Influence of NaCl concentrations on the crack-sealing behavior of superabsorbent polymers in cementitious materials

Construction and Building Materials 243 (2020) 118228

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Influence of NaCl concentrations on the crack-sealing behavior of superabsorbent polymers in cementitious materials Haitao Yang a,b,c, Juanhong Liu a,b,c,⇑, Xinshan Jia a,b,c, Yucheng Zhou a,b,c, Hongguang Ji a,b,c a

College of Civil and Resource Engineering, University of Science and Technology Beijing, Beijing 100083, China Beijing Key Laboratory of Urban Underground Space Engineering, University of Science and Technology Beijing, Beijing 100083, China c State Key Laboratory of High-efficient Mining and Safety of Metal Mines, Ministry of Education, University of Science and Technology Beijing, Beijing 100083, China b

h i g h l i g h t s

g r a p h i c a l a b s t r a c t


SAPs in realistic cracks can partly seal

The w/b ratios of C1 and C2 are 0.5 and 0.23; the crack widths of C1 and C2 are 0.5 mm and 0.25 mm.

the original void in 1 M NaCl solution.
The sealing capacity of SAPs in realistic cracks is weaker than that in artificial cracks.
The S value and r value will reduce when NaCl concentration reaches the Ccrit value.
The Ccrit value is seriously affected by the volume of original void and crack width.
SAPs promote the healing of the mortar during wet-dry (0.5 M NaCl) cycles.

a r t i c l e

i n f o

Article history: Received 29 October 2019 Received in revised form 14 December 2019 Accepted 19 January 2020

Keywords: Superabsorbent polymers NaCl concentration Sealing efficiency Self-sealing concrete

a b s t r a c t The crack-sealing behavior of superabsorbent polymers (SAPs) in high-concentration NaCl solutions is unclear. In this study, the influences of NaCl concentrations on the filled area (S) and sealing efficiency (r) of SAPs were, respectively, analyzed with a self-designed soaking test and a water permeability test. Furthermore, the 3D morphologies of a single SAPs particle and the effects of SAPs on the healing of mortars in NaCl solutions were investigated. The results indicate that the S value and the r value of SAPs are almost constant in low-concentration NaCl solutions, whereas they decrease with an increasing concentration until the concentration reaches the critical value. Additionally, the crack-sealing capacity of SAPs in realistic cracks is weaker than that in artificial cracks. Moreover, SAPs can promote the healing of mortar in wet-dry (0.5 M NaCl) cycles. The healing products are mainly CaCO3 and C-S-H gel. Ó 2020 Elsevier Ltd. All rights reserved.

1. Introduction Concrete is widely used in the construction of structures, e.g., ports, dams, bridges, tunnels, and mines. It is prone to cracking under the effects of self-shrinkage, plastic settlement, and external ⇑ Corresponding author. https://doi.org/10.1016/j.conbuildmat.2020.118228 0950-0618/Ó 2020 Elsevier Ltd. All rights reserved.

load because of its high brittleness [1]. Cracks aggravate the intrusion of harmful ions and the dissolution of Ca(OH)2, resulting in reduced durability of structures. Fortunately, concrete has the ability to heal cracks with widths less than 50 lm [2]. Incorporating supplementary cementitious materials (e.g., fly ash and slag [3,4]), minerals admixtures (e.g., sulphoaluminate-based expansion agent and crystalline admixtures [5,6]), autotrophic and

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heterotrophic bacteria [7,8], and superabsorbent polymers (SAPs [9]) can further improve the self-healing ability of concrete. As a new type of multifunctional admixtures in concrete, SAPs can swell during the mixing procedures and release the absorbed water when the internal humidity of concrete is low. The latter results in the relief of the self-shrinkage of concrete with a low water-to-binder (w/b) ratio (internal curing effect) [10]. SAPs will shrink after releasing moisture, leaving original voids inside concrete. These voids act like air-entraining agents and can improve the freeze-thaw resistance of concrete [9]. The most prominent advantage of SAPs is their ability to seal cracks. SAPs can rapidly swell in cracked concrete and seal cracks with large widths (0.3 mm [11] or 0.35 mm [12]), resulting in the remarkable recovery of water tightness during a short time (1 min [13] or 5 min [12]) [14–16]. Additionally, SAPs can absorb moisture from the environment and transfer it to the concrete, thus promoting the hydration of the unhydrated cement and the carbonization of Ca (OH)2 and accelerating the self-healing of the concrete in highhumidity environments (e.g., an R.H. of 90%) [17–19]. Moreover, SAPs can contribute to the recovery of the mechanical properties of cracked concrete when combined with fine fibers [20,21]. Excellent swelling capacity is the key parameter for SAPs to block cracks. The swelling capacity of SAPs is affected by many factors, such as the particle sizes and cross-linking densities of SAPs, the valences and concentrations of ions in solutions, and ambient temperatures and pressures [9]. Large SAPs particles have a stronger swelling capacity than small SAPs particles due to the higher activity in the bulk of SAPs than that in the surface zone [22]. The expansion of SAPs is controlled by a water diffusion process after initial water absorption; small SAPs particles with a larger surface area have a faster expansion rate than large SAPs particles [23]. SAPs can exchange ions with solutions, this process is characterized by the absorption of Ca2+ and the release of Na+ and K+ into solutions [24]. Multivalent ions, e.g., Ca2+, Mg2+, and Al3+, can combine with negatively charged SAPs to create a charge screening effect, which increases the cross-linking density (ionically) and decreases the efficient charge density of anionic groups, resulting in a lower swelling capacity of SAPs [25–27]. The swelling capacity of SAPs in fresh mortars is weaker than that in the filtrate of cement slurry because of the higher ion concentration in fresh mortars [12,28]. Increasing the w/b ratio of mortars can decrease the ion concentration of pore solutions, thus resulting in a higher swelling capacity of SAPs [29]. Additionally, the highconcentration of monovalent ions, e.g., Cl, K+, and Na+, can reduce the osmotic pressure between SAPs and solutions, leading to a lower swelling capacity of SAPs [30]. The chloride ion, a common ion in coastal environments, typically induces corrosion of steel bars and leads to the reduced durability of concrete structures. It can also affect the swelling behavior of SAPs. The swelling capacity of SAPs in solutions gradually decreases with increasing NaCl concentrations [24]. The swollen SAPs can seal or partly seal cracks with a width of 0.1 mm– 0.4 mm in a low-concentration NaCl solution (0.12 wt%) [11]. In a 0.5 M NaCl solution, the mass ratio (the mass of water absorbed per mass of SAPs) of SAPs is only one-tenth of that in deionized water [24]. Obviously, the water absorption capacity of SAPs in a high-concentration NaCl solution is reduced. The swelling behavior of SAPs in realistic cracks is very different from that in solutions. The expansion of SAPs in cracks is not only related to ion concentrations but also affected by the volume of the original void, the shape of SAPs particles, and the crack width. The X-ray computed microtomography (lCT) test revealed that the swollen SAPs mainly sealed the original voids, resulting in a noteworthy reduction of the flow rate of water in cracked concrete [12]. Supposedly, in high-concentration NaCl solutions, as long as the volume of swollen SAPs is equal to or slightly smaller than that

