Effect of triisopropanolamine on compressive strength ...

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Construction and Building Materials 179 (2018) 89–99

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Effect of triisopropanolamine on compressive strength and hydration of cement-fly ash paste Baoguo Ma, Ting Zhang, Hongbo Tan ⇑, Xiaohai Liu, Junpeng Mei, Huahui Qi, Wenbin Jiang, Fubing Zou State Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology, Wuhan 430070, PR China

h i g h l i g h t s  TIPA can increase compressive strength of cement-FA system.  TIPA can accelerate the hydration of both cement and FA.  TIPA can facilitate the dissolution of FA.  Air-entraining effect of TIPA negatively affects compressive strength.

a r t i c l e

i n f o

Article history: Received 6 November 2017 Received in revised form 26 March 2018 Accepted 12 May 2018

Keywords: Triisopropanolamine Pore Hydration Ettringite Dissolution

a b s t r a c t This paper aims to investigate the effect of triisopropanolamine (TIPA) on compressive strength and hydration of cement-fly ash paste. The samples with various dosages of TIPA were prepared with 30% fly ash (FA) and 70% cement (water/binder ratio by weight = 0.38), and cured under the standard condition. The compressive strength, pore structure, hydration process, and hydration products were investigated. The results show that TIPA can obviously increase the compressive strength of cement-FA system at the age of 7 d and 60 d, and the reasons are involved in pore structure and hydration of cement-FA system. Pore structure was characterized with mercury intrusion porosimetry, and the results show that TIPA can reduce total porosity but increase the amount of pore with size more than 50 nm, implying the air-entraining effect with negative effect on compressive strength. The result suggests that TIPA and defoaming agent should be used together to minimize the negative effect in real concrete. Furthermore, analysis of hydration products shows that TIPA can accelerate the hydration of both cement and FA, and this can also be illustrated from solid-state nuclear magnetic resonance. It is noticed that TIPA can hasten the conversion of AFt to AFm, which can be indicated from hydration heat. Additionally, the acceleration of pozzolanic reaction of FA is because TIPA can accelerate the dissolution of aluminate, silicate, and ferric into liquid paste which was demonstrated from morphology characterization and the change of ions in pore solution. Such results would be expected to provide experience for the use of alkanolamine in promoting the performance of cement-based materials. Ó 2018 Elsevier Ltd. All rights reserved.

1. Introduction In recent decades, the use of supplementary components in cement-based materials increasingly attracts attention in industrial area, because of both environmental and economic benefits [1–4]. Fly ash (FA), a by-product in coal-fueled power plants, has been developed as one of the most popular supplementary components. Even though many advantages can be found with addition of FA [5–8], one concerned issue cannot be ignored that poor mechanical performance would be found if the threshold replace⇑ Corresponding author. E-mail address: [email protected] (H. Tan). https://doi.org/10.1016/j.conbuildmat.2018.05.117 0950-0618/Ó 2018 Elsevier Ltd. All rights reserved.

ment was surpassed [9–12]. The effect of FA on mechanical performance is involved in two aspects: one is that finer particles in FA can fill into the pore [13], with positive effect on refining the pore structure to contribute to the mechanical performance. The other is that FA can react with the calcium hydroxide (CH) [14], known as pozzolanic reaction, promoting the formation of hydrates to contribute to the mechanical performance. However, if excessive FA were used, the CH generated by the hydration of cement minerals would be not enough for activating the pozzolanic reaction of FA, and in this case, the hydration degree of whole system would be reduced and the mechanical performance would be significantly declined.

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Table 1 Chemical composition of cement and FA.

Cement FA

wt% wt%

Loss

SiO2

Al2O3

Fe2O3

SO3

CaO

MgO

K2O

Na2O

3.82 5.97

24.08 48.33

4.72 31.69

2.46 4.14

2.31 1.37

58.24 4.12

1.95 0.50

1.02 1.34

0.27 0.37

Pore structure was investigated with mercury intrusion porosimetry (MIP). The hydration products were characterized with scanning electron microscope (SEM), thermos-gravimetric analysis (TGA), and X-ray diffraction (XRD). The reaction degree of cement and FA was assessed with solid-state nuclear magnetic resonance (NMR). The dissolution of FA was assessed with SEM and inductive coupled plasma emission spectrometer (ICP). Finally, the mechanism behind the improvement of TIPA in mechanical performance was revealed in the terms of pore structure and hydration process. Such results were expected to broaden the use of alkanolamine in cement-based materials and also provide useful experience to promote the utilization of FA in real concrete. 2. Materials and test methods 2.1. Materials Fig. 1. Schematic diagram of molecular structure of TIPA.

