Environ Sci Pollut Res DOI 10.1007/s11356-014-3816-5
RESEARCH ARTICLE
Solidification/stabilization and leaching behavior of PbCl2 in fly-ash hydrated silicate matrix and fly-ash geopolymer matrix Yang Li & Xingbao Gao & Qi Wang & Jie He & Dahai Yan
Received: 27 August 2014 / Accepted: 3 November 2014 # Springer-Verlag Berlin Heidelberg 2014
Abstract Fly ash (FA) for reuse as a construction material is activated using two methods, to produce hydrated silicate and geopolymer gels. We investigated the solidification/ stabilization and leaching behavior of PbCl2 in a geopolymer matrix (GM) and hydrated silicate matrix (HSM), based on FA as the source material, to evaluate the environmental and health risks. The GM and HSM synthetic conditions were 60 °C, 20 % relative humidity (RH), and 12 wt% (6 mol/L) NaOH, and 20±2 °C, ≥90 % RH, and 30 wt.%, respectively, based on their compressive strength performances. X-ray diffraction (XRD) showed that Pb participated in hydration and geopolymerization, and was incorporated in the structural components of the hydrated silicate and geopolymer. In leaching experiments, the solidification/stabilization effects of Pb and Cl in the HSM and GM improved with increasing curing time. After long-term curing (28 days), the immobility of Pb in the GM was better than that in the HSM. Sodalite improved the Cl-stabilizing ability of the GM compared with that of the HSM. In static monolithic leaching experiments, HSM and GM had the same Pb-leaching behaviors. Based on the changes in the location of the neutral sphere layer with decreasing acid-neutralizing capacity, Pb release was divided into alkaline-release, stagnation, and acid-release stages. The Responsible editor: Philippe Garrigues Electronic supplementary material The online version of this article (doi:10.1007/s11356-014-3816-5) contains supplementary material, which is available to authorized users. Y. Li : X. Gao : Q. Wang (*) : J. He : D. Yan State Key Laboratory of Environmental Criteria and Risk Assessment, Chinese Research Academy of Environmental Sciences, Beijing 100012, China e-mail: [email protected] Y. Li : Q. Wang College of Water Sciences, Beijing Normal University, Beijing 100875, China
neutral sphere layer contained the highest Pb concentration during permeation toward the block center from the block edge. This behavior regulation could also apply to other amphoteric metals immobilized by GMs and HSMs. Keywords Fly ash . Construction material . Hydrated silicate . Geopolymer . Lead . Leaching . Solidification/ stabilization
Introduction The Chinese electric power industry depends mainly on thermal power. Fly ash (FA) produced by coal-fired power stations is a major industrial waste, and much attention has been focused on its reuse. “The 12th five-year plan for major industrial solid waste comprehensive use”, proposed by the Ministry of Industry and Information Technology of China, stated that the comprehensive reuse rate of FA should reach 70 % by 2015, i.e., 396 million tons of FA should be reused (Ministry of Industry and Information Technology of the People’s Republic of China 2011). FA is rich in elements such as Si, Al, and Ca, and the production of construction materials is the most important way of digesting and using FA. In 2011, the Chinese cement industry produced 2.085 billion tons of cement, including 1.307 billion tons of clinker, and reused more than 0.88 billion tons of industrial solid waste (including FA) (China Building Materials Federation 2012). According to statistics from the China Bricks & Tiles Industrial Association, over 300 million tons of FA were reused in brick and tile production (China Bricks and Tiles Industrial Association 2011). At present, two activation methods for producing FAs with gel properties are used. One is reusing FA as a filler in Portland cement, in which FA hydration is activated by the large amount of Ca(OH)2 released from the reactions of
Environ Sci Pollut Res
cement and water; the FA is transformed into a hydrated silicate matrix (HSM) (Chen et al. 2009; Cocke 1990). The other is geopolymerization, using FA as the source material and an alkali-metal hydroxide or silicate as the activator; this transforms the FA into a geopolymer matrix (GM) (Buchwald et al. 2007; Temuujin et al. 2009). A geopolymer is a crosslinked long-chain inorganic polymeric material, with tetrahedral AlO4 and SiO4 units forming three-dimensional structures. The linkages of the AlO4 and SiO4 units require charge balancing from alkali-metal ions such as Li+, Na+, and K+ (Hanjitsuwan et al. 2014). Geopolymers are highly amorphous materials, and research has shown that the structures of geopolymers are similar to those of zeolites (Álvarez-Ayuso et al. 2008; Provis and Van Deventer 2009; Rovnaník 2010). Geopolymers have excellent acid and weather resistances, and good thermal and mechanical properties (Olivia and Nikraz 2012; Temuujin et al. 2011). More importantly, geopolymer production reduces CO2 emissions by 80–90 %, compared with cement production (Duxson et al. 2007; Izquierdo et al. 2009; Temuujin and van Riessen 2009; Van Deventer et al. 2012). Many researchers believe that geopolymers can replace cement as new environmentally friendly building materials (Duxson et al. 2007; Phoo-ngernkham et al. 2014; Van Deventer et al. 2012; Xu et al. 2010). Some researchers have found FA contained trace toxic elements, such as Pb, Cd, Cr, so the reusing of FA might pose health risks (Shaheen et al. 2014). Pb bioaccumulates and has biological toxicity; it causes chronic damage to the hematopoietic and digestive systems, and seriously damages the human nervous system, especially the immature nervous system, so babies, children, and pregnant women exposed to Pb pollution face high health risks (Banfalvi et al. 2012; Lee and Freeman 2014; Yoshinaga et al. 2014). Environmental and health safety has therefore become a key consideration when using FA as a source material for construction, and solidification/stabilization (S/S) of Pb by HSMs and GMs has recently received considerable attention. Not all heavy metals can be solidified efficiently by HSMs, and many studies have shown that the immobilization of Pb in HSMs is unstable (Janusa et al. 2000; Lin et al. 1996; Zain et al. 2004). However, a number of laboratory-scale studies have shown that GMs work well for Pb S/S (Chindaprasirt et al. 2009; Guo et al. 2010; Lloyd et al. 2009). In terms of engineering applications, Ogundirana found that after immobilization by a GM, the release of Pb in many leaching experiments was below the detection limit of the instrument (Ogundiran et al. 2013), but
Onisei observed that the Pb solubility increased significantly at pH ≥11 (Onisei et al. 2012). In this study, the immobility and release of Pb in HSMs and GMs were compared to provide a basis for the evaluation of environmental and health safety when FA is reused as a construction material only or as a raw material for the building and treatment of hazardous wastes (such as municipal solid waste incineration FA).
Materials and methods Materials The FA was obtained from a lignite-fired power plant (42.26° N, 116.02° E) Inner Mongolia province, and was secondary FA (Ф 45∼100 μm), according to the Chinese National Standard for Fly Ash for Silicate Building Products (JC/T 409–2001). The fineness of FA is 17.5 % (80-μm square hole sieve), ignition loss is 6.55 % and specific surface area 609 m2 kg−1. The major constituents of the FA are shown in Table 1. Cement clinker was obtained from a cement plant (40.03° N, 116.84° E) located in Hebei province. It was used as an HSM activator after crushing (Ф 45∼80 μm) using a ball mill (SMΦ500×500, Shangyushi Xiaojin Assay Equipment Co., Ltd., China) and mixing with 5 % gypsum. The fineness of cement clinker powder is 9.5 % (80-μm square hole sieve), ignition loss is 2.04 %, and specific surface area is 321 m2 kg−1. The main constituents of the clinker are shown in Table 2. Sample preparation FA (500 g) was used to prepare a cement-activated FA hydrated silicate and an alkali-activated FA geopolymer. The added amount of Pb was 5 g (1 % with respect to FA), so the added amount of PbCl2 was 6.714 g. The best curing conditions and activator dosage were determined using compressive strength tests. FA and heavy metal was blending by rotary oscillator (Zhonglv Shiye Co., Ltd., China) for 8 h before the HSM and GM making and the well stirring and compacting by vibration was done during the HSM and GM making for homogeneity. HSM and GM blended with PbCl2 (denoted by HSM-Pb and GM-Pb) were used after curing for 7 or 28 days. All chemical reagents used were analytically pure.
