Damage in cement pastes exposed to NaCl solutions

Construction and Building Materials 171 (2018) 120–127

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Damage in cement pastes exposed to NaCl solutions Chunyu Qiao a, Prannoy Suraneni b, Jason Weiss a,⇑ a b

School of Civil and Construction Engineering, Oregon State University, Corvallis 97331, USA Department of Civil, Architectural and Environmental Engineering, University of Miami, Coral Gables 33146, USA

h i g h l i g h t s
More Friedel’s salt forms in post-exposure cement pastes before staying constant.
Little volume change is associated with the formation of Friedel’s salt.
Cement pastes with a higher FA level shows a smaller flexural strength reduction.
The damage may be due to the crystallization pressure of Friedel’s salt in pores.

a r t i c l e

i n f o

Article history: Received 10 November 2017 Received in revised form 11 March 2018 Accepted 16 March 2018

Keywords: Friedel’s salt Sodium chloride Flexural strength Volume change

a b s t r a c t Friedel’s salt forms in cementitious materials exposed to NaCl solutions. This paper quantifies the amount of Friedel’s salt that forms and relates the formation of Friedel’s salt to the damage. Thermogravimetric analysis (TGA) is used to quantify the amount of Friedel’s salt. The volume change associated with the formation of Friedel’s salt is measured. Flexural strength is measured for cement pastes exposed to NaCl solutions using the ball-on-three-ball test. While Friedel’s salt was observed, no calcium oxychloride was detected in cement pastes exposed to NaCl solutions. As the NaCl concentration increases, the amount of Friedel’s salt formed in the cement pastes increases before remaining constant at high concentrations. For the same NaCl concentration, similar amounts of Friedel’s salt are obtained in cement pastes with varying water-to-cementitious materials ratios (w/cm), while a greater amount of Friedel’s salt exists in cement pastes with a higher fly ash replacement level. As the NaCl solution concentration increases, the flexural strength of the cement paste decreases. There is a greater flexural strength reduction in the cement pastes with higher w/cm. For cement pastes with a higher fly ash replacement, the flexural strength shows a smaller decrease when exposed to NaCl. Since TGA confirmed little leaching of Ca(OH)2 in the cement paste, the strength reduction appears to be mainly due to the formation of Friedel’s salt. Little volume change is associated with the formation of Friedel’s salt, which indicates that the damage may primarily be attributed to the crystallization pressure associated with the formation of Friedel’s salt in pores. Ó 2018 Elsevier Ltd. All rights reserved.

1. Introduction Chloride-based deicers such as sodium chloride (NaCl), calcium chloride (CaCl2), and magnesium chloride (MgCl2) are widely applied on concrete flatwork and pavements to melt ice and improve driving and walking conditions in cold regions during winter [1–3]. When chloride ions, coupled with the corresponding cations (Na+, Ca2+, and Mg2+), penetrate the cementitious materials, they interact with the hydrated and unhydrated phases through several different mechanisms [4]. For the NaCl, the common interaction mechanisms between cement paste and chloride include the ⇑ Corresponding author. E-mail address: [email protected] (J. Weiss). https://doi.org/10.1016/j.conbuildmat.2018.03.123 0950-0618/Ó 2018 Elsevier Ltd. All rights reserved.

adsorption of chloride ions onto the surface of calcium silicate hydrate (C-S-H) [5], and the reaction of chloride ions with aluminate and aluminoferrite phases to form Kuzel’s salt (Ca4Al2(OH)12Cl(SO4)0.55H2O, KS) [6] and Friedel’s salt (Ca4Al2(OH)12Cl24H2O, FS) [7–9]. However, in addition to the absorption of chlorides and the formation of Friedel’s and Kuzel’s salt, concrete exposed to CaCl2 and MgCl2 has been observed to develop an additional potential reaction [10–12]. CaCl2 reacts with calcium hydroxide (Ca(OH)2) to form calcium oxychloride (3Ca(OH)2CaCl212H2O) [10,11], which results in cracks [12,13] and strength reduction [14–16]. When cementitious materials are in contact with MgCl2, Mg2+ ions react with Ca(OH)2 and C-S-H to form brucite (Mg(OH)2), calcium oxychloride, and magnesium silicate hydrate (M-S-H), respectively

