Effects of metakaolin on properties and microstructure of magnesium phosphate cement

Construction and Building Materials 234 (2020) 117353

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Effects of metakaolin on properties and microstructure of magnesium phosphate cement Zhaohui Qin a, Cong Ma b,⇑, Zhiqin Zheng c, Guangcheng Long b, Bing Chen a a

Department of Civil Engineering, Shanghai Jiao Tong University, Shanghai 210240, PR China School of Civil Engineering, Central South University, Changsha, Hunan Province 410075, PR China c School of Environment and Resource, Southwest University of Science and Technology, Mianyang 621010, PR China b

h i g h l i g h t s

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


Strength and durability of MPC

The introduction of metakaolin into magnesium phosphate cement (MPC) can improve the compressive strength, volume stability, water resistance and freeze-thaw resistance significantly. Two possible reasons were proposed in this study. Firstly, metakaolin decreases the total porosity of MPC and improves the pore structure which is directly related to the mechanical properties. Secondly, part of metakaolin can dissolve in MPC and take part in hydration, even though no new hydration products can be observed in XRD patterns. This study not only gives a compressive evaluation on the application of metakaolin in magnesium phosphate cement, but also explains the possible reason for the improvement of the properties of magnesium phosphate cement.

incorporating metakaolin were systematically studied.
Possible mechanism of metakaolin influencing MPC was proposed by multi-analysis.
The more homogeneous microstructure in metakaolin modified MPC can explain the property improvement.

a r t i c l e

i n f o

Article history: Received 19 June 2019 Received in revised form 6 September 2019 Accepted 22 October 2019

Keywords: Magnesium phosphate cement Metakaolin

⇑ Corresponding author. E-mail address: [email protected] (C. Ma). https://doi.org/10.1016/j.conbuildmat.2019.117353 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.

a b s t r a c t Metakaolin has been generally used as a filler in Portland cement-based materials, and it was proved to be able to enhance the performance of magnesium phosphate cement composites. In this research, metakaolin was introduced into MPC, and compressive strength, tensile bonding strength, porosity and durability of MPC were measured. The microstructure of MPC before and after water immersion was evaluated. The experimental results indicate that the presence of metakaolin can lead to higher plastic viscosity of fresh MPC mortar, 28-day compressive strength, higher tensile bonding strength, better freeze-thaw resistance, better water resistance, lower drying shrinkage and better optimized microstructure. Physical effects of metakaolin including filling effect and inducing effect play the leading role in

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Notation

et s s0 l

r_ ADP CRT fCn fTn L0 Lt MgO MK

MPC n RC

Drying shrinkage at t days Shear stress Yield stress Plastic viscosity Shear rate Ammonium dihydrogen phosphate, NH4H2PO4 Composite retarder Compressive strength at n days of soaking Tensile bonding strength at n days of soaking Initial length of specimens Length of specimens at t days Dead burnt magnesia Metakaolin

Properties Microstructure Mechanism

RCC RCFT RCT PD

Magnesium phosphate cement Curing time, days Strength retention coefficient of specimens after soaking Compressive strength retention coefficient of specimens after soaking Strength retention coefficient after freeze-thaw measurements Tensile bonding strength retention coefficient of specimens after soaking Polymeric dispersant

MPC. Metakaolin particles can fill in the pores or cracks, resulting in the decrease in porosity. In addition, metakaolin particles can provide lots of sites for crystallization of hydration products, inducing in the generation of more homogeneous microstructure. Finally, the dissolution and chemical reaction of metakaolin particles were confirmed by BSE analysis. Ó 2019 Elsevier Ltd. All rights reserved.

1. Introduction In the late 1930s, Magnesium phosphate cements (MPC) were invented, and they attract more and more attention due to the rapid hardening, high early compressive strength and durability during the past decades [1–4]. The hardening mechanism is attributed to acid-base reaction between magnesia powder providing alkaline environment and acidic phosphates providing hydrogen ion [2,5]. The phosphates used are mainly ammonium dihydrogen phosphate (NH4H2PO4, ADP) and potassium dihydrogen phosphate (KH2PO4, PDP). The common used magnesia is dead burnt magnesia because light burnt magnesia may result in the flash hardening of MPC. Even so, retarders including boric acid, borax and metaphosphate should be employed to improve the setting property of MPC [6–8]. The type of hydration products is related to the acid-base property of reaction condition, and the main crystalline products is newberyite (MgHPO43H2O) under acidic condition (pH = 5 or 6) [9,10]. Under the neutral and alkalescent conditions, struvite (MgNH4PO46H2O or MgKPO46H2O) is generated according to the following well-known chemical equation [4,10].

