Influence of various sodium salt species on formation mechanism of lightweight aggregates made from coal fly ash-based material

Construction and Building Materials 239 (2020) 117890

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Influence of various sodium salt species on formation mechanism of lightweight aggregates made from coal fly ash-based material Yu-Ling Wei ⇑, Shao-Hsian Cheng, Wen-Jing Chen, Yueh-Hua Lu, Ken Chen, Pei-Ching Wu Department of Environmental Science and Engineering, Tunghai University, 40704, Taiwan

h i g h l i g h t s
Sintering temperature depends on type of sodium additives.
Sodium salt thermal behavior matters in reducing sintering energy for LWA preparation.
Solubility of coal fly ash/glass in melt NaCl is too low to save sintering energy.

a r t i c l e

i n f o

Article history: Received 6 June 2019 Received in revised form 17 November 2019 Accepted 16 December 2019

Keywords: Coal fly ash Flux Lightweight aggregate Sintering Recycle Sodium salt

a b s t r a c t To save energy in sintering to produce construction and building materials, alkali compounds are always recognized as a good flux component. We added various sodium salts, to coal fly ash/glass mixture, to investigate the effect of sodium salt species type on formation mechanism of lightweight aggregates. TGA/DTA, and XRD are used in this study. Results indicate that, despite the great similarity in melting point of the salt additives, a change of additive from one sodium salt to another leads to considerably different lightweight aggregate formation mechanism, thereby affecting the aggregate properties. Thermal behavior of sodium salt additives, as revealed in their TGA/DSC curves, might play a critical factor in influencing lightweight aggregate formation. All sodium salts can indirectly, following two-step sequential reactions, form sodium silicate with SiO2 present in coal fly ash/glass mixture. Sodium silicates are suggested as major components of viscous layer during sintering; they can envelope bloating gases to expand the size of lightweight aggregates. The solubility of coal fly ash/glass mixture in melt NaCl during sintering, is not as much as in melt Na2O and Na2SO4, thus requiring higher sintering temperature to prepare lightweight aggregates. As compared with the lightweight aggregates prepared in industry, all sodium salt additives used in present study can effectively reduce sintering energy, from the aspects of lightweight aggregate particle density. Effectiveness for the sodium salt additives to reduce sintering temperature for successful lightweight aggregate preparation is, in increasing order, sodium carbonate < sodium sulfate < sodium chloride. Sintering temperature can be potentially reduced to 1050 °C by proper addition of the sodium salts. Ó 2019 Elsevier Ltd. All rights reserved.

1. Introduction In foreseeable future, global power demand will continue to increase due to growing economy and population, and coal will still be the major energy source for power plant. Approximately only a Abbreviations: CFA, coal fly ash; ICP/AES, inductively coupled plasma/atomic emission spectroscope; LWA, lightweight aggregate; LWC, lightweight concrete; TGA/DSC, thermogravimetry analyzer/differential scanning calorimetry; TGA/DTA, thermogravimetry analyzer/differential thermal analyzer; XRD, x-ray diffractometer. ⇑ Corresponding author. E-mail address: [email protected] (Y.-L. Wei). https://doi.org/10.1016/j.conbuildmat.2019.117890 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.

quarter of worldwide coal fly ash (CFA) was recycled [1]. Improper disposal of CFA with open dump proved to cause water contamination problem [1]. Majority of the globally recycled CFA has been used as construction materials, such as brick, cement raw material, concrete additive, concrete block, geotechnical material [1]. However, so far only very limited studies have previously sintered CFA with additives to prepare lightweight aggregate (LWA) [2–5]. Specifically, Kourti and Cheeseman sintered mixtures of glass and lignite CFA to prepare LWAs with properties comparable to commercialized LWAs [3]. Kockal and Ozturan prepared LWAs with high particle density and strength by co-sintering CFA, bentonite, and glass powder [2]. Qin et al. co-sintered CFA, lime mud dis-

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Y.-L. Wei et al. / Construction and Building Materials 239 (2020) 117890

