Construction and Building Materials 134 (2017) 123–130 Contents lists available at ScienceDirect Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat Immobilization mechanism of Pb in fly ash-based geopolymer ⇑ Bin Guo a, De’an Pan a, , Bo Liu a, Alex A. Volinsky b, Mustafa Fincan b, Jinfeng Du c, Shengen Zhang a,1 a Institute for Advanced Materials and Technology, University of Science and Technology Beijing, Beijing 100083, PR China b Department of Mechanical Engineering, University of South Florida, Tampa, FL 33620, USA c Center of Technology Research, Shenhua Guohua Electric Power Research Institute Co., LTD, PR China highlights The behavior of Pb in geopolymer depends on the chemical property of the Pb compound. Pb participates in the formation of geopolymer network, forming leaded amorphous gel. Dissolution of lead compounds is a vital step in the formation of leaded geopolymer. Pb compounds, soluble in alkali, are chemically bonded into the geopolymer gel. PbS, inert to alkali, is trapped in the geopolymer by physical encapsulation. article info abstract Article history: Geopolymer possesses good immobilization capacity for Pb. There are two rival interpretations regarding Received 14 October 2015 the immobilization mechanism of Pb. This research investigates the behavior of 3 Pb compounds in Received in revised form 17 October 2016 geopolymer and clarifies the immobilization mechanism. When Pb contamination is added in the form, Accepted 21 December 2016 soluble in sodium hydroxide solution, the Pb is converted to an amorphous form and participates in the formation of geopolymer network. Successful immobilization of these species relies on chemical bonding and physical encapsulation. On the contrary, the Pb compound inert to sodium hydroxide solution is seg- Keywords: regated from the binder and trapped by physical encapsulation. Lead Ó 2016 Elsevier Ltd. All rights reserved. Leaded geopolymer Immobilization mechanism Chemical property 1. Introduction be determined, and (b) the physical properties should be stable, not only to encapsulate toxic metals, but also to make the product Geopolymer, the reaction product of a solid aluminosilicate suitable for further building applications. with a highly concentrated aqueous alkali hydroxide or silicate A great many hazardous solid wastes are proved to be suitable solution [1], is a type of cementitious material with an amorphous as a source material for making good geopolymeric materials, such or semi-crystalline nature [2]. Geopolymer can provide compara- as municipal solid waste incineration fly ash [2], lead smelting slag ble strength performance to traditional binders in practical appli- [11], and primary lead slag [12]. The synthesized geopolymer pos- cations [1]. Due to its low permeability, long-term durability and sess a comparable compressive strength with Ordinary Portland resistance to acid attack [3], geopolymer is believed to be a good Cement. In addition, most of the heavy metals that are presents immobilization system for various hazardous waste. There have in these wastes can be effectively immobilized in geopolymer. It been an increasing number of studies that focus on the potential is worth noting that these synthesized geopolymer possess good application of geopolymer to immobilize heavy metals during the immobilization capacity for Pb. last decade [4–9]. Research into this field have two main objectives The immobilization mechanism of heavy metals in geopolymer [10], i.e. (a) the immobilization efficiency and mechanism should is always a popular research, especially of Pb [4,5,13–15]. There are two rival interpretations regarding the immobilization mechanism of Pb. One is that Pb is likely to be bonded into the amorphous ⇑ Corresponding author at: Institute for Advanced Materials and Technology, University of Science and Technology Beijing, 30 Xueyuan Road, Haidian District, geopolymer gel, which was proposed by van Jaarsveld et al. [4]. Beijing 100083 PR China. It means that the Pb is in an amorphous form. Perera et al. also con- E-mail addresses: [email protected] (D. Pan), zhangshengen@mater. cluded that Pb may be mainly in the amorphous aluminosilicate ustb.edu.cn (S. Zhang). structure [13]. The other interpretation is that Pb is immobilized 1 Co-corresponding author. http://dx.doi.org/10.1016/j.conbuildmat.2016.12.139 0950-0618/Ó 2016 Elsevier Ltd. All rights reserved. 