Table 1 FA constituents Element Content/% Element Content/%
SiO2 53.64 SO3 0.43
Al2O3 20.04 P2O5 0.45
Fe2O3 6.36 MnO 0.08
CaO 7.34 PbO 0.04
K2O 1.77 Cr2O3 0.03
TiO2 1.23 ZnO 0.03
MgO 3.19 Ga2O3 0.01
Na2O 4.98 CuO 0.01
Environ Sci Pollut Res Table 2 Cement constituents Element SiO2 Al2O3 Fe2O3 CaO MgO K2O Na2O P2O5 SO3 Content/ 22.26 5.0 3.38 64.95 0.25 0.6 0.19 0.1 0.41 %
Compressive strength tests The best curing conditions and activator dosage for the GM was determined based on the compressive strength performance. The curing conditions for GM have six levels of temperature factor from 20 to 70 °C and two levels of humidity factor including 20 % and ≥95 % relative humidity (RH), the activator (NaOH) dosage has five levels, i.e., 3 wt% (1.5 mol/L), 6 wt% (3 mol/L), 9 wt% (4.5 mol/L), 12 wt% (6 mol/L), and 15 wt% (7.5 mol/L). The compressive strength tests complied with the Chinese National Standard Cement Mortar Strength Testing Method (GB/T17671-1999). FA, standard sand (China ISO Standard Sand, Xiamen ISO Standard Sand Co., Ltd., China), and deionized water (Milli-Q Gradient, Millipore Co., Ltd., U S A ) w e r e u s e d . T h e m a t e r i a l s ( FA + a c t i v a t o r (NaOH/cement clinker powder): standard sand: water = 1:3:0.5/w:w:w) were mixed uniformly and injected into a mold (40 mm×40 mm×160 mm). After vibration compaction, the samples were cured in a curing box (YH-40B, Beijing Zhongjiao Engineering Instrument Research Institute, China) at constant temperature and humidity. Samples cured for 3 days were used in the compressive strength tests. An electrohydraulic compression testing machine (TZA-300, Wuxi Xinluda Instrument Equipment Co., Ltd., China) was used, with a loading rate of 2400±200 N/s. The final value of the compressive strength was the average of six blocks. Leaching method The leaching method conformed to the Chinese National Standard for Solid Waste Leaching Toxicity Method— Sulfate Nitrate Leaching Method (HJ/T299-2007). The leaching reagent was prepared at pH 3.20±0.05, using a mixed acid (H2SO4/HNO3 =2:1/m:m) and deionized water. After curing for 7 or 28 days, the crushed sample (150 g) and 1.5 L of leaching reagent (L/S=10 L/1 kg) were placed in a 2-L bottle. The bottle was oscillated for 18±2 h on a rotary oscillator (Zhonglv Shiye Co., Ltd., China), at a rotating speed of 30±2 r/min at 23±2 °C. The leachate was examined after filtration. Static monolithic leaching tests Static monolithic leaching tests were performed at a constant pH of 3.20, L/S =50 L/1 kg. The leaching reagent was
prepared at pH 3.20 using a mixed acid (H2SO4/HNO3 =2:1/ m:m) and deionized water, the pH-stat automatic titrator (ZDJ400DH Beijing Xianquweifeng technology development Co., Ltd., China) was used to keep the leaching reagent at the same pH with time. The test block (cured for 28 days, 40 mm× 40 mm×40 mm) was leached for 21 days. The leachate was sampled at different times. At the beginning 3 days of the experiments, it was sampled three times a day at 8 am, 4 pm, and 12 pm, respectively; after 3 days, it was sampled once a day at 12 am until the end of the experiments. The test block was dried naturally and drilled for sampling when the static monolithic leaching test was finished. Twelve sampling points were located on one center cross-section at different depths within the block (Fig. S1). A desktop drill (ZQS4116, Shanghai ShuaiLi Hardware Machinery Co., Ltd., China) was used to obtain samples. The drilled samples were analyzed after acid digestion. The Kriging interpolation method was used for calculations and data analysis. Surfer 8.0 software was used to draw the isogram of Pb concentration in a 40×40 boundary. Detection The presence of crystalline phases in the matrix cured for 28 days was investigated using X-ray diffraction (XRD; X’Pert Pro MPD, PANalytical Co., Ltd., the Netherlands). The XRD patterns were collected by diffractometer using Cu Kα radiation (40 kV and 40 mA) in the 2θ range 10°∼80°, with a step size of 0.033°/step and collection speed of 22 s/step. Jade 5.0 (PDF2004) software was used for crystal identification. Microstructural images were obtained using scanning electron microscopy (SEM; S-4800, Hitachi High-Tech Instruments Co., Ltd., Japan) at an accelerating voltage of 10.0 kV, a working distance of 8.0 mm, and a magnification of ×200,000. The Pb concentrations were determined using inductively coupled plasma mass spectrometry (Agilent 7500a, Agilent Technology Co., Ltd., USA) with a detection limit of 0.01 μg/L. Cl ion concentrations were determined using ion chromatography (ICS-1000, DIONEX Co., Ltd., USA), with a detection limit of 10 μg/L.
Results and discussion Compressive strength and synthetic conditions The compressive strength decides whether FA can be reused as building materials, so the best curing conditions and activator dosage for the HSM and GM were determined based on the compressive strength performance.
Environ Sci Pollut Res
Geopolymer microstructure Figure 3 shows the geopolymer microstructure at a resolution of 200 nm, and the Na, Al, Si, and O are major elements in geopolymer gel (Fig. S2). Three characteristics are clearly 30 20%RH 25
≥95%RH
MPa
20 15 10 5 0 20
30
40
50
60
70 °C
Fig. 1 GM compressive strengths under different curing conditions
20 18 16 14 12
MPa
For the synthetic conditions of GM, the curing conditions are the basis of the activator (NaOH) dosage, so the curing conditions were confirmed firstly. The curing conditions include the temperature and humidity (Rovnaník 2010). Figure 1 shows that the best presence of compressive strength was obtained at ≥60 °C and 20 % RH. Some studies have shown that overheating leads to weakening of the geopolymer mechanical properties (Rovnaník 2010) and wastes energy; therefore, 60 °C was selected as the curing temperature (Chindaprasirt et al. 2009; Guo et al. 2010). The activator dosage has a significant influence on geopolymerization, because it determines the amounts of Si and Al dissolved out from the FA (Hanjitsuwan et al. 2014). Five activator dosages, from 3 wt% (1.5 mol/L) to 15 wt% (7.5 mol/L), were tested. The maximum compressive strength was achieved with a 12 wt% (6 mol/L) dosage (Fig. 2). The conditions used for FA-GM synthesis were therefore 60 °C, 20 % RH, and a 12 wt% (6 mol/L) dosage of NaOH. For the HSM, the curing conditions were 20±2 °C and ≥90 % RH, in compliance with the Cement Mortar Strength Testing Method (GB/T17671-1999). Preliminary experiments showed that an activator (cement) dosage of 30 wt% gave an average HSM compressive strength of over 10 MPa, and the characteristics of the FA-HSM were not destroyed by excess cement. Additionally, the cement dosage for non-burnt brick industrial production is in commonly less than 30 % (3∼28 %) (Zhengjia and Yanjun 2008). The conditions, 20±2 °C, ≥90 % RH, and 30 wt% dosage of cement, were therefore used for FA-HSM synthesis.
10 8 6 4 2 0
3
6
9
12
15 %
Fig. 2 GM compressive strengths at different activator dosages
observed. The gel formed by dissolution of Si and Al in the alkali-activated environment is amorphous. The structure is porous, implying nucleation. The compact layer structure shows the capacity for physical encapsulation. These structural characteristics are consistent with numerous observations in other studies (Álvarez-Ayuso et al. 2008; Izquierdo et al. 2009; Nath and Kumar 2013; Rovnaník 2010).
XRD analysis Erionite, sodalite, KPbO2, and a Pb–Si–Al polymer were identified in GM-Pb using XRD (Fig. 4a). These substances have been found in previous studies (Álvarez-Ayuso et al. 2008; De Silva and Sagoe-Crenstil 2008). The presence of erionite indicated the occurrence of geopolymerization of the Si–Al phase (Provis and Van Deventer 2009); some researchers have suggested that zeolites can influence the mechanical properties and S/S effects of heavy metals (De Silva and Sagoe-Crenstil 2008). The presence of sodalite means that some Cl ions in GM-Pb are no longer in a free state, but incorporated into sodalite. The presence of KPbO2 showed the amphoteric nature of Pb. The Pb–Si–Al polymer illustrated that Pb could enter into the microcrystalline structure and act as a charge balancer and cross-linker for AlO4 and SiO4 units, like Na+ and K+ (Ogundiran et al. 2013). In HSM-Pb, a Pb-containing silicate and hydroxide, and plumbate were identified by XRD (Fig. 4b). Pb acted as an amphoteric heavy metal, and Pb bonded with Si–O and formed Pb-containing silicate. This phenomenon has been reported in many papers. In Lee’s research, it was concluded that the formation of Pb-containing silicate was based on the adsorption and encapsulation of Pb in the form of hydroxide and carbonate by a hydrated silicate gel (Lee 2006). Gollmann found that adding Pb2+ to silicate mortar during the hydration process led to the loss of Ca2+ from the gel; therefore, Pb could replace Ca during hydration (Gollmann et al. 2010).