charged from paper industry, and diatomite to produce ceramsite with near 40% water sorption capacity and near 50% porosity [4]. Anorthite (CaAl2Si2O8) and wollastonite (CaSiO3) is abundant in their ceramsite [4]. Wei et al. rapidly co-sintered F-class CFA and household window glass to prepare LWAs [5]. Containing 12.5% sodium oxide and being rich in SiO2 (81%), the window glass promotes flux content in the mixed raw materials, thus reducing sintering temperature. Diopside (MgCaSi2O6) and wollastonite developed as new crystalline phases during sintering [5]. Despite very limited studies on sintering CFA to LWAs, it is a global trend to sinter solid wastes of various sources to LWAs to ease the negative environmental impact caused by mining natural resources, such as clay and pumice as raw materials and LWA, respectively, and to abide by the principle of circular economy for a sustainable society [1,6]. Waste-derived LWAs can be used to replace coarse aggregates in preparing concrete [7–10]. Wastes, with potential for being used as raw materials to prepare LWAs, cover a wide variety: sewage sludge ash [11], harbor sediment [12,13], incinerated fly ash and bottom ash [14–17], sludge [15,18,19,20], and other industrial hazardous/non-hazardous wastes [6–10,15,21–24]. Coal represents approximately half of power demand in Taiwan. It has been being the main power source in Taiwan since long time ago, due to a very slow progress in diverting energy source from coal to natural gas and to renewable energy, such as solar, sea tide, wind, and biomass-derived bio-fuel. This scenario results in considerable output of CFA, two million tons per year. Despite a high CFA recycling rate in Taiwan, each year approximately 200 thousand tons of CFA is accumulatively stored in ash pond, and this has been a troublesome problem due to a rather limited plant space for storing the ash by local power plant owners. On the other hand, CFA can be successfully converted to LWAs by sintering. With waste glass addition in CFA, sintering temperature for preparing LWA could be lowered from 1300 °C (for glass-free CFA) to 1200 °C [5]. From an energy-saving aspect, extra flux components like sodium salts can be added to further reduce sintering temperature. Used as flux component for sintering, sodium salts is more effective than alkali earth metallic salts. They are relatively cheap and readily accessible from nature resources. For example, approximately two thirds of Na2SO4 with decahydrate has been produced from mirabilite, a water-soluble mineral. Due its instability, the decahydrate rapidly releases its crystalline water, resulting in anhydrous Na2SO4. As a second example, halite, a sedimentary rock salt, is a natural mineral form of NaCl. The main source of NaCl is sea water and salty lake. As another example, Na2CO3 has in tradition been used as a flux to reduce melting point of silica to produce sodium silicates, with various Na/Si molar ratios, as a critical glassy component in window glass. Upon being heated alone, Na2CO3 decomposes at >851 °C into CO2 and Na2O. Melting and boiling points of Na2O are 1132 °C and 1950 °C, respectively. Sodium carbonate can be naturally present in arid areas, especially in dry lake bottom. The aim of this study is to add the abovementioned sodium salts, having considerably diverse thermal behavior, to coal fly ash/glass mixture to prepare LWAs by sintering at 1050 °C–1175 °C, and investigate the effect of sodium salt type on LWA formation mechanism. To date and to our best knowledge, no such study has been published.

2. Materials and methods F-class CFA studied in present study was sampled from a local power plant [5]. It is fine in particle size and its aggregates could be easily ground in a few seconds to pass a 297 lm sieve. For

the waste panel glass from household window, it was also pulverized. The respective D50 cut diameters of CFA and waste glass powder are approximately 18 lm and 213 lm. [5]. So far, waste window glass is not mandatorily recycled by Taiwan government, thus often being packed with household waste and disposed of in municipal incinerators. It ends up as a component in incinerator bottom ash [5]. The sodium salt additives in CFA/waste glass mixture are as follows. Sodium carbonate (Na2CO3: 99.9%, G.R. grade, Merck, Germany), Sodium sulfate dehydrate (Na2SO4: 99%, G.R. grade, Merck, Germany), and Sodium chloride (NaCl: 99.8%, G.R. grade, Ridel-de Haën, Germany). Acidic reagents used for raw material digestion include nitric acid (HNO3: 69%, G.R. grade, Merck, Germany), hydrofluoric acid (HF: 48%, G.R. grade, Ridel-de Haën, Germany), and boric acid (H3BO3: 99.5–100.5%, G.R. grade, Merck, Germany) [5]. Compositional elements, except Si, in raw materials were quantitatively determined using an inductively coupled plasma atomic emission spectrometer (ICP-AES, Profile plus, Teledyne Leeman Labs, New Hampshire, USA) after a two-step microwave digestion at 230 °C. A stock solution containing known concentrations of combined metallic ions was used to construct the calibration curves for all target ions in the digest liquid [5]. It is noteworthy that complete digestion of raw materials is necessary in order to accurately determine their compositional contents. Here the two-step digestion method left no residue after the digestion [25]. To determine Si content in raw materials, the microwave digest using combined acids including HF is not appropriate because some Si content will form SiF4 and vaporize into atmosphere when the Teflon vessels are uncapped even after cooling of the liquid digest. Thus a gravimetric method was used to determine Si content [26]. Briefly, concentrated HF alone is used to react with Si content in target sample under heating condition in a platinum crucible in a ventilated hood. Si content completely reacts with the HF to form SiF4 gas that vaporizes away from sample. Si content can thus be determined from sample weight loss [26]. The analytical results show that major chemical compositions, expressed in oxides, in CFA are SiO2 (65.1%) and Al2O3 (18.8%), followed by Fe2O3 (6.18%), CaO (3.93%), MgO (1.32%), K2O (0.8%) and others in negligible percentage [5]. The waste window glass powder contains approximately 81% SiO2, 13% Na2O, 5.0% CaO, 2.3% MgO, 1.0% Al2O3, 0.9% K2O and 0.6% FeO [5]. Total flux, the sum of Na2O, K2O, CaO, MgO and Fe2O3 contents, of the glass is approximately 22%. Thus, addition of the waste glass to CFA can promote both silica and flux contents in raw materials to prepare LWA by sintering at elevated temperatures. An increase in silica content would benefit the development of three-dimensional silicate structure, forming viscous layer, during sintering [27]. A promotion in flux content would reduce sintering temperature necessarily to enable LWA formation. Thermal behavior of sodium salt additives and the CFA/glass mixture with/without the additive was recorded using a thermogravimetric analyzer coupled with differential thermal analyzer (TGA/DTA: Pryis Diamond, Perkin Elmer, Massachusetts, USA) [5]. The equipment was conducted in 100 mL min1 air within 25 °C1200 °C range at a heating rate of 10 °C min1. A laser diffraction particle analyzer (LS230, Beckman Coulter, Washington, USA) was employed to determine particle size distribution of the raw materials CFA and glass powders [5]. Crystalline phase present in CFA and in sodium salt-added CFA/glass mixture sintered at 1050 °C and 1175 °C were determined with an X-ray diffractometer (XRD) (D8 Advantac, Bruker AXS, Germany) using copper target (wavelength 0.15406 nm). The XRD was run under 40 kV voltage, 40 mA current, 3° min1 scanning speed, 0.05° step size, and 5°80° scanning range (2h). The experimental procedure for mixing raw materials, forming green pellets, sintering green pellets, and cooling and characteriz-