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[17–20] resulting in damage and strength reduction [20]. Cracking and damage have been detected using acoustic emission [19]. While much of the recent work on salt damage to concrete has focused on CaCl2 and MgCl2 in contrast, fewer studies have focused on the potential damage to cementitious materials exposed to NaCl solutions [3,21–23]. Studies indicate that exposure to NaCl has a relatively small negative effect on the dynamic modulus of concrete [21]. Calcium leaching [22] and the formation of Friedel’s salt [23], have previously been claimed to cause damage to cementitious materials exposed to NaCl. The solubility of Ca(OH)2 increases with increasing concentrations of NaCl solutions, especially at concentrations above 0.5 mol/L [24]. This could potentially lead to dissolution and leaching of Ca(OH)2 and C-S-H. Leaching of calcium at the surface of the mortar samples has been experimentally and numerically confirmed [25]. It should be noted that calcium leaching and the extent to which it occurs depends on the experimental procedure used. It is important to determine calcium leaching in different specific scenarios and discuss its relation to the potential damage. The crystallization of Friedel’s salt has been reported to cause expansion in cement paste exposed to NaCl solutions, which leads to cracks in the paste [23,26]. The crystallization of Friedel’s salt can occur through a dissolution and precipitation mechanism as shown in Eq. (1) [9]:

ð1Þ

There is another mechanism that can potentially result in the formation of Friedel’s salt: ionic exchange between the chloride from the exposure solution and sulfate ions from the hydrated monosulfoaluminate (C3ACaSO412H2O, AFm) [7,9]. The ionic exchange progress is shown in Eq. (2) [27]:

AFmðsÞ þ 2NaClðaqÞ ! FSðsÞ þ Na2 SO4 ðaqÞ þ 2H2 OðaqÞ

2.1.2. NaCl solutions Granular NaCl (EMD Millipore Inc., reagent grade) was dissolved in deionized water to make NaCl solutions with concentrations of 5.3%, 10.6%, 16.0%, 21.4% and 26.7% w.t, respectively. They have the same chloride molar concentrations as 5%, 10%, 15%, 20% and 25% w.t. CaCl2 solutions previously studied [14], which makes the flexural strength reduction comparable in cement pastes exposed to NaCl and CaCl2 solutions in further work. 2.2. Experimental methods

C3 AðsÞ þ 2CHðsÞ þ 2NaClðaqÞ þ 10H2 OðaqÞ ! FSðsÞ þ 2NaOHðaqÞ

The specific gravity of the cement and fly ash is 3.15 and 2.66, respectively. Cement pastes were made with three water-to-cementitious material ratios (w/cm) (0.36, 0.42, and 0.50) and four fly ash replacement levels by volume (0%, 20%, 40%, and 60%). The mixture proportions are listed in Table 2. A programmable vacuum mixer was used to provide a constant high vacuum level during mixing, which minimizes the presence of entrapped air. The mixing procedure is described elsewhere [29]. After mixing, the paste was cast into cylindrical molds (50.8 mm in diameter and 101.6 mm tall) and sealed with lids. The cylinders were cured at 23 ± 1 °C for 3 days and then maintained at 50 ± 1 °C for 25 days, as an accelerated curing regime, to reach an equivalent age of 91 days (at 23 °C), based on the previous maturity calculations [30].

