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Effect of polyvinyl alcohol powder on the bonding mechanism of a new magnesium phosphate cement mortar
Construction and Building Materials 239 (2020) 117871
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Effect of polyvinyl alcohol powder on the bonding mechanism of a new magnesium phosphate cement mortar Yudong Xie, Xujian Lin ⇑, Hanfei Li, Tao Ji College of Civil Engineering, Fuzhou University, Fuzhou 350116, China
h i g h l i g h t s PVA can significantly improve the bonding performance between MSPM and OPM. PVA film can encapsulate the surface of Ca(OH)2 and ettringite crystals at the interface. The covering effect of PVA film will affect the normal process of hydration.
a r t i c l e
i n f o
Article history: Received 11 July 2019 Received in revised form 8 December 2019 Accepted 13 December 2019
Keywords: MSPM PVA powder Bonding mechanism Bonding interface
a b s t r a c t Silica-Magnesium Potassium Phosphate cement mortar (MSPM) which is made up of dead burnt magnesia powder, Dipotassium phosphate K2HPO43H2O (P), silica fume and a certain proportion of ISO standard sand, is used as repairing materials helping study the bonding performance between MSPM and ordinary Portland cement mortar (OPM). In this research, the effect of polyvinyl alcohol powder on MSPM bonding strength (flexural bonding strength and flexural two-sided bonding strength) was investigated. The bonding mechanism was studied with X-ray diffraction (XRD), Mercury intrusion porosimetry (MIP) and environmental scanning electron microscopy (ESEM). The experimental results show that PVA powder has good film forming properties. The PVA film formed by PVA powder will encapsulate reactants (silica fume, magnesium oxide) and hydration products (struvite-K) in MSPM system, and form structuring O-Mg-O with Mg2+. In addition, the PVA film encapsulates the surface of Ca(OH)2 and ettringite crystals at the interface, and complexes with Ca2+ to form O-Ca-O molecular chain structure, reducing the number of crystals at the interface. When the PVA content reaches 0.4%, the bonding performance between the OPM and the MSPM is greatly improved due to the good bonding properties of the PVA film and the ionic complexation reaction occurring at the bonding interface. However, when the content of PVA reaches 0.6%, PVA cannot be uniformly dispersed in the reaction system due to agglomeration. In addition, the covering effect of PVA film has affected the normal progress of hydration reaction, so the improvement in bonding performance at the bonding interface is limited. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction In daily life, there is a large number of service concrete structure is destroyed phenomenon. The damaged concrete is in urgent need of repair and reinforcement, otherwise there are serious safety risks. Generally speaking, these repair materials used to repair damaged concrete structures need to reach a high bond strength with the matrix in a short time to ensure a strong bond between the old and new materials [1]. In recent years, many new repair materials [2,3] have been applied in the field of concrete structure repair. Commonly used repair materials mainly include: ordinary ⇑ Corresponding author. E-mail address: [email protected] (X. Lin). https://doi.org/10.1016/j.conbuildmat.2019.117871 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.
Portland cement mortar and concrete [4]; Special cement, such as alkali activated cement [5] and sulphoaluminate cement [6]; Organic polymer materials, such as epoxy resin [7–9]. However, these repair materials have obvious shortcomings, such as low bond strength, slow development of early strength, poor compatibility with the matrix, and the existence of safety problems (e.g., water seepage caused by cracking). The defects of these repair materials are also an important reason for the failure in a large number of concrete repair projects. Especially when the temperature changes greatly, due to the poor volume compatibility of the new and old materials, the interface will crack, resulting in the failure of repair. This is due to the different thermal expansion coefficient between the old and new materials. As a new type of environment-friendly cement-based
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Y. Xie et al. / Construction and Building Materials 239 (2020) 117871
repairing material, magnesium phosphate cement (MPC) has similar thermal expansion coefficient to ordinary concrete and good bonding performance [10–13]. Many scholars have studied the repair effect of MPC. Ding et al. [14] studied the effect of polymer powder on the performance of MPC mortar. The results show that the bonding performance and bending strength of the hardened samples are improved. Ma et al. [15] studied the effect of redispersible emulsion powder (RPP) on comprehensive properties of magnesium potassium phosphate cement (MKPC) mortar. The results show that the bending strength and bond strength of MKPC mortar are improved due to the high viscosity of RPP. It is well-known that in addition to the mechanical strength of repair materials, the bonding performance between old and new concrete is also a key index to evaluate the repair quality [16,17]. The bonding properties include the chemical compatibility, bulk compatibility and bonding strength of the bonding interface. OPC mortar [11] has good volume compatibility with MPC mortar. This is because the drying shrinkage rate of OPC mortar is 6 104/°C, and the thermal expansion coefficient is 7 106/°C. The dry shrinkage rate of MPC mortar measured under the same conditions is 0.34 104/°C, and the thermal expansion coefficient is 9.6 106/°C. This shows that the thermal expansion performance of OPC mortar is similar to that of MPC mortar, and there is little difference in volume change between them when the temperature changes [18]. Therefore, Abdelrazig et al. [19] think that the main reason for the good compatibility of MPC mortar and OPC mortar is that the volume fraction of hydration products of MPC is small and the hydration product of the struvite-K belongs to the crystal, while the hydration product of OPC is hydrated calcium silicate gel. In actual repairing engineering, due to the different parts of structural damage, it is usually necessary to repair in different directions. Therefore, different test methods are needed to accurately evaluate the bonding performance of the interface. Researchers often use square bonding plate, ‘‘800 glyph, cylindrical core drawing and other ways [20,21] to test the bonding properties between old and new materials. Qian et al. [22] studied a new method for testing the bond properties of structures, which is using a circular platform to study the bond properties of structures. The outer side of the circular table is poured with old matrix material, and the inner side of the circular table is reinforced with reinforcement material. This method can be used to measure the volume stability of the repair material and the effect of drying shrinkage on the bonding properties of the material. In today’s building materials industry, a large number of polymers are used to improve the properties of cement-based materials. Common polymers include vinyl acetate [24,25], polyvinyl alcohol [26–30] and styrene-acrylic emulsion [23], where polyvinyl alcohol is one of the main polymers. Due to its excellent chemical and physical properties, PVA is used as the modifier of cement-based materials, mainly to improve the workability of cement-based materials [26,27]. Current studies show that PVA can significantly improve the bonding performance of cementbased materials [13,31]. PVA, which is a kind of water-soluble polymer, will dehydrate and form a highly viscous PVA film during cement hydration [28,32]. Although PVA has been widely used in the modification of OPC, the modification mechanism in MPC system is uncertain. Wang et al. [33] found that PVA fiber can improve the flexural strength and fracture energy of MPC, but the existing form of PVA powder (not fiber) is also very important for studying the modification mechanism (especially the bonding mechanism). Different forms of existence may lead to different test conclusions. Therefore, when studying the bonding mechanism of MPC, it is necessary to make further research on the effect of adding PVA powder. To sum up, a large number of studies have shown that MPC can be used as the repair material for the repair of concrete, and adding