Environ Sci Pollut Res Fig. 3 SEM images of GM microstructure
Identification for Extraction Toxicity (GB5085.3-2007). It was observed that the leaching concentrations of Pb from GM-Pb and HSM-Pb both decreased with increasing curing time, implying that this phenomenon can be attributed to increasing matrix volume density with increasing curing time (Rhan and Kürklü 2014). The decrease in Pb leaching with increasing curing time was larger for GM-Pb than for HSMPb, and GM showed better S/S effects for Pb after 28 days
Leaching experiments Pb-leaching concentration Figure 5 shows the Pb concentrations in GM-Pb and HSMPb-leaching experiments. Both of the them below the limit value (5 mg/L) required by Chinese National Standard Identification Standards for Hazardous Wastes— 2000
Fig. 4 XRD patterns of GM-Pb and HSM-Pb
Maricopaite - Pb7Ca2(Si,Al)48O100!32H2O KPbO2 - Potassium Lead Oxide Sodalite (Se-doped) - Na7.84(Al6Si6O24)Cl1.86 Erionite-K - K0.28Na0.192Al2Si10.43O25.86!xH2O
3
1 2 3 4
Intensity(Counts)
1500
1000
4
500
4
2
2
2
3
1
1 4
3
0 10
20
30
40 2-Theta(¡ã
50
60
70
(a) GM-Pb 3000
Dundasite - PbAl2(CO3)2(OH)4(H2O) Na6PbO4 - Hexasodium lead oxide Zn2PbO4 - Zinc Lead Oxide K4PbO4 - Potassium Lead Oxide Pb6O(OH)6(ClO4)4(H2O) - Lead Oxide Hy droxide Chlorate Hy drate Margarosanite - PbCa2Si3O9 Pb2ZnSi2O7 - Lead Zinc Silicate K2Pb4Si8O21 - Potassium Lead Silicate
Intensity(Counts)
2500
2000
6 1500
1000
36
500
1
2
5 1
4
7
5
4 3
1
8
2
7
0 10
20
30
40 2-Theta(¡ã
(b) HSM-Pb
50
60
70
1 2 3 4 5 6 7 8
Environ Sci Pollut Res 1400 7d
28d
1200
ug / L
1000 800 600 400
nature and the chlorine group orientates within the center of the framework of β-cage (Philip and Mark. 1995). In the aspect of leaching, Hwan-Young found the sodalite phase synthesized by zeolite and chloride contains Cl and has very low solubility in water (Hwan-Young et al. 2009). So, we infer that the sodalite is one way to immobilize Cl besides physical encapsulation. Static monolithic leaching experiments
200
Release behavior of Pb
0 GM
HSM
Fig. 5 Pb release in leaching of GM-Pb and HSM-Pb
curing than HSM did. The Pb fixation rates of GM and HSM were 99.9985 and 99.9917 % after 28-day curing, respectively. Release of Cl Chlorides are the principal cause of steel reinforcement corrosion in concrete structures (Arya and Xu 1995); therefore, Cl ion release was investigated (Fig. 6). It was concluded that the immobility of Cl ions was positively correlated with the curing time, and GM always performed better than HSM. Isabella also found clear inhibition of Cl ion release by geopolymers, using municipal solid waste incineration fly ash as the source material (Lancellotti et al. 2010). The sodalite was identified by XRD, which proved that sodalite can be synthesized at low temperature. JosefChristian also found that high alkaline salt bearing gels are adequate educts to synthesize sodalite at a very low temperature of 60 °C, and he thought the amorphous contribution within source material acts as a sodalite precursor during low temperature crystallization (Josef-Christian et al. 2011). In the aspect of structure, Philip thought the sodalite has stabilizing
The changes in the accumulative Pb concentrations in GM-Pb and HSM-Pb are shown in Fig. 7. It is obvious that the amount of Pb released from HSM-Pb was higher than that from GMPb; this is consistent with the results of the leaching experiments. The Pb-release behavior was analyzed. The analysis was based on two assumptions: the extrinsic mass-transfer power of Pb from the solid to the liquid was constant, because the pH of the environment was constant, and the microporous structures of both matrixes were homogeneous, so the main masstransfer resistance arose from the acid-neutralizing capacity (ANC) of the matrix. Pb is an amphoteric metal (detected by XRD) and is soluble in acid and alkaline environments (Lee 2006; Onisei et al. 2012). It has been found that leaching behavior of lead, zinc (amphoteric metals) strongly depends on the leachate pH in many literatures (Radu et al. 2005; Valérie and Rudy 2008; Onisei et al. 2012). Therefore, the release behavior of Pb with decreasing ANCs of the GM and HSM was divided into three stages (Fig. 8). Stage one was 450
HSM HSM step
400
GM GM step
350 300
1800
28d
ug / L
7d 1600 1400
250 200
mg / L
1200
150
1000 800
100
600
50
400
0 200
6h
12h
24h
3d
6d
9d
12d
15d
18d
21d
time
0 GM
HSM
Fig. 6 Cl ion release in leaching of GM-Pb and HSM-Pb
Fig. 7 Changes in Pb accumulative concentrations in leaching in GM-Pb and HSM-Pb
Environ Sci Pollut Res 100
3
90
2
80
1
concentration
70 60 50 40 30 20 10 0 1
2
3
4
5
6
7
8
9
10
time Fig. 8 Schematic diagram of Pb-release behavior
defined as the alkaline-release stage. In this stage, Pb, being alkali soluble, was released quickly, because the environment around the test block was alkaline as a result of the adequate ANC of the matrix, and the slope of the curve was high. Stage two was defined as the stagnation stage. With attenuation of the ANC, the pH value around the block, even at the block edge, approached 7; we call this pH environment as the neutral sphere layer, and Pb was precipitated in the neutral sphere layer. Consequently, because of the blockade formed by the neutral sphere layer, Pb mass transfer almost stopped. The third stage was defined as the acid-release stage. As the neutral sphere layer moved toward the center of the block
(a) GM Fig. 9 Pb distributions inside GM-Pb and HSM-Pb after leaching test
increasingly, acid-soluble Pb was rapidly released in the acid-permeated matrix and the curve became steeper again. Mohammad found the consumption of ANC in cement matrix forms two consecutive leaching fronts, the leaching of Zn and Pb only occurs between the second front and the specimen surface, and the leaching behavior of metals is modeled by the shift of the fronts (Mohammad et al. 2004a). Mohammad also found the shift of the fronts is influenced by pH (Mohammad et al. 2004b). These conclusions have supported our inference about the three-stage release. In Jianguo’s researches, the leaching behavior of Pb from geopolymer mortar with 1 % Pb in H2SO4 solution has presented threestage, i.e., released quickly, stagnated, and then, accelerated to released again (Jianguo et al. 2008). Using these behavior rules (Fig. 8) to reanalyze the leaching phenomenon (Fig. 7), it was found that the mass transfer of Pb occurred in the acid-release stage for both GMPb and HSM-Pb; this state suggested that the acid had at least already broken through the ANC barrier at the edge of the block. Secondly, it was observed that, for GM-Pb, the alkaline-release stage terminated at around 2 days and the acid-release stage started at approximately 6 days, while for HSM-Pb, the two time points discussed above were about 3 and 10 days, respectively; so, the three stages for GM-Pb were always ahead of those for HSM-Pb. It was therefore concluded that the ANC of the GM is lower than that of the HSM. Distribution of Pb inside matrix The distribution of Pb in the HSM was low at the center and high at the margin (Fig. 9b); conversely, in the GM, the Pb content at the center was higher than that around the margin
(b) HSM
Environ Sci Pollut Res
(Fig. 9a). There are two reasons for this: one is the ANC, and the other is the amphotericity of Pb. During leaching, the environment at the center of the matrix was highly alkaline at beginning, Pb was present as Pb(OH)3− here, and moved outwards. As the ANC decreased, the neutral sphere layer invaded the matrix and became deeply immersed, so the pH decreased from the center to the margin inside the matrix. The closer Pb(OH)3− approached the neutral sphere layer, the more Pb(OH)3− was converted to Pb(OH)2, and the slower the Pb moved. Consequentially, the Pb in the neutral sphere layer therefore stagnated. However, outside the neutral sphere layer, the Pb became acid soluble and it accelerated to spread outwards. As a result, the neutral sphere layer contained the highest concentration of Pb. Based on the above analysis, the pattern in Fig. 9b verifies the conclusion drawn in the “Release behavior of Pb”, i.e., that the acid had at least already broken through the ANC barrier at the edge of the block. Because the pattern in Fig. 9b resulted from the ANC of the matrix and the amphotericity of Pb, it was concluded that this Pb distribution mode also occurred in GM-Pb, and the pattern in Fig. 9a shows the state in which the neutral sphere layer has already reached the center at 21 days. This state is also consistent with the release status of GM-Pb shown in Fig. 7, i.e., the mass transfer of Pb occurred in the acid-release stage for GM-Pb after 21 days. The analysis suggested that the performances of Pb in both matrixes, shown in Figs. 7 and 9, obeyed one behavior rule, and the patterns in Figs. 7 and 9 just show this behavior regulation at different stages and different phases. Because this behavior is caused by the ANC of the matrix and amphotericity of Pb, it was inferred farther that this behavior regulation during leaching also applies to other amphoteric metals immobilized by GMs and HSMs, such as Zn and Cr.
Conclusions Pb participated in the hydration and geopolymerization of a FA source material, and became part of the structural components of the hydrated silicate and geopolymer. The S/S effects of Pb and Cl in the HSM and GM improved with increasing curing time. After long-term curing (28 days), the immobility of Pb in the GM was better than that in the HSM. Sodalite improved the ability of the GM to stabilize Cl compared with that of the HSM. The HSM and GM had the same Pb-releasing behavior in static monolithic leaching. As the ANC decreased, the neutral sphere layer moved from the liquid to the block surface, completed the solid–liquid interface transition, and continued to permeate toward the center of the block. Correspondingly, the Pb-release process was divided into alkaline-release, stagnation, and acid-release stages. The neutral sphere layer had
the highest Pb concentration during the period of permeation toward the block center from the block edge. The Pb concentration declined toward both sides of the neutral sphere layer, and this distribution pattern disappeared when the ANC was completely exhausted. It was concluded that this behavior regulation would also apply to other amphoteric metals immobilized by GMs and HSMs. Acknowledgments This work was supported by the State Environment Protection Commonweal Special Program, China (No. 201209023); the National Science & Technology Pillar Program, China (No. 2014BAL02B01); the Sino-Norwegian project phase II (CHN 2150 09/ 059).
