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Roles of metakaolin in magnesium phosphate cement: Effect of the replacement ratio of magnesia by metakaolin with different particle sizes
Construction and Building Materials 227 (2019) 116675
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Roles of metakaolin in magnesium phosphate cement: Effect of the replacement ratio of magnesia by metakaolin with different particle sizes Zhaohui Qin a, Shengbo Zhou b,c, Cong Ma d,⇑, Guangcheng Long a, Youjun Xie a, Bing Chen a a
Department of Civil Engineering, Shanghai Jiao Tong University, Shanghai 210240, PR China Guangxi Key Lab of Road Structure and Materials, Guangxi Xinfazhan Communications Group Co., Ltd., Nanning, Guangxi Province 530029, PR China Guangxi Transportation Research & Consulting CO., Ltd., Nanning, Guangxi Province 530029, PR China d School of Civil Engineering, Central South University, Changsha, Hunan Province 410075, PR China b c
h i g h l i g h t s Metakaolin improves the mechanical properties and durability of MPC. Metakaolin decreases the early-term hydration heat of MPC and the total porosity. Mechanisms of metakaolin in MPC include providing nucleation cites, filling effect and pozzolanic effect.
a r t i c l e
i n f o
Article history: Received 11 March 2019 Received in revised form 4 August 2019 Accepted 6 August 2019
Keywords: Magnesium phosphate cement Metakaolin Properties Microstructure Mechanism
a b s t r a c t Different particle sizes of metakaolin were employed to replace magnesia partially, and it goes MK1, MK2 and MK3 in order of the specific surface area. Experimental results indicate that the presence of metakaolin indeed improves the compressive strength, tensile bonding strength and water resistance of MPC mortar through sacrificing the workability to some extent, and 30% MK2 blended MPC mortar has the best performances except fluidity. Metakaolin decreases the porosity and improves the pore structure of MPC mortar, especially for specimens after water immersion. The dissolution and reaction of metakaolin in MPC are confirmed by BSE analysis. Nevertheless, no new hydration products can be detected in MPC paste with metakaolin through XRD and TG-DTG analysis. The condensation of intermediate products, crystallization and crystal growth occurring on the surface of metakaolin promote the formation of the more compacted and homogeneous microstructure, which is the physical mechanism of metakaolin improving the properties of MPC. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction Magnesium phosphate cements (MPC) invented in the late 1930s 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 acidbase 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). And the common used magnesia is dead burnt magnesia because light burnt magnesia may result in the flash hardening ⇑ Corresponding author. E-mail address: [email protected] (C. Ma). https://doi.org/10.1016/j.conbuildmat.2019.116675 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.
of MPC. Even so, retarders, such as boric acid, borax and metaphosphate should be employed to prolong the setting time of MPC [6– 8]. The types of hydration products are related to the pH value of the reaction environment, and the main crystalline product 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
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Z. Qin et al. / Construction and Building Materials 227 (2019) 116675 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. Three types of metakaolin obtained from a single manufacturer were employed and the oxide composition is also shown in Table 1. The main difference in the three metakaolin is the fineness as represented by the mean particle size (dm) and BET surface area (SBET). The retarder (CRT) used is a type of composite additive consisting of sodium borate (Na2B4O710H2O) and sodium tripolyphosphate at 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) was used.
has unique advantages of higher bonding strength and lower shrinkage. 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 cementitious materials commonly used in Portland cement including fly ash, slag, silica fume and metakaolin were introduced into MPC [16–22]. 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). And 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) [17,23,24]. However, the reaction mechanism and products are not entirely clear. Even though fly ash can improve the strength to some extent, there is no significant improvement in the water resistance of MPC [23]. 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. The usage of metakaolin in Portland cement-based materials is very common, but there are only very a few studies on metakaolin blended MPC [25–27]. Judged from the previous results, metakaolin can significantly increase the strength, prolong the setting time and improve the water resistance of MPC [22]. And the high content of active alumina phase may be responsible for the improvement of the properties of MPC. However, the influencing mechanism of metakaolin in MPC is not entirely clear. Moreover, there are many kinds of commercial metakaolin with different fineness and calcination temperature in China, which may affect the properties of MPC admixed with metakaolin. In this study, three types of metakaolin with different fineness were employed to prepare MPC mortar or paste. The effects of metakaolin on the properties of MPC mortar were investigated by testing the fluidity, compressive strength, tensile bonding strength, porosity and water resistance characterized by the retention coefficients of compressive strength and tensile bonding strength. Microstructure and mineral changes through introducing metakaolin into MPC paste were analyzed by BSE, XRD and TG-DTG analysis. The results not only provide the guidance for employing different types of metakaolin to improve the properties of MPC but also give a perspective of the mechanism of metakaolin in MPC.
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 are fixed 0.2 and 1.0, respectively. Retarder was added by weight of MgO. Metakaolin was introduced into MPC by replacing MgO, and the MK replacements were 0%, 15% and 30%, respectively, by weight of MgO. Series S1, S2 and S3 are specimens with MK1, MK2 and MK3, respectively. 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 mixture and the wet mixing lasted about three minutes until the flowing mixture was obtained. If necessary, several measurements are carried on the fresh MPC mortar in order to investigate the fresh property. And then, the fresh mortar was transferred into PVC moulds (40 mm 40 mm 160 mm) and homogeneous mixtures can be obtained by some forms of vibration. After casting, the moulds were placed in a standard curing room with a stable temperature of 20 ± 2 °C and relative humidity of 95 ± 3%. The specimens were demoulded after 1 day of curing, and the demoulded specimens were transferred to the standard curing room until the measuring ages. Fresh property of MPC mortar was investigated through measuring the initial fluidity. According to Chinese Standard GB/T2419-2005, electric jumping table is employed to measure the fluidity of MPC mortar. During the measurements, the mortar was immediately transferred onto the jumping table after mixing. The fluidity is the mean value of diameters of thin-pancake mortar after 25 times of vibration. At 1 h, 7 days and 28 days of curing, specimens were taken out from standard curing room for the measurements of compressive strength in accordance to ASTM C348. The final strength results are the mean values of at least three reproducible specimens, and it is notable that the measured results with mean standard deviations of over 10% should be eliminated. 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. 1a. Sand blasting surface treatment was carried on to obtain the rough surface of concrete substrate. 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 into the PVC moulds, the specimen was cured under standard condition for 1 and 28 days. As shown in Fig. 1a, 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 higher strength retention coefficient indicates better water resistance of materials [11,22,28]. In this study, not only the retention coefficient of compressive strength
2. Experimental methodology 2.1. Materials The 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
Table 1 Oxide composition of MgO and metakaolin. Materials
SiO2
Al2O3
Fe2O3
MgO
CaO
K2O
Na2O
LOI
dm (um)
SBET (m2/g)
MgO MK1 MK2 MK3
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 23.5 14.9 10.6
1.26 2.37 9.63 13.95
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Z. Qin et al. / Construction and Building Materials 227 (2019) 116675 Table 2 Mix proportions. Sample No.
CON S11 S12 S21 S22 S31 S32
Binder components (referred to 1 g ADP) MgO
ADP
MK1
MK2
MK3
CRT
PD
3.0 2.55 2.1 2.55 2.1 2.55 2.1
1.0 1.0 1.0 1.0 1.0 1.0 1.0
0 0.45 0.9 0 0 0 0
0 0 0 0.45 0.9 0 0
0 0 0 0 0 0.45 0.9
0.18 0.153 0.126 0.153 0.126 0.153 0.126
0 0.005 0.009 0.005 0.009 0.005 0.009
s/b
w/b
1.0 1.0 1.0 1.0 1.0 1.0 1.0
0.2 0.2 0.2 0.2 0.2 0.2 0.2
Disk
Disk
MPC
water
Roughness surface
MPC
Roughness surface Concrete substrate
Concrete substrate
(a)
(b)
Fig. 1. Schematic diagrams of experiments for flexural bonding strength of MPC mortar (a) under standard curing condition and (b) after water immersion.