MgO þ NH4 H2 PO4 þ 5H2 O ! MgNH4 PO4  6H2 O

ð1Þ

Different from the C-S-H gels in Portland cement, the hydration products are mainly crystalline phases, and the volume stability of MPC is superior to Portland cement-based materials [6,11,12]. Compared with the majority of inorganic repairing materials in constructional engineering, MPC mortar used as repairing material has unique advantages of higher bonding strength and lower shrinkage [3,6,11]. Moreover, because some of the heavy metal ions, such as Zn2+, Pb2+ and Cu2+ can participate in the acid-base reaction, MPC is also employed to stabilize the low-level nuclear wastes or wastes with heavy metals [13–15]. In order to decrease the hydration heat, improve the strength and durability and reduce the cost of MPC, some supplementary cementious materials commonly used in Portland cement

including Class F fly ash, granulated blast-furnace slag, silica fume and metakaolin were introduced into MPC [16–23]. Among these materials, fly ash is the most widely used and cost-efficient mineral admixture. The mechanism of fly ash in MPC is not in conformity with that in Portland cement because of the alkalescence or neutral of most MPCs. In the early-stage studies, plenty of fly ash particles were observed in scanning electron microscope (SEM) images and the diffraction peak representing new hydration products can hardly be detected through X-ray diffraction (XRD). It was regarded that fly ash in MPC works only as an insert filler. Recent studies reveal that the glassy minerals in fly ash indeed dissolve in MPC and the chemical reaction between fly ash and MPC was confirmed by solid-state nuclear magnetic resonance (NMR) [11,17,24]. Even though fly ash could enhance the strength at a certain extent, there is no significant improvement in the water resistance of MPC [24]. Metakaolin is a type of aluminosilicate admixture obtained by calcinating kaolin clay at temperature from about 600 °C to 900 °C, and its pozzolanic activity is much higher than fly ash [25,26]. The usage of metakaolin in Portland cement-based materials is very common, but there are only very a few studies on metakaolin blended MPC [22,27]. Judged from the previous results, metakaolin can significantly increase the strength, improve the hardening property and water-resisting property. And the high content of active alumina phase is valuable for improving the MPC properties [28,34]. However, the effects of metakaolin on the bonding strength, durability and porosity of MPC are not entirely clear. In this study, properties of magnesium phosphate cement incorporating metakaolin were systematically investigated by measuring compressive strength, tensile bonding strength, porosity and durability. The possible mechanism of metakaolin influencing the properties was studied by back-scattered electron analysis. This study not only gives a compressive evaluation on the application of metakaolin in magnesium phosphate cement, but also explains the possible reason for the improvement of MPC properties.

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condition whose relative humidity is 95 ± 3% was employed during the experiments, and the curing temperature is always 20 ± 2 °C. Yield stress (s0) and plastic viscosity (l) are important indexes to characterize the rheological properties of cement-based materials. Therefore, it is necessary to measure these two indexes for studying the rheological properties of cement-based materials. Evenly mixed fresh mortar of different proportioning was transferred into the sample container, then fresh mortar was presheared according to the procedure presented in Fig. 1. The purpose of pre-shear is to make fresh mortar with different proportioning have the same shearing state and improve the accuracy of testing. The shear rate of pre-shear increased linearly from 0 s1 to 200 s1 within 30 s, and then decreased linearly from 200 s1 to 0 s1 within the same time. Next, the mortar was stood for 30 s and then was formal sheared. The formal shear is divided into three parts, which are the rising stage of shear rate, the constant stage of shear rate and the falling stage of shear rate. The maximum shear rate was 200 s1. First, the shear rate increased linearly from 0 s1 to 200 s1 within 60 s, then the shear rate kept constant for 30 s, and finally reduced linearly from 200 s1 to 0 s1 within 60 s. At 1 h, 3 days, 7 days and 28 days, the compressive strength test was conducted according to Chinese standard GB/T 50081. The load was applied at a speed of 2 kN/s during the whole testing process. Over three specimens were measured for each strength testing, and the strength data used in next section is the mean value. In order to achieve the tensile bonding strength of MPC mortar, OPC concrete substrate with a 28-day compressive strength of about 42.5 MPa was prepared as presented in Fig. 2a. The rough surface of concrete substrate was obtained after surface treatment. The wettability of the rough surface should be improved through a water-spray process as the practical operation in engineering, which is conducive to evaluate the bonding property more accurately. And then, PVC moulds with the width and length of 40 mm and height of 6 mm were placed on the rough surface of concrete substrate. After pouring MPC mortar onto the PVC moulds, the specimen was cured under standard condition for 1 day, 3 days and 28 days. As shown in Fig. 2a, the pull-off force acts on a pull pin which is bonded with the MPC mortar by an epoxy resin. The tensile bonding strength was calculated by the maximum pull-off force. It is noteworthy that more than five rectangle MPC specimens were prepared and measured in order to guarantee the accuracy of testing results. Water resistance mainly represented by strength retention coefficient is one key indicator to investigate the durability of MPC. The strength retention coefficient was defined and used by many researchers to analyze the compressive strength degradation of cement-based materials quantitatively. In other words, the