of the original void, SAPs may block the cracks and improve the water tightness of cracked concrete. However, the crack-sealing behavior of SAPs in NaCl solutions and the influence of NaCl concentration on the sealing efficiency of SAPs are not clear yet. The influences of NaCl concentrations on the crack-sealing behavior of a single SAPs particle in an artificial crack and the sealing efficiency of SAPs in cracked mortars were studied. The effect of SAPs on the healing of a cracked mortar in wet-dry (0.5 M NaCl) cycles was also investigated. This study is helpful in understanding the crack healing mechanism of SAPs in a monovalent cation solution with high concentration and evaluating the feasibility of the application of smart concrete containing SAPs in environments with high NaCl concentration. 2. Materials and methods The influences of NaCl concentrations (0 M–2 M) on the filled area of SAPs in hardened mortar with an artificial crack were first studied using a self-designed soaking device. Subsequently, the sealing efficiencies of SAPs in NaCl solutions (0 M–2 M) were investigated with a water permeability test. Moreover, the effect of SAPs on the healing of the crack in wet-dry (0.5 M NaCl) cycles was analyzed. The healing products were characterized with a laser scanning confocal microscope (LSCM) and a scanning electron microscope (SEM) with energy-dispersive X-ray spectroscopy (EDS). Finally, the 3D morphologies of a single SAPs particle in cracked paste after being immersed in NaCl solutions (0 M and 1 M) was studied with a lCT test. 2.1. Materials and mixing procedures The polyacrylate-co-acrylamide based SAPs (Wote-I, Shengli Oil Field Changan Holding Group Co., Ltd., China) were crosslinked copolymers of acrylic acid (30%–40%) and acrylamide (50%–60%), neutralized with potassium hydroxide. They were produced through bulk polymerization and had irregular shapes. The SAPs with a range of particle sizes from 0.38 mm to 0.83 mm and a density of 1.1 g/cm3 were used because of their low price and high efficiency with regard to internal curing [29]. The water uptake of these SAPs was measured by suction filtration [11]; the mass ratios of SAPs after absorption in deionized water, synthetic groundwater, 0.9% NaCl, and a synthetic pore solution for 4 h were 294, 88, 35, and 30 gwater/gSAPs, respectively. The compositions of these solutions were as follows (in mmol/L): for synthetic groundwater, NaHCO3 (8.2), CaSO4 (1.04), MgSO4 (2.08), and CaCl2 (0.14) [24]; for synthetic pore solution, CaSO4 (20.6), K2SO4 (163.4), KOH (71.2), and NaOH (73.9) [31]. The absorbency under load (applied pressure 0.3 psi) of SAPs in 0.9% NaCl was measured according to the literature [32], and its value was 12 gwater/gSAPs. The compressive strength of portland cement at 28 days was 50.2 MPa. The specific surface areas of ground granulated blast furnace slag (GGBS) and silica fume were 495 m2/kg and 24,000 m2/kg, respectively. The chemical compositions of the cementitious materials are given in Table 1. The mix designs of mortars are given in Table 2. C1 to C4 were prepared to investigate the effect of w/b ratios and mineral admixtures on the crack-sealing behavior of SAPs. The dosage of SAPs was 1% (relative to the weight of binders) which has been typically used to obtain self-healing [18,20]. The mortars without SAPs were also prepared and marked with asterisks. Extra water was added due to the water uptake of SAPs during the mixing processes. Its amount was determined when the slump flow diameter of fresh mortars satisfied that of the mortars without SAPs (according to Chinese standard GB/T 8077-2012) [33,34]. The quantity of extra water (15–30 gwater/gSAPs) in Table 2 was slightly larger than that

