Ladle slag-based binder for the solidification/stabilization of heavy-metal-rich industrial waste.

Heavy metal pollution from industrial sources is a major environmental and health hazard on a global scale. This study introduces a solidification/stabilization method of industrial waste using a waste-based, ettringite-rich solid binder from ladle slag and gypsum for the immobilization of an industrial waste material with extremely high contents of several heavy metals. The importance of sulfate and water content on the immobilization efficiency and the use of citric acid to increase the processing time of the binder were studied. The leaching of Pb, Hg, Se, As, Cd, Cu, and Ni was measured, and X-ray powder diffraction, field-emission scanning electron microscopy, and field-emission electron probe microanalysis combined with wavelength-dispersive X-ray spectroscopy were used to analyze the structure of the hardened binder and the location of the heavy metals within. The study shows that the ladle slag/gypsum binder is suitable for the solidification/stabilization of heavy-metal-rich solid industrial waste. Hg, As, Cd, Cu, and Ni were fully immobilized in all scenarios covered in the study, whereas Pb and Se showed more complicated behaviors. The main immobilization method was encapsulation, and partial Se incorporation into ettringite was observed. The presence of citric acid increased the processing time of the binder without harming the immobilization, unless combined with low sulfate content.

Effects of thermal treatment on the characteristics of pulp mill residue.

In this study, the effects of thermal treatment at moderately low temperatures (Tmax: 525 °C) on the composition and properties of two distinct green liquor dregs (GLD) samples from two different pulp mills were investigated. This in-depth characterization was conducted with the aim of enabling the use of GLD as raw materials in cementitious applications (e.g., alkali-activated binders). The elemental composition of the GLD samples primarily comprised Ca, Mg, S, Na, and Mn. Analysis using x-ray diffractometry, an electron probe microanalyzer, and scanning electron microscopy revealed that calcite was the main crystalline phase, whereas layered double hydroxides with varying compositions constituted the primary amorphous phase. Characterization through Fourier-transform infrared spectroscopy and thermogravimetric analysis coupled with mass spectrometer analysis, indicated the loss of water and carbon during thermal treatment. Total organic carbon analysis showed that most of the removed carbon was organic, despite some remaining after thermal treatment at 525 °C. A leaching test was conducted to quantify the soluble Ca, Mg, Na, and K content, as well as pH variations. Overall, the findings suggest that thermal treatment enhances the potential of GLD samples for use in binder materials in the construction sector.

Fly ash classification efficiency of electrostatic precipitators in fluidized bed combustion of peat, wood, and forest residues.

The increasing use of biomasses in the production of electricity and heat results in an increased amount of burning residue, fly ash which disposal is becoming more and more restricted and expensive. Therefore, there is a great interest in utilizing fly ashes instead of just disposing of it. This study aimed to establish whether the utilization of fly ash from the fluidized bed combustion of peat, wood, and forest residues can be improved by electrostatic precipitator separation of sulfate, chloride, and some detrimental metals. Classification selectivity calculations of electrostatic precipitators for three different fuel mixtures from two different power plants were performed by using Nelson's and Karnis's selectivity indices. Results showed that all fly ashes behaved similarly in the electrostatic separation process SiO2 resulted in coarse fractions with Nelson's selectivity of 0.2 or more, while sulfate, chloride, and the studied detrimental metals (arsenic, cadmium, and lead) enriched into fine fractions with varying selectivity from 0.2 to 0.65. Overall, the results of this study suggest that it is possible to improve the utilization potential of fly ashes from fluidized bed combustion in concrete, fertilizer, and earth construction applications by using electrostatic precipitators for the fractionating of fly ashes in addition to their initial function of collecting fly ash particles from flue gases. The separation of the finer fractions (ESP 2 and 3) from ESP 1 field fly ash is recommended.

Optimizing activating solution and environmental leaching characteristics of Fe-rich alkali-activated Zn slag.

