Hydration Of Slag Research Articles

Influence mechanism of additives on the crystal structure and desulfurization performance of magnesium slag

In this thesis, three additives, BaO, B2O3 and P2O5, were added to modify the original magnesium slag in the process of quenching hydration of magnesium slag. The crystal phase composition, crystal structure parameters and crystal morphology of different modified magnesium slags were studied by various characterization analysis methods. Combined with the corresponding calcium conversion rate of the samples, the influence mechanism of different additive modification conditions on the desulfurization performance of magnesium slag was obtained. The results showed that the incorporation of BaO, B2O3 and P2O5 could inhibit the nucleation of γ-C2S and prevented the crystal phase transformation of β-C2S during cooling, thus increasing the content of β-C2S with high activity. Among them, MS-950–4% BaO desulfurizer had the best adsorption effect. The β-C2S content, lattice distortion and calcium conversion rate of the samples were 49.6%, 0.057% and 29.8%. The higher activity of β-C2S could generate C–S–H through hydration and then enhanced the physical adsorption capacity of magnesium slag; the increase in crystal structure defects could enhance the chemical adsorption capacity of magnesium slag, so as to improve the overall adsorption capacity of magnesium slag, and then improved the desulfurization performance of magnesium slag. The influence of chemical adsorption capacity on desulfurization performance was greater than that of physical adsorption capacity.

Properties of calcium aluminate blended cement incorporating iron-rich slag: Evolution over a curing period of 1 year

With an outlook directed towards circular economy, the valorization of municipal waste streams and industrial residues has recently conveyed a variety of novel industrial by-products to various applications such as supplementary cementitious materials (SCM) in cement blends or as precursors to inorganic polymers. Despite the synergistic benefits on recycling, reducing the cost and the carbon footprint of cements, the use of such non-conventional SCM is often limited to ordinary Portland cement (OPC)-based formulations. This study investigates the properties of a calcium aluminate cement (CAC)-based blend incorporating 30 wt% of iron (Fe)-rich slag produced at pilot scale from an industrial lead–zinc production. Compressive and flexural strength (EN 196-1 standard mortars), setting time, dimensional stability (Walter + Bai shrinkage measuring test), early hydration reactions (isothermal calorimetry), and phase assemblage evolution (XRD and TGA) were followed from 1 day to 1 year of curing period for both the slag-containing- and a corresponding reference cement formulation. Additionally, a fast non-destructive technique for quantifying the degree of slag hydration is demonstrated using a combination of X-ray computed tomography (XCT) and volume analysis based on grayscale threshold segmentation. The results from the different techniques suggest the long term (>28 days) reactivity of the slag with the slag-containing mortars attaining a comparable compressive strength of 39 MPa versus 41 MPa of the corresponding reference mortars at 90 days. Likewise, the prominent contribution of the slag to the hydration reactions is manifested by the differences in the evolution of the phase assemblage of the two formulations. The degree of slag hydration was calculated to progressively increase over time reaching up to 49% after 1 year of hydration based on the XCT scans. These results along with the long term dimensional stability of the slag-containing formulation support the potential for valorization of the Fe-rich slag as SCM in the CAC-based blend.

Effects of nanosilica on the hydration and hardening properties of slag cement

Ground granulated blast furnace slag (GGBS) has commonly been used in concrete to reduce energy consumption and pollution emission by replacing cement, but the early-age performance of GGBS-cement samples was unsatisfactory. Although nanosilica (NS) can significantly improve the performance of GGBS-replaced cementitious materials, the understanding of the effect of NS on slag was not consistent and some contradictory conclusions have been reported. In this paper, qualitative and quantitative analysis were used to study the effects of NS on the evolution of hydration and phase assemblage of the GGBS-cement system to reveal the source of performance changes at GGBS substitution of up to 70 wt%. Studies have shown that the compressive strength of GGBS-added samples was lower than control sample at early ages. When the addition amount was not more than 50%, the later age strength was higher, but when the substitution fraction was 70%, negative influence was displayed. The addition of NS continuously improved the mechanical strength, especially at early ages. The hydration heat release rate and cumulative heat of samples decreased with the increase of GGBS content, and NS promoted both parameters. TG results demonstrated that NS consumed CH to generate additional C-S-H gel and promoted hydration. This effect was due to the simultaneous promotion of the hydration of clinker and slag as shown by the analysis of XRD and selective dissolution results. And the enhancement effect of NS was most significant in the early stages, thus compensating the disadvantage of the poor early performance of high-volume-GGBS-replacement cementitious materials.