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in the literatures (7–20 [11], 20–25 [35], and 16.4 [36] gwater/gSAPs). This may be due to the large particle size (0.38 mm–0.83 mm) of the chosen SAPs. It is known that the swelling capacity of SAPs increases with the increase of the particle size [22]. Mixing was performed in a pan mixer. The cementitious materials, quartz sand, and dry SAPs were adequately mixed for 2 min. The tap-water was then added and mixed for another 2 min [29]. 2.2. Re-swelling of SAPs in hardened mortar with an artificial crack SAPs swell during mixing and re-swell in cracks when exposed to water. A method to investigate the re-swelling behavior of SAPs in hardened mortars was given here: A single SAPs particle (10 mg ± 0.5 mg) was fixed with adhesive tape, 50 g ± 1 g of fresh mortar (C1*–C4*) was cast (in a cylindroid plastic mold with height 30 mm and diameter 45 mm) on its surface. After curing in the standard condition (20 ± 2 °C, R.H. > 95%) for 7 days, a transparent glass sheet (25 mm  76 mm  1 mm) was pasted on the surface of the hardened mortar (using glue) with several steel needles between them. The space between the hardened mortar and the glass sheet was the artificial crack, its widths (0.25 mm and 0.5 mm) can be adjusted by changing the diameters of the steel needles. Details of the sample preparation can be found in [37]. Subsequently, the samples with an artificial crack were immersed in NaCl solutions with different concentrations (0 (deionized water), 0.05, 0.1, 0.2, 0.5, 1, and 2 M) as shown in Fig. 1. After each test, the residual solution was removed with filter paper to reduce the salt crystallization. Six samples were prepared and the filling area (mm2) of the artificial crack with swollen SAPs can be obtained using a digital camera and Auto CAD software. In this immersion test, the geometry evolution of swollen SAPs in the artificial cracks can be accurately acquired by avoiding the damage of SAPs particles and the tortuosity variation of cracks. SAPs swell during the mixing procedures and then release water to concrete when the humidity in concrete decreases, leaving original voids in the hardened concrete. The volumes of the original voids were measured to investigate the swelling behavior of SAPs in fresh mortar. First, the shrunk SAPs particle was removed from the original void. A small amount of deionized water was added to ensure that there were no residual SAPs. Second, the sample was dried at 50 °C to a constant weight, then the epoxy (epoxy resin and ethylenediamine, a commonly used mounting material [38]) was injected into the original void. After hardening of the epoxy, the sample was slightly polished with 2000 grit sand paper. The weight of the epoxy can be calculated by the weight differences between samples with and without epoxy. This method assumes that the volume of the hardened epoxy equals to that of the original void due to the high fluidity of epoxy. Accordingly, the volume of the original void (V) can be calculated by Eq. (1).

V¼

mepoxy

ð1Þ

qepoxy

where mepoxy and qepoxy are the weight and density (1.16 g/cm3) of epoxy, respectively.

Moreover, 0.1 g of SAPs (particle size of 0.38 mm–0.83 mm) was immersed in NaCl solutions (0 M–2 M) for 4 h. Its mass ratios were measured by suction filtration [11] to study the influences of NaCl concentrations on the swelling capacity of SAPs in a free state (without artificial cracks). 2.3. Water permeability test The sealing efficiencies of SAPs in NaCl solutions (0 M–2 M) were investigated using a water permeability test. After mixing, six fresh mortars for each group (see Table 2) were cast into the plastic molds (U 50 mm  20 mm) and cured in the standard condition for 3 days. The hardened mortars were then demolded and cured for another 25 days in the same condition. Subsequently, a loading device was used to create a through-thickness crack at the center of the sample (Fig. 2a). Afterward, the cracked mortar was slightly taken apart. Two steel needles were fitted in the crack. The sample was reassembled with a steel clamp to create a through-thickness crack [15] (Fig. 2b). Samples with crack widths of 0.25 mm and 0.5 mm were prepared by changing the diameters of the steel needles. The normal crack widths were checked with a crack width monitor (KON-FK), and their deviations generally ranged between 0.03 mm and 0.07 mm. Afterward, the sides of the crack were sealed with silica gel. The assembled samples were then put into a plastic pipe (30 mm in height and 75 mm in diameter). The space between the assembled sample and the pipe was filled with fresh mortars (w/b = 0.5, sand/cement = 2) to make sure the solutions could flow through the crack. After curing for 3 days in the standard condition, the samples were connected to the device of the water permeability test with clamps and preservative film to prevent water leakage (Fig. 2c, d). The constant height distance between the water level in the water tank and the bottom of the sample was 60 cm. The masses of NaCl solutions that flow through the cracked sample were recorded with an electronic balance, thus the flow rate of solutions (g/s) could be obtained. It is known that the crack width variation has a significant effect on the water flow rate [39]. To reduce this impact, the water permeability tests with different NaCl concentrations were performed on the same sample. After each water permeability test, the sample was immediately dried at 50 °C for 5 h to decrease the influences of the self-healing of the mortar on the variation of crack width [40]. The water permeability test was stopped when the water flow rate was stable. The sealing efficiency (r) of SAPs can be calculated based on the reduction ratio of flow rate, as given in Eq. (2) [12]:

r¼

Qi - Qf Qi

ð2Þ

where Qi and Qf are the initial and final flow rate (g/s) of cracked mortars in NaCl solutions (0 M–2 M), respectively. 2.4. X-ray computed microtomography test A novel method to investigate the 3D morphologies of a single SAPs particle in cracks after being immersed in NaCl solutions was given here: The paste (C1* and C2* without quartz sand)

Table 1 Chemical compositions of cementitious materials (mass %). Materials

SiO2

Al2O3

Fe2O3

MgO

CaO

Na2O

SO3

K2O

Cement Fly ash GGBS Silica fume

21.9 41.0 30.6 85–96

4.5 33.7 14.6 1 ± 0.2

3.5 9.8 0.57 0.9 ± 0.3

2.4 0.6 9.0 0.7 ± 0.1

64.7 8.5 39.1 0.3 ± 0.1

0.51 – – 1.3 ± 0.2

2.4 1.0 2.53 –

– 1.0 – –

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Table 2 Mix designs of the mortars (g). Series

Cement

Fly ash

GGBS

Silica fume

Quartz sand

Water

SAPs

Extra water

C1* C1 C2* C2 C3* C3 C4* C4

1000 1000 1000 1000 580 580 420 420

0 0 0 0 250 250 250 250

0 0 0 0 170 170 170 170

0 0 0 0 0 0 160 160

1728 1728 1728 1728 1728 1728 1728 1728

500 500 230 230 230 230 230 230

0 10 0 10 0 10 0 10

0 300 0 225 0 290 0 150

Notes: *Mixes marked with asterisks are reference control mortars without SAPs.