an appropriate amount of PVA can improve the bonding performance of the repair material. However, no one has studied the bonding performance of MSPM mortar prepared with K2HPO43H2O (not KH2PO4), PVA powder (not fiber) and silica powder. In this paper, flexural bonding strength test and flexural two-sided bonding strength test were used to study the bonding properties of MSPM and OPM, and the influence of PVA powder on MSPM bond properties was studied. In addition, the bonding mechanism of MSPM system was analyzed by means of pore structure tester, XRD and ESEM-EDS 2. Materials The magnesium oxide for experiment was supplied by Liaoning Haicheng Qunli Mining Co., Ltd. China, which was produced by calcining magnesite at 1500 . The magnesium oxide was industrial pure grade, the MgO content was more than 95%, and the average particle size of magnesium oxide in the experiment was 33.29 lm. The K2HPO43H2O used was chemically pure grade, which was manufactured by Xilong Chemical Co., Ltd. China. DHP (K2HPO43H2O, P) that was analytical grade with 99% K2HPO43H2O content was provided by Xilong Chemical Co., Ltd. (Fuzhou, China). Silica fume was obtained from Xining Ferroalloy Factory with SiO2 content of 90%, and its average particle size was 0.1– 0.2 lm. Polyvinyl alcohol (abbreviated as PVA) was a white powder, which was provided by Shanghai Yanye Industrial Co., Ltd. It had excellent film-forming property and cohesiveness. It had a fineness of 160 mesh and a viscosity of 7000 mPas, and the pH of the PVA after mixing with water was measured by a pH meter of 5.6. ISO standard sand was used for MSPM; river sand and 42.5 ordinary Portland cement were used for OPM, and each performance index of river sand was measured according to GB/T 14684-2001 [34], as shown in Table 1. The tap water taken from Fuzhou (China) was used. The main chemical components of magnesia and silica fume are shown in Table 2. 3. Experimental 3.1. Mix proportion The sand-to-binder ratio (S/C) was 0.14, the mass ratio (P/M) of 1/3, the water-to-binder ratio (W/C) was 0.14 and the silica fume content was 15%. The mass ratio of PVA to gelling materials was varied. P/M refers to the mass ratio of n(K2HPO43H2O)/n(MgO), and W/C indicates the mass ratio of water consumption (crystal water in hydrated DHP should be considered) to gelling materials (DHP, silica fume, magnesium oxide and PVA). The PVA contents of P-0, P-1, P-2 and P-3 were 0%, 0.2%, 0.4% and 0.6%, respectively. Additionally, sand-binder ratio (S/C) and water-binder ratio (W/C) of OPM were 2 and 0.4, respectively; and the compressive strength and flexural strength of OPM at 28 days were 47.75 MPa and 12.25 MPa, respectively. 3.2. Test methods 3.2.1. Flexural bonding strength test After mixing magnesium oxide, silica fume and PVA powder in cement mortar mixer, the prepared K2HPO4 solution was added to the mixer for 60 s and then add sand for 60 s to get MSPM. The OPM test block of 40 mm 40 mm 80 mm was maintained in a 40 mm 40 mm 160 mm mold for 28 days (20 ± 2 , 95% >RH), and then the prepared MSPM repair mortar was poured into the mold to form new and old mortar joint specimens. According to the provisions of GB/T17671-1999 [35], the flexural bond strength of the samples was tested by the electro-hydraulic servo pressure
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Y. Xie et al. / Construction and Building Materials 239 (2020) 117871 Table 1 Performance index of the river sand. Bulk density (kg/m3)
Fineness modulus
Water absorption (%)
Apparent density (kg/m3)
Mud Content (%)
1480
2.5
0.1
2591
3.53
Table 2 Chemical composition (% wt.).
Magnesia Silica fume
SiO2
Al2O3
Fe2O3
MgO
CaO
K2O
F.C
LOI
2.92 97.57
2.34 0.06
1.3 0.02
93.06 0.05
0.22 0.03
– 0.78
– 0.38
0.16 2.26
testing machine controlled by microcomputer (YAW 4000). The samples are shown in Fig. 1(a) and (b). The formula for calculating the flexural bond strength is as follows Formula (1):
r¼
2
M Fðl - 20Þ bh ¼ = ¼ 3:75F W 2 6
ð1Þ
where r represents the flexural bonding strength (MPa); F is the maximum load of cuboid specimen when it breaks (kN); L represents the distance between two fulcrums (L=100 mm); h is the width of cuboid specimen (h=40 mm); b stands for the height of cuboid specimen (b=40 mm). 3.2.2. Flexural two-sided bonding strength test Two ordinary cement mortar blocks (40 mm 40 mm 75 mm) maintained for 28 days were put into 40 mm 40 mm 160 mm mold with a 10 mm interval between the two blocks, and then the fresh MSPM was poured into the gap. After hardening, the mold was removed and cured to the prescribed age. The testing process is the same as section 3.2.1. The test specimens are shown in Fig. 2(a) and (b). The flexural twosided bonding strength was calculated by the formula according to Formula (1). 3.2.3. XRD and ESEM-EDS tests In order to analyze the phases presented in MSPM, small dry particle samples are taken from the oven and ground into fine powder (less than 100 mesh). XRD which was made by PANalytical, the Netherlands. The localized morphology of the interface of OPM and MSPM was observed in ESEM (Quanta 250, FEI, USA). The energy dispersive spectrometer (EDS) was XFlash6T|30 manufactured by Bruker Corp. (USA). In the ESEM/EDS test, samples were selected from near the bonding interface. 3.2.4. Pore structure test In this study, the pore structure analysis adopted mercury injection analysis (MIP, PoreMaster-60) to determine the pore diameter
and porosity characteristics of mortar samples. The maximum pressure value of the high pressure was set as 55,000 psia. Sample preparation: after the test specimen is broken, particles with a diameter of about 3 mm are selected and placed in liquid nitrogen to terminate hydration. Before the test, they are placed in a vacuum drying box (60 ) and dried in vacuum for 1 month. Then the pore structure test is carried out. 4. Results and discussion 4.1. Flexural bonding strength Fig. 3 shows the effect of PVA content on the flexural bond strength of MSPM. When the PVA content is 0.2%, 0.4% and 0.6%, the flexural bond strength of PMSPM in each group at 28 days of age is 15.98 MPa, 16.75 MPa and 17.1 MPa, respectively. Compared with 14.21 MPa in the control group, the flexural bond strength increases by up to 20.34%. This is because the PVA film has very strong bonding properties, the tensile bonding strength of MSPM increases gradually with the increase in PVA content. However, when the content of PVA exceeds 0.4%, the strengthening effect of PVA is not so obvious, and the value of flexural bond strength is basically stable. Additionally, at the early stage of hydration reaction, the bonding strength is lower because of the less gel material generated in the MSPM reaction system. At this time, PVA can greatly improve the bond strength of MSPM. However, with the increase in age, the bonding phases provided by hydration products increase in MSPM system, so the effect of PVA on bonding strength is not as obvious as that of early stage. 4.2. Flexural two-sided bonding strength Fig. 4 shows the effect of PVA on the flexural double-sided bond strength of MSPM. When the content of PVA was 0.2%, 0.4% and 0.6%, the flexural double-sided bond strength of each group was 10.97 MPa, 11.48 MPa and 11.76 MPa, respectively. Compared with
OPM
(a) Force diagram
MSPM
(b) Specimens Fig. 1. Flexural bonding strength test.