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. 1b and the water immersion lasted 28 days. And then, the specimens were taken out for measuring the after-soaking compressive strength and tensile bonding strength of MPC mortar. The retention coefficients can be expressed as the following equations [35,36].
f RC C ¼ Cn fC
ð2Þ
f Tn fT
ð3Þ
RC T ¼
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. Mercury intrusion porosimetry (MIP) was employed to characterize the porosity of MPC mortar. Some crushed samples are collected from the hardened specimens after compressive strength measurements before and after water immersion. The sizes of selected samples are about 2–4 mm. Before porosity tests, the samples should be oven-dried at a temperature lower than 60 °C. And the measured smallest pore size is about 5.5 nm. During the measurements, the intrusive volume of Hg should be normalized according to the weight of samples. An in-situ isothermal calorimeter named as TAM Air was employed to carry out isothermal calorimetry tests. TAM Air apparatus has four parallel twin-chamber measuring channels, and one chamber contains the MPC paste [29]. In order to avoid considerable temperature differences between the sample and the isothermal environment, the TAM Air apparatus was adjusted close to the measurement temperature of 20 ± 0.02 °C before sample preparation. The fresh paste is mixed in standard curing room and immediately transferred into an ampoule. And then, the ampoule is placed into the apparatus, and the heat flow could be recorded. The recording duration is about 20 h, and the cumulative heat can also be normalized. MPC pastes corresponding to MPC mortars were prepared for characterizing the effects of metakaolin on microstructure and mineral phases. Samples for X-Ray Powder Diffraction (XRD) required grinding and the maximal particle size was smaller than 45 um. The ultra-fine power is made into thin sheets which can be scanned on an X-ray diffractometer with a CuKa source [21,33]. The scanning region ranges from 10° 2h to 60° 2h with a scanning rate of 0.01°/step. Some rectangle or cubic samples are prepared for back-scattered electron (BSE) [37,38]. The samples are impregnated with isopropanol, and then dried in a vacuum oven at 40 °C for about 2 days. The oven-dried samples are stuck to the stubs by using conducting resin. Before BSE analysis, carbon-coating process is essential due to the poor electrical conductivity of inorganic powder. Samples for thermogravimetric
and derivative thermogravimetry (TG/DTG) analysis were also ground and ovendried. TG/DTG analysis was performed on a thermal analyzer, from 20 °C (room temperature) to 990 °C with a heating rate of 10 °C/min.
3. Results and discussion 3.1. Properties of MPC mortar 3.1.1. Fluidity Fig. 2 shows the influence of different types of metakaolin on the fluidity of MPC mortar. It is observed that the presence of metakaolin decreases the fluidity and reduces the workability of MPC mortar, even though the polymeric dispersant is used. MPC mortar without metakaolin has the largest fluidity and its value is about 26.5 cm. And the fluidity of specimen with 30% MK3 is
Fig. 2. The fluidity of different MPC mortar specimens.
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only 18.6 cm, only 81.6% of that with 30% MK1. On the one hand, the presence of high content of metakaolin can generate much higher total specific surface area, and this results in the sharp increase of the water requirement. On the other hand, the microstructure of metakaolin is the irregular sheet structure which is completely different from fly ash, in other words, the morphological effect generated by the spherical beads of fly ash can not occur in metakaolin blended cement composites [30,31]. 3.1.2. Compressive strength The influence of different types and contents of metakaolin on the compressive strength of MPC mortar is presented in Fig. 3. There is only a slight difference in the ultra-early strength of different specimens, and the 1-hour strengths of MPC mortar specimens CON, S12, S22 and S32 are 26.7 MPa, 27.8 MPa, 29.5 MPa and 24.9 MPa, respectively. For MPC mortar without metakaolin, the strength increases slowly with increasing curing time to 28 days, and the 1-hour strength can reach about seventy percent of the 28-day strength. The usage of metakaolin indeed increases the middle-term compressive strength, and the highest strength is observed in MPC mortar with 30% MK2. MPC mortar with 30% metakaolin can achieve the 28-day strength of over 60 MPa, about fifty percent higher than that of MPC mortar without metakaolin. The dominant factors for the compressive strength are the degree of hydration reaction and the compactness of microstructure. Although the addition of metakaolin may decrease the amount of hydration products, the filling effect of metakaolin is also a helpful contributor to the compressive strength. More importantly, the increase in compressive strength of MPC mortar with metakaolin from 1 h to 28 days is much higher in comparison to that of specimen without metakaolin. The 1-hour strengths of specimens S12 and S22 are only 49.5% and 44.8% of the 28-day strengths, respectively. This indicates that the presence of metakaolin improves the middle- and long-term strength of MPC mortar due to the possible pozzolanic reaction and the improvement of microstructure. However, the mechanism of metakaolin in MPC has not been really known yet, which need further study. 3.1.3. Tensile bonding strength Fig. 4 presents the influence of different types and contents of metakaolin on the tensile bonding strength of MPC mortar. The development of tensile bonding strength of different MPC mortar specimens is nearly consistent with the tendency observed in compressive strength. In other words, the higher compressive strength
Fig. 4. Tensile bonding strength of MPC mortars with different types of metakaolin.
Fig. 5. Compressive strength, tensile bonding strength and strength retention coefficients of MPC mortar after 28 days of soaking.
the greater tensile bonding strength. And the addition of metakaolin increases the bonding strength of MPC mortar significantly. The 28-day tensile bonding strength of specimen S22 is 3.5 MPa, about 65.7% higher than that of specimen without metakaolin. The bonding strength relates to the amount of hydration products and pore structure in the interface between MPC mortar and substrate concrete. Firstly, a portion of metakaolin particles take participate in pozzolanic reaction, which may relieve the adverse effect of the reduction in the struvite amount. Secondly, the porosity and pore structure of MPC mortar may be improved by adding a moderate amount of metakaolin. Another interesting phenomenon is that the 1-day and 28-day strengths of 30% MK3 blended MPC mortar are almost the same as those of specimens with 15% MK3. The gathering of unreacted metakaolin in the interface may be responsible for this phenomenon.
Fig. 3. Compressive strength of MPC mortars with different types of metakaolin.
3.1.4. Water resistance Fig. 5 presents the residual compressive strength, tensile bonding strength and strength retention coefficients of different MPC specimens after 28 days of water immersion. The RCC value of MPC mortar without metakaolin is only 0.76 and the correspond-
Z. Qin et al. / Construction and Building Materials 227 (2019) 116675
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ing residual compressive strength (fC28) is only 30.3 MPa. The presence of metakaolin increases the fC28 significantly and the maximum fC28 is observed in specimen S22. The values of fC28 and RCC of specimen S22 are 59.9 MPa and 0.91, 97.7% and 19.7% higher than those of specimen without metakaolin. The tendency in tensile bonding strength is similar to the above observation, and the maximum residual bonding strength (fT28) also occurs in the presence of MK2. The RCT value of 30% MK2 blended MPC mortar is about 0.82, 26.2% higher than that of MPC mortar without metakaolin. 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 that in compressive strength. The possible reason for this phenomenon is 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.
3.1.5. Porosity The MIP results expressed by cumulative porosity of the selected MPC mortar specimens before and after water immersion are presented in Fig. 6. The addition of 10% MK1 decreases the total porosity of MPC mortar significantly, and the porosity decreases further with increasing the metakaolin content from 10% to 30%. The total porosities of specimens CON, S11 and S12 are 15.90%, 14.01% and 12.69%, respectively. In comparison to metakaolin content, the metakaolin fineness can be regarded as the secondary influencing factor for porosity of MPC mortar. In addition, the specimen S22 has the lowest porosity among the measured specimens, which is almost consistent with the tendency presented in compressive strength. The possible reason may be that the gathering and concentrating of metakaolin particles with the largest specific surface area decrease the compactness of hardened MPC mortar. Another interesting observation is that the MPC mortar after water immersion has a much higher porosity. More importantly, water immersion makes the pore structure of MPC mortar much coarser. This can explain the strength degradation of MPC mortar after water immersion. In comparison to Portland cement-based materials, the pore size in MPC mortar, especially in MAPC mortar with ADP, was proved to be larger by previous researchers [7,32]. The pores smaller than 50 um are considered as capillary pores and Fig. 7 presents
Fig. 6. MIP results expressed by cumulative porosity curves for several MPC mortar specimens before and after water immersion.
Fig. 7. The proportion of different sizes of pores in MPC mortar.
the distribution of different sizes of pores. The presence of metakaolin decreases the number of capillary pores remarkably. The decrease in the number of air pores larger than 50 um can be explained by the adsorption effect of the metakaolin with huge specific surface. And the ammonia gas during the preparation of MPC with metakaolin smells much thinner than that without metakaolin. The filling effect of metakaolin can be responsible for the change in capillary pores. For specimens after water immersion, the proportion of larger capillary pores (1–50 mm) becomes much higher. The number of larger capillary pores and air pores in specimen CON after soaking is almost twice of that before soaking. And the increase in large pores of specimen with metakaolin is less significant. This is one of the reasons for the improvement of water resistance by adding metakaolin. 3.2. Characterization of MPC paste 3.2.1. Heat release rate of hydration Fig. 8 shows the normalized heat flow of hydration of MPC pastes with different contents and types of metakaolin. The heat release rate increases sharply within the first several minutes for each specimen. As reported in previous studies [33,34], the hydra-
Fig. 8. Normalized heat flow of hydration of MPC pastes.