In cement-FA system, in order to improve the mechanical performance, accelerating hydration of both FA and cement should be considered. In fact, the essence of the pozzolanic reaction of FA is the dissolution of ions from FA surface into liquid phase to participate in hydration, resulting in the formation of hydrates, with contribution to mechanical performance. Taking sodium sulphate for instance, sodium sulphate has been accepted as one of the most popular activators for FA [15]. One explanation for the mechanism is that sulphate can facilitate the formation of ettringite with nano particle size as nucleation seeds, which can hasten the hydration of aluminates and also induce the hydration of other phase [16]. Another reason can be explained that sodium ions provide higher alkalinity to accelerate the depolymerization of silicon and aluminum structure in FA, which means that the dissolution of FA can be promoted. Additionally, organic alkali can also facilitate the dissolution of FA to obviously increase the early strength of cement-FA system. Taking triethanolamine (TEA) for example, TEA can accelerate the dissolution of aluminum, ferric, and calcium of FA into pore solution, resulting from the complexation of TEA with Al3+ and Fe3+ [17]. In terms of the acceleration of cement hydration, as reported in the literatures, triisopropanolamine (TIPA) shows high efficiency [18], and the probable reason is due to the accelerated formation of hydrated calcium sulphoaluminate and the induced dissolution of ferric by forming a complex TIPA-Fe [19]. Since the ferric phase, with much lower temperature of melting point, would exist on the surface of clinkers, the promoted dissolution of ferric phase would hasten the dissolution of other phases. It is deduced that in cement-FA system, the promoted cement hydration by TIPA can facilitate the formation CH, and possibly, this would provide stronger alkali environment to hasten the pozzolanic reaction of FA. Additionally, another reason should be noticed that TIPA might induce the dissolution of aluminum, ferric, and silicate into liquid phase, which could also facilitate the pozzolanic reaction of FA. In this case, the addition of TIPA to promote the strength of cement-FA system would be expected. However, no direct evidence can be found. In this study, the effect of TIPA on compressive strength and hydration of cement-FA system was systematically investigated.

2.1.1. Cement and fly ash A Portland cement (P.I 42.5, Wuhan Yadong Cement Co., Ltd.) in accordance with the requirements of GB 175-2007 (Chinese standard) and class F-Ⅱ fly ash (FA) in accordance with the requirements of GB/T 1596-2005 (Chinese standard) were used in this study. The chemical composition of cement and FA was obtained with X-ray Fluorescence (XRF, Axios advanced, made by PANalytical B.V., Holland), and the results are shown in Table 1.

2.1.2. TIPA A reagent-grade triisopropanolamine (TIPA, anhydrous white solid, 95.0% purity, made by Aladdin Biochemical Technology Co., Ltd., Shanghai, China) was used. Additionally, the added dosage of TIPA was recorded as the solid amount. The molecular structure of TIPA are shown in Fig. 1.

2.1.3. Preparation for specimens Cement-FA paste (C-FA: 30% FA and 70% cement) with different dosages of TIPA (0%, 0.03%, 0.06%, and 0.10%) was prepared with a water/binder ratio of 0.38 by weight. TIPA was dissolved in water in advance and the solution was then mixed with the binder to prepare the paste. The fresh pastes were cast in 40 mm  40 mm  40 mm cubic metallic moulds and cured in the >90% R.H. and 20 ± 1 °C chamber for 24 h, and then demoulded and further cured with the same condition. At the age of 7 d and 60 d, the compressive strength of the sample was measured. The samples were also broken into small pieces and immediately immersed into ethanol in order to stop hydration. The pieces were dried in a vacuum drier at 20 °C, and then prepared for the measurement of pore structure. Additionally, these specimens were also grinded by hand, and the powder, which could pass through a 63 lm sieve, was prepared for analysis of hydration products.

2.2. Test methods 2.2.1. Compressive strength The samples were tested with the compressive machine at a rate of 0.6 MPa/s. For each mix, three samples were tested, and the average value was the result.

2.2.2. Ions dissolution of fly ash Firstly, pore solution with different concentrations of TIPA (0–20.0 g/L) was prepared with KOH and NaOH (K+/Na+ = 1:1; pH = 13.0). One gram of FA was added into these solutions (20.0 g), respectively, and mixed. The suspensions was sealed in a plastic container, with constant temperature of 20 ± 1 °C. For each 12 h, the containers were shocked in order to make the suspension to be even. At the age of 12 h, 1 d, 7 d, 14 d, 28 d, and 60 d, the suspension was centrifuged at 3600 r/min for 10 min in a centrifuge, and the content of Al, Fe and Si in supernatant solution were tested with inductive coupled plasma emission spectrometer (ICP, Optima 4300 DV, made by Perkin Elmer Ltd., USA) to investigate the effect of TIPA on the dissolution of FA. In addition, the solid was dried in a vacuum drier with the temperature of 20 ± 1 °C, and then the surficial morphology was characterized with field emission scanning electron microscope (SEM).

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Fig. 2. Effect of TIPA on compressive strength of 7 d and 60 d.