In this work, slag from Zn processing was used to produce Fe-rich alkali-activated materials (AAMs) with low environmental impact. The interconnection between activating solution composition, compressive strength, and environmental leaching characteristics was assessed. The reaction products characterised with FT-IR, XRD, and SEM-WDS were represented by Fe-rich C-S-H gel of tobermorite-related structure. The local aggregation of Na and Mg suggests the minor role of these elements in the reaction product. The reaction product seems to be undependable on the alkali cation used in the solution. Besides, the hardening reaction took place fast, and the maximum compressive strength of 70 MPa was determined only after 1 day after mixing with silicate solutions. To decrease the economic and environmental impact, 1) simultaneous decreasing Na2O/slag and SiO2/slag ratios or 2) decreasing SiO2/Na2O ratio can be applied without prominent deterioration of the strength. Environmental leaching results showed an increase in the leached content of several metal(loid)s (e.g., As, Mo, Cr, Sb, Se, V) as a results of alkali activation, but also some immobilization effect for Ba, Pb and Zn. Also, the presence of liquid silica in the activating solutions or higher water content reduced the leaching of some elements.

Characterization of an aged alkali-activated slag roof tile after 30 years of exposure to Northern Scandinavian weather.

Alkali-activated materials (AAMs) have been known as an alternative cementitious binder in construction for more than 120 years. Several buildings utilizing AAMs were realized in Europe in the 1950s-1980s. During the last 30 years, the interest towards AAMs has been reinvigorated due to the potentially lower CO2 footprint in comparison to Portland cement. However, one often-raised issue with AAMs is the lack of long-term studies concerning durability in realistic conditions. In the present study, we examined a roof tile, which was prepared from alkali-activated blast furnace slag mortar and exposed to harsh Northern Scandinavian weather conditions in Turku, Finland, for approximately 30 years. Characterization of this roof tile provides unique and crucial information about the changes occurring during AAM lifetime. The results obtained with a suite of analytical techniques indicate that the roof tile had maintained excellent durability properties with little sign of structural disintegration in real-life living lab conditions, and thus provide in part assurance that AAM-based binders can be safely adopted in harsh climates. The phase assemblage and nanostructural characterization results reported here further elucidate the long-term changes occurring in AAMs and provide reference points for accelerated durability tests and thermodynamic modelling.

The Effect of Fibrous Reinforcement on the Polycondensation Degree of Slag-Based Alkali Activated Composites.

Alternative cementitious binders, based on industrial side streams, characterized by a low carbon footprint, are profitably proposed to partially replace Portland cement. Among these alternatives, alkali-activated materials have attracted attention as a promising cementitious binder. In this paper, the chemical stability of the matrix, in fiber-reinforced slag-based alkali-activated composites, was studied, in order to assess any possible effect of the presence of the reinforcement on the chemistry of polycondensation. For this purpose, organic fiber, cellulose, and an inorganic fiber, basalt, were chosen, showing a different behavior in the alkaline media that was used to activate the slag fine powders. The novelty of the paper is the study of consolidation by means of chemical measurements, more than from the mechanical point of view. The evaluation of the chemical behavior of the starting slag in NaOH, indeed, was preparatory to the understanding of the consolidation degree in the alkali-activated composites. The reactivity of alkali-activated composites was studied in water (integrity test, normed leaching test, pH and ionic conductivity), and acids (leaching in acetic acid and HCl attack). The presence of fibers does not favor nor hinder the geopolymerization process, even if an increase in the ionic conductivity in samples containing fibers leads to the hypothesis that samples with fibers are less consolidated, or that fiber dissolution contributes to the conductivity values. The amorphous fraction was enriched in silicon after HCl attack, but the structure was not completely dissolved, and the presence of an amorphous phase is confirmed (C-S-H gel). Basalt fibers partly dissolved in the alkaline environment, leading to the formation of a C-N-A-S-H gel surrounding the fibers. In contrast, cellulose fiber remained stable in both acidic and alkaline conditions.