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10 mm
OPM MSPM OPM
(a) Force diagram
(b) Specimens
Fig. 2. Flexural two-sided bonding strength test.
bonding strength. However, after the addition of PVA, the flexural double-sided bond strength of each group at 56 days of age was basically unchanged compared with that at 28 days of age. Therefore, PVA can effectively improve the bonding performance of MSPM. In general, the influence of different PVA content on the flexural double-sided bond strength of MSPM is basically the same as that on the flexural bond strength.
Flexural bonding strength (MPa)
18 17 16 15 14 3d 7d 28d 56d
13 12 11 0.0
0.1
0.2
0.3
0.4
0.5
0.6
The content of PVA (%)
Flexural bi-sided bonding strength (MPa)
Fig. 3. Influence of PVA content on flexural bonding strength of MSPM.
12 11 3d 7d 28d 56d
10 9 8 7 6 0.0
0.1
0.2 0.3 0.4 0.5 The content of PVA (%)
0.6
Fig. 4. Influence of PVA content on the flexural two-sided bonding strength of MSPM.
the P-0 group of 9.56 MPa, the flexural bond strength increased by up to 2.2 MPa. As the age increases, the bending strength of P-0 group was 6.89 MPa, 9.31 MPa, 9.56 MPa and 8.8 MPa, respectively. The flexural double-sided bond strength of P-0 group increased by 35.12% in the early stage. After 7 days of age, the bond strength decreased slightly at 56 days, which was 7.9% lower than that at 28 days. This may be due to the increase of shrinkage caused by the secondary hydration reaction, resulting in a slight decrease in
4.3. ESEM/EDS analysis Fig. 5(a) shows the interface between OPM and MSPM of P-0 group. It shows that the interface between OPM and MSPM is dense, and a small amount of MgKPO46H2O combined with unreacted MgO [37] to form a dense structure. However, there are holes near the interface of MSPM. This may be due to the poor fluidity of MSPM slurry, the less compact bonding at the interface and the occurrence of cracks perpendicular to the bonding interface near the voids. Fig. 5(b) is an enlarged view of the interface gap in Fig. 5(a). It can be found that a large amount of ettringite and Ca(OH)2 crystals appear at the bonding interface of OPM, and the crystals extend along the gap towards the interface near MSPM. Additionally, it can be seen from Fig. 5(b) that many irregular rod crystals appear near the bonding interface of OPM. EDS analysis of the irregular crystals selected in Fig. 5(c) shows the results as shown in Fig. 5(d). The results show that the composition of the crystal is complex, and the contents of K+ and Ca2+ are relatively high. Due to the free Ca2+ in OPM system + can diffuses into MSPM system, Mg2+, PO3 4 and K in MSPM system also diffuses into OPM system, and finally forms an ion diffusion layer composed of calcium phosphate compounds, which enhances the complexation reaction between ions at the interface and improves the chemical bonding effect [38,39]. As a result, the bonding properties at the interface has been enhanced. Through the above analysis, the schematic diagram of bond layer structure can be obtained, as shown in Fig. 6. Fig. 7(a) shows the bonding interface between OPM and MSPM in P-2 group. The experimental results show that the bonding between OPM and MSPM is good. Compared with Fig. 5(a), there are no holes in the bonding interface, and the bonding between MSPM and OPM is more compact, and there are no cracks perpendicular to the bonding interface. This is because the PVA particles aggregate at the bonding interface [26,28], and fill in the capillary holes at the bonding interface, improving the density of the interface. By comparing Figs. 5(b) and 7(b), it can be found that there were a large number of ettringite crystals in the pores of the interface of the control group, but the content of crystals in the interface decreased significantly after PVA was added. This is due to the film-forming effect of the PVA, which covers on the surface of
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Y. Xie et al. / Construction and Building Materials 239 (2020) 117871
Crack AFt
MSPM
Interface
OPM
(a)
(b)
(c)
(d) Fig. 5. ESEM/EDS of bond interface of group P-0.
excellent bonding properties [28,32], so the bonding properties of P-1 group, P-2 group and P-3 group are higher than those of P-0 group. 4.4. XRD analysis
Fig. 6. Bonding interface model diagram of MSPM.
the crystal [36], leading to decrease in the crystal morphology. In addition, in alkaline environment, the –OH in PVA particles will react with Ca2+ in OPM to form the molecular chain structure of O-Ca-O, which reduces the formation of crystals at the bonding interface. It can be found in Fig. 7(c) that the PVA film can also enrich on the surface of struvite-K and form O-Mg-O structure with Mg2+ to enhance chemical bonding. Moreover, PVA film has
According to Figs. 8(a) and 8(b), the crystal morphology of PVA powder mixed with MSPM is as follows: quartz (Quartz, PDF#720439), hydration product struvite-K (MgKPO46H2O, PDF#751076) and unreacted MgO (Periclase, PDF#87-0651). Compared with the P-0 group, when the content of PVA increased from 0% to 0.2% (P-1 group), 0.4% (P-2 group) and 0.6% (P-3 group), the types of hydration products in each group were the same, but the corresponding diffraction peaks of the same substances in each group were different. With the increase in PVA content, the peak values of the main characteristic peaks of struvite-K decreased gradually in the range of 20.94° and 27.378°. There was no significant difference in struvite-K characteristic peaks between P-1 group and P-0 group. Some struvite-K characteristic peaks were not obvious in P-3 group. This is because too much PVA powder
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Y. Xie et al. / Construction and Building Materials 239 (2020) 117871
Infinitive crystal
OPM MSPM
PVA film
Interface
AFt
(a)
(b)
PVA film
(c) Fig. 7. ESEM of bond interface of group P-2.
