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Highlights • SPC exhibits excellent effectiveness in treating soils even at 256 days of curing. • SPC treatment significantly reduces the leachable heavy metals and organics. • Acid neutralization capacity and XRD analyses can interpret leaching mechanisms. • Ex-situ treatment well improves spatial homogeneity of contaminated soil. Abstract This study presents a field trial of ex-situ solidification/stabilization of heavy metal contaminated soils using a new hydroxyapatite based binder SPC (superphosphate and calcium oxide). The field performance including electrical conductivity (EC), heavy metal and chemical oxygen demand (COD) leachability, and penetration resistance of stabilized soils up to 256 days after treatment is investigated. Moreover, acid neutralization capacity (ANC), metal speciation, and soil mineralogy analyses are performed to elucidate the heavy metal immobilization mechanisms. The results indicate that the stabilized soils have lower EC values than untreated soils, indicating the lower soluble salt contents in stabilized soils. Ex-situ treatment can significantly reduce acid soluble fraction contents and leachability of lead, zinc and cadmium in soils as a consequence. Decrease in COD leachability is observed during the curing period up to 256 days. Additionally, SPC stabilized soils give superior mechanical properties as indicated by their higher penetration resistance values as compared to the untreated soils. The improvement in ANC and reduction in acid soluble fraction resulting from the formation of phosphate-based precipitates are mainly responsible for the reduced heavy metal leachability of stabilized soils. Additionally, ex-situ SPC treatment is found to be effective in improving soil spatial homogeneity. [ABSTRACT FROM AUTHOR]

Highlights • KMP stabilized soil possesses superior early strength performance. • GM shows higher Cl immobilization, while KMP is better on immobilizing Zn. • Zn-bearing precipitates and Friedel's salt form in GM stabilized soil. • Zn and/or Cl-bearing phosphate precipitates form in KMP stabilized soil. Abstract This study presents a multi-scale evaluation of the effectiveness of two novel binders of reactive magnesia activated blast furnace slag-based binder named as GM and phosphate-based binder named as KMP to stabilize mixed zinc (Zn) and chloride (Cl) contaminated soil collected from an abandoned industrial electroplating company site. The stabilized soils cured at 1, 3, 7, and 28 days were subjected to soil pH, unconfined compressive strength, leachability and Zn speciation tests. The X-ray diffraction, scanning electron microscope and energy dispersive spectroscopy analyses were also performed to investigate the immobilization mechanisms of Zn and Cl in the stabilized soils. The test results showed that the unconfined compressive strength of the soils stabilized with 6% wt GM or KMP after 28 days curing was significantly improved, and the leached Zn and Cl concentrations were well below their corresponding remediation goals. In general, the KMP stabilized soil possessed superior performance in terms of higher unconfined compressive strength in early curing (7 days) and lower leached Zn concentration. While the GM exhibited superior Cl immobilization in the stabilized soil. The X-ray diffraction and energy dispersive spectroscopy results indicated that the Zn-bearing precipitates and Cl-bearing Friedel's salt were the predominant mechanisms of immobilizing Zn and Cl by using GM, while the formation of phosphate-bearing precipitates containing Zn and Cl were the predominant mechanisms of immobilizing Zn and Cl by using KMP. The results demonstrate that both GM and KMP are promising binders in treating Zn and Cl-contaminated soils at electroplating industry sites. [ABSTRACT FROM AUTHOR]