Fig. 1. The immersion test for SAPs in the hardened mortar with an artificial crack.

Fig. 2. The loading device to create a through-thickness crack (a), the cracked mortar (b), and the setup for water permeability test (c, d).

was cast in a plastic tube (U 14 mm  40 mm) (Fig. 3a). A single SAPs particle (5 mg ± 0.5 mg) was then placed in the center of the paste surface (Fig. 3b). Afterward, another plastic tube was fixed above the sample using tapes (Fig. 3c) and filled with pastes (Fig. 3d). After curing for 7 days, the sample was split at the position of SAPs using a three-point bending test (Fig. 3f) and then put into a plastic tube (U 14 mm  80 mm) with notches on both sides. Two steel needles (0.25 mm or 0.5 mm in diameter) were placed in the crack (at the notch). A small amount of glue was used to bond the sample and the plastic tube. The reassembled sample was then immersed in the 0 M NaCl solution for 1 h. The residual solution on the sample surface was removed before the lCT test (nanoVoxel-3502E). The X-ray tube was set at a voltage of 85 kV with a tube current around 14 mA. The obtained pixel size was 21 lm. After the lCT test, the sample was dried at 50 °C for 24 h and immersed in the 1 M NaCl solution for another lCT test.

2.5. LSCM, SEM, and EDS SAPs in cracks can swell in solutions and release water in dry environments, thus promoting the healing of cracked concrete [17]. The swelling of SAPs and the healing of the matrix can reduce the water permeability of cracked concrete [20]. In this section, the influences of SAPs on the healing of the matrix were studied in wet-dry cycles. Six cracked mortars (width in 0.25 mm) for each group (C2 and C2*) were prepared as described in Fig. 2. Their sealing efficiencies (r) were studied in wet-dry cycles (alternatively stored in a 0.5 M NaCl solution for 1 h and at an R.H. of 60% for 23 h [17]) after curing for 28 days. The test period was 20 days, and the water permeability tests were conducted before and after the wet-dry cycles. It is difficult to distinguish the healing products from the matrix in realistic cracks [3]. Therefore, the pastes (after the removal of quartz sand in Table 2) with artificial cracks were prepared

H. Yang et al. / Construction and Building Materials 243 (2020) 118228

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Fig. 3. Schematic diagrams (a–d) and physical map (e) of sample preparations; the setup of a three-point bending test (f); the sample for the lCT test (g).

(Fig. 1) to study the surrounding healing products of SAPs in NaCl solutions. After curing for 28 days, the surface layer (approximately 5 mm) of the sample was removed using sand papers to achieve a fresh surface. Subsequently, the samples with artificial cracks (width: 0.5 mm) were immersed in the wet-dry cycles (as described above) 20 times. Afterward, the artificial crack was removed and the healing products were examined with the LSCM (OLYMPUS 4100), SEM (FEI QUANTA 250), and EDS tests. 3. Results and discussion 3.1. Influences of NaCl concentrations on the filled area (S value) The volumes of the original voids (V) in hardened mortars were calculated by measuring the masses of epoxy that was injected into the original void and are presented in Fig. 4. The V values in descending order were C1, C3, C2, and C4. The w/b ratio of C1 (0.5) was larger than that of C2 (0.23). A higher w/b ratio means a lower ion concentration of the pore solution, which is beneficial to the expansion of SAPs in fresh mortars [29], thus resulting in a larger V value in C1 than that in C2. The Ca2+ generated by the dissolution of cement can reach 26.5 mM after mixing for 13 min [31] and can be absorbed by osmosis in the SAPs, decreasing the swelling capacity of SAPs [41]. The replacement of cement with fly ash and GGBS in C3 reduces the Ca2+ concentration, thus, the V value in C3 is larger than that in C2. C4 is prepared by adding a fine silica fume (particle size of 0.1 lm–0.3 lm) on the base of C3. The silica fume can absorb water and reduce the effective w/b ratio of mortars [25], resulting in a smaller V value in C4 than that in C3. The crack-sealing behavior of a single SAPs particle in the artificial crack was investigated by installing a transparent glass sheet on the surface of the hardened mortar. Fig. 5 illustrates the morphologies and filled area (S) of SAPs in C1 before and after being immersed in NaCl solutions (0 M–2 M). SAPs shrunk in the mortar before the immersion (Fig. 5a-1, b-1). Furthermore, the bonding between SAPs and mortars caused severe deformation of the shrunk SAPs (Fig. 5b-1). After the immersion, the original void and its surrounding crack were sealed with the swollen SAPs particle (Fig. 5b-2). The S value significantly increased with increasing crack width (Fig. 5b-2). The swelling of SAPs in a crack is affected by many factors, e.g., mortar properties, crack widths, and ion concentrations of solutions [37]. A smaller crack width limits the expansion of SAPs, resulting in a lower swelling capacity of SAPs in a crack than that in a solution. Increasing the crack width can reduce the physical restricting effect of a crack, thus leading to a larger S value.

