Eco-friendly treatment of low-calcium coal fly ash for high pozzolanic reactivity: A step towards waste utilization in sustainable building material

Journal of Cleaner Production 238 (2019) 117962

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Eco-friendly treatment of low-calcium coal fly ash for high pozzolanic reactivity: A step towards waste utilization in sustainable building material Jin Yang a, c, Jianxiang Huang a, Ying Su a, c, Xingyang He a, c, *, Hongbo Tan b, Wei Yang e, Bohumír Strnadel a, d a

School of Civil Engineering, Architecture and Environment, Hubei University of Technology, Wuhan, 430068, China State Key Laboratory of Silicate Materials for Architecture, Wuhan University of Technology, Wuhan, 430070, China Building Waterproof Engineering and Technology Research Center of Hubei Province, Hubei University of Technology, Wuhan, 430068, China d Center of Advanced Innovation Technologies, V
SB-Technical University of Ostrava, 708 33, Ostrava-Poruba, Czech Republic e China Construction Third Bureau Industry Investment Co., Ltd, Wuhan, 430100, China b c

a r t i c l e i n f o

a b s t r a c t

Article history: Received 3 June 2019 Received in revised form 3 August 2019 Accepted 7 August 2019 Available online 8 August 2019

Fly ash is a coal combustion by-product with low pozzolanic reactivity which limits its resource utilization and engineering properties in building materials. The present study investigates a wet-milling treatment to activate the coal fly ash and promote its sustainable utilization. It was found that wetmilling is a suitable, efficient and eco-friendly technology for solid waste refinement. The pozzolanic reactivity is greatly improved after wet-milling treatment, as high as 140% at 60 days with particle size of 2.51 mm. It was observed that ettringite formation has occurred during the wet-milling. Physical structure, chemical evolution and ion leaching behavior during the wet-milling were discussed. It was proved that activation mechanism of wet-milling is a combined effect of physical breakage and ion dissolution acceleration. Binding energy was found decreased and SiQn structural units were found less polymerized which are helpful for the depolymerization during the pozzolanic reaction. Furthermore, technical applicability of wet-milling for various industrial solid wastes was also investigated. © 2019 Elsevier Ltd. All rights reserved.

Handling Editor: Prof. Jiri Jaromir Kleme
s Keywords: Wet-milling Coal fly ash Pozzolanic reactivity Industrial solid waste Cement-based material

1. Introduction Coal fly ash (FA) is the by-product from burning pulverized coal in thermal power generating plants (Yang et al., 2018). The output of fly ash is predicted to continue to grow for many more years, as a result of the world’s increasing reliance on coal-fired power generation (Belviso, 2018; Yao et al., 2014). Coal-fired generation is more economically attractive in countries which are rich in coal resources, such as China, US and India (Yao et al., 2015). China is the largest coal consumer and contributes more than half the worldwide generation of fly ash. According to the statistics of the National Development and Reform Commission of China, annual generation of FA reached 580 million tons and kept growing. The accumulated coal fly ash not only occupies land but also pollutes

* Corresponding author. School of Civil Engineering, Architecture and Environment, Hubei University of Technology, 28# Nanli Road, Wuhan, 430068, China. E-mail address: [email protected] (X. He). https://doi.org/10.1016/j.jclepro.2019.117962 0959-6526/© 2019 Elsevier Ltd. All rights reserved.

the environment. Thus the resource utilization of fly ash is of great significance. Recycling coal fly ash brings significant economic and environmental benefits (Yao et al., 2014). Coal fly ash, with main chemical composition of SiO2, Al2O3, Fe2O3 and CaO, has potential pozzolanic reactivity. There is a large amount of research on the pozzolanic reactivity of fly ash, covering reactivity evolution in cementitious system (Schwarz and Neithalath, 2008; Tangpagasit et al., 2005; Yamamoto et al., 2006), reactivity testing method (Bentz et al., ^ncio, 2007) and reac2011; Donatello et al., 2010; Gava and Prude lu et al., 2009; tivity improvement (Aydın et al., 2010; Felekog Shvarzman et al., 2002). Usage of fly ash as pozzolanic material to replace cement has been proved to be an efficient way. Fly ash as alternative material used in cement and concrete industry (Deschner et al., 2012; Donatello et al., 2013; Telesca et al., 2017), alkali activated material (Kumar and Kumar, 2011; Rashad, 2015; Shekhovtsova et al., 2018), stabilisation/solidification materials (Chen et al., 2019; Wang, L. et al., 2019a; Wang et al., 2018), surface

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J. Yang et al. / Journal of Cleaner Production 238 (2019) 117962

Nomenclature FA0 FA1 FA2 FA3 FA4 FA5

jn RFA R0 R W/FA PSD ICP Itot DE h c v

raw fly ash fly ash wet-milled for 15 min fly ash wet-milled for 30 min fly ash wet-milled for 70 min fly ash wet-milled for 180 min fly ash wet-milled for 360 min reactivity index compressive strength of FA blended mixture compressive strength of reference mixture basicity, CaO/SiO2 water-to-fly ash mass ratio Particle size distribution area of the crystalline peaks area of the total domain change in silicon oxide bond energy Plank’s constant velocity of light wave numbers of SieO vibration