2. Experimental methodology 2.1. Materials Raw materials for preparing MPC slurry or mortar include cementitious materials, retarders and mineral admixture. The cementitious materials are mainly dead burnt magnesia (MgO) and ammonium dihydrogen phosphate (NH4H2PO4, ADP). The flavescens MgO powder used after the calcination of about 1600 °C was obtained from Xingyou Engineering Materials Co., Ltd, China. The chemical composition of MgO powder is shown in Table 1. The ADP used is commercially pure and the purity is greater than 96%. The mineral admixture used in this study is only metakaolin with a calcination temperature of about 800 °C. The average size of particle (dm) and specific area (SBET) of MgO and MK are also presented in Table 1. The retarder (CRT) used is a type of composite additive consisting of sodium borate (Na2B4O710H2O) and sodium tripolyphosphate with the mass ratio of 1:1. Silica sand with continuous gradation was employed as fine aggregates with the maximum size of 2 mm to prepare MPC mortar. In order to improve the workability of metakaolin blended MPC, a type of polymeric dispersant (PD) named as sodium polyacrylate was used. 2.2. Methods 2.2.1. Specimen preparation Table 2 presents the designed mix proportions of MPC mortar including binder, sand and water. Contrast specimen without metakaolin was designed. The mass ratio of MgO to ADP was fixed 3.0 which had been determined by series of experiments. The mass ratios of water-to-binder and sand-to-binder were fixed at 0.2 and 1.0, respectively. Retarder was added by the weight of MgO, and the mass ratio of retarder to MgO is always 0.06. Metakaolin was introduced into MPC by replacing MgO, and the MK replacements were 0%, 7.5%, 15%, 30% and 45%, respectively, by the weight of MgO. The polymeric dispersant was used to improve the workability of MPC, and the mass ratio of polymeric dispersant to metakaolin is fixed at about 0.01. 2.2.2. Experimental methods MPC paste or mortar was prepared in two steps. Firstly, solid materials including binder, additives and sand were added into a mortar mixer for two minutes of dry mixing. Secondly, the projected mass of water was introduced into the solid materials and the continued mixing was about three minutes until the flowing mixture was obtained. And then, the fresh mortar was transferred into PVC moulds (40 mm  40 mm  160 mm) and vibrating process was used to achieve the homogeneous slurry. Standard curing

Table 1 Oxide composition of MgO and metakaolin. Materials

SiO2

Al2O3

Fe2O3

MgO

CaO

K2O

Na2O

LOI

dm (lm)

SBET (m2/g)

MgO MK

4.67 54.83

2.12 42.15

1.35 0.37

89.21 0.02

2.65 0.07

– 0.14

– 0.24

– 1.36

39.2 14.9

1.26 9.63

s/b

w/b

Molar ratio of MgO to ADP

1.0 1.0 1.0 1.0 1.0

0.2 0.2 0.2 0.2 0.2

8.56 7.92 7.28 5.99 4.71

Table 2 Mix proportions. Sample No.

M00 M07 M15 M30 M45

Binder components (referred to 1 g ADP) MgO

ADP

MK

CRT

PD

3.0 2.775 2.55 2.1 1.65

1.0 1.0 1.0 1.0 1.0

0 0.225 0.45 0.9 1.35

0.18 0.167 0.153 0.126 0.099

0 0.002 0.005 0.009 0.014

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measure the remaining compressive strength. The strength retention coefficient for freeze-thaw test is proposed as follows.

constant stage

200

RCFT ¼

shear rate (1/s)

150

falling stage

rising stage

50

0 0

50

100

150

200

ð4Þ

where RCFT is strength retention coefficient after freeze-thaw measurements, fC100 is compressive strength of MPC mortar after 100 freeze-thaw cycles and fC is the 28-day compressive strength before freeze-thaw measurements. According to ASTM C490-93a, the MPC mortar specimens with a size of 25 mm  25 mm  285 mm were prepared to measure the drying shrinkage. The initial length (L0) of each specimen was measured at 3 h of curing, and the length (Lt) of specimens at 1, 3, 7, 14, 28 and 60 days of curing was recorded. The temperature and relative humidity of the curing condition are 20 ± 2 °C and 60 ± 5%, respectively. The drying shrinkage of MPC mortar can be calculated by the following equation.

preshearing

100

f C100 fC

250

time (s)

et ¼

Fig. 1. Testing procedure of the rheology of MPC mortar.

higher strength retention coefficient indicates better water resistance of materials [29]. In this study, not only the retention coefficient of compressive strength but also that of tensile bonding strength was measured. After 28 days of standard curing, the specimens were transferred into water tank as shown in Fig. 2b and the water immersion lasted 28 days. And then, the samples were fetched for measuring the after-soaking strength and tensile bonding strength. The retention coefficients can be expressed as the following equations.

