Effect of calcination condition on the microstructure and pozzolanic activity

Effect of calcination condition on the microstructure and pozzolanic activity of calcined coal gangue

Zhao Cao a ,b ,?,Yongdan Cao a ,b ,Hongjuan Dong a ,b ,Jinshan Zhang a ,b ,Chunbao Sun a

a Institute of Mining Engineering,Inner Mongolia University of Science and Technology,Baotou 014010,China b

Inner Mongolia Engineering Center of Coal Mining &Comprehensive Utilization,Baotou 014010,China

a b s t r a c t

a r t i c l e i n f o Article history:

Received 12October 2014

Received in revised form 11September 2015Accepted 23November 2015Available online xxxx In this paper,a systematic study was conducted to investigate the in ?uence of calcination conditions including grinding time of raw material,temperature,holding time and heating rate on the pozzolanic activity of calcined coal gangue (abbreviated as CCG hereinafter).Furthermore,the changes of mineral composition,chemical structure and morphology of coal gangue during calcination were characterized by means of thermogravimetric-differential scanning calorimeter (TG-DSC),X-ray diffraction (XRD),Fourier transform infrared spectroscopy (FTIR)and scanning electron microscope (SEM)analyses.The results show that heating temperature affects directly the mineralogy and crystallinity of CCG.Kaolinite in coal gangue transforms into metakaolin which is an irregular and amorphous phase as calcined at 600°C –800°C because of the dehydroxyl-ation of Al –(O,OH)octahedrons together with the depolymerization of Si –O tetrahedrons.The amorphous metakaolin is of high pozzolanic activity;however,it will lose its activity due to the recrystallization into mullite when heating temperature is over 1000°C.The pozzolanic activity of CCG is mainly dependent on the non-crystallizing degree of kaolinite after calcination.

?2015Elsevier B.V.All rights reserved.

Keywords:Coal gangue

Calcination condition Metakaolin

Pozzolanic activity

1.Introduction

Coal gangue is a complex industrial solid waste discharged when coal is excavated and washed in the production course of coal mine.Its major chemical compositions are SiO 2and Al 2O 3,and major mineral-ogical compositions are kaolinite,illite,and quartz.The amount of coal gangue accumulated in China has already reached 3.8billion tons;moreover,the stockpile of coal gangue is increasing at a rate of 0.2billion tons per year (Gu,1997).Existing coal gangue in such a large quantity has occupied a lot of land and caused many serious environ-mental problems.

Presently,two major methods are applied for the reuse of coal gangue:combustion for power generation (Liu and Liu,2010)and used as building materials,such as cement admixture (Li et al.,2006)and goaf back ?ll (Yao and Sun,2012),among these,cement admixture accounts for the largest proportion of the utilization of coal gangue.However,the raw gangue cannot be utilized directly as cement admixture due to its weak pozzolanic activity (Zhang,2006).Therefore,improving the pozzolanic activity of coal gangue is a critical process in its utilization as building materials.Many studies have been carried out on the activation of coal gangue containing clay minerals and

thermal treatment is the most common method used by these researchers (Buchwald et al.,2009;Seiffarth et al.,2013).

The main reaction that happened in the activation of coal gangue is dehydroxylation of clay minerals such as kaolin and the formation of metakaolin,which is an amorphous mineral with high pozzolanic activ-ity (Siddique and Klaus,2009;Dellisanti and Valde,2012).Thermal treatment with a low temperature or short holding time gives metakaolin which is not very reactive and contains residues of kaolin and carbon,however,a temperature over 925°C leads to the beginning of crystallization and a decline of pozzolanic activity (Konan et al.,2009).Therefore,in order to get a better pozzolanic material,the calcination condition such as temperature,holding time and heating rate should be controlled accurately.

