature and relative humidity. To study the pore solution zone formation needs further study.

#### **4.3 Study of chemical sulfate attack mechanism**

2


As we know, the main hydrated phases of cement paste are calcium silicate hydrate (C-S-H), calcium hydroxide (CH), calcium aluminate hydrate (C-A-H) ettringite (AFt) and monosulfoaliuminate (AFm). However, these three hydrated phases are not stable in the external environment containing sulfates. The following reactions can occur [62]:

$$\text{Ca(OH)}\_{2} + \text{C-S-H} + \text{SO}\_{4} \text{2} + \text{H}\_{2}\text{O} \rightarrow \text{CaSO}\_{4} \cdot 2\text{H}\_{2}\text{O} \tag{7}$$

$$\text{Ca(OH)}\_{2} + \text{C-S-H} + \text{MgSO}\_{4} + \text{H}\_{2}\text{O} \rightarrow \text{CaSO}\_{4}\cdot2\text{H}\_{2}\text{O} + \text{Mg(OH)}\_{2} + \text{SiO}\_{2}\text{xH}\_{2}\text{O} \tag{8}$$

3CaO·Al2O3·Ca(OH)2·(12-18)H2O+SO42-·2H2O+H2O→3CaO·Al2O3·3CaSO4·32H2O (9)

$$\text{Ca(OH)}\_{2} + \text{C-S-H} + \text{SO}\_{4} \text{2} + \text{CO}\_{2} \text{2} + \text{H}\_{2}\text{O} \rightarrow \text{CaSiO}\_{3}\text{CaCO}\_{3}\text{CaSO}\_{4}\text{H}\_{2}\text{O} \tag{10}$$

In our previous study [61], the pore solution expression test method was used to squeeze the pore solution in the cement paste. Cement paste samples were partially exposed to 10% Na2SO4 solution under the constant storage condition (20°C and 60% RH). The sulfate concentrations of pore solution in the lower part under solution (labeled L), film zone (labeled M) and efflorescence zone (labeled U) (shown in Fig. 4) were measured

2345678

w e e k s )

2- concentration of different parts of the cement paste partially exposed to the

A g e (

The results confirm that a pore solution zone can be formed in the efflorescence zone in the concrete, and the sulfate concentration was much higher than the lower part under solution and even the exposure solution (10% by mass). The strong chemical reactions occurring in this high concentration pore solution cause severe concrete decay. This also confirms the

Certainly, the ambient temperature and relative humidity of environment are always fluctuating. The boundary, the sulfate concentration and the formation time of pore solution zone were controlled by the evaporation rate due to the interactive effect of temperature and

As we know, the main hydrated phases of cement paste are calcium silicate hydrate (C-S-H), calcium hydroxide (CH), calcium aluminate hydrate (C-A-H) ettringite (AFt) and monosulfoaliuminate (AFm). However, these three hydrated phases are not stable in the external

Ca(OH)2 + C-S-H + SO42- + H2O → CaSO4·2H2O (7)

Ca(OH)2 + C-S-H + MgSO4 + H2O → CaSO4·2H2O + Mg(OH)2 + SiO2·xH2O (8)

3CaO·Al2O3·Ca(OH)2·(12-18)H2O+SO42-·2H2O+H2O→3CaO·Al2O3·3CaSO4·32H2O (9)

Ca(OH)2 + C-S-H + SO42- +CO32- + H2O → CaSiO3·CaCO3·CaSO4·H2O (10)

relative humidity. To study the pore solution zone formation needs further study.

environment containing sulfates. The following reactions can occur [62]:

L

M

U

1 0 % N a 2 S O 4

respectively. Fig. 19 gives the results.

Fig. 19. SO4

10% Na2SO4 solution [61]

wick action theory.

**4.3 Study of chemical sulfate attack mechanism** 

S O

(

g/l

)

2 -

4

The main reaction products are gypsum, ettringite, thaumasite, brucite and silica gel. Gypsum and ettringite are the common products of sulfate attack. Brucite and silica gel are found in case of magnesium sulfate. Thaumasite is formed when CO32- is presented.

However, the product formation depends on the exposure conditions, such as sulfate content and pH value of sulfate environment, temperature and relative humidity.

Concerning gypsum, Bellmann et al have discussed the influence of sulfate concentration and pH value of solution on the gypsum formation in detail [52]. They indicate that portlandite will react to gypsum at a minimal sulfate concentration of approximately 1400 mg/l (pH=12.45). With rising pH, higher concentrations of sulfate ions are needed for the reaction to proceed. Between pH values of 12.45 and 12.7, the sulfate concentration slowly increases, whereas it rises dramatically from that level on. In solutions in which sodium ions are the counterpart of the hydroxide ions, the precipitation of gypsum can take place until pH values of approximately **12.9.** Beyond that mark, a further increase of the sulfate concentration is unable to lead to the formation of gypsum [52].