2.2.3. Hydration heat TIPA (0–0.10 wt% of cement-FA binder, 30% FA and 70% cement) was added into water in advance, and then the solution and cement-FA were mixed together with a water/binder ratio of 0.5 by weight. Hydration heat was obtained with an isothermal calorimetry (TAM AIR, C80, SETARAM company, France). 2.2.4. MIP The pore structure of the samples was tested with mercury intrusion porosimetry (Poremaster GT-60, Kangta Instrument Company, USA) at a maximum pressure of 420 MPa and contact angle of 140°. 2.2.5. Phase analysis The effect of TIPA on hydration products was investigated with scanning electron microscope (SEM), thermo gravimetric analysis (TGA), solid-state nuclear magnetic resonance (NMR) and X-ray diffraction (XRD). 2.2.5.1. XRD. The powder was measured with an X-ray diffractometer (XRD, D/MaxRB) with Cu (Ka) radiation and a current of 40 mA, 40 kV. The test rate was at a speed of 4°/min and a step of 0.02° within the range from 5 to 70°. 2.2.5.2. SEM. Field Emission Scanning Electron Microscope (FE-SEM, QUANTA FEG 450, FEI Co, USA) was used for SEM microstructural characterization. 2.2.5.3. TG-DTG. TGA was conducted with the comprehensive thermal analyzer (German-resistant STA449F3). The heating rate was 10 °C/min, using nitrogen as purging gas and the temperature was ranged from the room temperature to

1000 °C. CH was decomposed at the temperature range from 400 to 500 °C, and calcium carbonate resulting from the carbonation of CH in the process of preparing the samples was decomposed at 500–700 °C, as shown in follows:

Ca ðOHÞ2 ! CaO þ H2 O CaCO3 ! CaO þ CO2 The total content of CH in hydration products can be calculated as follow: MCaðOHÞ2 ¼ 74 M H2 O þ 74 M CO2 ; 18 44 MCaðOHÞ2 : the mass of calcium hydroxide; MH2 O : at 400–500 °C, the weight loss resulting from water; MCO2 : at 500–700 °C, the weight loss resulting from carbon dioxide. Additionally, the weight loss at the temperature range from 50 to 200 °C, due to evaporation of free water, dehydration of C-S-H gel and decomposition of ettringite (AFt), is of great interesting. Therefore, the peak of TGA at this temperature range was divided into two peaks, which were processed by the software XPSPEAK 4.1. One with lower temperature is related to the evaporation of free water and dehydration of C-S-H gel, and the other with higher temperature is involved in evaporation of free water, dehydration of C-S-H gel and decomposition of AFt. The relative ratio of these two peaks can be calculated from area of each peak. As the total weight loss from 50 to 200 °C can be obtained directly from the data of TG, the weight loss related to each peak can be calculated. Such results can provide supplementary evidence to illustrate the effect of TIPA on the formation of AFt.

Fig. 3. Pore size distribution of cement-fly ash pastes at different ages.

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Table 2 Pore structures of cement-fly ash pastes. Sample

Porosity (mL/g)

200 nm

7d

C-FA C-FA + 0.06% TIPA

0.1728 0.1444

0.0332 0.0009

0.0337 0.0330

0.0524 0.0561

0.0535 0.0544

60 d

C-FA C-FA + 0.06% TIPA

0.1272 0.1020

0.0394 0.0000

0.0252 0.0200

0.0323 0.0432

0.0303 0.0388

2.2.5.4. NMR. To further verify the effect of TIPA on hydration of cement-FA paste, the hydration products were characterized with 29Si MAS NMR. As reported in the literatures [20], six peaks can be found in NMR spectrum of hydrated cement-FA paste: Q1 (chain-end groups), Q2 (middle-chain groups), and Q2 (1Al) (middle-chain groups where one of the adjacent tetrahedral sites is occupied by Al) represent the Si-O tetrahedron in hydration products; Q0 represents the Si-O tetrahedron in unhydrated cement minerals, and Q3 and Q4 represent the Si-O tetrahedron in FA. Due to different chemical surroundings of Si sites in cementitious materials, the polymerization degree of Si-O tetrahedron and substitution of Si by Al ratios in C-S-H can be evaluated. 29 Si NMR (solid-state nuclear magnetic resonance) was conducted with a Bruker Advance III400 spectrometer operating at 79.5 MHz. The rotation frequency was 5 kHz and the delay time was 10 s. Tetramethylsilane was used as a standard for 29Si. The data was processed with commercial solid-state NMR software package. It was firstly fitted, and then the phasing and baseline were corrected, followed by subsequently iterative fitting. During the deconvolution of 29Si NMR spectra, the peak shapes were constrained with Gaussian function. The main chain length (MCL) of C-S-H gel and the ratio of Si in C-S-H substituted by Al were calculated as follows [21]:

MCL ¼

Al=Si ¼

2IðQ 1 Þ þ 2IðQ 2 Þ þ 3I½Q 2 ðAlÞ Q1 0:5I½Q 2 ðAlÞ IðQ 1 Þ þ IðQ 2 Þ þ I½Q 2 ðAlÞ

Reaction degree of FA and cement was also calculated as follows:

AFA ð%Þ ¼ 1 

AC ð%Þ ¼ 1 

IðQ 3 þ Q 4 Þ I0 ðQ 3 þ Q 4 Þ IðQ 0 Þ

I0 ðQ 0 Þ

where, I(Q0), I(Q1), I(Q2) and I[Q2(Al)] represent the integrated intensities of signals Q0, Q1, Q2 and Q2 (Al) in hydrated cement-FA paste, respectively; I0(Q0), I0(Q3), and I0(Q4) represent the integrated intensities of signals Q0, Q3, and Q4 in unhydrated cement-FA mixture.