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Y.-L. Wei et al. / Construction and Building Materials 239 (2020) 117890

ing the sintered pellets is the same as a previous study [5]. CFA and glass powders were sieved through a 50 mesh sieve (<297 lm), mixed with the sodium salts at a weight ratio of 65/25/10, formed into disc-type green pellets using a 3000-psi press. These green pellets were then placed on porous refractory plates and delivered into an electric furnace having already been pre-heated to a pre-set sintering temperature (1050 °C, 1100 °C, 1150 °C and 1175 °C). After a 10-minute sintering time, the sintered pellets were taken out from the electric furnace and placed in the atmosphere for a natural cooling to room temperature. All sintering experiments in present study use a single-step process requiring no pre-heating step at 400–500 °C. The pre-heating step was conducted in previous studies [12,13,18,22,24,28] to release majority of gases that would otherwise be generated so rapidly, in a single-step sintering process, as to blast the pellets while sintering. The sintered pellets were determined in properties, such as morphology of their core fragments, particle outer appearance, 24-hour water sorption, particle density, and crystalline phases according to the same methods previously used [5]. The sintering temperature were chosen as 1050 , 1100 and 1175 based on one of our previous studies [5]. It has indicated that coal fly ash alone and coal fly ash/glass mixture can be successfully converted to LWAs with LWA particle density <1.6 g cm3 by sintering for 10 min at  1200 °C and  1150 °C, respectively [5]. Sodium salts has traditionarily been recognized as an effective flux component to lower sintering energy by reduce sintering temperature, thus, this study chose the sintering temperature as 1050 , 1100 and 1175 . In general, successful sintering of LWA is justified by collective evaluation of LWA particle density, water sorption rate, and LWA appearance [3,11,14]. Practical construction sector prefers LWAs with particle density <1.6 g cm3, 24-hour water sorption rate <20% (by weight), and glassy surface [3,11]. 3. Results and discussion 3.1. TGA/DSC behavior of sodium salt additives Fig. 1 shows thermal behavior of sodium salt additives alone. For the Na2CO3 TGA/DSC curves (top compartment of Fig. 1), the minor TGA weight loss at 100 °C accompanied by a DSC endothermic peak can be attributed to moisture vaporization. Its endothermic peak starting at 851 °C is due to Na2CO3 decomposition to Na2O and CO2. Such decomposition reaction rate gradually and steadily increases from 851 °C to the end of the TGA/DSC experiment, 1200 °C, with a total weight loss of 38.5%. Such weight loss

Na2CO3

95

0 -50

Na2SO4

-25 0

0 100

-50

NaCl

-25

Weight remaining (%)

0 100

50

-25

-25

TGA DSC

50

100

Heat flow endo up (mW)

50

Weight remaining (%)

0

-50

Na2CO3 added

-50 0

90 100

-25

Na2SO4 added

95

-50

90

0

100 -25 95

NaCl added -50

0

0 0

200

400

600

800

1000

1200

Heating Temperature (°C) Fig. 1. Thermal behavior of sodium salts Na2CO3, Na2SO4 and NaCl.