References Álvarez-Ayuso E, Querol X, Plana F, Alastuey A, Moreno N, Izquierdo M, Font O, Moreno T, Diez S, Vázquez E, Barra M (2008) Environmental, physical and structural characterisation of geopolymer matrixes synthesised from coal (co-)combustion fly ashes. J Hazard Mater 154:175–183. doi:10.1016/j.jhazmat.2007. 10.008 Arya C, Xu Y (1995) Effect of cement type on chloride binding and corrosion of steel in concrete. Cem Concr Res 25:893–902 Banfalvi G, Sarvari A, Nagy G (2012) Chromatin changes induced by Pb and Cd in human cells. Toxicol In Vitro 26:1064–1071. doi:10. 1016/j.tiv.2012.03.016 Buchwald A, Hilbig H, Kapps C (2007) Alkali-activated metakaolin-slag blends—performance and structure in dependence on their composition. J Mater Sci 42:3024–3032. doi:10.1007/s10853-006-0525-6 Chen QY, Tyrer M, Hills CD, Yang XM, Carey P (2009) Immobilisation of heavy metal in cement-based solidification stabilisation a review. Waste Manag 29:390–403. doi:10.1016/j.wasman.2008.01.019 China Bricks & Tiles Industrial Association (2011) The 12th five-year plan from China Bricks & Tiles Industrial Association. http://www. cbtia.com/news/2012/5-22/H14910705.shtml. Accessed May 2012 (In Chinses) China Building Materials Federation (2012) Statistical analysis report of cement output and productivity in 2011. China Cem 3:10–12 (In Chinses) Chindaprasirt P, Jaturapitakkul C, Chalee W, Rattanasak U (2009) Comparative study on the characteristics of fly ash and bottom ash geopolymers. Waste Manag 29:539–543. doi:10.1016/j.wasman. 2008.06.023 Cocke DL (1990) The binding chemistry and leaching mechanisms of hazardous substances in cementitious solidification/stabilization systems. J Hazard Mater 24:231–253 De Silva P, Sagoe-Crenstil K (2008) Medium-term phase stability of Na2O–Al2O3–SiO2–H2O geopolymer systems. Cem Concr Res 38:870–876. doi:10.1016/j.cemconres.2007.10.003 Duxson P, Provis JL, Lukey GC, Van Deventer JJS (2007) The role of inorganic polymer technology in the development of ‘Green concrete’. Cem Concr Res 37:1590–1597. doi:10.1016/j.cemconres. 2007.08.018 Gollmann MAC, Da Silva MM, Masuero NB, Dos Santos JOHZ (2010) Stabilization and solidification of Pb in cement matrices. J Hazard Mater 179:507–514. doi:10.1016/j.jhazmat.2010.03.032 Guo X, Shi H, Dick WA (2010) Compressive strength and microstructural characteristics of class C fly ash geopolymer. Cem Concr Compos 32:142–147. doi:10.1016/j.cemconcomp.2009.11.003
Environ Sci Pollut Res Hanjitsuwan S, Hunpratub S, Thongbai P, Maensiri S, Sata V, Chindaprasirt P (2014) Effects of NaOH concentrations on physical and electrical properties of high calcium fly ash geopolymer paste. Cem Concr Compos 45:9–14. doi:10.1016/j.cemconcomp.2013.09. 012 Hwan-Young K, Jeong-Guk K, In-Tae K, Hwan-Seo P (2009) Study on the leaching behavior of salt waste forms prepared by using zeolite and lime glass. J Ind Eng Chem 15:293–298. doi:10.1016/j.jiec. 2008.11.009 Izquierdo M, Querol X, Davidovits J, Antenucci D, Nugteren H, Fernández-Pereira C (2009) Coal fly ash-slag-based geopolymers: microstructure and metal leaching. J Hazard Mater 166:561–566. doi:10.1016/j.jhazmat.2008.11.063 Janusa MA, Champagne CA, Fanguy JC, Heard GE, Laine PL, Landry AA (2000) Solidification/stabilization of lead with the aid of bagasse as an additive to Portland cement. Microchem J 65:255–259 Jianguo Z, John LP, Dingwu F, Jannie SJVD (2008) Geopolymers for immobilization of Cr6+, Cd2+, and Pb2+. J Hazard Mater 157:587– 598. doi:10.1016/j.jhazmat.2008.01.053 Josef-Christian B, Karsten S, Lars R (2011) Nanocrystalline sodalite grown from superalkaline NaCl bearing gels at low temperature (333 K) and the influence of TEA on crystallization process. Microporous Mesoporous Mater 142:666–671. doi:10.1016/j. micromeso.2011.01.020 Lancellotti I, Kamseu E, Michelazzi M, Barbieri L, Corradi A, Leonelli C (2010) Chemical stability of geopolymers containing municipal solid waste incinerator fly ash. Waste Manag 30:673–679. doi:10. 1016/j.wasman.2009.09.032 Lee D (2006) Effect of calcite on Pb-doped solidified waste forms in leaching. Chemosphere 63:1903–1911. doi:10.1016/j.chemosphere. 2005.10.018 Lee J, Freeman JL (2014) Zebrafish as a model for investigating developmental lead (Pb) neurotoxicity as a risk factor in adult neurodegenerative disease: a mini-review. Neurotoxicology. doi:10.1016/j. neuro.2014.03.008 Lin SL, Cross WH, Chian ESK, Lai JS, Giabbai M, Hung CH (1996) Stabilization and solidification of lead in contaminated soils. J Hazard Mater 48:95–110 Lloyd RR, Provis JL, Van Deventer JJS (2009) Microscopy and microanalysis of inorganic polymer cements. 1: remnant fly ash particles. J Mater Sci 44:608–619. doi:10.1007/s10853-008-3077-0 Ministry of Industry and Information Technology of the People’s Republic of China (2011) The 12th five-year plan of major industrial waste solid comprehensive utilization. http://www.miit.gov.cn/ n11293472/n11293832/n11293907/n11368223/14416612.html. Accessed Jan 2012. (In Chinses) Mohammad ZI, Lionel JJC, Ernest KY (2004a) A two-front leach model for cement-stabilized heavy metal waste. Environ Sci Technol 38: 1522–1528. doi:10.1021/es0348400 Mohammad ZI, Lionel JJC, Ernest KY (2004b) Effect of remineralization on heavy-metal leaching from cement-stabilized/solidified waste. Environ Sci Technol 38:1561–1568. doi:10.1021/es034659r Nath SK, Kumar S (2013) Influence of iron making slags on strength and microstructure of fly ash geopolymer. Constr Build Mater 38:924– 930. doi:10.1016/j.conbuildmat.2012.09.070 Ogundiran MB, Nugteren HW, Witkamp GJ (2013) Immobilisation of lead smelting slag within spent aluminate—fly ash based geopolymers. J Hazard Mater 248:29–36. doi:10.1016/j.jhazmat. 2012.12.040 Olivia M, Nikraz H (2012) Properties of fly ash geopolymer concrete designed by Taguchi method. Mater Des 36:191–198. doi:10.1016/ j.matdes.2011.10.036
Onisei S, Pontikes Y, Van Gerven T, Angelopoulos GN, Velea T, Predica V, Moldovan P (2012) Synthesis of inorganic polymers using fly ash and primary lead slag. J Hazard Mater 205:101–110. doi:10.1016/j. jhazmat.2011.12.039 Philip JM, Mark TW (1995) Synthesis, structure, and characterization of halate sodalites:M8[AlSiO4]6(XO3)x(OH)2-x; M=Na, Li, or K; X= Cl, Br, or I. Zeolites 15:561–568 Phoo-ngernkham T, Chindaprasirt P, Sata V, Hanjitsuwan S, Hatanaka S (2014) The effect of adding nano-SiO2 and nano-Al2O3 on properties of high calcium fly ash geopolymer cured at ambient temperature. Mater Des 55: 58–65. doi:10.1016/j.matdes.2013.09.049 Provis JL, Van Deventer JJS (2009) Geopolymers: structures, processing, properties and industrial applications. CRC Press, Boca Raton Radu B, Zoltan R, Ligia TB (2005) Release dynamic process identification for a cement based material in various leaching conditions. Part I Influence of leaching conditions on the release amount. J Environ Manag 74:141–151. doi:10.1016/ j.jenvman.2004.06.016 Rhan G, Kürklü GK (2014) The influence of the NaOH solution on the properties of the fly ash-based geopolymer mortar cured at different temperatures. Compos Part B 58:371–377. doi:10.1016/j. compositesb.2013.10.082 Rovnaník P (2010) Effect of curing temperature on the development of hard structure of metakaolin-based geopolymer. Constr Build Mater 24:1176–1183. doi:10.1016/j.conbuildmat.2009.12.023 Shaheen SM, Hooda PS, Tsadilas CD (2014) Opportunities and challenges in the use of coal fly ash for soil improvements—a review. J Environ Manag 145:249–267. doi:10.1016/j.jenvman.2014.07.005 Temuujin J, van Riessen A (2009) Effect of fly ash preliminary calcination on the properties of geopolymer. J Hazard Mater 164:634–639. doi:10.1016/j.jhazmat.2008.08.065 Temuujin J, van Riessen A, Williams R (2009) Influence of calcium compounds on the mechanical properties of fly ash geopolymer pastes. J Hazard Mater 167:82–88. doi:10.1016/j.jhazmat.2008.12. 121 Temuujin J, Minjigmaa A, Lee M, Chen-Tan N, van Riessen A (2011) Characterisation of class F fly ash geopolymer pastes immersed in acid and alkaline solutions. Cem Concr Compos 33:1086–1091. doi: 10.1016/j.cemconcomp.2011.08.008 Valérie C, Rudy S (2008) The application of pHstat leaching tests to assess the pH-dependent release of trace metals from soils, sediments and waste materials. J Hazard Mater 158:185–195. doi:10. 1016/j.jhazmat.2008.01.058 Van Deventer JSJ, Provis JL, Duxson P (2012) Technical and commercial progress in the adoption of geopolymer cement. Miner Eng 29:89– 104. doi:10.1016/j.mineng.2011.09.009 Xu H, Li Q, Shen L, Wang W, Zhai J (2010) Synthesis of thermostable geopolymer from circulating fluidized bed combustion (CFBC) bottom ashes. J Hazard Mater 175:198–204. doi:10.1016/j. jhazmat.2009.09.149 Yoshinaga J, Yamasaki K, Yonemura A, Ishibashi Y, Kaido T, Mizuno K, Takagi M, Tanaka A (2014) Lead and other elements in house dust of Japanese residences—source of lead and health risks due to metal exposure. Environ Pollut 189:223–228. doi:10.1016/j.envpol.2014. 03.003 Zain MFM, Islam MN, Radin SS, Yap SG (2004) Cement-based solidification for the safe disposal of blasted copper slag. Cem Concr Compos 26:845–851. doi:10.1016/j. cemconcomp.2003.08.002 Zhengjia Y, Yanjun H (2008) The practical production technology of nonburnt brick. Chemical Industry Press, Beijing (In Chinses)
