Leaching of elements from cement activated ﬂy ash and slag amended soils Masrur Mahedi, Bora Cetin * Department of Civil, Construction and Environmental Engineering 813 Bissell Road, Town Engineering Building Iowa State University, Ames, IA, 50011, USA highlights  Fly ash/slag had less effects on the leaching of elements when is used with cement.  TCLP leached higher amounts of Cr, Cu and S than those of SPLP and WLT.  Cement addition resulted consistent pH-dependent leaching behavior of the elements.  Cr and Cu had amphoteric leaching patterns, when Fe and S showed cationic patterns.  Leaching of Cr, Cu, Fe, and S were found to be solubility controlled. article info Article history: Received 25 March 2019 Received in revised form 10 June 2019 Accepted 23 June 2019 Available online 26 June 2019 Handling Editor: Lena Q. Ma Keywords: Metal leaching Soil stabilization Cement Fly ash Slag Geochemical modeling abstract Very few studies have investigated the leaching characteristics of cement activated ﬂy ash and slag treated soils, although the inclusion of cement signiﬁcantly enhances the material pH and may alter the leachability of elements. In this study the leaching behavior and mechanisms of chromium (Cr), copper (Cu), iron (Fe) and sulfur (S) from cement activated ﬂy ash and slag stabilized soils were evaluated. An array of synthetic precipitation leaching procedure (SPLP), batch water leach test (WLT), toxicity char- acteristic leaching procedure (TCLP) and pH-Static leach tests were conducted. A geochemical equilib- rium model Visual MINTEQ was implemented to identify the leaching controlling mechanisms of the metals. Results indicated that, the leached concentrations of Cr, Cu, Fe and S in SPLP, WLT and TCLP efﬂuents were in the range of 0.016e0.74 mg/L, 0.013e0.17 mg/L, 0.019e0.27 mg/L and 1.78e234 mg/L, respectively. Quantitative comparisons between the standard test procedures suggested the necessity of multiple test methods for a comprehensive leaching assessment. Cr and Cu showed amphoteric leaching behaviors, whereas Fe and S followed cationic leaching patterns. According to the geochemical analyses, amorphous Cr(OH) 3 ; tenorite and Cu(OH) 2 ; ferrihydrite and goethite; gypsum and anhydrite; could control the leaching of Cr, Cu, Fe and S, respectively. The efﬂuent Cr concentrations frequently exceeding the U.S. EPA speciﬁed maximum contaminant level of 0.1 mg/L. Yet, the use of cement activated ﬂy ash and slag mixed soils could be beneﬁcial, since less toxic trivalent Cr (III) was identiﬁed through geochemical modeling. © 2019 Elsevier Ltd. All rights reserved. 1. Introduction The use of recycled materials in construction purposes consid- erably reduces the natural resource depletion, cost, labor and declination of landﬁll spacing. Fly ash and steel slag are the two widely implemented recycled materials in pavement constructions with great applicability in subgrade soil stabilization (Tyagi and Soni, 2019). Fly ashes are byproducts from coal burning electric power plants, whereas steel slags are the non-metallic offshoots generated in different stages of steel production (Cetin et al., 2014; Labrincha et al., 2017). Some of the ﬂy ashes (e.g. Class C) have self- cementing properties resulting from higher amount of calcium oxides (CaO), while others (e.g. Class F, high-carbon ﬂy ash) are effective pozzolans owing to the presence of silica (SiO 2 ), alumina (Al 2 O 3 ) and iron oxides (Fe 2 O 3 ), capable of producing cementitious compounds with the presence of an activator such as lime and cement (Cetin et al., 2010). Besides, steel slags are frequently * Corresponding author. E-mail addresses: mmahedi@iastate.edu (M. Mahedi), bcetin@iastate.edu (B. Cetin). Contents lists available at ScienceDirect Chemosphere journal homepage: www.elsevier.com/locate/chemosphere https://doi.org/10.1016/j.chemosphere.2019.06.178 0045-6535/© 2019 Elsevier Ltd. All rights reserved. Chemosphere 235 (2019) 565e574