RCC ¼

f Cn fC

ð2Þ

RCT ¼

f Tn fT

ð3Þ

where RC is retention coefficient of compressive strength (RCC) and tensile bonding strength (RCT), n is the immersion time, fCn and fTn are compressive strength and tensile bonding strength at n days of soaking, fC and fT are 28-day compressive strength and tensile bonding strength of MPC mortar. Before freeze-thaw tests, the specimens after 28 days of standard curing were immersed in water for 24 h. And then, the soaked specimens were transferred into the freeze-thaw testing apparatus. Quick-freezing method was employed, in other words, one freeze-thaw cycle lasted only 3 h including 1 h of frozen time and the 2 h of thaw time. The freezing and thawing temperatures were 17 ± 3 °C and 5 ± 2 °C. After 25, 50, 75 and 100 freezethaw cycles, three specimens for each mixture were taken out to

Lt -L0 250

ð5Þ

where 250 is the effective length (mm) of specimens. Mercury porosimetry (MIP) is a common method for analyzing the porosity of MPC mortar. The broken specimens are selected from the hardened specimens after compressive strength measurements before and after water immersion. The minimal pore size measured by MIP is smaller than 10 nm. MPC pastes corresponding to MPC mortars were prepared for characterizing the microstructure. The specimens for backscattered electron (BSE) are mainly rectangular rectangular or cubic. The samples are impregnated in isopropanol for terminating the hydration, and the oven-dried temperature is about 40 °C. Samples for X-Ray Powder Diffraction (XRD) required grinding and the maximal particle size was smaller than 45 lm. The scanning rates during XRD tests are fixed at 0.01°/step. The samples for Fourier Transform Infrared Spectroscopy (FT-IR) were prepared by the KBr tablet method. The solid sample mixed with a transparent alkali halide (KBr) in a mold which is subjected to a clamping force to obtain a clear pill that allows its intrusion in the analysis equipment. The spectral range that was used to characterize MPC was between 400 and 4000 cm1 [15]. 3. Results and discussion 3.1. Rheological behavior of MPC mortar The relationships between shear stress and shear rate measured on fresh MPC mortar are presented in Fig. 3. Thixotropic loop can be observed in each rheological curve, and its ar increases with increasing the metakaolin content. This implies that the presence of metakaolin improves the thixotropy of MPC mortar. The rela-

Disk

Disk

MPC

water

MPC

Roughness surface Concrete substrate

(a)

Roughness surface Concrete substrate

(b)

Fig. 2. Sketch for the tests of flexural bonding strength of MPC mortar (a) under standard curing condition and (b) after water immersion.

Z. Qin et al. / Construction and Building Materials 234 (2020) 117353

tionship of shear stress and shear rate can be analyzed by Bingham model as the follows.

s ¼ s0 þ l  r_

ð6Þ

where s (Pa) is shear stress, s0 (Pa) is yield stress, l (Pa∙s) is plastic viscosity and r_ (s1) is shear rate. The measured results can be simulated by the Bingham model, and the fitted curves are also shown in Fig. 3. The obtained yield stress and plastic viscosity of different MPC mortars are enumerated in Table 3. Metakaolin has different influences on the yield stress and plastic viscosity. When the metakaolin content is lower than 15%, the yield stresses in both ascent and descent stages change a little. This indicates that a small quantity of metakaolin does not affect the yield stresses of MPC mortar. There is a sharp increase in yield stress when the metakaolin content is higher than 15%. The plastic viscosity increases with increasing the metakaolin content. That is to say, metakaolin in MPC has tackifying effect, even though the polymeric dispersant is used.