modifier of recycled aggregate (Shaban et al., 2019) and fill material (Wang, L. et al., 2019b) has attracted wide interest. However, compared with pozzolanic materials like ground blast-furnace slag and silica fume, most fly ash shows poor reactivity. It was reported in (Zeng et al., 2012) that the FA reaction extent is still at a low level even after three months. On the other hand, with the gradual consumption of high-quality fly ash, fly ash with low quality is increasing. Negative influence of fly ash on mechanical strength is usually observed. Obvious strength reduction before 28 days was observed even at a low replacement level of 10 m% (Yang et al., 2018).This limits a lot its resource utilization and engineering properties in building materials. Generally, the threshold replacement level of fly ash in concrete is usually limited as 30% (Tan et al., 2019), and typically in the range of 15e25% (Kumar et al., 2007). This means that once the limit was surpassed, the strength (especially the early strength) development of concrete would be greatly slowed down. For this regard, reducing the particle size and improving the pozzolanic reactivity is usually considered to promote the recycling utilization of fly ash in sustainable building materials (Kumar et al., 2007). Particle refinement method is usually used to improve the pozzolanic reactivity of fly ash. When fly ash is pulverized to increase fineness, its pozzolanic activity increases significantly (Siddique and Khan, 2011; Supit et al., 2014; Zhang et al., 2011). Typical refinement method for finer fly ash includes mechanical grinding or particle separation process (for example, by wind separator and elec-trostatic precipitator). It was reported in (Li et al., 2014; Wang et al., 2017) that the separated fly ash microsphere shows a relatively high level of early activity. However, ultrafine particle separation method usually has low outcome and efficiency, thus is not the best choice for large-scale disposal. Mechanical grinding (for example, dry-milling) method has been a widely used technique for physical breakage and activation of fly ndez-Jime nez et al., 2019; Kotake et al., 2011; Seo et al., ash (Ferna 2016). It was reported in (Kumar and Kumar, 2011) that the heat evolution and setting time of cement-based material is accelerated by dry-milling fly ash (median size ~5 mm). Mechanical strength of cement-based material is also enhanced by dry-milling ultrafine FA (Marjanovi c et al., 2014). However, the particle agglomeration may easily occur during the dry-milling (Kumar et al., 2017), due to the high particle surface energy. This is negative for the grinding limit

(Kotake et al., 2011). As a result, the efficiency and the energy consumption of dry-milling for higher fineness cannot be accepted (Tan et al., 2019). Thus it is meaningful to develop new recycling techniques for coal fly ash. According to previous work (Tan et al., 2018), it is possible to grind the solids with potential activity in a water environment (namely wet-milling) to reduce the high surface energy and avoid the particle agglomeration. The wet-milling technique is usually used in paper-making industry, ceramic industry and raw meal milling for cement clinker to simply reduce the particle fineness and meet the requirement of production. However, the purpose of wet-milling for solid waste proposal in present work is to greatly improve the potential reactivity and promote the waste utilization in sustainable building materials. In present work, mechanical activation by grinding in water environment was conducted to improve the fineness and activity quality of coal fly ash. Low-calcium raw fly ash with low reactivity was wet-milled for different fineness. Five kinds of wet-milled fly ash were prepared and the median particle diameter was reduced as low as 2.5 mm. The pozzolanic reactivity was evaluated by compressive strength activity index. Wet-milling activation mechanism was investigated with physical structure, chemical evolution and leaching behavior. Physical structure and phase characteristic was studied by scanning electron microscope, X-ray diffraction and thermal gravimetry. Chemical structure was analyzed by infrared spectroscopy, nuclear magnetic resonance and X-ray photoelectron spectroscopy. Ion leaching behavior during the wet-milling was evaluated by inductively coupled plasma spectrometer. The technical advantages and applicability of wet-milling for various solid wastes were also discussed. 2. Materials and methods 2.1. Materials A low-calcium, rich-silicon and aluminum raw fly ash (FA), belongs to Class F (ASTM C618), was used to prepare ultrafine particles by wet-milling. Its chemical composition in wt.% includes SiO2 (48.21), Al2O3 (28.18), Fe2O3 (6.02), CaO (8.34), MgO (0.96), SO3 (1.91) and Na2O (0.8). According to (Sharonova et al., 2010), the basicity R ¼ CaO/SiO2 of the raw FA used in present study is quite low, around 0.17. With R ˂ 0.5, there is no independent binding capacity (Sharonova et al., 2010). This means that the raw FA used here is with a low reactivity. In order to explore the wide application potential of wet-milling technology for solid waste disposal, we also investigated three other types of industrial solid wastes such as blast-furnace slag, steel slag and phosphorus slag as a comparison of reactivity improvement. The initial d50 particle size of above mentioned three solid wastes is 18.5, 54.5 and 20.7 mm respectively, and the particle size after wet-milling disposal is around 2.5 mm. A Portland cement CEM I 52.5 N, according to EN 197e1, was used for FA activity index evaluation which will be given in the following test methods. The specific surface and density of CEM is 360 m2/kg and 3.11 g/cm3, respectively. A standard ISO sand (GB/T 17671) with fineness modulus of 2.38 and specific density of 2.60 g/ cm3 was used. The CEM and ISO sand were only used for the strength activity index evaluation. 2.2. Wet-milling treatment Wet-milling technology was used in present work to modify the fly ash both physically and chemically. The utilization of wetmilling technology for solid waste disposal can be found in previous work (Tan et al., 2018; Wang, Y. et al., 2019). The industrial wetmilling process and equipment is illustrated in Fig. 1. Take fly ash for