In this paper,we systematically investigated the in ?uence of calcina-tion conditions including ?neness of raw material,temperature,holding time and heating rate on the pozzolanic activity of CCG.Furthermore,the mineral composition,chemical structure and morphology of CCG under different temperatures were characterized comparatively by means of XRD,FTIR and SEM analyses.2.Experimental procedures 2.1.Raw materials

Coal gangue used in this study was obtained from JingYang Coal Washing Plant,which is located in Baotou City of China.The mineralogical

International Journal of Mineral Processing 146(2016)23–28

?Corresponding author at:Institute of Mining Engineering,Inner Mongolia University of Science and Technology,Baotou 014010,China.

E-mail address:btcaoz@https://www.wendangku.net/doc/cc13398783.html, (Z.

Cao).

https://www.wendangku.net/doc/cc13398783.html,/10.1016/j.minpro.2015.11.0080301-7516/?2015Elsevier B.V.All rights

reserved.

Contents lists available at ScienceDirect

International Journal of Mineral Processing

j o u r na l ho m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /i j m i n p r o

phase was characterized by XRD as shown in Fig.1.It indicates kaolinite (Al 2O 3·2SiO 2·2H 2O)and quartz (SiO 2)are the major crystallized minerals presented in the raw coal gangue.The chemical composition and physical properties of raw coal gangue are presented in Table 1.The mineral compositions of raw and calcined coal gangues are shown in Table 2.The result shows that the raw coal gangue has a high purity of kaolinite which can be calcined to prepare a high reactive pozzolan.2.2.Thermal treatment

The raw samples were wet ground with a stainless steel rod mill at a rotation speed of 400rpm for 10min,20min,30min and 40min.The speci ?c surface area (measured by N 2adsorption multi-point BET method)and ?neness (residue on 30μm sieve,%)of ground samples are presented in Table 3.The ground samples were dried and then calcined at different temperatures (400°C,500°C,600°C,700°C,800°C,900°C and 1000°C)with different heating rates (10°C·min ?1,30°C·min ?1,50°C·min ?1and 70°C·min ?1)for different holding times (1.0h,1.5h,2.0h and 2.5h).The in ?uences of grinding time,calcination temperature,holding time and heating rate on the pozzolanic activities of calcined products were evaluated by Chapelle test.2.3.Test methods

The chemical compositions of samples were performed on a PANalytical X-ray ?uorescence analyzer.

Thermogravimetric and differential thermal analysis (TG-DSC)were obtained simultaneously using a Mettler Toledo 851instrument.In order to separately analyze the combustion reaction of carbon and de-hydroxylation of kaolin during the thermal treatment of coal gangue,two ground raw samples were heated from ambient temperature to 1200°C at a rate of 10°C·min ?1under 50mL·min ?1air and argon gas,respectively.

The mineralogical phase and crystal structure were characterized by X-ray diffraction using a Rigaku D/max-RB powder diffractometer,with Cu K αradiation (40kV;100mA).

The chemical structures of CCG under different roasting tempera-tures were analyzed by Fourier transform infrared spectra using a Spectrum-One,Perkin Elmer FTIR spectrometer in absorbance mode with KBr pellet technique (1–2mg sample with 200mg KBr).

SEM test was carried out on a Hitachi SC-2500scanning electron microscope for observing the morphology and microstructure of CCG.

The pozzolanic activity was determined according to the Chapelle test.A gram of CCG was mixed with 1g of Ca(OH)2and 200mL boiling water.The suspension was boiled for 16h and the free Ca(OH)2was

determined by means of sucrose extraction and titration with a HCl solution(Kakali et al.,2001).3.Results and discussion 3.1.TG-DSC measurement

Fig.2presents the TG-DSC curves of coal gangues under air and Ar gas separately.Calcining coal gangue in absence of oxygen (in Fig.2(b)),the weight loss of 15.2%is attributed to the decomposition of minerals such as kaolinite and dolomite;and the combustion of car-bon and organic matter causes the weight loss to increase by 11.4%under aerobic calcination (in Fig.2(a)).