Concerning ettringite, ettringite is not stable in an environment with pH value below 11.5- 12.0. At this low pH range, ettringite decomposes and forms gypsum [63, 64].

Concerning thaumasite, a number of experimental studies show that a high pH value (above 10.5) is in favor of the thaumasite formation [65-67]. If the pH value drops below 10.5 and even further towards 7, thaumasite is unstable, calcite and another calcium-bearing phase will be generated in the field cases [68, 69]. Thaumasite formation needs a relatively cold condition (below 15oC) [70].

In summary, the sulfate concentration, pH value and temperature control the reaction products.

Normally, in the full immersion test, 5% sulfate solutions stored at 23.0 ± 2.0°C are used in laboratories [71]. Compared to ground water in the field, a 5% sulfate solution used in the tests is much more concentrated [72]. Thus, the concrete immersed in the 5% sulfate solution can be regarded as an accelerated test. However, a high sulfate contents pore solution (higher than 5% and 10%) can be formed in the concrete in contact with air (Fig. 19) [61]. Due to concrete carbonation the pH value of pore solution in the concrete will decrease. The ambient temperature during the process of salt weathering in the field is always fluctuating. The exposure conditions of "salt weathering" on concrete are different from the full immersion tests. According to the XRD analysis, the results of long term field tests and indoor tests indicated that gypsum likely was the main reaction products and responsible for the concrete damage [2, 45, 46, 53]. Certainly, the mechanism of chemical sulfate attack should be further and systematically studied.

#### **4.4 Study of the role of mineral additions in "Salt weathering" on concrete**

An important result of long term field tests is the negative role of mineral additions in the concrete sulfate resistance. Normally, the indoor tests [74-80] always showed that the mineral additions can improve the sulfate resistance of cementitious materials based on the full immersion in 5% sulfate solutions stored at 23.0 ± 2.0°C. However, the long term field tests showed that the mineral additions accelerated the concrete decay.

"Salt Weathering" Distress on Concrete by Sulfates? 455

pastes modified by different mineral additions were ground to a fineness of <200 μm and were mixed with stoichiometric parts of high quality gypsum powder (CaSO4·2H2O) and chemically produced calcite (CaCO3) as well as with an excess of 20% of distilled water to investigate the thaumasite formation without the physical obstacle. The products identification showed unexpected results: (1) concerning cement-FA paste, the amount of ettringite increased with an increasing Al2O3 content at 20 °C while at 6 oC fly ash promoted a little more thaumasite formation; (2) slag cements which are generally classified as high sulfate resisting cements showed the most intensive thaumasite formation; (3) the use of micro-silica strongly accelerated the thaumasite formation. These findings indicate that mineral additions have a potentially negative effect in the concrete's resistance to sulfate attack depending on the exposure conditions. In the paper [87], the negative effect emerged

In the fly ash the aluminum phase existing as solid glass spheres is stable, but can be activated in a thermal, mechanical or chemical way [88]. It should be noted that Na2SO4 is an effective activator which is often used to activate the pozzolanic fly ash reaction in cementfly ash pastes [89, 90]. What is worse, the ambient temperature may rise and also play a positive role in activating the aluminum phase, promoting the ettringite formation. According to the thermal analysis results [61], the cement and cement – fly ash (25%) pastes were immersed in the 5% Na2SO4 solution at 30 oC for 6 months. The amount of ettringite in the cement and cement-FA pastes amounted to 0.173 mg /mg and 0.217 mg/mg respectively. On the other hand, more gypsum was also detected in the cement – fly ash paste than cement paste. Fig. 21 is the thermal analysis of pastes exposed to 15% Na2SO4 solution at 30oC for 6 months. More ettringite and gypsum were generated in the cement – fly ash paste than in cement paste. Moreover, according to the wick action, 15% Na2SO4 can be formed in the upper portion of concrete in contact with air during the process of "salt

> -5 -4 -3 -2 -1 0 1 DTG /(%/min

主主 2010-01-26 11:53 用户: Administrator

100 200 300 400 500 600 700 800 900 温温 ℃ /


[4]

[4]

[3

Fig. 21. Thermal analysis of pastes partially immersed in the 15% Na2SO4 solution at 30oC

In the paper [87], the negative effect of mineral additions on concrete sulfate resistance opposite to the normal results was attributed to no physical obstacle. In the process of "salt weathering" on concrete, a similar no physical obstacle appearance also can be defined. As abovementioned, solution goes into concrete by capillary suction. For porous materials, the capillary suction is a kind of active process, i.e. the solution is invited into the concrete. This can also be regarded as a no physical obstacle process. The sulfates can homogeneously

[3]

Cement Cement – fly ash

due to no physical obstacle.

weathering" on field concrete.