RESEARCH ARTICLE
Solidification/stabilization and leaching behavior of PbCl2 in fly-ash hydrated silicate matrix and fly-ash geopolymer matrix Yang Li & Xingbao Gao & Qi Wang & Jie He & Dahai Yan
Received: 27 August 2014 / Accepted: 3 November 2014 # Springer-Verlag Berlin Heidelberg 2014
Abstract Fly ash (FA) for reuse as a construction material is activated using two methods, to produce hydrated silicate and geopolymer gels. We investigated the solidification/ stabilization and leaching behavior of PbCl2 in a geopolymer matrix (GM) and hydrated silicate matrix (HSM), based on FA as the source material, to evaluate the environmental and health risks. The GM and HSM synthetic conditions were 60 °C, 20 % relative humidity (RH), and 12 wt% (6 mol/L) NaOH, and 20±2 °C, ≥90 % RH, and 30 wt.%, respectively, based on their compressive strength performances. X-ray diffraction (XRD) showed that Pb participated in hydration and geopolymerization, and was incorporated in the structural components of the hydrated silicate and geopolymer. In leaching experiments, the solidification/stabilization effects of Pb and Cl in the HSM and GM improved with increasing curing time. After long-term curing (28 days), the immobility of Pb in the GM was better than that in the HSM. Sodalite improved the Cl-stabilizing ability of the GM compared with that of the HSM. In static monolithic leaching experiments, HSM and GM had the same Pb-leaching behaviors. Based on the changes in the location of the neutral sphere layer with decreasing acid-neutralizing capacity, Pb release was divided into alkaline-release, stagnation, and acid-release stages. The Responsible editor: Philippe Garrigues Electronic supplementary material The online version of this article (doi:10.1007/s11356-014-3816-5) contains supplementary material, which is available to authorized users. Y. Li : X. Gao : Q. Wang (*) : J. He : D. Yan State Key Laboratory of Environmental Criteria and Risk Assessment, Chinese Research Academy of Environmental Sciences, Beijing 100012, China e-mail: [email protected] Y. Li : Q. Wang College of Water Sciences, Beijing Normal University, Beijing 100875, China
neutral sphere layer contained the highest Pb concentration during permeation toward the block center from the block edge. This behavior regulation could also apply to other amphoteric metals immobilized by GMs and HSMs. Keywords Fly ash . Construction material . Hydrated silicate . Geopolymer . Lead . Leaching . Solidification/ stabilization
Introduction The Chinese electric power industry depends mainly on thermal power. Fly ash (FA) produced by coal-fired power stations is a major industrial waste, and much attention has been focused on its reuse. “The 12th five-year plan for major industrial solid waste comprehensive use”, proposed by the Ministry of Industry and Information Technology of China, stated that the comprehensive reuse rate of FA should reach 70 % by 2015, i.e., 396 million tons of FA should be reused (Ministry of Industry and Information Technology of the People’s Republic of China 2011). FA is rich in elements such as Si, Al, and Ca, and the production of construction materials is the most important way of digesting and using FA. In 2011, the Chinese cement industry produced 2.085 billion tons of cement, including 1.307 billion tons of clinker, and reused more than 0.88 billion tons of industrial solid waste (including FA) (China Building Materials Federation 2012). According to statistics from the China Bricks & Tiles Industrial Association, over 300 million tons of FA were reused in brick and tile production (China Bricks and Tiles Industrial Association 2011). At present, two activation methods for producing FAs with gel properties are used. One is reusing FA as a filler in Portland cement, in which FA hydration is activated by the large amount of Ca(OH)2 released from the reactions of
Environ Sci Pollut Res
cement and water; the FA is transformed into a hydrated silicate matrix (HSM) (Chen et al. 2009; Cocke 1990). The other is geopolymerization, using FA as the source material and an alkali-metal hydroxide or silicate as the activator; this transforms the FA into a geopolymer matrix (GM) (Buchwald et al. 2007; Temuujin et al. 2009). A geopolymer is a crosslinked long-chain inorganic polymeric material, with tetrahedral AlO4 and SiO4 units forming three-dimensional structures. The linkages of the AlO4 and SiO4 units require charge balancing from alkali-metal ions such as Li+, Na+, and K+ (Hanjitsuwan et al. 2014). Geopolymers are highly amorphous materials, and research has shown that the structures of geopolymers are similar to those of zeolites (Álvarez-Ayuso et al. 2008; Provis and Van Deventer 2009; Rovnaník 2010). Geopolymers have excellent acid and weather resistances, and good thermal and mechanical properties (Olivia and Nikraz 2012; Temuujin et al. 2011). More importantly, geopolymer production reduces CO2 emissions by 80–90 %, compared with cement production (Duxson et al. 2007; Izquierdo et al. 2009; Temuujin and van Riessen 2009; Van Deventer et al. 2012). Many researchers believe that geopolymers can replace cement as new environmentally friendly building materials (Duxson et al. 2007; Phoo-ngernkham et al. 2014; Van Deventer et al. 2012; Xu et al. 2010). Some researchers have found FA contained trace toxic elements, such as Pb, Cd, Cr, so the reusing of FA might pose health risks (Shaheen et al. 2014). Pb bioaccumulates and has biological toxicity; it causes chronic damage to the hematopoietic and digestive systems, and seriously damages the human nervous system, especially the immature nervous system, so babies, children, and pregnant women exposed to Pb pollution face high health risks (Banfalvi et al. 2012; Lee and Freeman 2014; Yoshinaga et al. 2014). Environmental and health safety has therefore become a key consideration when using FA as a source material for construction, and solidification/stabilization (S/S) of Pb by HSMs and GMs has recently received considerable attention. Not all heavy metals can be solidified efficiently by HSMs, and many studies have shown that the immobilization of Pb in HSMs is unstable (Janusa et al. 2000; Lin et al. 1996; Zain et al. 2004). However, a number of laboratory-scale studies have shown that GMs work well for Pb S/S (Chindaprasirt et al. 2009; Guo et al. 2010; Lloyd et al. 2009). In terms of engineering applications, Ogundirana found that after immobilization by a GM, the release of Pb in many leaching experiments was below the detection limit of the instrument (Ogundiran et al. 2013), but
Onisei observed that the Pb solubility increased significantly at pH ≥11 (Onisei et al. 2012). In this study, the immobility and release of Pb in HSMs and GMs were compared to provide a basis for the evaluation of environmental and health safety when FA is reused as a construction material only or as a raw material for the building and treatment of hazardous wastes (such as municipal solid waste incineration FA).
Materials and methods Materials The FA was obtained from a lignite-fired power plant (42.26° N, 116.02° E) Inner Mongolia province, and was secondary FA (Ф 45∼100 μm), according to the Chinese National Standard for Fly Ash for Silicate Building Products (JC/T 409–2001). The fineness of FA is 17.5 % (80-μm square hole sieve), ignition loss is 6.55 % and specific surface area 609 m2 kg−1. The major constituents of the FA are shown in Table 1. Cement clinker was obtained from a cement plant (40.03° N, 116.84° E) located in Hebei province. It was used as an HSM activator after crushing (Ф 45∼80 μm) using a ball mill (SMΦ500×500, Shangyushi Xiaojin Assay Equipment Co., Ltd., China) and mixing with 5 % gypsum. The fineness of cement clinker powder is 9.5 % (80-μm square hole sieve), ignition loss is 2.04 %, and specific surface area is 321 m2 kg−1. The main constituents of the clinker are shown in Table 2. Sample preparation FA (500 g) was used to prepare a cement-activated FA hydrated silicate and an alkali-activated FA geopolymer. The added amount of Pb was 5 g (1 % with respect to FA), so the added amount of PbCl2 was 6.714 g. The best curing conditions and activator dosage were determined using compressive strength tests. FA and heavy metal was blending by rotary oscillator (Zhonglv Shiye Co., Ltd., China) for 8 h before the HSM and GM making and the well stirring and compacting by vibration was done during the HSM and GM making for homogeneity. HSM and GM blended with PbCl2 (denoted by HSM-Pb and GM-Pb) were used after curing for 7 or 28 days. All chemical reagents used were analytically pure.