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Z. Qin et al. / Construction and Building Materials 227 (2019) 116675
tion kinetics of MPC can be divided into three steps, dissolution and sol formation, acid-base reaction and gel condensation, saturation and crystallization of the gel. The dissolution of ADP and the acid-base reaction is very quick, resulting in the ultrafast exothermicity during the initial hydration process. When metakaolin was introduced to replace magnesia, the number of hydroxide ions from MgO dissolution reduces and the intensity of acid-base reaction weakens, which decreases the maximal heat release rate and prolongs the arrival time of the peak value. And the maximal heat release rate decreases further with increasing metakaolin content from 15% to 30%. This explains the improvement of setting time of MPC mortar by adding metakaolin reported in [22]. Although the metakaolin contents in specimens S12, S22 and S32 are the same, there is obvious differences in the maximal heat release rate and arrival time of the peak value due to the different particle sizes of metakaolin. The maximal heat release rate increases, and the arrival time of the peak value is shortened with the increasing fineness of metakaolin. Although there is little difference in the acidbase reaction of specimens with different types of metakaolin, the gel condensation on the surface of metakaolin becomes faster as the specific surface area of metakaolin increases. In addition, the heat release rate in the descent stage of metakaolin blended MPC is slightly higher than that of pure MPC. This indicates the presence of metakaolin increases the speed of saturation and crystallization of the gel. 3.2.2. BSE analysis Fig. 9 presents the microstructure of hydrated pastes without metakaolin and with 30% MK2 analyzed by BSE at 28 days. A relatively simple microstructure is observed in the MPC paste without metakaolin, CON, and many unreacted magnesia particles embeds in the struvite crystals of short columnar and wedge-shaped [23,24]. 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 sheet-shaped 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 mentioned before, the presence of metakaolin influences the processes of gel condensation and crystallization. With the combination of thermal analysis and BSE analysis, the schematic representation the struvite formation is presented in Fig. 10. As shown in Fig. 10a, the quick crystallization and concentration of struvite generate the dense layer filled with struvite between the two magnesia particles. As the hydration proceeds, the continuous crystal growth on the surface of magnesia may lead to the internal stress, hence the micro cracks. In metakaolin blended MPC, metakaolin with huge specific surface area provides a lot of sites for crystallization as indicated in Fig. 10b, which can promote the crystal growth more orderly. And the more homogeneous struvite clusters can be formed on the surface of metakaolin. It is noteworthy that the degree of crystallization of gelatinous phases in metakaolin blended MPC may be higher. Furthermore, the presence of metakaolin increases the distance between the adjacent magnesia particles, which can reduce the internal force caused by crystal growth. 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 through BSE analysis is shown in Fig. 11. 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 metakao-
(a) Paste CON
MgO
(b) Paste S22 MK
MgO
Fig. 9. Microstructure of MPC pastes without metakaolin (a) and with 30% MK2 (b) at 28 days.
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Z. Qin et al. / Construction and Building Materials 227 (2019) 116675
Fig. 10. Schematic representation of the formation of struvite clusters in MPC.
(b)
(a)
MK
Mg
P
Al
Si Fig. 11. (a) BSE image and (b) Element maps of specimen S22 at 28 days.
O
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Z. Qin et al. / Construction and Building Materials 227 (2019) 116675
lin particles. Although the Mg, P, Si and Al elements mainly distribute 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 [17,22]. 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. 12 presents the microstructure of MPC pastes without metakaolin and with 30% MK2 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 blended MPC specimens, there is no significant difference in microstructure before and after water immersion. This observation corresponds to the results of water resistance and porosity tests.
3.2.3. XRD analysis The XRD patterns of MPC pastes without or with 30% metakaolin at 28 days of curing are presented in Fig. 13. The diffraction peaks appearing between 40°and 70° 2h in the XRD patterns mainly refer to the unreacted MgO [11,16,32]. The unreacted ADP can hardly be detected, implying the almost complete reaction of ADP in MPC paste. For each specimen, struvite is the dominant
Fig. 13. XRD patterns of MPC pastes containing 30% metakaolin at 28 days.
crystal phase. And the diffraction peaks in MPC paste without metakaolin are thin and sharp, which indicates the well crystallinity of hydration products. A broad hump between about 25° and 27 °2h possibly resembling the diffraction peak of aluminum silicon oxide existing in unreacted metakaolin particles is observed in MPC pastes with metakaolin. However, no new diffraction peaks resembling some different hydration products after adding metakaolin can be detected by XRD analysis. In fact, the dissolution and chemical reaction of metakaolin in MPC paste was confirmed by BSE analysis. The reaction products may be gelatinous phases whose diffraction peaks can be covered by the prominent peaks
(a) Paste CON after soaking
(b) Paste S22 after soaking
Fig. 12. Microstructure of MPC pastes CON and S22 after 28 days of water immersion.
Z. Qin et al. / Construction and Building Materials 227 (2019) 116675
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extent. And the type of metakaolin has little influence on the total weight loss. Judged by the DTG analysis, the MPC paste without metakaolin has the most struvite, followed by specimen with 10% metakaolin, closely followed by specimens with 30% metakaolin. There is little difference among the amount of struvite in specimens S12, S22 and S32. With the combination of BSE analysis, the more dense and homogeneous microstructure of metakaolin blended MPC can be regarded as the dominant factor for the significant improvement of strength in comparison to the pozzolanic effect of metakaolin in MPC-based materials. 4. Conclusion
Fig. 14. XRD patterns of MPC pastes after water immersion.
Fig. 15. TG/DTG curves of MPC pastes with different contents and types of metakaolin at 28 days.
of crystal minerals. This observation is similar to the results reported in fly ash or slag blended MPC [17,23,24]. Fig. 14 presents the XRD patterns of MPC pastes after 28 days of water immersion. The main hydration products are also struvite, and there in 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.
3.2.4. TG-DTG analysis Fig. 15 presents the TG/DTG curves of MPC pastes with different contents and types of metakaolin at 28 days of curing. For each MPC paste, the weight loss mainly occurs within 200 °C, referring to the dehydration of K-struvite. After introducing metakaolin into MPC, the weight losses within 200 °C and 1000 °C decrease to some
This study investigated the influence of partial replacement of magnesia by metakaolin with different fineness on workability, compressive strength, tensile bonding strength and water resistance of MPC mortar. The possible mechanism of metakaolin was systematically analyzed through characterizing heat release rate of hydration and microstructure of MPC paste. The following conclusions can be drawn. 1) Metakaolin decreases the fluidity of MPC mortar, and the decreasing degree has a direct correlation to the content and fineness of metakaolin. The 28-day strength increases significantly with the metakaolin replacement increasing to 30%, and its maximal values occurs in MPC mortar with a middle fineness of metakaolin (MK2). And metakaolin has a slight influence on the 1-hour strength, and the maximal value of MPC mortar with 30% MK2 is only 10.5% higher than that of specimen without metakaolin. Likewise, metakaolin can also improve the bonding property of MPC mortar significantly, and the replacement of 30% MK2 increases the 28-day tensile bonding strength by 65.7%. 2) Judged from the compressive strength and tensile bonding strength measured after water immersion, metakaolin increases the strength retention coefficients, indicating the improvement of water resistance of MPC mortar. The retention coefficients of compressive strength are higher than those of tensile bonding strength. And 30% MK2 blended MPC mortar has the best water resistance, and its retention coefficients of compressive strength and tensile bonding strength are 0.91 and 0.82, respectively. 3) The presence of metakaolin reduces the total porosity of MPC mortar, but the influence the fineness of metakaolin on the pore structure is not so prominent as the strength and water resistance observed. BSE images reveal that unreacted metakaolin particles can embed in the interlayer of struvite, leading to the more compacted structure. Water immersion increases the total porosity of MPC mortar and makes the pore structure much coarser. A huge microcrack with a width of several micrometers can be observed in after-soaking specimen without metakaolin. However, the change in the microstructure of after-soaking specimen with metakaolin is not very significant. 4) Metakaolin decreases the maximal heat release rate and prolongs the arrival time of peak value by decreasing the alkalinity of MPC paste. When the finer metakaolin was employed, the maximal heat release rate increases due to the gel condensation and crystallization on the surface of metakaolin. The mineralogical characterization by XRD and TG-DTG analysis indicate that there are no new hydration products generated in metakaolin blended MPC. Nevertheless, the dissolution and reaction of metakaolin are confirmed according to the invasion of silicon and aluminum elements into struvite by BSE analysis.
