- Email: [email protected]
O Pickering emulsions
Colloids and Surfaces A 584 (2020) 124073
Contents lists available at ScienceDirect
Colloids and Surfaces A journal homepage: www.elsevier.com/locate/colsurfa
Synthesis of cement shell microcapsules via W/O Pickering emulsions ⁎
T
Yumin Ren, Guangming Zhu , Jiaoning Tang College of Materials Science and Engineering, Shenzhen University, Shenzhen, 518060, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Cement shell microcapsule Pickering emulsion Self-healing cementitious materials
The synthesis of inorganic shell microcapsules is challenging in the field of self-healing cementitious materials. In this study, cement shell microcapsules were synthesized via hydration reaction of cement particles at the water-oil interface by using water-in-oil (W/O) Pickering emulsion method. The shell of the microcapsules is integrated with the cementitious materials, and incorporation of the microcapsules slightly effects the mechanical properties of cementitious materials.
1. Introduction Concrete is the most widely used construction materials in the world. However, the cracks caused by imposed deformations and external loads are unavoidable [1]. These cracks provide an easy path for harmful substances and thus reduce the durability of concrete structures [2]. Conventional repair method is deliberate external intervention after the cracks are discovered. However, the small and deep cracks are difficult to repair. Inspired by biological systems, the concept of self-healing cementitious materials was proposed, and it is especially useful to achieve repair of deep-micro cracks [3–5]. Self-healing cementitious materials containing microcapsules enclosing healing agents have attracted considerable attentions recently. However, most efforts have focused on organic shell microcapsules [6–11]. Compared with organic polymer shell materials, inorganic shell materials have advantages of high stability, high compactness and high strength. The core-shell structure is more stable, which can effectively improve the defects of the traditional organic shell microcapsules, such as poor compactness and easy loss of core materials. The size of microcapsules also plays an important role in the cementitious materials application. The microcapsules are required to have a large particle size (d > 100 μm) because of the porous structure of concrete. If the size of the microcapsules is smaller than the pore size, the microcapsules will not rupture under the tip stress of cracks, thus losing the self-healing effect. At present, the synthesis of inorganic shell microcapsules is carried out generally through the hydrolysis of tetraethyl silicate [12]. However, the particle size is very small, mostly below 10 μm, not exceeding 50 μm [13,14]. Normally, the synthesis of large-sized inorganic shell microcapsules adopts Pickering emulsion method [15]. Pickering emulsions is an emulsion that is stabilized by the solid particles rather than traditional polymeric surfactants. However, it is difficult to ⁎
crosslink between inorganic particles since there are no functional groups attached to the inorganic particles. So far, very few articles adopted the organic-inorganic composite method [16,17]. In this investigation, cement shell microcapsules with large particle size were synthesized via W/O Pickering emulsion method. The distribution of cement shell microcapsules in cementitious materials has been explored by XCT (X-ray computed tomography). Finally, the influences of cement shell microcapsules and urea-formaldehyde shell microcapsules on the mechanical properties of cementitious materials were compared. In addition, the approach will be applicable to the synthesis of other geopolymers shell microcapsules. 2. Experimental procedure 2.1. Synthesis of cement shell microcapsules Fig. 1 shows the synthetic route of the cement shell microcapsules, where C, S, H, and A refer to CaO, SiO2, H2O, and Al2O3, respectively. The synthetic procedure is described as follows: 0.8 g of organic montmorillonite (model is I.44 P, vacuum drying at 105 °C for 8–10 h before use, US Nanocor Co., LTD) was dispersed in 75 ml of liquid paraffin (Shanghai Maclean Biochemical Technology Co., LTD) and was stirred rapidly (500 r/min) for 15 min. Then, 1.6 g of low alkalinity sulphoaluminate cement (passed through a 500 mesh sieve, Guangzhou Zhujiang Cement Co., LTD) was added and stirring was continued until a uniform stable continuous phase was formed. Further, 7.5 ml of a sodium silicate solution (Jinan Jinhui Chemical Co., LTD) was slowly added drop wise, and was stirred (500 r/min) for 30 min to form a stable Pickering emulsion. The rotation speed was reduced (300 r/min) and the mixture was stirred at room temperature for 5 h. The final products were filtered, washed successively with toluene (Tianjin Baishi
Corresponding author. E-mail address: [email protected] (G. Zhu).