The endothermic peak presented at 500°C in Fig.2is contributed by the dehydroxylation of kaolinite and formation of metakaolinite;the exo-thermic peak at 1000°C is due to the transformation of metakaolinite into mullite.It is deduced that kaolinite (Al 2O 3·2SiO 2·H 2O)in coal gangue be-gins to decompose into amorphous metakaolinite (Al 2O 3·2SiO 2)at 500°C and transforms to the crystallized mullite (3Al 2O 3·2SiO 2)when tem-perature reaches up to 1000°C (Ilic et al.,2010).The two thermal induced processes can be described by the following reactions:Al 2O 3á2SiO 2á2H 2O →500 C

Al 2O 3á2SiO 2t2H 2O g eTe1T3Al 2O 3á2SiO 2eT→1000 C

3Al 2O 3á2SiO 2t4SiO 2:

e2T

The exothermic peak at 600°C in Fig.2(b)may be caused by the combustion of carbon and organic matter,which indicates that higher temperature is needed for the removal of carbonaceous minerals than amorphization of kaolinite.This scheme is represented by the following chemical equations:C tO 2→CO 2

e3T

C m H n t2m t12

O 2→600 C

m CO 2t12

n H 2O :

e4T

3.2.XRD analysis

XRD patterns of calcined coal gangues under different temperatures

are presented in Fig.3.Kaolinite and quartz are the two major crystal-lized minerals in the uncalcined coal gangue.Kaolinite is a hydrous layer silicate clay mineral.The structural unit of kaolinite consists of a Si-O tetrahedral sheet and an Al-O(OH)octahedral sheet (Frost,1997).The crystallinity of kaolinite in the coal gangue can be evaluated on the basis of XRD background in the range 2θ=20–30°and the

width

Fig.1.XRD pattern of raw coal gangue.

Table 1

Chemical composition and physical properties of raw coal gangue.SiO 2Al 2O 3

CaO

MgO

K 2O

Na 2O

Fe 2O 3

TiO 2

LOI

Speci ?c gravity g·cm ?3%39.08

31.06

0.38

0.24

0.26

0.16

1.20

0.98

26.60

2.16

Table 2

Mineral compositions of raw and calcined coal gangues.Raw coal gangue

Content (%)Calcined coal gangue Content (%)Kaolinite 78.55Metakaolinite 92.09Quartz

2.55Quartz

3.47Carbon and organic matter 15.64Else

4.44

Else

3.26

24Z.Cao et al./International Journal of Mineral Processing 146(2016)23–28

of the (002)diffraction peak (d =0.358nm)at half of the maximum height (Cheng et al.,2010).As can be seen,the kaolinite contained in the sample is well-crystallized.The two main peaks of uncalcined kaolinite at 12.4°and 24.9°are due to the diffraction of (001)and (002)crystal planes,respectively.

As temperature rises,the intense peaks at 12.4°and 24.9°in the CCG gradually disappear,which is caused by the breakages of Al –OH bonds and the deterioration of lamellar structure of kaolinite.As show in Fig.3,the kaolinite in calcined coal gangue becomes completely amor-phous from 600°C and transforms to mullite at 1000°C.This rule is in harmony with the TG-DSC result.Previous studies have proved that metakaolin can be obtained from 650°C to 900°C and transforms to silicon-spinel at 925°C and mullite is generated above 1000°C.It is noted that the silicon-spinel and mullite both have low activities (Brindley and Nakahira,1958).Therefore,in order to get better pozzola-nic properties,the temperature should be controlled accurately.Fig.3indicates that quartz is not activated in the CCG during calcination because the peaks at 20.8°and 26.6°are retained.According to C.Li et al.(2010),the inactive quartz has a detrimental effect on the cementitious property of blended mortar.Therefore,our next research

will focus on how to improve the pozzolanic activity of quartz in coal gangue.