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Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The authors appreciate the financial support from the National Natural Science Foundation of China, Grant No. 51808560, Natural Science Foundation of Hunan Province, China (2019JJ50820), the Open fund of Guangxi Key Laboratory of Road Structure and Materials, China (2018gxjgclkf004) and the Opening Fund of Key Laboratory of Unconventional Oil & Gas Development (China University of Petroleum (East China)), Ministry of Education (19CX05005A-8). This study is also funded by China Postdoctoral Science Foundation funded project (2018M633617XB), the Fundamental Research Funds for the Central Universities, Shock and Vibration of Engineering Materials and Structures Key Laboratory of Sichuan Province, China (18kfgk06), Special Project of Nanning Science and Technology Base (20185071-1). References [1] Prosen EM. Dental investment or refractory material. Google Patents, 1939. [2] S.S. Seehra, S. Gupta, S. Kumar, Rapid setting magnesium phosphate cement for quick repair of concrete pavements-characterization and durability aspects, Cem. Concr. Res. 23 (2) (1993) 254–266. [3] Q.B. Yang, X.L. Wu, Factors influencing properties of phosphate cement-based binder for rapid repair of concrete, Cem. Concr. Res. 29 (1999) 389–396. [4] F. Qiao, C.K. Chau, Z.J. Li, Property evaluation of magnesium phosphate cement mortar as patch repair material, Constr. Build. Mater. 24 (5) (2010) 695–700. [5] Z. Ding, B.Q. Dong, F. Xing, N. Han, Z.J. Li, Cementing mechanism of potassium phosphate based magnesium phosphate cement, Ceram. Int. 38 (8) (2012) 6281–6288. [6] M. Tuyan, Ö. Andiç-Çakir, K. Ramyar, Effect of alkali activator concentration and curing condition on strength and microstructure of waste clay brick powder-based geopolymer, Compos. B Eng. 135 (2018) 242–252. [7] H.Y. Ma, B.Y. Xu, J. Liu, H. Pei, Z.J. Li, Effects of water content, magnesia-tophosphate molar ratio and age on pore structure, strength and permeability of magnesium potassium phosphate cement paste, Mater. Des. 64 (2014) 497– 502. [8] N. Yang, C.J. Shi, J. Yang, Y. Chang, Research progresses in magnesium phosphate cement-based materials, J. Mater. Civil Eng. 26 (10) (2013) 04014071. [9] S.A. Holt, C.F. Jones, G.S. Watson, A. Crossley, Surface modification of MgO substrates from aqueous exposure: an atomic force microscopy study, Thin Solid Films (1997) 1547–1554. [10] J. Xie, J. Wang, R. Rao, et al., Effects of combined usage of GGBS and fly ash on workability and mechanical properties of alkali activated geopolymer concrete with recycled aggregate, Compos. B Eng. 164 (2019) 179–190. [11] C. Ma, B. Chen, Properties of magnesium phosphate cement containing redispersible polymer powder, Constr. Build. Mater. 113 (2016) 255–263. [12] Z. He, X. Zhu, J. Wang, et al., Comparison of CO2 emissions from OPC and recycled cement production, Constr. Build. Mater. 211 (2019) 965–973. [13] I. Buj, J. Torras, M. Rovira, J. de Pablo, Leaching behaviour of magnesium phosphate cements containing high quantities of heavy metals, J. Hazard. Mater. 175 (1–3) (2010) 789–794. [14] T. Meng, Y. Yu, Z. Wang, Effect of nano-CaCO3 slurry on the mechanical properties and micro-structure of concrete with and without fly ash, Compos. B Eng. 117 (2017) 124–129.
[15] J. Torras, I. Buj, M. Rovira, J. de Pablo, Semi-dynamic leaching tests of nickel containing wastes stabilized/solidified with magnesium potassium phosphate cements, J. Hazard. Mater. 186 (2–3) (2011) 1954–1960. [16] Z. Ding, Z.J. Li, High-early-strength magnesium phosphate cement with fly ash, ACI Mater. J. 102 (6) (2005) 375. [17] L.J. Gardner, S.A. Bernal, S.A. Walling, C.L. Corkhill, J.L. Provis, N.C. Hyatt, Characterisation of magnesium potassium phosphate cements blended with fly ash and ground granulated blast furnace slag, Cem. Concr. Res. 74 (2015) 78–87. [18] Y. Shi, G. Long, C. Ma, Y. Xie, J. He, Design and preparation of ultra-high performance concrete with low environmental impact, J Clean Prod 214 (2019) 633–643. [19] Y. Huang, H. Li, Z. Jiang, X. Yang, Q. Chen, Migration and transformation of sulfur in the municipal sewage sludge during disposal in cement kiln, Waste Manage. 77 (2018) 537–544. [20] C. Ma, B. Chen, Experimental study on the preparation and properties of a novel foamed concrete based on magnesium phosphate cement, Constr. Build. Mater. 137 (2017) 160–168. [21] N.M. Agyei, C.A. Strydom, J.H. Potgieter, The removal of phosphate ions from aqueous solution by fly ash, slag, ordinary Portland cement and related blends, Cem. Concr. Res. 32 (12) (2002) 1889–1897. [22] L.J. Vandeperre, M. Liska, A. Al-Tabbaa, Microstructures of reactive magnesia cement blends, Cem. Concr. Compos. 30 (8) (2008) 706–714. [23] Y. Huang, C. Xu, H. Li, Z. Jiang, Z. Gong, X. Yang, Q. Chen, Utilization of the black tea powder as multifunctional admixture for the hemihydrate gypsum, J Clean Prod 210 (2019) 231–237. [24] C. Ma, G. Long, Y. Shi, Y. Xie, Preparation of cleaner one-part geopolymer by investigating different types of commercial sodium metasilicate in China, J. Clean. Prod. 201 (2018) 636–647. [25] X. Yang, J. Liu, H. Li, L. Xu, Q. Ren, L. Li, Effect of triethanolamine hydrochloride on the performance of cement paste, Constr. Build. Mater. 200 (2019) 218– 225. [26] B.W. Xu, B. Lothenbach, H.Y. Ma, Properties of fly ash blended magnesium potassium phosphate mortars: effect of the ratio between fly ash and magnesia, Cem. Concr. Compos. 90 (2018) 169–177. [27] B.W. Xu, H.Y. Ma, H. Shao, Z. Li, B. Lothenbach, Influence of fly ash on compressive strength and micro-characteristics of magnesium potassium phosphate cement mortars, Cem. Concr. Res. 99 (2017) 86–94. [28] S. Wild, J.M. Khatib, A. Jones, Relative strength, pozzolanic activity and cement hydration in superplasticised metakaolin concrete, Cem. Concr. Res. 26 (10) (1996) 1537–1544. [29] Haoxin Li, Hui Zhang, Lin Li, Qiang Ren, Xiaojie Yang, Zhengwu Jiang, Zhenlei Zhang, Utilization of low-quality desulfurized ash from semi-dry flue gas desulfurization by mixing with hemihydrate gypsum, Fuel 255 (2019) 115783. [30] N.M. Al-Akhras, Durability of metakaolin concrete to sulfate attack, Cem. Concr. Res. 36 (9) (2006) 1727–1734. [31] C. Ma, B. Chen, Properties of a foamed concrete with soil as filler, Constr. Build. Mater. 76 (2015) 61–69. [32] G.C. Long, Y.Y. Li, C. Ma, Y.J. Xie, Y. Shi, Hydration kinetics of cement incorporating different nanoparticles at elevated temperatures, Thermochim. Acta 664 (2018) 108–117. [33] C. Ma, B. Zhao, S. Guo, G. Long, Y. Xie, Properties and characterization of green one-part geopolymer activated by composite activators, J. Clean. Prod. 220 (2019) 188–199. [34] M. Clausi, S.C. Tarantino, L.L. Magnani, M.P. Riccardi, C. Tedeschi, M. Zema, Metakaolin as a precursor of materials for applications in cultural Heritage: geopolymer-based mortars with ornamental stone aggregates, Appl. Clay Sci. 132 (2016) 589–599. [35] H. Ma, B. Xu, Z. Li, Magnesium potassium phosphate cement paste: degree of reaction, porosity and pore structure, Cem. Concr. Res. 65 (2014) 96–104. [36] A.S. Wagh, S.Y. Jeong, Chemically bonded phosphate ceramics: I, a dissolution model of formation, J. Am. Ceram. Soc. 86 (11) (2003) 1838–1844. [37] J. Xie, J. Liu, F. Liu, J. Wang, P. Huang, Investigation of a new lightweight green concrete containing sludge ceramsite and recycled fine aggregates, J. Clean. Prod. (2019). [38] N. Ostrowski, A. Roy, P.N. Kumta, Magnesium phosphate cement systems for hard tissue applications: a review, ACS Biomater. Sci. Eng. 2 (7) (2016) 1067– 1083.