https://doi.org/10.1016/j.colsurfa.2019.124073 Received 21 August 2019; Received in revised form 30 September 2019; Accepted 2 October 2019 Available online 03 October 2019 0927-7757/ © 2019 Elsevier B.V. All rights reserved.
Colloids and Surfaces A 584 (2020) 124073
Y. Ren, et al.
Fig. 1. Schematic illustration of cement shell microcapsules via Pickering emulsion method.
and there are few free particles in the continuous phase. The emulsion system was still stable after several hours indicating that the organic montmorillonite has an excellent emulsification. Fig. 2(c) shows the dispersion of organic montmorillonite and cement particles in liquid paraffin. It can be seen that both adsorb each other to form a stable suspension. Fig. 2(d) shows the emulsion stabilized by the synergistic emulsification of organic montmorillonite and cement particles. The droplets show a good spherical shape. The inorganic particles were cross-linked to form a dense microcapsule shell through the hydration of cement particles, and the hydration reaction was controlled at the water-oil interface by interfacial tension. Fig. 2(e) and (f) display the morphology of the cement shell microcapsules. These microcapsules exhibit a good spherical shape with relatively smooth and compact shell. Fig. 2(g), (h), and (i) demonstrate that the microcapsules exhibit good hollow structure, and the shell thickness is about 15 μm. Since the shell material has been hydrophobized, it will not react with the core material sodium silicate solution, which is the key factor to the formation of hollow structure. The particle size distribution of the microcapsules is shown in Fig. 3(a), and the average particle size is 173.1 μm. Therefore, the cement shell microcapsules have a size suitable for self-healing cementitious materials. A phenolphthalein indicator was used to detect sodium silicate in the core material. As shown in Fig. 3(b), the whole microcapsules and broken microcapsules were respectively immersed in 2% phenolphthalein indicator solution. The left test tube shows the soaking solution of whole microcapsules, and it is colorless. The right one is a soaking solution of broken microcapsules, and it exhibits a purple-red color. Thus, it is evident that sodium silicate is present in the core material.
Chemical Co., LTD) and ethanol (Tianjin Baishi Chemical Co., LTD), and dried at room temperature for 2 h. The details of the synthesis of urea-formaldehyde shell microcapsules can be found in reference [18]. 2.2. Preparation of microcapsules and cementitious materials composite samples The samples were made by using microcapsules (the particle size is about 150 μm), ordinary Portland cement (CEMI 42.5 N), and deionised water. The mass ratio of deionised water to cement was always 1 : 4, and microcapsules were incorporated with different volume ratios, accounting for 2%, 4%, 8%, 16%, and 32% of the total volume of the samples, respectively. Moulds with dimensions of 4 cm × 4 cm × 4 cm and 4 cm × 4 cm × 16 cm were used. The former size was used for the compression test and the latter was used for the bending test. After casting, moulds were placed in a room with temperature 20 °C and relative humidity of more than 90% for 24 h. After demoulding, the samples were placed in the same room for 28 days. 2.3. Characterization The morphology of the emulsion droplets and the distribution of the inorganic particles in the continuous phase were obtained by optical microscopy (OM; JPL1350). The morphology of the microcapsules was obtained by scanning electron microscopy (SEM; Hitachi S4700). The particle size distribution of the microcapsules was determined by laser particle size distribution analyzer (BT-9300ST). The distribution of microcapsules in samples was obtained by three-dimensional reconstruction imaging X-ray microscope (XCT-400). The mechanical properties of samples were tested by microcomputer controlled universal material testing machine (CMT4204).