3.3.FTIR analysis

Fig.4shows the IR spectra of calcined coal gangues under different temperatures.In the uncalcined sample,the characteristic bands of kaolinite can be found:O –H stretching vibrations at 3695cm ?1,3670cm ?1and 3620cm ?1;Si –O –Si stretching and bending vibrations at 1050cm ?1,797cm ?1and 754cm ?1;Si –O vibrations at 1020cm ?1,693cm ?1and 470cm ?1;Al –OH bending vibrations at 915cm ?1;Si –O –Al VI bending vibrations at 540cm ?1(Vizcayno et al.,2010).The bands at 3435cm ?1and 1630cm ?1correspond to the O –H stretching and bending vibrations of absorbed water,respectively.

According to H.Ming (2004),the band at 3695cm ?1is related to the stretching vibration of hydroxyl groups at the edges of the kaolinite platelets.The band at 3670cm ?1is a contribution of the hydroxyl groups at the surface of the octahedral layers that interact with the ox-ygen atoms of the adjacent tetrahedral layers.The band at 3620cm ?1is connected with the internal hydroxyl groups of kaolinite.None of the bands are present on the spectra of the products calcined at tempera-ture above 600°C,which indicates the hydroxyl groups of kaolinite have been removed and metakaolin generates.Absence of peaks at

Table 3

Physical characteristics of coal gangue with different grinding time.Grinding time (min)

10203040Speci ?c surface area (m 2·g ?1)

0.41 2.85 5.948.66Fineness (residue on 30μm sieve,%)

43.69

13.37

3.91

1.76

Fig.2.TG-DSC curves of coal gangue after grinding for 30min:(a)under air ?ow(with ox-ygen);(b)under argon ?

ow(oxygen-free).Fig.3.XRD patterns of coal gangue calcined at different temperatures (grinding for 30min;holding time 2h;heating rate 50

°C/min).

Fig.4.IR spectra of coal gangue calcined at different temperatures (grinding for 30min;holding time 2h;heating rate 50°C/min).

25

Z.Cao et al./International Journal of Mineral Processing 146(2016)23–28

915cm?1and540cm?1demonstrates the breakages of Al–OH and Si–O–Al VI bands in the thermal treated kaolinite.

In the calcined products,the bands related to Si–O–Si and Si–O vibrations at1050cm?1,1020cm?1,797cm?1,754cm?1and 693cm?1vanish and two broader bands at1100cm?1and810cm?1 appear.It is due to the depolymerization of silica tetrahedrons induced by thermal treatment.The same result has been obtained through29Si MAS NMR analysis by Zhang et al.(2010).

3.4.SEM test

The SEM images in Fig.5illustrate the morphological characteristics of coal gangues(grinding for30min)uncalcined and calcined at different temperatures for2h with a heating rate50°C·min?1. Fig.5(a)shows the uncalcined coal gangue has a“scale-shaped”lamellar structure and remains almost unchanged at500°C(in Fig.5(b));however,the structure turns to irregular and porous when coal gangue heated from600°C to800°C(in Fig.5(c)–(e)),which may be caused by the dehydroxylation of kaolinite and phase change to metakaolin.The well crystalline structure presented in Fig.5(f)indicates that metakaolin will transform to mullite when heated above1000°C.The SEM images agree well with the XRD and IR results.3.5.Pozzolanic activity of the CCG

The durability of cement can be improved by mixing metakaolin, due to the reaction between metakaolin and the excess Ca(OH)2pro-duced by cement hydration to form calcium silicate hydrate(C-S-H) gel,which reduces the porosity of the blinder.Therefore,the pozzolanic activity of CCG(mainly metakaolin)can be directly assessed by the amount of Ca(OH)2consumed per gram of CCG according to the Chapple test.