Pore volume distribution (mL/g)

3. Results and discussion 3.1. Compressive strength The effect of TIPA on the compressive strength of cement-FA paste is shown in Fig. 2. As can be seen from the figure, in comparison with the reference (i.e. Without TIPA), the compressive strength of paste at the age of 7 d is reduced with addition of 0.03%, but further increase of TIPA in dosage to 0.06% increases the compressive strength more than the reference, with an increase by 15.6%; with the dosage of 0.10%, the compressive strength of paste reaches 37.9 MPa, with an increase by 25.5%. Furthermore, the compressive strength of 60 d is increased with the increasing dosage of TIPA; when the dosage of TIPA was 0.10%, the compressive strength is increased by 16.7%, compared to the reference. It is worth noting that 0.03% TIPA can slightly decline the compressive strength of paste, a little below the reference, and this is probably attributed to the air-entraining effect [22]. Even though, air-entraining effect would be enhanced with further increase in TIPA dosage, the effect of TIPA on accelerating the hydration of cement-FA system can also be promoted. Probably, this promotion should be obviously greater that the negative effect resulting from air-entraining performance, thereby resulting in increase in compressive strength. The air-entraining effect was further discussed with the analysis of pore structure characterized by MIP, as shown in Section 3.2. Furthermore, except the pore structure, hydration is one of the main factors affecting the compressive strength, involved in the accelerated cement hydration and the promoted pozzolanic reaction of FA. The cement hydration can be facilitated via accelerating the release of ions into liquid phase, and after oversaturation, precipitation would take place to form hydrates. As reported in the liter-

Fig. 4. Hydration heat of cement-FA paste with TIPA.

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Fig. 5. XRD patterns of cement-fly ash paste hydrated for 7 d.

Fig. 6. XRD patterns of cement-FA paste hydrated for 60 d.

Fig. 7. TG-DTG patterns of the paste hydrated for 7 d.

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atures, TIPA can significantly promote the dissolution of ferrite in clinkers, and the dissolved Fe3+ in pore solution would be precipitated, resulting in the consumption of CH and inducing the formation of AFt [23,24]. And the main mechanism behind the facilitated transport of ferric ions is ascribed to the formation of ferricalkanolamine complexes. These have been widely accepted in the literatures [25]. The pozzolanic reaction of FA also depends on dissolution of silicon and aluminum into aqueous phase. As reported [17], the reason why the presence of triethanolamine (TEA) can increase the early strength of cement-FA system is due to the accelerated ions dissolution of calcium, aluminum, and ferric from FA into liquid to form more amount of AFt. Therefore, the dissolution of FA could be probably facilitated by TIPA to hasten the hydration of FA contributing to the compressive strength, and this could also induce the hydration of the whole system. More details were illustrated in the following text.

structure, the air-entraining effect of TIPA might predominate. It is noticed that the presence of TIPA would reduce the porosity of the paste, in spite of its air-entraining effect. In fact, the porosity depends on two aspects: one is the accelerated formation of hydrates to refine the pore structure, and another is the airentraining effect of TIPA to increase the amount of harmful pore and more-harmful pore. Based on the results shown in Table 2, it is concluded that the former is predominated in most case, and therefore, the porosity is obviously lower than that of paste without TIPA. Additionally, with addition of 0.03%, air-entraining effect of TIPA is a little stronger than the filling effect, and in this case, a little decrease in compressive strength at 7 d age can be observed. Based on discussion above, it is concluded that addition of TIPA has air-entraining effect on cement-FA paste, and this may exert negative effect on mechanical performance. The combining use of TIPA and defoaming agent is suggested.

3.2. Pore structure

3.3. Hydration heat

Pore structure is one of the main factors affecting the mechanical performance of cement-based materials. As reported in the literature [26–28], pores in cementitious composites can be divided into four types: harmless pore (200 nm). The refined pore structure would benefit the compressive strength, while more amounts of pore with greater size tend to reduce the strength. The pore structure of the paste was characterized with MIP, and the pore distributions are shown in Fig. 3 and Table 2. As can be seen clearly from Table 2, the porosity of the reference paste at 60 d age is obviously lower than that at 7 d age. This is mainly attributed to the formation of hydrates with size of nano scale to fill the pore. Furthermore, with the addition of TIPA, the porosity of both 7 d and 60 d age is reduced, and the volume of pore with size less than 20 nm is reduced but the pore with size more than 50 nm is increased obviously. In cement-FA paste with addition of TIPA, the pore structure depends on the formation of hydrates and the air-entraining properties of TIPA [22]. The hydrates formed with the size of nano scale can fill the pores to refine the pore structure, and the disappear of harmless pore at both 7 d age and 60 d age is probably related to the formation of these hydrates as a filler in pores. However, the increase in amount of harmful pores and more-harmful pores at both 7 d age and 60 d age can be seen clearly in Table 2. Despite the accelerated hydration of both cement and FA to refine pore

