Utilization of iron tailings as substitute in autoclaved aerated concrete: physico-mechanical and microstructure of hydration products

Journal of Cleaner Production xxx (2016) 1e10

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Utilization of iron tailings as substitute in autoclaved aerated concrete: physico-mechanical and microstructure of hydration products Bao-guo Ma, Li-xiong Cai, Xiang-guo Li*, Shou-wei Jian State Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology, Wuhan 430070, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 13 September 2015 Received in revised form 30 March 2016 Accepted 30 March 2016 Available online xxx

Based on the background that a large amount of iron tailings was stockpiled in China, the most appropriate preparation conditions of producing autoclaved aerated concrete (AAC) with iron tailing was studied. The slurry properties were tested to evaluate the effects of the raw material factors on gas forming character. The compressive and specific strengths were measured to assess the feasibility of producing B05, A2.5 AAC blocks. The analysis of morphology, mineral components and thermal property were carried out in order to determine the mineral composition of iron tailing AAC blocks. Leaching toxicity was determined to ensure the environmental safety of iron tailing AAC blocks. The results shows that under the following conditions, cement 8%, quicklime 21%e27%, 20 min ball milled siliceous 62% e68% (with 40%e60% substituted by iron tailings), gypsum 3%, ratio of water to raw material (W/R) 0.6, aluminum (Al) powder 0.14% and at 1.4 MPa steam pressure maintaining for 8 h, the bulk density can be between 490 and 525 kg/m3, compressive strength higher than 2.5 MPa and specific strength higher than 4700 N m/kg. The main minerals in the AAC were dough-like CSH gel, flake-like tobermorite and longstrip anhydrate besides quartz and other residual minerals from the iron tailings. The compressive strength is mainly attributed to the interconnected microstructure constituted of CSH gel and tobermorite. The thermal analysis also proved the existing of the main minerals. © 2016 Elsevier Ltd. All rights reserved.

Keywords: Iron tailing Autoclaved aerated concrete Compressive strength Specific strength Crystalline components Leaching toxicity

1. Introduction Autoclaved aerated concrete (AAC) is a lightweight and highly porous material with excellent thermal insulation ability which can be used both in load bearing wall and filler wall (Narayanan and Ramamurthy, 2000; Karakurt et al., 2010; Jerman et al., 2013). The physical and mechanical properties of AAC are mainly determined by the hole-wall composition of hydration products and the pore size distribution (Alexanderson, 1979; Abbate, 2004). The promotion of Chinese government on building energy saving and carbon emission gives autoclaved aerated concrete a

Abbreviations: AAC, autoclaved aerated concrete; W/R, ratio of water to raw material; Al, aluminum; CaO, calcium oxide; P.O. 42.5, ordinary portland cement 42.5; MSIS, mass substitution ratio of iron tailing to silicon sand; FESEM, fieldemission high resolution transmission electron microscopy; EDX, Energy Dispersive X-ray; XRD, X-ray diffraction; TG, thermogravimetric; DSC, differential scanning calorimetry; ICP-AES, inductively coupled plasma atomic emission spectrometry; CSH, calcium silicate hydrate; CASH, calcium aluminum silicate hydrate; DM, dry mixture; HAC, hardened aerated concrete; AFt, ettringite. * Corresponding author. E-mail address: [email protected] (X.-g. Li).

broad application prospect. The commercialized AAC is usually produced with cement and lime as calcareous materials, with quartz sand or fly ash as siliceous materials, and small quantities of aluminum powder as gas forming material. Recent trends in AAC have heightened the need for industrial wastes utilization in AAC production. Several researchers have investigated the possibility of replacing the traditional raw materials of AAC by industrial waste, such as fly ash (Andre et al., 1999), air-cooled slag (Mostafa, 2005), coal bottom ash (Kurama et al., 2009), efflorescence sand (Mirza and Al-Noury, 1986), copper tailings (Huang et al., 2012) and carbide slag (Fan et al., 2014), etc. Iron tailings in China have been nearly totally piled up through the history of iron ore mining. As a by-product of iron ore, the iron tailings production ratio is 1:2.5e3.0 iron ore. By 2013, the total cumulative stockpiling of iron tailings in China was 5 billion tons and the number is keeping increasing as the rising of iron tailing emission, posing a severe threat to the environmental condition (Zhang et al., 2006). Nowadays, the allowable disposal method for iron tailings is outdoor stack after solidify with curing agent, which may cause soil contamination, river and underground water pollution and