Fig. 4. Influence of mortar composition on the volume of the original voids, the mix designs of C1–C4 are given in Table 2.

Moreover, the NaCl concentrations could significantly affect the S value. In the crack with a width of 0.25 mm, the S value did not change much with the increase of concentrations from 0 M to 0.5 M, whereas it decreased when the concentration reached 1 M. The S value in the 2 M NaCl solution was only 77% of that in the 0 M NaCl solution. In the crack with a width of 0.5 mm, the S value was almost constant when the concentration increased from 0 M to 0.05 M, whereas it decreased with the further increase in concentrations. The swelling of SAPs in a 0 M NaCl solution is controlled by the restricting effect of a crack. The increase of concentrations reduces the difference in ion concentrations between the gel and the external solution, which results in a decrease of the driving force for the swelling of SAPs [42]. When the swelling of SAPs is controlled by ion concentrations, the S value decreases with increasing concentrations. The above results indicate that the NaCl concentration is an important factor affecting the crack-sealing behavior of SAPs in artificial cracks. Fig. 6a shows the effects of NaCl concentrations on the mass ratios of SAPs in a free state and the filled area (S) of SAPs in artificial cracks. The mass ratios rapidly decreased with increasing concentrations (0 M–0.05 M) and then flattened gradually. This finding agrees well with that in the literature [24]. In the artificial crack, the S value was affected by the crack widths and mortar properties (Fig. 6a). The descending order of the S value in the 0 M NaCl solution was C1, C3, C2, and C4, which

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Fig. 5. The morphologies (below transparent glasses) and filled area (S/mm2) of SAPs (in C1) in artificial cracks with widths of 0.25 mm (a) and 0.5 mm (b) before (1) and after (2–8) immersion; No. 2–8 were 0, 0.05, 0.1, 0.2, 0.5, 1, and 2 M NaCl solutions. The immersion period was 1 h.

was well consistent with that of the V value (Fig. 4). Moreover, the S value increased with increasing crack widths (0.25 mm–0.5 mm). The swollen SAPs can fill the original void and the crack above the original void. The larger the V value is, the larger the area of the crack above the original void. Accordingly, the S value increases with the increase of the V value. The further swollen SAPs can fill the cracks outside the original void. The increase of crack width reduces the restricting effect of the crack and thus increases the S value. Furthermore, the S value was influenced by NaCl concentrations (Fig. 6a). The S value was almost constant in low-concentration NaCl solutions, while it reduced with the further rise of NaCl concentrations (except for C4-0.25). For ease of analysis, the NaCl concentration corresponding to the decrease in the S value was defined as the critical NaCl concentration (Ccrit). The results of the Ccrit values are plotted in Fig. 6b. The S value of C4-0.25 did not decrease with increasing concentrations. The Ccrit values of C1-0.5 and C1-0.25 were 0.1 M and 0.5 M, respectively. The Ccrit value of the remaining samples was 1 M. Obviously, the Ccrit value was markedly affected by the mortar properties and crack widths. The swelling of SAPs in high-concentration NaCl solutions is controlled by ion concentrations. The further increase of concentrations decreases the filled area of SAPs (Fig. 6a). The volume of SAPs required to fill a smaller volume of the original void and crack width is lower relative to a larger volume of the original void and crack width. When the volume of the original void and the crack width are both small, SAPs are able to maintain a constantly filled area in high-concentration NaCl solutions, thus resulting in a larger Ccrit value.

The above results (Fig. 6) indicate that the S value of SAPs in artificial cracks is affected by the NaCl concentrations, mortar properties, and crack widths. A high-concentration NaCl solution (2 M) may lead to a decrease in the S value in artificial cracks (Fig. 6a). The Ccrit value corresponding to the reduction of the S value is influenced by the mortar properties and crack widths. Additionally, the swollen SAPs can fill the original void (Fig. 5a-8, b-8), indicating that SAPs may block cracks and improve the water tightness of cracked concrete in high-concentration NaCl solutions. This inference is verified with the water penetration test in the next section. 3.2. Influences of NaCl concentrations on the sealing efficiency The influences of NaCl concentrations on the sealing efficiency (r) of SAPs were evaluated by the reduction ratio of the flow rate in cracked mortars (Fig. 7). In the 0 M NaCl solution, the r value increased with the increase of crack width (0.25 mm–0.5 mm) because of the restricting effect of cracks on the swelling of SAPs [37]. For different mortars, the descending order of the r value was C1, C3, C2, and C4. This order is consistent with that of the S value (Fig. 6a). The swollen SAPs can seal the original voids and cracks, thereby effectively reducing the length of cracks perpendicular to the direction of water flow and decreasing the flow rate of water. Consequently, a larger S value corresponds to a larger r value. Additionally, the r value was influenced by the NaCl concentration. A higher NaCl concentration led to a decrease in the r value (Fig. 7a). Fig. 7b illustrates the critical NaCl concentration (Ccrit) corresponding to the reduction in the r value. The Ccrit value in

Fig. 6. Influences of NaCl concentrations on the mass ratios of SAPs in the free state and the filled areas (S) of SAPs in artificial cracks (width in 0.25 mm and 0.5 mm) (a); the Ccrit values of SAPs (b); the immersion period was 1 h.