ð2Þ

It has been shown that in cementitious materials, it is only the formation of Friedel’s salt by dissolution and precipitation that causes damage to the cementitious materials [23]. The relationship between the volume change due to the formation of Friedel’s salt and the strength reduction is not known. There are very few studies focusing on the quantification of Friedel’s salt [25,28]. These studies that quantify the formation of Friedel’s salt are important to link the formation of Friedel’s salt to the damage of cementitious materials exposed to NaCl solutions. The influence of supplementary cementitious materials on the formation of Friedel’s salt also still remains unclear. This paper aims to relate the reaction between cement pastes and NaCl solutions, with a focus on the quantification of Friedel’s salt, to the corresponding reduction in flexural strength. The influence of varying exposure conditions (NaCl concentrations) and mixture proportions (water-to-cementitious materials ratios, and fly ash replacement levels) will be examined. Other potential damage mechanisms, such as calcium leaching and the formation of calcium oxychloride, are also considered. The volume change associated with the formation of Friedel’s salt is measured to help explain the damage mechanism. 2. Materials and experimental methods 2.1. Materials 2.1.1. Cement paste Type I ordinary Portland cement and class C fly ash were used in this study. Their chemical and mineral compositions are listed in Table 1. The type I cement has a Blaine fineness of 392 m2/kg.

2.2.1. Low temperature differential scanning calorimetry (LT-DSC) A LT-DSC instrument (Q20, TA Instruments) was used to measure the heat flow in the cement paste-NaCl solution system. Approximately 10 mg hydrated cement paste powder (with an equivalent age of 91 days at 23 °C as discussed in Section 2.1.1) was mixed with 10 mg NaCl solutions with varying concentrations (5.3%, 10.6%, 21.4% and 26.7% by mass). Mixing was done in high volume stainless steel pans, and the pans were then sealed and placed in the LT-DSC chamber. The samples were then exposed to the following temperature cycle [14]: isothermal at 25 °C for one hour; 3 °C/min cooling until 90 °C; low temperature loop from 90 °C to 70 °C and back to 90 °C at 3 °C/min; and 0.25 °C/min heating until 50 °C. 2.2.2. Volume change measurement Approximately 10 g of ground cement paste powder placed in a glass vial (a capacity of 24 ml, VWR Inc.). The 21.4% NaCl solution was added to the ground cement paste powder and the amount of NaCl solution was calculated to fit the volume of the glass vial, as shown in Table 3. After filling the glass vials with cement paste and NaCl solution, a rubber stopper was placed in the container with a

Table 1 Composition of raw materials (in wt.%). Type I cement

Fly ash

Chemical Data SiO2 Al2O3 Fe2O3 CaO MgO SO3 Na2Oeq. Loss on ignition

20.20 4.60 3.30 63.40 0.80 3.10 0.54 2.70

38.97 21.43 5.42 22.31 4.58 1.11 1.80 0.64

Bogue Phase Calculation C3S C2S C3A C4AF

60 12 7 10

– – – –

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Table 2 Mixture proportions of cement pastes. Group

Cement (g)

Fly ash (g)

Water (g)

w/cm

Fly ash replacement (% by volume)