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potential danger (Dudka and Adriano, 1997; Licsk et al., 1999; Moreno and Neretnieks, 2006). Thus, for environmental protection and sustainable development, utilization of iron tailings has become an issue. Many studies engaged in utilizing iron tailings such as recovery (Sirkeci et al., 2006; Das et al., 2002), fired blocks (Yang et al., 2014), ceramsite (Das et al., 2000), concrete aggregate (Zhao et al., 2014), and additives in the ceramic industry (da Silvaa et al., 2014). Producing building materials is a mature technical route for utilizing waste, and a great number of methods in this field have been reported. Most of the studies reported the utilization of waste on autoclaved brick with good quality (Ahmari and Zhang, 2012; Kumar and Kumar, 2013; Zhang, 2013; Zhao et al., 2009; Du et al., 2014). Some of them have studied the leaching behavior to ensure the service safety of the material (Tanrıverdi, 2006; Ahmari and Zhang, 2013). As for AAC, most scholars focused on the preparation of B06 AAC blocks which were classified in the national standard GB/T 11968-2006, but few studies involved B05 or much lighter AAC blocks. Meanwhile, most of the studies were reported on the macro-property and micro-structure of AAC blocks, but they neglected the influence of slurry properties and gas forming process. The objective of this study is to investigate the feasibility of producing B05, A2.5 AAC blocks with iron tailings. The SiO2 in iron tailings used to replace quartz sand as silicon material. In addition, calcium oxide (CaO) in iron tailings was considered as partly alternative calcareous resource to reduce the consumption of lime. This paper mainly discussed the influence of quick-lime content, iron tailing content and siliceous fineness on the slurry gas forming character and AAC mechanical property, meanwhile, the influence of steam pressure maintaining time on AAC mechanical property is also discussed. Additionally, to make primary understanding of reaction mechanism during the process of dry mixing, pre-curing and autoclaving, the micro-structural and phase compositions of the AAC blocks prepared by iron tailings were investigated. Leaching toxicity was measured to examine the environmental safety of iron tailing AAC blocks. 2. Material and methods 2.1. Raw materials Raw materials contained silicon sand, iron tailing, cement, quicklime and calcium sulfate dihydrate (analytically pure). All the raw materials used were from the same batch to ensure the stability of the chemical compositions. The chemical components of raw materials were shown in Table 1. Iron tailing discharged by ore-dressing machinery was sampled from the Wuhan Iron and Steel (Group) Company in China. Cement and quicklime were used as calcareous materials to provide CaO in the autoclaved hydrothermal reactions. Quicklime had 71.6% active CaO and its residue on 80 mm sieve was 9.3% with 12 min digestion time, 87
C digestion temperature. The cement was commercial ordinary portland cement 42.5 (P.O. 42.5) provided by Hubei Yadong cement Co., Ltd. The calcium sulfate dihydrate (analytically pure) was made by Sinopharm Chemical Reagent Co., Ltd. Al powder was used as a gas producing agent for the slurry foaming,