J. Yang et al. / Journal of Cleaner Production 238 (2019) 117962

example, the raw FA and liquid medium, i.e. water in present study, with a water-to-solid ratio (W/FA ratio) of 0.5, were mixed in the regulating tank first. In order to improve the wet-milling efficiency, chemical additive (polycarboxylate-based superplasticizer) as a dispersing agent was also used with a fixed dosage (0.6 m% of FA weight). Then the raw FA slurry was transferred into the buffer tank equipped with stirring apparatus to avoid the sedimentation. Straight after, the raw FA slurry was pumped into the mill and ground with a rotation frequency of 40 Hz, ball-to-FA ratio of 7:5 and duration of 15 min to 6 h, to obtain FA with varying fineness. The wet-milling FA slurry was stored in the stirring tank in a room temperature. The storage tank is equipped with cover to avoid water evaporation and ensure the stability of the water content. The FA slurry can be used directly as a raw material in concrete with no need being dried. The water content in the FA slurry can be determined and deducted from the mixing water before using. The raw fly ash was wet-milled for 15, 30, 70, 180 and 360 min and labeled as FA1, FA2, FA3, FA4 and FA5 respectively. The raw fly ash without milling treatment was recorded as FA0. 2.3. Particle size distribution (PSD) analysis A Malvern Mastersizer 2000 was used to test the FA particle size in a range from 0.02 mm to 2000 mm. Alcohol was used as dispersion medium. The particle refractive index of 1.500 with an absorption of 0.3 were chosen. Specific surface area (SSA) as well as characteristic particle size (d10, d50 and d90) was calculated automatically. With d90 as the upper bound, and d10 as the lower bound, d90d10 is defined as the PSD width, thus the distribution width ratio c1 can be represented by the following equation (Kotake et al., 2011; Nakach et al., 2004):

c1 ¼ ðd90  d10 Þ=d50

(1)

The distribution can also be represented by the particle size ratio c2, which is the ratio of d90 to d10:

c2 ¼ d90 = d10

(2)

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mode, with an accelerating voltage of 20 kV and working distance around 12 mm. All the samples were coated with Pt.

2.5. Mineralogical and chemical characterization 2.5.1. X-ray diffraction (XRD) analysis A D8 Advance X-ray diffractometer was used to study the mineralogical composition of FA during the wet-milling. The diffractometer with copper-target X-ray tube (l ¼ 1.5406 Å) was operated at 40 mA and 40 kV. The XRD patterns were collected from 5 to 80 2q in increments of 2 per minute. All the samples were oven dried at 50  C for 24 h and filtrated with a 74 mm sieve before testing. The XRD deconvolution method (Park et al., 2010) using a curvefitting process was used to separate amorphous and crystalline contributions and calculate the crystallinity index (Eq. (3)) of FA samples. The amorphous peak in the XRD pattern is fitted first, and then the crystal diffraction peak is added to be fitted until the fitting spectrum is basically consistent with the measured spectrum. Detailed information can be found in (Ciolacu et al., 2011; Karimi and Taherzadeh, 2016; Park et al., 2010). It is assumed in this study that the diffraction peak with full width at half maxima (FWHM) larger than 3 belongs to amorphous peak. It was reported that FWHMs of single crystals were less than 1 (Barrales-Rienda and Fatou, 1971), and for semi-crystalline polymer, the FWHM was found larger than 1 and less than ~3 (Khulbe et al., 2000), and wider amorphous peaks were found with FWHM around 5 (Khulbe et al., 2000). In present work, amorphous peak is identified around 24 with FWHM value above 7, and the FWHM of all the other peaks (crystalline phase) is found below 1.

Crystallinity ¼

ICP  100% Itot

(3)

Where ICP is the area of the crystalline peaks and Itot is the area of the total domain.

When c1 and c2 are smaller, the PSD is narrower. 2.4. Scanning electron microscope (SEM) analysis A FEI Quanta FEG 450 SEM was used to observe the morphologies of the FA particles. The FA samples were diluted and dispersed with alcohol first and then dried in the oven with temperature of 50  C for 24 h. The microscope was operated in the high vacuum

2.5.2. Thermal gravimetry (TG) analysis A German STA 449 F3 simultaneous thermogravimetric analyzer was used in this study. Approximately 20 mg of sample was placed in alumina pan and heated from room temperature to 800  C at a rate of 10  C/min under the nitrogen atmosphere with a flow rate of 20 mL/min. All the samples were oven dried at 50  C for 24 h and filtrated with a 74 mm sieve before testing.

Fig. 1. Illustration for (a) wet-milling process and (b) the laboratory equipment (c) raw FA and wet-milling FA slurry.

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2.5.3. Infrared spectroscopy (FTIR) analysis Mid-infrared transmission spectrum was tested with a Nexus Fourier transform infrared spectrometer. The wave range is among 400 cm1 and 4000 cm1. All the samples were oven dried at 50  C for 24 h before testing. The change in silicon oxide bond energy (DE) is determined using the method in (Liao et al., 2011):

of at least 5 specimens for each mixture was tested with loading rate of 2400 N/s. The reactivity index, jn (%), was calculated by the equation below:

DE ¼ hcðn1  n2 Þ

where RFA is the compressive strength of FA blended mixture at a certain age (n); R0 is the compressive strength of reference mixture at curing age n. In addition, reactivity index of raw/wet-milling blast-furnace slag, steel slag and phosphorus slag were also tested using the same method as a comparison.

