PE wax composite shell microcapsules containing TDI for self-healing of cementitious materials

Construction and Building Materials 231 (2020) 117060

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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat

Preparation and characterization of nano-SiO2/paraffin/PE wax composite shell microcapsules containing TDI for self-healing of cementitious materials Wei Du, Jianying Yu ⇑, Bianyang He, Yanheng He, Peng He, Ying Li, Quantao Liu State Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology, Wuhan 430070, PR China

h i g h l i g h t s
Nano-SiO2/paraffin/PE wax microcapsules containing TDI were prepared.
The mechanical property of microcapsules is improved by adding nano-SiO2 in shell.
Crack of mortars with nano-SiO2/paraffin/PE wax microcapsules was self-healed in 4 h.

a r t i c l e

i n f o

Article history: Received 25 April 2019 Received in revised form 30 August 2019 Accepted 21 September 2019

Keywords: Microcapsules Cementitious materials Nano-SiO2/paraffin/PE wax composite shell Compactness Micromechanical properties Self-healing capability

a b s t r a c t Cement concrete materials are prone to form internal micro-cracks during service, which can be selfhealed via microcapsule method. In this paper, self-healing microcapsules were prepared through melt condensation method with nano-SiO2/paraffin/PE wax composite shell and toluene-di-isocyanate (TDI) healing agent. The core fraction and compactness of microcapsules were measured. Particle size distributions, morphologies, micromechanical properties and chemical structure of microcapsules were characterized by laser particle size analyzer, scanning electron microscopy (SEM), nanoindentation test and Fourier transform infrared spectroscopy (FTIR), respectively. Self-healing capability of mortars containing microcapsules was investigated. The results depicted that core fraction of nano-SiO2/paraffin/PE wax composite shell microcapsule was 72.6%, and the size of the microcapsules was mainly between 400 and 600 mm. Since nano-SiO2 was added to the shell material, the elastic modulus and hardness of the microcapsule reached 1.87 GPA and 61.67 MPa respectively, and the weight loss rate decreased by only 2.6% within 60 d. SEM showed that nano-SiO2/paraffin/PE wax composite shell microcapsules were regular spherical with rough surface, and the shell thickness was about 1/20 of the diameter. The result of FTIR indicated that TDI was encapsulated into the nano-SiO2/paraffin/PE wax composite shell successfully. The compressive strength recovery rate of the mortar containing nano-SiO2/paraffin/PE wax composite shell microcapsules was 87.8% after 60% fc0 pre-load for 10 d self-healing in air. Moreover, the nano-SiO2/paraffin/PE wax composite shell microcapsule could completely self-heal the surface crack of mortar with a maximum width of 0.48 mm within 4 h, which showed that it had good self-healing capability for cracks in cementitious materials. Ó 2019 Elsevier Ltd. All rights reserved.

1. Introduction As an important building material, cementitious materials are world-widely used in the construction field [1]. However, microcracks tend to form during the service of concrete due to its brittleness. Unless these micro-cracks are effectively detected and repaired, they may cause larger cracks, which will reduce the

⇑ Corresponding author. E-mail address: [email protected] (J. Yu). https://doi.org/10.1016/j.conbuildmat.2019.117060 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.

durability of concrete [2,3]. Therefore, the detection and maintenance technology of concrete structure has attracted a wide spread attention. However, it was difficult to continuously detect, maintain and heal the concrete cracks by conventional methods, and it also required a lot of labors and funds. In the United States, the annual maintenance cost of concrete buildings was estimated at about $20 billion (2006–2020) [4]. Meanwhile, more than 50% of total construction funds were used for inspection and maintenance in Europe (2009) [5]. In this case, concrete self-healing technology has received an increasing attention because it can repair cracks