As reported in the literatures, the hydration heat of cement-FA paste includes the initial reaction, a period of slow reaction, an acceleration period, and a deceleration period, as described by Taylor [29]. As shown in Fig. 4(a), these four steps can be seen clearly, which means that the addition of TIPA cannot change these steps. However, it is noticed that the extra peak at about 30 h can be seen clearly with addition of TIPA. As reported, the effect of TIPA on cement hydration can be divided into three phases: (1) adsorb on the surface of cement particles and slightly delay the hydration [30,31], which would affect the first and the second steps; (2) hasten the dissolution of ions, especially the ferric ion, to accelerate the formation of AFt, which would affect the second and the third steps; (3) facilitate the conversion of AFt to AFm [32], which would affect the third and the forth steps. It can be inferred that the extra peak should be related to that conversion of AFt to AFm. The reason can be revealed: in cement paste, at the very beginning of the hydration, AFt should be formed, because of the much faster dissolution speed of gypsum to provide enough sulfates, with lower Al/S ratio. With time going on, the dissolved aluminates would offer more amount of aluminum to increase the Al/S ratio, thereby hastening the conversion of AFt to AFm. In cement-FA paste with TIPA, the dissolution of aluminum in FA and cement minerals could be hastened by TIPA to significantly increase the aluminum in solution, and the Al/S ratio would be much higher than that without TIPA. In this case, the conversion of AFt to AFm can be facilitated.

Fig. 8. TG-DTG patterns of the paste hydrated for 60 d.

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B. Ma et al. / Construction and Building Materials 179 (2018) 89–99 Table 3 Calcium hydroxide content in cement-fly ash system (wt%). Temperature range/°C

400–500 °C 500–700 °C CH content

7d

60 d

Blank

0.06% TIPA

Blank

0.06% TIPA

2.56 3.50 16.40

2.62 2.35 14.72

2.61 2.64 15.16

2.52 2.32 14.25

Fig. 9. SEM images of the paste hydrated for 7 d.

Furthermore, the accumulated hydration heat of the paste with different dosages of TIPA is shown in Fig. 4(b). It can be seen that the addition of TIPA can increase the hydration heat, which also indicates that TIPA can accelerate the hydration of cement-FA paste at the early age.

3.4. Analysis of hydration products 3.4.1. XRD As reported, CH (calcium hydroxide) and AFt (ettringite) are typical hydration products of cement, and their amounts can be used to illustrate the hydration process. Different amounts of CH and AFt can be reflected in the characteristic peak in XRD. The XRD patterns of the pastes hydrated for 7 d in the presence and absence of TIPA (0.06%) are presented in Fig. 5. As can be seen from Fig. 5(a), the peak of AFt and CH can be observed clearly, and by contrast, the addition of TIPA increases the intensity of AFt. More details can be seen in Fig. 5(b) that TIPA can accelerate the formation of AFt and AFm. Furthermore, at the 60 d age, as shown in

Fig. 10. SEM images of the paste hydrated for 60 d.

Fig. 6(a) and (b), the addition of TIPA can also increase the peak intensity of AFt and AFm, in agreement with the results of 7 d. As a result, XRD results indicate that TIPA can increase the amount of AFt and AFm in hydrates at the 7 d age and 60 d age. 3.4.2. TG-DTG In order to further discuss the effect of TIPA on hydration, the hydration products were characterized with TG-DTG. At the temperature range from 0 to 400 °C, the weight loss is related to the loss of free water, decomposition of C-S-H gel and hydrated calcium sulphoaluminate; that for 400–500 °C is involved in the decomposition of calcium hydroxide; that for 500–700 °C is due to the decomposition of carbon dioxide which was formed in the process of preparing the samples [33,34]. TG-DTG patterns of the paste hydrated for 7 d are shown in Fig. 7. From the figure, the weight loss at the temperature from 50 to 200 °C, mainly resulting from evaporation of free water and the decomposition of hydrates (C-S-H gel and AFt), shows great interest. However, it is very difficult to distinguish the accurate loss weight among these three. It is noticed that in Fig. 7(a), the

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Fig. 11. Deconvolution of the

29

Si MAS NMR spectrum of the sample at the age of 60 d.

Table 4 Deconvolution results of the sample at the age of 60 d. Q0 (%) (a) Unhydrated C-FA (b) C-FA (c) C-FA with 0.06% TIPA

62.27 18.39 12.90

Q1 (%) 19.30 20.69

Q2 (1Al) (%) 10.00 14.44

loss weight, resulting from evaporation of free water, decomposition of C-S-H gel and decomposition of AFt, was calculated as 2.01%; with addition of TIPA (0.06%), as shown in Fig. 7(b), that is 3.85%, which is much greater. As reported in the literatures, AFt would be decomposed at the temperature range from 120 to 150 °C [35]. Therefore, that greater weight loss should be related to the higher content of AFt, providing supplementary evidence to prove that TIPA can facilitate formation of AFt, in agreement with the XRD results. Fig. 8 shows TG-DTG patterns of the paste with and without addition of TIPA hydrated for 60 d. The weight loss at each temperature range can be seen clearly. However, the amount of CH in hydration products attracts more interest. As shown in Table 3, for 7 d age, the amount of CH in reference (without TIPA) is greater than that with TIPA, and the same result can also be found at 60 d age. These results indicate that TIPA can reduce the amount of CH in hydration products at both of 7 d and 60 d age.