3.2. Compressive strength Fig. 4 shows the compressive strength of MPC mortar with different contents of metakaolin. MPC mortar gains high strength at 1 h of hydration, and the 1-hour strength of MPC mortar without metakaolin is about 67.1% of the 28-day strength. The strength increases gradually with curing time. The presence of metakaolin has slight influence on the 1-hour strength and increases the subsequent strength significantly. The 1-hour strengths of MIXs M00, M15 and M30 are 26.7 MPa, 27.5 MPa and 29.5 MPa, respectively. The 28-day strength increases significantly with the metakaolin content increases from 0 to 30%, and there is only a small increase in the 28-day strength when 45% metakaolin was employed. This observation is consistent with the results reported by Lu and Chen [22]. The 28-day strength of MIX M30 is 65.9 MPa, 65.6% higher than that of MIX M00. Hydration degree and microstructure are the dominant factors for cement composites. Although the replacement of MgO by metakaolin reduces the hydration degree and decreases the quantity of hydration products to some extent, the possible filling effect of metakaolin is also a helpful contributor to the compressive strength. As the hydration reaction proceeds, metakaolin may take participate in pozzolanic reaction, hedging the adverse effect of the decrease in the amount of struvite.

1200

M00 M07 M15 M30 M45

Shear stress (Pa)

1000

800

5

3.3. Tensile bonding strength Fig. 5 presents the tensile bonding strength (fT) of MPC mortars measured. The development in tensile bonding strength of different specimens is nearly consistent with the tendency observed in compressive strength. In other words, the higher compressive strength the greater tensile bonding strength. The 1-day fT increases with increasing the metakaolin content from 0 to 45%. The 1-day fT of MIX M45 is 1.83 MPa, only 33.3% higher than that of MIX M00. As curing time proceeds, the influence of metakaolin on tensile bonding strength is more significant. The 3-day and 28day fT values of MIX M45 are 2.94 MPa and 3.71 MPa, 73.9% and 75.1% higher in comparison to the strength of MIX M00. The failure locations of MIXs M00 and M07 were often in MPC mortar, and those of MIXs with metakaolin higher than 15% were generally in the substrate OPC concrete. This indicates that the presence of metakaolin improves the bonding property of MPC significantly. The bonding strength relates to the quantity of reaction products and pore structure in the interface [30]. Hence, the porosity and pore structure could be ameliorated through introducing a moderate amount of metakaolin into MPC. 3.4. Porosity Fig. 6 presents the MIP results expressed by cumulative porosity of MPC mortars with different contents of metakaolin. It is observed that only about half of the pores in MPC mortar are smaller than 1 um. This indicates that the pore structure of MPC mortar is much coarser than that of Portland cement composites, which was also observed in previous studies [3,31,32]. The addition of 7.5% metakaolin decreases the total porosity of MPC mortar significantly, and the porosity decreases further with increasing the metakaolin content from 15% to 30%. And there is only a small decrease in porosity as the metakaolin content increases from 30% to 45%. The total porosities of MIXs M00, M07, M15 and M30 are 15.90%, 14.70%, 12.86% and 11.48%, respectively. MIX M45 has the minimum porosity, which is almost consistent with the tendency presented in compressive strength. This observation can also explain the influence of metakaolin on tensile bonding strength. In other words, the decrease in porosity increases the quantity of hydration products bonding on the substrate concrete and improves the pore structure of the interface. Power function was proved to be suitable to describe the relation between compressive strength and porosity of MPC mortar [3]. Likewise, this function can also be used to analyze the development of bonding strength with porosity. Fig. 7 shows the relationship and fitted curves of compressive strength and tensile bonding strength to porosity of MPC mortars with different contents of metakaolin at 28 days. The fitted formulae have very high coefficients of determination greater than 0.95. This confirms the increase in strengths following the decreasing porosities in an inversely proportional way. 3.5. Drying shrinkage

600

400

200

0 0

50

100

150

Shear rate (s-1) Fig. 3. The rheological curves of MPC mortar.

200

The development of drying shrinkage of MPC mortar with curing time is presented in Fig. 8. It is observed that the drying shrinkage increases rapidly within 14 days of hydration, and the 14-day shrinkages reach about seventy percent of the 60-day shrinkage. And the 60-day shrinkage of MPC mortar is only about 60 microstrain, which is only one tenth of that of Portland cement composites whose 60-day drying shrinkages can reach about 1000 microstrain [23]. This is one of the most important reasons for the superiority of MPC composites used as repairing materials. The presence of metakaolin reduces the drying shrinkage, and the 60-day shrinkage decreases firstly and then increases with

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Table 3 The fitted results of yield stress and plastic viscosity from Fig. 3. MIX

M00 M07 M15 M30 M45

Ascent stage

Descent stage

s0

l

R

s0

l

R2

149.9 137.3 154.1 246.5 272.9

1.63 2.27 2.53 2.75 3.91

0.912 0.933 0.927 0.773 0.932

22.0 17.1 16.4 32.6 84.8

1.95 2.39 2.73 3.09 4.18

0.996 0.997 0.999 0.999 0.996

2

18

M00 M07 M15 M30 M45

15

Porosity (%)

12

9

6

3

0 0.01

0.1

1

10

100

Pore diameter (μm) Fig. 4. Compressive strength of MPC mortar with different contents of metakaolin. Fig. 6. MIP results expressed by cumulative porosity curves for MPC mortars.