H. Yang et al. / Construction and Building Materials 243 (2020) 118228

the crack with a width of 0.5 mm was smaller than that in the crack with a width of 0.25 mm, indicating that the r value in a larger crack width will decrease first with the increase of NaCl concentration. For a given crack width, the increasing order of the Ccrit values was C1, C3 (C2), and C4. As discussed in Section 3.1, the Ccrit value is influenced by the mortar properties and crack widths. Moreover, there were some differences in the Ccrit values between cracked mortars (Fig. 7b) and artificial cracks (Fig. 6b), which may be due to the following reasons: First, the rough surface and large tortuosity of a realistic crack increase the difficulty in crack-sealing of SAPs. Second, SAPs are intact in artificial cracks, whereas in cracked mortars, SAPs may be split during the crack-creating processes, which reduce the swelling capacity of SAPs [12]. Increasing the crack widths and the V value could increase the r value of SAPs in the 0 M NaCl solution (Fig. 7a). While in highconcentration NaCl solutions, it would result in a decrease in the Ccrit value (Fig. 7b). Moreover, the r value of C1-0.5 significantly reduced and was smaller than that of C2-0.5, C3-0.5, and C4-0.5 in 0.5 M–2 M NaCl solutions. The above results indicate that the best healing efficiency cannot be obtained only by rising the V value and crack widths in high-concentration NaCl solutions; the influences of NaCl concentrations on the healing efficiency must be considered. The morphologies of SAPs on the surface of cracked mortars after the water permeability test are shown in Fig. 8. The swollen SAPs were white gels and sealed the crack in the 0 M NaCl solution. In the 1 M NaCl solution, the volume of swollen SAPs reduced but SAPs still blocked part of the crack, indicating that SAPs can seal cracks in such an environment. This corresponds to the fact that SAPs have a certain sealing efficiency in high-concentration NaCl solutions (Fig. 7a). 3.3. The 3D morphologies of SAPs after being immersed in NaCl solutions The 3D morphologies of SAPs in realistic cracks after being immersed in the 0 M and 1 M NaCl solutions were investigated using the lCT test and are shown in Fig. 9. SAPs almost completely sealed the original voids in the 0 M NaCl solution (Fig. 9a-2, a-3, b2, and b-3). Additionally, the volume of swollen SAPs in C1 was larger than that in C2. This is because C1 has a larger original void (Fig. 4). Moreover, SAPs could not seal cracks (Fig. 9b-3) or could seal limited parts of cracks (Fig. 9a-3) around the original void. Similar results revealed that SAPs in realistic cracks mainly sealed the original void in deionized water [12]. The above results indicate that the crack-sealing capacity of SAPs in realistic cracks is weaker than that in artificial cracks (Fig. 5). This may be due to the differences in the integrity of SAPs, the tortuosity of cracks,

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and the roughness of crack surfaces in the two types of cracks, as discussed previously. When the concentration reached 1 M, the volume of swollen SAPs reduced and the reduction ratio of the volume in C1 (63%) was larger than that in C2 (21%) (Fig. 9c, d). The swelling of SAPs in high-concentration NaCl solutions is controlled by ion concentrations. C1 has a larger V value and crack width, which increases the difficulty of SAPs to seal the original void completely. Furthermore, the swollen SAPs partly sealed the original void (Fig. 9c), which reduced the crack length perpendicular to the crack width. This may be the reason why SAPs in C2 have a certain cracksealing efficiency in high-concentration NaCl solutions (Fig. 7a). 3.4. Influences of SAPs on the self-healing of mortars The r values of mortars with and without SAPs were measured before and after wet-dry (0.5 M NaCl) cycles to study the effect of SAPs on the healing of the mortar (Fig. 10). Before wet-dry cycles, the r value of mortars without SAPs was small (1%). This is because the healing of the matrix mainly occurs within the first 3–5 days [43]. It is difficult to produce healing during the water permeability test (approximately 20 min). By contrast, the r value of mortars containing SAPs was large (41%) due to the rapid swelling of SAPs [11]. After wet-dry cycles, the r values of mortars with and without SAPs increased. Additionally, the increase in the r value of mortars with SAPs was more significant, indicating that SAPs can promote the healing of the mortar in wet-dry cycles. SAPs can absorb water in the wet period and release it to the matrix in the dry period, thus promoting the hydration of unhydrated cementitious materials and the carbonation of Ca(OH)2 [17,20]. These healing products can seal the crack [18] and reduce the flow rate of water [14,16]. The LSCM, SEM, and EDS tests were used to study the morphologies and the compositions of healing products of pastes in wet-dry (0.5 M NaCl) cycles (Fig. 11). The LSCM photos revealed that the surface of the paste was covered with grayish white healing products that consisted of irregularly precipitated crystal-like particles (SEM topography). The EDS results indicated that the elements of these particles were mainly C, O, Si, and Ca. It is known that the Ca/Si ratio of C-S-H gel is around 2 [45]. However, the Ca/Si ratios in Fig. 11 were larger than 2. Based on the aforementioned results and analyses, CaCO3 and C-S-H gel, which, respectively, originated from the carbonation of Ca(OH)2 and the hydration of unhydrated cementitious materials [44], were the main healing products, as reported in many studies [45,46,50]. Moreover, the increase of Ca/Si ratio reflects an increase in the proportion of CaCO3 in the healing products [47,48]. According to the Ca/Si ratios in the EDS results, the proportions of CaCO3 in descending order were C1, C2, C4, and C3.

Fig. 7. Influences of NaCl concentrations on the r value in cracked mortars with different crack widths (0.25 mm and 0.5 mm) (a), the Ccrit values of SAPs (b).

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Fig. 8. Morphologies of SAPs in C2 with a crack width of 0.25 mm after being immersed in 0 M (a) and 1 M (b) NaCl solutions.