Q2 (%)

Q3 + Q4 (%)

MCL

Al/Si

AC (%)

AFA (%)

23.37 24.35

37.73 28.94 27.62

5.98 6.45

0.095 0.121

70.47 79.28

23.30 26.87

The amount of CH depends on the cement hydration and pozzolanic reaction of FA. Cement hydration with greater degree would generate more amount of CH, and pozzolanic reaction of FA with greater degree would consume more amount of CH. The predominant aspect would determine the relative amount of CH. Obviously, the acceleration of FA, namely consumption of CH, should be predominated. This implies that TIPA can obviously hasten the pozzolanic reaction of FA. 3.4.3. SEM Figs. 9 and 10 shows the SEM images of the paste hydrated for 7 d and 60 d in the presence and absence of TIPA. More serious erosion can be seen clearly in Fig. 9(b) than that of Fig. 9(a), which implies that the addition of TIPA can hasten pozzolanic reaction of FA. Furthermore, as shown in Fig. 10, the similar result can also be found. As a result, SEM presents supplementary evidence to prove the accelerated pozzolanic reaction of FA by addition of TIPA.

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Fig. 12. Ions dissolution of FA in the pore solution with TIPA.

Based on discussion above, it is concluded that TIPA can accelerate the pozzolanic reaction of FA at both 7 d and 60 d, and can also hasten the formation of AFt and AFm.

3.5. NMR analysis The deconvoluted 29Si MAS NMR spectra of the samples obtained from the fitting are plotted in Fig. 11, and the results of reaction ratio, MCL and Al/Si ratio was calculated and shown in Table 4. As shown in Fig. 11, Q0, Q1, Q2 (1Al), Q2, Q3, and Q4 can be seen clearly. In comparison with the unhydrated C-FA (as shown in Fig. 11(a)), the appearance the peak of Q1, Q2 (1Al), and Q2 can be observed clearly, and the decline in Q0, Q3, and Q4 can also be found. These results illustrate the hydration of cement and the pozzolanic reaction of FA. More details can be found in Table 4: in comparison with the reference sample (i.e. C-FA system), 0.06% TIPA obviously reduces the amount of Q0, and declines the amount of Q3 + Q4; these results demonstrate that TIPA can promote the cement hydration as well as the pozzolanic reaction of FA; by contrast, this promoting effect on cement hydration is slightly stronger than that of FA. Furthermore, C-FA had an MCL of 5.98, while that for C-FA-TIPA (0.06%) was 6.45, which indicates that the addition of TIPA can increase the degree of silicate polymerization; this can also show that a higher degree of hydration has occurred. Moreover, the Al/Si ratio is also increased with 0.06% TIPA, in comparison with reference sample (i.e. C-FA

system). This increase definitely confirms that the incorporation of TIPA into cement-FA system induces the substitution of Si by Al (Al [4]) into C-S-H, resulting in an increase in the length of silicate chain of C-S-H, in agreement with the results of MCL. Additionally, the reaction degree of cement without TIPA is 70.47%, while that for addition of TIPA (0.06%) is 79.28%, which indicates the accelerated hydration of cement by TIPA. The same results can also be found in FA: the reaction degree of FA is increased from 23.30% in reference to 26.87% in the presence of 0.06% TIPA. As a consequence, the addition of TIPA not only hastens the hydration of cement but also accelerates the pozzolanic reaction of FA, and it also increase the degree of silicate polymerization, the length of C-S-H and substitution of Si by Al, with contribution to the mechanical performance.

3.6. Dissolution of fly ash The accelerated hydration of FA by TIPA has been confirmed above, and the mechanism behind this is closely related to the dissolution of ions from FA into liquid. Accordingly, the reaction of FA in pore solution with various dosages of TIPA was investigated to further reveal the accelerated hydration of FA by TIPA. The effect of TIPA on dissolution of FA in pore solution is shown in Fig. 12. As shown in the figure, the dissolution of aluminum ions, ferric ions, and silicon ions into pore solution was increased over time, regardless of the added dosage of TIPA. Furthermore, as

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Fig. 13. Morphology of FA with pore solution including 20 g/L TIPA.