Nanostructured and Advanced Designs from Biomass and Mineral Residues: Multifunctional Biopolymer Hydrogels and Hybrid Films Reinforced with Exfoliated Mica Nanosheets.

Transforming potential waste materials into high-value-added sustainable materials with advanced properties is one of the key targets of the emerging green circular economy. Natural mica (muscovite) is abundant in the mining industry, which is commonly regarded as a byproduct and gangue mineral flowing to waste rock and mine tailings. Similarly, chitin is the second-most abundant biomass resource on Earth after cellulose, extracted as a byproduct from the exoskeleton of crustaceans, fungal mycelia, and mushroom wastes. In this study, exfoliated mica nanosheets were individualized using a mechanochemical process and incorporated into regenerated chitin matrix through an alkali dissolution system (KOH/urea) to result in a multifunctional, hybrid hydrogel, and film design. The hydrogels displayed a hierarchical and open nanoporous structure comprising an enhanced, load-bearing double-cross-linked polymeric chitin network strengthened by mica nanosheets possessing high stiffness after high-temperature curing, while the hybrid films (HFs) exhibited favorable UV-shielding properties, optical transparency, and dielectric properties. These hybrid designs derived from industrial residues pave the way toward sustainable applications for many future purposes, such as wearable devices and tissue engineering/drug delivery.

Characterization of mineral wool waste chemical composition, organic resin content and fiber dimensions: Aspects for valorization.

Despite mineral wool waste is only a small fraction of total construction and demolition waste (CDW) by mass, it requires large transportation and landfilling capacities due to its low bulk density, and its utilization remains low compared to other CDW types. It is essential to understand the physical and chemical properties of this waste fraction in order to utilize it, e.g. as fiber reinforcement in composites or as supplementary cementitious material. Here, we provide a chemical and physical characterization of 15 glass wool and 12 stone wool samples of different ages collected from various locations across Europe. In addition, the chemical compositions of 61 glass and stone wool samples obtained from the literature are presented. Glass wool samples show little variation in their chemical composition, which resembles the composition of typical soda-lime silicate glass. Stone wool presents a composition similar to basaltic glass but with variability between samples in terms of calcium, magnesium, and iron content. Potentially toxic elements, such as Cr, Ba, and Ni, are present in mineral wools, but in low concentrations (<0.2%). Both wool types contain organic resin, which may decompose into smaller molecular fragments and ammonia upon heating or contact with alkaline solution. Mineral wool wastes have relatively similar length and width distributions, despite the age and type of the mineral wool. Overall, both mineral wool waste types have homogenous chemical and physical properties as compared to many other mineral wastes which makes their utilization as a secondary raw material promising.

Sustainable batching water options for one-part alkali-activated slag mortar: Sea water and reverse osmosis reject water.

Concrete production is globally a major water consumer, and in general, drinking-quality water is mixed in the binder. In the present study, simulated sea water and reverse osmosis reject water were used as batching water for one-part (dry-mix) alkali-activated blast furnace slag mortar. Alkali-activated materials are low-CO2 alternative binders gaining world-wide acceptance in construction. However, their production requires approximately similar amount of water as regular Portland cement concrete. The results of the present study revealed that the use of saline water did not hinder strength development, increased setting time, and did not affect workability. The salts incorporated in the binder decreased the total porosity of mortar, but they did not form separate phases detectable with X-ray diffraction or scanning electron microscopy. Leaching tests for monolithic materials revealed only minimal leaching. Furthermore, results for crushed mortars (by a standard two-stage leaching test) were within the limits of non-hazardous waste. Thus, the results indicated that high-salinity waters can be used safely in one-part alkali-activated slag to prepare high-strength mortars. Moreover, alkali-activation technology could be used as a novel stabilization/solidification method for reverse osmosis reject waters, which frequently pose disposal problems.

Production and properties of ferrite-rich CSAB cement from metallurgical industry residues.