1 2 3 4 5 6

3900 3600 3300 2880 2160 1440

– – – 608 1216 1824

1404 1512 1650 1465 1418 1371

0.36 0.42 0.50 0.42 0.42 0.42

0 0 0 20 40 60

color-coded reusable glass volumetric pipette embedded in it. An illustration of the experimental setup has been shown in a previous study [31]. The pipette has a capacity of 1 ml and a resolution of 0.01 ml (VWR Inc.). Care was taken to ensure that no air bubbles were entrapped in the sealed glass vials. Water-insoluble red liquid dye was added onto the top of the solution meniscus in the glass pipettes. The sample preparation and mixing was carried out at 50 ± 1 °C in accordance with previous experimental procedures [31,32], which theoretically prevents the potential to form calcium oxychloride during preparation. A programmable water bath (VWR Inc.) was used to control the temperature during the test. The sealed glass vials were placed in a sample holder in the water bath and subjected to a cooling (50 °C– 0 °C) and heating (0 °C–50 °C) cycle with the specific time and temperature steps described elsewhere [31,32]. Each step was maintained for 8 h after the temperature was stabilized to ensure equilibration of the volume change. Each test required 11 days to complete a cooling-heating cycle. A digital thermometer (Testo Inc.) was used to record the temperature in the water bath, while a camera was used to automatically track the location of the red marks in the glass pipettes. The changes in the location of the red marks are converted to the total volume change in each paste-NaCl solution mixture. 2.2.3. Exposure of cement pastes to NaCl solutions Cement paste cylinders were cut into disc shaped specimens with a thickness of 2.6 ± 0.2 mm using a diamond blade wet saw. The specimens were then placed in limewater at 50 ± 1 °C for 2 days in order to achieve full saturation. Subsequently, 5 specimens were placed in 500 g NaCl solutions (5.3%, 10.6%, 16.0%, 21.4%, and 26.7% by mass) at 50 ± 1 °C for 7 days, which is long enough to reach equilibration between the pore solution and the external NaCl concentrations via diffusion. Since this paper studies the potential to form calcium oxychloride in the cement paste exposed to NaCl solutions, a high temperature of 50 ± 1 °C was chosen to minimize calcium oxychloride formation during the specimen conditioning [31]. The specimens, together with the NaCl solutions, were then moved into a chamber maintained at 5 ± 1 °C for 2 days. The temperature change was selected to enable the potential formation, if any, of calcium oxychloride. After the low temperature exposure, the specimens were moved back to a 50 ± 1 °C oven for one day. Five specimens were placed in limewater as control, which underwent the same temperature cycle as the other samples.

Table 3 Masses of the mixtures of paste and 21.4% NaCl solution (g).

2.2.4. Thermogravimetric analysis (TGA) A TGA instrument (Q50, TA Instruments) was used to measure the mass losses of the hydrated cement paste after exposure to NaCl solutions. Approximately 60–70 mg ground cement paste powder was placed in the sample pan and transferred into the TGA chamber. The temperature was increased from 25 °C to 500 °C at a rate of 10 °C/min and the test was performed under an inert N2 atmosphere. The amounts of Friedel’s salt and Ca(OH)2 in the cement paste can be calculated based on the mass loss during 230–410 °C [28,33] and 400–500 °C [34], respectively. 2.2.5. Ball-on-three-ball flexural strength test (B3B test) After exposure to NaCl solutions, the specimens were removed from the solutions and the specimen was dried to surface saturated dry (SSD) using paper towels. The specimen was then placed in the B3B test set-up. Load was applied through one ball pressing downward at the center of the specimen; the specimen was supported on three balls placed at the corners of an equilateral triangle [35]. The load was increased until the specimen cracked, and the peak load was recorded. The B3B flexural strength can be calculated using the equation [35]:

r ¼ f ða; b; mÞ

F t2

ð3Þ

where: r is the flexural strength (MPa), a and b are two geometry parameters, m is the Poisson’s ratio of the material, F is the peak load (N), and t is the thickness of the specimen (m) [35]. 3. Results and discussion 3.1. Leaching of Ca(OH)2 The leaching of Ca(OH)2 from the cement paste into NaCl solutions was investigated by comparing the amounts of Ca(OH)2 in the cement paste before and after the exposure. The amounts of Ca (OH)2 in the cement paste before exposure to NaCl solutions were measured. Fig. 1 shows that the measured amounts of Ca(OH)2 by TGA in the cement paste after exposure to NaCl solutions are similar to the original values, regardless of the NaCl solution concentrations (in Fig. 1a) and the mixture proportions (in Fig. 1b). These results indicate there is no obvious Ca(OH)2 leaching when the cement paste was exposed to NaCl solutions in the conducted experiments here. Therefore, the leaching of Ca(OH)2 does not appear to be an explanation for the strength reduction in the cement paste exposed to NaCl solutions herein (discussed in the later sections). 3.2. Reaction between cement pastes and NaCl solutions