(4)

where h is the Plank’s constant, c is the velocity of light, v1 and v2 is the wave numbers of the SieO vibration of FA before and after wetmilling respectively. 2.5.4. Solid-state nuclear magnetic resonance (NMR) analysis A Bruker AVANCE III 400 WB solid-state NMR was used to collect the chemical shift signal of 29Si. Before the test, the instrument was shimmed and tuned. During the test, the 7 mm sample tube and the applied magnetic field were rotated at 54.736 magic angle, and the spinning rate was 6000 Hz. The spectra were acquired with 2048 scans using the Onepulse mode with excitation pulse length of 4.97 ms, acquisition time of 0.021 s and pulse recycle delay of 5 s. As for FA sample, the main peaks for silicate tetrahedron are Q3 (88~98 ppm) and Q4 (98~-129 ppm). Quantitative information on the fractions of Si present in silicate tetrahedral with different con~o nectivities can be obtained by deconvolution of the spectra (Gira et al., 2010). The spectra were fitted using three peaks, i.e. Q3 and Q4, at approximately 89, 98 and 106 ppm. Compared with Q4 units, the Q3 units present a relatively lower degree of polymerization. And the relative polymerization degree of FA sample was roughly evaluated by the area fraction of Q3 units, i.e. Q3/(Q3þQ4). 2.6. X-ray photoelectron spectroscopy (XPS) analysis An ESCALAB 250Xi spectrometer from Thermo Fisher Scientific Ltd. was used to collect the surface spectra. Scans were operated using monochromatic Al Ka X-ray (hv ¼ 1486.68 eV) with a beam diameter of 500 mm. The spectra were recorded in the CAE (constant analyzer energy) scanning mode (CAE ¼ 100 eV for survey spectra and CAE ¼ 30 eV for high resolution spectra). High resolution spectra for Si 2p, Al 2p, Ca 2p and O 1s were recorded with step size of 0.1 eV to obtain the elemental and chemical composition of FA during wet-milling. High resolution analyses were calibrated to C 1s signal of 284.8 eV. 2.7. Inductively coupled plasma spectrometer (ICP) analysis An Optima 4300DV spectrometer from PerkinElmer Ltd. was used to investigate the ion dissolution of FA during the wet-milling. Some wet-milled FA slurries (FA3 and FA5) were filtrated right after the wet-milling process. Ion dissolution of raw fly ash (FA0) was also considered. The raw FA was mixed with deionized water (W/ FA ¼ 0.5) and stirred hermetically for 360 min. All the liquid samples were diluted by nitric acid solution with a certain concentration to dissolve the potential sedimentation. The dilution of nitric acid solution on the testing ions was finally considered and calibrated. 2.8. Reactivity index (jn) analysis Reactivity index of raw FA and wet-milling FA was evaluated according to GB/T 1596e2005. FA blended mixtures (FA: CEM: ISO sand: water ¼ 3: 7: 30: 5) and reference mixture (CEM: ISO sand: water ¼ 10: 30: 5) were placed in 40  40  40 mm3 steel molds. The specimens were cured indoor and demoulded after 24 h, and then cured in water at 20 ± 1  C until testing. Compressive strength

Jn ¼

RFA  100 R0

(5)

3. Results and discussion 3.1. Particle size and morphological characteristic In present work, wet-milling technology was used to prepare FA particles with different fineness. The initial specific surface area of as-received raw FA (FA0) is 476 m2/kg. FA1 to FA5 are ground FA with different particle fineness obtained after the wet-milling treatment of FA0, as shown in Fig. 2. One can note that the d10, d50 and d90 values of the fly ash particle group showed a decreasing trend with the wet-milling duration. Importantly, the d10, d50 and d90 values were dramatically reduced from the initial 5.71, 19.70 and 63.78 mm to 1.62, 2.51 and 3.94 mm respectively, after wet milling for 360 min. The specific surface area increased significantly from the initial 476 m2/kg to 2520 m2/kg. The continuous growth of surface area in Fig. 2b indicates the high efficiency of wet-milling. It was also observed that the particle size was dropped from 19.70 mm to 7.23 mm after only 15 min. From 15 min to 360 min, the d50 was slightly reduced by 4.7 mm, but the specific surface area was notably increased by 1360 m2/kg. This indicates that a small decrease in particle size can result in a significant increase in specific surface area, when the particle size is small enough. The increase of specific surface area helps to increase the reaction area of fly ash and improve the ion dissolution (result can be found in section 3.5). Interestingly, the decrease of particle size keeps happening with no abnormal increase, even after a long milling time. This is quite different from the dry-milling technology (Kotake et al., 2011; Kumar and Kumar, 2011; Kumar et al., 2017), for which the particle size reduction may stop and even increase after a long duration. Typically, the difference between classical dry-milling and wet-milling technology in present work is the milling environment. The former takes air as milling medium, while the later adopts liquid medium. The abnormal increase in dry-milling technology can easily happen due to the intense particle interaction, i.e. agglomeration (Kumar et al., 2017). At the same time, it can be seen that the particle size distribution curve in Fig. 2a from FA0 to FA5 is gradually narrowed and the shape is closer to the normal distribution, which indicates the uniformity of wet-milling particles. The particle size distribution (PSD) width can describe the concentrated and uniform characteristics of the powder. The narrower the particle size distribution, the more uniform the particle size. Fig. 3 shows two specific particle size ratios which can reflect the particle uniformity of wet-milling FA. With the decrease of particle fineness, the particle size ratio (d90-d10)/d50 and d90/d10 was significantly reduced from initial 2.95 and 11.18 to 0.92 and 2.44, respectively. This indicates that the distribution width of FA particles was narrowed (Fig. 2a) and the uniformity of the particles is increasing during the wet-milling. Both the (d90-d10)/d50 and d90/ d10 were decreased continuously with milling time. In a previous study (Kotake et al., 2011), concerning the quartz powder, the size

J. Yang et al. / Journal of Cleaner Production 238 (2019) 117962

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Fig. 2. Particle size distribution (a) and characteristic particle parameters (b) of wet-milling FA.

used herein, since liquid (water) acts as dispersion medium, the agglomeration is inhibited and the grinding efficiency is effectively improved. As a result, the PSD is narrower and the particle distribution has higher homogeneity (Figs. 2a and 3). Similarly, notably lower particle size ratios of wet-milling quartz than the dry-milling one can also be found in (Kotake et al., 2011). The particle refinement and uniform distribution of wet-milling FA is also confirmed by the morphological images (Fig. 4). The spherical and smooth morphology of raw FA is clearly visible (Fig. 4a). Furthermore, the raw FA contains both large and micro spheres in a broad size range, confirming the large distribution width of raw FA in Fig. 3. Large spheres are broken in irregular shape, while microspheres almost retain their original spherical shape (Fig. 4bef). During the brittle breakage, the glassy aluminosilicate phase and crystal phase in fly ash can be broken up and