shown in Fig. 12(a), the dissolution of Al into solution was significantly promoted by TIPA, and more amount of TIPA used results in stronger promoting effect. Specifically, TIPA makes the aluminum ions about 2 times of the reference (without TIPA) at 1, 7 and 14 d, and about 2.5 times of the reference at 28 d and 60 d, indicating that TIPA can improve the dissolution of aluminum ions continuously. As shown in Fig. 12(b), very little amount of Fe can be found without TIPA, which implies that ferric in FA is not that easy to be dissolved into pore solution in the hydration process. However, with addition of TIPA, surprisingly, the dissolution of ferric ions was considerably hastened, in agreement with the results in the literatures [19,23]. As shown in Fig. 12(c), the dissolution of silicate can also be facilitated, with almost the same change tendency with Al. With 20 g/L TIPA, silicate in solution was increased by 15% at 7 d and 45% at 14 d compared to the reference, and that for 28 and 60 d is about 2 times of the reference. The morphology of FA immersed into pore solution was observed with SEM, as a supplementary evidence to prove the dissolution of ions of FA in pore solution. Compared with Fig. 13(a), more serious erosion of FA can be found in Fig. 13(b), and more amount of flocculent reaction product on the surface can also be observed, indicating that even at 7 d age, TIPA can accelerate the dissolution of FA, thereby hastening the pozzolanic reaction of FA. Furthermore, at 60 d age, more serious erosion on FA surface can be found in Fig. 13(d) in comparison with Fig. 13(c), indicating that TIPA can also hasten pozzolanic reaction of FA at the age of 60 d. Based on discussion above, the promoted ions dissolution of FA into solution by addition of TIPA can be concluded, and this can

significantly affect the hydration of cement-FA system. Firstly, the promotion of TIPA on cement hydration is confirmed, which can directly contribute to the mechanical performance, and this promotion can also generate more amount of CH to activate the pozzolanic reaction of FA. Secondly, the accelerated dissolution of Al in the pore solution can hasten the formation of AFt at the early time. With time going on, the dissolved Al would accelerate the conversion of AFt to AFm, and can also substitute the silicate to form C-A-S-H gel, which would hasten the hydration of cement minerals and the formation of hydrates. Thirdly, the dissolved Fe3+ can be easily precipitated in the pore, with the size of nano scale, which can be used as nano-crystal nucleus to accelerate the other hydration. Furthermore, the accelerated dissolution of Si into pore solution would benefit the formation of C-S-H gel to directly contribute to the strength. Additionally, those hydrates formed with nano particle size can fill the pores to refine the pore structure and compensate its negative effect on pore structure resulting from air-entraining effect. 4. Conclusions (1) TIPA can increase the mechanical performance of cement-FA system, and this is mainly attributed to the accelerated hydration of both cement and FA. (2) Because TIPA can accelerate the hydration of cement minerals, more amount of CH can be generated to activate the pozzolanic reaction of FA, which is one of the reasons for the promoted hydration of FA. Another reason is that TIPA can also accelerate the dissolution of the silicate, ferric, and alu-

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minate of FA into pore solution, which can hasten the formation of C-(Al)S-H gel and hydrated calcium sulphoaluminate to contribute to the compressive strength. (3) It is noticed that TIPA can increase the volume of harmful and more-harmful pore, which probably exert negative effect on the compressive strength. Conflict of interest Baoguo Ma, Ting Zhang, Hongbo Tan, Xiaohai Liu, Junpeng Mei, Huahui Qi, Wenbin Jiang and Fubing Zou declare that there is no conflict of interest regarding the publication of this paper. Acknowledgment The financial support from National key R&D program of China (2016YFC0701003-5) is gratefully acknowledged. References [1] P. Suraneni, J. Weiss, Examining the pozzolanicity of supplementary cementitious materials using isothermal calorimetry and thermogravimetric analysis, Cem. Concr. Compos. 83 (2017) 273–278. [2] B. Lothenbach, K. Scrivener, R.D. Hooton, Supplementary cementitious materials, Cem. Concr. Res. 41 (12) (2011) 1244–1256. [3] W. Shen, Y. Liu, B. Yan, J. Wang, P. He, C. Zhou, X. Huo, W. Zhang, G. Xu, Q. Ding, Cement industry of China: driving force, environment impact and sustainable development, Renewable Sustainable Energy Rev. 75 (2017) 618–628. [4] W. Shen, L. Cao, Q. Li, W. Zhang, G. Wang, C. Li, Quantifying CO2 emissions from China’s cement industry, Renewable Sustainable Energy Rev. 50 (2015) 1004– 1012. [5] S.R. da Silva, J.J. de Oliveira Andrade, Investigation of mechanical properties and carbonation of concretes with construction and demolition waste and fly ash, Constr. Build. Mater. 153 (2017) 704–715. [6] D. Shen, X. Shi, S. Zhu, X. Duan, J. Zhang, Relationship between tensile Young’s modulus and strength of fly ash high strength concrete at early age, Constr. Build. Mater. 123 (2016) 317–326. [7] B. Xu, H. 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. [8] J. Yu, C. Lu, C.K.Y. Leung, G. Li, Mechanical properties of green structural concrete with ultrahigh-volume fly ash, Constr. Build. Mater. 147 (2017) 510– 518. [9] I. De la Varga, R.P. Spragg, C. Di Bella, J. Castro, D.P. Bentz, J. Weiss, Fluid transport in high volume fly ash mixtures with and without internal curing, Cem. Concr. Compos. 45 (2014) 102–110. [10] X.Y. Wang, K.B. Park, Analysis of compressive strength development of concrete containing high volume fly ash, Constr. Build. Mater. 98 (2015) 810–819. [11] R. Siddique, Performance characteristics of high-volume Class F fly ash concrete, Cem. Concr. Res. 34 (3) (2004) 487–493. [12] V.M. Malhotra, M.H. Zhang, P.H. Read, Long-term mechanical properties and durability characteristics of high-strength/high-performance concrete incorporating supplementary cementing materials under outdoor exposure conditions, ACI Struct. J. 97 (5) (2000) 518–525.