Group

Paste

Solution

1 2 3 4 5 6

10.02 10.04 10.02 10.02 10.02 10.03

22.82 23.97 24.52 22.50 23.39 23.19

Fig. 2 shows the heat flow of the cement paste-NaCl solution mixtures obtained from LT-DSC. Two distinct endothermic peaks are noted from the LT-DSC results, which correspond to the melting of ice (marked as ‘‘I”) and the melting of eutectic compounds (marked as ‘‘II”), respectively. These results are in accordance with observations that NaCl cannot react significantly with Ca(OH)2 or

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20 Ca(OH)2 amount after exposure (wt. %)

25

Ca(OH)2 amount (wt. %)

(a) w/cm=0.42

20

15

10

5

(b) 21.4% NaCl solution w/cm=0.42

15

w/cm=0.36

w/cm=0.50

20% FA

10 40% FA

5 60% FA

0

0 0

5

10

15

20

25

30

NaCl concentration (wt. %)

0

5

10

15

20

Original Ca(OH)2 amount (wt. %)

Fig. 1. The content of Ca(OH)2 in cement paste exposed to NaCl solutions: (a) varying NaCl concentrations; (b) different cement pastes (FA indicates fly ash).

C-S-H to form new compounds [24]. Fig. 2 indicates that there is no detectable peak associated with the formation of calcium oxychloride (from 0 °C to 50 °C [32]) in cement pastes exposed to NaCl solutions, which is consistent with the previous study [36]. Therefore, calcium oxychloride does not appear to be the reason for damage in cement pastes exposed to NaCl solutions. Fig. 3a shows the differential thermogravimetry (DTG, the first derivative of the TGA data) curves for the cement paste with w/cm = 0.42 exposed to NaCl solutions of varying concentrations. In the temperature range of 230–410 °C, there is no peak in the control exposed to lime water, while peaks are observed in the specimens exposed to NaCl solutions. This peak is due to the mass loss of the main layer water in Friedel’s salt [25]. The mass loss for the main layer water can be determined by subtracting the DTG curve (the first derivative of TGA curve with respect to temperature) of the non chloride-ingressed specimens from the corresponding DTG curve of chloride-ingressed specimens in the temperature range of 230–410 °C [25]. Based on the crystal structure of Friedel’s salt (Ca4Al2(OH)12Cl24H2O), there are 6 water molecules belonging to the main layer water [7]. Thus, the mass fraction of Friedel’s salt, mFS (wt.%) can be calculated using the equation below [25]:

mFS ¼

M FS mH2 O 6M H2 O

ð4Þ

where, mH2 O is the mass loss from the TGA for the main layer water in Friedel’s salt (wt.%), MFS and MH2 O are the molar mass of Friedel’s salt and water with values of 561.3 g/mol and 18 g/mol, respectively. The amount of Friedel’s salt as a function of NaCl concentration is shown in Fig. 3b. As the NaCl concentration increases, the amount of formed Friedel’s salt increases. However, the increase is not linear and begins to reach a plateau shortly after the NaCl concentration exceeds 10.7%. Under the exposure condition described in this paper, the amount of chloride ions in the NaCl solutions exceed that which will react with the aluminate phases in the cement paste. As the reaction to form Friedel’s salt is assumed to be able to be complete within the NaCl concentrations examined [7,27], the aluminate phases were completely consumed to form similar amounts of Friedel’s salt in the cement paste after exposure. The one exception is the lower amount of Friedel’s salt formed in the cement paste exposed to 5.3% NaCl solution. This may be due to slower reaction in the solution with lowest concentration of NaCl, which results in the incomplete consumption of aluminate phases to form Friedel’s salt in the cement paste during exposure. Fig. 4 shows the amount of Friedel’s salt formed in cement pastes with varying w/cm and fly ash replacement levels when exposed to 21.4% NaCl solution. There are similar amounts of Friedel’s salt formed in cement pastes with varying w/cm (Fig. 4a). This indicates that w/cm has little influence on the formation of Friedel’s salt in cementitious materials exposed to NaCl solutions. There is a higher amount of formed Friedel’s salt in cement pastes with a higher fly ash replacement level (Fig. 4b). Fig. 5 shows that there is a linear relationship between the amount of Friedel’s salt and the amount of Al2O3 in the paste, which is in accordance with the previous studies [37,38]. Due to similar Al2O3 amounts in cement pastes with varying w/cm, it can be assumed that similar amounts of Friedel’s salt formed (in Fig. 4a). Although there is less Al2O3 from the cement as the fly ash replacement level increases, there is more Al2O3 available from fly ash to react with chloride ions and thus, a higher amount of Friedel’s salt is formed, as shown in Fig. 4b. 3.3. Volume change due to Friedel’s salt formation