in MSPM system will reduce the degree of hydration reaction due to its physical adsorption and chemical combination [25]. Therefore, the hydration products struvite-K of P-3 group had lower crystallinity. Fig. 9 shows the XRD analysis results of MgO characteristic peaks in MSPM system with different PVA content. The results show that with the increase in PVA content, the characteristic peak of MgO in MSPM appears first flat and then sharp, indicating that the crystallinity of unreacted MgO in the system first decreases and then increases. When the PVA content reaches 0.6%, the characteristic peak of MgO is more obvious, and the integral area of the curve increases significantly. This is because when the content of PVA is low, the –OH group in the molecular structure of PVA will react with Mg2+ in the alkaline environment of the reaction system, promoting the hydrolysis of magnesium oxide, leading to the decline of crystallinity of magnesium oxide. When the content of PVA is 0.4%, the PVA polymer film wraps MgO particles in the system [36], which has no significant impact on the hydration reaction, reducing the degree of hydrolysis of MgO, but slightly increasing the crystallinity of MgO. When the content of PVA reaches 0.6%, PVA cannot be uniformly dispersed in the system due to agglomeration. In addition, the encapsulation effect of PVA film hinders the normal progress of the hydration reaction [36]. This is consistent with the peak reduction of MKP characteristic peaks of hydration products in P-3 group in Fig. 8(b). 4.5. Pore structure analysis It can be seen from Table 3 that the porosity of P-2 and P-3 groups is 5.87% and 9.16%, respectively. Compared with the control group without PVA, the total porosity of the control group is 6.74%, which first decreases and then increases. This is because adding
the right amount of PVA powder can achieve a better filling effect and improve the pore characteristics of MSPM. However, when the polymer content is too large, due to the agglomeration of PVA, the packing and filling effects are reduced and the pores become larger. This result is also corresponding to the bond strength test. When the PVA content is too large, the improvement degree of flexural bond strength and flexural double-sided bond strength are limited or even decreases. According to Fig. 10, the proportion of holes larger than 100 nm in P-0, P-2 and P-3 groups is 74.06%, 68.71% and 70.47%, respectively. The proportion of large holes in MSPM decreased first and then increased. This is because PVA powder forms film by absorbing water and wraps on the surface of sand and hydrated product particles, enhancing the interface compactness between fine aggregate and cement slurry, thus reducing the proportion of big holes and improving the bonding performance, which is consistent with the results in section 4.1 and 4.2. However, when the PVA content increases to 0.6%, because too many polymer particles are wrapped on the surface of reactant particles, the hydration reaction degree inside the system is reduced, so the gelation composition in the system is reduced, resulting in reduced structure compactness and increased number of macropores. 4.6. Bonding mechanism This study attempts to explain the effects of PVA on the bonding mechanism between OPM and MSPM. In the initial stage of bonding interface: the PVA is dissolved and dispersed in the MSPM system in water, and adsorbed on the surface of silica fume and MgO particles. At this time, the hydration product of struvite-K is less formed. Because of the alkaline environment inside the OPM system, the polymer particles move towards the interface near the
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♣
♦♦♦♦ ♦ ♦♦
♦
♣
0.016
♣∇
dv/dlog (d) [cc/g]
♦
♣
∇
♦
P-0
♦
♦
♦♦♦♦ ♦ ♦♦
♣
♦
♣
P-0 P-2 P-3
0.018
∇ Periclase
∇
♦ ♦
P-1
0.020
♦ MKP ♣ Quartz
0.014 0.012 0.010 0.008 0.006 0.004
∇
0.002 0.000
10
20
30
40 2θ (°)
50
-0.002
60
10
100 1000 Diameter (nm)
10000
(a) P-0 and P-1 Fig. 10. Pore size distribution curve of MSPM with different PVA content.
♣
♦ ♦
P-3
♦ MKP ♣ Quartz
♣ ♦
∇ Periclase
∇ ♦♦♦♦ ♦ ♦♦
♦
∇
♣
♣
♣
♣∇
50
60
♣
P-2
10
∇
♦ ♦
♦
20
♦♦♦♦
30
♦ ♦♦
♦
40 2θ (°)
(b) P-2 and P-3 Fig. 8. XRD spectra of MSPM with different PVA content at 28 days.
P-0 P-1 P-2 P-3
Periclase
42.7
42.8
42.9
43.0
43.1
43.2
2θ (°) Fig. 9. XRD spectra of MgO diffraction peak at 28 days with different PVA content.
OPM and gather at the interface. Therefore, the number of PVA particles increase at the interface and fills in the capillary pores at the interface, thus greatly increasing the density of the interface. In the bonding interface formation stage before the final setting of MSPM, with the further hydration reaction, the PVA particles at the interface gradually increase and fuse to form a PVA film. The PVA will also be enriched on the surface of the hydrated product of struvite-K and some amorphous magnesium silicate hydrated products gel, and forms an O-Mg-O structure with Mg2+ to improve the chemical action, finally forming a closed pocket film, that is, the blend of the polymer membrane and MSPM hydration product. Subsequently, due to the adsorption of the polymer film at the interface, the free blends in the system move towards the PVA particle film at the bonding interface. After hardening, the MSPM and OPM interface composition stage: the PVA particle film at the bonding interface was further developed, forming a polymer film close to the OPM interface. Fig. 5(b) shows that a large amount of ettringite crystals and Ca (OH)2 form a bonding interface without adding PVA, but the existence of a large number of crystals is not conducive to the bonding force at the interface so that the bonding strength of the control group is the lowest. After adding the PVA powder, the PVA film formed by the PVA powder will be covered on the surface of ettringite crystals and Ca(OH)2 at the interface [40], as shown in Fig. 7 (b). However, the –OH in the PVA particles will react with Ca2+ [41] at the interface to form the molecular chain structure of OCa-O, reducing the number of crystals at the interface and enhance the chemical bonding. Subsequently, the PVA film interacts with the struvite-K gel layer film near the interface to form a bilayer film blend of polymer and the struvite-K gel [42], it can be seen from Fig. 7(c). So the interface has a very good adhesive force. However, when the PVA content is too high, the encapsulation effect of the PVA film hinders the normal progress of the hydration reaction [36], which can be explained by the experimental results of the reduction of the characteristic peak of struvite-K, a hydration product of group P-3 in Fig. 8(b), which is also consistent with the
Table 3 Effect of PVA content on Pore structure parameters of MSPM at 28 days. No
Porosity (%)
P-0 P-2 P-3
6.74 5.87 9.16
Pore size distribution (%) <20 nm
20–50 nm
50–100 nm
>100 nm
20.04 26.54 19.07
2.51 2.04 5.67
3.39 2.71 4.79
74.06 68.71 70.47
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increase in setting time and fluidity. Therefore, the bonding performance between MSPM and OPM can be greatly improved by adding appropriate amount of PVA. 5. Conclusions 1. Two different bonding experiments show that adding a small amount of PVA into MSPM can significantly improve the bonding performance between OPM and MSPM. However, when the content of PVA reaches 0.6%, the bond strength will not change or even decrease. This is because PVA powder is excessive in the system, due to its physical adsorption and chemical combination, the hydration reaction degree of MSPM will be reduced and its bond performance will be affected. 2. By analyzing the microstructure of the binding interface between MSPM and OPM, it is found that MgKPO46H2O, the hydration product infiltrated by MSPM into OPM, will bind with OPM and improve the binding performance of OPM and MSPM. Compared with the control group, PVA enriched on the surface of the struvite-K and formed O-Mg-O structure with Mg2+, enhanced the chemical action, greatly improved the bond strength between OPM and MSPM. 3. When the content of PVA reaches 0.4%, the PVA polymer film has a certain covering effect in the system, which covers and adsorbs on the surface of the reactant particles. At this point, the physical adsorption and chemical interaction effect of PVA film play a certain role in promoting the hydration reaction. However, when the content of PVA increases to 0.6%, the covering effect of PVA film affects the normal occurrence of the hydration reaction, so that the bonding strength remains unchanged or even decreases.