which had 80%solid content, 86%active Al content, and its coating surface on water was 5417 cm2/g. 2.2. Procedure The mixture proportion and Kalk (Du et al., 2014) of each sample were shown in Table 2. The percentage content of cement, gypsum, Al powder and water were constant. Sample T1-T5 was set to investigate the effect of different quicklime contents on the properties of slurry and AAC blocks. The quicklime content varied between 19% and 27%, meanwhile, the siliceous content changed from 70% to 62%, and the mass substitution ratio of iron tailing to silicon sand (MSIS) was constant at 40%. The samples T2, T6-T11 were set to investigate the effect of MSIS on the properties of slurry and AAC blocks by maintaining the dosage of quicklime at 21% and varying MSIS with 0%, 20%, 40%, 50%, 60%, 80% and 100%. The effect of the siliceous fineness on the properties of slurry and AAC blocks was investigated by maintaining the dosage of quick lime at 21% and MSIS at 40% with the samples T2, T12-T15. The flow chart of preparation of raw materials and samples was shown in Fig 1. Raw materials in each sample were weighed according to the mixture proportions in Table 2. The margins of weight errors of powder materials and water were controlled in ±0.2 g, and that of Al powder was controlled in ±0.02 g. The powder materials were thoroughly dry mixed before adding warm water (50 ± 1
C) and then stirred for 2 min. After that, Al powder was added and mixed with the slurry for another 45 s. The ultimate slurry was poured into 100  100  100 mm3 moulds, and it pre
cured at the temperature of 50 ± 2 C under a steam saturated condition for 2.5 h. After that, cutting the swollen up surface to flat, and demoulded to getting the green body. Finally, the green body was put into an industrial autoclave for hydrothermal reaction. The pressure in the autoclave was kept at 14 bars for 8 h before obtaining the final products. 2.3. Analysis methods The true density and specific surface area of raw materials were determined according to GB/T 208-2014 and GB/T 8074-2008, respectively. The particle distributions were detected by particle size analyzer, Marlvern Mastersizer 2000. The gas-foaming rate were carried out by recording the slurry volume value in a 250 ml measuring cylinder with 100 ml initial slurry every 2 min until the slurry stopped expanding. The compressive strength was tested according to GB/T 11969-2008. In order to compare the compressive strengths of the samples with different bulk densities, a formula of specific strength(S) was defined as S ¼ s(MPa)/D (kg/m3) (s:tested compressive strength of the AAC samples; D: bulk density of the AAC samples). Nine samples were carried out in each performance test. The chemical components of the raw materials were determined by X-ray fluorescence spectrometer. The crystal structure of the iron tailing and AAC samples were detected by a D/8 Adwance
X-ray diffractometer with Cu Ka radiation from 5 to 70
. The microtopography of iron tailing and AAC samples was characterized by using JSM-5610LV scanning electron microscope and JEM-2100F field-emission high resolution transmission electron microscopy

Table 1 Chemical composition of the raw materials (%). Component

SiO2

Al2O3

Fe2O3

CaO

MgO

Na2O

K2O

SO3

CO3

Cl

Ignition loss (%)

Silicon sand Iron tailing Cement Quicklime

90.21 42.90 17.76 2.78

4.51 10.75 3.94 1.02

1.24 7.51 4.04 0.73

0.40 12.97 61.11 73.64

0.10 7.10 1.78 1.45

0.27 2.06 e e

2.54 1.96 0.29 0.13

0.03 9.04 3.52 0.33

e e 6.32 12.7

0.03 0.10 e 0.01

0.47 4.48 0.73 6.94

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Table 2 Mixture proportion and Kalk of each sample. Sample numbera

T T T T T T T T T T T T T T T

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Siliceous materialb

Calcium material Cement

Quicklime

Silicon sand

Iron tailing

8% 8% 8% 8% 8% 8% 8% 8% 8% 8% 8% 8% 8% 8% 8%

19% 21% 23% 25% 27% 21% 21% 21% 21% 21% 21% 21% 21% 21% 21%

42% 40.8% 39.6% 38.4% 37.2% 68% 54.4% 34% 27.2% 13.6% 0% 40.8% 40.8% 40.8% 40.8%

28% 27.2% 26.4% 25.6% 24.8% 0% 13.6% 34% 40.8% 54.4% 68% 27.2% 27.2% 27.2% 27.2%

Gypsum

Al powder

W/R

Kalkc

3% 3% 3% 3% 3% 3% 3% 3% 3% 3% 3% 3% 3% 3% 3%

0.14% 0.14% 0.14% 0.14% 0.14% 0.14% 0.14% 0.14% 0.14% 0.14% 0.14% 0.14% 0.14% 0.14% 0.14%

0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6

0.42 0.46 0.51 0.55 0.60 0.33 0.39 0.51 0.56 0.68 0.86 0.46 0.46 0.46 0.46

a

Nine pieces for each sample. Siliceous material of Sample T1-T11were ball milled for 20 min, T12, T13, T14, T15 were ball milled for 10 min, 15 min, 25 min, 30 min, respectively. Kalk is the alkaline value of materials. It is usually used to quantitatively depict the property of materials and to determine the acidebase properties of materials for making autoclaved materials. b c

Fig. 1. Flow chart of preparation of raw materials and samples.

(FESEM), respectively. Thermal analysis of AAC sample was carried out with the NETZSCH STA449F3 simultaneous thermal analyzer to detect the thermogravimetric (TG) and differential scanning calorimetry (DSC) curve from 50
C to 1000
C. The heavy metal elements in iron tailings, and AAC samples were determined according to HJ/T 299-2007. The concentrations of heavy metals in the leachates were determined by Perkin Elmer Optima4300DV inductively coupled plasma atomic emission spectrometry (ICPAES).