DTA (μV)

100

percentage is close to the theoretically complete decomposition weight loss of Na2CO3, which is 41.5% (44/106 = 41.5%). In addition, the broad DSC endothermic peak around 1130 °C indicates melting of Na2O. It is noteworthy that Na2O boiling point is relatively high, 1950 °C, thus might not be easy to vaporize at the sintering temperature, 1050 °C1175 °C, used in this study. For the Na2SO4 TGA/DSC curves (middle compartment of Fig. 1), the entire TGA path for heating Na2SO4 from room temperature up to 1200 °C only causes 1.48% loss of total weight; no notable decomposition reaction was observed. Also, no de-watering was observed because the Na2SO4 used in this study is in anhydrous form. Its DSC curve has two endothermic peaks starting at 260 °C and 885 °C, respectively. At 260°C, sodium sulfate is converted from orthorhombic into hexagonal crystal. The endothermic peak at 885 °C is due to Na2SO4 melting. For NaCl TGA/DSC curves (bottom compartment of Fig. 1), its TGA curve reveals that there is barely any weight loss upon heating from room temperature until approximately 800 °C that is NaCl melting point. An DSC endothermic peak, starting at 800 °C attributed to NaCl melting, was observed. Despite of its 1465 °C boiling point, the TGA curve shows that NaCl sample is completely vaporized during the heating path up to 1120 °C. It is interesting to recall that, in contrast, no notable weight loss of the other salt, Na2SO4, was detected during the heating path up to 1200 °C, although its boiling point, 1429 °C, is relatively similar to NaCl melting point. As to the coal fly ash, its TGA/DSC curves and detailed discussion on them were given in a previous study [5]. In short, in DSC curve of coal fly ash, there is a broad endothermic peak, centering at approximately 1000 °C, which results from lattice formation of silicate. The coal fly ash loses approximately 4.0% of total weight along with the TGA heating path up to 1200 °C [5]. Fig. 2 shows thermal behavior of mixtures of CFA/glass/sodium salt additives. All three TGA curves have a mild sample weight loss in 100 °C  750 °C range, which can be attributed to CFA thermal behavior, namely dehydration of calcium hydroxide at ca. 580 °C and de-CO2 calcination of calcium carbonate at ca. 750 °C, according to one of our previous studies [5]. Top compartment of Fig. 2 indicates that, when co-existent with CFA/glass mixture, Na2CO3 starts its decomposition at a temperature slightly lower, and with a decomposition rate higher, than it would be when present alone. The difference in decomposition temperature is approximately 100 °C, and most Na2CO3 decomposition, in presence of CFA/glass mixture, appears to take place in 750 °C830 °C range (see Fig. 2), compared to 850 °C1200 °C range for pure Na2CO3 (see Fig. 1). Middle compartment of Fig. 2 indicates that, when

90

0

200

400

600

800

1000

1200

Heating Temperature (°C) Fig. 2. Thermal behavior of and CFA/glass mixtures added with sodium salts.

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Y.-L. Wei et al. / Construction and Building Materials 239 (2020) 117890

co-existent with CFA/glass mixture, Na2SO4 thermal behavior seems to be the same as when it is present alone (compared with Fig. 1). Bottom compartment of Fig. 2 shows that, when copresent with CFA/glass mixture, NaCl thermal behavior is also the same as when it is present alone (compared with Fig. 1). In summary, although all above-mentioned sodium salts have low melting point, 800–900 °C, thus they are expected to be good flux candidates in promoting sintering of CFA/glass mixture for saving sintering energy. However, the sodium salts selected for this study show different thermal behavior from each other, thus they are also expected to show different effectiveness as a flux and/or bloating promoter during LWA preparation from CFA/glass mixture.

500 C

M Q W D

Na2 CO3 ! Na2 O + CO2ðgÞ

ð1Þ

Na2 O + SiO2 ! Na2 SiO3

ð2Þ

According to TGA/DSC results (Figs. 1 and 2), Na2CO3 decomposes at 750 °C851 °C; thus, reaction (1) readily takes place at all sintering temperatures in present study. Although our TGA/DSC curves show that Na2SO4 alone does not decompose up to 1200 °C (Fig. 1), it has been previously reported that in the presence of SiO2, Na2SO4 can react with SiO2 with the release of SO3, at a relatively lower temperature, to generate Na2SiO3 [31] according the following vitrification reaction:

ð3Þ

M Na 2CO3 1175 oC

1000 500

M C Q

0 1000

Intensity (a.u.)

