Construction and Building Materials 47 (2013) 496–501

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Construction and Building Materials

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Characterization on a cementitious material composed of redmud and coal industry byproducts

0950-0618/$ - see front matter Published by Elsevier Ltd.http://dx.doi.org/10.1016/j.conbuildmat.2013.05.030

⇑ Corresponding author. Address: 3601 Pacific Ave., University of the Pacific,Stockton, CA 95211, USA. Tel.: +1 9518244476.

E-mail address: y_yao@u.pacific.edu (Y. Yao).1 Contribution to this paper equals to the first author.

Yuan Yao a,b,⇑, Yu Li c,1, Xiaoming Liu c, Shushu Jiang d, Chao Feng b, Ester Rafanan b

a Shanghai Advanced Research Institute, Chinese Academy of Science, Pudong, Shanghai 201210, Chinab School of Engineering and Computer Science, University of the Pacific, Stockton, CA 95211, USAc State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, Beijing 100083, Chinad Department of Pathology and Microbiology, University of California–Riverside, Riverside 92507, USA

h i g h l i g h t s

� RCC satisfy with the durability testing requirement.� Amorphous gel-like becomes the dominant structure.� RCC material meet with EPA TCLP requirement.

a r t i c l e i n f o

Article history:Received 10 May 2012Received in revised form 30 April 2013Accepted 8 May 2013

Keywords:Red mudCoal wasteCementitious materialPerformanceLeaching

a b s t r a c t

This research was to investigate the possibility of incorporating red mud and coal industry byproducts asthe raw material for producing cementitious material. Systematic mechanical strength tests were con-ducted to evaluate the performance of this cementitious material. Results showed that the designedred mud–coal industry byproducts based cementitious material had higher strength in the middle to latecuring age (47.5 MPa in 180 days and 48.7 MPa in 360 days) than the OPC control group. The series ofdurability tests indicated that the cementitious material met with the ASTM requirement. Moreover,the toxicity characteristic leaching tests demonstrated that this cementitious material had good stabiliza-tion/solidification ability to bind the heavy metal in the red mud as raw material. Microanalysis revealedthat the amorphous gel was the dominant structure of the material at the middle to late curing age,which possibly played a significant role on the heavy metal binding properties through the polymeriza-tion during the hydration process of this cementitious material. In essence, this designed red mud–coalindustry byproducts based cementitious material not only meet with the physical and mechanicalrequirements of the ASTM standards, but also meet with the EPA regulation on the environmental heavymetal leaching limitation. This proves the designed material can possibly be used as a clean technology torecycle the red mud from alumina industries and byproduct from coal industries.

Published by Elsevier Ltd.

1. Introduction

Coal is still considered as the primary energy resource in theworld and coal waste from mining system turns out to be one ofthe greatest amounts of solid waste in coal industries. For the tra-ditional coal waste resources utilization, the coal solid waste is ma-jorly divided into two categories: combustion and non-combustion. For the combustion part, bottom ash and fly ash areconsidered as major categories of byproduct after the combustionand they have successfully been applied in the cement and con-

struction material in the last two decades [1]. On the other hand,the non-combustion part faces more challenges in the recyclingand utilization, because the coal refuse is a low thermal-valuebyproduct of the coal mining industry and mainly consists ofnon-combustible rock and some attached carbon materials thatcannot be effectively separated [2]. Coal mines in the US generatean estimated 109 million metric tons (120 million short tons) ofcoal refuse from 600 coal preparation plants in 21 coal-producingstates annually [3]. Currently, large volume of coal refuse is stillaccumulated at the coal mining sites and raise lots of environmen-tal concern.

Red mud is the solid waste residue of the digestion of bauxiteores with caustic soda for alumina (Al2O3) production. Approxi-mately 35–40% of the processed bauxite ore goes into the wasteas alkaline red mud slurry which consists of 15–40% solids [4,5].

Y. Yao et al. / Construction and Building Materials 47 (2013) 496–501 497

The chemical and mineralogical composition of the complex indus-trial waste is widely different, depending on the source of bauxiteand the technological process parameters. However, six major oxi-des in the red mud are CaO, SiO2, Fe2O3, Al2O3, TiO2 and Na2O [6]. Itis estimated that annually 70 million tons of red mud is producedall over the world [7].