Figs.6–8illustrate the in?uences of grinding time,calcining temper-ature,holding time and heating rate on the pozzolanic activity of CCG.In Fig.6,the pozzolanic activity of CCG increases with calcination temper-ature and reaches to the maximal value at800°C then decreases dramatically when temperature rises to1000°https://www.wendangku.net/doc/cc13398783.html,bined with the results of XRD and SEM(Figs.3and5),we con?rm that the amorphous phase in CCG calcined at600–800°C contributes to the increase of poz-zolanic activity and the activity starts to decrease with the formation of mullite when heating temperature is over1000°C.Consequently,the pozzolanic activity of CCG mainly depends on the non-crystallizing degree of kaolinite after calcination.On the other hand,the activity of CCG increases with the grinding time of raw material and reaches the plateau value at30min,which shows the same rule with the growth of speci?c surface area of ground coal gangue.The pozzolanicity of coal gangue can be improved by extending the grinding time.It

is Fig.5.SEM images of coal gangue calcined at different temperatures:(a)uncalcined;(b)500°C;(c)600°C;(d)700°C;(e)800°C;(f)1000°C.

26Z.Cao et al./International Journal of Mineral Processing146(2016)23–28

believed that the mechanical treatment does not only dissociate the interlocked minerals such as kaolinite and carbon,but also breaks the bridge bonds between polymerized silica tetrahedrons;thus leads to the further disordering of kaolinite.This rule has been proved by Mitrovic and Zdujic (2014)through mechanochemical treatment of kaolin.

Fig.7displays the pozzolanic activity of coal gangue after calcining at 800°C for different holding time.The activity is relatively low when heating time is less than 2h,which can be due to the incomplete decomposition of kaolinite and unburned carbon in coal gangue.Ch.Bich et al.(2009)pointed out that the dehydroxylation degree of kaolin is incomplete with insuf ?cient calcination time and hence makes calcined products inert.Fig.8shows that the heating rate has little impact on the pozzolanic activity of CCG.However,a moderate rate around 50°C·min ?1is recommended because it is easy to be con-trolled and shortens the production period in the industrial-scale calcination.

From the analyses above,we con ?rm that coal gangue can be ther-mal activated to a high reactive pozzolan under appropriate conditions,which has a promising application prospect in high performance con-crete.Lots of investigations have demonstrated that this pozzolan can improve the strength,durability and workability of the blended mortar and concrete (Al-Akhras,2006;Alonso and Palomo,2001;Gruber et al.,2001).Moreover,taking CCG as a cement admixture cannot only

consume large quantities of coal waste,but also save a lot of nature resources for the cement clinker.Therefore,it is of great economic and environmental signi ?cances to realize the reutilization of coal waste.4.Conclusions

From the above studies,the following conclusions can be drawn:(1)Kaolinite,quartz and unburned carbon are the main mineral

compositions in the coal gangue.TG-DSC analysis indicates that kaolinite starts its amorphization at 500°C and then recrystal-lizes into mullite as heating temperature reaches over 1000°C.(2)XRD analysis shows that the kaolinite peaks in coal gangue begin

to weaken at 500°C because of its transformation to metakaolin,which can be proved by the disappearance of O –H stretching vi-bration peaks of kaolinite.Moreover,the crystallinity of kaolinite can be reduced by thermal treatment through the destruction of the layered structure and depolymerization of silica tetrahedrons.(3)Morphology observation proves that the lamellar structure of kao-linite is destroyed into an irregular and porous phase heated at 600–800°C and then transforms into a crystalline structure above 1000°C.

(4)Heating temperature has a substantial impact on the mineralogy

and crystallinity of calcined coal gangue.Whereas heating rate and holding time affect the dehydroxylation degree of kaolinite and the removal rate of impurities such as carbon and carbonates.All the calcination conditions could be optimized to get a highly reactive pozzolan with low heat consumption,which will make it possible to reuse coal gangue as a building material industrially.