This study aims to evaluate interlayer and surface characteristics of carboxymethylcellulose (CMC) and tetramethylammonium (TMA) modified bentonite, named as TCMB, as a potential material in contamination containment systems. The study employed a variety of experimental methods, including zeta-potential, scanning electron microscopy, X-ray diffraction (XRD), transmission electron microscopy, and Fourier transform infrared spectroscopy (FTIR), along with molecular dynamics (MD) simulations. Zeta potential results revealed increased electronegativity in TCMB compared to the conventional bentonite. By forming three-dimensional hydrogel networks capable of filling inter clusters pore spaces, CMC effectively maintained the low hydraulic conductivity of TCMB. XRD results confirmed the presence of TMA in the interlayer of montmorillonite. MD simulations and FTIR analyses validated the successful adsorption of CMC and TMA, which can be jointly attributed to hydrogen bonds between hydroxyl groups in CMC and the surface oxygen of montmorillonite, and electrostatic attraction between TMA and montmorillonite. The comparatively weak bonding between CMC and montmorillonite may facilitate CMC elution in the TCMB exposed to aggressive permeants. Furthermore, the electrical double layer thickness of TCMB and CB was estimated as 31.54 Å and 18.76 Å, respectively. Overall, the CMC and TMA treatments effectively enhance the suitability of bentonite for contamination containment applications. [Display omitted] • Interlayer and surface characteristics of the modified bentonite were determined. • CMC formed three-dimensional networks, effectively filling inter-aggregate pores. • TMA intercalated in a monolayer configuration in montmorillonite interlayer. • Modified bentonite possessed thicker electric double layer. [ABSTRACT FROM AUTHOR]

Ground granulated blast furnace slag (GGBS) is widely used to stabilize soils due to its environmental and economic merits. The strength and durability of reactive MgO activated GGBS (GGBS-MgO) stabilized lead (Pb)-contaminated soils have been explored by previous studies. However, the effects of simulated acid rain (SAR) on the leachability and micro-properties of GGBS-MgO stabilized Pb-contaminated soils are hardly investigated. This research studies the leachability and microstructural properties of GGBS-MgO stabilized Pb-contaminated kaolin clay exposed to SAR with initial pH values of 2.0, 4.0 and 7.0. A series of tests are performed including the semi-dynamic leaching tests using SAR as the extraction liquid, acid neutralization capacity (ANC), mercury intrusion porosimetry (MIP), and X-ray diffraction (XRD) tests. The results demonstrate that as the SAR pH decreases from 7.0 to 4.0, the Pb cumulative fraction leached ( CFL ) and observed diffusion coefficient ( D obs ) increases significantly whereas the leachate pH decreases. Meanwhile, increasing the GGBS-MgO content from 12% to 18% results in the decrease of CFL and D obs . Further decreasing the SAR pH to 2.0 results in the dissolution-controlled leaching mechanism regardless of the binder dosage. The differences in the leaching properties under different pH conditions are interpreted based on the cemented soil acid buffering capacity, hydration products and pore size distributions obtained from the ANC, MIP, and XRD tests, respectively. [ABSTRACT FROM AUTHOR]

Synthetic hydroxyapatite (HA) is an efficient and environment-friendly material for the remediation of heavy metal contaminated soils. However, the application of conventional HA powder in stabilizing contaminated soils is limited, due to high cost of final products, difficulties in synthesizing purified HA crystals. A new binder named SPC, which composes of single superphosphate (SSP) and calcium oxide (CaO), is presented as an alternative in this study. HA can form in the soil matrix by an acid-base reaction between SSP and CaO, resulting in a dense structure and improved mechanical properties of treated soils. Therefore, the SPC is capable of effectively immobilizing heavy metals and elevating strength of contaminated soils, meanwhile, maintaining relatively low cost. This paper presents a systematic investigation of the performance, reaction products, and microstructural properties of a lead (Pb), zinc (Zn), and cadmium (Cd) contaminated industrial site soil stabilized with SPC binder. The effects of SPC content and curing time on the pH, leachability and strength properties of the stabilized soils are evaluated. Furthermore, modified European Communities Bureau of Reference (BCR) sequential extraction procedure (SEP), mercury intrusion porosimetry (MIP), X-ray diffraction (XRD), and scanning electron microscope (SEM) analyses are performed to interpret the mechanisms controlling the changes in these macro-properties. The results show that the soil pH and unconfined compressive strength (UCS) increase with increasing SPC content and curing time. After 28 days of curing, the UCS values of stabilized soils are approximately 2.2–5.7 times those of the untreated soil. The leachability of Pb, Zn and Cd is significantly reduced after stabilization, and the SPC content and curing time have considerable influences on the leached concentrations of heavy metals. The SEP results confirm that SPC significantly reduces the acid soluble fractions of Pb, Zn and Cd while increases their residual fractions. The MIP test results show that pore volume reduces notably and pore profile of the soil changes remarkably after SPC stabilization. The mineralogical (XRD) and microstructural (SEM) analyses reveal that the formation of heavy metal-bearing hydroxyapatites and phosphate-based precipitates are the primary mechanisms of immobilization of Pb, Zn and Cd in the SPC stabilized soil. [ABSTRACT FROM AUTHOR]