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Roles of metakaolin in magnesium phosphate cement: Effect of the replacement ratio of magnesia by metakaolin with different particle sizes Zhaohui Qin a, Shengbo Zhou b,c, Cong Ma d,⇑, Guangcheng Long a, Youjun Xie a, Bing Chen a a
Department of Civil Engineering, Shanghai Jiao Tong University, Shanghai 210240, PR China Guangxi Key Lab of Road Structure and Materials, Guangxi Xinfazhan Communications Group Co., Ltd., Nanning, Guangxi Province 530029, PR China Guangxi Transportation Research & Consulting CO., Ltd., Nanning, Guangxi Province 530029, PR China d School of Civil Engineering, Central South University, Changsha, Hunan Province 410075, PR China b c
h i g h l i g h t s Metakaolin improves the mechanical properties and durability of MPC. Metakaolin decreases the early-term hydration heat of MPC and the total porosity. Mechanisms of metakaolin in MPC include providing nucleation cites, filling effect and pozzolanic effect.
a r t i c l e
i n f o
Article history: Received 11 March 2019 Received in revised form 4 August 2019 Accepted 6 August 2019
Keywords: Magnesium phosphate cement Metakaolin Properties Microstructure Mechanism
a b s t r a c t Different particle sizes of metakaolin were employed to replace magnesia partially, and it goes MK1, MK2 and MK3 in order of the specific surface area. Experimental results indicate that the presence of metakaolin indeed improves the compressive strength, tensile bonding strength and water resistance of MPC mortar through sacrificing the workability to some extent, and 30% MK2 blended MPC mortar has the best performances except fluidity. Metakaolin decreases the porosity and improves the pore structure of MPC mortar, especially for specimens after water immersion. The dissolution and reaction of metakaolin in MPC are confirmed by BSE analysis. Nevertheless, no new hydration products can be detected in MPC paste with metakaolin through XRD and TG-DTG analysis. The condensation of intermediate products, crystallization and crystal growth occurring on the surface of metakaolin promote the formation of the more compacted and homogeneous microstructure, which is the physical mechanism of metakaolin improving the properties of MPC. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction Magnesium phosphate cements (MPC) invented in the late 1930s 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 acidbase 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). And the common used magnesia is dead burnt magnesia because light burnt magnesia may result in the flash hardening ⇑ Corresponding author. E-mail address: [email protected] (C. Ma). https://doi.org/10.1016/j.conbuildmat.2019.116675 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.
of MPC. Even so, retarders, such as boric acid, borax and metaphosphate should be employed to prolong the setting time of MPC [6– 8]. The types of hydration products are related to the pH value of the reaction environment, and the main crystalline product 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
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Z. Qin et al. / Construction and Building Materials 227 (2019) 116675 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. Three types of metakaolin obtained from a single manufacturer were employed and the oxide composition is also shown in Table 1. The main difference in the three metakaolin is the fineness as represented by the mean particle size (dm) and BET surface area (SBET). The retarder (CRT) used is a type of composite additive consisting of sodium borate (Na2B4O710H2O) and sodium tripolyphosphate at 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) was used.
has unique advantages of higher bonding strength and lower shrinkage. 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 cementitious materials commonly used in Portland cement including fly ash, slag, silica fume and metakaolin were introduced into MPC [16–22]. 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). And 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) [17,23,24]. However, the reaction mechanism and products are not entirely clear. Even though fly ash can improve the strength to some extent, there is no significant improvement in the water resistance of MPC [23]. 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. The usage of metakaolin in Portland cement-based materials is very common, but there are only very a few studies on metakaolin blended MPC [25–27]. Judged from the previous results, metakaolin can significantly increase the strength, prolong the setting time and improve the water resistance of MPC [22]. And the high content of active alumina phase may be responsible for the improvement of the properties of MPC. However, the influencing mechanism of metakaolin in MPC is not entirely clear. Moreover, there are many kinds of commercial metakaolin with different fineness and calcination temperature in China, which may affect the properties of MPC admixed with metakaolin. In this study, three types of metakaolin with different fineness were employed to prepare MPC mortar or paste. The effects of metakaolin on the properties of MPC mortar were investigated by testing the fluidity, compressive strength, tensile bonding strength, porosity and water resistance characterized by the retention coefficients of compressive strength and tensile bonding strength. Microstructure and mineral changes through introducing metakaolin into MPC paste were analyzed by BSE, XRD and TG-DTG analysis. The results not only provide the guidance for employing different types of metakaolin to improve the properties of MPC but also give a perspective of the mechanism of metakaolin in MPC.
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 are fixed 0.2 and 1.0, respectively. Retarder was added by weight of MgO. Metakaolin was introduced into MPC by replacing MgO, and the MK replacements were 0%, 15% and 30%, respectively, by weight of MgO. Series S1, S2 and S3 are specimens with MK1, MK2 and MK3, respectively. 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 mixture and the wet mixing lasted about three minutes until the flowing mixture was obtained. If necessary, several measurements are carried on the fresh MPC mortar in order to investigate the fresh property. And then, the fresh mortar was transferred into PVC moulds (40 mm 40 mm 160 mm) and homogeneous mixtures can be obtained by some forms of vibration. After casting, the moulds were placed in a standard curing room with a stable temperature of 20 ± 2 °C and relative humidity of 95 ± 3%. The specimens were demoulded after 1 day of curing, and the demoulded specimens were transferred to the standard curing room until the measuring ages. Fresh property of MPC mortar was investigated through measuring the initial fluidity. According to Chinese Standard GB/T2419-2005, electric jumping table is employed to measure the fluidity of MPC mortar. During the measurements, the mortar was immediately transferred onto the jumping table after mixing. The fluidity is the mean value of diameters of thin-pancake mortar after 25 times of vibration. At 1 h, 7 days and 28 days of curing, specimens were taken out from standard curing room for the measurements of compressive strength in accordance to ASTM C348. The final strength results are the mean values of at least three reproducible specimens, and it is notable that the measured results with mean standard deviations of over 10% should be eliminated. 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. 1a. Sand blasting surface treatment was carried on to obtain the rough surface of concrete substrate. 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 into the PVC moulds, the specimen was cured under standard condition for 1 and 28 days. As shown in Fig. 1a, 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 higher strength retention coefficient indicates better water resistance of materials [11,22,28]. In this study, not only the retention coefficient of compressive strength
2. Experimental methodology 2.1. Materials The 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
Table 1 Oxide composition of MgO and metakaolin. Materials
SiO2
Al2O3
Fe2O3
MgO
CaO
K2O
Na2O
LOI
dm (um)
SBET (m2/g)
MgO MK1 MK2 MK3
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 23.5 14.9 10.6
1.26 2.37 9.63 13.95
3
Z. Qin et al. / Construction and Building Materials 227 (2019) 116675 Table 2 Mix proportions. Sample No.
CON S11 S12 S21 S22 S31 S32
Binder components (referred to 1 g ADP) MgO
ADP
MK1
MK2
MK3
CRT
PD
3.0 2.55 2.1 2.55 2.1 2.55 2.1
1.0 1.0 1.0 1.0 1.0 1.0 1.0
0 0.45 0.9 0 0 0 0
0 0 0 0.45 0.9 0 0
0 0 0 0 0 0.45 0.9
0.18 0.153 0.126 0.153 0.126 0.153 0.126
0 0.005 0.009 0.005 0.009 0.005 0.009
s/b
w/b
1.0 1.0 1.0 1.0 1.0 1.0 1.0
0.2 0.2 0.2 0.2 0.2 0.2 0.2
Disk
Disk
MPC
water
Roughness surface
MPC
Roughness surface Concrete substrate
Concrete substrate
(a)
(b)
Fig. 1. Schematic diagrams of experiments for flexural bonding strength of MPC mortar (a) under standard curing condition and (b) after water immersion.