Blast furnace slag from the steel industry is commercially utilized as a cement replacement material without major processing requirements; however, there are many unutilized steel production slags which differ considerably from the blast furnace slag in chemical and physical properties. In this study, calcium sulfoaluminate belite (CSAB) cement clinkers were produced using generally unutilized metallurgical industry residues: AOD (Argon Oxygen Decarburisation) slag from stainless steel production, Fe slag from zinc production, and fayalitic slag from nickel production. CSAB clinker with a target composition of ye'elimite-belite-ferrite was produced by firing raw materials at 1300 °C. The phase composition of the produced clinkers was identified using quantitative XRD analyses, and the chemical composition of the clinker phases produced was established using FESEM-EDS and mechanical properties were tested through compressive strength test. It is demonstrated that these metallurgical residues can be used successfully as alternative raw materials for the production of CSAB cement that can be used for special applications. In addition, it is shown that the available quantities of these side-streams are enough for significant replacement of virgin raw materials used in cement production.

Towards designing reactive glasses for alkali activation: Understanding the origins of alkaline reactivity of Na-Mg aluminosilicate glasses.

Alkali-activated materials (AAMs), sometimes called geopolymers, are eco-friendly cementitious materials with reduced carbon emissions when compared to ordinary Portland cement. However, the availability of most precursors used for AAM production may decline in the future because of changes in industrial sectors. Thus, new precursors must be developed. Recently there has been increased interest in synthetic glass precursors. One major concern with using synthetic glasses is ensuring that they react sufficiently under alkaline conditions. Reactivity is a necessary, although not sufficient, requirement for a suitable precursor for AAMs. This work involves the synthesis, characterization, and estimation of alkaline reactivity of Na-Mg aluminosilicate glasses. Structural characterization showed that replacing Na with Mg led to more depolymerization. Alkaline reactivity studies indicated that, as Mg replaced Na, reactivity of glasses increased at first, reached an optimal value, and then declined. This trend in reactivity could not be explained by the conventional parameters used for estimating glass reactivity: the non-bridging oxygen fraction (which predicts similar reactivity for all glasses) and optical basicity (which predicts a decrease in reactivity with an increase in Mg replacement). The reactivity of the studied glasses was found to depend on two main factors: depolymerization (as indicated by structural characterization) and optical basicity. Depolymerization dominated initially, which led to an increase in reactivity, while the effect of optical basicity dominated later, leading to a decrease in reactivity. Hence, while designing reactive synthetic glasses for alkali activation, structural study of glasses should be given due consideration in addition to the conventional factors.

Production of Lightweight Alkali Activated Mortars Using Mineral Wools.

This experimental study aimed to develop a fiber-reinforced lightweight mineral wool-based alkali activated mortar. The lightweight mineral wool-based alkali activated mortars were produced using premade foam and reinforced by polypropylene (PP) fibers. They were assessed in terms of fresh and hardened-state properties. Fresh-state properties were investigated by mini-slump tests. Hardened-state characteristics were assessed by ultrasonic pulse velocity, dry density, compressive and flexural strengths, drying shrinkage, efflorescence, water absorption, and permeable porosity. For the first time, the resistance of the synthesized lightweight mineral wool-based alkali activated mortars against harsh conditions (carbonation, freeze and thaw, and high temperature) were evaluated. The porous structures of the developed lightweight alkali activated mortars were also analyzed using an X-ray micro-computed tomography (CT) technique. Lightweight mix compositions with densities in a range of 770-1510 kg/m3, compressive strengths of 1-9 MPa, and flexural strengths of 2.6-8 MPa were developed. Increases in both density and strength after carbonation were also recorded, while a decrease of strength was noticed after exposure to freeze/thaw and high temperatures of up to 500 °C.

Construction and Demolition Waste as Recycled Aggregates in Alkali-Activated Concretes.