Table 1 FA constituents Element Content/% Element Content/%
SiO2 53.64 SO3 0.43
Al2O3 20.04 P2O5 0.45
Fe2O3 6.36 MnO 0.08
CaO 7.34 PbO 0.04
K2O 1.77 Cr2O3 0.03
TiO2 1.23 ZnO 0.03
MgO 3.19 Ga2O3 0.01
Na2O 4.98 CuO 0.01
Environ Sci Pollut Res Table 2 Cement constituents Element SiO2 Al2O3 Fe2O3 CaO MgO K2O Na2O P2O5 SO3 Content/ 22.26 5.0 3.38 64.95 0.25 0.6 0.19 0.1 0.41 %
Compressive strength tests The best curing conditions and activator dosage for the GM was determined based on the compressive strength performance. The curing conditions for GM have six levels of temperature factor from 20 to 70 °C and two levels of humidity factor including 20 % and ≥95 % relative humidity (RH), the activator (NaOH) dosage has five levels, i.e., 3 wt% (1.5 mol/L), 6 wt% (3 mol/L), 9 wt% (4.5 mol/L), 12 wt% (6 mol/L), and 15 wt% (7.5 mol/L). The compressive strength tests complied with the Chinese National Standard Cement Mortar Strength Testing Method (GB/T17671-1999). FA, standard sand (China ISO Standard Sand, Xiamen ISO Standard Sand Co., Ltd., China), and deionized water (Milli-Q Gradient, Millipore Co., Ltd., U S A ) w e r e u s e d . T h e m a t e r i a l s ( FA + a c t i v a t o r (NaOH/cement clinker powder): standard sand: water = 1:3:0.5/w:w:w) were mixed uniformly and injected into a mold (40 mm×40 mm×160 mm). After vibration compaction, the samples were cured in a curing box (YH-40B, Beijing Zhongjiao Engineering Instrument Research Institute, China) at constant temperature and humidity. Samples cured for 3 days were used in the compressive strength tests. An electrohydraulic compression testing machine (TZA-300, Wuxi Xinluda Instrument Equipment Co., Ltd., China) was used, with a loading rate of 2400±200 N/s. The final value of the compressive strength was the average of six blocks. Leaching method The leaching method conformed to the Chinese National Standard for Solid Waste Leaching Toxicity Method— Sulfate Nitrate Leaching Method (HJ/T299-2007). The leaching reagent was prepared at pH 3.20±0.05, using a mixed acid (H2SO4/HNO3 =2:1/m:m) and deionized water. After curing for 7 or 28 days, the crushed sample (150 g) and 1.5 L of leaching reagent (L/S=10 L/1 kg) were placed in a 2-L bottle. The bottle was oscillated for 18±2 h on a rotary oscillator (Zhonglv Shiye Co., Ltd., China), at a rotating speed of 30±2 r/min at 23±2 °C. The leachate was examined after filtration. Static monolithic leaching tests Static monolithic leaching tests were performed at a constant pH of 3.20, L/S =50 L/1 kg. The leaching reagent was
prepared at pH 3.20 using a mixed acid (H2SO4/HNO3 =2:1/ m:m) and deionized water, the pH-stat automatic titrator (ZDJ400DH Beijing Xianquweifeng technology development Co., Ltd., China) was used to keep the leaching reagent at the same pH with time. The test block (cured for 28 days, 40 mm× 40 mm×40 mm) was leached for 21 days. The leachate was sampled at different times. At the beginning 3 days of the experiments, it was sampled three times a day at 8 am, 4 pm, and 12 pm, respectively; after 3 days, it was sampled once a day at 12 am until the end of the experiments. The test block was dried naturally and drilled for sampling when the static monolithic leaching test was finished. Twelve sampling points were located on one center cross-section at different depths within the block (Fig. S1). A desktop drill (ZQS4116, Shanghai ShuaiLi Hardware Machinery Co., Ltd., China) was used to obtain samples. The drilled samples were analyzed after acid digestion. The Kriging interpolation method was used for calculations and data analysis. Surfer 8.0 software was used to draw the isogram of Pb concentration in a 40×40 boundary. Detection The presence of crystalline phases in the matrix cured for 28 days was investigated using X-ray diffraction (XRD; X’Pert Pro MPD, PANalytical Co., Ltd., the Netherlands). The XRD patterns were collected by diffractometer using Cu Kα radiation (40 kV and 40 mA) in the 2θ range 10°∼80°, with a step size of 0.033°/step and collection speed of 22 s/step. Jade 5.0 (PDF2004) software was used for crystal identification. Microstructural images were obtained using scanning electron microscopy (SEM; S-4800, Hitachi High-Tech Instruments Co., Ltd., Japan) at an accelerating voltage of 10.0 kV, a working distance of 8.0 mm, and a magnification of ×200,000. The Pb concentrations were determined using inductively coupled plasma mass spectrometry (Agilent 7500a, Agilent Technology Co., Ltd., USA) with a detection limit of 0.01 μg/L. Cl ion concentrations were determined using ion chromatography (ICS-1000, DIONEX Co., Ltd., USA), with a detection limit of 10 μg/L.
Results and discussion Compressive strength and synthetic conditions The compressive strength decides whether FA can be reused as building materials, so the best curing conditions and activator dosage for the HSM and GM were determined based on the compressive strength performance.
Environ Sci Pollut Res
Geopolymer microstructure Figure 3 shows the geopolymer microstructure at a resolution of 200 nm, and the Na, Al, Si, and O are major elements in geopolymer gel (Fig. S2). Three characteristics are clearly 30 20%RH 25
≥95%RH
MPa
20 15 10 5 0 20
30
40
50
60
70 °C
Fig. 1 GM compressive strengths under different curing conditions
20 18 16 14 12
MPa
For the synthetic conditions of GM, the curing conditions are the basis of the activator (NaOH) dosage, so the curing conditions were confirmed firstly. The curing conditions include the temperature and humidity (Rovnaník 2010). Figure 1 shows that the best presence of compressive strength was obtained at ≥60 °C and 20 % RH. Some studies have shown that overheating leads to weakening of the geopolymer mechanical properties (Rovnaník 2010) and wastes energy; therefore, 60 °C was selected as the curing temperature (Chindaprasirt et al. 2009; Guo et al. 2010). The activator dosage has a significant influence on geopolymerization, because it determines the amounts of Si and Al dissolved out from the FA (Hanjitsuwan et al. 2014). Five activator dosages, from 3 wt% (1.5 mol/L) to 15 wt% (7.5 mol/L), were tested. The maximum compressive strength was achieved with a 12 wt% (6 mol/L) dosage (Fig. 2). The conditions used for FA-GM synthesis were therefore 60 °C, 20 % RH, and a 12 wt% (6 mol/L) dosage of NaOH. For the HSM, the curing conditions were 20±2 °C and ≥90 % RH, in compliance with the Cement Mortar Strength Testing Method (GB/T17671-1999). Preliminary experiments showed that an activator (cement) dosage of 30 wt% gave an average HSM compressive strength of over 10 MPa, and the characteristics of the FA-HSM were not destroyed by excess cement. Additionally, the cement dosage for non-burnt brick industrial production is in commonly less than 30 % (3∼28 %) (Zhengjia and Yanjun 2008). The conditions, 20±2 °C, ≥90 % RH, and 30 wt% dosage of cement, were therefore used for FA-HSM synthesis.
10 8 6 4 2 0
3
6
9
12
15 %
Fig. 2 GM compressive strengths at different activator dosages
observed. The gel formed by dissolution of Si and Al in the alkali-activated environment is amorphous. The structure is porous, implying nucleation. The compact layer structure shows the capacity for physical encapsulation. These structural characteristics are consistent with numerous observations in other studies (Álvarez-Ayuso et al. 2008; Izquierdo et al. 2009; Nath and Kumar 2013; Rovnaník 2010).
XRD analysis Erionite, sodalite, KPbO2, and a Pb–Si–Al polymer were identified in GM-Pb using XRD (Fig. 4a). These substances have been found in previous studies (Álvarez-Ayuso et al. 2008; De Silva and Sagoe-Crenstil 2008). The presence of erionite indicated the occurrence of geopolymerization of the Si–Al phase (Provis and Van Deventer 2009); some researchers have suggested that zeolites can influence the mechanical properties and S/S effects of heavy metals (De Silva and Sagoe-Crenstil 2008). The presence of sodalite means that some Cl ions in GM-Pb are no longer in a free state, but incorporated into sodalite. The presence of KPbO2 showed the amphoteric nature of Pb. The Pb–Si–Al polymer illustrated that Pb could enter into the microcrystalline structure and act as a charge balancer and cross-linker for AlO4 and SiO4 units, like Na+ and K+ (Ogundiran et al. 2013). In HSM-Pb, a Pb-containing silicate and hydroxide, and plumbate were identified by XRD (Fig. 4b). Pb acted as an amphoteric heavy metal, and Pb bonded with Si–O and formed Pb-containing silicate. This phenomenon has been reported in many papers. In Lee’s research, it was concluded that the formation of Pb-containing silicate was based on the adsorption and encapsulation of Pb in the form of hydroxide and carbonate by a hydrated silicate gel (Lee 2006). Gollmann found that adding Pb2+ to silicate mortar during the hydration process led to the loss of Ca2+ from the gel; therefore, Pb could replace Ca during hydration (Gollmann et al. 2010).