[Display omitted] • Novel composite polymer-amended bentonite is developed. • PAC-HPMC mass ratio for bentonite amendment is optimized. • Amended bentonite possesses superior k when exposed to Cd(NO 3) 2 solution. • Yield stress is a viable indicator for evaluating polymer stability in amended bentonite. This study developed a novel composite polymer-amended bentonite consisting of two linear polymers (i.e., polyanionic cellulose (PAC) and hydroxypropyl methylcellulose (HPMC)) for use as an engineered barrier material in environmental containment applications. Several series of laboratory experiments were conducted on bentonite and polymer-amended bentonites with different PAC-HPMC mass ratios to evaluate free swell index and liquid limit using deionized water and 20 mM cadmium nitrate (Cd(NO 3) 2) as a typical contaminant. Hydraulic conductivity of bentonite and polymer-amended bentonite filter cakes was determined via modified fluid loss (MFL) tests using 20 mM Cd(NO 3) 2 solution under increasing pressure conditions. Rheological properties, such as apparent viscosity and shear stress, were measured, and yield stresses were estimated using the Bingham plastic and Herschel–Bulkley models to evaluate polymer stability. The results showed that exposure to 20 mM Cd(NO 3) 2 solution reduced both free swell index and liquid limit of bentonites with and without polymer-amendment. The bentonite amended with PAC-HPMC mass ratio of 7 (i.e., (P7H1)B) possessed the lowest hydraulic conductivity among the bentonites tested. Its hydraulic conductivity was one to two orders of magnitude lower than that of unamended bentonite. The ultraviolet absorption spectra analyses revealed the value of the wavelength of maximum absorption bands for (P7H1)B slurry was 11.9% greater than that for PAC-amended bentonite slurry, indicating stronger hydrogen bonding strength of (P7H1)B slurry. The total organic carbon in the filtrate for HPMC-amended bentonite slurries was 64.0–67.7% lower than that for PAC-amended bentonite slurries in the MFL tests. In addition, the total organic carbon in the filtrate decreased with increasing yield stress of polymer-amended bentonite slurries, indicating yield stress can be used as a good indicator for evaluating polymer stability in the polymer-amended bentonite filter cakes. [ABSTRACT FROM AUTHOR]

The grinding of coffee beans and subsequent brewing of coffee generates spent coffee grounds (CG), an insoluble waste material rich in organic content. Ground granulated blast furnace slag (S), is a waste by-product derived from steel production. The combustion of coal in power plants produces fly ash (FA) as a waste by-product. The reuse of CG, S and FA as green and sustainable construction materials is of global concern nowadays. The objective of this study was to evaluate the possibility of combining a highly organic waste (CG) with industrial wastes (S and FA) into a sustainable subgrade construction material by a geopolymerization process. A geopolymeric material was synthesized using CG as the base material, and a controlled ratio of S and FA as precursors. A mixture of sodium silicate solution (Na 2 SiO 3 ) and sodium hydroxide solution (NaOH) was used as the alkaline liquid activator (L). Strength development of these geopolymers was assessed using the unconfined compressive strength (UCS) test. Particle bonding and geopolymer matrix pattern of the geopolymers were observed with scanning electron microscopy (SEM). With the base material-to-precursor ratio fixed at a ratio of 70:30, the following factors are found to influence strength development in the geopolymer: (1) ratio of S:FA, (2) ratio of Na 2 SiO 3 :NaOH, (3) curing time, and (4) curing temperature. When compacted under optimum liquid content (OLC), all the material combinations investigated meet the structural strength requirement for subgrade materials in road embankments specified by various road authorities. It was found that the mixture with CG:S:FA = 70:30:0, Na 2 SiO 3 :NaOH = 70:30, and L = 55% was the optimum geopolymer blend recommended, as it produces a relatively high UCS with a relatively low L content. This green geopolymer comprising essentially of waste materials was found to be viable as a stabilized subgrade material. The research findings have the potential to transform the construction industry in the sustainable usage of waste by-products in future road subgrades. [ABSTRACT FROM AUTHOR]