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automatically timely, decrease maintenance funds and extend service life [6,7]. Cementitious materials self-healing technology mainly includes microcapsule method, bacteria method and crystalline admixtures method [8,9,10]. Compared with other methods, the microcapsule method can self-heal the surface cracks and internal micro-cracks of concrete quickly and effectively, thus recovering the mechanical properties of concrete [11]. White [12] first proposed the concept of self-healing of polymer composites based on microcapsules in 2001, and then many researchers applied the self-healing technology of microcapsules to the field of concrete self-healing. Beglarigale [7] coated sodium silicate in polyurethane to prepare microcapsules for self-healing cementitious materials. Giannaros [13] prepared two kinds of microcapsules (gelatin-gum Arabic coated sodium silicate solution and polyurethane coated solid sodium silicate) to seal the cracks in cement-based materials. Kanellopoulos [14] reported the microcapsules that was the gelatin-acacia gum containing sodium silicate solution healing agent for self-healing application in concrete. Dong [15,16] applied urea formaldehyde/epoxy microcapsules to self-heal cementitious material and the permeability of concrete restored by self-healing system based on microcapsules was also studied. Yang [17] selfhealed cementitious composite by oil core/silica gel shell microcapsules. The self-healing principle of microcapsules is when cracks appear in cementitious materials, the propagation of the cracks can rupture the microcapsules, and healing and curing agent in the microcapsules will be released to heal the cracks. In recent decades, many thermosetting materials have been used as shell materials, such as urea formaldehyde [18,19], polyurea [20,21], polyurea formaldehyde [22,23] and melamine– formaldehyde (MF) [24,25]. However, due to the high strength of thermosetting materials [26], it is difficult to break the thermosetting shell and release healing agent while the micro-cracks occur in cementitious materials. Janssen [27] had tried to solve the problem by using paraffin as shell materials. In our previous research work, the paraffin microcapsules contained toluene-di-isocyanate (TDI) have been prepared and applied in self-healing of concrete [28]. The self-healing mechanism of TDI agent is that the microcapsules are ruptured under the stress of the micro-cracks tip, then TDI flowing into micro-cracks will react with moisture. The reaction products eventually self-heal micro-cracks. The chemical reaction process of diisocyanate with water is shown in Fig. 1. The curing time between TDI and water is more than a few minutes, which indicates that TDI is suitable as a healing agent to develop onecomponent microcapsules for concrete self-healing. However, although these microcapsules are easier to break under external forces, the mechanical properties of shell materials are relatively low, and there is a high risk of breakage in concrete mixing process. In order to improve the strength of microcapsules, Krupa [29,30] tried to prepare shell materials for microcapsules by blending thermoplastic materials paraffin, polyethylene (PE), and polypropylene, while Karkri [31] used high density PE and MF mixture to prepare shell materials. Besides the mechanical property, the leakage of core materials is also a major factor affecting the self-healing ability and durability of microcapsules. Recently, some researchers have synthesized the shell materials by adding nanomaterials to improve the compactness of microcapsules. Jiang [32] investigated the synthesis of nano-Al2O3 modified poly (methyl methacrylate-co-methyl acrylate) encapsulated paraffin. Yang [33] used modified MF resin and nano-CaCO3 particles as shell materials to coat aromatic oil rejuvenator. These researches indicate that it is feasible to improve the compactness of microcapsules by incorporating nanomaterials into shell materials. To improve the micromechanical properties and compactness of paraffin microcapsules containing TDI, nano-SiO2/paraffin/PE

Fig. 1. The chemical reaction process of diisocyanate with water.

wax composite was used as shell materials in this paper. The core fraction and compactness of the microcapsules were measured. Size distributions, morphologies, micromechanical properties and chemical structure of the microcapsules were characterized. Compressive strength recovery rate of the pre-damage mortars containing microcapsules was measured. Self-healing of microcapsules on cracks in mortars was evaluated. 2. Experimental 2.1. Materials Paraffin (melting point: 58–60 °C, relative molecular weight: 500–1000, density: 0.88–0.91 g/cm3) was obtained from Sinopharm Chemical Reagent Co., Ltd. PE wax (melting point: 105 °C, relative molecular weight: 3000–5000, density: 0.93–0.98 g/cm3) was supplied from Dingguo Plastic Chemical Co., Ltd. TDI was provide by Chengdu Micxy Chemical Co., Ltd. Nano-SiO2 with a particle size of 20 nm was provided by Nanjing Tansail Advanced Materials Co., Ltd. Perfluoro-tributylamine (PFTBA) was purchased from Beijing Letai Chemical Reagent Co., Ltd. Ordinary Portland cement (CEMI 42.5 N) was provided by Huaxin Cement Co., Ltd