5

fC

70

fT

fC=1039.4/φ -25.89,R =0.993

60

3

50 2 2

fT=56.36/φ -1.46, R =0.986 40

30 10

1

Tensile bonding strength, fT (MPa)

Compressive strength, fC (MPa)

4 2

0 11

12

13

14

15

16

Porosity,φ (%) Fig. 5. Tensile bonding strength of MPC mortars with different contents of metakaolin.

increasing the metakaolin content. And the MIX M30 with 30% metakaolin has the minimum 60-day shrinkage, about half of that of MPC mortar without metakaolin. Previous studies concluded that the hydration degree of MPC is the dominant factor for the drying shrinkage [4,32]. Hence, the addition of mineral admixtures, such as fly ash, slag and metakaolin can reduce the drying shrinkage of MPC mortar. Moreover, the more compact microstructure in MPC mortar with metakaolin may be also responsible for the improvement of shrinkage deformation.

Fig. 7. Relationship of compressive strength and tensile bonding strength to porosity of MPC mortars at 28 days.

3.6. Freeze-thaw resistance The compressive strength of MPC mortars after different freezethaw cycles and the corresponding strength retention coefficient are shown in Fig. 9. The decrease ratios (%) of compressive strength of MPC mortars after freeze-thaw cycles are presented in Table 4. For each MPC mortar, the compressive strength decreases gradually with increasing the freeze-thaw cycles. It is also observed that

Z. Qin et al. / Construction and Building Materials 234 (2020) 117353

the lower freeze-thaw resistance. The reduction in porosity and improvement of pore structure through using metakaolin decrease the expansive force, which explains the improvement of freezethaw resistance in MPC mortar.

0

M00 M07 M15 M30 M45

Drying shrinkage (microstrain)

-10

-20

3.7. Water resistance

-30

-40

-50

-60 0

10

20

30

40

50

7

60

Curing time (days) Fig. 8. Drying shrinkage of MPC mortars with different contents of metakaolin.

there is only a slight decrease in compressive strength within 50 cycles and the remaining strength more significantly decreases with increasing the freeze-thaw cycles from 50 to 100. This implies that the freezing resistance of MPC is weaker than that of Portland cement-based materials. This observation is consistent with the results reported previously [22]. The presence of metakaolin increases the remaining strength remarkably, and the strength values after different freeze-thaw cycles increase with increasing the metakaolin content. The remaining strength of MPC mortar with 45% metakaolin after 100 cycles is 56.1 MPa, more than twice of that of MIX M00. Moreover, the strength retention coefficient also increases with the increasing metakaolin content, and that of MIX M45 is about 0.81 which is 28.6% higher than that of MIX M00. The mechanism of freeze-thaw damage in cement-based materials was explained by the expansive force caused by the transformation of water to ice in pore structure. Hence, the degree of freeze-thaw damage is dependent upon the ice content which has a direct correlation with porosity of cement-based materials. Because MPC mortars have higher porosities and the pore structure is much coarser than that of Portland cement mortar, the internal expansive force during freeze-thaw tests is much higher, resulting in

Fig. 10 presents the residual compressive strength, tensile bonding strength and strength retention coefficients of MPC mortars after 28 days of water immersion. The RCC value of MPC mortar without metakaolin is only 0.76 and the corresponding residual compressive strength (fC28) is only 30.3 MPa. The presence of metakaolin increases the fC28 significantly and the maximum fC28 is observed in MIX M45. The RCC value increases significantly with increasing the metakaolin content from 0 to 15% and then increases slightly when the metakaolin content is higher than 15%. The values of fC28 and RCC of MIX M45 are 63.7 MPa and 0.92, 102.3% and 21.1% higher than those of MIX M00, respectively. The influencing mechanism of metakaolin in water resistance of MPC mortar is similar to that in freeze-thaw resistance, which can be explained by the more compact microstructure obtained by adding metakaolin. The tendency in tensile bonding strength is similar to the above observation, and the maximum residual bonding strength (fT28) and strength retention coefficient (RCT) also occur in MIX M45. The values of fT28 and RCT of MIX M45 are 3.11 MPa and 0.83, 144.8% and 22.1% higher than those of MIX M00, respectively. It is noteworthy that the RCT value is lower than the RCC value for each MPC mortar, which indicates a more significant degradation occurs in tensile bonding strength in comparison to compressive strength. The possible reason for this phenomenon may be that the origin of the bonding strength between MPC mortar and substrate concrete is the interfacial contact and the water migration into the interface decreases the cohesive force of the hydration products to substrate concrete more significantly. 3.8. XRD analysis Fig. 11 shows the XRD patterns of specimens M00 and M30 before and after 28 days of water immersion. The main hydration products are also struvite, and no new diffraction peaks referring to some newly generated products by adding metakaolin can be detected by XRD analysis. There is no significant difference in the diffraction peaks of MPC pastes before and after water immersion. This indicates that the crystal products change little under water curing condition. The broad hump can also be observed in metakaolin blended MPC paste after water immersion, indicating the strong combination between unreacted metakaolin particles and hydration products. And the leaching loss of metakaolin could not occur, which is beneficial to keeping the compactness of the microstructure. In addition, the leaching loss of the gelatinous intermediate products could not be detected by XRD analysis. 3.9. FT-IR analysis

