Industrial by-products-derived binders for in-situ remediation of high Pb content pyrite ash: Synergistic use of ground granulated blast furnace slag and steel slag to achieve efficient Pb retention and CO2 mitigation.

Ordinary Portland cement (OPC) is a cost-effective and conventional binder that is widely adopted in brownfield site remediation and redevelopment. However, the substantial carbon dioxide emission during OPC production and the concerns about its undesirable retention capacity for potentially toxic elements strain this strategy. To tackle this objective, we herein tailored four alternative binders (calcium aluminate cement, OPC-activated ground-granulated blast-furnace slag (GGBFS), white-steel-slag activated GGBFS, and alkaline-activated GGBFS) for facilitating immobilization of high Pb content pyrite ash, with the perspectives of enhancing Pb retention and mitigating anthropogenic carbon dioxide emissions. The characterizations revealed that the incorporation of white steel slag efficiently benefits the activity of GGBFS, herein facilitating the hydration products (mainly ettringite and calcium silicate hydrates) precipitation and Pb immobilization. Further, we quantified the cradle-to-gate carbon footprint and cost analysis attributed to each binder-Pb contaminants system, finding that the application of these alternative binders could be pivotal in the envisaged carbon-neutral world if the growth of the OPC-free roadmap continues. The findings suggest that the synergistic use of recycled white steel slag and GGBFS can be proposed as a profitable and sustainable OPC-free candidate to facilitate the management of lead-contaminated brownfield sites. The overall results underscore the potential immobilization mechanisms of Pb in multiple OPC-free/substitution binder systems and highlight the urgent need to bridge the zero-emission insights to sustainable in-situ solidification/stabilization technologies.

Mechanistic insights into Pb and sulfates retention in ordinary Portland cement and aluminous cement: Assessing the contributions from binders and solid waste.

Identifying immobilization mechanisms of potentially toxic elements (PTEs) is of paramount importance in the field application of solidification/stabilization. Traditionally, demanding and extensive experiments are required to better access the underlying retention mechanisms, which are usually challenging to quantify and clarify precisely. Herein, we present a geochemical model with parametric fitting techniques to reveal the solidification/stabilization of Pb-rich pyrite ash through conventional (ordinary Portland cement) and alternative (calcium aluminate cement) binders. We found that ettringite and calcium silicate hydrates exhibit strong affinities for Pb at alkaline conditions. When the hydration products are unable to stabilize all the soluble Pb in the system, part of the soluble Pb may be immobilized as Pb(OH)2. At acidic and neutral conditions, hematite from pyrite ash and newly-formed ferrihydrite are the main controlling factors of Pb, coupled with anglesite and cerussite precipitation. Thus, this work provides a much-needed complement to this widely-applied solid waste remediation technique for the development of more sustainable mixture formulations.

Detecting Early-Stage Cohesion Due to Calcium Silicate Hydration with Rheology and Surface Force Apparatus.

Extremely robust cohesion triggered by calcium silicate hydrate (C-S-H) precipitation during cement hardening makes concrete one of the most commonly used man-made materials. Here, in this proof-of-concept study, we seek an additional nanoscale understanding of early-stage cohesive forces acting between hydrating model tricalcium silicate (C3S) surfaces by combining rheological and surface force measurements. We first used time-resolved small oscillatory rheology measurements (SAOSs) to characterize the early-stage evolution of the cohesive properties of a C3S paste and a C-S-H gel. SAOS revealed the reactive and viscoelastic nature of C3S pastes, in contrast with the nonreactive but still viscoelastic nature of the C-S-H gel, which proves a temporal variation in the cohesion during microstructural physicochemical rearrangements in the C3S paste. We further prepared thin films of C3S by plasma laser deposition (PLD) and demonstrated that these films are suitable for force measurements in the surface force apparatus (SFA). We measured surface forces acting between two thin C3S films exposed to water and subsequent in situ calcium silicate hydrate precipitation. With the SFA and SFA-coupled interferometric measurements, we resolved that C3S surface reprecipitation in water was associated with both increasing film thickness and progressively stronger adhesion (pull-off force). The lasting adhesion developing between the growing surfaces depended on the applied load, pull-off rate, and time in contact. These properties indicated the viscoelastic character of the soft, gel-like reprecipitated layer, pointing to the formation of C-S-H. Our findings confirm the strong cohesive properties of hydrated calcium silicate surfaces that, based on our preliminary SFA measurements, are attributed to sharp changes in the surface microstructure. In contact with water, the brittle and rough C3S surfaces with little contact area weather into soft, gel-like C-S-H nanoparticles with a much larger surface area available for forming direct contacts between interacting surfaces.