Environ Sci Pollut Res Fig. 3 SEM images of GM microstructure
Identification for Extraction Toxicity (GB5085.3-2007). It was observed that the leaching concentrations of Pb from GM-Pb and HSM-Pb both decreased with increasing curing time, implying that this phenomenon can be attributed to increasing matrix volume density with increasing curing time (Rhan and Kürklü 2014). The decrease in Pb leaching with increasing curing time was larger for GM-Pb than for HSMPb, and GM showed better S/S effects for Pb after 28 days
Leaching experiments Pb-leaching concentration Figure 5 shows the Pb concentrations in GM-Pb and HSMPb-leaching experiments. Both of the them below the limit value (5 mg/L) required by Chinese National Standard Identification Standards for Hazardous Wastes— 2000
Fig. 4 XRD patterns of GM-Pb and HSM-Pb
Maricopaite - Pb7Ca2(Si,Al)48O100!32H2O KPbO2 - Potassium Lead Oxide Sodalite (Se-doped) - Na7.84(Al6Si6O24)Cl1.86 Erionite-K - K0.28Na0.192Al2Si10.43O25.86!xH2O
3
1 2 3 4
Intensity(Counts)
1500
1000
4
500
4
2
2
2
3
1
1 4
3
0 10
20
30
40 2-Theta(¡ã
50
60
70
(a) GM-Pb 3000
Dundasite - PbAl2(CO3)2(OH)4(H2O) Na6PbO4 - Hexasodium lead oxide Zn2PbO4 - Zinc Lead Oxide K4PbO4 - Potassium Lead Oxide Pb6O(OH)6(ClO4)4(H2O) - Lead Oxide Hy droxide Chlorate Hy drate Margarosanite - PbCa2Si3O9 Pb2ZnSi2O7 - Lead Zinc Silicate K2Pb4Si8O21 - Potassium Lead Silicate
Intensity(Counts)
2500
2000
6 1500
1000
36
500
1
2
5 1
4
7
5
4 3
1
8
2
7
0 10
20
30
40 2-Theta(¡ã
(b) HSM-Pb
50
60
70
1 2 3 4 5 6 7 8
Environ Sci Pollut Res 1400 7d
28d
1200
ug / L
1000 800 600 400
nature and the chlorine group orientates within the center of the framework of β-cage (Philip and Mark. 1995). In the aspect of leaching, Hwan-Young found the sodalite phase synthesized by zeolite and chloride contains Cl and has very low solubility in water (Hwan-Young et al. 2009). So, we infer that the sodalite is one way to immobilize Cl besides physical encapsulation. Static monolithic leaching experiments
200
Release behavior of Pb
0 GM
HSM
Fig. 5 Pb release in leaching of GM-Pb and HSM-Pb
curing than HSM did. The Pb fixation rates of GM and HSM were 99.9985 and 99.9917 % after 28-day curing, respectively. Release of Cl Chlorides are the principal cause of steel reinforcement corrosion in concrete structures (Arya and Xu 1995); therefore, Cl ion release was investigated (Fig. 6). It was concluded that the immobility of Cl ions was positively correlated with the curing time, and GM always performed better than HSM. Isabella also found clear inhibition of Cl ion release by geopolymers, using municipal solid waste incineration fly ash as the source material (Lancellotti et al. 2010). The sodalite was identified by XRD, which proved that sodalite can be synthesized at low temperature. JosefChristian also found that high alkaline salt bearing gels are adequate educts to synthesize sodalite at a very low temperature of 60 °C, and he thought the amorphous contribution within source material acts as a sodalite precursor during low temperature crystallization (Josef-Christian et al. 2011). In the aspect of structure, Philip thought the sodalite has stabilizing
The changes in the accumulative Pb concentrations in GM-Pb and HSM-Pb are shown in Fig. 7. It is obvious that the amount of Pb released from HSM-Pb was higher than that from GMPb; this is consistent with the results of the leaching experiments. The Pb-release behavior was analyzed. The analysis was based on two assumptions: the extrinsic mass-transfer power of Pb from the solid to the liquid was constant, because the pH of the environment was constant, and the microporous structures of both matrixes were homogeneous, so the main masstransfer resistance arose from the acid-neutralizing capacity (ANC) of the matrix. Pb is an amphoteric metal (detected by XRD) and is soluble in acid and alkaline environments (Lee 2006; Onisei et al. 2012). It has been found that leaching behavior of lead, zinc (amphoteric metals) strongly depends on the leachate pH in many literatures (Radu et al. 2005; Valérie and Rudy 2008; Onisei et al. 2012). Therefore, the release behavior of Pb with decreasing ANCs of the GM and HSM was divided into three stages (Fig. 8). Stage one was 450
HSM HSM step
400
GM GM step
350 300
1800
28d
ug / L
7d 1600 1400
250 200
mg / L
1200
150
1000 800
100
600
50
400
0 200
6h
12h
24h
3d
6d
9d
12d
15d
18d
21d
time
0 GM
HSM
Fig. 6 Cl ion release in leaching of GM-Pb and HSM-Pb
Fig. 7 Changes in Pb accumulative concentrations in leaching in GM-Pb and HSM-Pb
Environ Sci Pollut Res 100
3
90
2
80
1
concentration
70 60 50 40 30 20 10 0 1
2
3
4
5
6
7
8
9
10
time Fig. 8 Schematic diagram of Pb-release behavior
defined as the alkaline-release stage. In this stage, Pb, being alkali soluble, was released quickly, because the environment around the test block was alkaline as a result of the adequate ANC of the matrix, and the slope of the curve was high. Stage two was defined as the stagnation stage. With attenuation of the ANC, the pH value around the block, even at the block edge, approached 7; we call this pH environment as the neutral sphere layer, and Pb was precipitated in the neutral sphere layer. Consequently, because of the blockade formed by the neutral sphere layer, Pb mass transfer almost stopped. The third stage was defined as the acid-release stage. As the neutral sphere layer moved toward the center of the block
(a) GM Fig. 9 Pb distributions inside GM-Pb and HSM-Pb after leaching test
increasingly, acid-soluble Pb was rapidly released in the acid-permeated matrix and the curve became steeper again. Mohammad found the consumption of ANC in cement matrix forms two consecutive leaching fronts, the leaching of Zn and Pb only occurs between the second front and the specimen surface, and the leaching behavior of metals is modeled by the shift of the fronts (Mohammad et al. 2004a). Mohammad also found the shift of the fronts is influenced by pH (Mohammad et al. 2004b). These conclusions have supported our inference about the three-stage release. In Jianguo’s researches, the leaching behavior of Pb from geopolymer mortar with 1 % Pb in H2SO4 solution has presented threestage, i.e., released quickly, stagnated, and then, accelerated to released again (Jianguo et al. 2008). Using these behavior rules (Fig. 8) to reanalyze the leaching phenomenon (Fig. 7), it was found that the mass transfer of Pb occurred in the acid-release stage for both GMPb and HSM-Pb; this state suggested that the acid had at least already broken through the ANC barrier at the edge of the block. Secondly, it was observed that, for GM-Pb, the alkaline-release stage terminated at around 2 days and the acid-release stage started at approximately 6 days, while for HSM-Pb, the two time points discussed above were about 3 and 10 days, respectively; so, the three stages for GM-Pb were always ahead of those for HSM-Pb. It was therefore concluded that the ANC of the GM is lower than that of the HSM. Distribution of Pb inside matrix The distribution of Pb in the HSM was low at the center and high at the margin (Fig. 9b); conversely, in the GM, the Pb content at the center was higher than that around the margin
(b) HSM
Environ Sci Pollut Res
(Fig. 9a). There are two reasons for this: one is the ANC, and the other is the amphotericity of Pb. During leaching, the environment at the center of the matrix was highly alkaline at beginning, Pb was present as Pb(OH)3− here, and moved outwards. As the ANC decreased, the neutral sphere layer invaded the matrix and became deeply immersed, so the pH decreased from the center to the margin inside the matrix. The closer Pb(OH)3− approached the neutral sphere layer, the more Pb(OH)3− was converted to Pb(OH)2, and the slower the Pb moved. Consequentially, the Pb in the neutral sphere layer therefore stagnated. However, outside the neutral sphere layer, the Pb became acid soluble and it accelerated to spread outwards. As a result, the neutral sphere layer contained the highest concentration of Pb. Based on the above analysis, the pattern in Fig. 9b verifies the conclusion drawn in the “Release behavior of Pb”, i.e., that the acid had at least already broken through the ANC barrier at the edge of the block. Because the pattern in Fig. 9b resulted from the ANC of the matrix and the amphotericity of Pb, it was concluded that this Pb distribution mode also occurred in GM-Pb, and the pattern in Fig. 9a shows the state in which the neutral sphere layer has already reached the center at 21 days. This state is also consistent with the release status of GM-Pb shown in Fig. 7, i.e., the mass transfer of Pb occurred in the acid-release stage for GM-Pb after 21 days. The analysis suggested that the performances of Pb in both matrixes, shown in Figs. 7 and 9, obeyed one behavior rule, and the patterns in Figs. 7 and 9 just show this behavior regulation at different stages and different phases. Because this behavior is caused by the ANC of the matrix and amphotericity of Pb, it was inferred farther that this behavior regulation during leaching also applies to other amphoteric metals immobilized by GMs and HSMs, such as Zn and Cr.
Conclusions Pb participated in the hydration and geopolymerization of a FA source material, and became part of the structural components of the hydrated silicate and geopolymer. The S/S effects of Pb and Cl in the HSM and GM improved with increasing curing time. After long-term curing (28 days), the immobility of Pb in the GM was better than that in the HSM. Sodalite improved the ability of the GM to stabilize Cl compared with that of the HSM. The HSM and GM had the same Pb-releasing behavior in static monolithic leaching. As the ANC decreased, the neutral sphere layer moved from the liquid to the block surface, completed the solid–liquid interface transition, and continued to permeate toward the center of the block. Correspondingly, the Pb-release process was divided into alkaline-release, stagnation, and acid-release stages. The neutral sphere layer had
the highest Pb concentration during the period of permeation toward the block center from the block edge. The Pb concentration declined toward both sides of the neutral sphere layer, and this distribution pattern disappeared when the ANC was completely exhausted. It was concluded that this behavior regulation would also apply to other amphoteric metals immobilized by GMs and HSMs. Acknowledgments This work was supported by the State Environment Protection Commonweal Special Program, China (No. 201209023); the National Science & Technology Pillar Program, China (No. 2014BAL02B01); the Sino-Norwegian project phase II (CHN 2150 09/ 059).