Fig. 9. The 3D morphologies (1), top views (2), and cross-section views (3) of SAPs in C2 (crack width in 0.25 mm) (a, c) and C1 (crack width in 0.5 mm) (b, d) after being immersed in the 0 M (a, b) and 1 M (c, d) NaCl solutions, the immersion period was 1 h, the scale bar was 1 mm, the grayscale in descending order was cracks, SAPs, and pastes.

The lower w/b ratio in C2 leads to the insufficient hydration of cement, which results in a lower dissolution of Ca2+ in C2 than that in C1 [49]. Therefore, C2 has a lower Ca/Si ratio compared to C1. The replacement of cement with fly ash and slag decreases the concentration of Ca2+ that is produced by the cement hydration. Moreover, fly ash and GGBS can consume Ca2+ by the pozzolanic reaction [49,50]. The above reasons lead to a lower Ca/Si ratio in C3 than that in C2. The C4 was prepared by adding a highactivity silica fume that can react with Ca(OH)2 after casting for 1–3 days, resulting in a lower Ca2+ concentration [51] and a smaller pH value [52]. Additionally, the entry of water causes a further decrease in the pH value of solutions in a crack. Reportedly, slag and fly ash can only be activated in solutions with pH values higher than 12 and 13, respectively [53]. The addition of silica fume may inhibit the hydration of unhydrated fly ash and GGBS, causing a

lower proportion of C-S-H gel in the healing products and a larger Ca/Si ratio in C4 than that in C3. To study the influences of SAPs on the compressive strengths of mortars, three specimens (100 mm cubic) for each group were fabricated following the procedures in Section 2.3 and cured in the standard condition for 28 days. The results are plotted in Fig. 12. For mortars without SAPs, the compressive strength of C2 was larger than that of C3 and C1. This is because the increase of the w/b ratio and the addition of supplementary cementitious materials reduce the compressive strengths [54,55]. The loss in compressive strength of C4 may be due to the decrease in hydration products with increasing supplementary cementitious admixtures [8]. When SAPs were incorporated, the compressive strength significantly reduced, the reduction ratios of compressive strength in descending order were C1, C3, C2, and C4 (Fig. 12). This order is

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Fig. 10. The r values of mortars with and without SAPs before and after wet-dry cycles.

consistent with that of the V value (Fig. 4). The original voids in mortars can be viewed as macro-defects [56], a large original void reduces the effective cross-sectional area of samples and increase the reduction ratio of compressive strength.

4. Sealing mechanism of SAPs in NaCl solutions The mass ratio of SAPs in the 0 M NaCl solution is 294 gwater/gSAPs (Section 2.1) which is much larger than that in

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fresh mortars (15–30 gwater/gSAPs in Table 2). Accordingly, SAPs can seal a large area of artificial cracks in the 0 M NaCl solution (Fig. 5b-2). When the NaCl concentration exceeds the Ccrit value, the filled area and the sealing efficiency of SAPs will decrease. Additionally, the Ccrit value is affected by the mortar properties and crack widths (Figs. 6b and 7b). It is assumed that each spherical SAPs particle leaves an original void in concrete and the cracks that propagate along those voids are modelled as flat planes. In cracked concrete, the ingress of NaCl solutions triggers the re-swelling of SAPs on the crack surface, leading to the sealing of cracks. Without considering the bonding between SAPs and cement matrix [12], the split of SAPs during crack creating processes [12], and the influences of ion exchange between SAPs and solutions on the swelling capacity of SAPs [27]. The crack-sealing mechanism of SAPs in NaCl solutions is analyzed based on the volume difference between swollen SAPs in cracks (V1) and in NaCl solutions (V2) (Fig. 13). SAPs swell during the mixing processes and shrink when the humidity inside cracks decreases (Fig. 13a, e). When the volume of the original void and crack width are both small (Fig. 13b), the swelling of SAPs in low-concentration NaCl solutions is controlled by the restricting effect of a crack. Accordingly, the volume of the swollen SAPs in NaCl solutions (V2) is much larger than that in cracks (V1). In medium-concentration NaCl solutions (Fig. 13c), the difference in ion concentrations between SAPs and solutions decreases (Fig. 6a), resulting in a decrease in the expansion capacity of SAPs and a reduction in the V2 value. The swelling of SAPs is still controlled by the restricting effect of cracks (V2 > V1).

Fig. 11. LSCM (lower magnification image), SEM (higher magnification image), and EDS results of the healing products on the surfaces of C1 (a), C2 (b), C3 (c), and C4 (d).

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Fig. 12. Influences of SAPs on the compressive strengths of mortars at 28 days.

Therefore, increasing the NaCl concentration does not result in a decrease in the V1 value. In high-concentration NaCl solutions (Fig. 13d), the difference in ion concentrations between SAPs and solutions further reduces, which decreases the driving force of SAPs swelling and the V2 value. The swelling of SAPs in such a condition is controlled by ion concentrations (V2 = V1). Reducing the V2 value causes a decrease in the V1 value, which reduces the crack filling area of SAPs. When the volume of the original void and crack width are both large, there is a significant increase in the V1 value (Fig. 13f). The swelling of SAPs in low-concentration NaCl solutions is controlled by the restricting effect of cracks. In medium-concentration NaCl solutions (Fig. 13g), the swelling of SAPs is controlled by ion concentrations (V2 = V1). The increase in concentrations leads to a reduction in the V2 (V1) value, thus resulting in a decreased filled area of SAPs. In high-concentration NaCl solutions (Fig. 13h), the swelling of SAPs is also controlled by ion concentrations (V2 = V1). Increasing the NaCl concentration results in a further decrease in the swelling capacity of SAPs, which reduces the V2 (V1) value and the filled area of SAPs. The Ccrit value corresponding to the reduction of the filled area is influenced by the volumes of the original voids and crack widths. When the NaCl concentration reaches the Ccrit value, the control of SAPs swelling is transformed from the restricting effect of cracks (V1 < V2) to ion concentrations (V1 = V2). A larger volume of the original void and crack width means a larger V1 value, resulting in a