Retention of phosphorus and fluorine in phosphogypsum for cemented paste backfill: Experimental and numerical simulation studies.

The solidification/stabilization of phosphogypsum using cemented paste backfill (OCPB) provides a low-cost and alternative in-situ technique for recycling phosphogypsum stockpiles. But the OCPB is far from obtaining steady states in which the pollutants would redistribute as a response to dynamic environmental conditions. Further, the associated chemical interactions and the mineralogy information of the solubility-controlling phases of contaminants (fluorine and phosphorus) have not been thoroughly studied or fully understood. In this study, a framework coupling the chemical, mineralogical, and morphological analyses is used to determine the fluoride and phosphate retention mechanisms of immobilized OCPB. Then the pH-dependent leaching tests and numerical simulation is applied as a useful tool to identify the minerals controlling stabilized OCPB leaching behavior. The overall findings proved that aluminate-rich calcium silicate hydrates play an essential role in fluoride and phosphate retention. Both experimental and simulational acid neutralization and leaching curves indicate that the cementitious matrix works as a strong buffering material ensuring high pH conditions that are necessary for fluorine and phosphorus retention. Although discrepancies were observed in absolute fluorine and phosphorus leaching values at highly acidic conditions, the simulations are able to describe highly amphoteric leaching behavior. The simulation suggests that the aluminum species and calcium phosphates governed the solubility of fluorine and phosphorus, respectively. The results of this work would have implications for predicting the leaching behavior of OCPB in detrimental and multiple environments.

Pursuing unprecedented anisotropic morphologies of halide-free Pd nanoparticles by tuning their nucleation and growth.

In this paper, a highly effective and scalable polyol-based modified procedure is reported, yielding shape-controlled Pd nanoparticles (NPs) formed via two distinct growth mechanisms as a function of apparent pH. Starting from tetraethylammonium tetrahydroxypalladate (TEA)2[Pd(OH)4], a halide-free precursor, the resulting shape of the NPs ranged from highly defective worm-like nanostructures to well-defined polyhedra (tetrahedra, octahedra and 5-fold twins) as shown by TEM, HRTEM, and STEM. The effect of the different synthesis parameters was thoroughly investigated, finding that apparent pH - modulated by adding diluted HNO3 - is the key parameter in determining the final size and shape of Pd NPs, whose evolution was followed during the reaction. A rational explanation of the observed shape modification as a function of apparent pH was proposed. The as-prepared Pd NPs, once dried, were analysed by means of XRD. DRIFT spectroscopy was used to show how CO binds on the Pd NPs after deposition on γ-Al2O3 as catalytic support.

An Atomistic Model Describing the Structure and Morphology of Cu-Doped C-S-H Hardening Accelerator Nanoparticles.

Calcium silicate hydrate (C-S-H) is the main binding phase in Portland cement. The addition of C-S-H nanoparticles as nucleation seeds has successfully been used to accelerate the hydration process and the precipitation of binding phases either in conventional Portland cement or in alternative binders. Indeed, the modulation of the hydration kinetics during the early-stage dissolution-precipitation reactions, by acting on the nucleation and growth of binding phases, improves the early strength development. The fine-tuning of concrete properties in terms of compressive strength and durability by designed structural modifications can be achieved through the detailed description of the reaction products at the atomic scale. The nano-sized, chemically complex and structurally disordered nature of these phases hamper their thorough structural characterization. To this aim, we implement a novel multi-scale approach by combining forefront small-angle X-ray scattering (SAXS) and synchrotron wide-angle X-ray total scattering (WAXTS) analyses for the characterization of Cu-doped C-S-H nanoparticles dispersed in a colloidal suspension, used as hardening accelerator. SAXS and WAXTS data were analyzed under a unified modeling approach by developing suitable atomistic models for C-S-H nanoparticles to be used to simulate the experimental X-ray scattering pattern through the Debye scattering equation. The optimization of atomistic models against the experimental pattern, together with complementary information on the structural local order from 29Si solid-state nuclear magnetic resonance and X-ray absorption spectroscopy, provided a comprehensive description of the structure, size and morphology of C-S-H nanoparticles from the atomic to the nanometer scale. C-S-H nanoparticles were modeled as an assembly of layers composed of 7-fold coordinated Ca atoms and decorated by silicate dimers and chains. The structural layers are a few tens of nanometers in length and width, with a crystal structure resembling that of a defective tobermorite, but lacking any ordering between stacking layers.