Na 2SO4 1050 oC

M Q

500

Our previous study indicates that major crystalline phases detected in the coal fly ash/glass raw mixture are quartz (SiO2), mullite (Al6Si2O13), corundum (Al2O3), hematite (Fe2O3) and wustite (FeO) [5]. Crystalline phases present in the sodium salt-added LWAs produced at 1050 °C and 1175 °C are shown in Fig. 3. For clarity, Crystalline phase present in CFA raw material and in sodium salt-added CFA sintered at 1050 °C and 1175 °C are collected in Table 1. According to Fig. 3 and Table 1, sintering sodium saltadded coal fly ash/glass mixture at 1050 °C and 1175 °C causes a disappearance of corundum, hematite, and wustite phases, with a concomitant formation of cristobalite phase (SiO2). In Fig. 3, for Na2CO3- and Na2SO4-added LWAs prepared at same temperature, they have same diffraction patterns (same crystalline phases), while different diffraction intensity. Fig. 3 shows that an increase in sintering temperature by 125 °C, from 1050 °C to 1175 °C, results in notable reduction in not only the diffraction pattern intensities, but also the number of crystalline phases present. Reduction in diffraction intensity implies an increase in glassy phase. Five crystalline phases are present in both Na2CO3- and Na2SO4-added LWAs produced at 1050 °C, while the number of crystalline phases reduces to three in both 1175 °C LWAs. Other than cristobalite (SiO2), wollastonite (CaSiO3) and diopside (MgCaSi2O6) phases are formed by sintering in both Na2CO3- and Na2SO4-added LWAs produced at 1050 °C. Both wollastonite (CaSiO3) and diopside (MgCaSi2O6) phases were also detected in a previous study preparing LWAs by sintering the mixtures of glass and lignite CFA [3]. Formation of wollastonite and diopside was reported to promote the development of glassy layer with a good sealing property [14,29,30]. These two crystalline phases, not detected in the 1175 °C Na2CO3- and Na2SO4-added LWAs, are suggested to convert into amorphous phase. In addition, neither Na2O, a product of Na2CO3 decomposition, nor Na2SO4 phase is detectable in any LWAs shown in Fig. 3. Na2SiO3 is suggested to form in present study, in presence of Na2CO3 and SiO2, according to the following sequential reactions:

C - cristobalite W - wollastonite D - diopside M - mullite N - NaCl Q - quartz

0

3.2. Change in crystalline phases due to sintering

SiO2 + Na2 SO4 ! SiO2 + Na2 O + SO3ðgÞ

Na 2CO3 1050 oC

1000

W D

C

M

0 Na 2SO4 1175 oC

1000 500

M C Q

0 M Q

1000

NaCl 1050 oC

N N

C W D W D

500 Q

M

N

N

0 M Q

1000 500

C

NaCl 1175 oC N

WW DD

N

M

N

N

0 10

20

30

40

50

60

70

80

o

2-theta ( ) Fig. 3. Crystalline phases present in the sodium salt-added LWAs made by sintering at 1050 °C and 1175 °C.

SiO2 + Na2 O ! Na2 SiO3

ð4Þ

For the NaCl-added LWAs (bottom two compartments of Fig. 3), in contrast to Na2CO3- and Na2SO4-added LWAs, Fig. 3 shows that an increase in sintering temperature, from 1050 °C to 1175 °C, results in a notable reduction in diffraction pattern intensity of both cristobalite and NaCl, while no change in the number of crystalline phases present. Six crystalline phases are present in all NaCl-added LWAs. Crystalline phase of NaCl is clearly present in both 1050 °C and 1175 °C NaCl-added LWAs. Although the prominent diffraction pattern intensities suggest that NaCl does not have a good solubility in coal fly ash/glass mixture upon sintering, it can somewhat lower sintering temperature of coal fly ash/glass mixture through eutectic effect. In general, a mixture containing more components tends to have lower eutectic temperature, thus melts at lower temperature. The reason that crystalline phase of the NaCl is sharply detected in 1175 °C LWA without being completely vaporized might be attributed to trapping of fractional NaCl by the viscous layer formed during sintering. The viscous layer turns into glassy phase after sample cooling. It is worth mentioning that although not too compatible to each other, partial amount of NaCl can chemically react with SiO2 to give SiCl4 (shown in reaction 5) and Na2SiO3 (reaction 6) [32], a major glassy phase, according to the following sequential reactions:

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Y.-L. Wei et al. / Construction and Building Materials 239 (2020) 117890 Table 1 Crystalline phase present in CFA raw material and in sodium salt-added CFA sintered at 1050 °C and 1175 °C. sample identity

coal fly ash 1050 °C Na2CO3 LWA 1175 °C Na2CO3 LWA 1050 °C Na2SO4 LWA 1175 °C Na2SO4 LWA 1050 °C NaCl LWA 1175 °C NaCl LWA

crystalline phase quar1

cristo2

corun3

mull4

hema5

wust6

diops7

wollas8

NaCl

+ + + + + + +

– + + + + + +

+ – – – – – –

+ + + + + + +

+ – – – – – –

+ – – – – – –

– + – + – + +

– + – + – + +

– – – – – + +

+detected in sample. -: not detected in sample. 1 Quartz. 2 Cristobalite. 3 Corundum. 4 Mullite. 5 Hematite. 6 Wustite. 7 Diopside. 8 Wollastonite.