主主 2010-01-26 11:52 用户: Administrator

for 6 months [61]

100 200 300 400 500 600 700 800 900 温温 ℃ /

As pointed out by Mehta [73], when concrete is fully immersed in the sulfate solution, for the prevention of sulfate attack "control of permeability is more important than control of the chemistry of cement". The pore size refinement due to mineral additions will prevent the sulfates to penetrate into concrete and lighten the negative effect of sulfate attack on concrete. Therefore, a number of previous researches all supported the idea that the mineral additions, such as fly ash, slag powder, silica fume and metakaolin, play a positive role in making sulfate-resisting concrete [74-80], not depending on the exposure conditions (sodium sulfate, magnesium sulfate, or ammonium sulfate).

However, in the case of partial immersion the pore size refinement due to mineral addtions will contribute to an increase in the capillary sorption height following Eq. 5. The pore solution expression tests showed that this process can draw more sulfates into fly ash concrete than into normal concrete, resulting in a pore solution with a higher sulfate concentration as shown in Fig. 20 [61].

Fig. 20. SO42- concentration of the pore solution in efflorescence and film zone of cement paste and cement-FA paste exposed to 5% Na2SO4 solution [61]

Another reason for mineral additions to lighten the negative effect of sulfate attack on concrete is the dilution effect induced by the partial cement replacement since it entails a reduction in the C3A content [81]. Thus, based on laboratory tests mineral additions have always been regarded as an effective constituent to increase concrete's sulfate resistance in the field [82]. However, people maybe just remember the good things and ignore the bad ones. In the 1960s and 1970s extensive studies at the U.S. Bureau of Reclamation had reminded that [83, 84] concretes containing 30 percent low-calcium fly ashes showed greatly improved sulfate resistance to a standard sodium sulfate solution. However, the use of highcalcium fly ashes generally reduced the sulfate resistance. The high-calcium fly ashes containing highly reactive alumina in the form of C3A or C4A3Ŝ are therefore less suitable than low-calcium fly ashes for improving the sulfate resistance of concrete. Taylor also pointed out that if slag has low alumina content, it improves the sulfate resistance, but with a high content of alumina, the reverse is the case [85,86]. M. Nehdi[47] also pointed out that it should not be overlooked that fly ash contains a large amount of reactive aluminum and that binders with an increased Al2O3 content can be more susceptive to the formation of ettringite. P. Nobst and J. Stark [87] carried out a very interesting test. Hardened cement

As pointed out by Mehta [73], when concrete is fully immersed in the sulfate solution, for the prevention of sulfate attack "control of permeability is more important than control of the chemistry of cement". The pore size refinement due to mineral additions will prevent the sulfates to penetrate into concrete and lighten the negative effect of sulfate attack on concrete. Therefore, a number of previous researches all supported the idea that the mineral additions, such as fly ash, slag powder, silica fume and metakaolin, play a positive role in making sulfate-resisting concrete [74-80], not depending on the exposure conditions (sodium

However, in the case of partial immersion the pore size refinement due to mineral addtions will contribute to an increase in the capillary sorption height following Eq. 5. The pore solution expression tests showed that this process can draw more sulfates into fly ash concrete than into normal concrete, resulting in a pore solution with a higher sulfate

2345678

w e e k s )

Cement + Fly ash

5% Na2SO4

Cement

A g e (

Another reason for mineral additions to lighten the negative effect of sulfate attack on concrete is the dilution effect induced by the partial cement replacement since it entails a reduction in the C3A content [81]. Thus, based on laboratory tests mineral additions have always been regarded as an effective constituent to increase concrete's sulfate resistance in the field [82]. However, people maybe just remember the good things and ignore the bad ones. In the 1960s and 1970s extensive studies at the U.S. Bureau of Reclamation had reminded that [83, 84] concretes containing 30 percent low-calcium fly ashes showed greatly improved sulfate resistance to a standard sodium sulfate solution. However, the use of highcalcium fly ashes generally reduced the sulfate resistance. The high-calcium fly ashes containing highly reactive alumina in the form of C3A or C4A3Ŝ are therefore less suitable than low-calcium fly ashes for improving the sulfate resistance of concrete. Taylor also pointed out that if slag has low alumina content, it improves the sulfate resistance, but with a high content of alumina, the reverse is the case [85,86]. M. Nehdi[47] also pointed out that it should not be overlooked that fly ash contains a large amount of reactive aluminum and that binders with an increased Al2O3 content can be more susceptive to the formation of ettringite. P. Nobst and J. Stark [87] carried out a very interesting test. Hardened cement

Fig. 20. SO42- concentration of the pore solution in efflorescence and film zone of cement

sulfate, magnesium sulfate, or ammonium sulfate).