The growth of global construction has contributed to an inevitable increase in the amount of construction and demolition (C&D) waste, and the recycling of C&D waste as aggregates in concrete is receiving increased interest, resulting in less demand for normal aggregates and bringing a potential solution for the landfilling of wastes. Recently, several studies have focused on the use of C&D waste in alkali-activated concrete to move one step closer to sustainable concretes. This paper focuses on the main mechanisms of using C&D waste in the resulting physical, mechanical, and durability properties of alkali-activated concrete in fresh and hardened state properties. The main difficulties observed with recycled aggregates (RA) in concrete, such as high levels of water demand, porous structure, and low mechanical strength, occur in RA alkali-activated concretes. These are associated with the highly porous nature and defects of RA. However, the high calcium concentration of RA affects the binder gel products, accelerates the hardening rate of the concrete, and reduces the flowability of alkali-activated concretes. For this reason, several techniques have been investigated for modifying the water content and workability of the fresh matrix and for treating RA and RA/alkali-activated binder interactions to produce more sustainable alkali-activated concretes.

Using Carbonated BOF Slag Aggregates in Alkali-Activated Concretes.

This experimental study aimed to develop alkali-activated concretes containing carbonated basic oxygen furnace (BOF) slag aggregates. In the first stage, the impacts of replacing normal aggregates with carbonated BOF slag aggregates in different alkali-activated concretes were determined by assessing mechanical properties (compressive and flexural strengths), morphology, thermogravimetric analyses (TGA), differential thermogravimetry (DTG) and the crystalline phases using X-ray diffraction analysis. Second, the developed plain alkali-activated concrete was reinforced by different fibre types and dosages to limit the negative impacts of the drying shrinkage and to improve strength. Therefore, the effects of using different fibre contents (1% and 1.5% in Vol.) and types (Polyvinyl alcohol [PVA], Polypropylene [PP], basalt, cellulose and indented short-length steel) on hardened state properties were evaluated. These evaluations were expressed in terms of the compressive and flexural strengths, ultrasonic pulse velocity, mass changes, drying shrinkage and efflorescence. Then, the impacts of aggressive conditions on the hardened properties of fibre-reinforced alkali-activated concretes were evaluated under carbonation, high temperature and freeze/thaw tests. The results showed that using carbonated BOF slag aggregates led to obtain higher strength than using normal aggregates in alkali activated concretes. Moreover, the maximum enhancement due to reinforcing the mixtures was recorded in alkali-activated concretes with steel fibres.

Sonication-assisted surface modification method to expedite the water removal from cellulose nanofibers for use in nanopapers and paper making.

This paper addresses the issue of high water retention by cellulose nanofibers (CNFs) that lead to exorbitant time consumption in the dewatering of CNF suspensions. This has been a bottleneck, which is restricting the commercialization of CNF derived products such as nanopapers and CNF reinforced paper sheets. As a remedy, we suggest an eco-friendly water-based approach that involves the use of sonication energy and lactic acid (LA) to modify the surface of CNFs. The suggested modification resulted in rapid water drainage, and dewatering was completed in 10 min; with unmodified CNFs, it took around 45 min. We have also compared the draining characteristics of LA modification of CNF suspensions with a common draining agent (NaCl); LA modification drains water 56% faster than the use of NaCl, and produced mechanically superior dimensionally stable nanopaper. Additionally, LA modification allows the addition of 10 wt.% CNF in paper sheets, with dewatering done in 2 min (while the unmodified CNFs took 23 min).

Stereoselectively water resistant hybrid nanopapers prepared by cellulose nanofibers and water-based polyurethane.

