Slag Grains Research Articles

On the action mechanism of phosphate-based superplasticizers in one-part alkali-activated slag

The mechanisms responsible for the compromised or lost dispersing capability of polycarboxylate ether (PCE) superplasticizers in two-part alkali-activated slag (AAS) have been extensively studied, but limited scientific understanding is available on how PCEs function or lose their dispersing efficiency in one-part AAS systems, particularly those prepared using greener carbonate -based solid activators. This study investigates the dispersing and adsorption behaviors of phosphate-based PCE in one-part AAS activated by K2CO3 or a combination of K2CO3 and CaO. Our findings indicate that in K2CO3-activated slag systems, the loss of phosphate-based PCE efficiency primarily results from the conformational contraction of PCE molecules, which deteriorates steric repulsion in the alkaline activating solution, rather than the insolubility of PCEs in alkaline solution or competitive adsorption between PCEs and carbonate anions on dissolving slag grains. In K2CO3-CaO-activated slag systems, when PCE dosage is below 5 mg/g, the consumption of PCE by early precipitates (e.g., C-A-S-H, gaylussite, calcite) contributes substantially to its lost efficiency; however, once the PCE dosage surpasses this threshold, the dominant factor influencing PCE efficiency shifts to the conformational change of PCE.

Exploring the Efficiency of Magnetic Separation and Gravity Concentration for Valorizing Pb-Zn Smelter Slag in a Circular Economy Framework.

The presented work offers an innovative process scheme for valorizing Pb-Zn slag, which involves crushing, grinding, and separation techniques to concentrate valuable components (non-ferrous metals). This methodology could have a significant impact on the global beneficiation of metallurgical slags since it is significantly more simple, environmentally friendly, and cost-effective than standard pyro- and hydrometallurgical procedures. According to previous physicochemical and mineralogical studies, Pb-Zn slag is a valuable secondary raw material. This inhomogeneous technogenic resource contains substantial amounts of non-ferrous metals (Pb, Zn, Cu, and Ag). However, laboratory tests have indicated that the Pb-Zn slag contains highly uneven amounts of valuable metals, ranging from several g/ton to tens of g/ton. The main issue is that traditional metallurgical procedures for releasing beneficial elements are not commercially viable since the elements are "trapped" within the amorphous aluminosilicates or intergrowths of alloy grains and glassy phases. Gravity concentration (Wilfley 13 shaking table) and magnetic separation (Davis separator and disk separator) were used to obtain the final concentrate following comminution and grindability testing. The gravity concentration proved more effective. Namely, magnetic separators could not process nor adequately separate beneficial non-ferrous elements because they were merged together with iron-bearing minerals and aluminosilicates in amorphous Pb-Zn slag grains. With the gravity concentration approach, 12.99% of the processed slag belonged to ∆T fraction (concentration of non-ferrous metal alloys), while remaining 87% corresponded to the tailings fraction (∆L). The total amounts of recovered Pb, Zn, Cu, and Ag from ∆T and ∆L fractions were 5.28%, 6.69%, 0.58%, and 76.12 ppm and 1.22%, 6.05%, 0.43%, and 15.26 ppm, respectively. This streamlined approach to valorizing Pb-Zn slag can reduce the need for hazardous chemicals used in hydrometallurgical refinement operations, as well as the extremely high temperatures required for pyrometallurgical processing. This is the first study to investigate the viability of this novel methodology, which involves the direct examinations of the Pb-Zn slag feed with various alternative technologies for separation and concentration. After extracting the valuable metals, the amorphous aluminosilicate part of the Pb-Zn slag can be reapplied as an alternative raw material in the building sector, adding to the circularity of the suggested approach.