Fig. 2. Heat flow for the mixture of cement paste (w/cm = 0.42)-NaCl solutions.

The formation of Friedel’s salt is a chemical reaction involving both liquid and solid phases, and the volume change associated with the reaction was studied. The volume change can be normalized by the mass of the cement paste using the equation below:

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Fig. 3. (a) DTG curves (solid lines) of cement pastes exposed to NaCl solutions (dashed line represents the baseline in control exposed to lime water); (b) amounts of Friedel’s salt (FS) in the cement pastes exposed to NaCl solutions.

10

10

(b) Friedel's salt amount (wt. %)

Friedel's salt amount (wt. %)

(a) 8

6

4

2

8

6

4

2

0

0 0.36

0.42

0

0.50

20

40

60

Fly ash replacement level (%)

w/cm

Fig. 4. The amounts of Friedel’s salt formed in the cement pastes with (a) varying w/cm and (b) fly ash replacement levels exposed to 21.4 wt% NaCl solution.

DV re ¼

DV Total  msol



qNaCl ðT 0 Þ  qNaCl ðT 1 Þ 1

mpaste

1

 ð5Þ

Friedel's salt amount (wt. %)

10 40% FA

8

60% FA

20% FA

6

R2=0.961 w/cm=0.50

w/cm=0.36 w/cm=0.42

4

2

0 0

2

4

6

8

10

Al2O3 amount (wt. %) Fig. 5. The relationship between the amounts of Al2O3 and Friedel’s salt.

where, DV re is the normalized volume change (ml/g paste) due to the reaction between the cement paste and NaCl solutions from an initial temperature T0 to a final temperature T1; DV Total is the total volume change (ml), msol is the mass of NaCl solution (g), and mpaste is the mass of cement paste (g). Fig. 6 shows the normalized volume change due to the interaction between the cement paste and NaCl solutions. It indicates that the normalized volume change is similar with an average value of 0.0165 ml/g paste, regardless of the w/cm and fly ash replacement level. Compared to the volume change in the mixtures of cement paste-CaCl2 solutions [14], no dramatic change is noticed for normalized volume change (in Fig. 6). This, along with the previous observations using LT-DSC, implies that there is no significant formation of calcium oxychloride in cement pastes exposed to NaCl solutions. The volume change due to the interaction between the cement paste and NaCl solutions may arise from two processes. One is further hydration of cement paste due to the exposure of unhydrated

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0.00

Normalized volume change (ml/g paste)

Normalized volume change (ml/g paste)

0.00

-0.01

-0.02

-0.03

(a) varying w/cm -0.04

w/cm=0.36 w/cm=0.42 w/cm=0.50

-0.05

-0.01

-0.02

-0.03

(b) varying FA replacement levels w/cm=0.42 w/cm=0.42+20% FA w/cm=0.42+40% FA w/cm=0.42+60% FA

-0.04

-0.05

0

10

20

30

40

50

0

10

Temperature (°C)

20

30

40

50

Temperature (°C)

Fig. 6. The volume change in mixtures of cement paste-NaCl solutions: (a) varying w/cm; (b) varying fly ash replacement levels (FA indicates fly ash).