SiO2 + 4 NaCl ! 2Na2 O + SiCl4ðgÞ

ð5Þ

SiO2 + Na2 O ! Na2 SiO3

ð6Þ

In addition, the SiCl4 formed in reaction (5) can further react with oxygen present in air at >400 °C to form SiO2, or can be trapped by the viscous layer during sintering reaction to act as a bloating agent for pellet expansion to form LWA. Finally, it is noted that the present study detects no sodium silicate crystalline phase in XRD patterns from all sodium salt-added LWAs, suggesting that sodium silicate is amorphous and present as glassy phase. 3.3. Mechanism of influence of sodium salt on lowering sintering temperature It has been generally recognized that to successfully form LWAs through thermal process, two main conditions have to be met. Firstly, during green pellet sintering, considerable amount of viscous layer with proper viscosity must form. Secondly, after formation of sufficient viscous layer, the green pellets being sintered should release sufficient amount of gases to bloat the pellets that are being sintered. In other words, the viscous layer then should be able to envelope the gases thermally generated. The sintering reaction needs to occur no later than the bloating process to effectively envelope the gas under the viscous layer that becomes glassy surface after pellet cooling. In many LWA formation cases, gases are released in a wide range of temperature, whilst considerable amount of viscous layer with proper viscosity is formed in a narrow temperature window. Thus, to reduce LWA formation temperature, thereby saving energy, most current researches are focusing on how to develop considerable amount of viscous layer with proper viscosity at lower temperature. In the present study, the mechanism of the influence of sodium salts on lowering the sintering temperature and saving energy is mainly based on the concept of lowering eutectic point and forming low-melting-point viscous layer. At eutectic point, co-melting of sodium salt with coal fly ash compositions could take place at a temperature considerably lower than melting point of coal fly ash alone. 3.4. Pore structure, particle density, and compressibility of fired pellets One of the most important properties of LWAs is their particle density which is closely related to the pore volume of LWAs. To gain some knowledge on the pore size of our LWAs, Fig. 4

presents the side-viewed photos of all half-cut pellets. Because surface of the 1100 °C–1175 °C Na2CO3-added pellets and the 1150 °C–1175 °C Na2SO4-added ones melted during sintering, these pellets were stuck to their sample holder after cooling. Although the melted pellets still captured gases inside the pellets, their compressibility is expected to be low. Two trends are observed in Fig. 4 if the melted samples are not considered. First, pore size of the LWAs generally increases with sintering temperature, explained as follows. Sintering at higher temperature would develop more viscous layer with less viscosity, therefore, the bloating gases enveloped by such viscous layer would be easier to expand to create bigger pore size. However, it is noteworthy that if the viscosity is too low, it would be easier for the bloating gas molecules to penetrate the layer, leading to less LWA particle size. Second, LWA particle density is influenced by type of sodium salt additive, in decreasing order: NaCl-added LWA > Na2SO4-added LWA > Na2CO3-added LWA, as shown in Table 2. The main criterion to justify successful formation of LWA is relatively light in particle density, <1.6 g cm3, as compared with normal-weight aggregate of which the particle density is approximately 2.6–2.8 g cm3. Table 2 indicates that LWAs containing any sodium salt are lighter than the no-salt-added LWA in particle density, it is thus concluded that the sodium salt additives can reduce sintering energy. It is interesting to note that the type of sodium salt additives considerably influences LWA pore size. Addition of Na2CO3 appears to be the most effective way in preparing LWAs with the least particle density (Table 2). The Na2CO3-added LWAs made by sintering at  1100 °C all melted. Particle density measurement was infeasible for these samples because they were tightly stuck to the sample holder after cooling and could not be separated. Given that Na2O, formed by gradual and steady decomposition of Na2CO3 at >851 °C, has considerably higher melting point (Na2O m.p., 1130 °C) and boiling point than the other two sodium salt additives, Na2SO4 (m.p. 885 °C) and NaCl (m.p. 804 °C), it seems reasonable for one to expect that Na2CO3 might be the least effective additive in preparing low-density LWAs. However, this is not the case actually happened in present study (Table 2). One explanation for this is that Na2O has great tendency to form sodium silicate by reaction with SiO2 present in CFA/glass mixture, while the other two sodium salt additives are less apt to form silicates with SiO2 component. Sodium silicates, such as sodium orthosilicate and sodium metasilicate (Na2SiO3), can be formed by fusing Na2O and SiO2. Na2SiO3 has a relatively low melting point, 1088 °C, and its anion is a polymeric structure with the repeating unit being [SiO4]2- tetrahedral, with electric charge being balanced by two

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Y.-L. Wei et al. / Construction and Building Materials 239 (2020) 117890

Na2 CO3 added

Na2SO4 added

NaCl added

no salt added

Fig. 4. Side-viewed photos of half-cut LWAs made by sintering at 1050 °C (bottom-most row), 1100 °C (bottom-second row), 1150 °C (top-second row) and 1175 °C (top-most row).