Effect of Slag on the Porosity and Microstructure of HPC Reinforced with Hybrid Steel Fiber

Due to their structural and economic performance, high performance metal fiber concretes are increasingly used in construction, but with the addition of metal fibers, workability decreases as the amount of fibers added increases. In order to improve workability, a larger quantity of fines should be used than for ordinary concrete. The use of granulated slag reduces the amount of clinker needed to make cement. It is considered that granulated slag improves the workability of concrete. According to Manai, this is due to the very low rates of water adsorption by the granulated slag grains at the start of mixing. Also, the addition of granulated slag has a positive effect on the porosity, but also on the microstructure of the hydrates. These two characteristics are factors determining the compressive strength and durability of the cement paste. The aim of this study is to define the effect of blast furnace slag on the porosity and microstructure of high performance fibre-reinforced concrete. The results of SEM and DRX observation of the microstructure of the concrete samples show that the addition of ultra-fine granulated slag resulted in a relative improvement in porosity and microstructure.

3-Dimensional insight into zonation within slag rims of aged blended cement

Through the integration of SEM-BSE and TEM, we gained a comprehensive 3-dimensional understanding of different distribution patterns of inner hydration products of slag. For fully hydrated small slag grains, two distinct sub-zones were formed in the rims. Lath-like, well-crystalline hydrotalcite-like crystals were found to precipitate, grow, and accumulate near the boundary, forming a layer with a thickness slightly exceeding 0.5 μm. In the center, entrapped calcium and silicon played roles in the formation of a homogeneous and fibrous C−(A)–S–H gel phase. The concentration equilibrium between cement matrix and grain core led to the establishment of a similar grey pixel value and Ca/Si atomic ratio of gel phase at ~1.10. As the size of slag grains increased, three sub-zones became visible. Hydrotalcite-like phase was enriched near the boundary, followed by a sandwiched area abundant in C–(A)–S–H gel phase. Due to the low mobility and increased migration distance, newly released magnesium from reaction front accumulated locally to form a new Mg-rich region.

On the chemo-mechanical evolution process of high-volume slag cement paste

This study investigated the evolution process of high-volume slag cement (HVSC) paste from a chemo-mechanical standpoint. HVSC specimens with a 70 w.t. % slag replacement rate were studied at various ages. Evolution of phase assemblage, microstructure development, and micromechanical properties were analyzed using TGA/XRD/MIP/SEM-EDS and nano-/micro-indentation techniques. A two-scale micromechanical model was built to predict the effective elastic modulus based on the nanoindentation results. Key findings include: 1) Between 7 and 28 days, the formation of calcium silicate hydrate (C-S-H) gel phase improves the effective elastic modulus by filling capillary pores; 2) From 28 to 90 days, the phase assemblage and microstructure remain stable, with a transition from low-density to high-density C-S-H; 3) Between 90 days and 2 years, slag rims produced by slag grains result in increased elastic modulus; 4) The two-scale micromechanical model, combined with nanoindentation data, accurately predicts the effective modulus of HVSC composites, although the unhydrated slag grains-hydrated cement matrix interface may cause an overestimation at an early age. With longer curing time, this interface disappears owing to the continuous hydration of large slag particles and therefore a good match is found between the modelling and experimental results.

An evaluation of the feasibility of electrostatic separation for physical soil washing

We present the first application of electrostatic separation for soil washing. Soil samples were collected from the PTE-containing area of La Cruz in Linares, southern Spain. Using a single-phase high-tension roll separator with voltages ranging from 20 kV to 41.5 kV, we achieved yield values between 0.69% and 9%, with high recovery rates for certain elements such as Zn, Cu, and Mo. SEM-EDX analysis revealed three particle types, including a non-conductive fraction composed of feldspar, a middling fraction composed of mica, and a conductive fraction consisting of PTE-bearing slag grains. Attributive analysis showed that 41.5 kV was the optimal voltage for maximizing PTE concentration. Overall, electrostatic separation is a promising approach for treating soils contaminated with PTEs, particularly in dry climate areas impacted by mining activities.

EDS Microanalysis of Unhydrated Blast Furnace Slag Grains in Field Concrete with Different Service Life.