99

[13] E.E. Berry, R.T. Hemmings, B.J. Cornelius, Mechanisms of hydration reactions in high volume fly ash pastes and mortars, Cem. Concr. Compos. 12 (4) (1990) 253–261. [14] L. Lam, Y.L. Wong, C.S. Poon, Degree of hydration and gel/space ratio of highvolume fly ash/cement systems, Cem. Concr. Res. 30 (2000) 747–756. [15] S. Donatello, A. Fernández-Jimenez, A. Palomo, Very high volume fly ash cements. Early age hydration study using Na2SO4 as an activator, J. Am. Ceram. Soc. 96 (3) (2013) 900–906. [16] D.-Y. Lei, L.-P. Guo, W. Sun, J.-P. Liu, C.-W. Miao, Study on properties of untreated FGD gypsum-based high-strength building materials, Constr. Build. Mater. 153 (2017) 765–773. [17] D. Heinz, M. Göbel, H. Hilbig, L. Urbonas, G. Bujauskaite, Effect of TEA on fly ash solubility and early age strength of mortar, Cem. Concr. Res. 40 (3) (2010) 392– 397. [18] P.J. Sandberg, F. Doncaster, On the mechanism of strength enhancement of cement paste and mortar with triisopropanolamine, Cem. Concr. Res. 34 (6) (2004) 973–976. [19] E. Gartner, D. Myers, Influence of tertiary alkanolamines on Portland cement hydration, J. Am. Ceram. Soc. 76 (6) (1993) 1521–1530. [20] C.A. Love, I.G. Richardson, A.R. Brough, Composition and structure of C–S–H in white Portland cement–20% metakaolin pastes hydrated at 25 °C, Cem. Concr. Res. 37 (2) (2007) 109–117. [21] L. Wang, Z. He, Quantitative of fly ash-cement hydration by 29Si MAS NMR, J. Chin. Ceram. Soc. 38 (11) (2010) 2212–2216. [22] Z. Xu, W. Li, J. Sun, Y. Hu, K. Xu, S. Ma, X. Shen, Research on cement hydration and hardening with different alkanolamines, Constr. Build. Mater. 141 (2017) 296–306. [23] J.P. Perez, A. Nonat, S. Pourchet, M. Garrault, C. Canevet, Why TIPA leads to an increase in the mechanical properties of mortars whereas TEA does not, ACI Mater. J. 217 (38) (2003) 583–594. [24] H. Huang, X.-R. Li, X.-D. Shen, Hydration of ternary cement in the presence of triisopropanolamine, Constr. Build. Mater. 111 (2016) 513–521. [25] H. Huang, X. Shen, J. Zheng, Modeling, analysis of interaction effects of several chemical additives on the strength development of silicate cement, Constr. Build. Mater. 24 (10) (2010) 1937–1943. [26] W. Zhongwei, L. Huizhen, High Performance Concrete, China Railway Publishing House, Beijing, 1999. [27] B. Zhang, H. Tan, B. Ma, F. Chen, Z. Lv, X. Li, Preparation and application of finegrinded cement in cement-based material, Constr. Build. Mater. 157 (24) (2017) 34–41. [28] B. Pang, Z. Zhou, H. Xu, Utilization of carbonated and granulated steel slag aggregate in concrete, Constr. Build. Mater. 84 (2015) 454–467. [29] H. Taylor, Cement Chemistry, Thomas Telford, London, 1997. [30] F. Zou, H. Tan, Y. Guo, B. Ma, X. He, Y. Zhou, Effect of sodium gluconate on dispersion of polycarboxylate superplasticizer with different grafting density in side chain, J. Ind. Eng. Chem. 55 (2017) 91–100. [31] Y. Guo, B. Ma, Z. Zhi, H. Tan, M. Liu, S. Jian, Y. Guo, Effect of polyacrylic acid emulsion on fluidity of cement paste, Colloids Surf., A 535 (2017) 139–148. [32] Z. Shi, C. Shi, H. Liu, P. Li, Effects of triisopropanol amine, sodium chloride and limestone on the compressive strength and hydration of Portland cement, Constr. Build. Mater. 125 (2016) 210–218. [33] S. Yang, J. Wang, S. Cui, H. Liu, X. Wang, Impact of four kinds of alkanolamines on hydration of steel slag-blended cementitious materials, Constr. Build. Mater. 131 (2017) 655–666. [34] S. Ma, W. Li, S. Zhang, Y. Hu, X. Shen, Study on the hydration and microstructure of Portland cement containing diethanol-isopropanolamine, Cem. Concr. Res. 67 (2015) 122–130. [35] K. Ndiaye, M. Cyr, S. Ginestet, Durability and stability of an ettringite-based material for thermal energy storage at low temperature, Cem. Concr. Res. 99 (2017) 106–115.