phases in the ground cement pastes to NaCl solutions. The other is the formation of Friedel’s salt. To separate the volume change due to the formation of Friedel’s salt from that due to further hydration, the total volume change in cement paste-water mixtures was measured. It should be noted that the accelerated hydration by NaCl is not considered. Fig. 7 shows the normalized volume change due to the interaction between the cement paste and water. The normalized volume change is similar when cement pastes are exposed to water and NaCl solutions. This indicates that the formation of Friedel’s salt causes very little volume change and the normalized volume change that is observed in Fig. 6 is primarily attributed to the further hydration. 3.4. B3B flexural strength of cement pastes exposed to NaCl solutions The B3B flexural strength of cement pastes exposed to NaCl solutions is normalized to the strength of pastes exposed to lime water using the equation below:

RB3B ¼

rNaCl  100 r0

ð6Þ

Normalized volume change (ml/g paste)

0.00

-0.01

-0.02

-0.03

w/cm=0.42

-0.04

Water NaCl

-0.05 0

10

20

30

40

50

Temperature (°C) Fig. 7. The comparison of volume change in the mixtures of cement paste and water and NaCl solutions.

where, RB3B is the relative flexural strength (%), rNaCl is the B3B flexural strength of cement paste exposed to NaCl solutions (MPa), and r0 is the B3B flexural strength of cement paste exposed to lime water (MPa). The relative flexural strength reduction (DRB3B , in %) is defined below as:

DRB3B ¼ 100  RB3B

ð7Þ

Fig. 8 shows that the relative flexural strength of cement paste decreases after the cement paste is exposed to NaCl solutions, which indicates that NaCl solutions cause damage to the hydrated cement paste. In general, the flexural strength reduces as the NaCl solution concentration increases. Since there is no calcium oxychloride formed or Ca(OH)2 leaching when these samples are exposed to NaCl solutions in this study (see Section 3.1), it can be hypothesized that the strength reduction is primarily due to the formation of Friedel’s salt. As shown in Fig. 8a, there is a greater flexural strength reduction in cement pastes with a higher w/cm. Fig. 8b shows that the flexural strength reduction decreases as the fly ash replacement increases. For the cement paste with 60% fly ash replacement, there is virtually no strength reduction after exposed to NaCl solutions. This indicates that the damage caused by the formation of Friedel’s salt can be mitigated by lowering w/cm and replacing cement with fly ash. Fig. 9 shows the relationship between the amount of formed Friedel’s salt and the relative flexural strength reduction. For the cement paste with w/cm = 0.42, the flexural strength reduction is related to the amount of formed Friedel’s salt (Fig. 9a). This indicates that a higher amount of formed Friedel’s salt at a higher NaCl concentration can cause more damage to the same cement paste. However, there is no clear trend between the amount of Friedel’s salt and the strength reduction of different cement pastes containing fly ash exposed to the same NaCl solution (Fig. 9b). There is a smaller reduction in the flexural strength of cement pastes with higher fly ash replacement levels, although these have higher amounts of Friedel’s salt. The scatter in Fig. 9b may be due to the fact that the relative flexural strength reduction is calculated based on the original flexural strength for each cement paste, which varies among different cement pastes due to the different microstructure. As a crystallization process, the formation of Friedel’s salt by dissolution and precipitation may lead to damage through two mechanisms. One is hydraulic pressure [39], which is claimed to

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100

Relative flexural strength (%)

Relative flexural strength (%)

100

80

60

40

(a) varying w/cm

20

w/cm=0.36 w/cm=0.42 w/cm=0.50

0 0

5

80

60

40

(b) varying FA replacement levels w/cm=0.42 w/cm=0.42+20% FA w/cm=0.42+40% FA w/cm=0.42+60% FA

20

0

10

15

20

25

0

30

5

10

15

20

25

30

NaCl concentration (wt. %)

NaCl concentration (wt. %)

Fig. 8. The reduction of B3B flexural strength in varying cement pastes: (a) varying w/cm; (b) varying fly ash replacement levels.