Because the essential quality metrics of blast furnace slag are based on its oxide composition, the determination of chemical compositions of unhydrated slag grains in an aged concrete could be useful for understanding its past performance and in predicting the remaining service life of existing slag-bearing concrete. In this research, the authors explored the feasibility of using standard-based energy-dispersive X-ray spectroscopy (EDS) microanalysis, in tandem with electron imaging, as a tool for quantitative measurement of the chemical composition of blast furnace slag grains in cement/concrete. In the experimental study, seven concrete samples representing various service life durations were collected in the Netherlands. The microanalysis results of the samples revealed that the change in slag chemistry is insignificant for samples B (1985) to F (2006); however, elevated CaO and SiO2 contents are found in slag used for sample G (2015), opposite to that of Al2O3 and MgO. After discussing compositional characterization, the paper discusses favorable microanalysis protocols for acceptable elemental quantification accuracy. It was concluded that quantitative EDS microanalysis is a strong tool to characterize the chemical composition of unhydrated slag used in field concrete, which could potentially contribute to understanding the correlations between composition and long-term performance in slag concrete structures.

Role of the grain size on the hydration characteristics of slag in an aged field concrete

This paper studies chemical composition of partially and fully hydrated slag grains in a (nearly) 40-year-old field concrete from the Netherlands. The concrete samples were assumed to be sufficiently aged to contain fully hydrated slag grains as well as partially hydrated large slag particles with thick rims. Our analysis showed that three different elemental zoning could be identified depending on the original slag grain size. Upon full hydration of a small slag grain (i.e., <8 μm), two distinct regions were identified corresponding to a hydrotalcite-like phase in the outer rim and a C–S–H gel phase in the core, respectively. As for medium (8–17 μm) and large (>15 μm) slag grains, three distinct regions were clearly visible. Hydrotalcite-like phase was mainly observed in the outer rim and the core. C–S–H gel phase was found to be precipitated in the region between the outer rim and the core.

On the action mechanism of lignosulfonate plasticizer in alkali-activated slag-based system

Workability improvement of alkali-activated materials by common plasticizers used for ordinary Portland cement is usually an issue, but certain effectiveness is often found for lignosulfonate-based plasticizers (LS), especially for activation with sodium hydroxide (its silicate modulus considered as zero). Therefore, the action of LS depending on the silicate modulus of the activator (0, 0.10, 0.25, 0.50) was studied in this paper, using non-traditional oscillation strain sweep tests. The results showed that LS works for silicate moduli of 0 and 0.10 especially in terms of great reduction in the viscoelastic moduli due to a significant decrease in zeta potential magnitude. However, LS had a marginal effect for the silicate moduli of 0.25 and 0.50, which was attributed to the competitive adsorption of LS and silicates from waterglass onto the slag grains. Although splitting of LS chains in NaOH was proven using X-ray photoelectron spectroscopy, electrostatic repulsion of smaller fragments is preserved.

Kinetic studies of Ni(II) ions adsorption from aqueous solutions using the blast furnace slag (BF slag)

In this work, we used the blast furnace slag for the nickel adsorption in aqueous solution. The physico-chemical characterization showed that the BF slag consists mainly of the silica, lime, and alumina. The specific surface area of the BF slag grains is of the order of 275.8 m2/g. The optimum elimination parameters are the agitation speed 200 rpm, pH 4.5, the adsorption temperature 20 °C, and particle size between 200 and 500 μm. The adsorption capacity and the efficiency of nickel removal by the BF slag after 90 min of agitation are respectively 53.58 mg/g and 92.7%.The experimental adsorption data showed that the pseudo-second-order model was the most appropriate in nickel adsorption kinetics; the adsorption isotherm could be described well by the Langmuir model indicating that the process was monolayer, and intra-particle diffusion is not the sole mechanism involved in this process. Thermodynamic study showed that the Ni(II) elimination by BF slag process is spontaneous, exothermic, and less entropic.