Fig. 3. Particle size ratio d90/d10 and (d90-d10)/d50 of wet-milling FA.

ratio (d90-d10)/d50 tends to remain constant in the range of d50 > 1 mm. It was explained by the authors that breakage property of the solid material may change in the micronized stage and affected the fine grinding mechanism (Kotake et al., 2011). In addition, solid hardness and grinding condition of FA in present work are also different from the quartz in (Kotake et al., 2011). We have known that solid particles become more difficult to grind as particles become finer. During the mostly used dry-milling process, agglomeration of fine particles is easy to happen, due to the high energy state and electric charge on the surface, thus limiting the further reduction of particle size. And the resulted particles usually exhibit wide PSD and multi-peaks (indicating agglomeration) (Kotake et al., 2011). In the wet-milling process

Fig. 4. Morphological images of FA: (a) FA0; (b) FA1; (c) FA2; (d) FA3; (e) FA4; (f) FA5.

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J. Yang et al. / Journal of Cleaner Production 238 (2019) 117962

stripped, which has been evidenced by (Kumar et al., 2017) using TEM morphologies. Besides, the surface of the fly ash has changed from glassy smooth to irregular and rough.

3.2. Phase composition 3.2.1. XRD Fig. 5 shows the mineral phase changes of FA due to wet-milling. The main crystalline phases in raw FA are identified as mullite and quartz. The first important observation is that new phases are generated after wet-milling (Fig. 5a, FA3 and FA5). The diffraction peak location (around 9.1 and 29.2 ) indicates that the new phases are ettringite (calcium sulfoaluminate hydrate) and calcium carbonate respectively. This is importantly different from the drymilling process in which no apparent change in mineralogy is observed (Kumar and Kumar, 2011). One may have doubt about the formation of ettringite in wet-milling. Chemically, ettringite is the hydration product of aluminate phase and sulfate. The existence of aluminate phase and sulfate is confirmed by the chemical composition in Section 2.1. It has also been reported by (van der Merwe et al., 2014) that small amount of gypsum and/or anhydride can exist in FA. In a previous study, ettringite was synthesized with Al2(SO4)3 and Ca(OH)2 solution (Goetz-Neunhoeffer et al., 2006). Hence, the dissolved aluminate and sulfate ion groups in the water medium in present work can provide a good formation environment for ettringite. Besides, the presence of calcium carbonate can be the result of carbonization of Ca2þ which is dissolved out during the wet-milling. The formation of ettringite in present work may indicate that some pre-reaction or preliminary reaction of FA has occurred in the wet-milling. It is usually believed that the reaction of ordinary FA can only happen in the presence of strong alkali (e.g. NaOH and water glass) (Duan et al., 2016; Kumar et al., 2017) and the pozzolanic reaction of FA in concrete occurs lately. In this regard, it seems that wet-milling treatment can help bring the reaction process forward. This is evidenced by the obviously promoted hydration heat by dry-milling ultrafine fly ash (Kumar et al., 2007). One the other hand, the rapid and primary formation of ettringite is beneficial for the early strength of resulted binders. This is similar to the early strength of sulphoaluminate cement of which ettringite is one of the main hydrates (Tang et al., 2015). The early happened pre-reaction of wet-milling FA in present work seems inspiring and may give some new understanding towards the reaction condition of FA Pozzolan.

Fig. 5. XRD patterns of FA before and after wet-milling: (a) 5e80 ; (b) 25e28 .

The second observation is the shifting of crystal diffraction peak and the changing of interplanar spacing, d, as shown in Fig. 5b. It is observed that the main diffraction peak of quartz shifts to a high 2q value, while the mullite diffraction peak shifts to a low value after wet-milling. The shifting of diffraction peak indicates the changing of d value. Take FA5 for example, the d value (~26.3 ) of mullite is increased by 0.0044 Å, while the d value (~26.6 ) of quartz is decreased by 0.0025 Å. This indicates that lattice distortion occurs inside the fly ash and residual stress is generated after the grinding action. Furthermore, one can also notice the slight decrease in peak intensity and broadening of crystalline phase after wet-milling (Fig. 5b). Similar result has also been detected in dry-milling FA (Kumar and Kumar, 2011). The broadening of diffraction peak is the result of damaged crystalline structure and decreased crystalline phase, which can be described by the crystallinity index (Fig. 6) according to Eq. (3). One can observe the decreased crystallinity with milling time, from the initial 53.96%e40.73% after 360 min, not so different from the reported crystalline phase content (37.9%) of a coal FA from South African (van der Merwe et al., 2014). In fact, the crystalline content of FA can be in a wide range of 5e80% (Sharonova et al., 2010), due to different FA sources. Similar reduction of crystallinity was also reported by (Liao et al., 2011) in case of cassava stillage residue activation by dry-milling. The reduced crystallinity index indicates the deformation of crystalline structure and the increase of amorphous phase.

Fig. 6. XRD pattern peak fitting and crystallinity of FA before and after wet-milling.