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and the chemical composition of cement is given in Table 1. Medium-sized sand with soil and water content less than 1%. 2.2. Microcapsule preparation The microcapsules were prepared through the melt condensation method [28]. Compared with frequently-used interfacial polymerization [34,35] and in-situ polymerization [11,15], melt condensation dispersion is a physical method with simple process, high preparation efficiency and no chemical reaction occurs during the preparation. There were three main steps to prepare microcapsules: (1) the raw materials of shell (a. paraffin; b. paraffin/PE wax; c. nano-SiO2/paraffin/PE wax) were melted and dispersed; (2) TDI (core material) was added to the shell mixture, heated and stirred continuously; (3) PFTBA (cooling agent) was poured into the mixture to form microcapsules; (4) the suspension containing the microcapsules was oscillated with ultrasonic waves for 60 min; (5) the microcapsules were filtered and dried (40 °C, 24 h). Preparation parameters and shell/core mass ratio of various microcapsules were illustrated in Table 2. For example, the preparation procedure of microcapsules (MC3) was summarized in Fig. 2. 2.3. Measurement and characterization 2.3.1. Core fraction The core fraction of microcapsules was investigated according to the following steps [28]: Ⅰ. weighted the microcapsules which were filtered from the mixture; Ⅱ. added the deionized water into left mixture (PFTBA and un-encapsulated TDI), and some white precipitation were produced; Ⅲ. once the quantity of the sediments stayed stable (the TDI was completely reacted), filtered the sediments; Ⅳ. the remaining mixture could be layered automatically, the lower one was PFTBA while the upper one was water; Ⅴ. weighed the residue PFTBA. The core fraction (x) of microcapsules were measured through Eq. (1).

x¼

M1  ðm1  m2 Þ  100% M

ð1Þ

Table 1 Chemical composition of cement (%). Sample

SiO2

Al2O3

CaO

MgO

SO3

Fe2O3

Loss

Cement

24.06

6.34

59.89

0.98

2.24

3.56

1.31

M: mass of microcapsules; M1: original mass of TDI; m1: mass of un-encapsulated TDI and PFTBA (left mixture); m2: mass of residue PFTBA. 2.3.2. Compactness of microcapsules All batches of microcapsules were weighted directly after preparation. Then, the microcapsules were stored in a curing chamber (20 ± 2 °C, 50% RH). Weight loss of the microcapsule was monitored in time after 1 d, 3 d, 7 d, 10 d, 15 d, 30 d, 45 d and 60 d, respectively. The weight loss rate of the microcapsule (P) was calculated by Eq. (2).

P¼

M0  Mc  100% M0

ð2Þ

M0: the original mass of microcapsules; Mc: the present mass of microcapsules. 2.3.3. Size distributions of microcapsules The size distributions of prepared microcapsules were determined by a laser particle size analyzer (Mastersizer 2000, Malvern Instruments Ltd., England). 2.3.4. Morphologies of microcapsules The morphologies of microcapsules were observed through a scanning electron microscopy (SEM) (S-4800, Hitachi, Japan). Before SEM observation, some nano-SiO2/paraffin/PE wax microcapsules were cut through sharp blades to observe the shell thickness. All the intact and fractured microcapsules were mounted on a conductive adhesive tape and a thin layer of Pt was sputter-coated on the surface. The SEM images were taken at 3.0 kV accelerating voltage. 2.3.5. Fourier transform infrared spectroscopy (FTIR) A Fourier transform infrared spectrometer (Nexus, Thermo Nicolet Corporation, America) was used to characterize nanoSiO2, paraffin, PE wax, TDI and microcapsules. Among them, paraffin, PE wax, nano-SiO2 were compressed into slices after mixed with KBr, respectively. The TDI was tested as soon as it was brushed on KBr tablets. The microcapsules with KBr were compressed into slices after mixed and grinded, which were measured through FTIR immediately. With a resolution for 4 cm1, the FTIR spectra were recorded within the range of 400–4000 cm1 in absorption mode.

Table 2 Preparation parameters and shell/core mass ratio of various microcapsules. Sample

Raw Shell Material Using by Mass Ratio

Raw Core Material Using by Mass Ratio

Heating Temperature

Agitation Rate

Viscosity of Shell at Heating Temperature

MC1 MC2 MC3

Paraffin, 33 Paraffin, 16.5; PE Wax, 16.5 Paraffin, 15; PE Wax, 15; Nano-SiO2, 3

TDI, 67 TDI, 67 ‘TDI, 67

75 °C 120 °C 120 °C

600 rpm 800 rpm 800 rpm

3.6 mPas 10 mPas 40 mPas

Fig. 2. Preparation procedure of nano-SiO2/paraffin/PE wax composite shell microcapsules.