Kalk ¼

sulfur were the main components of iron tailing, while magnesium, sodium, potassium and chlorine were the minor components. Kalk shown in Eq. (1). The Kalk of iron tailing was 0.178, indicating that the material was characteristically acidic, and that alkaline materials needed to be added to improve the Kalk. Consequently, correction materials, like cement and quicklime, were added to adjust the Kalk of mixed materials to be the most appropriate value range for making autoclaved aerated concrete blocks.

½ðCaO þ 0:93MgO þ 0:6R2 OÞ  ð0:55Al2 O3 þ 0:35Fe2 O3 þ 0:7SO3 Þ 0:93SiO2

3. Result and discussion 3.1. Characteristics of iron tailings 3.1.1. Chemical analysis The results of chemical analysis of the iron tailing are listed in Table 1. It can be observed that calcium, iron, silicon, aluminum and

(1)

3.1.2. Mineral composition The crystalline components of iron tailings are given in Fig 2. The spectra shows the major crystalline components are gypsum (CaSO4$2H2O), muscovite (KAl2Si3AlO10(OH)2) and quartz (SiO2), accompanied by minor phases including albite ((Na,Ca)Al(Si,Al)3O8) and terranovaite (NaCaAl3Si17O40$H2O). The crystalline silicate phases might be beneficial for generating calcium silicate hydrate

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Fig. 2. XRD patterns of iron tailing. Fig. 4. Particle distribution of different milling time iron tailings.

seen that the percentage of the particle size below 60 mm increased mostly during the ball milling process. 3.3. Determination of the preparation conditions

Fig. 3. SEM photograph of 20 min milled iron tailing.

(CSH) gel or calcium aluminum silicate hydrate (CASH) (Yang and Yue, 2000).

3.1.3. Morphology The SEM photograph of 20 min ball milled iron tailing is shown in Fig 3. The shape of the particles is irregular. The majority of particle size is estimated between 10 mm and 80 mm. 3.2. Pretreatment of siliceous Iron tailing is ground in a ball mill. The true density and specific surface area of different fineness iron tailing are shown in Table 3, and the particle distributions of different milling time iron tailing are presented in Fig 4. The grading analysis of iron tailing with different grinding time showed a falling tendency in increasing milling time. The specific surface area increased from 2.63 m2/g to 5.69 m2/g, while the grinding time prolonged from 10 min to 30 min. Meanwhile, from the results of particle size distribution presented in Fig 4, it can be

Table 3 True density and specific surface area of different milling time iron tailings. Milling time 3

True density (g/cm ) Specific surface area (m2/g) Particle size <80 mm (%)

10 min

15 min

20 min

25 min

30 min

2.794 2.63 59.86

2.886 3.02 76.83

2.941 4.85 83.57

2.986 5.21 87.9

3.093 5.69 91.38

3.3.1. Effects of quicklime content on properties of slurry and AAC blocks The effects of quicklime content on the slurry properties and AAC blocks mechanical property are shown in Fig 5 and Fig 6, respectively. Fig 5(a) shows the fresh slurry fluidity fluctuated between 200 mm and 230 mm with no significant tendency, meanwhile, the gas-foaming time decreased from 136 min to 90 min as quicklime content increased from 19% to 27%.With further analysis the gas-foaming rate curve depicted in Fig 5(b) indicated that all the 5 groups' slurry expanded nearly 60% in the first 20 min and then increased steadily. An increasing speed in the first 20 min was indicated when quicklime content increased from 19% to 27%. This is probably ascribed to the increasing heating rate by the increase of quicklime content. Finally, the reaction speed of Al powder was slightly accelerated by solution temperature and alkalinity. The effect of quicklime content on the properties of the AAC block was examined. As Fig 6 shows, the compressive strength increased from 2.05 MPa to 3.0 MPa, meanwhile, the specific strength increased from 3945.6 N$m/kg to 5698.3 N$m/kg as quick lime mass ratio increased from 19% to 27% which caused the Kalk value (Table 3) changed from 0.42 to 0.6. AAC is an autoclaved calcium silicon system, which strength was mainly contributed by CSH (B) and tobermorite, generated by the hydrothermal reaction between Ca(OH)2 and SiO2. The increasing of the dosage of quicklime improves the ratio of calcium to silicon of the system, and promotes the formation of CSH (B) and tobermorite, so that the compressive strength increased significantly. 3.3.2. Effects of MSIS on properties of slurry and AAC The effects of MSIS on the slurry properties are shown in Fig 7. As it's excessively congested to exhibition seven groups' gas foaming curve in one graph, there are only 4 typical dosage groups shown in the diagram, which can also demonstrate the law of effects of MSIS. As shown in Fig 7(a), fresh slurry fluidity exhibited significant ascendant tendency, meanwhile, the gas-foaming time reached its peak of 136 min under the MSIS of 60% and then decreased to 122 min keep on increasing the MSIS value. From Fig 7(b), it can be seen that with the increasing of the value of MSIS, the slurry gas-forming rate reached 0.8 nearly 10 min earlier, which