A Fresh View on Limestone Calcined Clay Cement (LC3) Pastes.

In this work, the factors controlling the fresh state properties of limestone calcined clay cement (LC3) are assessed and compared to Portland and binary cements, extending the scope of previous research by combining rheological measurements with setting time determination and the evaluation of plastic shrinkage by a novel method. Yield stress and elastic modulus are considered indicators for the structural build-up/breakdown process when stress is applied to the system. On the other hand, plastic shrinkage occurs from the mixing to the setting of fresh paste and plays an important role in governing microstructural changes due to settlement and evaporation. Evaluation of the rheological properties with time was appropriate to give an overview of the influence and behavior of different added materials. The elastic modulus of all binders (clinker, LC3, clinker-limestone, and clinker-calcined clay) was increased from mixing to 60 min of curing as follows: 5.27 × 103 to 9.50 × 105 Pa, 5.94 × 103 to 9.87 × 105 Pa, 6.89 × 103 to 5.62 × 105 Pa and 7.85 × 103 to 1.27 × 106 Pa, respectively. Moreover, during the first three hours of curing, LC3 exhibited a reduction of plastic shrinkage by more than a factor of 2 compared to clinker cement. The use of calcined clay with clinker increases the elastic modulus of the system due to the flocculation effect and increased water absorption, while a dilution effect is contributed due to deflocculation and a free-water increase in the system when a high fraction of limestone is present in the binary cement. The combination of limestone and calcined clay with clinker can induce additional chemical reactions, which control the early age properties, such as plastic shrinkage. The obtained results can contribute to optimizing the fresh state properties of ternary blends of OPC, calcined clay, and limestone through a knowledge-based approach.

Cement-stabilized contaminated soil: Understanding Pb retention with XANES and Raman spectroscopy.

Cement-based stabilization is a widespread technique used for the treatment of contaminated soils. Despite its established application, the mechanisms involved in the stabilization of contaminants are not fully understood yet. This work aims to elucidate the fate of a real Pb contaminated soil amended with different binders, by studying Pb local environment prior and after the stabilization process. The study of a complex historically contaminated soil was coupled with the investigation of simplified artificial systems, developed to model Pb local structure in the unknown newly formed hybrid systems of soil and binders. The use of synchrotron-based element-specific X-ray absorption spectroscopy (XAS) permitted to probe the actual transformation of Pb environment in the real contaminated soil after the stabilization process. With the support of the model systems, we can propose as the main mechanism involved in Pb retention in sulfated soil treated with cement, the incorporation and/or adsorption of Pb on calcium silicate hydrates and ettringite.

Stabilization of lead contaminated soil with traditional and alternative binders.

The application of an innovative solidification/stabilization (S/S) process was investigated for the remediation of Pb contaminated soil. The performance of Pb stabilization was evaluated by comparing the use of calcium aluminate cement (CAC) and an alkali activated metakaolin binder vs the Ordinary Portland Cement (OPC). The phase composition of the stabilized products was investigated by XRD and correlated to the internal microstructure obtained by SEM-EDX imaging. Leaching tests were performed to ascertain the effectiveness of the proposed binders in the S/S of the contaminated soil, and Pb release was evaluated for each binding system. The overall results proved that multiple mechanisms are involved in Pb retention and that key parameters regulating the stabilization performance are strongly dependent on the type of applied binder system. Pb was found to be associated to C-S-H in the case of OPC, whereas ettringite played a key role in the retention of this contaminant using the CAC binder. The use of a NaOH activated metakaolin resulted in almost total retention of Pb, despite a lack of solidification, highlighting the importance of pH in the regulation of the leaching behavior.