The objective of this research was to evaluate the strength development of industrial by-products, namely Recycled Glass (RG) and Fly Ash (FA) in the manufacture of low carbon masonry units. A low carbon concept for manufacturing masonry units using RG-FA geopolymers was explored by applying low curing temperature of 50 °C and a low curing period of just 3–7 days. RG, being rich in silica was used as a filler material. FA, being a silica and alumina rich industrial by-product was used as a precursor in the RG-FA geopolymers. A liquid alkaline activator (L) comprising of a sodium hydroxide-sodium silicate solution was used for the alkali activation of 30% content of FA in the RG-FA blend. Factors found to affect strength development of the RG-FA geopolymers were: (1) the ratio of sodium hydroxide and sodium silicate in the liquid activator, (2) application of a low curing temperature of only 50 °C, (3) short curing periods of just 7 days and (4) the L/FA ratios. A geopolymer composed of a 70% Na 2 SiO 3 :30% NaOH ratio with a L/FA ratio of 0.625 was found to be the most efficient combination to provide the required unconfined compression strengths for manufacturing masonry. The usage of RG-FA geopolymers as low carbon masonry units was found to be viable using a low curing period of just 7 days and a curing temperature of just 50 °C. The RG-FA geopolymers had sufficient compressive strength be used as structural masonry units and needed only a low amount of heat treatment to achieve the minimum strength requirement. Optimally, 30% of FA was found to be sufficient for geopolymerization to occur for this novel low carbon RG-FA masonry units. Furthermore, the core materials needed to produce RG-FA masonry units were industrial by-products, the sustainable usage of which would contribute significantly to efficient waste management, through the production of aesthetically pleasing masonry units. [ABSTRACT FROM AUTHOR]

• Polyphosphate amended backfill exhibited excellent containment of metal contamination. • CaCl 2 concentration slightly affected hydraulic conductivity of amended backfill. • Polyphoshpate amendment retarded transport of Ca2+ through the backfill. • Polyphoshpate amendment significantly increased breakthrough time of Ca2+. This study aimed at assessing containment properties of slurry wall backfill containing sodium hexametaphosphate (SHMP) modified calcium bentonite to contaminated groundwater. Free swell index and flexible-wall hydraulic conductivity (k) tests were conducted to investigate swelling and hydraulic performances of this novel backfill. The relative concentration (RC) and cumulative mass ratio (CMR) methods were adopted to assess transport parameters including hydrodynamic dispersion coefficient (D) and the retardation factor (R d) of calcium ion (Ca2+) through the backfills. The results showed negligible to moderate decrease in the swell index of the modified bentonite and the modified backfill when exposed to 5 mM and 1 M calcium chloride (CaCl 2) solution. Permeation with CaCl 2 solutions increased the short-term k of the backfill by a factor of 0.9 to 1.1 as compared to the tap water permeation. The D values varied from 1.44 × 10–10 m2/s to 2.89 × 10–10 m2/s regardless of the CaCl 2 concentration, whereas R d was vulnerable to CaCl 2 concentration. The CMR method yielded a higher coefficient of determination than the RC method, but it showed no particular advantage in evaluating the transport parameters. The SHMP-modification delayed the breakthrough time of Ca2+ by a factor of 15 to 199 in a model slurry trench wall. [ABSTRACT FROM AUTHOR]