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2.3.6. Nanoindentation test Before the nanoindentation test, the test samples were prepared by following steps: firstly, the embedding agent (cold mounting resin) was poured into a U2.5 cm mold, then the microcapsules were added into it. After hardening of the embedding agent, the surface of the microcapsules embedded in the embedded agent was polished into a smooth sphere. The nanoindentation tests were performed by a Triboindenter (TI-900, Hysitron, America), and fitted with a Berkovich tip. The indenter contacted with the sample surface and had a calibrated trapezoidal load function, which was defined as the loading rate at 20.00 mN/min, the holding time of 5 s under the maximum load of 10 mN, and the unloading rate of 20.00 mN/min. The record of these mechanical values was plotted on a graph in order to create the load–displacement curve. The contact stiffness (S) was defined as the slope of the unloading curve which was calculated through Eq. (3).

S¼

dp 2 pffiffiffi ¼ Er A dh p

ð3Þ

The hardness H was defined as

H¼

Pmax A

ð4Þ

A: projected contact area at the peak load Pmax; Er: reduced elastic modulus. 2.3.7. Preparation of mortar with microcapsules As shown in Table 3, the mix designs of the mortars with microcapsules were illustrated. At first, the sand, cement and microcapsules were stirred for 120 s by a mortar agitator. Secondly, added the water and the mixture was stirred together for 120 s under a higher speed. In the third step, the mortars were poured into a mold with a size of 70.7 mm  70.7 mm  70.7 mm. The compaction method for mortars was mechanical vibration according to the ASTM C192-06. At last, the mortars were demolded and placed in a standard curing room (20 ± 2 °C, 95% RH) for 28 d. The microcapsules used to prepare the mortar were stored in the air for more than 60 d. 2.3.8. Pre-damage self-healing of mortars Each types of mortars were loaded with a compression tester (WYA-300, Xiyi, China) to determine its maximum compressive strength (denoted by fc0) for standard curing 28 d. The samples were then pre-damaged with loading strengths of 30% fc0, 60% fc0 and 80% fc0, respectively. The pre-load mortars were left for 1 d, 3 d, 7 d and 10 d self-healing in air, respectively. Finally, the samples were reloaded until destroying and the reserved strength were recorded. The average results per group were calculated by three samples. The recovery rate of compressive strength was measured according to formula (5).

g¼

Fig. 3. Schematic diagram of mortar pre-cracking by splitting test.

2.3.9. Cracks self-healing of mortars After 28 d of standard curing, some mortar samples were precracked via a splitting test through an automatic compression tester under the loading rate of 10 N/s. The schematic diagram of the pre-cracking process was shown in Fig. 3. The crack width of mortar with different composite shell microcapsules was tested through a crack test instrument before and after 4 h self-healing in air. 3. Results and discussion 3.1. Core fraction Fig. 4 showed that the core fraction of microcapsules containing TDI with paraffin, paraffin/PE wax or nano-SiO2/paraffin/PE wax composite shell. From Fig. 4, it could be observed that the core fraction of MC1, MC2, MC3 were 66.5%, 68.8%, 72.6%, respectively. This is mainly due to the different viscosities of the three kinds of microcapsule shell materials. The viscosities of paraffin/PE wax blends are higher than that of paraffin, thus reducing the dispersion of the materials. It decreases the time of cooling and solidification, improves the encapsulation capacity, and leads to the higher core fraction of microcapsules. Nano-SiO2 can further increase the viscosity of the paraffin/PE wax blends, reduce the dispersion of the materials, and improve the ability of shell materials to encapsulate TDI. 3.2. Compactness

f c1  100% f c0

ð5Þ

g: recovery rate of compressive strength; fc0: initial compressive strength of mortars; fc1: compressive strength of mortars with pre-damage after selfhealing.