Table 2 LWA particle density with/without sodium salt additive. Sintering temperature

1050 1100 1150 1175

°C °C °C °C

particle density (g/cm3) no salt added

NaCl added

Na2SO4 added

Na2CO3 added

1.78 1.63 1.29 1.13

1.29 1.14 0.99 0.99

0.91 0.71 – –

0.76 – – –

-: density is not available due to the sticking of fired pellets to sample holder.

Na+ ions. We thus infer that considerable amount of sodium silicates, representing the major component of viscous layer which turns into hard and impermeable glassy phase after cooling from sintering reaction, were readily formed at 1050 °C-1175 °C to facilitate the formation of LWAs with bigger pore size in present study. The fact, that relatively poor sintering reaction took place with NaCl additive, might be attributed to a low solubility of the major phase, coal fly ash/glass mixture, in the NaCl liquid phase, thus causing only little grain rearrangement. Such low-solubility phenomenon was also observed in our previous study which used CaCl2 additive [33]. The effect of sodium salt additives on the final compressive properties of LWA compared to the one without additives is presented in Table 3. Because shape of the pellets is so different from each other that compressive strength data, expressed in unit of kgf/cm2, is not used in this study. Rather, maximum compressive loading force (in kgf) is adapted for discussing LWA compressive property. The results in Tables 2 and 3 strongly correlate to each other, the LWA’s maximum compressive loading per pellet, each weighing approximately 7 g, generally decreases with decreasing LWA particle density. Such trend is observed no matter which sodium salt was used as an additive in coal fly ash/glass mixture.

carefully controlled. Excessive water would make the concrete body porous and permeable, while insufficient water leads to poor cement hydration reaction resulting in harsh and dry concrete body. For LWAs, their closed pore is irrelative to 24-hour water sorption capacity; only the pores open to atmosphere are related to water sorption capacity. Fig. 5 presents the dependence of 24hour water sorption capacity of LWAs, along with respective appearance, on both sintering temperature (1050 °C–1175 °C) and type of sodium salt additives. As a reference, bottom-row pictures of Fig. 5 display the no-salt-added sintered pellets; their pellet sizes were not well expanded due to an insufficient formation of glassy phase. With increasing temperature, water sorption capacity of the no-salt-added LWAs decreases from 12% (1050 °C-LWA) to 7.5% (1100 °C-LWA), and then increases to 8.6% (1175 °C-LWA) [5]. For the salt-added LWAs, their water sorption capacity appears to increase with increasing temperature. Water sorption capacity of NaCl-added LWAs increases from 7.7% (1050 °C) all the way to 12% (1175 °C) with increasing temperature. Glassy phase on these pellets’ surface does not appear to be as rich as that of Na2CO3- and Na2SO4-added LWAs. The 1050 °C Na2CO3-added and 1100 °C Na2SO4-added LWAs seem to be very rich in glassy phase to efficiently envelope bloating gases, thus they are well bloated.

3.5. Water sorption and appearance of LWA products 3.6 Weight. loss during green pellet sintering For construction sector, 24-hour water sorption capacity is an important property of LWAs when preparing lightweight concrete (LWC) that generally consists of LWAs, sand, cement, and water. In preparation of concrete, proper water/cement ratio needs to be

Upon sintering, fractional components in green pellets would vaporize, react with each other to generate gases, and/or decompose and lost as gases. These gases might act as effective bloating

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Y.-L. Wei et al. / Construction and Building Materials 239 (2020) 117890 Table 3 Maximum compressive loading of LWAs with/without sodium salt additive. Sintering temperature

1050 1100 1150 1175

maximum compressive loading (kgf)

°C °C °C °C

no salt added

NaCl added

Na2SO4 added

Na2CO3 added

2700 ± 323 1990 ± 631 1010 ± 530 659 ± 135

1460 ± 180 559 ± 259 49 ± 46 12 ± 19

66 ± 41 40 ± 24

92 ± 90

Na2CO3 added 11%

Na2SO4 added 6%

8.2%

7.7%

10%

11%

12%

12%

7.5%

7.7%

8.6%

NaCl added

No salt added 1050°C

1100°C

1150°C

1175°C

Fig. 5. Appearance and 24-hour water sorption capacity (inset number) of LWA made by sintering at 1050 °C, 1100 °C, 1150 °C and 1175 °C. The mark ‘‘-‘‘ in the pictures represents occurrence of melting of sample surface, thus pellets were stuck to sample holder when cooled after sintering.