Spatial zonation of a hydrotalcite-like phase in the inner product of slag: New insights into the hydration mechanism

The nano-structural evolution of the inner product of ground granulated blast furnace slag in blended cement at 7, 14, 28 56 and 180d was revealed. Three distinct layered inner hydration product region were observed because of the spatial zonation of hydrotalcite-like phase, C-A-S-H gel and Ca-Al LDH phase. The zonation of inner hydration product explains the presence of the multi-rims features of the slag grains with a relatively high reaction degree at very late ages. Diffusion of dissolved ion from slag grains and the effect of pore space on the reorganization of hydration products play a key role in forming the spatial zonation of inner hydration product. The findings of the zonation of inner region argue that the lack of pore space in the inner region makes the continued dissolution of slag hard to complete.

Possibility of Slag Sensible Heat Recovery on Drum-like Installations

&#x0D; &#x0D; &#x0D; A variant of utilizing of slag physical heat in drum-like installations has been considered. A high-temperature melt is delivered to movable metal bodies. Heat is picked up from the working bodies surface and newly generated surfaces of slag due to interaction with working bodies. Surface of slag grains, as they cool down, allows to pick up heat with various energy characteristics.&#x0D; Keywords: smelter slags, heat content, drum-like installations, picking up and utilization of heat with various energy characteristics&#x0D; &#x0D; &#x0D;

In-flame spheroid formation from non-spherical slag particles – A numerical and experimental study

In order to identify the key factors for successful and efficient production, the formation of spherical particles from highly non-spherical coal slag grains was investigated within this work. The main focus was on precise drag and heat transfer calculations in order to obtain suitable criteria to predict the later product quality. State-of-the-art combustion, radiation and multiphase models in combination with specially adapted closure relations for drag and heat transfer were used for this purpose. The second objective was to investigate the relationship between particle temperature, viscosity, surface tension, shape and time required by the particles to obtain a smooth, round surface while passing through the burner chamber. In the first step, the process was investigated in experimental work. Slag particles were injected into an experimental furnace with a thermal input of 70 kW. The slag particles heated up which caused a reduction of slag viscosity by several orders of magnitude. While passing through the furnace, the sharp-edged powder grains transformed into smooth, highly spherical particles due to their surface tension. Numerical calculations were used to identify the main influence factors of in-flame spheroid formation. The effects of particle temperature, size, initial shape and residence time on the sphericity of the particle were studied using a Volume of Fluid (VOF) multiphase model and single-particle simulations which allowed calculations of the interface between an isothermal particle and the surrounding gas phase, as well as the interface deformation due to surface tension. The calculations have shown, that particles with volume-equivalent diameters in the range 420 µm-840 µm and an initial sphericity of 0.65 transform to spheroids in timescales in the order of 10−1 – 10° s when heated to temperatures above 1373 K. The critical particle temperature value of 1373 K was also confirmed by Euler-Lagrangian calculations of the reactive multiphase flow in the furnace. It has been shown that the particle peak temperature is a sufficient criterion for the formation of spheroids from highly non-spherical particles and that fluid temperature and particle residence time are the most important parameters of the later production process.

Engineered addition of slag fines for the sequestration of phosphate and sulfide during mesophilic anaerobic digestion.