Fig. 9. Remaining compressive strength and strength retention coefficient after freeze-thaw cycles.

Fig. 12 presents three examples of characteristic bands of FT-IR measured at 28 days. Although the unreacted MgO is main material in hardened MPC pastes, the vibration peak of Mg-O can be only detected at 420 cm1. The peaks resembling hydration products including struvite can be observed. The stretching vibration 1 absorption peaks of NH and 1435 cm1. 4 appear at 3225 cm 3 The absorption peaks of PO4 appear multiple times at 640 cm1, 770 cm1 and 1060 cm1, respectively. It is noteworthy that the stretching vibration peak of H2PO 4 is also detected. This may represent the generation of newberyite, but that cannot be detected in XRD patterns. For this reason, H2PO 4 may be free state in hardened MPC. The peaks at 1660 cm1 and 3430 cm1 represent the bend-

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Table 4 Decrease ratio (%) of compressive strength of MPC mortars after freeze-thaw cycles. MIX ID

Decrease ratio (%) after freeze-thaw cycles

M00 M07 M15 M30 M45

25

50

75

100

7.3 5.1 5.4 3.2 2.0

13.8 9.1 8.3 8.1 8.0

23.1 16.9 14.3 12.1 11.9

36.9 32.1 25.2 20.0 19.0

50

M00 M15 M30

Transmitance (%)

40

30 O-H

20

PO43-

NH4+

PO43-

10

NH4+

Mg-O Si-O-Si

O-H -

H2PO4 PO43-

0 Fig. 10. Compressive strength, tensile bonding strength and strength retention coefficients of MPC mortar after 28 days of soaking.

500

1000

1500

2000

2500

3000

3500

4000

Wavenumber (cm-1) Fig. 12. FT-IR spectra of MPC pastes.

Fig. 11. XRD patterns of MPC pastes after water immersion.

ing vibration and stretching vibration of bonding water [41]. The introduction of metakaolin into MPC has little effect on the peak position and types, and only one stretching vibration peak resembling Si-O appears at 575 cm1. Even so, the peak intensity in the three samples changes significantly, indicating the sharp decrease in the amount of hydration products. However, the FT-IR spectra of MIX M15 and M30 are almost coincident. It can be deduced that physical effect of metakaolin play an important role in MPC.

3.10. BSE analysis Fig. 13 presents the microstructure of hydrated pastes without metakaolin and with metakaolin analyzed by BSE at 28 days. A

relatively simple microstructure is observed in the MPC paste without metakaolin, M00, and many unreacted magnesia particles embeds in the struvite crystals of short columnar and wedgeshaped [24,36,37]. And many micro cracks appear in specimen CON. After adding metakaolin, the microstructure becomes more complicated and less unreacted magnesia particles are noticed due to the possible pozzolanic reactions. And many metakaolin particles imbedded in the interlayer of struvite are observed. It is noteworthy that the presence of metakaolin decreases the number of micro cracks and improve the compactness of MPC. The filling effect of metakaolin is indeed conducive to form the more compacted microstructure. This is an evident for the improvement of compressive strength and porosity discussed in previous section. As reported in previous studies [7,33], the hydration process of MPC can be divided into three steps, dissolution and sol formation, acid-base reaction and gel condensation, saturation and crystallization of the gel. As the hydration proceeds, the continuous crystal growth on the surface of magnesia may lead to the internal stress, hence the micro cracks (Fig. 13a). Because metakaolin with a high specific surface area can provide a lot of sites for crystallization, gel condensation and crystallization can occur on the surface of metakaolin. This can promote the crystal growth more orderly, and the more homogeneous struvite clusters can be formed on the surface of metakaolin. Furthermore, the presence of metakaolin particles increases the distance between the adjacent magnesia particles, which can reduce the internal force caused by crystal growth [35,38,39]. These can be responsible for the observation of the more compacted and homogeneous microstructure in metakaolin blended MPC. Aluminum and silicon are the main elements in metakaolin, which is different from struvite and magnesia in hydrated MPC pastes. The element distribution of the randomly selected area