Acknowledgement This research was funded by four National Natural Science Foundations (NSFC) (Approval No. 51878179, 51479036, 51708120 and 51708119). References [1] D.R. Morgan, Compatibility of concrete repair materials and systems, Constr. Build. Mater. 10 (1996) 57–67. [2] M.G. Lee, Y.C. Wang, C.T. Chiu, A preliminary study of reactive powder concrete as a new repair material, Constr. Build. Mater. 21 (1) (2007) 182–189. [3] X. Song, J. Wei, T. He, A method to repair concrete leakage through cracks by synthesizing super-absorbent resin in situ, Constr. Build. Mater. 23 (1) (2009) 386–391. [4] A.M. Vaysburd, P.H. Emmons, How to make today’s repairs durable for tomorrow-corrosion protection in concrete repair, Constr. Build. Mater. 14 (4) (2000) 189–197. [5] L. Coppola, D. Coffetti, E. Crotti, Pre-packed alkali activated cement-free mortars for repair of existing masonry buildings and concrete structures, Constr. Build. Mater. 173 (2018) 111–117. [6] G. Cai, J. Zhao, Application of sulphoaluminate cement to repair deteriorated concrete members in chloride ion rich environment – a basic experimental investigation of durability properties, KSCE J. Civil Eng. 20 (7) (2016) 2832– 2841. [7] L.K. Aggarwal, P.C. Thapliyal, S.R. Karade, Properties of polymer-modified mortars using epoxy and acrylic emulsions, Constr. Build. Mater. 21 (2) (2007) 379–383. [8] J. Do, Y. Soh, Performance of polymer-modified self-leveling mortars with high polymer-cement ratio for floor finishing, Cem. Concr. Res. 33 (2003) 1497– 1505. [9] B. Pang, Y. Zhang, G. Liu, Study on the effect of waterborne epoxy resins on the performance and microstructure of cement paste, Constr. Build. Mater. 167 (2018) 831–845. [10] G. Yi, C. Ma, G.C. Long, et al., Effects of metakaolin on a novel aerated magnesium phosphate cement with high early strength, Constr. Build. Mater. 187 (2018) 1130–1133. [11] Q. Yang, X. Wu, Factors influencing properties of phosphate cement-based binder for rapid repair of concrete, Cem. Concr. Res. 29 (1999) 389–396.
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Effect of polyvinyl alcohol powder on the bonding mechanism of a new magnesium phosphate cement mortar Yudong Xie, Xujian Lin ⇑, Hanfei Li, Tao Ji College of Civil Engineering, Fuzhou University, Fuzhou 350116, China
h i g h l i g h t s PVA can significantly improve the bonding performance between MSPM and OPM. PVA film can encapsulate the surface of Ca(OH)2 and ettringite crystals at the interface. The covering effect of PVA film will affect the normal process of hydration.
a r t i c l e
i n f o
Article history: Received 11 July 2019 Received in revised form 8 December 2019 Accepted 13 December 2019
Keywords: MSPM PVA powder Bonding mechanism Bonding interface
a b s t r a c t Silica-Magnesium Potassium Phosphate cement mortar (MSPM) which is made up of dead burnt magnesia powder, Dipotassium phosphate K2HPO43H2O (P), silica fume and a certain proportion of ISO standard sand, is used as repairing materials helping study the bonding performance between MSPM and ordinary Portland cement mortar (OPM). In this research, the effect of polyvinyl alcohol powder on MSPM bonding strength (flexural bonding strength and flexural two-sided bonding strength) was investigated. The bonding mechanism was studied with X-ray diffraction (XRD), Mercury intrusion porosimetry (MIP) and environmental scanning electron microscopy (ESEM). The experimental results show that PVA powder has good film forming properties. The PVA film formed by PVA powder will encapsulate reactants (silica fume, magnesium oxide) and hydration products (struvite-K) in MSPM system, and form structuring O-Mg-O with Mg2+. In addition, the PVA film encapsulates the surface of Ca(OH)2 and ettringite crystals at the interface, and complexes with Ca2+ to form O-Ca-O molecular chain structure, reducing the number of crystals at the interface. When the PVA content reaches 0.4%, the bonding performance between the OPM and the MSPM is greatly improved due to the good bonding properties of the PVA film and the ionic complexation reaction occurring at the bonding interface. However, when the content of PVA reaches 0.6%, PVA cannot be uniformly dispersed in the reaction system due to agglomeration. In addition, the covering effect of PVA film has affected the normal progress of hydration reaction, so the improvement in bonding performance at the bonding interface is limited. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction In daily life, there is a large number of service concrete structure is destroyed phenomenon. The damaged concrete is in urgent need of repair and reinforcement, otherwise there are serious safety risks. Generally speaking, these repair materials used to repair damaged concrete structures need to reach a high bond strength with the matrix in a short time to ensure a strong bond between the old and new materials [1]. In recent years, many new repair materials [2,3] have been applied in the field of concrete structure repair. Commonly used repair materials mainly include: ordinary ⇑ Corresponding author. E-mail address: [email protected] (X. Lin). https://doi.org/10.1016/j.conbuildmat.2019.117871 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.
Portland cement mortar and concrete [4]; Special cement, such as alkali activated cement [5] and sulphoaluminate cement [6]; Organic polymer materials, such as epoxy resin [7–9]. However, these repair materials have obvious shortcomings, such as low bond strength, slow development of early strength, poor compatibility with the matrix, and the existence of safety problems (e.g., water seepage caused by cracking). The defects of these repair materials are also an important reason for the failure in a large number of concrete repair projects. Especially when the temperature changes greatly, due to the poor volume compatibility of the new and old materials, the interface will crack, resulting in the failure of repair. This is due to the different thermal expansion coefficient between the old and new materials. As a new type of environment-friendly cement-based
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Y. Xie et al. / Construction and Building Materials 239 (2020) 117871
repairing material, magnesium phosphate cement (MPC) has similar thermal expansion coefficient to ordinary concrete and good bonding performance [10–13]. Many scholars have studied the repair effect of MPC. Ding et al. [14] studied the effect of polymer powder on the performance of MPC mortar. The results show that the bonding performance and bending strength of the hardened samples are improved. Ma et al. [15] studied the effect of redispersible emulsion powder (RPP) on comprehensive properties of magnesium potassium phosphate cement (MKPC) mortar. The results show that the bending strength and bond strength of MKPC mortar are improved due to the high viscosity of RPP. It is well-known that in addition to the mechanical strength of repair materials, the bonding performance between old and new concrete is also a key index to evaluate the repair quality [16,17]. The bonding properties include the chemical compatibility, bulk compatibility and bonding strength of the bonding interface. OPC mortar [11] has good volume compatibility with MPC mortar. This is because the drying shrinkage rate of OPC mortar is 6 104/°C, and the thermal expansion coefficient is 7 106/°C. The dry shrinkage rate of MPC mortar measured under the same conditions is 0.34 104/°C, and the thermal expansion coefficient is 9.6 106/°C. This shows that the thermal expansion performance of OPC mortar is similar to that of MPC mortar, and there is little difference in volume change between them when the temperature changes [18]. Therefore, Abdelrazig et al. [19] think that the main reason for the good compatibility of MPC mortar and OPC mortar is that the volume fraction of hydration products of MPC is small and the hydration product of the struvite-K belongs to the crystal, while the hydration product of OPC is hydrated calcium silicate gel. In actual repairing engineering, due to the different parts of structural damage, it is usually necessary to repair in different directions. Therefore, different test methods are needed to accurately evaluate the bonding performance of the interface. Researchers often use square bonding plate, ‘‘800 glyph, cylindrical core drawing and other ways [20,21] to test the bonding properties between old and new materials. Qian et al. [22] studied a new method for testing the bond properties of structures, which is using a circular platform to study the bond properties of structures. The outer side of the circular table is poured with old matrix material, and the inner side of the circular table is reinforced with reinforcement material. This method can be used to measure the volume stability