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. 1b and the water immersion lasted 28 days. And then, the specimens were taken out for measuring the after-soaking compressive strength and tensile bonding strength of MPC mortar. The retention coefficients can be expressed as the following equations [35,36].
f RC C ¼ Cn fC
ð2Þ
f Tn fT
ð3Þ
RC T ¼
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. Mercury intrusion porosimetry (MIP) was employed to characterize the porosity of MPC mortar. Some crushed samples are collected from the hardened specimens after compressive strength measurements before and after water immersion. The sizes of selected samples are about 2–4 mm. Before porosity tests, the samples should be oven-dried at a temperature lower than 60 °C. And the measured smallest pore size is about 5.5 nm. During the measurements, the intrusive volume of Hg should be normalized according to the weight of samples. An in-situ isothermal calorimeter named as TAM Air was employed to carry out isothermal calorimetry tests. TAM Air apparatus has four parallel twin-chamber measuring channels, and one chamber contains the MPC paste [29]. In order to avoid considerable temperature differences between the sample and the isothermal environment, the TAM Air apparatus was adjusted close to the measurement temperature of 20 ± 0.02 °C before sample preparation. The fresh paste is mixed in standard curing room and immediately transferred into an ampoule. And then, the ampoule is placed into the apparatus, and the heat flow could be recorded. The recording duration is about 20 h, and the cumulative heat can also be normalized. MPC pastes corresponding to MPC mortars were prepared for characterizing the effects of metakaolin on microstructure and mineral phases. Samples for X-Ray Powder Diffraction (XRD) required grinding and the maximal particle size was smaller than 45 um. The ultra-fine power is made into thin sheets which can be scanned on an X-ray diffractometer with a CuKa source [21,33]. The scanning region ranges from 10° 2h to 60° 2h with a scanning rate of 0.01°/step. Some rectangle or cubic samples are prepared for back-scattered electron (BSE) [37,38]. The samples are impregnated with isopropanol, and then dried in a vacuum oven at 40 °C for about 2 days. The oven-dried samples are stuck to the stubs by using conducting resin. Before BSE analysis, carbon-coating process is essential due to the poor electrical conductivity of inorganic powder. Samples for thermogravimetric
and derivative thermogravimetry (TG/DTG) analysis were also ground and ovendried. TG/DTG analysis was performed on a thermal analyzer, from 20 °C (room temperature) to 990 °C with a heating rate of 10 °C/min.
3. Results and discussion 3.1. Properties of MPC mortar 3.1.1. Fluidity Fig. 2 shows the influence of different types of metakaolin on the fluidity of MPC mortar. It is observed that the presence of metakaolin decreases the fluidity and reduces the workability of MPC mortar, even though the polymeric dispersant is used. MPC mortar without metakaolin has the largest fluidity and its value is about 26.5 cm. And the fluidity of specimen with 30% MK3 is
Fig. 2. The fluidity of different MPC mortar specimens.
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Z. Qin et al. / Construction and Building Materials 227 (2019) 116675
only 18.6 cm, only 81.6% of that with 30% MK1. On the one hand, the presence of high content of metakaolin can generate much higher total specific surface area, and this results in the sharp increase of the water requirement. On the other hand, the microstructure of metakaolin is the irregular sheet structure which is completely different from fly ash, in other words, the morphological effect generated by the spherical beads of fly ash can not occur in metakaolin blended cement composites [30,31]. 3.1.2. Compressive strength The influence of different types and contents of metakaolin on the compressive strength of MPC mortar is presented in Fig. 3. There is only a slight difference in the ultra-early strength of different specimens, and the 1-hour strengths of MPC mortar specimens CON, S12, S22 and S32 are 26.7 MPa, 27.8 MPa, 29.5 MPa and 24.9 MPa, respectively. For MPC mortar without metakaolin, the strength increases slowly with increasing curing time to 28 days, and the 1-hour strength can reach about seventy percent of the 28-day strength. The usage of metakaolin indeed increases the middle-term compressive strength, and the highest strength is observed in MPC mortar with 30% MK2. MPC mortar with 30% metakaolin can achieve the 28-day strength of over 60 MPa, about fifty percent higher than that of MPC mortar without metakaolin. The dominant factors for the compressive strength are the degree of hydration reaction and the compactness of microstructure. Although the addition of metakaolin may decrease the amount of hydration products, the filling effect of metakaolin is also a helpful contributor to the compressive strength. More importantly, the increase in compressive strength of MPC mortar with metakaolin from 1 h to 28 days is much higher in comparison to that of specimen without metakaolin. The 1-hour strengths of specimens S12 and S22 are only 49.5% and 44.8% of the 28-day strengths, respectively. This indicates that the presence of metakaolin improves the middle- and long-term strength of MPC mortar due to the possible pozzolanic reaction and the improvement of microstructure. However, the mechanism of metakaolin in MPC has not been really known yet, which need further study. 3.1.3. Tensile bonding strength Fig. 4 presents the influence of different types and contents of metakaolin on the tensile bonding strength of MPC mortar. The development of tensile bonding strength of different MPC mortar specimens is nearly consistent with the tendency observed in compressive strength. In other words, the higher compressive strength
Fig. 4. Tensile bonding strength of MPC mortars with different types of metakaolin.
Fig. 5. Compressive strength, tensile bonding strength and strength retention coefficients of MPC mortar after 28 days of soaking.
the greater tensile bonding strength. And the addition of metakaolin increases the bonding strength of MPC mortar significantly. The 28-day tensile bonding strength of specimen S22 is 3.5 MPa, about 65.7% higher than that of specimen without metakaolin. The bonding strength relates to the amount of hydration products and pore structure in the interface between MPC mortar and substrate concrete. Firstly, a portion of metakaolin particles take participate in pozzolanic reaction, which may relieve the adverse effect of the reduction in the struvite amount. Secondly, the porosity and pore structure of MPC mortar may be improved by adding a moderate amount of metakaolin. Another interesting phenomenon is that the 1-day and 28-day strengths of 30% MK3 blended MPC mortar are almost the same as those of specimens with 15% MK3. The gathering of unreacted metakaolin in the interface may be responsible for this phenomenon.
Fig. 3. Compressive strength of MPC mortars with different types of metakaolin.
3.1.4. Water resistance Fig. 5 presents the residual compressive strength, tensile bonding strength and strength retention coefficients of different MPC specimens after 28 days of water immersion. The RCC value of MPC mortar without metakaolin is only 0.76 and the correspond-
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ing residual compressive strength (fC28) is only 30.3 MPa. The presence of metakaolin increases the fC28 significantly and the maximum fC28 is observed in specimen S22. The values of fC28 and RCC of specimen S22 are 59.9 MPa and 0.91, 97.7% and 19.7% higher than those of specimen without metakaolin. The tendency in tensile bonding strength is similar to the above observation, and the maximum residual bonding strength (fT28) also occurs in the presence of MK2. The RCT value of 30% MK2 blended MPC mortar is about 0.82, 26.2% higher than that of MPC mortar without metakaolin. 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 that in compressive strength. The possible reason for this phenomenon is 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.
3.1.5. Porosity The MIP results expressed by cumulative porosity of the selected MPC mortar specimens before and after water immersion are presented in Fig. 6. The addition of 10% MK1 decreases the total porosity of MPC mortar significantly, and the porosity decreases further with increasing the metakaolin content from 10% to 30%. The total porosities of specimens CON, S11 and S12 are 15.90%, 14.01% and 12.69%, respectively. In comparison to metakaolin content, the metakaolin fineness can be regarded as the secondary influencing factor for porosity of MPC mortar. In addition, the specimen S22 has the lowest porosity among the measured specimens, which is almost consistent with the tendency presented in compressive strength. The possible reason may be that the gathering and concentrating of metakaolin particles with the largest specific surface area decrease the compactness of hardened MPC mortar. Another interesting observation is that the MPC mortar after water immersion has a much higher porosity. More importantly, water immersion makes the pore structure of MPC mortar much coarser. This can explain the strength degradation of MPC mortar after water immersion. In comparison to Portland cement-based materials, the pore size in MPC mortar, especially in MAPC mortar with ADP, was proved to be larger by previous researchers [7,32]. The pores smaller than 50 um are considered as capillary pores and Fig. 7 presents
Fig. 6. MIP results expressed by cumulative porosity curves for several MPC mortar specimens before and after water immersion.
Fig. 7. The proportion of different sizes of pores in MPC mortar.
the distribution of different sizes of pores. The presence of metakaolin decreases the number of capillary pores remarkably. The decrease in the number of air pores larger than 50 um can be explained by the adsorption effect of the metakaolin with huge specific surface. And the ammonia gas during the preparation of MPC with metakaolin smells much thinner than that without metakaolin. The filling effect of metakaolin can be responsible for the change in capillary pores. For specimens after water immersion, the proportion of larger capillary pores (1–50 mm) becomes much higher. The number of larger capillary pores and air pores in specimen CON after soaking is almost twice of that before soaking. And the increase in large pores of specimen with metakaolin is less significant. This is one of the reasons for the improvement of water resistance by adding metakaolin. 3.2. Characterization of MPC paste 3.2.1. Heat release rate of hydration Fig. 8 shows the normalized heat flow of hydration of MPC pastes with different contents and types of metakaolin. The heat release rate increases sharply within the first several minutes for each specimen. As reported in previous studies [33,34], the hydra-
Fig. 8. Normalized heat flow of hydration of MPC pastes.