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Fig. 5. Effects of quicklime content on the slurry properties.

reduced from 3.15 MPa to 2.5 MPa; meanwhile, the specific strength decreased from 5974.6 N$m/kg to 4820.7 N$m/kg. The reduction of mechanical strength can still meet the requirements of B05, A2.5 grade in GB 11968-2006. However, as the MSIS continued increasing to 100%, it caused a more rapid reduction on compressive and specific strengths to 1.65 MPa and 3185.3 N$m/kg, respectively. It can be explained that with the MSIS increased from 0% to 100%, the mass percentage of iron tailing increased from 0% to 68%. As the fine crystalline SiO2 supplied by silicon sand gradually replaced by lower crystallinity SiO2 supplied by iron tailings, the CSH (B) and tobermorite generated during the autoclaved curing process became less dense, and finally result in the weaken of mechanical property.

Fig. 6. Effects of quicklime content on AAC blocks mechanical property.

means that the slurry gas-forming speed was accelerated only in the earlier stage, but the final gas-forming rate was not affected. The effect of MSIS on the mechanical properties of seven groups is presented in Fig 8. It can be seen that with the increase of MSIS from 0% to 50%, the compressive strength of AAC blocks gradually

3.3.3. Effects of siliceous fineness on properties of slurry and AAC The effects of siliceous fineness on the slurry properties and AAC blocks mechanical property are shown in Fig 9 and Fig 10, respectively. Fig 9(a) shows that the fresh slurry fluidity decreased when siliceous milling time was prolonged. Meanwhile, the gas-foaming time increased to 128 min at first and then decreased to 84 min. Furthermore, as shown in Fig 9(b), with the finer of siliceous

Fig. 7. Effect of MSIS value on the slurry properties.

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Fig. 8. Effects of MSIS value on AAC blocks' mechanical property.

Fig. 10. Effects of siliceous fineness on AAC blocks mechanical property.

particles, the gas-forming speed is slightly accelerated in the early stage but there is no significant influence on the final forming rate. The effect of siliceous fineness on AAC block properties is presented in Fig 10. It can be seen that longer milling time could result in higher compressive strength; meanwhile, the specific strength increased from 4728.5 N$m/kg to 5423 N$m/kg. The increasing of siliceous fineness can enhance the compressive strength efficiently. This mainly attributed to the higher reaction degree of SiO2 with finer siliceous, so that more CSH (B) and tobermorite could be formed during the autoclaving process (Isu et al., 1995). However, to obtain finer siliceous requires more energy, which increases the cost of production. Considering the mechanical property of final products and the ball mill energy consumption, the optimum milling time is 20 min and the corresponding proportion of particle size under 80 mm is 83.57%.

to 8 h, the compressive strength of AAC blocks was gradually enhanced. This is mainly attributed to the increasing quantity of CSH gel and tobermorite, which make the largest contribution to the strength among the hydration products, with the prolonging of autoclaved curing time. However, as the time reached to 10 h, the compressive strength dramatically decreased. For instance, the samples with 100% MSIS even reduced to 0.65 MPa. This may ascribe to the over-transforming from CSH gel to tobermorite, which makes the samples exceeding the optimum ratio of CSH gel to tobermorite, and finally resulted in the significant reduction of compressive strength. As the samples had similar bulk density, the specific strength shows the same law on the effects of steam pressure maintaining time in Fig 11(b). Furthermore, at any pressure maintaining time, the compressive strength decreased obviously when MSIS increased from 0% to 100%. Thus it can be seen that the silicon sand can't be 100% substituted by iron tailing as siliceous source. Consequently, based on the experimental results above, the most appropriate quicklime content, MSIS, siliceous fineness and steam pressure maintaining time are determined as follows: quicklime content 21%e27%, 20 min ball milled siliceous 62%e68% (with 40%e60% substituted by iron tailing), and at 1.4 MPa steam pressure maintaining for 8 h. Accordingly the optimum Kalk is 0.46e0.60 accordingly.