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3.2.2. TG-DTG The TG and derivative DTG curves of FA before and after wetmilling are plotted in Fig. 7. It is interesting to find that the thermogravimetry result of wet-milling FA is clearly different from the raw FA. Both the two wet-milling FA (FA3 and FA5) show a significant mass loss in TG curve and strong peak in DTG curve before around 100  C, which is mainly due to the loss of physically bound water. In cementitious system, the mass loss between 50 and 150  C is associated with the released physically bound water of hydration products, e.g. ettringite and CeSeH gel (Angulo-Ramírez et al., 2017; Ismail et al., 2014). It can be confirmed that this mass loss is inextricably linked to the loss of physically bound water within ettringite, together with the results of XRD. The remarkable mass loss of ettringite around 70  C was also evidenced in (Tang et al., 2015). This is strongly consistent with the ettringite hydrate identification in XRD patterns (Fig. 5). The evaporable water content before 150  C is around 2.5% for wet-milling FA, notably higher than that (~0.26%) of raw FA. One can also notice clearer gradual mass loss in wet-milling FA before 600  C. This is speculated to be the result of decomposition of amorphous silica-alumina hydrates (Gao et al., 2015) which may be formed during the wet-milling, although it is difficult to be identified in XRD pattern. The peak located around 700  C in the DTG curve is attributed to the thermal decomposition of little calcium carbonate, which is consistent with the results of XRD. 3.3. Chemical structure 3.3.1. FTIR The FTIR spectrums of the FA before and after wet-milling are presented in Fig. 8. The broad absorption band around 3423 cm1

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and the weak band around 1635 cm1 is attributed to the stretching vibrations and bending vibrations of OeH bonds respectively (Kumar et al., 2017). One can notice that the peak at ~3423 cm1 become more conspicuous in case of wet-milled FA. This is attributed to the break of SieO bonds and formation of SieOH groups during the high energy milling. Similar result is reported in (Paul et al., 2007). The absorption peaks at ~1099 cm1 and ~462 cm1 belongs to the asymmetric stretching vibrations and symmetric bending vibrations of SieOeSi from glassy phase, mullite and quartz (van der Merwe et al., 2014). The absorption band around 1099 cm1 relates to the SiQn (n ¼ 3e4) structural units. No clear bands can be found near 1014 cm1 which is attributed to SiQn (n ¼ 0e2) chain structure. This is the evidence of the threedimensional structure of glassy FA. The peaks at 558 cm1 originate from the AleO stretching vibrations (van der Merwe et al., 2014). Moreover, new absorption peak appears around 1432 cm1 after wet-milling. This should assign to the vibration of CeH bonds in organic dispersing agent which is added during the wet-milling process. Let us now focus more on the SieO vibrations around 1099 cm1. One can notice a shifting of SieO band towards lower wave number with the increasing milling duration. Recently, similarly lower frequency shifting of chemical bond (CO2 3 groups) with dry-milling time was also observed in ultrafine dolostone (Guzzo et al., 2019). The change of silicon oxide bond energy (DE) is calculated according to Eq. (4). It is shown that the SieO bond energy (~1099 cm1) of FA3 and FA5 is decreased by 1.24  103 eV and 1.61  103 eV respectively, referenced to the raw FA. This indicates that the stability of SieO bond is decreased, contributing to the further pozzolanic reaction in the cementing materials, which will be discussed later.

Fig. 7. TG-DTG curves of FA before and after wet-milling.

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J. Yang et al. / Journal of Cleaner Production 238 (2019) 117962

intense signal around 89 ppm is attributed to the SiQ3 structure (Criado et al., 2008; Palomo et al., 2007). The intense signal around 98 ppm and the weak signal at ~ -106 ppm is assigned to the SiQ4(2Al) and SiQ4(1Al) unit respectively (Kovalchuk et al., 2007; Oh et al., 2014). For a better understanding of the chemical structure evolution during the wet-milling, the proportion of SiQ3 and SiQ4 unit is illustrated in Fig. 9bed and Table 1. One can note the decreased proportion of SiQ4 unit after wet-milling. Chemically, the pozzolanic reaction of fly ash is a process in which the glassy network is first depolymerized (into a less polymerized structure) and then repolymerized. In this regard, the change of chemical environment, i.e. reduction of highly polymerized structure (SiQ4), would be beneficial for the further depolymerization during the pozzolanic reaction. It was also proved by (Shekhovtsova et al., 2018) that the lower value of degree of polymerization of silica in amorphous phase, the more actively the fly ash will dissociate. 3.4. Surface characteristic

Fig. 8. FTIR spectrums of FA before and after wet-milling.

3.3.2. NMR The 29Si NMR spectrums of FA samples are displayed in Fig. 9. The SiQn (n ¼ 3e4) structural units can be clearly identified at 88~-98 ppm for SiQ3 and -98~-129 ppm for SiQ4, whether or not it has been treated by wet-milling. This evidences again the polymerized 3D structure in FA, and consistent with the result of FTIR (Fig. 8). The deconvolution spectra are shown in Fig. 9bed. The

XPS analyses were conducted to study the influence of wetmilling on the surface characteristics of FA, as shown in Fig. 10. The high resolution scan curves of O 1s, Ca 2p, Si 2p and Al 2p are shown in the bottom of Fig. 10. As can be seen from the XPS survey spectrums, oxygen, calcium, silicon, aluminum and carbon are the main constituents of the fly ash surface, including small amounts of sulfur. Possible chemical states of the elements for fly ash were reported by (van der Merwe et al., 2014). Let us now focus more on the high resolution scan curves of oxygen, calcium, silicon and aluminum, since they are quite important for the cementitious reaction and hydration products of FA. It is interesting to find that the binding energies of O 1s, Ca 2p, Si 2p and Al 2p are all decreased with milling duration. To a certain extent, the decreased binding energy indicates that the binding of atoms to electrons is weakened, and chemical bonds are more likely to break (Xu and van Deventer, 2003). In Ref. (Xu and van Deventer, 2003), it was reported that amorphous FA with lower binding energy showed highest reactivity during geopolymerization. The reduced binding energy is similar to the decreased bond energy (DE) result in FTIR testing. It is also worth noting that the decreased Si 2p binding energy indicates the reduced degree of SieO polymerization (Li et al., 2010). This corresponds very well with the findings of 29Si NMR. The findings in present study concerning the low value shifting binding energy after wet-milling is different from the drymilling results by (Kanuchova et al., 2016) in which slight increase of Al 2p binding energy was observed. However, this special increase was only present in some FA samples. This may also reflect, to some extent, the different effects of dry- and wet-milling on the chemical environment of elements. In a previous study by (Li et al., 2010), regarding the compound wet-milling activation of coal gangue, similar decreased binding energies (Si 2p and Al 2p) were reported, which is consistent with the results in present study. The decreased binding energy of different elements is very encouraging as it may promote the cementitious activity in the further hydration.