3.2. The distribution of cement shell microcapsules in samples In recent years, XCT has emerged out as a new promising non-destructive testing technology owing to its unique advantages and potential, especially in the study of microstructure of cementitious materials. In this paper, the raw XCT images were acquired at an accelerating voltage of 75 kV with an exposure time of 3 s. The resolution of CT scans was set to 7.5 μm. Fig. 3(c) is the XCT photograph of the sample containing microcapsules and the inset shows the samples only containing pores. The shell of the microcapsules can not be observed. The pores in sample look black, while the cores of the microcapsules in sample appear dark gray because of sodium silicate. There are less black spots in the sample containing microcapsules. It can be concluded that the microcapsule shell materials have been integrated
3. Results and discussion 3.1. Synthesis and characterization of cement shell microcapsules Fig. 2(a) shows the laminar microstructure of organic montmorillonite, which was used as a synergistic emulsifying particle during the preparation of Pickering emulsion. The performance parameters of organic montmorillonite are shown in Table 1. The micrograph of emulsion stabilized by organic montmorillonite is shown in Fig. 2(b). It can be observed that the droplets show a good spherical shape, the montmorillonite particles are adsorbed on the surface of the droplets, 2
Colloids and Surfaces A 584 (2020) 124073
Y. Ren, et al.
Fig. 2. (a) SEM image of organic montmorillonite; Optical images of (b) sodium silicate solution/organic montmorillonite emulsion, (c) organic montmorillonite and cement particles dispersed in liquid paraffin, (d) sodium silicate solution/organic montmorillonite and cement particles emulsion; (e–i) SEM images of cement shell microcapsules.
of the samples in different degrees, and the downward trend became more obvious with the increase of the microcapsules. The urea-formaldehyde shell microcapsules have a great influence on the mechanical properties of samples. The compressive and bending strengths of the sample were respectively about 24% and 22% of the blank sample when the urea-formaldehyde shell microcapsules accounts for 32% of the sample volume. This results from the low strength of urea-formaldehyde shell materials and the poor interfacial adhesion between the microcapsules and the cementitious materials. Contrarily, the incorporation of cement shell microcapsules has little influence on the mechanical properties of the samples. The compressive and the bending strengths of the sample were respectively about 75% and 78% of the blank sample when the cement shell microcapsules accounts for the same percentage of the sample volume. The decrease in strength is much smaller than that reported by Lv et al. [19]. The compressive strength of the sample was about 68% of the blank sample by incorporation of only 4% phenol-formaldehyde shell microcapsules into cement paste, which is similar to that reported by Wang et al. [20] when they used the same volume of microencapsulated bacterial spores. This means that the mechanical properties of the composite are highly dependent on the capsule type. Cement shell material is homogenous to the cementitious materials, with a high strength, integrated with the matrix, and the slight decrease in strength is caused by the core-shell structure of the microcapsules. Therefore, the cement shell microcapsules are of great significance for the load design and practical applications of self-healing cementitious materials.
Table 1 Performance parameters of organic montmorillonite. Average particle size (μm)
Layer spacing (nm)
Zeta potential (mV)
16-20
2.4-2.6
35.5
with the cementitious materials. For self-healing microcapsules in cementitious materials, if the interfacial adhesion between the microcapsules and the matrix is poor, the cracks would spread along the interface, bypassing the microcapsules rather than passing through the microcapsules and causing them to rupture. Therefore, the cement shell microcapsules fundamentally overcome the above issue. Fig. 3(d) shows a three-dimensional reconstruction photograph of the samples. It can be seen that the cement shell microcapsules are evenly distributed in the samples, which lays a foundation for further research on the application of microcapsules in cementitious materials.