of the repair material and the effect of drying shrinkage on the bonding properties of the material. In today’s building materials industry, a large number of polymers are used to improve the properties of cement-based materials. Common polymers include vinyl acetate [24,25], polyvinyl alcohol [26–30] and styrene-acrylic emulsion [23], where polyvinyl alcohol is one of the main polymers. Due to its excellent chemical and physical properties, PVA is used as the modifier of cement-based materials, mainly to improve the workability of cement-based materials [26,27]. Current studies show that PVA can significantly improve the bonding performance of cementbased materials [13,31]. PVA, which is a kind of water-soluble polymer, will dehydrate and form a highly viscous PVA film during cement hydration [28,32]. Although PVA has been widely used in the modification of OPC, the modification mechanism in MPC system is uncertain. Wang et al. [33] found that PVA fiber can improve the flexural strength and fracture energy of MPC, but the existing form of PVA powder (not fiber) is also very important for studying the modification mechanism (especially the bonding mechanism). Different forms of existence may lead to different test conclusions. Therefore, when studying the bonding mechanism of MPC, it is necessary to make further research on the effect of adding PVA powder. To sum up, a large number of studies have shown that MPC can be used as the repair material for the repair of concrete, and adding
an appropriate amount of PVA can improve the bonding performance of the repair material. However, no one has studied the bonding performance of MSPM mortar prepared with K2HPO43H2O (not KH2PO4), PVA powder (not fiber) and silica powder. In this paper, flexural bonding strength test and flexural two-sided bonding strength test were used to study the bonding properties of MSPM and OPM, and the influence of PVA powder on MSPM bond properties was studied. In addition, the bonding mechanism of MSPM system was analyzed by means of pore structure tester, XRD and ESEM-EDS 2. Materials The magnesium oxide for experiment was supplied by Liaoning Haicheng Qunli Mining Co., Ltd. China, which was produced by calcining magnesite at 1500 . The magnesium oxide was industrial pure grade, the MgO content was more than 95%, and the average particle size of magnesium oxide in the experiment was 33.29 lm. The K2HPO43H2O used was chemically pure grade, which was manufactured by Xilong Chemical Co., Ltd. China. DHP (K2HPO43H2O, P) that was analytical grade with 99% K2HPO43H2O content was provided by Xilong Chemical Co., Ltd. (Fuzhou, China). Silica fume was obtained from Xining Ferroalloy Factory with SiO2 content of 90%, and its average particle size was 0.1– 0.2 lm. Polyvinyl alcohol (abbreviated as PVA) was a white powder, which was provided by Shanghai Yanye Industrial Co., Ltd. It had excellent film-forming property and cohesiveness. It had a fineness of 160 mesh and a viscosity of 7000 mPas, and the pH of the PVA after mixing with water was measured by a pH meter of 5.6. ISO standard sand was used for MSPM; river sand and 42.5 ordinary Portland cement were used for OPM, and each performance index of river sand was measured according to GB/T 14684-2001 [34], as shown in Table 1. The tap water taken from Fuzhou (China) was used. The main chemical components of magnesia and silica fume are shown in Table 2. 3. Experimental 3.1. Mix proportion The sand-to-binder ratio (S/C) was 0.14, the mass ratio (P/M) of 1/3, the water-to-binder ratio (W/C) was 0.14 and the silica fume content was 15%. The mass ratio of PVA to gelling materials was varied. P/M refers to the mass ratio of n(K2HPO43H2O)/n(MgO), and W/C indicates the mass ratio of water consumption (crystal water in hydrated DHP should be considered) to gelling materials (DHP, silica fume, magnesium oxide and PVA). The PVA contents of P-0, P-1, P-2 and P-3 were 0%, 0.2%, 0.4% and 0.6%, respectively. Additionally, sand-binder ratio (S/C) and water-binder ratio (W/C) of OPM were 2 and 0.4, respectively; and the compressive strength and flexural strength of OPM at 28 days were 47.75 MPa and 12.25 MPa, respectively. 3.2. Test methods 3.2.1. Flexural bonding strength test After mixing magnesium oxide, silica fume and PVA powder in cement mortar mixer, the prepared K2HPO4 solution was added to the mixer for 60 s and then add sand for 60 s to get MSPM. The OPM test block of 40 mm 40 mm 80 mm was maintained in a 40 mm 40 mm 160 mm mold for 28 days (20 ± 2 , 95% >RH), and then the prepared MSPM repair mortar was poured into the mold to form new and old mortar joint specimens. According to the provisions of GB/T17671-1999 [35], the flexural bond strength of the samples was tested by the electro-hydraulic servo pressure
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Y. Xie et al. / Construction and Building Materials 239 (2020) 117871 Table 1 Performance index of the river sand. Bulk density (kg/m3)
Fineness modulus
Water absorption (%)
Apparent density (kg/m3)
Mud Content (%)
1480
2.5
0.1
2591
3.53
Table 2 Chemical composition (% wt.).
Magnesia Silica fume
SiO2
Al2O3
Fe2O3
MgO
CaO
K2O
F.C
LOI
2.92 97.57
2.34 0.06
1.3 0.02
93.06 0.05
0.22 0.03
– 0.78
– 0.38
0.16 2.26
testing machine controlled by microcomputer (YAW 4000). The samples are shown in Fig. 1(a) and (b). The formula for calculating the flexural bond strength is as follows Formula (1):
r¼
2
M Fðl - 20Þ bh ¼ = ¼ 3:75F W 2 6
ð1Þ
where r represents the flexural bonding strength (MPa); F is the maximum load of cuboid specimen when it breaks (kN); L represents the distance between two fulcrums (L=100 mm); h is the width of cuboid specimen (h=40 mm); b stands for the height of cuboid specimen (b=40 mm). 3.2.2. Flexural two-sided bonding strength test Two ordinary cement mortar blocks (40 mm 40 mm 75 mm) maintained for 28 days were put into 40 mm 40 mm 160 mm mold with a 10 mm interval between the two blocks, and then the fresh MSPM was poured into the gap. After hardening, the mold was removed and cured to the prescribed age. The testing process is the same as section 3.2.1. The test specimens are shown in Fig. 2(a) and (b). The flexural twosided bonding strength was calculated by the formula according to Formula (1). 3.2.3. XRD and ESEM-EDS tests In order to analyze the phases presented in MSPM, small dry particle samples are taken from the oven and ground into fine powder (less than 100 mesh). XRD which was made by PANalytical, the Netherlands. The localized morphology of the interface of OPM and MSPM was observed in ESEM (Quanta 250, FEI, USA). The energy dispersive spectrometer (EDS) was XFlash6T|30 manufactured by Bruker Corp. (USA). In the ESEM/EDS test, samples were selected from near the bonding interface. 3.2.4. Pore structure test In this study, the pore structure analysis adopted mercury injection analysis (MIP, PoreMaster-60) to determine the pore diameter
and porosity characteristics of mortar samples. The maximum pressure value of the high pressure was set as 55,000 psia. Sample preparation: after the test specimen is broken, particles with a diameter of about 3 mm are selected and placed in liquid nitrogen to terminate hydration. Before the test, they are placed in a vacuum drying box (60 ) and dried in vacuum for 1 month. Then the pore structure test is carried out. 4. Results and discussion 4.1. Flexural bonding strength Fig. 3 shows the effect of PVA content on the flexural bond strength of MSPM. When the PVA content is 0.2%, 0.4% and 0.6%, the flexural bond strength of PMSPM in each group at 28 days of age is 15.98 MPa, 16.75 MPa and 17.1 MPa, respectively. Compared with 14.21 MPa in the control group, the flexural bond strength increases by up to 20.34%. This is because the PVA film has very strong bonding properties, the tensile bonding strength of MSPM increases gradually with the increase in PVA content. However, when the content of PVA exceeds 0.4%, the strengthening effect of PVA is not so obvious, and the value of flexural bond strength is basically stable. Additionally, at the early stage of hydration reaction, the bonding strength is lower because of the less gel material generated in the MSPM reaction system. At this time, PVA can greatly improve the bond strength of MSPM. However, with the increase in age, the bonding phases provided by hydration products increase in MSPM system, so the effect of PVA on bonding strength is not as obvious as that of early stage. 4.2. Flexural two-sided bonding strength Fig. 4 shows the effect of PVA on the flexural double-sided bond strength of MSPM. When the content of PVA was 0.2%, 0.4% and 0.6%, the flexural double-sided bond strength of each group was 10.97 MPa, 11.48 MPa and 11.76 MPa, respectively. Compared with
OPM
(a) Force diagram
MSPM
(b) Specimens Fig. 1. Flexural bonding strength test.