reagents to expand the pellets if they are trapped inside pellets. Some gases, non-trapped, would tunnel through the pellets and escape to the atmosphere as ‘‘weight loss”. Fig. 6 shows the weight loss percentage of the pellets sintered at various temperatures. According to the proposed reactions (1), (3) and (5) in section 3.2, the weight losses of Na2CO3-added LWA, Na2SO4-added LWA, and NaCl-added LWA can mainly be attributed to the escape loss

9

Weight loss (%)

8

Na 2 CO3 Na 2 SO4 NaCl

4. Conclusions

7 6 5 4 3 1050

of CO2, SO3, and SiCl4, respectively. In addition, based on the TGA/DSC results (Figs. 1 and 2), NaCl might also be vaporized at the sintering temperatures. However, Fig. 6 shows that NaCladded LWA lost the least amount of weight, implying that reaction (5) is relatively slow as compared with reactions 1 and 3, and that the evaporation loss of NaCl during sintering was minor. Certainly, such relatively low weight loss would affect the formation of LWAs. This argument is also justified by NaCl-added LWA’s low porosity (Fig. 4) and by the presence of the prominent XRD patterns from NaCl crystalline phase in Fig. 3.

1100

1150

1200

Temperature (°C) Fig. 6. Weight loss percentage of sodium salt-added LWAs sintered at 1050 °C, 1100 °C, 1150 °C and 1175 °C.

Various sodium salts were added into mixtures consisting of coal fly ash and waste glass powders, formed into disc pellets, and sintered at 1050 °C, 1100 °C, 1150 °C, and 1175 °C to manufacture LWAs. The influence of type of sodium salt additive on formation mechanism of lightweight aggregates was studied, giving the following conclusions. Although all having low melting point, 800–900 °C, the three sodium salts selected for this study show considerably different thermal behavior from each other, thus leading to different effectiveness as a flux and/or bloating promoter during LWA preparation. The above argument was supported by the great difference in LWA appearance, in crystalline phase, in LWA particle density, and in 24-h water sorption. Formation of sodium silicates, a viscous layer developed during sintering and becoming glassy

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Y.-L. Wei et al. / Construction and Building Materials 239 (2020) 117890

phase after pellet cooling, is suggested to be through a two-step chemical reaction. The effectiveness of adding sodium salt additives to create bigger pore size is different, in decreasing order: Na2CO3 > Na2SO4 > NaCl. Using LWAs prepared at 1050 °C as an example, the particle density in increasing order is as follows, Na2CO3-added LWA (0.76 g cm3) < Na2SO4-added LWA (0.91 g cm3) < NaCl-added LWA (1.29 g cm3) < no-salt-added LWA (1.78 g cm3). The fact, that relatively poor sintering reaction took place with NaCl additive, might be attributed to a low solubility of the major phase, coal fly ash/glass mixture, in the NaCl liquid phase, thus causing only little grain re-arrangement. 5. Authors’ contribution The corresponding author Yu-Ling Wei surveyed most literature, initiated research concept, designed research scheme, monitored the laboratory work, examined and explained the experimental data, wrote and revised the manuscript, and submitted it to Construction and Building Materials. Shao-Hsian Cheng, Wen-Jing Chen, Yueh-Hua Lu, Ken Chen, and Pei-Ching Wu are either Master program students or senior undergraduate student accepted and are working in Yu-Ling Wei’s research group. ShaoHsian Cheng made major contribution in the experiments and also did some paper survey. Wen-Jing Chen, Yueh-Hua Lu, Ken Chen, and Pei-Ching Wu worked together on the experiment in TGA/ DTA of mixed sample, preparing LWAs by sintering for the photos of half-cut LWA, and the compressive loading examination. WenJing Chen also coordinated the teammates and made some graphs and tables. 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. Acknowledgment We thank Taiwan Ministry of Science and Technology for sponsoring this study (NSC-99-2221-E-029-008-MY3). References [1] R.S. Blissett, N.A. Rowson, A review of the multi-component utilisation of CFA, Fuel 97 (2012) 1–23. [2] N.U. Kockal, T. Ozturan, Characteristics of lightweight fly ash aggregates produced with different binders and heat treatments, Cem. Concr. Compos. 33 (2011) 61–67. [3] I. Kourti, C.R. Cheeseman, Properties and microstructure of lightweight aggregate produced from lignite CFA and recycled glass, Resour. Conserv. Recy. 54 (2010) 769–775. [4] J. Qin, C. Cui, X.Y. Cui, A. Hussain, C.M. Yang, Preparation and characterization of ceramsite from lime mud and CFA, Constr. Build. Mater. 95 (2015) 10–17. [5] Y.L. Wei, S.H. Cheng, G.W. Ko, Effect of waste glass addition on lightweight aggregates prepared from F-class CFA, Constr. Build. Mater. 112 (2016) 773–782.

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