40

paste and cement-FA paste exposed to 5% Na2SO4 solution [61]

50

60

S O

(

)

g /l

2 -

4

70

80

90

concentration as shown in Fig. 20 [61].

pastes modified by different mineral additions were ground to a fineness of <200 μm and were mixed with stoichiometric parts of high quality gypsum powder (CaSO4·2H2O) and chemically produced calcite (CaCO3) as well as with an excess of 20% of distilled water to investigate the thaumasite formation without the physical obstacle. The products identification showed unexpected results: (1) concerning cement-FA paste, the amount of ettringite increased with an increasing Al2O3 content at 20 °C while at 6 oC fly ash promoted a little more thaumasite formation; (2) slag cements which are generally classified as high sulfate resisting cements showed the most intensive thaumasite formation; (3) the use of micro-silica strongly accelerated the thaumasite formation. These findings indicate that mineral additions have a potentially negative effect in the concrete's resistance to sulfate attack depending on the exposure conditions. In the paper [87], the negative effect emerged due to no physical obstacle.

In the fly ash the aluminum phase existing as solid glass spheres is stable, but can be activated in a thermal, mechanical or chemical way [88]. It should be noted that Na2SO4 is an effective activator which is often used to activate the pozzolanic fly ash reaction in cementfly ash pastes [89, 90]. What is worse, the ambient temperature may rise and also play a positive role in activating the aluminum phase, promoting the ettringite formation. According to the thermal analysis results [61], the cement and cement – fly ash (25%) pastes were immersed in the 5% Na2SO4 solution at 30 oC for 6 months. The amount of ettringite in the cement and cement-FA pastes amounted to 0.173 mg /mg and 0.217 mg/mg respectively. On the other hand, more gypsum was also detected in the cement – fly ash paste than cement paste. Fig. 21 is the thermal analysis of pastes exposed to 15% Na2SO4 solution at 30oC for 6 months. More ettringite and gypsum were generated in the cement – fly ash paste than in cement paste. Moreover, according to the wick action, 15% Na2SO4 can be formed in the upper portion of concrete in contact with air during the process of "salt weathering" on field concrete.

Fig. 21. Thermal analysis of pastes partially immersed in the 15% Na2SO4 solution at 30oC for 6 months [61]

主主 2010-01-26 11:53 用户: Administrator

主主 2010-01-26 11:52 用户: Administrator

In the paper [87], the negative effect of mineral additions on concrete sulfate resistance opposite to the normal results was attributed to no physical obstacle. In the process of "salt weathering" on concrete, a similar no physical obstacle appearance also can be defined. As abovementioned, solution goes into concrete by capillary suction. For porous materials, the capillary suction is a kind of active process, i.e. the solution is invited into the concrete. This can also be regarded as a no physical obstacle process. The sulfates can homogeneously

"Salt Weathering" Distress on Concrete by Sulfates? 457

almost the same, showing similar efflorescence zone due to salt weathering. The reason for the opposite appearance of MgSO4 in concrete may be the insoluble brucite due to chemical reaction that blocks the capillary. The role of sulfates in the "salt weathering" on concrete

also needs further research.

in the full immersion situation.

severer concrete damage.

Fig. 23. Surface tensions of NaCl, Na2SO4 and MgSO4 [92]

**4.6 Study of the role of concrete carbonation in "salt weathering" on concrete** 

The negative effect of carbonation on corrosion of reinforcing steel in concrete is well known. As to the sulfate attack on concrete, as a result of carbonation, the total porosity would be reduced and the permeability of concrete could be improved [93-94]. So, Gao [95] pointed out that the carbonation layer could mitigate diffusion of sulfate ions to some extent

However, when the concretes are partially exposed to sulfate solutions, the situation may be different. V.T. Ngala [94] studied the effect of carbonation on the ratio of capillary to total porosity of cement paste. The results showed that the capillary pore fraction greatly was improved after carbonation (shown in Fig. 24). This will promote the capillary suction of concrete, forming a more severe sulfate pore solution in the concrete and resulting more

Fig. 24. Ratio of capillary to total porosity of non-carbonated and carbonated paste [94]

distribute in the cement paste similar to the alkali activated cement in which Na2SO4 and powders are mixed before adding water, resulting in Na2SO4 homogeneously distributing in the cement paste.

In summary, concerning the role of mineral additions in the sulfate attack on partially exposed concrete, the exposure conditions and the solution transport mechanism are different from the full immersion cases. It needs further research.