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10 mm
OPM MSPM OPM
(a) Force diagram
(b) Specimens
Fig. 2. Flexural two-sided bonding strength test.
bonding strength. However, after the addition of PVA, the flexural double-sided bond strength of each group at 56 days of age was basically unchanged compared with that at 28 days of age. Therefore, PVA can effectively improve the bonding performance of MSPM. In general, the influence of different PVA content on the flexural double-sided bond strength of MSPM is basically the same as that on the flexural bond strength.
Flexural bonding strength (MPa)
18 17 16 15 14 3d 7d 28d 56d
13 12 11 0.0
0.1
0.2
0.3
0.4
0.5
0.6
The content of PVA (%)
Flexural bi-sided bonding strength (MPa)
Fig. 3. Influence of PVA content on flexural bonding strength of MSPM.
12 11 3d 7d 28d 56d
10 9 8 7 6 0.0
0.1
0.2 0.3 0.4 0.5 The content of PVA (%)
0.6
Fig. 4. Influence of PVA content on the flexural two-sided bonding strength of MSPM.
the P-0 group of 9.56 MPa, the flexural bond strength increased by up to 2.2 MPa. As the age increases, the bending strength of P-0 group was 6.89 MPa, 9.31 MPa, 9.56 MPa and 8.8 MPa, respectively. The flexural double-sided bond strength of P-0 group increased by 35.12% in the early stage. After 7 days of age, the bond strength decreased slightly at 56 days, which was 7.9% lower than that at 28 days. This may be due to the increase of shrinkage caused by the secondary hydration reaction, resulting in a slight decrease in
4.3. ESEM/EDS analysis Fig. 5(a) shows the interface between OPM and MSPM of P-0 group. It shows that the interface between OPM and MSPM is dense, and a small amount of MgKPO46H2O combined with unreacted MgO [37] to form a dense structure. However, there are holes near the interface of MSPM. This may be due to the poor fluidity of MSPM slurry, the less compact bonding at the interface and the occurrence of cracks perpendicular to the bonding interface near the voids. Fig. 5(b) is an enlarged view of the interface gap in Fig. 5(a). It can be found that a large amount of ettringite and Ca(OH)2 crystals appear at the bonding interface of OPM, and the crystals extend along the gap towards the interface near MSPM. Additionally, it can be seen from Fig. 5(b) that many irregular rod crystals appear near the bonding interface of OPM. EDS analysis of the irregular crystals selected in Fig. 5(c) shows the results as shown in Fig. 5(d). The results show that the composition of the crystal is complex, and the contents of K+ and Ca2+ are relatively high. Due to the free Ca2+ in OPM system + can diffuses into MSPM system, Mg2+, PO3 4 and K in MSPM system also diffuses into OPM system, and finally forms an ion diffusion layer composed of calcium phosphate compounds, which enhances the complexation reaction between ions at the interface and improves the chemical bonding effect [38,39]. As a result, the bonding properties at the interface has been enhanced. Through the above analysis, the schematic diagram of bond layer structure can be obtained, as shown in Fig. 6. Fig. 7(a) shows the bonding interface between OPM and MSPM in P-2 group. The experimental results show that the bonding between OPM and MSPM is good. Compared with Fig. 5(a), there are no holes in the bonding interface, and the bonding between MSPM and OPM is more compact, and there are no cracks perpendicular to the bonding interface. This is because the PVA particles aggregate at the bonding interface [26,28], and fill in the capillary holes at the bonding interface, improving the density of the interface. By comparing Figs. 5(b) and 7(b), it can be found that there were a large number of ettringite crystals in the pores of the interface of the control group, but the content of crystals in the interface decreased significantly after PVA was added. This is due to the film-forming effect of the PVA, which covers on the surface of
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Y. Xie et al. / Construction and Building Materials 239 (2020) 117871
Crack AFt
MSPM
Interface
OPM
(a)
(b)
(c)
(d) Fig. 5. ESEM/EDS of bond interface of group P-0.
excellent bonding properties [28,32], so the bonding properties of P-1 group, P-2 group and P-3 group are higher than those of P-0 group. 4.4. XRD analysis
Fig. 6. Bonding interface model diagram of MSPM.
the crystal [36], leading to decrease in the crystal morphology. In addition, in alkaline environment, the –OH in PVA particles will react with Ca2+ in OPM to form the molecular chain structure of O-Ca-O, which reduces the formation of crystals at the bonding interface. It can be found in Fig. 7(c) that the PVA film can also enrich on the surface of struvite-K and form O-Mg-O structure with Mg2+ to enhance chemical bonding. Moreover, PVA film has
According to Figs. 8(a) and 8(b), the crystal morphology of PVA powder mixed with MSPM is as follows: quartz (Quartz, PDF#720439), hydration product struvite-K (MgKPO46H2O, PDF#751076) and unreacted MgO (Periclase, PDF#87-0651). Compared with the P-0 group, when the content of PVA increased from 0% to 0.2% (P-1 group), 0.4% (P-2 group) and 0.6% (P-3 group), the types of hydration products in each group were the same, but the corresponding diffraction peaks of the same substances in each group were different. With the increase in PVA content, the peak values of the main characteristic peaks of struvite-K decreased gradually in the range of 20.94° and 27.378°. There was no significant difference in struvite-K characteristic peaks between P-1 group and P-0 group. Some struvite-K characteristic peaks were not obvious in P-3 group. This is because too much PVA powder
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Y. Xie et al. / Construction and Building Materials 239 (2020) 117871
Infinitive crystal
OPM MSPM
PVA film
Interface
AFt
(a)
(b)
PVA film
(c) Fig. 7. ESEM of bond interface of group P-2.