3.3. The influence of microcapsules on the mechanical properties of cementitious materials Cementitious materials are always used to bear load. It is necessary to study the influence of microcapsules on the mechanical properties of cementitious materials. The influences of cement shell microcapsules and urea-formaldehyde shell microcapsules on the mechanical properties of cementitious materials were compared. The contribution of the core materials to the mechanical strength of the cementitious materials could be negligible since they are all liquid. As shown in Fig. 4, the incorporation of cement shell and urea-formaldehyde shell microcapsules reduced the compressive strength as well as bending strength
4. Conclusions In 3
summary,
cement
shell
microcapsules
are
successfully
Colloids and Surfaces A 584 (2020) 124073
Y. Ren, et al.
Fig. 3. (a) Size distribution of cement shell microcapsules; (b) Detection of core material by phenolphthalein indicator; (c) XCT photographs of microcapsules in samples, inset shows pores in the samples; (d) Three-dimensional reconstruction photograph of samples.
Fig. 4. The influence of microcapsules on the (a) compressive strength and (b) bending strength of cementitious materials.
References
synthesized by using the Pickering emulsion method, which features a guiding significance for the synthesis of large particle size inorganic shell microcapsules. The cement shell microcapsules could be integrated with the cementitious materials, and have little influence on the mechanical properties of matrix, with a very good application prospect in cementitious materials.
[1] H. Huang, G. Ye, Simulation of self-healing by further hydration in cementitious materials, Cement Concrete Comp. 34 (4) (2012) 460–467. [2] K.V. Tittelboom, N.D. Belie, Self-healing in cementitious materials—a review, Materials 6 (6) (2013) 2182–2217. [3] M.D. Rooij, K.V. Tittelboom, N.D. Belie, et al., Self-healing phenomena in cementbased materials, RILEM State-of-the-Art Rep. (2013) 11. [4] S.R. White, N.R. Sottos, P.H. Geubelle, et al., Autonomic healing of polymer composites, Nature 409 (6822) (2001) 794–797. [5] N.D. Belie, E. Gruyaert, A. Altabbaa, et al., A review of self-healing concrete for damage management of structures, Adv. Mater. Interfaces 5 (17) (2018) 1–28. [6] B. Hilloulin, K.V. Tittelboom, E. Gruyaert, et al., Design of polymeric capsules for self-healing concrete, Cement Concrete Comp. 55 (2015) 298–307. [7] L. Lv, Z. Yang, G. Chen, et al., Synthesis and characterization of a new polymeric microcapsule and feasibility investigation in self-healing cementitious materials, Constr. Build. Mater. 105 (2016) 487–495. [8] W. Li, Z. Jiang, Z. Yang, et al., Self-healing efficiency of cementitious materials containing microcapsules filled with healing adhesive: mechanical restoration and healing process monitored by water absorption, PLoS One 8 (11) (2013).
Declaration of Competing Interest The authors declare that there are no conflicts of interest. Acknowledgement This research is supported by National Natural Science Fundation of China (No. 51778369). 4
Colloids and Surfaces A 584 (2020) 124073
Y. Ren, et al.
[15] H. Liu, C. Wang, Q. Gao, et al., Magnetic hydrogels with supracolloidal structures prepared by suspension polymerization stabilized by Fe2O3 nanoparticles, Acta Biomater. 6 (1) (2010) 275–281. [16] G.H. Teo, Y.H. Ng, P.B. Zetterlund, et al., Factors influencing the preparation of hollow polymer-graphene oxide microcapsules via Pickering miniemulsion polymerization, Polymer 63 (2015) 1–9. [17] W. Chen, X. Liu, Y. Liu, et al., Synthesis of microcapsules with polystyrene/ZnO hybrid shell by Pickering emulsion polymerization, Colloid Polym. Sci. 288 (2010) 1393–1399. [18] F. Xing, N.X. Han, Z. Ni, et al., Self-healing mechanism of a novel cementitious composite using microcapsules, Proceedings of the International Conference on Durability of Concrete Structures, Hangzhou, China, 2008, pp. 26–27. [19] L. Lv, E. Schlangen, Z. Yang, et al., Micromechanical properties of a new polymeric microcapsule for self-healing cementitious materials, Materials 9 (12) (2016). [20] J. Wang, H. Soens, W. Verstraete, et al., Self-healing concrete by use of microencapsulated bacterial spores, Cement Concrete Res. 56 (2) (2014) 139–152.