Mesoporous Carbon with Different Density of Thiophenic-Like Functional Groups and Their Effect on Oxygen Reduction.

The metal-support interactions between sulfur-doped carbon supports (SMCs) and Pt nanoparticles (NPs) were investigated, aiming at verifying how sulfur functional groups can improve the electrocatalytic performance of Pt NPs towards the oxygen reduction reaction (ORR). SMCs were synthetized, tailoring the density of sulfur functional groups, and Pt NPs were deposited by thermal reduction of Pt(acac)2 . The extent of the metal-support interaction was proved by X-ray photoelectron spectroscopy (XPS) analysis, which revealed a strong electronic interaction, proportional to the density of sulfur defects, whereas XRD spectra provided evidence of higher strain in Pt NPs loaded on SMC. DFT simulations confirmed that the metal-support interaction was strongest in the presence of a high density of sulfur defects. The combination of microstrain and electronic effects resulted in a high catalytic activity of supported Pt NPs towards ORR, with linear correlations of the half-wave potential E1/2 or the kinetic current jk with the sulfur content in the support. Furthermore, a mass activity value (550 A g-1 ) well above the United States Department of Energy target of 440 A g-1 at 0.9 V (vs. reversible hydrogen electrode, RHE), was determined.

Structural relaxation around Cr3+ in YAlO3-YCrO3 perovskites from electron absorption spectra.

The structural relaxation around Cr(3+) in YAl(1-x)Cr(x)O(3) perovskites was investigated and compared with analogous Cr-Al joins (corundum, spinel, garnet). Eight compositions (x(Cr)((3+)) from 0 to 1) were prepared by sol-gel combustion and were analyzed by a combined X-ray diffraction (XRD) and electron absorption spectroscopy (EAS) approach. The unit cell parameters and the XRD averaged octahedral (Cr,Al)-O and ([VIII])Y-O bond distances scale linearly with the chromium fraction. The optical parameters show an expected decrease of crystal field strength (10Dq) and an increase of covalency (B(35)) and polarizability (B(55)) toward YCrO(3), but a nonlinear trend outlines some excess 10Dq below x(Cr)((3+)) approximately 0.4. The local Cr-O bond lengths, as calculated from EAS, indicate a compression from 1.98 A (x(Cr)((3+)) = 1.0) down to 1.95 A (x(Cr)((3+)) = 0.035) so that the relaxation coefficient of perovskite (epsilon = 0.54) is the lowest in comparison with garnet (epsilon = 0.74), spinel (epsilon = 0.68), and corundum (epsilon = 0.58) in contrast with its structural features. The enhanced covalent character of the Cr(3+)-O-Cr(3+) bond in the one-dimensional arrangement of corner-sharing octahedra can be invoked as a factor limiting the perovskite polyhedral network flexibility. The increased probability of Cr-O-Cr clusters for x(Cr)((3+)) greater than approximately 0.4 is associated to diverging trends of nonequivalent interoctahedral angles. The relatively low relaxation degree of Y(Al,Cr)O(3) can be also understood by considering an additional contribution to 10Dq because of the electrostatic potential of the rest of the lattice ions upon the localized electrons of the CrO(6) octahedron. Such an "excess" of 10Dq increases when the point symmetry of the Cr site is low, as in perovskite, and would be affected by the change of yttrium effective coordination number observed by XRD for x(Cr)((3+)) greater than approximately 0.4. This would justify the systematic underestimation of local Cr-O bond distances, as inferred from EAS, compared to what is derived from X-ray absorption (XAS) studies implying a stronger degree of relaxation around Cr(3+) of all the structures considered and supporting the hypothesis that 10Dq from EAS contains more information than previously retained particularly an additional contribution from the next nearest neighboring ions.

Rietveld refinement on x-ray diffraction patterns of bioapatite in human fetal bones.

Bioapatite, the main constituent of mineralized tissue in mammalian bones, is a calcium-phosphate-based mineral that is similar in structure and composition to hydroxyapatite. In this work, the crystallographic structure of bioapatite in human fetuses was investigated by synchrotron radiation x-ray diffraction (XRD) and microdiffraction ( micro -XRD) techniques. Rietveld refinement analyses of XRD and micro -XRD data allow for quantitative probing of the structural modifications of bioapatite as functions of the mineralization process and gestational age.