50 Relative flexural strength reduction (%)

Relative flexural strength reduction (%)

40

(a) w/cm=0.42 30

20

R =0.997 2

10

(b) 21.4% NaCl solution

40 w/cm=0.50

30

w/cm=0.42 20%FA w/cm=0.42 40%FA

w/cm=0.42

20

w/cm=0.36 w/cm=0.42 60%FA

10 0

0 0

1

2

3

4

Friedel's salt amount (wt. %)

0

2

4

6

8

10

Friedel's salt amount (wt. %)

Fig. 9. The relationship between the amount of Friedel’s salt and the flexural strength reduction.

result in stress due to the volume increase during the phase transition. The other one is the internal crystallization pressure [26,40], which suggests that the crystallization itself results in damage and the volume change due to the crystallization is not necessary to cause damage. Considering the little volume change due to the formation of Friedel’s salt, hydraulic pressure due to the volume increase is not expected to lead to the damage in this study. Thus, the damage may result from the crystallization stress due to the formation of Friedel’s salt rather than the volume change. It should be noted that the crystallization pressure exerted on the pore walls of the cementitious materials is related to the temperature of crystallization, the pore size distribution, and the concentration of salt solutions [41] and further studies need to be carried out to better explain this aspect of the damage. 4. Conclusions In this paper, TGA and B3B tests were used to quantify the amounts of formed Friedel’s salt and strength reduction of the cement paste exposed to NaCl solutions, respectively. The influence of varying mixture proportions (w/cm and fly ash replace-

ment levels) and exposure conditions (NaCl concentrations) were studied. The potential calcium leaching was measured using TGA and no leaching was observed. The potential formation of calcium oxychloride was measured using LT-DSC and no calcium oxychloride was detected in cement pastes exposed to NaCl solutions. The volume change due to the formation of Friedel’s salt was determined. Friedel’s salt appears to be the primary reaction product formed in the exposure conditions here. Similar amounts of Friedel’s salt are formed in cement pastes exposed to varying NaCl solutions except the 5.3% w.t. NaCl solution. For the same NaCl concentration, similar amounts of Friedel’s salt form in cement pastes with varying w/cm, while higher amounts of Friedel’s salt form in cement pastes with a higher fly ash replacement level. The amount of Friedel’s salt is proportional to the Al2O3 content in the cement paste. As the NaCl concentration increases, the flexural strength of the cement paste decreases. There is a greater strength reduction in cement pastes with a higher w/cm. There is a lower flexural strength reduction in cement pastes with a higher fly ash replacement level. Therefore, using a low w/cm paste with a high replacement level of fly ash can help to mitigate the damage caused by

C. Qiao et al. / Construction and Building Materials 171 (2018) 120–127

NaCl solutions. Considering there is negligible calcium leaching in this study, the formation of Friedel’s salt seems to result in the observed flexural strength reductions in this study. For the same cement paste, the formation of more Friedel’s salt causes a greater strength reduction. However, for the different cement pastes exposed to the same NaCl solution, the trend between the amount of formed Friedel’s salt and the relative strength reduction is not clear. As little volume change is associated with the formation of Friedel’s salt, the damage is hypothesized to result from the crystallization pressure of Friedel’s salt, rather than the hydraulic pressure.

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The authors gratefully acknowledge financial support from the National Ready Mix Concrete Association (NRMCA), the Portland Cement Association (PCA), MIT Concrete Sustainability Hub and a pooled fund by the Oklahoma Department of Transportation (TP5(297)) ‘‘Improving Specifications to Resist Frost Damage in Modern Concrete”. The authors also acknowledge insightful discussions with Dr. Vahid Jafari Azad and Professor O. Burkan Isgor at Oregon State University. The authors also acknowledge the hard work by Myo Thiha Zaw for B3B specimen preparation. References

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