reduced concentration which corresponds to V1 = V2. Accordingly, the Ccrit value reduces with the increase of the volume of the original void and crack width (Figs. 6b and 7b). In realistic cracks, SAPs with a larger V1 value can effectively seal the cracks and have a higher sealing efficiency. Moreover, they can release more water to the matrix and promote the self-healing of the matrix on the crack surface (Fig. 10), which further improve the sealing efficiency. The swollen SAPs can fill the original voids (Figs. 5 and 9) and reduce the flow rate of cracked mortars (Fig. 7) in 0 M–2 M NaCl solutions. This key finding indicates that the smart cementitious material with SAPs has great potentials as structural materials in the environments containing NaCl, e.g., bridges, tunnels, mines, and dams in coastal areas. Reducing the volume of the original void and crack width can increase the Ccrit value (Fig. 7b). In practical applications, the volumes of the original voids can be reduced by the techniques as follows, the decrease of the swelling of SAPs during mixing, e.g., the decrease of w/b ratio and the addition of silica fume (Fig. 4), the modification of SAPs (e.g., pH-responsive SAPs [56] and ionresponsive SAPs [57]), and the coating of SAPs [58]. Additionally, the crack width of concrete can be controlled by the addition of fibers, e.g., engineered cementitious composites with typical crack widths of tens of microns [19,20]. Accordingly, the above strategies may help to raise the Ccrit value so that SAPs can maintain a constant sealing efficiency in high-concentration NaCl solutions. Increasing the volume of the original void and crack widths leads to a larger S value (Fig. 6a) and r value (Fig. 7a) of SAPs in the 0 M NaCl solution. While it also causes a decrease in the Ccrit value (Figs. 6b and 7b). Consequently, the effects of the volumes of the original voids, crack widths, and NaCl concentrations on the r value should be considered to achieve optimum healing efficiency.

5. Conclusions The effects of NaCl concentrations on the filled area (S) of SAPs in artificial cracks and the sealing efficiency (r) of SAPs in cracked mortars were evaluated in this paper. The 3D morphologies of SAPs in cracked paste after being immersed in NaCl solutions were studied with a lCT test. The healing products of the pastes during wetdry (0.5 M NaCl) cycles were characterized using the LSCM, SEM, and EDS tests. Key findings are as follows:

Fig. 13. The cross-section views of SAPs in hardened concrete with a small (a–d) and a large (e–h) V1 value before (a, d) and after being immersed in NaCl solutions with low (b, f), medium (c, g), and high (d, h) concentrations.

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(1) The volumes of the original voids (V) can be changed by adjusting the mortar properties. The reduction of the w/b ratio and the addition of silica fume reduce the V value. By contrast, for a given w/b ratio, the replacement of cement with fly ash and slag increases the V value. (2) The V values, crack widths, and NaCl concentrations have significant effects on the S value of SAPs in artificial cracks. The S value rises with the increase of the V value and crack widths, and increasing the NaCl concentration may reduce the S value. Moreover, the critical NaCl concentration (Ccrit), the concentration corresponding to the decrease in the S value, is affected by the V value and crack widths. (3) In the 0 M NaCl solution, the lCT results indicate that SAPs mainly seal the original void, and the crack-sealing capacity of SAPs in realistic cracks is lower than that in artificial cracks. Moreover, the volume of the swollen SAPs decreases when the concentration reaches 1 M, although SAPs can still partly seal the original void. (4) Increasing the NaCl concentration may decrease the r value of SAPs, and the Ccrit value of the r value is influenced by the V values and crack widths. Decreasing the V values and crack widths can increase the Ccrit value. The V values, crack widths, and NaCl concentrations are key factors influencing the crack-sealing behavior of SAPs in cracks. (5) SAPs can promote the healing of mortar and increase the r value in wet-dry (0.5 M NaCl) cycles. The healing products are mainly CaCO3 and C-S-H gel. The proportions of CaCO3 (in healing products) in descending order are C1, C2, C4, and C3. Furthermore, the incorporation of SAPs decreases the mortar strength. A larger V value results in a more pronounced decrease in the mortar strength. This study is the beginning of exploring the crack-sealing behavior of SAPs in solutions with high ionic concentrations. To accurately measure the dry mass and filled area of SAPs in artificial cracks, the particle size of SAPs (0.38 mm–0.83 mm) used in this study is slightly larger than that in the literatures (0.1 mm–0.7 m m) [11,12]. The small SAPs particle, with a smaller dry volume, has a weak swelling capacity due to the higher activity of the bulk SAPs than that of the surface [22]. Consequently, the small SAPs particle has a smaller V value, which may affect the r value and Ccrit value of SAPs in NaCl solutions. The crack-sealing behavior of small SAPs particles requires further investigations in the future. CRediT authorship contribution statement Haitao Yang: Conceptualization, Methodology, Software, Formal analysis, Investigation, Data curation, Writing - original draft, Writing - review & editing. Juanhong Liu: Supervision, Project administration, Funding acquisition. Xinshan Jia: Visualization, Methodology. Yucheng Zhou: Methodology. Hongguang Ji: Validation, Resources. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement This study was supported by the National Natural Science Foundation of China [grant numbers 51834001 and 51678049].

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