[9] X. Wang, J. Zhang, W. Zhao, et al., Permeability and pore structure of microcapsulebased self-healing cementitious composite, Constr. Build. Mater. 165 (2018) 149–162. [10] B. Dong, G. Fang, Y. Wang, et al., Performance recovery concerning the permeability of concrete by means of a microcapsule based self-healing system, Cement Concrete Comp. 78 (2017) 84–96. [11] L. Lv, E. Schlangen, Z. Yang, et al., Micromechanical properties of a new polymeric microcapsule for self-healing cementitious materials, Materials 9 (12) (2016). [12] H. Zhang, S. Sun, X. Wang, et al., Fabrication of microencapsulated phase change materials based on n-octadecane core and silica shell through interfacial polycondensation, Colloids Surf. A Physicochem. Eng. Asp. 389 (1) (2011) 104–117. [13] H. Zhang, X. Wang, D. Wu, et al., Silica encapsulation of n-octadecane via sol-gel process: a novel microencapsulated phase-change material with enhanced thermal conductivity and performance, J. Colloid Interface Sci. 343 (1) (2010) 246–255. [14] W. Guo, Y. Jia, K. Tian, et al., UV-triggered self-healing of a single robust SiO2 microcapsule based on cationic polymerization for potential application in aerospace coatings, ACS Appl. Mater. Interface 8 (32) (2016) 21046–21054.
5
Contents lists available at ScienceDirect
Colloids and Surfaces A journal homepage: www.elsevier.com/locate/colsurfa
Synthesis of cement shell microcapsules via W/O Pickering emulsions ⁎
T
Yumin Ren, Guangming Zhu , Jiaoning Tang College of Materials Science and Engineering, Shenzhen University, Shenzhen, 518060, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Cement shell microcapsule Pickering emulsion Self-healing cementitious materials
The synthesis of inorganic shell microcapsules is challenging in the field of self-healing cementitious materials. In this study, cement shell microcapsules were synthesized via hydration reaction of cement particles at the water-oil interface by using water-in-oil (W/O) Pickering emulsion method. The shell of the microcapsules is integrated with the cementitious materials, and incorporation of the microcapsules slightly effects the mechanical properties of cementitious materials.
1. Introduction Concrete is the most widely used construction materials in the world. However, the cracks caused by imposed deformations and external loads are unavoidable [1]. These cracks provide an easy path for harmful substances and thus reduce the durability of concrete structures [2]. Conventional repair method is deliberate external intervention after the cracks are discovered. However, the small and deep cracks are difficult to repair. Inspired by biological systems, the concept of self-healing cementitious materials was proposed, and it is especially useful to achieve repair of deep-micro cracks [3–5]. Self-healing cementitious materials containing microcapsules enclosing healing agents have attracted considerable attentions recently. However, most efforts have focused on organic shell microcapsules [6–11]. Compared with organic polymer shell materials, inorganic shell materials have advantages of high stability, high compactness and high strength. The core-shell structure is more stable, which can effectively improve the defects of the traditional organic shell microcapsules, such as poor compactness and easy loss of core materials. The size of microcapsules also plays an important role in the cementitious materials application. The microcapsules are required to have a large particle size (d > 100 μm) because of the porous structure of concrete. If the size of the microcapsules is smaller than the pore size, the microcapsules will not rupture under the tip stress of cracks, thus losing the self-healing effect. At present, the synthesis of inorganic shell microcapsules is carried out generally through the hydrolysis of tetraethyl silicate [12]. However, the particle size is very small, mostly below 10 μm, not exceeding 50 μm [13,14]. Normally, the synthesis of large-sized inorganic shell microcapsules adopts Pickering emulsion method [15]. Pickering emulsions is an emulsion that is stabilized by the solid particles rather than traditional polymeric surfactants. However, it is difficult to ⁎