[Display omitted] • The raw materials of MASB slurry are eco-friendly and cost-effective. • The MASB slurry has comparative fresh properties with the SCB slurry. • The harden MASB slurry presents excellent mechanical and impervious performances. • The pore structure of MASB slurry is superior to SCB slurry. Slag-cement–bentonite (SCB) slurry has been widely used for the construction of vertical cutoff walls to restrain the movement of groundwater at contaminated sites. However, the conventional cement-based slurry needs a long time to evolve satisfactory performance along with a relatively high environmental load. This paper proposes an innovative material for the self-hardening slurry system, consisting of reactive MgO, slag, bentonite and lots of water. The properties of fresh slurry, unconfined compressive strength (UCS) and permeability of the MgO activated slag and bentonite (MASB) slurry are investigated in comparison with SCB slurry. The results indicate that compared with SCB slurry, MASB slurry with appropriate proportions has comparative fresh properties, but possesses higher UCS and much lower hydraulic conductivity (close to 1.0 × 10−10 m/s) at a later age. A massive formation of expansive hydration products (hydrotalcite phases) could satisfactorily fill in the voids of matrix, leading to a dense microstructure which is responsible for the excellent mechanical and penetrative performance of MASB slurry. The overall findings from this study well demonstrate that the developed MASB slurry holds great potential as a novel, eco-friendly and cost-effective material in the construction of vertical cutoff walls. [ABSTRACT FROM AUTHOR]

[Display omitted] • The self-hardening slurry is highly universal for constructing vertical barriers. • The performances of four self-hardening slurries were reviewed and compared. • The current limitations and future perspectives of self-hardening slurry were proposed. • The chemical compatibility of self-hardening slurry needs to be improved in some specific conditions. • The potential use of green cementitious materials in self-hardening slurry is promising. Landfilling is still the most commonly adopted means for disposal of industrial and municipal solid wastes in the world. However, contaminants from these untreated wastes may spread out with lapse of time and pollute soil and groundwater nearby, which can destroy ecological environment and threaten human's health. Thus, the encapsulation technology is usually applied for preventing spreading of the contaminants. Self-hardening slurry is prevalently used to form cutoff wall as vertical barriers for the control of horizontal migration of contaminants at those sites, due to its technological efficiency, easy operation and handling over other vertical barrier materials. This study comprehensively reviewed and assessed key properties of four types of self-hardening slurry, namely, cement-bentonite (CB) slurry, slag-cement-bentonite (SCB) slurry, fly ash–cement-bentonite (FCB) slurry and Impermix slurry. Emphasis has been put on unconfined compressive strength (UCS), permeability coefficient and chemical compatibility, which characterize the overall performances of these slurries. Apart from conventional factors, some hidden factors, such as cast-into position, depth and confining pressure, water table and scale effect etc., affecting the performance of self-hardening slurries were simultaneously investigated to shed light on the limitations and perspectives in the application of these materials. [ABSTRACT FROM AUTHOR]

• MgO-activated GGBS, bentonite and local clayey sand are used to produce cutoff wall backfill. • Engineering properties and microstructural evolutions of MSB backfill are assessed. • Major hydration products are hydrotalcite-like phases, C-S-H, monosulfate, and portlandite. • Higher bentonite and lower binder content increase cumulative pore volumes. • Binding effect and pore filling by hydration products and swelling bentonite contribute to the engineering performance. An innovative cutoff wall backfill consisting of reactive MgO, ground granulated blast furnace slag (GGBS), bentonite and local clayey sand (MSB) was developed recently for land remediation applications. This paper investigates the engineering characteristics (e.g. strength and permeability) and reaction mechanisms of the MSB backfills with various MgO-activated GGBS (i.e., the binder) and bentonite contents. A series of analytical techniques are employed to identify the hydration products in this complex system. The engineering properties are tested via the unconfined compressive strength (UCS) test and flexible-wall permeation test. Results show that UCS and dry density decrease with increasing bentonite content, while the opposite trends are observed when increasing the binder content. The UCS values increase with curing time and become plateaued after ∼90 days. Meanwhile, the hydraulic conductivity (k) decreases distinctly with the increase of the binder content and bentonite content. All backfills reach UCS of >100 kPa UCS and k <10−8 m/s at 28 days while curing for 90 days leads to increase of UCS by >1.5 times and reduction of k by nearly one order of magnitude. The major hydration products of MSB backfills are identified as hydrotalcite-like phases (Ht), calcium silicate hydrates (C-S-H), monosulfate (AFm) and portlandite (CH). The hydration products, binding adjacent soil particles, filling pores, together with the swelling of bentonite, contribute to the mechanical performance and impermeability of the backfills. [ABSTRACT FROM AUTHOR]