Table 3 Mix designs of the mortars with microcapsules according to the mass ratio. Mix

Cement

Water

Sand

Microcapsules

AM0 AM1 AM2 AM3

100 100 100 100

50 50 50 50

300 300 300 300

0 3 (MC1) 3 (MC2) 3 (MC3)

Fig. 5 indicated the weight loss rate of microcapsules with different composite shells. From Fig. 5, it could be observed that the weight loss rate of prepared microcapsules increased rapidly from 1 d to 10 d, and then levelled off. Among them, the weight loss rates of MC1, MC2 and MC3 were 13.5%, 7.2% and 2.6% respectively after 60 d of preparation, which indicated that the compactness of MC3 was the best. This could be attributed to the high crystallinity of PE wax, while nano-SiO2 further improved the compactness of paraffin/PE wax. 3.3. Particle size distributions Particle size distributions of microcapsules with paraffin, paraffin/PE wax or nano-SiO2/paraffin/PE wax composite shell were

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Fig. 4. Core fraction of microcapsules with different shells.

Fig. 6. Particle size distributions of different microcapsules.

Table 4 Particle sizes of different microcapsules. Microcapsule

D10 values / lm

D50 values / lm

D90 values / lm

MC1 MC2 MC3

35 105 208

90 320 480

240 724 954

* The values of D10, D50 and D90 indicate that the volume diameter of microcapsules is less than 10%, 50% and 90% of that value, respectively.

Fig. 5. Weight loss rates of microcapsules with different shell composition.

illustrated in Fig. 6. The particle size distributions of MC1 were from 30 lm to 300 lm. For MC2, the particle size distributions were from 100 lm to 800 lm. The particle size distributions of MC3 were from 100 lm to 1100 lm, and it was mostly from 400 lm to 600 lm. The particle sizes values of D10, D50 and D90 of microcapsules with different shell compositions were shown in Table 4. As can be observed from Table 4, the average particle sizes of MC1, MC2 and MC3 were 90 lm, 320 lm and 480 lm, respectively. This can be explained by the fact that the viscosity of paraffin is lower than that of PE wax, and the paraffin is easier to disperse in the mixing process. Therefore, the particle size of MC1 is smaller than that of MC2 under the action of shear force. Nano-SiO2 can increase the viscosity of paraffin/PE wax mixture and reduce the dispersion of the mixture, thus increasing the particle size of microcapsules at a constant agitation rate. 3.4. Morphologies The surface morphologies of different microcapsules can be seen in Fig. 7. As shown in Fig. 7(a), the microcapsules (MC1) have regular globe shape, and the particle size distribution is within 75– 150 lm. Compared with MC1, it could be seen from Fig. 7(b) that microcapsules (MC2) have a rougher surface, and the particle size distribution is between 200 and 400 lm. The reason is that the viscosity of paraffin is lower than that of PE wax, and the

dispersibility of paraffin is better. Under the effect of stirring force, it is easy to form microcapsules with smaller particle size and smoother surface. In Fig. 7(c), it can be observed that the surface of MC3 is very rough with a particle size distribution of 400– 600 lm. This is mainly because the nano-SiO2 is added to the paraffin/PE wax mixture, which increases the viscosity of the mixture and makes its dispersion worse, resulting in rough surface and larger particle size of microcapsules. As can be seen from Fig. 7(d), the size of MC3 was about 480 lm and the shell thickness was 24.5 lm, which illustrated that MC3 had a low shell thickness/diameter rate (about 1/20) and could provide ideal storage capacity for encapsulating TDI.

3.5. FTIR analysis To prove the successful preparation of microcapsules, FTIR was used to characterize the prepared microcapsules. Fig. 8(a)–(e) illustrated the FTIR spectra of nano-SiO2, paraffin, PE wax, TDI and MC3, respectively. As shown in Fig. 8(a) and Fig. 8(e), the peaks at 1094 cm1 and 464 cm1 represented the symmetric stretching vibration characteristic and the flexural vibration characteristic peak of Si-O-Si of nano-SiO2. The peaks at 2939 cm1, 2860 cm1, as shown in Fig. 8 (b), (c), (e), were attributed to the asymmetric and symmetric stretching vibration of –CH2 and –CH3 groups of paraffin and PE wax. In Fig. 8 (d) and Fig. 8 (e), the absorption peak at 2270 cm1 is attributed to the asymmetric tensile vibration of –NCO, and 1464 cm1 is the skeleton vibration peak of the benzene ring in TDI. Moreover, since the chemical reactivity of TDI is strong, TDI may react with moisture in air and form –NHCOO groups before the FTIR test, so absorption peaks caused by N–H stretch vibration were found at 3302 cm1 and 1540 cm1[28].

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Fig. 7. SEM image of microcapsules: (a) MC1, (b) MC2, (c) MC3, (d) fractured MC3.