crosslink between inorganic particles since there are no functional groups attached to the inorganic particles. So far, very few articles adopted the organic-inorganic composite method [16,17]. In this investigation, cement shell microcapsules with large particle size were synthesized via W/O Pickering emulsion method. The distribution of cement shell microcapsules in cementitious materials has been explored by XCT (X-ray computed tomography). Finally, the influences of cement shell microcapsules and urea-formaldehyde shell microcapsules on the mechanical properties of cementitious materials were compared. In addition, the approach will be applicable to the synthesis of other geopolymers shell microcapsules. 2. Experimental procedure 2.1. Synthesis of cement shell microcapsules Fig. 1 shows the synthetic route of the cement shell microcapsules, where C, S, H, and A refer to CaO, SiO2, H2O, and Al2O3, respectively. The synthetic procedure is described as follows: 0.8 g of organic montmorillonite (model is I.44 P, vacuum drying at 105 °C for 8–10 h before use, US Nanocor Co., LTD) was dispersed in 75 ml of liquid paraffin (Shanghai Maclean Biochemical Technology Co., LTD) and was stirred rapidly (500 r/min) for 15 min. Then, 1.6 g of low alkalinity sulphoaluminate cement (passed through a 500 mesh sieve, Guangzhou Zhujiang Cement Co., LTD) was added and stirring was continued until a uniform stable continuous phase was formed. Further, 7.5 ml of a sodium silicate solution (Jinan Jinhui Chemical Co., LTD) was slowly added drop wise, and was stirred (500 r/min) for 30 min to form a stable Pickering emulsion. The rotation speed was reduced (300 r/min) and the mixture was stirred at room temperature for 5 h. The final products were filtered, washed successively with toluene (Tianjin Baishi
Corresponding author. E-mail address: [email protected] (G. Zhu).
https://doi.org/10.1016/j.colsurfa.2019.124073 Received 21 August 2019; Received in revised form 30 September 2019; Accepted 2 October 2019 Available online 03 October 2019 0927-7757/ © 2019 Elsevier B.V. All rights reserved.
Colloids and Surfaces A 584 (2020) 124073
Y. Ren, et al.
Fig. 1. Schematic illustration of cement shell microcapsules via Pickering emulsion method.
and there are few free particles in the continuous phase. The emulsion system was still stable after several hours indicating that the organic montmorillonite has an excellent emulsification. Fig. 2(c) shows the dispersion of organic montmorillonite and cement particles in liquid paraffin. It can be seen that both adsorb each other to form a stable suspension. Fig. 2(d) shows the emulsion stabilized by the synergistic emulsification of organic montmorillonite and cement particles. The droplets show a good spherical shape. The inorganic particles were cross-linked to form a dense microcapsule shell through the hydration of cement particles, and the hydration reaction was controlled at the water-oil interface by interfacial tension. Fig. 2(e) and (f) display the morphology of the cement shell microcapsules. These microcapsules exhibit a good spherical shape with relatively smooth and compact shell. Fig. 2(g), (h), and (i) demonstrate that the microcapsules exhibit good hollow structure, and the shell thickness is about 15 μm. Since the shell material has been hydrophobized, it will not react with the core material sodium silicate solution, which is the key factor to the formation of hollow structure. The particle size distribution of the microcapsules is shown in Fig. 3(a), and the average particle size is 173.1 μm. Therefore, the cement shell microcapsules have a size suitable for self-healing cementitious materials. A phenolphthalein indicator was used to detect sodium silicate in the core material. As shown in Fig. 3(b), the whole microcapsules and broken microcapsules were respectively immersed in 2% phenolphthalein indicator solution. The left test tube shows the soaking solution of whole microcapsules, and it is colorless. The right one is a soaking solution of broken microcapsules, and it exhibits a purple-red color. Thus, it is evident that sodium silicate is present in the core material.