Previous studies have been conducted on recycling these twocategories of industrial solid waste by activating them into pozzo-lanic materials. For example, Zhang et al. have successfully recy-cled red mud and coal refuse into cementitious material bythermal activation at 600 �C [8,9]. Zhang has demonstrated howcoal refuse contains good pozzolanic properties after thermal acti-vation [10–12]. The use of coal refuse and red mud in buildingmaterial has also been reported [13–15]. However, two disadvan-tages of utilizing coal refuse as cementitious material have beenfound. The first one is that the cementitious strength of the coal re-fuse is still low due to the low CaO content in the coal refuse [7].But the CaO content in red mud is high which is potential mixedwith coal refuse together to increase the cementitious perfor-mance. Another disadvantage of calcinated coal refuse is in thelow flowability. On the other hand, the round sphere particle ofcoal fly ash has been proven to have good flowability in some con-struction materials [16–18]. In this paper, a new research has beenconducted to investigate a new cementitious material composed ofred mud (15%), coal refuse (15%) and fly ash (15%) as its majorcomposition besides the OPC (53%). The scenario of this composi-tion determination is to keep CaO content between 40% and 50%and increase the (SiO2 + Al2O3)/CaO ratio for better durability per-formance without sacrificing mechanical properties [6,8]. Thephysical and mechanical properties, durability, microstructureand metal leaching of this cementitious material will be discussedin detail within this paper.

2. Materials and experiments

The raw materials used in this cementitious material contained red mud, coalrefuse, coal fly ash, flue gas desulphurization (FGD) gypsum and ordinary Portlandcement (OPC). The red mud used in this experiment was the byproducts from analumina industry in Texas. The coal refuse, fly ash and FGD gypsum were all fromWest Virginia. US type I/II cement was used in this experiment to provide the prop-er cohesion and strength values in the early strength. The recipe design of red mud–coal industry byproduct based cementitious material is listed in Table 1. In order toanalyze the chemical and mineral composition of the raw materials, X-ray fluores-cence (XRF-1700) analyzer and X-ray diffraction (XRD) system (Rigaku Ultimate)were applied. In this designed composition of this material, the coal refuse andred mud were activated at 600 �C for 30 min in a furnace (Lindberg Blue M, ThermoScientific) before utilizing as raw material in RCC.

2.1. Mechanical properties test

The recipe design of the cementitious material is listed in Table 1. In thestrength test, the compressive strength test followed the ASTM C109 (CompressiveStrength of Hydraulic Cement Mortars) [19] and the flexural test was according tothe ASTM C348 (Flexural Strength of Hydraulic-Cement Mortars) [20]. All mortarsamples were mixed according to ASTM C305 (Mechanical Mixing of Hydraulic Ce-ment Pastes and Mortars of Plastic Consistency) [21]. The procedures were per-formed as described in the standard: all mixing water was placed into the bowl;cement was added to the water and the mixer was started at a low speed(140 ± 5 r/min) for 30 s; aggregate was then added over a 30-s period while mixingat a low speed; mixing was then increased to a medium speed (285 ± 10 r/min) for30 s. The mixer was stopped for 90 s to scrape down all of the mortar paste followedby mixing for 90 s at medium speed. Finally, the mixer was stopped, and mortarwas cast into the mold. Cubic specimens (50 mm � 50 mm � 50 mm) were castfor each mixture for the unconfined compressive strength test. The molds of the

Table 1Recipe design of red mud–coal waste based cementitious material.

(%) By mass Coal refuse Red mud Fly ash Cement FGD gypsum

RCC 15 15 15 53 2

flexural test were prisms in size (40 mm � 40 mm � 160 mm). The strength testswere performed at different curing ages (3–360 days). It was necessary to unmoldcarefully after the first curing day due to the low strength of the specimens whichshould be cured in a moist cabinet at 95% humidity and 23 �C after unmolding.

2.2. Flowability measurement

In order to measure the flowability of the mortar paste which was just mixedout of the machine, the flowability test was conducted according to ASTM C230(Flow Table for Use in Tests of Hydraulic Cement) [22] to determine the water con-tent needed for a cement paste sample to obtain a given flow spread of 110 ± 5%.According to these standards, mortar samples from the different mixtures wereplaced on the same flow table, subjected to 25 repetitions of a standard table dropand the spread diameters of the samples were measured [23].

2.3. Durability test

In this research, freezing–thawing test, chloride permeability test and alkali–sil-ica reaction (ASR) test were completed as durability performance tests according tothe relevant ASTM standards.