Cellulose nanopapers, known for excellent mechanical properties, loses 90% of their stiffness in the wet conditions. In this study, we attempt to improve the wet mechanical properties of cellulose nanopaper by incorporating polyurethane by a novel and ecofriendly method. Water based PU was dispersed along with CNFs in water and hybrid nanopapers were prepared by draining water under vacuum followed by forced drying. These hybrid nanopapers have a gradient interpenetrating structure with PU concentrated towards one side and CNFs towards the other, which was confirmed by scanning electron microscopy, x-ray photoelectron spectroscopy and contact angle measurements. Because of this, the nanopapers are water resistant on one surface (PU rich side) and hydrophilic on the other (cellulose rich side), making them stereoselectively water resistant. When wetted with water on the PU side, the hybrid nanopaper with 10% PU is able to retain 65% modulus; on the other hand, the reference retains only 10% of the modulus. Similar results are seen in the tensile and the yield strength. Additionally, the hybrid nanopapers have higher elongation and improved thermal stability. The reported material is relevant to the applications such as flexible electronics and transparent displays.

The effect of seasonal variations on floc morphology in the activated sludge process.

The effect of seasonal variations on floc formation in the activated sludge process (ASP) was studied in a municipal wastewater treatment plant in Finland nearly 16 months. Floc formation was measured with an online optical monitoring device, and results were correlated with the temperature of the upcoming wastewater and the treatment efficiency of the ASP. Results showed that floc formation has a clear, seasonal pattern, with flocs in summer being larger and rounder and having fewer filaments and small particles. In addition, treatment efficiency increased in summer. The study correlated the results of image analysis with the composition (chemical oxygen demand and suspended solids content) and temperature of the wastewater before and after the ASP. Results showed that the composition of upcoming wastewater has no clear correlation with floc morphological parameters. However, the wastewater temperature clearly correlated with floc formation. Results indicated that cold winter conditions enhanced the growth of filamentous bacteria in wastewater, decreasing treatment efficiency. Furthermore, these results confirmed that floc formation has seasonal variations.

Utilization of Mineral Wools as Alkali-Activated Material Precursor.

Mineral wools are the most common insulation materials in buildings worldwide. However, mineral wool waste is often considered unrecyclable because of its fibrous nature and low density. In this paper, rock wool (RW) and glass wool (GW) were studied as alkali-activated material precursors without any additional co-binders. Both mineral wools were pulverized by a vibratory disc mill in order to remove the fibrous nature of the material. The pulverized mineral wools were then alkali-activated with a sodium aluminate solution. Compressive strengths of up to 30.0 MPa and 48.7 MPa were measured for RW and GW, respectively, with high flexural strengths measured for both (20.1 MPa for RW and 13.2 MPa for GW). The resulting alkali-activated matrix was a composite-type in which partly-dissolved fibers were dispersed. In addition to the amorphous material, sodium aluminate silicate hydroxide hydrate and magnesium aluminum hydroxide carbonate phases were identified in the alkali-activated RW samples. The only crystalline phase in the GW samples was sodium aluminum silicate. The results of this study show that mineral wool is a very promising raw material for alkali activation.

Alkali activation of recovered fuel-biofuel fly ash from fluidised-bed combustion: Stabilisation/solidification of heavy metals.

Recovered fuel-biofuel fly ash from a fluidized bed boiler was alkali-activated and granulated with a sodium-silicate solution in order to immobilise the heavy metals it contains. The effect of blast-furnace slag and metakaolin as co-binders were studied. Leaching standard EN 12457-3 was applied to evaluate the immobilisation potential. The results showed that Ba, Pb and Zn were effectively immobilised. However, there was increased leaching after alkali activation for As, Cu, Mo, Sb and V. The co-binders had minimal or even negative effect on the immobilisation. One exception was found for Cr, in which the slag decreased leaching, and one was found for Cu, in which the slag increased leaching. A sequential leaching procedure was utilized to gain a deeper understanding of the immobilisation mechanism. By using a sequential leaching procedure it is possible fractionate elements into watersoluble, acid-soluble, easily-reduced and oxidisable fractions, yielding a total 'bioavailable' amount that is potentially hazardous for the environment. It was found that the total bioavailable amount was lower following alkali activation for all heavy metals, although the water-soluble fraction was higher for some metals. Evidence from leaching tests suggests the immobilisation mechanism was chemical retention, or trapping inside the alkali activation reaction products, rather than physical retention, adsorption or precipitation as hydroxides.