in MSPM system will reduce the degree of hydration reaction due to its physical adsorption and chemical combination [25]. Therefore, the hydration products struvite-K of P-3 group had lower crystallinity. Fig. 9 shows the XRD analysis results of MgO characteristic peaks in MSPM system with different PVA content. The results show that with the increase in PVA content, the characteristic peak of MgO in MSPM appears first flat and then sharp, indicating that the crystallinity of unreacted MgO in the system first decreases and then increases. When the PVA content reaches 0.6%, the characteristic peak of MgO is more obvious, and the integral area of the curve increases significantly. This is because when the content of PVA is low, the –OH group in the molecular structure of PVA will react with Mg2+ in the alkaline environment of the reaction system, promoting the hydrolysis of magnesium oxide, leading to the decline of crystallinity of magnesium oxide. When the content of PVA is 0.4%, the PVA polymer film wraps MgO particles in the system [36], which has no significant impact on the hydration reaction, reducing the degree of hydrolysis of MgO, but slightly increasing the crystallinity of MgO. When the content of PVA reaches 0.6%, PVA cannot be uniformly dispersed in the system due to agglomeration. In addition, the encapsulation effect of PVA film hinders the normal progress of the hydration reaction [36]. This is consistent with the peak reduction of MKP characteristic peaks of hydration products in P-3 group in Fig. 8(b). 4.5. Pore structure analysis It can be seen from Table 3 that the porosity of P-2 and P-3 groups is 5.87% and 9.16%, respectively. Compared with the control group without PVA, the total porosity of the control group is 6.74%, which first decreases and then increases. This is because adding
the right amount of PVA powder can achieve a better filling effect and improve the pore characteristics of MSPM. However, when the polymer content is too large, due to the agglomeration of PVA, the packing and filling effects are reduced and the pores become larger. This result is also corresponding to the bond strength test. When the PVA content is too large, the improvement degree of flexural bond strength and flexural double-sided bond strength are limited or even decreases. According to Fig. 10, the proportion of holes larger than 100 nm in P-0, P-2 and P-3 groups is 74.06%, 68.71% and 70.47%, respectively. The proportion of large holes in MSPM decreased first and then increased. This is because PVA powder forms film by absorbing water and wraps on the surface of sand and hydrated product particles, enhancing the interface compactness between fine aggregate and cement slurry, thus reducing the proportion of big holes and improving the bonding performance, which is consistent with the results in section 4.1 and 4.2. However, when the PVA content increases to 0.6%, because too many polymer particles are wrapped on the surface of reactant particles, the hydration reaction degree inside the system is reduced, so the gelation composition in the system is reduced, resulting in reduced structure compactness and increased number of macropores. 4.6. Bonding mechanism This study attempts to explain the effects of PVA on the bonding mechanism between OPM and MSPM. In the initial stage of bonding interface: the PVA is dissolved and dispersed in the MSPM system in water, and adsorbed on the surface of silica fume and MgO particles. At this time, the hydration product of struvite-K is less formed. Because of the alkaline environment inside the OPM system, the polymer particles move towards the interface near the
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Y. Xie et al. / Construction and Building Materials 239 (2020) 117871
♣
♦♦♦♦ ♦ ♦♦
♦
♣
0.016
♣∇
dv/dlog (d) [cc/g]
♦
♣
∇
♦
P-0
♦
♦
♦♦♦♦ ♦ ♦♦
♣
♦
♣
P-0 P-2 P-3
0.018
∇ Periclase
∇
♦ ♦
P-1
0.020
♦ MKP ♣ Quartz
0.014 0.012 0.010 0.008 0.006 0.004
∇
0.002 0.000
10
20
30
40 2θ (°)
50
-0.002
60
10
100 1000 Diameter (nm)
10000
(a) P-0 and P-1 Fig. 10. Pore size distribution curve of MSPM with different PVA content.
♣
♦ ♦
P-3
♦ MKP ♣ Quartz
♣ ♦
∇ Periclase
∇ ♦♦♦♦ ♦ ♦♦
♦
∇
♣
♣
♣
♣∇
50
60
♣
P-2
10
∇
♦ ♦
♦
20
♦♦♦♦
30
♦ ♦♦
♦
40 2θ (°)
(b) P-2 and P-3 Fig. 8. XRD spectra of MSPM with different PVA content at 28 days.
P-0 P-1 P-2 P-3
Periclase
42.7
42.8
42.9
43.0
43.1
43.2
2θ (°) Fig. 9. XRD spectra of MgO diffraction peak at 28 days with different PVA content.
OPM and gather at the interface. Therefore, the number of PVA particles increase at the interface and fills in the capillary pores at the interface, thus greatly increasing the density of the interface. In the bonding interface formation stage before the final setting of MSPM, with the further hydration reaction, the PVA particles at the interface gradually increase and fuse to form a PVA film. The PVA will also be enriched on the surface of the hydrated product of struvite-K and some amorphous magnesium silicate hydrated products gel, and forms an O-Mg-O structure with Mg2+ to improve the chemical action, finally forming a closed pocket film, that is, the blend of the polymer membrane and MSPM hydration product. Subsequently, due to the adsorption of the polymer film at the interface, the free blends in the system move towards the PVA particle film at the bonding interface. After hardening, the MSPM and OPM interface composition stage: the PVA particle film at the bonding interface was further developed, forming a polymer film close to the OPM interface. Fig. 5(b) shows that a large amount of ettringite crystals and Ca (OH)2 form a bonding interface without adding PVA, but the existence of a large number of crystals is not conducive to the bonding force at the interface so that the bonding strength of the control group is the lowest. After adding the PVA powder, the PVA film formed by the PVA powder will be covered on the surface of ettringite crystals and Ca(OH)2 at the interface [40], as shown in Fig. 7 (b). However, the –OH in the PVA particles will react with Ca2+ [41] at the interface to form the molecular chain structure of OCa-O, reducing the number of crystals at the interface and enhance the chemical bonding. Subsequently, the PVA film interacts with the struvite-K gel layer film near the interface to form a bilayer film blend of polymer and the struvite-K gel [42], it can be seen from Fig. 7(c). So the interface has a very good adhesive force. However, when the PVA content is too high, the encapsulation effect of the PVA film hinders the normal progress of the hydration reaction [36], which can be explained by the experimental results of the reduction of the characteristic peak of struvite-K, a hydration product of group P-3 in Fig. 8(b), which is also consistent with the
Table 3 Effect of PVA content on Pore structure parameters of MSPM at 28 days. No
Porosity (%)
P-0 P-2 P-3
6.74 5.87 9.16
Pore size distribution (%) <20 nm
20–50 nm
50–100 nm
>100 nm
20.04 26.54 19.07
2.51 2.04 5.67
3.39 2.71 4.79
74.06 68.71 70.47
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Y. Xie et al. / Construction and Building Materials 239 (2020) 117871
increase in setting time and fluidity. Therefore, the bonding performance between MSPM and OPM can be greatly improved by adding appropriate amount of PVA. 5. Conclusions 1. Two different bonding experiments show that adding a small amount of PVA into MSPM can significantly improve the bonding performance between OPM and MSPM. However, when the content of PVA reaches 0.6%, the bond strength will not change or even decrease. This is because PVA powder is excessive in the system, due to its physical adsorption and chemical combination, the hydration reaction degree of MSPM will be reduced and its bond performance will be affected. 2. By analyzing the microstructure of the binding interface between MSPM and OPM, it is found that MgKPO46H2O, the hydration product infiltrated by MSPM into OPM, will bind with OPM and improve the binding performance of OPM and MSPM. Compared with the control group, PVA enriched on the surface of the struvite-K and formed O-Mg-O structure with Mg2+, enhanced the chemical action, greatly improved the bond strength between OPM and MSPM. 3. When the content of PVA reaches 0.4%, the PVA polymer film has a certain covering effect in the system, which covers and adsorbs on the surface of the reactant particles. At this point, the physical adsorption and chemical interaction effect of PVA film play a certain role in promoting the hydration reaction. However, when the content of PVA increases to 0.6%, the covering effect of PVA film affects the normal occurrence of the hydration reaction, so that the bonding strength remains unchanged or even decreases.
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