3.3.4. Effect of steam pressure maintaining time on properties of AAC block The samples of different steam pressure maintaining time are prepared according to sample T2 and autoclaved cured with the pressure maintaining for 4 h, 6 h, 8 h, and 10 h at 1.4 MPa steam pressure. The results are shown in Fig 11. It can be seen from Fig 11(a) that with the pressure maintaining time increasing from 4 h

Fig. 9. Effects of siliceous fineness on slurry properties.

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Fig. 11. Effect of MSIS value and pressure remain time on mechanical property.

3.4. Strength mechanism of AAC blocks 3.4.1. FESEM and EDX analysis The micro-morphology of wall section between artificial pores is shown in Fig 12(a). The dough-like particles is believed to be CSH gel partly incorporating with Mg, Al, K and Fe, which is generated at the crust surface of the silicon sand. The surface microstructure of one artificial pores in AAC sample (Fig 12(b)) shows that, long-strip shaped anhydrite interpenetrating in CSH gel with tobermorite crystals were presented. In Fig 13, flake-like tobermorite was formed by interfingering and overlapping with each other on the CSH gel; meanwhile, each CSH gel unit is combined through the skeletons formed by tobermorite crystals. The interconnected microstructure constituted the main source of compressive strength of the AAC samples (Bonaccorsi et al., 2005; Oh et al., 2012). The interconnected and cavity microstructure resulted in the high compressive strength, good thermal insulation and heat preservation performance of AAC blocks. 3.4.2. X-ray diffraction (XRD) analysis XRD analysis is executed to investigate the phase change in the AAC block during the process of procedure of T2. The XRD patterns of the dry mixture (DM) of raw materials, hardened aerated concrete (HAC) sample pre-cured under the saturated steam curing for 12 h after being mixed with warm water and the final AAC sample are shown in Fig. 14. As indicated in DM's spectrum, most minerals in iron tailing are identified, except albite and terranovalte. The non-detection of these two minerals is likely due to the fact that the overall amount of those minerals originally comprising small amount in iron tailing were in more minor quantities after addition into the dry mixture with only 27.2% in weight, and XRD was not sensitive enough to allow detection at such low level. As added with silicon sand, gypsum and cement, the quartz, gypsum and alite were detected as expected. Meanwhile, the diffused peaks at 18
and 34
of 2q are caused by low crystallinity portlandite from quicklime and cement. It can be seen from HAC spectrum that, after being pro-cured under the saturated steam curing for 12 h, the diffraction peak of alite disappeared and that of gypsum weakened obviously. Meanwhile, a new phase, with diffraction peaks at 9.1
, 15.8
and 27.5
of 2q indicated the formation of ettringite (Aft). The broad band at




around 20 e35
of 2q indicated the existence of CSH gel (Li et al., 2011; Bensted and Barnes, 2002). The transformation of the diffraction peaks was attributed to the hydration reaction between gypsum and cement clinker. From the AAC spectrum, it can be seen that the final AAC products mainly consisted of tobermorite-11 Å, anhydrite, hydrogarnet, and some residual minerals including quartz and calcite accompanied by ferric oxide and muscovite in minor quantities. Through the autoclaving process, the hydration products, AFt and CSH gel, which are formed during the pre-curing, transformed into tobermorite and anhydrite. The reduction of the characteristic peaks intensity quartz indicated that the fine quartz particles participated in the hydrothermal reaction. In addition, the chemical elements decomposed from iron tailing are absorbed into the tobermorite crystals through the transfer of AFt and CSH gel which leaded to numerous tobermorite crystals with different lattice parameters and broad half width of the tobermorite peaks in XRD patterns. The gypsum diffraction peaks disappeared after autoclave curing which is ascribed to the gypsum decomposition at the temperature of 180e220
C in autoclaving. 3.4.3. Thermal analysis The result of DSC and TG analysis of AAC sample of T2 between
50 Ce1000
C is presented in Fig 15. According to FESEM-EDX and XRD analysis of AAC sample of T2, the main composition included CSH(B), tobermorite, quartz and calcite, accompanied by ferric oxide, xonotlite and muscovite in minor quantities. As the complex mineral composition of AAC sample, the TG and DSC curve are compounded with multi minerals, nevertheless, some mineral can still be reflected on the outstanding peaks of the curves. CSH (B) and tobermorite removed the bound water and transformed into CSH (A) with 4.6% mass decrement between 100
C to 350
C. The 472
C and 541.6
C exothermic peaks are related to the oxidation reactions of the unreacted ferric oxide in iron tailings with slight weight increment in the TG curve. With further heating, 714
C endothermic peak is related to the decomposition of calcite, from which generated CaO accompanied with obvious weight loss. The 804.1
C endothermic peak is related to the dehydration of xonotlite, transforming into beta-wollastonite. The peak at 843.8
C supposed to be the transformation of the dehydrated CSH (B) and tobermorite into beta-wollastonite with highly exothermic. Therefore, the thermal analysis present the similar mineral