2.3.1. Freezing–thawing testsThe freezing–thawing tests were performed as described in ASTM C666 [24].

The concrete beams were subjected to 300 freezing–thawing cycles, and the rela-tive dynamic modulus was recorded (at 100, 200, 300 cycles in this test) to repre-sent the ability in resisting freeze–thaw weathering. Relative dynamic modulus ofelasticity Pc = (n12/n2) � 100 (where Pc = relative dynamic modulus of elasticity,after c cycles of freezing and thawing; n = fundamental transverse frequency at0 cycles of freezing and thawing; and n1 = fundamental transverse frequency afterc cycles of freezing and thawing).

2.3.2. Chloride permeability testAccording to ASTM C1202 [25], the chloride permeability test involves monitor-

ing the amount of electrical current (coulombs) passed through a 102-mm diameterby 51-mm thick concrete disc with a potential difference of 60 V DC maintainedacross the specimen for a period of 6 h. If the number of coulombs passed lies be-tween 2000 and 4000, the chloride permeability of concrete is considered low, andit is considered very low for the 100–1000 range [26–29].

2.3.3. Alkali–silica reaction (ASR) testThe 25 � 25 � 285 mm prismatic mortar bar was used for the ASR test accord-

ing to ASTM C1260 [30] and ASTM C1567 [31]. After the casting, the prismatic mor-tar bars were aged in a sodium hydroxide (NaOH) solution at 80 �C continuously for28 days with intermittent readings of the length change of the bars taken during thecourse of the test. ASR-related expansions less than 0.10% at 16 days after castingare indicative of innocuous behavior, while those between 0.10% and 0.20% at thesame age are indicative of both innocuous and deleterious behavior in field perfor-mance; expansion greater than 0.20% at 16 days of age are indicative of potentiallydeleterious expansion.

2.4. Microanalysis

Scanning electron microscope (Philips XL30 FEG) was used to analyze themicrostructure of the specimens at different curing ages. Before the SEM observa-tion, the target samples were first immersed in acetone for 10 days to stop thehydration process followed by drying under vacuum for 7 days.

2.5. Leaching test

The toxicity characteristic leaching procedure (TCLP) was used to determine themobility the heavy metal according to the EPA-TCLP 1311 procedure. The leachingheavy metals were analyzed using ICP-OES and ICP-AES [32].

3. Results and discussion

3.1. Characterization of raw material and red mud–coal waste basedcementitious material (RCC)

Chemical analysis showed that the total carbon in the coal re-fuse was 3.43%, and the Higher Heating Value (HHV) was only342 British thermal units per pound (Btu/lb), suggesting that thecoal refuse was not suitable for combustion use. Generally, forcombustion use, bituminous coals have heating values of 10,500–14,000 Btu/lb on a wet, mineral-matter-free basis; The heating val-ues of sub-bituminous coals range from 8300 to 11,500 Btu/lb on a

Table 2Chemical and mineral analysis of raw material.

Coal refuse R Fly ash R Red mud R Gypsum R Cement R

SiO2 (%) 47.23 0.9 49.34 1.1 18.92 1.0 0.83 0.04 13.83 0.9Al2O3 (%) 14.61 0.7 19.81 1.0 7.11 0.4 0.16 0.06 2.56 0.3Fe2O3 (%) 11.94 0.5 13.93 0.7 12.55 0.4 0.18 0.05 3.72 0.4CaO (%) 4.55 0.2 6.58 0.2 38.33 1.3 42.33 1.1 61.12 1.1MgO (%) 1.68 0.2 0.62 0.1 1.19 0.1 0.88 0.04 1.42 0.1SO3 (%) 0.53 0.1 0.44 0.1 0.49 0.08 53.19 0.9 2.29 0.1LOI(%) 11.5 0.8 2.78 0.3 15.44 0.9 1.33 0.2 2.62 0.4Mineral Quartz Mullite Calcite Gypsum BrownmilleritePhase Muscovite Quartz Quartz Hatrurite

Chlorite Gibbsite LamiteKaolinite HematiteHematite PerovskiteCalcite

R: range = maximum – minimum; LOI = Loss on ignition.

Table 3Chemical analysis of RCC.

Oxides (%) SiO2 Al2O3 CaO Fe2O3 MgO SO3

RCC 17.59 7.59 40.69 7.74 1.29 2.49

0

5

10

15

20

25

30

35

40

45

50

55

60

Com

pres

sive

str

engt

h (M

Pa)

OPC RCC

3 7 28 90 180 360

Curing Stage (days)

Fig. 1. Compressive strengths of RCC.

498 Y. Yao et al. / Construction and Building Materials 47 (2013) 496–501

wet, mineral-matter-free basis [33–35]. The fly ash contains CaO ata 6.58% abundance, which belongs to class F according to the des-ignation in ASTM C618 [36]. The chemical and mineral analysis ofother materials is listed in Table 2.