References Álvarez-Ayuso E, Querol X, Plana F, Alastuey A, Moreno N, Izquierdo M, Font O, Moreno T, Diez S, Vázquez E, Barra M (2008) Environmental, physical and structural characterisation of geopolymer matrixes synthesised from coal (co-)combustion fly ashes. J Hazard Mater 154:175–183. doi:10.1016/j.jhazmat.2007. 10.008 Arya C, Xu Y (1995) Effect of cement type on chloride binding and corrosion of steel in concrete. Cem Concr Res 25:893–902 Banfalvi G, Sarvari A, Nagy G (2012) Chromatin changes induced by Pb and Cd in human cells. Toxicol In Vitro 26:1064–1071. doi:10. 1016/j.tiv.2012.03.016 Buchwald A, Hilbig H, Kapps C (2007) Alkali-activated metakaolin-slag blends—performance and structure in dependence on their composition. J Mater Sci 42:3024–3032. doi:10.1007/s10853-006-0525-6 Chen QY, Tyrer M, Hills CD, Yang XM, Carey P (2009) Immobilisation of heavy metal in cement-based solidification stabilisation a review. Waste Manag 29:390–403. doi:10.1016/j.wasman.2008.01.019 China Bricks & Tiles Industrial Association (2011) The 12th five-year plan from China Bricks & Tiles Industrial Association. http://www. cbtia.com/news/2012/5-22/H14910705.shtml. Accessed May 2012 (In Chinses) China Building Materials Federation (2012) Statistical analysis report of cement output and productivity in 2011. China Cem 3:10–12 (In Chinses) Chindaprasirt P, Jaturapitakkul C, Chalee W, Rattanasak U (2009) Comparative study on the characteristics of fly ash and bottom ash geopolymers. Waste Manag 29:539–543. doi:10.1016/j.wasman. 2008.06.023 Cocke DL (1990) The binding chemistry and leaching mechanisms of hazardous substances in cementitious solidification/stabilization systems. J Hazard Mater 24:231–253 De Silva P, Sagoe-Crenstil K (2008) Medium-term phase stability of Na2O–Al2O3–SiO2–H2O geopolymer systems. Cem Concr Res 38:870–876. doi:10.1016/j.cemconres.2007.10.003 Duxson P, Provis JL, Lukey GC, Van Deventer JJS (2007) The role of inorganic polymer technology in the development of ‘Green concrete’. Cem Concr Res 37:1590–1597. doi:10.1016/j.cemconres. 2007.08.018 Gollmann MAC, Da Silva MM, Masuero NB, Dos Santos JOHZ (2010) Stabilization and solidification of Pb in cement matrices. J Hazard Mater 179:507–514. doi:10.1016/j.jhazmat.2010.03.032 Guo X, Shi H, Dick WA (2010) Compressive strength and microstructural characteristics of class C fly ash geopolymer. Cem Concr Compos 32:142–147. doi:10.1016/j.cemconcomp.2009.11.003
Environ Sci Pollut Res Hanjitsuwan S, Hunpratub S, Thongbai P, Maensiri S, Sata V, Chindaprasirt P (2014) Effects of NaOH concentrations on physical and electrical properties of high calcium fly ash geopolymer paste. Cem Concr Compos 45:9–14. doi:10.1016/j.cemconcomp.2013.09. 012 Hwan-Young K, Jeong-Guk K, In-Tae K, Hwan-Seo P (2009) Study on the leaching behavior of salt waste forms prepared by using zeolite and lime glass. J Ind Eng Chem 15:293–298. doi:10.1016/j.jiec. 2008.11.009 Izquierdo M, Querol X, Davidovits J, Antenucci D, Nugteren H, Fernández-Pereira C (2009) Coal fly ash-slag-based geopolymers: microstructure and metal leaching. J Hazard Mater 166:561–566. doi:10.1016/j.jhazmat.2008.11.063 Janusa MA, Champagne CA, Fanguy JC, Heard GE, Laine PL, Landry AA (2000) Solidification/stabilization of lead with the aid of bagasse as an additive to Portland cement. Microchem J 65:255–259 Jianguo Z, John LP, Dingwu F, Jannie SJVD (2008) Geopolymers for immobilization of Cr6+, Cd2+, and Pb2+. J Hazard Mater 157:587– 598. doi:10.1016/j.jhazmat.2008.01.053 Josef-Christian B, Karsten S, Lars R (2011) Nanocrystalline sodalite grown from superalkaline NaCl bearing gels at low temperature (333 K) and the influence of TEA on crystallization process. Microporous Mesoporous Mater 142:666–671. doi:10.1016/j. micromeso.2011.01.020 Lancellotti I, Kamseu E, Michelazzi M, Barbieri L, Corradi A, Leonelli C (2010) Chemical stability of geopolymers containing municipal solid waste incinerator fly ash. Waste Manag 30:673–679. doi:10. 1016/j.wasman.2009.09.032 Lee D (2006) Effect of calcite on Pb-doped solidified waste forms in leaching. Chemosphere 63:1903–1911. doi:10.1016/j.chemosphere. 2005.10.018 Lee J, Freeman JL (2014) Zebrafish as a model for investigating developmental lead (Pb) neurotoxicity as a risk factor in adult neurodegenerative disease: a mini-review. Neurotoxicology. doi:10.1016/j. neuro.2014.03.008 Lin SL, Cross WH, Chian ESK, Lai JS, Giabbai M, Hung CH (1996) Stabilization and solidification of lead in contaminated soils. J Hazard Mater 48:95–110 Lloyd RR, Provis JL, Van Deventer JJS (2009) Microscopy and microanalysis of inorganic polymer cements. 1: remnant fly ash particles. J Mater Sci 44:608–619. doi:10.1007/s10853-008-3077-0 Ministry of Industry and Information Technology of the People’s Republic of China (2011) The 12th five-year plan of major industrial waste solid comprehensive utilization. http://www.miit.gov.cn/ n11293472/n11293832/n11293907/n11368223/14416612.html. Accessed Jan 2012. (In Chinses) Mohammad ZI, Lionel JJC, Ernest KY (2004a) A two-front leach model for cement-stabilized heavy metal waste. Environ Sci Technol 38: 1522–1528. doi:10.1021/es0348400 Mohammad ZI, Lionel JJC, Ernest KY (2004b) Effect of remineralization on heavy-metal leaching from cement-stabilized/solidified waste. Environ Sci Technol 38:1561–1568. doi:10.1021/es034659r Nath SK, Kumar S (2013) Influence of iron making slags on strength and microstructure of fly ash geopolymer. Constr Build Mater 38:924– 930. doi:10.1016/j.conbuildmat.2012.09.070 Ogundiran MB, Nugteren HW, Witkamp GJ (2013) Immobilisation of lead smelting slag within spent aluminate—fly ash based geopolymers. J Hazard Mater 248:29–36. doi:10.1016/j.jhazmat. 2012.12.040 Olivia M, Nikraz H (2012) Properties of fly ash geopolymer concrete designed by Taguchi method. Mater Des 36:191–198. doi:10.1016/ j.matdes.2011.10.036
Onisei S, Pontikes Y, Van Gerven T, Angelopoulos GN, Velea T, Predica V, Moldovan P (2012) Synthesis of inorganic polymers using fly ash and primary lead slag. J Hazard Mater 205:101–110. doi:10.1016/j. jhazmat.2011.12.039 Philip JM, Mark TW (1995) Synthesis, structure, and characterization of halate sodalites:M8[AlSiO4]6(XO3)x(OH)2-x; M=Na, Li, or K; X= Cl, Br, or I. Zeolites 15:561–568 Phoo-ngernkham T, Chindaprasirt P, Sata V, Hanjitsuwan S, Hatanaka S (2014) The effect of adding nano-SiO2 and nano-Al2O3 on properties of high calcium fly ash geopolymer cured at ambient temperature. Mater Des 55: 58–65. doi:10.1016/j.matdes.2013.09.049 Provis JL, Van Deventer JJS (2009) Geopolymers: structures, processing, properties and industrial applications. CRC Press, Boca Raton Radu B, Zoltan R, Ligia TB (2005) Release dynamic process identification for a cement based material in various leaching conditions. Part I Influence of leaching conditions on the release amount. J Environ Manag 74:141–151. doi:10.1016/ j.jenvman.2004.06.016 Rhan G, Kürklü GK (2014) The influence of the NaOH solution on the properties of the fly ash-based geopolymer mortar cured at different temperatures. Compos Part B 58:371–377. doi:10.1016/j. compositesb.2013.10.082 Rovnaník P (2010) Effect of curing temperature on the development of hard structure of metakaolin-based geopolymer. Constr Build Mater 24:1176–1183. doi:10.1016/j.conbuildmat.2009.12.023 Shaheen SM, Hooda PS, Tsadilas CD (2014) Opportunities and challenges in the use of coal fly ash for soil improvements—a review. J Environ Manag 145:249–267. doi:10.1016/j.jenvman.2014.07.005 Temuujin J, van Riessen A (2009) Effect of fly ash preliminary calcination on the properties of geopolymer. J Hazard Mater 164:634–639. doi:10.1016/j.jhazmat.2008.08.065 Temuujin J, van Riessen A, Williams R (2009) Influence of calcium compounds on the mechanical properties of fly ash geopolymer pastes. J Hazard Mater 167:82–88. doi:10.1016/j.jhazmat.2008.12. 121 Temuujin J, Minjigmaa A, Lee M, Chen-Tan N, van Riessen A (2011) Characterisation of class F fly ash geopolymer pastes immersed in acid and alkaline solutions. Cem Concr Compos 33:1086–1091. doi: 10.1016/j.cemconcomp.2011.08.008 Valérie C, Rudy S (2008) The application of pHstat leaching tests to assess the pH-dependent release of trace metals from soils, sediments and waste materials. J Hazard Mater 158:185–195. doi:10. 1016/j.jhazmat.2008.01.058 Van Deventer JSJ, Provis JL, Duxson P (2012) Technical and commercial progress in the adoption of geopolymer cement. Miner Eng 29:89– 104. doi:10.1016/j.mineng.2011.09.009 Xu H, Li Q, Shen L, Wang W, Zhai J (2010) Synthesis of thermostable geopolymer from circulating fluidized bed combustion (CFBC) bottom ashes. J Hazard Mater 175:198–204. doi:10.1016/j. jhazmat.2009.09.149 Yoshinaga J, Yamasaki K, Yonemura A, Ishibashi Y, Kaido T, Mizuno K, Takagi M, Tanaka A (2014) Lead and other elements in house dust of Japanese residences—source of lead and health risks due to metal exposure. Environ Pollut 189:223–228. doi:10.1016/j.envpol.2014. 03.003 Zain MFM, Islam MN, Radin SS, Yap SG (2004) Cement-based solidification for the safe disposal of blasted copper slag. Cem Concr Compos 26:845–851. doi:10.1016/j. cemconcomp.2003.08.002 Zhengjia Y, Yanjun H (2008) The practical production technology of nonburnt brick. Chemical Industry Press, Beijing (In Chinses)