Chemical Co., LTD) and ethanol (Tianjin Baishi Chemical Co., LTD), and dried at room temperature for 2 h. The details of the synthesis of urea-formaldehyde shell microcapsules can be found in reference [18]. 2.2. Preparation of microcapsules and cementitious materials composite samples The samples were made by using microcapsules (the particle size is about 150 μm), ordinary Portland cement (CEMI 42.5 N), and deionised water. The mass ratio of deionised water to cement was always 1 : 4, and microcapsules were incorporated with different volume ratios, accounting for 2%, 4%, 8%, 16%, and 32% of the total volume of the samples, respectively. Moulds with dimensions of 4 cm × 4 cm × 4 cm and 4 cm × 4 cm × 16 cm were used. The former size was used for the compression test and the latter was used for the bending test. After casting, moulds were placed in a room with temperature 20 °C and relative humidity of more than 90% for 24 h. After demoulding, the samples were placed in the same room for 28 days. 2.3. Characterization The morphology of the emulsion droplets and the distribution of the inorganic particles in the continuous phase were obtained by optical microscopy (OM; JPL1350). The morphology of the microcapsules was obtained by scanning electron microscopy (SEM; Hitachi S4700). The particle size distribution of the microcapsules was determined by laser particle size distribution analyzer (BT-9300ST). The distribution of microcapsules in samples was obtained by three-dimensional reconstruction imaging X-ray microscope (XCT-400). The mechanical properties of samples were tested by microcomputer controlled universal material testing machine (CMT4204).
3.2. The distribution of cement shell microcapsules in samples In recent years, XCT has emerged out as a new promising non-destructive testing technology owing to its unique advantages and potential, especially in the study of microstructure of cementitious materials. In this paper, the raw XCT images were acquired at an accelerating voltage of 75 kV with an exposure time of 3 s. The resolution of CT scans was set to 7.5 μm. Fig. 3(c) is the XCT photograph of the sample containing microcapsules and the inset shows the samples only containing pores. The shell of the microcapsules can not be observed. The pores in sample look black, while the cores of the microcapsules in sample appear dark gray because of sodium silicate. There are less black spots in the sample containing microcapsules. It can be concluded that the microcapsule shell materials have been integrated
3. Results and discussion 3.1. Synthesis and characterization of cement shell microcapsules Fig. 2(a) shows the laminar microstructure of organic montmorillonite, which was used as a synergistic emulsifying particle during the preparation of Pickering emulsion. The performance parameters of organic montmorillonite are shown in Table 1. The micrograph of emulsion stabilized by organic montmorillonite is shown in Fig. 2(b). It can be observed that the droplets show a good spherical shape, the montmorillonite particles are adsorbed on the surface of the droplets, 2
Colloids and Surfaces A 584 (2020) 124073
Y. Ren, et al.
Fig. 2. (a) SEM image of organic montmorillonite; Optical images of (b) sodium silicate solution/organic montmorillonite emulsion, (c) organic montmorillonite and cement particles dispersed in liquid paraffin, (d) sodium silicate solution/organic montmorillonite and cement particles emulsion; (e–i) SEM images of cement shell microcapsules.
of the samples in different degrees, and the downward trend became more obvious with the increase of the microcapsules. The urea-formaldehyde shell microcapsules have a great influence on the mechanical properties of samples. The compressive and bending strengths of the sample were respectively about 24% and 22% of the blank sample when the urea-formaldehyde shell microcapsules accounts for 32% of the sample volume. This results from the low strength of urea-formaldehyde shell materials and the poor interfacial adhesion between the microcapsules and the cementitious materials. Contrarily, the incorporation of cement shell microcapsules has little influence on the mechanical properties of the samples. The compressive and the bending strengths of the sample were respectively about 75% and 78% of the blank sample when the cement shell microcapsules accounts for the same percentage of the sample volume. The decrease in strength is much smaller than that reported by Lv et al. [19]. The compressive strength of the sample was about 68% of the blank sample by incorporation of only 4% phenol-formaldehyde shell microcapsules into cement paste, which is similar to that reported by Wang et al. [20] when they used the same volume of microencapsulated bacterial spores. This means that the mechanical properties of the composite are highly dependent on the capsule type. Cement shell material is homogenous to the cementitious materials, with a high strength, integrated with the matrix, and the slight decrease in strength is caused by the core-shell structure of the microcapsules. Therefore, the cement shell microcapsules are of great significance for the load design and practical applications of self-healing cementitious materials.