Table 1 Deconvolution data of Q

Q4

FA0

89.48 ppm I ¼ 31.21% 90.03 ppm I ¼ 35.43% 90.39 ppm I ¼ 48.22%

98.01 ppm I ¼ 9.46% 98.51 ppm I ¼ 11.13% 99.88 ppm I ¼ 10.01%

FA5 29

Si NMR spectrums of FA before and after wet-milling.

Si NMR spectrum of FA.

Sample

FA3

Fig. 9. Onepulse

29

3

Q4/(Q3þQ4) 105.43 ppm I ¼ 59.32% 106.40 ppm I ¼ 53.44% 106.85 ppm I ¼ 41.77%

68.78% 64.57% 51.78%

J. Yang et al. / Journal of Cleaner Production 238 (2019) 117962

9

Fig. 10. XPS survey spectrums and high resolution scan curves of FA before and after wet-milling.

3.5. Leaching behavior The leaching solution chemistry of FA in the liquid environment of wet-milling is shown in Table 2. The liquid environment and the mechanical exfoliation can necessarily accelerate the ion dissolution from the surface of broken FA particles. This is a unique feature of wet-milling technology, which is not available in dry-milling process. The promoted dissolution of Si, Al and Fe ionic groups evidences directly the effect of wet-milling process. As discussed earlier, the silica-alumina group in the glassy phase is the main contributor to the cementing reactivity of fly ash. The pre-dissolved ionic groups form a pre-hydrated liquid environment rich in silicon-aluminum groups during the wet-milling stage, and contribute to the subsequent pozzolanic reaction in the cementing and hardening stage. One may notice the abnormal and significant decrease of ion concentration for Ca and S, as well the pH value and conductivity of leaching solution. The chemical state of sulfur in combusted fly ash

Table 2 Ion leaching of FA with different fineness (mg/L). Sample

Ca

Si

Al

Fe

S

pH

Conductivity (mS/cm)

FA0 FA3 FA5

494.03 8.95 4.18

3.89 2.56 8.58

0.27 1.71 18.24

0.038 0.204 0.275

730.34 128.23 10.64

11.83 11.55 11.06

8.34 3.04 2.27

is sulfate type which is easily leached by water (Hirokawa and Danzaki, 1984). Chemically, ettringite is the hydration product of calcium-aluminate phase and sulfate. The decreased (consumed) calcium and sulfate evidence again the formation of ettringite during the wet-milling (see XRD result in Fig. 5). It is believed that alkali plays an important role in the pozzolanic reaction of FA. The leaching solution of raw FA presents alkaline characteristic (pH ¼ 11.83), due to the existence of soluble alkali in FA. The pH value is reduced to 11.06 after 360 min wet-milling, due to the alkali consumption. Actually, the pozzolanic reaction is a reaction of active silicate and/or aluminate with alkali (Na-based or Ca-based). The small amount of calcium and sulfate contained in the fly ash itself may be not enough to cause an obvious pozzolanic reaction. However, in a pozzolanic reaction system, the externally provided calcium and sulfate is usually sufficient. Theoretically, the pozzolanic contribution is more related to the dissolved silicate and aluminate which are promoted in the wet-milling (see Table 2), which are the main precursor of hydration product and the contributor of mechanical strength. One can note that the main ions in leaching solution are calcium and sulfur. The consumption of these two ions to form ettringite results in the conductivity reduction in Table 2. Reduced electrical conductivity was also reported by (Schwarz and Neithalath, 2008) in which FA was mixed into saturated Ca(OH)2 solution. The decreased conductivity can be interpreted as an indication of high pozzolanicity.

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J. Yang et al. / Journal of Cleaner Production 238 (2019) 117962

3.6. FA reactivity index The reactivity index (Jn) of FA is displayed in Fig. 11. The reactivity index is evaluated basing on the compressive strength contribution of FA in cement mortar with curing time. The increased reactivity index indicates the pozzolanic reaction progress of FA. One can observe that the reactivity index of wet-milling FA is notably higher than that of ordinary FA. Take theJn at 60 days for example, theJn of FA3 (111%) and FA5 (140%) is greatly higher than ordinary FA (96%). The increase of pozzolanic reactivity of FA is favorable for its eco-friendly utilization in cement and concrete materials. The activation mechanism of wet-milling treatment can be understood from the physical and chemical evolutions of FA. The activation mechanism is generally summarized in Fig. 12. Different from the classical dry-milling process, the wet-milling treatment used in present study is a combined effect of mechanical activation and liquid environment. The mechanical breakage notably decreased the particle size and improved the specific surface area (as shown in Figs. 2 and 4), which is helpful to loosen the physical structure and increase the reaction area of pozzolanic hydration. The high energy density of the mechanical grinding is also beneficial to improve the amorphous phase and produce more lattice distortion and defects (see Figs. 5 and 6) in the physical structure. The bonding energy was found decreased after wet-milling (Figs. 8 and 10) which would be helpful for the bond breaking during the reaction. In addition, the SiQn (n ¼ 3e4) structural units were found less polymerized (Fig. 9). These two chemical changes are helpful for the depolymerization during the pozzolanic reaction. More importantly, the mechanical activation happens in the hygrothermal water environment which plays a significant role governing the pozzolanic activation. In this hot liquid environment, dissolution of ion groups was accelerated (Table 2). The soluble ions help to form a pre-hydrated ion environment and contribute to the subsequent pozzolanic reaction, since the hydration of reactive phase is a first dissolving and re-reacting process.