The potential for weathered steel slag fines (D50 <1.6mm) to sequester dissolved phosphorus and sulfur from fermenting biosolids was determined. Batch studies showed that sorption equilibrium between phosphates and sulfides was reached in less than48hr after adding basic oxygen furnace (BOF) slag grains to municipal digester sludge. When these same particles were added to pilot digesters (75L) operating at a 30-day SRT, a classic dose-response was observed with respect to PO4 and H2 S sequestration, and an inhibitory performance threshold emerged above 20gBOF/L. Only a fraction of the BOF slag solids contributed to the digester TS mass, because a substantial portion dissolved into the supernatant, some of which contributed to alkalinity. After 160days of continuous operation, approximately 63% of the total phosphate was sequestered from the supernatant and 78% of the hydrogen sulfide from the biogas, when a steady-state BOF slag dose of 10gBOF/L was applied. At or below this level, BOF slag fines had no significant effect on mesophilic digester performance as judged by conventional operational metrics. This study demonstrated that upcycling BOF slag directly into the anaerobic digestion process could offer wastewater resource recovery facilities a novel nutrient management tool. PRACTITIONER POINTS: Basic Oxygen Furnace (BOF) Slag fines can be engineered as a cost-effective alternative to conventional phosphorus and sulfur control strategies. A rapid phosphate and sulfide removal from anaerobic digestates occurs within hours after BOF slag addition. 63% of phosphate and 78% of hydrogen sulfide were removed when digester sludge was dosed with 10gBOF/L, without impacting digester performance. Only a fraction of the dosed BOF slag contributed to digester TS, likely due to partial dissolution into the digestate. BOF slag can potentially increase the nutrient content of biosolids, facilitate biogas recovery, and reduce odors.

Pore Structure Characterization of Sodium Hydroxide Activated Slag Using Mercury Intrusion Porosimetry, Nitrogen Adsorption, and Image Analysis.

The pore structure of alkali-activated slag has a significant influence on its performance. However, the literature shows insufficient studies regarding the suitability of different techniques for characterizing the pore structure and the influences of Na2O and curing age on pore structure development. In pursuit of a better understanding, the pore structure of sodium hydroxide activated slag paste was characterized by multiple techniques, e.g., mercury intrusion porosimetry (MIP), nitrogen (N2) adsorption, and scanning electron microscopy (SEM) image analysis. The sodium hydroxide activated slag pastes were prepared with three different contents of Na2O (Na2O/slag = 4, 6, and 8%) and cured for different times up to 360 days. The microstructure observation reveals that outer C–(N–)A–S–H and inner C–(N–)A–S–H grow successively around the reacting slag grains, along with crystalline reaction products which are formed in the empty coarse pore space. The increase of Na2O content and curing age lead to a finer pore structure. The MIP measurements show that the total porosity drops about 70% within the first day, and that one peak at most, corresponding to gel pores, was identified in the differential curves of all the investigated samples from 1 to 360 days. On the contrary, only one peak, corresponding to capillary pores, was identified by SEM-image analysis. The differential curves derived from N2 adsorption generally reveal two peaks, and the trend that the pore diameters of those two peaks vary with curing age depends on the content of Na2O. Compared to Portland cement, sodium hydroxide activated slag has a higher pore space filling capacity (χ, Vproducts/Vslag-reacted), while the capacity decreases with increasing Na2O content and curing age.

Appraisal on Strength Characteristics of Alkali-Activated GGBFS with Low Concentrations of Sodium Hydroxide

This paper investigates the effect of low concentrations of sodium hydroxide (NaOH) and alkaline ratio (wt. of alkali silicate/alkali hydroxide) on the physical and mechanical properties of alkali-activated GGBFS. Parameters considered are activator concentration (1, 2, 3, 4, 5, 6, 7, 8, 9M) and alkaline ratio (1, 1.5, 2, 2.5, 3). Normal consistency, initial setting time and compressive strength tests were performed considering GGBFS as binder and alkaline solution as fluid medium with varying NaOH concentration and alkaline ratio. The obtained results indicated that change in NaOH concentration has significant effect on consistency, initial setting time and compressive strength of alkali-activated GGBFS. Change in alkaline ratio showed a significant effect in initial setting time and compressive strength of alkali-activated GGBFS. Compressive strength of mortar cubes cast with 4M NaOH with an alkaline ratio of 1.5 cured at room temperature produced a strength of 41 MPa at 28 days. XRD analysis indicated that increase in NaOH concentration resulted in increase in the intensity of albite peaks. SEM analysis indicated a complete dissolution of slag grains, and the gel was observed to be homogenous on slag particles at higher activator concentration.