Fig. 9. Elastic modulus (E) and hardness (H) of microcapsules with different shell composition.

Fig. 8. FTIR spectra: (a) nano-SiO2, (b) paraffin, (c) PE wax, (d) TDI, (e) MC3.

Thus, the FTIR result indicated that the TDI had been coated into the paraffin/PE wax/nano-SiO2 shell successfully. 3.6. Micromechanical properties of microcapsules Fig. 9 described the elastic modulus and hardness of microcapsules measured by nanoindentation test. The elastic modulus of MC1, MC2 and MC3 were 0.48 GPa, 0.83 GPa and 1.87 GPa, respectively. The hardness of MC1, MC2 and MC3 were 4.06 MPa, 5.90 MPa and 61.67 MPa. The results showed that the elastic modulus and hardness of MC2 were higher than those of MC1. This can be explained that the molecular weight of PE wax is larger than that of paraffin, thus the intermolecular force is stronger and the movement of molecular chains is limited, which enhances the

Fig. 10. Force-displacement curves of different microcapsules.

W. Du et al. / Construction and Building Materials 231 (2020) 117060

strength of microcapsule shell materials accordingly. In addition, compared with the paraffin, the higher crystallinity of PE wax increases the elastic modulus and hardness of MC2 [36–38]. It could be also observed from Fig. 9 that the elastic modulus and hardness of MC3 are significantly higher than those of MC2. This is because the nano-SiO2 can transfer stress and disperse stress after blending with paraffin/PE wax according to the particle strengthening mechanism [39,40]. That is, when one molecular chain of PE wax is subjected to external force, nano-SiO2 can transfer the stress to other molecular chains and disperse the stress, so that even if the molecular chain breaks somewhere, other molecular chains can still withstand the stress, and the structure of the material is stronger, thus enhancing the strength of MC3. Force-displacement curves of microcapsules were shown in Fig. 10. The elastic modulus and hardness reflect the ability of

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microcapsules to resist plastic deformation caused by press. Normally, the material with higher elastic modulus and hardness is subjected to less elastic deformation [38]. In Fig. 10, it could be found that the displacement of MC3 is 2000 nm, while the displacement of MC2 and MC1 exceeds 8000 nm and 10000 nm, respectively. This could be attributed to the increase of the elastic modulus and hardness of microcapsule shell by nano-SiO2, thus reducing the displacement of the microcapsules. 3.7. Compressive strength The compressive strength of mortars containing different microcapsules for standard curing 28 d was demonstrated in Fig. 11. The compressive strength of AM0, AM1, AM2 and AM3 was 30.5 MPa, 39.1 MPa, 29.6 MPa and 28.6 MPa, respectively. Compared with AM0, the compressive strength of AM1 was increased by 28.2%, which indicated that moderate microcapsules significantly increased the compressive strength of mortar. The reason is that appropriate dosage of microcapsules could fill the internal pores of mortar, thereby increasing the compressive strength of mortar. Compared with AM0, the compressive strength of AM2 and AM3 was declined by 3% and 6.2%. This may be due to the large size of MC2 and MC3, which enlarges the internal pore of mortars, and reduces the compressive strength of mortars accordingly. 3.8. Pre-damage self-healing of mortars

Fig. 11. Compressive strength of mortars containing different microcapsules.

Fig. 12 showed the compressive strength recovery rate of mortars containing different microcapsules. As shown in Fig. 12(a), under all pre-loading conditions, the recovery rate of compressive strength of AM0 declined and did not increase with the prolongation of self-healing time. As the pre-loading raised from 30% fc0 to

Fig. 12. Compressive strength recovery rate of mortars containing different microcapsules. a. AM0, (b) AM1, (c) AM2, (d) AM3.