In Table 3, the chemical analysis of the produced red mud–coalwaste based cementitious material (RCC) showed that the primarycomponents found in RCC were SiO2, Al2O3 and CaO. Unlike the or-dinary Portland cement (OPC) containing much higher amount ofCaO, the CaO content of this produced RCC was reduced by theintroduction of the solid waste. The value of (SiO2 + Al2O3)/CaOwas 0.62, much larger than that of OPC (0.27). The general physicalproperties of RCC are reported in Table 4. It showed that the settingtime and flowability of RCC perfectly satisfied the requirement ofASTM standards when the water to binder ratio was controlledat 0.485. For example, although the setting time of RCC was longerthan that of OPC, it still met with the requirement of ASTM C150(Initial P 45 min; Final 6 375 min) [37]. The consistency of RCCmortar using a standard flow table reached to 109 (requirementof 105–115 in the case of ASTM C109 [19]), indicating the flowabil-ity of the mortar paste in the experiment was acceptable.

3.2. Mechanical properties of RCC

In order to compare the mechanical properties of RCC, a type I/IIordinary Portland cement (OPC) was used as control in accordancewith ASTM C109 [19]. The compressive and flexural strength re-sults of RCC mortars comparing with those of OPC are presentedin Figs. 1 and 2 respectively. According to the figures, both com-pressive and flexural strengths of the RCC developed well withinthe curing time (3–360 days). At the 3-day age, the compressiveand flexural strengths of RCC reached 12.2 MPa and 3.5 MPa, whichwas lower than those of OPC respectively. The initial period is char-acterized by rapid reactions between C3S and water that begin

Table 4Physical properties of RCC.

Sample Density (t/m3) Specific surface area (Blaine value)

RCC 3.12 463OPC 3.14 425

immediately upon wetting, C3Sþ 3H2O! 3Ca2þ þH2SiO2�4 þ

4OH� and the cement composition reduce in RCC will decreasethe C3S of the mixture, which might result in the loss of earlystrength when compared with OPC [38,39]. However, the RCCshowed higher strength performance than the OPC at curing agefrom 180 days and 360 days (47.5 MPa in 180 days and 48.7 MPain 360 days). This is probably due to the pozzolanic reactions ofthe blended solid byproduct material to help to continue thestrength development of RCC at middle to late stages of hardening[6,8,18,30].

3.3. Durability test results

Table 5 lists the results of the durability test, the freeze–thaw-ing resistance of RCC in relative dynamic modulus are 94.4%, 92.1%and 91.3% respectively after 100, 200, 300 cycles, which is higherthan the relative dynamic modulus of OPC that remains by thefreezing damage. It is obviously seen that the alkali silica reaction

(m2/kg) Setting time (min) Flowability (%)

Initial Final

158 251 109145 202 111

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

Flex

ural

str

engt

h (M

Pa)

OPC RCC

3 7 28 90 180 360

Curing Stage (days)

Fig. 2. Flexural strengths of RCC.

Table 5Durability test results.

RCC OPC

Freezing–thawing (relative dynamic modulus %) 100 cycle 94.4 91.3200 cycle 92.1 87.2300 cycle 91.3 84.9

Alkali silica reaction (expansion rate %) 16 days 0.112 0.15928 days 0.209 0.231

Cl penetration (number of coulombs) – 811 2478

Fig. 3. SEM image of RCC at 3 days elapsed.

Fig. 4. SEM image of RCC at 28 days elapsed.

Y. Yao et al. / Construction and Building Materials 47 (2013) 496–501 499

expansion in 16 days of RCC is only 0.112% at 16 days, which islower than 0.159% of OPC sample. When the curing time reachesto 28 days, the expansion rate of RCC is 0.209% which is still lessthan 0.231% of OPC sample. By measuring the number of coulombspassed through CRC and OPC samples, it shows the number cou-lombs passed through RCC is only 811, which is below 1000 as con-sidered to be very low according to the ASTM C1202 requirement[25]. Therefore, the application of combined red mud, fly ash andcoal refuse to substitute for OPC cement is able to reduce the po-tential damage caused by ASR and Cl penetration as well as thefreezing–thawing corrosion.