Hydration mechanisms of supersulfated cement

Considerable attention has been given to special cements, capable of reducing CO2 emissions, energy and limestone consumption. Supersulfated cements are made of blast furnace slag (GBFS), calcium sulfate (CS), and small quantities of activator, but achieving their optimal proportions is complex. In this paper, the effects of the both CS and alkali activator (KOH) contents were studied. The main results showed that the compressive strength, heat of hydration, and consumption of anhydrite phase were strongly influenced by the alkaline content, while low calcium sulfate or alkaline content increased the formation of CSH. The instability of ettringite was verified: with low CS, the probable hypothesis was its conversion into monosulfate due to the scarcity of sulfate; with high CS, it was associated with intense, rapid consumption of anhydrite with high KOH content, followed by the precipitation of ettringite on the surface of slag grains and its conversion into monosulfate.

Effect of fly ash microsphere on the rheology and microstructure of alkali-activated fly ash/slag pastes

The highly viscous property of alkali silicate-activated cements is one of the critical challenges that hinder their wide application. The present study focuses on ameliorating the rheological performance of sodium silicate-activated fly ash/slag pastes by using fly ash microsphere (FAM), which are highly spherical particles collected from fly ash with electrostatic adsorption classification technology. The FAM particles work as ‘ball-bearings’ in the pastes to reduce the internal friction between fly ash and slag grains, and meanwhile mitigate the agglomeration of flocs and fragmentation to release the locked water. The interrelationship between the FAM particle geometry and plastic viscosity of the paste is well described by the Krieger-Dougherty equation, which supports the proposed mechanisms of ‘ball-bearings’ effects. FAM can work as an inorganic dispersing agent to improve the workability of alkali-activated cement products for a variety of application aspects.

Continuous treatment of dairy effluent in a downflow anaerobic filter packed with slag grains: Reactor performance and kinetics

ABSTRACT A downflow anaerobic filter packed with blast-furnace slag (BFS) grains was evaluated for treating cheese whey, a high-strength dairy effluent. The reactor demonstrated effective and stable biomethanation of whey in continuous mode without pH control. The chemical oxygen demand (COD) removal remained at around 80%, with the reactor pH being maintained neutral without buffering, in the organic loading rate (OLR) range of 0.8–2.4 g COD/l∙d. Although the COD removal decreased down to below 60% at higher OLRs along with acid accumulation, the methane production rate continued to increase with increasing OLR. On the other hand, the methane yield was maintained at 0.28–0.34 l/g COD throughout the experiment. The substrate removal and methane production kinetics were successfully described by the modified Stover–Kincannon model. The overall results suggest that BFS grains could be used as economic filter media for anaerobic treatment of cheese whey and other easily acidifying waste streams.

The effect of prilling and sintering on the characteristics of steel slag for phosphorus removal

Steel slag was made into slag ceramisites to obtain better characteristic as filter material for phosphorus removal. Comparison between slag ceramisites and steel slag grains was made in physicochemical property, phosphate removal efficiency, isotherm adsorption, and filter operation. It was found that slag ceramisites were light and porous, with higher phosphate removal rate than that of steel slag grains. The isotherm adsorption of both materials followed Langmuir model at 20°C; and the maximum adsorption of slag ceramisites was 3.3 times that of steel slag grains. As for P removal by slag media filtering from micro-aerobic effluent, TP saturation value of slag ceramisites was 1.96 g/kg, which was 2.7 times that of steel slag. Mineralogical and chemical investigations revealed that the mechanisms for P removal by slag media were combination of adsorption and Ca phosphate precipitation. Adsorption was overwhelming in respect to slag ceramisites, nevertheless, Ca phosphate precipitation was dominant for steel slag.