Acknowledgments

This work was funded by the National “863”Project of China (No.2012AA06A118),the Natural Science Foundation of Inner Mongolia (No.2014MS0521)and the Key Science &Technology Development Project of Baotou City (No.2013Z1016).The authors are grateful to Li Han,Institute of Rare Earth for her help on SEM measurements.References

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一般过去式 时间状语：yesterday just now (刚刚) the day before three days ag0 a week ago in 1880 last month last year 1. I was in the classroom yesterday. I was not in the classroom yesterday. Were you in the classroom yesterday. 2. They went to see the film the day before. Did they go to see the film the day before. They did go to see the film the day before. 3. The man beat his wife yesterday. The man didn’t beat his wife yesterday. 4. I was a high student three years ago. 5. She became a teacher in 2009. 6. They began to study english a week ago 7. My mother brought a book from Canada last year. 8.My parents build a house to me four years ago . 9.He was husband ago. She was a cooker last mouth. My father was in the Xinjiang half a year ago. 10.My grandfather was a famer six years ago. 11.He burned in 1991

学生造句--Unit 1

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介词后的动词要用—ing形式。介词加代词时，代词要用宾格。例如：give up her（him）这种形式是正确的，而give up she（he）这种形式是错误的。 7.冠词：冠词是指修饰名词，表名词泛指或特指。冠词有a an the 。 8.叹词：叹词表示一种语气。例如：OH. Ya 等 9.连词：连词是指连接两个并列的成分，这两个并列的成分可以是两个词也可以是两个句子。例如：and but or so 。 10.数词：数词是指表示数量关系词，一般分为基数词和序数词 第二章节：英语句子成分 主语：动作的发出者，一般放在动词前或句首。由名词. 代词. 数词. 不定时. 动名词. 或从句充当。 谓语：指主语发出来的动作，只能由动词充当，一般紧跟在主语后面。 宾语：指动作的承受着，一般由代词. 名词. 数词. 不定时. 动名词. 或从句充当. 介词后面的成分也叫介词宾语。 定语：只对名词起限定修饰的成分，一般由形容

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base on的例句

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英语造句大全

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(完整版)主谓造句

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初中英语造句

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The Kite Runner-美句摘抄及造句

《The Kite Runner》追风筝的人--------------------------------美句摘抄 1.I can still see Hassan up on that tree, sunlight flickering through the leaves on his almost perfectly round face, a face like a Chinese doll chiseled from hardwood: his flat, broad nose and slanting, narrow eyes like bamboo leaves, eyes that looked, depending on the light, gold, green even sapphire 翻译：我依然能记得哈桑坐在树上的样子，阳光穿过叶子，照着他那浑圆的脸庞。他的脸很像木头刻成的中国娃娃，鼻子大而扁平，双眼眯斜如同竹叶，在不同光线下会显现出金色、绿色，甚至是宝石蓝。 E.g.: A shadow of disquiet flickering over his face. 2.Never told that the mirror, like shooting walnuts at the neighbor's dog, was always my idea. 翻译：从来不提镜子、用胡桃射狗其实都是我的鬼主意。E.g.:His secret died with him, for he never told anyone. 3.We would sit across from each other on a pair of high

翻译加造句

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改写句子练习2标准答案

The effective sentences:(improve the sentences!) 1.She hopes to spend this holiday either in Shanghai or in Suzhou. 2.Showing/to show sincerity and to keep/keeping promises are the basic requirements of a real friend. 3.I want to know the space of this house and when it was built. I want to know how big this house is and when it was built. I want to know the space of this house and the building time of the house. 4.In the past ten years,Mr.Smith has been a waiter,a tour guide,and taught English. In the past ten years,Mr.Smith has been a waiter,a tour guide,and an English teacher. 5.They are sweeping the floor wearing masks. They are sweeping the floor by wearing masks. wearing masks,They are sweeping the floor. 6.the drivers are told to drive carefully on the radio. the drivers are told on the radio to drive carefully 7.I almost spent two hours on this exercises. I spent almost two hours on this exercises. 8.Checking carefully,a serious mistake was found in the design. Checking carefully,I found a serious mistake in the design.

用以下短语造句

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英语造句

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