3.4. Toxicity characteristic leaching procedure (TCLP)

As shown in the raw materials Table 1, there are three major so-lid waste materials contained in this RCC material: red mud, coalrefuse and coal fly ash. Previous researches have been conductedon various tests to evaluate the environmental heavy metal leach-ing regarding to the potential hazardous composition of the solidwaste [40]. Based on this consideration, the EPA TCLP tests wereconducted for raw materials and RCC sample to evaluate the heavy

Table 6TCLP result of raw red mud and RCC sample at 28 days elapsed (ppb).

B Mg Al Ca Cr Mn Fe

Red mud 101 353 43 158,093 11,889 0.1 10OPC 758 39.5 159 492,558 45 2.1 79Flyash 579 15,557 9442 78,556 150 554 718Coal refuse 105 19,585 8442 65,001 102 258 545RCC 19 460 57 75,449 64.9 0.1 7.6EPA STD 5000

metal stabilization/solidification ability. As shown in Table 6, noneof the hazardous elements exceeded the EPA limits. Although 50%of the cement increased the leaching rate of the calcium, other ele-ments occurred at very low concentrations that would not havehazardous effects on the environment. However, RCC showed sig-nificant binding ability on chromium (Cr III and Cr VI as total)when compared with the raw red mud leachate. The Cr leachateof raw mud was 11889.6 ppb, which was higher than the5000 ppb EPA limitation, indicating that raw material cannot beused alone as cementitious material due to hazardous effect onthe environment. However, the Cr leachate of RCC was only64.96 ppb, which was lower than the EPA limitation. Therefore,the cementitious material showed good stabilization/solidificationability of the heavy metal material. This was possibly due to the

Cu Zn As Se Cd Sb Ba Tl Pb

8.6 19 34 134 0.19 1.5 153 0.5 1.96.9 29.8 0.8 1.4 0.38 0.1 117 0.12 6.981.2 155 1.9 6.10 17.8 89 398 1.4 4.812 110 2.1 14 10.2 11 334 2.9 5.56.0 6.5 2.2 13.5 0.06 3.2 88.9 0.2 1.6

5000 1000 1000 100,000 5000

Fig. 5. SEM image of RCC at 180 days elapsed.

Fig. 6. SEM image of RCC at 360 days elapsed.

500 Y. Yao et al. / Construction and Building Materials 47 (2013) 496–501

introduced coal refuse has certain amount of hematite usuallyincorporate the Cr [41]. In conclusion, this RCC is an environmen-tally acceptable material that can utilize red mud and coal waste ascementitious material.

3.5. Microstructure of RCC

The microstructure has significantly changed in 1 year of curingtime. From Figs. 3–6 the microstructure of the hydration productsof the RCC showed different morphological characteristics at days3, 28, 180, and 360 of curing age. At the early age from 3 days to28 days (Figs. 3 and 4), the dominant structure was the needle-shaped ettringite (3CaO�Al2O3�3CaSO4�32H2O) and rod-like crystal-lized calcium hydroxide [Ca(OH)2]. However, in the middle to lateage from 180 days to 360 days (Figs. 5 and 6), gel structure coveredthe coating area of the material and created a denser structure witha lower porosity, which was important for the stabilization/solidi-fication of the RCC. According to stabilization/solidification charac-terization by Spence and Shi, it can be generally divided into threestages according to the SEM image structure: (1) early hydrationand aggregate settlement, (2) intermediate hydration and earlysolidification and (3) late hydration and late solidification [42].Thus, the early hydration process is similar to the cement hydra-tion process and includes a large degree of ettringite and Ca(OH)2

crystallization as well as C–S–H gel formation, which occurs withinthe first 2 or 3 weeks. However, in the middle to late age, the C–S–H gel or C–A–S–H gel might contribute more to the stabilization/solidification in hydration process.

4. Conclusion

This study evaluated the performance of a new cementitiousmaterial (RCC) that was largely composed of red mud, coal refuseand coal fly ash. RCC performed well in the physical and mechan-ical tests to meet with the requirement of ASTM C230, ASTM C1437and ASTM C109 standards [19,22, 43]. At the same time, RCC sat-isfy with the durability testing requirement.

RCC material was environmentally acceptable according to EPAstandards. RCC had very good stabilization/solidification ability forthe Cr to prevent it from leaching into environment.

At an early age, needle-shaped ettringite and rod-like Ca(OH)2

contributed to strength development. However, increasingly irreg-ular crystalline and amorphous gel-like became the dominantstructure at middle to late curing age, and this structure develop-ment might also play important role in the heavy metal stabiliza-tion/solidification performance.

Acknowledgment

The authors gratefully acknowledge financial support frominternal funding from University of the Pacific.

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