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Fig. 12. FESEM and EDX analysis of T2 AAC sample.

Fig. 13. FESEM photograph of T2 AAC sample.

Fig. 14. XRD patterns of DM, HAC and AAC samples.

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The main minerals in the AAC are dough-like CSH gel, flake-like tobermorite and long-strip anhydrate besides quartz and other residual minerals from the iron tailings. The interconnected microstructure of CSH gel and tobermorite constituted the main source of compressive strength. The thermal analysis also proved the existence of the main minerals. According to the result of the toxic metal elements leaching test, the iron tailing AAC blocks are not harmful to the environment. Consequently, it is applicable to manufacture AAC blocks with iron tailing. Acknowledgments This work was supported by “The National Technology Support Project of China” (Project NO: 2011BAJ03B03). Fig. 15. DSC and TG analysis of AAC sample T2.

References Table 4 Concentrations of heavy metals in the leachates of iron tailings and autoclaved blocks (mg/L). Heavy metals

Cd

Pb

Cr

Leachates of un-milled iron tailing Leachates of 20 min milled iron tailing Leachates of iron tailings autoclaved block (8 h) Regulatory limit of GB 5085.3e2007

0.015

N.D

N.D

Zn 0.052

0.004

0.021

N.D

N.D

0.055

0.005

N.D

N.D

N.D

0.011

0.002

1.0

5.0

15

100

As

5.0

*N.D: None Detected.

composition with the XRD and morphology analysis of AAC sample (Yang and Yue, 2000; Klimesch and Ray, 1999). 3.5. Leaching toxicity To examine and compare the leaching toxicity of iron tailings and its AAC block prepared under the preparation condition, heavy metals such as Cd, Pb, Cr, Zn, and As in the leachates are measured by using sulfuric acid and nitric acid leaching methods (The State Environmental Protection Administration of the People's Republic of China, 2007), and their concentrations are shown in Table 4. The concentrations of Pb and Cr were lower than the detection limit. The concentrations of Cd, Zn and As in 20 min milled iron tailing were higher than those of Cd, Zn and As in un-milled iron tailing which is mainly ascribed to the smaller particles after milling. The concentrations in the leachates of 8 h autoclaved iron tailings AAC block are much lower than the iron tailing leachates and far below the regulatory limit of GB 5085.3e2007, which indicated that the block presents no harm to the environment. 4. Conclusions In this study, the technological parameters for preparing lightweight iron tailing AAC blocks are recommended as follows: cement 8%, quicklime 21%e27%, 20 min ball milled siliceous 62%e 68% (with 40%e60% substituted by iron tailings), gypsum 3%, W/R 0.6, Al powder 0.14% and at 1.4 MPa steam pressure maintaining for 8 h. Under this conditions, the bulk density can be obtained between 490 and 525 kg/m3, compressive strength higher than 2.5 MPa and specific strength higher than 4700 N m/kg. Dry bulk density and compressive strength can satisfy the requirements of B05, A2.5 grade in the Chinese national standard GB 11968-2006.

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Please cite this article in press as: Ma, B.-g., et al., Utilization of iron tailings as substitute in autoclaved aerated concrete: physico-mechanical and microstructure of hydration products, Journal of Cleaner Production (2016), http://dx.doi.org/10.1016/j.jclepro.2016.03.172

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Please cite this article in press as: Ma, B.-g., et al., Utilization of iron tailings as substitute in autoclaved aerated concrete: physico-mechanical and microstructure of hydration products, Journal of Cleaner Production (2016), http://dx.doi.org/10.1016/j.jclepro.2016.03.172