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tion kinetics of MPC can be divided into three steps, dissolution and sol formation, acid-base reaction and gel condensation, saturation and crystallization of the gel. The dissolution of ADP and the acid-base reaction is very quick, resulting in the ultrafast exothermicity during the initial hydration process. When metakaolin was introduced to replace magnesia, the number of hydroxide ions from MgO dissolution reduces and the intensity of acid-base reaction weakens, which decreases the maximal heat release rate and prolongs the arrival time of the peak value. And the maximal heat release rate decreases further with increasing metakaolin content from 15% to 30%. This explains the improvement of setting time of MPC mortar by adding metakaolin reported in [22]. Although the metakaolin contents in specimens S12, S22 and S32 are the same, there is obvious differences in the maximal heat release rate and arrival time of the peak value due to the different particle sizes of metakaolin. The maximal heat release rate increases, and the arrival time of the peak value is shortened with the increasing fineness of metakaolin. Although there is little difference in the acidbase reaction of specimens with different types of metakaolin, the gel condensation on the surface of metakaolin becomes faster as the specific surface area of metakaolin increases. In addition, the heat release rate in the descent stage of metakaolin blended MPC is slightly higher than that of pure MPC. This indicates the presence of metakaolin increases the speed of saturation and crystallization of the gel. 3.2.2. BSE analysis Fig. 9 presents the microstructure of hydrated pastes without metakaolin and with 30% MK2 analyzed by BSE at 28 days. A relatively simple microstructure is observed in the MPC paste without metakaolin, CON, and many unreacted magnesia particles embeds in the struvite crystals of short columnar and wedge-shaped [23,24]. 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 sheet-shaped 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 mentioned before, the presence of metakaolin influences the processes of gel condensation and crystallization. With the combination of thermal analysis and BSE analysis, the schematic representation the struvite formation is presented in Fig. 10. As shown in Fig. 10a, the quick crystallization and concentration of struvite generate the dense layer filled with struvite between the two magnesia particles. As the hydration proceeds, the continuous crystal growth on the surface of magnesia may lead to the internal stress, hence the micro cracks. In metakaolin blended MPC, metakaolin with huge specific surface area provides a lot of sites for crystallization as indicated in Fig. 10b, which can promote the crystal growth more orderly. And the more homogeneous struvite clusters can be formed on the surface of metakaolin. It is noteworthy that the degree of crystallization of gelatinous phases in metakaolin blended MPC may be higher. Furthermore, the presence of metakaolin increases the distance between the adjacent magnesia particles, which can reduce the internal force caused by crystal growth. 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 through BSE analysis is shown in Fig. 11. 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 metakao-
(a) Paste CON
MgO
(b) Paste S22 MK
MgO
Fig. 9. Microstructure of MPC pastes without metakaolin (a) and with 30% MK2 (b) at 28 days.
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Fig. 10. Schematic representation of the formation of struvite clusters in MPC.
(b)
(a)
MK
Mg
P
Al
Si Fig. 11. (a) BSE image and (b) Element maps of specimen S22 at 28 days.
O
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lin particles. Although the Mg, P, Si and Al elements mainly distribute 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 [17,22]. 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. 12 presents the microstructure of MPC pastes without metakaolin and with 30% MK2 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 blended MPC specimens, there is no significant difference in microstructure before and after water immersion. This observation corresponds to the results of water resistance and porosity tests.
3.2.3. XRD analysis The XRD patterns of MPC pastes without or with 30% metakaolin at 28 days of curing are presented in Fig. 13. The diffraction peaks appearing between 40°and 70° 2h in the XRD patterns mainly refer to the unreacted MgO [11,16,32]. The unreacted ADP can hardly be detected, implying the almost complete reaction of ADP in MPC paste. For each specimen, struvite is the dominant
Fig. 13. XRD patterns of MPC pastes containing 30% metakaolin at 28 days.
crystal phase. And the diffraction peaks in MPC paste without metakaolin are thin and sharp, which indicates the well crystallinity of hydration products. A broad hump between about 25° and 27 °2h possibly resembling the diffraction peak of aluminum silicon oxide existing in unreacted metakaolin particles is observed in MPC pastes with metakaolin. However, no new diffraction peaks resembling some different hydration products after adding metakaolin can be detected by XRD analysis. In fact, the dissolution and chemical reaction of metakaolin in MPC paste was confirmed by BSE analysis. The reaction products may be gelatinous phases whose diffraction peaks can be covered by the prominent peaks
(a) Paste CON after soaking
(b) Paste S22 after soaking
Fig. 12. Microstructure of MPC pastes CON and S22 after 28 days of water immersion.
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extent. And the type of metakaolin has little influence on the total weight loss. Judged by the DTG analysis, the MPC paste without metakaolin has the most struvite, followed by specimen with 10% metakaolin, closely followed by specimens with 30% metakaolin. There is little difference among the amount of struvite in specimens S12, S22 and S32. With the combination of BSE analysis, the more dense and homogeneous microstructure of metakaolin blended MPC can be regarded as the dominant factor for the significant improvement of strength in comparison to the pozzolanic effect of metakaolin in MPC-based materials. 4. Conclusion
Fig. 14. XRD patterns of MPC pastes after water immersion.
Fig. 15. TG/DTG curves of MPC pastes with different contents and types of metakaolin at 28 days.
of crystal minerals. This observation is similar to the results reported in fly ash or slag blended MPC [17,23,24]. Fig. 14 presents the XRD patterns of MPC pastes after 28 days of water immersion. The main hydration products are also struvite, and there in 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.
3.2.4. TG-DTG analysis Fig. 15 presents the TG/DTG curves of MPC pastes with different contents and types of metakaolin at 28 days of curing. For each MPC paste, the weight loss mainly occurs within 200 °C, referring to the dehydration of K-struvite. After introducing metakaolin into MPC, the weight losses within 200 °C and 1000 °C decrease to some
This study investigated the influence of partial replacement of magnesia by metakaolin with different fineness on workability, compressive strength, tensile bonding strength and water resistance of MPC mortar. The possible mechanism of metakaolin was systematically analyzed through characterizing heat release rate of hydration and microstructure of MPC paste. The following conclusions can be drawn. 1) Metakaolin decreases the fluidity of MPC mortar, and the decreasing degree has a direct correlation to the content and fineness of metakaolin. The 28-day strength increases significantly with the metakaolin replacement increasing to 30%, and its maximal values occurs in MPC mortar with a middle fineness of metakaolin (MK2). And metakaolin has a slight influence on the 1-hour strength, and the maximal value of MPC mortar with 30% MK2 is only 10.5% higher than that of specimen without metakaolin. Likewise, metakaolin can also improve the bonding property of MPC mortar significantly, and the replacement of 30% MK2 increases the 28-day tensile bonding strength by 65.7%. 2) Judged from the compressive strength and tensile bonding strength measured after water immersion, metakaolin increases the strength retention coefficients, indicating the improvement of water resistance of MPC mortar. The retention coefficients of compressive strength are higher than those of tensile bonding strength. And 30% MK2 blended MPC mortar has the best water resistance, and its retention coefficients of compressive strength and tensile bonding strength are 0.91 and 0.82, respectively. 3) The presence of metakaolin reduces the total porosity of MPC mortar, but the influence the fineness of metakaolin on the pore structure is not so prominent as the strength and water resistance observed. BSE images reveal that unreacted metakaolin particles can embed in the interlayer of struvite, leading to the more compacted structure. Water immersion increases the total porosity of MPC mortar and makes the pore structure much coarser. A huge microcrack with a width of several micrometers can be observed in after-soaking specimen without metakaolin. However, the change in the microstructure of after-soaking specimen with metakaolin is not very significant. 4) Metakaolin decreases the maximal heat release rate and prolongs the arrival time of peak value by decreasing the alkalinity of MPC paste. When the finer metakaolin was employed, the maximal heat release rate increases due to the gel condensation and crystallization on the surface of metakaolin. The mineralogical characterization by XRD and TG-DTG analysis indicate that there are no new hydration products generated in metakaolin blended MPC. Nevertheless, the dissolution and reaction of metakaolin are confirmed according to the invasion of silicon and aluminum elements into struvite by BSE analysis.