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60% fc0, the compressive strength recovery rates of AM1, AM2 and AM3 were increased. However, the recovery rates of mortars were declined once the pre-load raised to 80% fc0. The reason is that micro-cracks occurred in the mortar after pre-load. The microcapsules were ruptured under the stress of the micro-cracks tip, then TDI flowing into micro-cracks would react with moisture. The reaction product eventually self-healed micro-cracks. Nevertheless, when the preloading reached 80% fc0, a large number of microcracks occurred inside the mortar, which seriously damaged the internal structure of the mortar. In this case, it is difficult for microcapsules to self-heal all the micro-cracks, which ultimately results in a lower compressive strength of mortars. Fig. 12 also demonstrated that the compressive strength recovery rates of mortars changed with different self-healing period. For AM1, AM2 and AM3, the compressive strength recovery rate of pre-load mortar increased significantly from 1 d to 3 d, changed slightly from 3 d to 7 d, and remained unchanged from 7 d to 10

d. It indicated that the compressive strength of pre-load mortar had been repaired after 7 d, and the self-healing process had been completed. This can be explained by that, the TDI flowing out of the ruptured microcapsule could react with the moisture in the microcracks quickly, and form a healing product with network crosslinking structure to fill the micro-cracks and increase the recovery rate of compressive strength of mortar. Compared Fig. 12(b), (c) and (d), the highest compressive strength recovery rate of AM1, AM2 and AM3 was 78.3%, 82.4% and 87.8% respectively after 10 d. This is mainly because the initial core fraction of MC3 is higher than that of MC2 and MC1, and the compactness is better (the microcapsules used in the mortar predamage self-healing experiment were prepared 60 d earlier), resulting in the ultimate compressive strength recovery rate of AM3 is higher than that of AM2 and AM1. Moreover, since the mechanical properties of MC3 are better than MC1 and MC2, the risk of rupture during mortar mixing is avoided, and TDI is not lost

Fig. 13. Surface crack self-healing of mortars containing different microcapsules. (a) AM1 before self-healing, (b) AM1 for 4 h self-healing, (c) AM2 before self-healing, (d) AM2 for 4 h self-healing, (e) AM3 before self-healing, (f) AM3 for 4 h self-healing.

W. Du et al. / Construction and Building Materials 231 (2020) 117060

during the preparation process, thus improving the pre-damage self-healing capability of cementitious materials. 3.9. Cracks self-healing of mortars The widths of the self-healing cracks of different composite shell microcapsule mortars were tested. As shown in Fig. 13, cracks of AM1, AM2 and AM3 with initial widths of 0.1–0.28 mm, 0.1– 0.34 mm and 0.4–0.48 mm can be completely self-healed after 4 h. It indicates that the prepared microcapsules can effectively self-heal cracks in mortar, and the self-healed crack width of AM3 is wider than that of others within a certain time. The three kinds of microcapsules for crack self-healing experiment were prepared 60 d ago, and the compactness of MC3 was higher than that of MC1 and MC2, which resulted in the wider crack self-healing width of AM3. Meanwhile, the initial core fraction of MC3 is higher than that of MC1 and MC2, which is also conducive to the repair of wider cracks. In addition, MC3 has good mechanical properties, which avoids the risk of breaking during the mortar mixing and prevents the loss of TDI, thus improving the crack self-healing capability of cementitious materials. 4. Conclusions In this article, nano-SiO2/paraffin/PE wax composite shell microcapsules containing TDI for self-healing of cementitious materials were prepared. The core fraction, the compactness, the size distributions, the morphologies and micromechanical properties of microcapsules were measured. Self-healing ability of mortars containing microcapsules has been investigated. Some conclusions could be obtained from this research as below: 1) Compared microcapsules with paraffin shell, microcapsules with paraffin/PE wax composite shell had better encapsulation ability, compactness and mechanical properties. When nano-SiO2 was added into paraffin/PE wax composite shell, the encapsulation ability, mechanical properties and compactness of microcapsules were further improved. The core fraction, elastic modulus and hardness of the nanoSiO2/paraffin/PE wax composite shell microcapsules were 72.6%, 1.87 GPA and 61.67 MPa respectively, while the weight loss rate was only 2.6% after 60 d. 2) The particle size distributions of microcapsules with nanoSiO2/paraffin/PE wax shell was mainly from 400 lm to 600 lm. The microcapsules had a rough surface and the shell thickness was about 1/20 of the diameter. The result of FTIR indicated that TDI had been coated into the nanoSiO2/paraffin/PE wax composite shell successfully. 3) The mortar containing nano-SiO2/paraffin/PE wax composite shell microcapsules had higher compressive strength recovery rate (87.8%, 60% fc0 pre-load) with 10 d self-healing, and the width of surface cracks self-healed within 4 h was wider (less 0.48 mm), which indicated the microcapsules had good self-healing capability for cementitious materials. 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. Acknowledgments This research was supported by the National Key R&D Program of China (2017YFB0309905), thanks for the financial help.

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