124 B. Guo et al. / Construction and Building Materials 134 (2017) 123–130 in the form of Pb3SiO5, which is an insoluble crystalline compound 2. Experimental program [14]. Zhang et al. observed the Pb-rich regions in the SEM micro- graphs of the geopolymer with 0.5% Pb as Pb(NO3)2 [15]. The com- 2.1. Materials position of the Pb-rich regions was approximately consistent with the Pb3SiO5 phase. He concluded that the effect of heavy metal Fly ash, class F, according to the ASTM C618, was obtained from immobilization in geopolymer depends on the form of the supplied the Shenhua Power Station, Taicang, Jiangsu Province, China. The contaminant. In sum, the difference between the two interpreta- fly ash is of coal origin. The chemical composition, determined by tions is the existing form of Pb bonded into geopolymer. X-ray fluorescence, is shown in Table 2. Analytical grade reagents Thus, it is clear that more work is required to settle the contro- PbO, PbSO4, PbS and NaOH were used. The silicate modulus (Ms) versy and find out the exact immobilization mechanism. In this of the sodium silicate solution is 3.6. Distilled water was used paper, Pb compounds were used as the simulated lead-laden throughout. wastes to mix with aluminosilicate source for synthesis of geopoly- mer. Fly ash was used as the aluminosilicate source, because it is 2.2. Synthesis the most common source material for geopolymerization and its cost effectiveness [16]. This paper focused on the geopolymer con- Fly ash and Pb compounds were mixed and ground in a ball mill taining three types of Pb compounds: PbO, PbSO4 and PbS. These for 30 min. NaOH, distilled water and sodium silicate solution were Pb compounds are chosen based on the following aspects: mixed and heated at 65 °C for 10 min. Then the mixed powder was added in the alkaline activator (1.3SiO2ÁNa2OÁ14.4H2O), followed 1. PbO, PbSO4 and PbS are three types of the primary Pb contam- by further 5 min mixing. Table 3 shows the summary of the matri- inants existing in the environment [17]. In soil, lead exists ces under discussion. The mass ratio of activator to solid material mainly as PbSO4 and PbO [18]. In the atmosphere, lead is pre- and the composition of the activator were chose based on previous sent mainly as PbSO4 [19]. Lead particles emitted from mining work in which the geopolymer prepared from same fly ash and and smelting processes are mainly in the form of PbSO4, activator showed the optimal strength. The pastes were cast in Á PbO PbSO4, and PbS [19,20]. steel 20  20  20 mm cubic molds for the strength test and in 2. Lead ion is amphoteric specie. This character of Pb has not been u8.5 mm  4 mm plastic cylindrical molds for the leaching test. considered in the previous researches on the immobilization All the samples were cured in an airtight container at 85 °C for mechanism of Pb in geopolymer. The activators provide alkaline 24 h. After that the samples were sealed in plastic bags for curing condition for the geopolymeric reaction. Many Pb compounds, at room temperature, until other tests were carried out after 7 days such as PbO and PbSO4, can dissolve in excess alkali to form of ageing. The mole ratios of Si/Al of the designed geopolymer are plumbite ions. Toxicity Characteristic Leaching Procedure also showed in Table 3, which could be calculated as follows, (TCLP) [21] leaching test is often used to evaluate the immobi- :  WSiO2 ð À Þþ   1 3 MSiO2 lization efficiency. Acetic acid solution is used as extraction 100 WPb 100 RA=S :  þ þ :  100 1 3 MSiO2 MNa2O 14 4 MH2O Si=AlMole ¼ fluid in the TCLP test. Some Pb compounds can also react as a WAl2O3 ð100 À WPbÞ base and dissolve in acetic acid solution, such as PbO. The 100 immobilization mechanism of Pb in geopolymer may be related  MAl2O3  to the chemistry property of Pb contamination. Thus, PbO, 2 MSiO2 ð Þ PbSO4 and PbS were chose to simulate lead-laden wastes based 1 on their solubility in acetic acid and alkali. Table 1 shows the where W is the SiO content in the fly ash (%); W is the solubility of these compounds. SiO2 2 Al2O3 Al2O3 content in the fly ash (%); WPb is the amount of Pb compound added into the fly ash (%); RA/S is the ratio of activator (g) to solid In this paper, research was conducted to establish links À1 material (g); MSiO2 is the molar mass of SiO2 (gÁmol ); MNa2O is between the immobilization mechanism of Pb and chemical prop- À the molar mass of Na O(gÁmol 1); M is the molar mass of H O erties of the supplied Pb compounds. The amounts of Pb added as 2 H2O 2 (gÁmolÀ1). different Pb compounds were 1%, 2%, 8% of fly ash weight. The pur- poses of this research were to (a) investigate the change of Pb before and after geopolymerization; (b) study the distribution of 2.3. Samples analysis Pb in geopolymer; (c) assess the immobilization efficiency of dif- ferent Pb compounds by geopolymer; (d) investigate the upper The compressive strength tests were conducted after 7 days of limit of the Pb content could be immobilized in the geopolymer. ageing. Three samples were tested for each mechanical test. X-ray powder diffraction data was obtained using Rigaku Ultima IV X-ray diffractometer with Cu Ka radiation, 40 kV voltage and Table 1 40 mA current at 20°/min scanning rate, from 5° to 60° 2h range.