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Z. Qin et al. / Construction and Building Materials 234 (2020) 117353

(a) ᧤

(b) ᧤

(c) ᧤

(d) ᧤

Fig. 13. Microstructure of MPC pastes M00 (a), M07 (b), M15 (c) and M30 (d) at 28 days.

through BSE analysis is shown in Fig. 14. The grey level of the outer rim of metakaolin particles is different from that of the central area, indicating the dissolution and chemical reaction of metakaolin particles. Although the Mg, P, Si and Al elements mainly dis-

tribute in unreacted magnesia particles, struvite and unreacted metakaolin particles, the fuzzy boundary of metakaolin particles confirms the chemical reaction. And the reaction products may be potassium aluminosilicate or alumina phosphate hydrate

(b)

(a)

MK

Mg

P

Al

Si

Fig. 14. (a) BSE image and (b) Element maps of MIX M30 at 28 days.

O

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Z. Qin et al. / Construction and Building Materials 234 (2020) 117353

(a) ᧤

(b) ᧤

(c) ᧤

(d) ᧤

Fig. 15. Microstructure of MPC pastes M00 (a), M07 (b), M15 (c) and M30 (d) after 28 days of water immersion.

[17,22,40]. In addition, there is a large difference in the invasion depths of Si and Al elements into hydration products. This implies that the reaction of the aluminum and silicon phases in MPC is inconsistent. Fig. 15 presents the microstructure of MPC pastes M00, M07, M15 and M30 after 28 days of water immersion. Except many fine cracks exist in the specimen after soaking, a huge and long crack with a width of about several micrometers is also observed in MPC pastes for the first time. The micro cracks become much larger, mainly due to the water migration in interconnected capillary pores. Although the hydration products of MPC are mainly crystalline phases, there are still some gelatinous intermediate products whose dissolvability is much higher than struvite in hydrated MPC. The gel dissolution out of crystallization structure decreases the compactness of microstructure, resulting in the increase of the porosity and degradation of strength. For metakaolin was added into MPC, the width of the microcrack reduces as shown in Fig. 15b. And when the metakaolin content was 30%, there is no significant difference in microstructure before and after water immersion. This observation corresponds to the results of water resistance and porosity tests. 4. Conclusion This study investigated the influence of partial replacement of magnesia by metakaolin on compressive strength, tensile bonding strength, drying shrinkage and water resistance of MPC mortar. The influence of metakaolin on the microstructure of MPC paste was studied by BSE. The following conclusions can be drawn. (1) The presence of metakaolin increases the 28-day compressive strength and tensile bonding strength of MPC mortar. The strengths increase significantly with increasing the metakaolin content to 30%. When the metakaolin content is higher than 30%, the increase in the strengths is slight. It

is noteworthy that metakaolin has little influence over the 1-hour compressive strength. The decrease in hydration degree and improvement of microstructure are responsible for this observation. (2) The drying shrinkage increases rapidly within 14 days of hydration, and the 14-day shrinkages reach about seventy percent of the 60-day shrinkage. MPC mortar blended with 30% metakaolin has the minimum drying shrinkage, about half of that of MPC mortar without metakaolin. Metakaolin improves the freeze-thaw and water resistance of MPC mortar significantly. The remaining strengths of MPC mortar with 45% metakaolin after 100 freeze-thaw cycles and 28 days of water immersion are more than twice of those of MPC mortar without metakaolin. (3) Only about half of the pores are smaller than 1 um, which confirms the coarser pore structure of MPC mortar in comparison to Portland cement-based materials. Metakaolin decreases the total porosity of MPC mortar, which can explain the improvement of freeze-thaw resistance and water resistance by adding metakaolin. And the relationships of compressive strength and tensile bonding strength to porosity can be analyzed by power functions. (4) Metakaolin provides a lot of sites for crystallization of hydration products, which promotes the crystal growth more orderly. As a result, metakaolin decreases the number of micro cracks and improve the compactness of MPC. And the dissolution and chemical reaction of metakaolin in MPC are confirmed, even though the mineralogical characterization by XRD and TG-DTG analysis indicate that there are no new hydration products generated in metakaolin blended MPC. After water immersion, the micro cracks in MPC paste become much larger, and the change in microstructure of MPC paste with metakaolin is not significant. This can further explain the reason for the improvement of freeze-thaw and water resistance.

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