Fig. 11. Reactivity index of FA before and after wet-milling.

Fig. 12. Illustration for activation mechanism of wet-milling treatment.

3.7. Advantages and applicability of wet-milling technique One of the most important disposal technologies for industrial solid waste is mechanical refinement, for example dry-milling, to improve the potential reactivity in building materials. Recently years, wet-milling technology has also been developed for solid waste refinement (Kotake et al., 2011; Wang, Y. et al., 2019). The most important difference between dry-milling and wet-milling is that the latter is proceeded in water medium (as shown in Table 3). This brings a different activation mechanism for wet-milling, since water flushing and ion dissolution happens during the physical breakage. The good dispersion effect of water and the surface energy reduction of solid particle by water can greatly improve the grinding limit and avoid particle agglomeration. Thus, the milling efficiency can be improved and the energy consumption can be reduced (Kotake et al., 2011; Tan et al., 2019). The potential grinding limit of dry-milling and wet-milling was reported around 2 mm and 0.5e0.6 mm (Kotake et al., 2011) respectively. This means that wetmilling has higher grinding efficiency and lower energy consumption than dry-milling under the same particle size conditions. Compared to dry-milling, wet-milling technology seems more ecofriendly. With the existing of water, dust generation can be avoided during the grinding. More importantly, the raw solid waste can be feed in the wet-mill in wet state with no need for pre-drying process like dry-milling. In fact, most solid waste is in a watery state, for instance, wet-discharged waste or exposed to the rain. The omission of the pre-drying process helps save energy. The energy consumption, cost and CO2 emission of wet-milling FA disposal were listed in Table 4. Energy consumption was calculated according to the milling duration and power of the equipment. It is shown that wet-milling FA for cement replacement has environmental and economic benefits. The disposal cost and CO2 emission of wet-milling FA due to the energy consumption is notably lower than Portland cement (600 RMB/t, 930 kg/t) (Wu et al., 2018). With the super refinement of wet-milling, pozzolanic reactivity of solid waste can be notably improved. A comparison of pozzolanic reactivity index of coal fly ash, including raw FA and dry-milling FA

J. Yang et al. / Journal of Cleaner Production 238 (2019) 117962

11

Table 3 A brief comparison of wet-milling with classical dry-milling.

Wet-milling Dry-milling

Dispersion medium

Activation mechanism

Grinding limit

Agglomeration

Dust generation

Pre-drying treatment

Water Air

Physical breakage; Flushing and dissolution Physical breakage

~0.5e0.6 mm* ~2 mm*

Without With

Without With

Without With

Note:*Data from reference (Kotake et al., 2011).

Table 4 Environmental and economic parameters of wet-milling FA. Wet-milling FA Wet-milling FA mass (kg) Milling duration (h) Power (kW) Energy consumption (kWh/t) Disposal cost (RMB/t)a CO2 emission of electricity (kg/t)b FA3 FA5 Note:

100 100 a

1.17 6

The cost of electricity in Wuhan is 1.0 RMB/kWh;

3 3 b

35 180

35 180

27.5 141.3

The CO2 emission of electricity is 0.785 kg/kWh, according to Ref. (Tan et al., 2019).

Fig. 13. Comparison of pozzolanic reactivity index of coal fly ash (water-to-binder ratio around 0.5, FA content 20e40 w.t.%).

in public reports, is summarized in Fig. 13. One can observe that the reactivity of wet-milling FA in present study is very encouraging and higher than current reactivity level. This seems to be more attractive for the disposal of low-grade fly ash, since it helps to improve the quality of fly ash and increase its replacement limit in sustainable building materials. The highly activated fly ash may also be considered as efficient stabilization binder (Li et al., 2018). This also helps reduce the CO2 emission and save the cost. In addition to coal fly ash, we also investigated the technical applicability of wet-milling for various industrial solid wastes, like blast-furnace slag, steel slag and phosphorus slag (Fig. 14). It indicates that wet-milling technology is efficient for solid wastes with potential reactivity. All the investigated wastes showed clearly improved reactivity index. Thus wet-milling can be a suitable, efficient and eco-friendly technology for solid waste refinement. It is worth noting that the ultrafine particle output is in a slurry state which can be directly utilized in concrete mixture without drying. However, the potential challenge of slurry storage and transport may be considered in the future industrialized application.

4. Conclusions The following conclusions can be drawn from the experimental results: (1) Wet-milling is a suitable, efficient and eco-friendly technique for solid waste refinement and resource utilization in building materials. (2) Pozzolanic reactivity of solid waste, for example coal fly ash, is greatly improved after wet-milling treatment. The reactivity activation of wet-milling technology is a combined effect of physical breakage and liquid environment which is different from dry-milling. (3) The mechanical breakage of wet-milling notably decreased the particle size, improved the specific surface area, distribution homogeneity, amorphous phase content and produced more lattice distortion and defects in the physical structure.

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J. Yang et al. / Journal of Cleaner Production 238 (2019) 117962

Fig. 14. Technical applicability of wet-milling for various industrial solid wastes.

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