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Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The authors appreciate the financial support from the National Natural Science Foundation of China, Grant No. 51808560, Natural Science Foundation of Hunan Province, China (2019JJ50820), the Open fund of Guangxi Key Laboratory of Road Structure and Materials, China (2018gxjgclkf004) and the Opening Fund of Key Laboratory of Unconventional Oil & Gas Development (China University of Petroleum (East China)), Ministry of Education (19CX05005A-8). This study is also funded by China Postdoctoral Science Foundation funded project (2018M633617XB), the Fundamental Research Funds for the Central Universities, Shock and Vibration of Engineering Materials and Structures Key Laboratory of Sichuan Province, China (18kfgk06), Special Project of Nanning Science and Technology Base (20185071-1). References [1] Prosen EM. Dental investment or refractory material. Google Patents, 1939. [2] S.S. Seehra, S. Gupta, S. Kumar, Rapid setting magnesium phosphate cement for quick repair of concrete pavements-characterization and durability aspects, Cem. Concr. Res. 23 (2) (1993) 254–266. [3] Q.B. Yang, X.L. Wu, Factors influencing properties of phosphate cement-based binder for rapid repair of concrete, Cem. Concr. Res. 29 (1999) 389–396. [4] F. Qiao, C.K. Chau, Z.J. Li, Property evaluation of magnesium phosphate cement mortar as patch repair material, Constr. Build. Mater. 24 (5) (2010) 695–700. [5] Z. Ding, B.Q. Dong, F. Xing, N. Han, Z.J. Li, Cementing mechanism of potassium phosphate based magnesium phosphate cement, Ceram. Int. 38 (8) (2012) 6281–6288. [6] M. Tuyan, Ö. Andiç-Çakir, K. Ramyar, Effect of alkali activator concentration and curing condition on strength and microstructure of waste clay brick powder-based geopolymer, Compos. B Eng. 135 (2018) 242–252. [7] H.Y. Ma, B.Y. Xu, J. Liu, H. Pei, Z.J. Li, Effects of water content, magnesia-tophosphate molar ratio and age on pore structure, strength and permeability of magnesium potassium phosphate cement paste, Mater. Des. 64 (2014) 497– 502. [8] N. Yang, C.J. Shi, J. Yang, Y. Chang, Research progresses in magnesium phosphate cement-based materials, J. Mater. Civil Eng. 26 (10) (2013) 04014071. [9] S.A. Holt, C.F. Jones, G.S. Watson, A. Crossley, Surface modification of MgO substrates from aqueous exposure: an atomic force microscopy study, Thin Solid Films (1997) 1547–1554. [10] J. Xie, J. Wang, R. Rao, et al., Effects of combined usage of GGBS and fly ash on workability and mechanical properties of alkali activated geopolymer concrete with recycled aggregate, Compos. B Eng. 164 (2019) 179–190. [11] C. Ma, B. Chen, Properties of magnesium phosphate cement containing redispersible polymer powder, Constr. Build. Mater. 113 (2016) 255–263. [12] Z. He, X. Zhu, J. Wang, et al., Comparison of CO2 emissions from OPC and recycled cement production, Constr. Build. Mater. 211 (2019) 965–973. [13] I. Buj, J. Torras, M. Rovira, J. de Pablo, Leaching behaviour of magnesium phosphate cements containing high quantities of heavy metals, J. Hazard. Mater. 175 (1–3) (2010) 789–794. [14] T. Meng, Y. Yu, Z. Wang, Effect of nano-CaCO3 slurry on the mechanical properties and micro-structure of concrete with and without fly ash, Compos. B Eng. 117 (2017) 124–129.
[15] J. Torras, I. Buj, M. Rovira, J. de Pablo, Semi-dynamic leaching tests of nickel containing wastes stabilized/solidified with magnesium potassium phosphate cements, J. Hazard. Mater. 186 (2–3) (2011) 1954–1960. [16] Z. Ding, Z.J. Li, High-early-strength magnesium phosphate cement with fly ash, ACI Mater. J. 102 (6) (2005) 375. [17] L.J. Gardner, S.A. Bernal, S.A. Walling, C.L. Corkhill, J.L. Provis, N.C. Hyatt, Characterisation of magnesium potassium phosphate cements blended with fly ash and ground granulated blast furnace slag, Cem. Concr. Res. 74 (2015) 78–87. [18] Y. Shi, G. Long, C. Ma, Y. Xie, J. He, Design and preparation of ultra-high performance concrete with low environmental impact, J Clean Prod 214 (2019) 633–643. [19] Y. Huang, H. Li, Z. Jiang, X. Yang, Q. Chen, Migration and transformation of sulfur in the municipal sewage sludge during disposal in cement kiln, Waste Manage. 77 (2018) 537–544. [20] C. Ma, B. Chen, Experimental study on the preparation and properties of a novel foamed concrete based on magnesium phosphate cement, Constr. Build. Mater. 137 (2017) 160–168. [21] N.M. Agyei, C.A. Strydom, J.H. Potgieter, The removal of phosphate ions from aqueous solution by fly ash, slag, ordinary Portland cement and related blends, Cem. Concr. Res. 32 (12) (2002) 1889–1897. [22] L.J. Vandeperre, M. Liska, A. Al-Tabbaa, Microstructures of reactive magnesia cement blends, Cem. Concr. Compos. 30 (8) (2008) 706–714. [23] Y. Huang, C. Xu, H. Li, Z. Jiang, Z. Gong, X. Yang, Q. Chen, Utilization of the black tea powder as multifunctional admixture for the hemihydrate gypsum, J Clean Prod 210 (2019) 231–237. [24] C. Ma, G. Long, Y. Shi, Y. Xie, Preparation of cleaner one-part geopolymer by investigating different types of commercial sodium metasilicate in China, J. Clean. Prod. 201 (2018) 636–647. [25] X. Yang, J. Liu, H. Li, L. Xu, Q. Ren, L. Li, Effect of triethanolamine hydrochloride on the performance of cement paste, Constr. Build. Mater. 200 (2019) 218– 225. [26] B.W. Xu, B. Lothenbach, H.Y. Ma, Properties of fly ash blended magnesium potassium phosphate mortars: effect of the ratio between fly ash and magnesia, Cem. Concr. Compos. 90 (2018) 169–177. [27] B.W. Xu, H.Y. Ma, H. Shao, Z. Li, B. Lothenbach, Influence of fly ash on compressive strength and micro-characteristics of magnesium potassium phosphate cement mortars, Cem. Concr. Res. 99 (2017) 86–94. [28] S. Wild, J.M. Khatib, A. Jones, Relative strength, pozzolanic activity and cement hydration in superplasticised metakaolin concrete, Cem. Concr. Res. 26 (10) (1996) 1537–1544. [29] Haoxin Li, Hui Zhang, Lin Li, Qiang Ren, Xiaojie Yang, Zhengwu Jiang, Zhenlei Zhang, Utilization of low-quality desulfurized ash from semi-dry flue gas desulfurization by mixing with hemihydrate gypsum, Fuel 255 (2019) 115783. [30] N.M. Al-Akhras, Durability of metakaolin concrete to sulfate attack, Cem. Concr. Res. 36 (9) (2006) 1727–1734. [31] C. Ma, B. Chen, Properties of a foamed concrete with soil as filler, Constr. Build. Mater. 76 (2015) 61–69. [32] G.C. Long, Y.Y. Li, C. Ma, Y.J. Xie, Y. Shi, Hydration kinetics of cement incorporating different nanoparticles at elevated temperatures, Thermochim. Acta 664 (2018) 108–117. [33] C. Ma, B. Zhao, S. Guo, G. Long, Y. Xie, Properties and characterization of green one-part geopolymer activated by composite activators, J. Clean. Prod. 220 (2019) 188–199. [34] M. Clausi, S.C. Tarantino, L.L. Magnani, M.P. Riccardi, C. Tedeschi, M. Zema, Metakaolin as a precursor of materials for applications in cultural Heritage: geopolymer-based mortars with ornamental stone aggregates, Appl. Clay Sci. 132 (2016) 589–599. [35] H. Ma, B. Xu, Z. Li, Magnesium potassium phosphate cement paste: degree of reaction, porosity and pore structure, Cem. Concr. Res. 65 (2014) 96–104. [36] A.S. Wagh, S.Y. Jeong, Chemically bonded phosphate ceramics: I, a dissolution model of formation, J. Am. Ceram. Soc. 86 (11) (2003) 1838–1844. [37] J. Xie, J. Liu, F. Liu, J. Wang, P. Huang, Investigation of a new lightweight green concrete containing sludge ceramsite and recycled fine aggregates, J. Clean. Prod. (2019). [38] N. Ostrowski, A. Roy, P.N. Kumta, Magnesium phosphate cement systems for hard tissue applications: a review, ACS Biomater. Sci. Eng. 2 (7) (2016) 1067– 1083.