Table 1 Performance parameters of organic montmorillonite. Average particle size (μm)
Layer spacing (nm)
Zeta potential (mV)
16-20
2.4-2.6
35.5
with the cementitious materials. For self-healing microcapsules in cementitious materials, if the interfacial adhesion between the microcapsules and the matrix is poor, the cracks would spread along the interface, bypassing the microcapsules rather than passing through the microcapsules and causing them to rupture. Therefore, the cement shell microcapsules fundamentally overcome the above issue. Fig. 3(d) shows a three-dimensional reconstruction photograph of the samples. It can be seen that the cement shell microcapsules are evenly distributed in the samples, which lays a foundation for further research on the application of microcapsules in cementitious materials.
3.3. The influence of microcapsules on the mechanical properties of cementitious materials Cementitious materials are always used to bear load. It is necessary to study the influence of microcapsules on the mechanical properties of cementitious materials. The influences of cement shell microcapsules and urea-formaldehyde shell microcapsules on the mechanical properties of cementitious materials were compared. The contribution of the core materials to the mechanical strength of the cementitious materials could be negligible since they are all liquid. As shown in Fig. 4, the incorporation of cement shell and urea-formaldehyde shell microcapsules reduced the compressive strength as well as bending strength
4. Conclusions In 3
summary,
cement
shell
microcapsules
are
successfully
Colloids and Surfaces A 584 (2020) 124073
Y. Ren, et al.
Fig. 3. (a) Size distribution of cement shell microcapsules; (b) Detection of core material by phenolphthalein indicator; (c) XCT photographs of microcapsules in samples, inset shows pores in the samples; (d) Three-dimensional reconstruction photograph of samples.
Fig. 4. The influence of microcapsules on the (a) compressive strength and (b) bending strength of cementitious materials.
References
synthesized by using the Pickering emulsion method, which features a guiding significance for the synthesis of large particle size inorganic shell microcapsules. The cement shell microcapsules could be integrated with the cementitious materials, and have little influence on the mechanical properties of matrix, with a very good application prospect in cementitious materials.
[1] H. Huang, G. Ye, Simulation of self-healing by further hydration in cementitious materials, Cement Concrete Comp. 34 (4) (2012) 460–467. [2] K.V. Tittelboom, N.D. Belie, Self-healing in cementitious materials—a review, Materials 6 (6) (2013) 2182–2217. [3] M.D. Rooij, K.V. Tittelboom, N.D. Belie, et al., Self-healing phenomena in cementbased materials, RILEM State-of-the-Art Rep. (2013) 11. [4] S.R. White, N.R. Sottos, P.H. Geubelle, et al., Autonomic healing of polymer composites, Nature 409 (6822) (2001) 794–797. [5] N.D. Belie, E. Gruyaert, A. Altabbaa, et al., A review of self-healing concrete for damage management of structures, Adv. Mater. Interfaces 5 (17) (2018) 1–28. [6] B. Hilloulin, K.V. Tittelboom, E. Gruyaert, et al., Design of polymeric capsules for self-healing concrete, Cement Concrete Comp. 55 (2015) 298–307. [7] L. Lv, Z. Yang, G. Chen, et al., Synthesis and characterization of a new polymeric microcapsule and feasibility investigation in self-healing cementitious materials, Constr. Build. Mater. 105 (2016) 487–495. [8] W. Li, Z. Jiang, Z. Yang, et al., Self-healing efficiency of cementitious materials containing microcapsules filled with healing adhesive: mechanical restoration and healing process monitored by water absorption, PLoS One 8 (11) (2013).
Declaration of Competing Interest The authors declare that there are no conflicts of interest. Acknowledgement This research is supported by National Natural Science Fundation of China (No. 51778369). 4
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