Internal Curing Effect of Pre-Soaked Zeolite Sand on the Performance of Alkali-Activated Slag.

This study clarifies the effects of pre-soaked zeolite sand as an internal curing material on the hydration, strength, autogenous shrinkage, and durability of alkali-activated slag (AAS) mortars. The liquid-to-binder ratio (L/b) of all of the AAS mortars was 0.55. Sodium hydroxide solution was used as an alkali activator and an internal curing liquid. Calcined zeolite and natural zeolite sand replaced the standard sand at 15% and 30%, respectively. The setting time, autogenous shrinkage, compressive strength, ultrasonic pulse velocity, and surface electrical resistivity were tested. The following conclusions were drawn: (1) The addition of zeolite significantly reduces the autogenous shrinkage of AAS mortar. Compared with the control group, 30% calcined zeolite reduced the autogenous shrinkage by 96.4%. Moreover, the autogenous shrinkage of the AAS mortars was noticed in two stages (a variable temperature stage and an ambient temperature stage), and the two stages split at one day of age. (2) The compressive strength of all of the specimens increased as the zeolite sand content increased, and the highest compressive strength was obtained for AAS combined with 30% natural zeolite sand. (3) Internal curing accelerated the formation of the second peak of heat flow and reduced the accumulated heat release. (4) Calcined zeolite sand delayed the setting time of the AAS mortars. (5) The addition of zeolite significantly reduced the surface electrical resistivity of the AAS mortars. In summary, zeolite sand is extremely useful as an internal curing agent to reduce autogenous shrinkage and to increase the compressive strength of AAS mortars.

Evolution of ettringite content and its effects on hydration properties of CaO/fluorgypsum-activated granulated blast furnace slag binders

To investigate the influence of ettringite on the hydration properties of CaO/fluorgypsum-activated granulated blast furnace slag binders, the ettringite content in hydrated pastes with different amounts of fluorgypsum added was measured by selective dissolution in an ethylene glycol/methanol solvent system. The effects of ettringite content on the compressive strength development, the hydration products, the hydration degree of slag, the polymerization degree of calcium aluminum silicate hydrate (C-A-S-H), and the microstructure of hardened pastes were studied using a suite of characterization methods. The quantity of ettringite was found to increase with the amount of fluorgypsum added during the entire curing process. Only a moderate amount of ettringite was shown to improve the compressive strength due to the reinforcing effect of the fiber-like microstructure of ettringite. X-ray diffraction analysis revealed that hydrotalcite-like phases were strongly inhibited with the addition of fluorgypsum while ettringite formation was promoted. 29Si nuclear magnetic resonance spectroscopy and scanning electron microscopy analysis suggested that excessive ettringite caused a reduction in the hydration degree of slag, poor polymerization of C-A-S-H gels, and a loose microstructure of hydrated pastes. We concluded that an optimal strength was achieved by controlling the level of ettringite content to exploit the emergent reinforcing effect while avoiding both a poor microstructure and insufficient slag hydration. Moderate ettringite content improves the hydration properties of CaO-activated slag binder due to the fiber-like reinforcing effect.

Effect of recycled fine aggregates on alkali-activated slag concrete properties

Normally, recycled fine aggregate (RFA) is an unwanted by-product of construction demolished waste crushing, with high contaminant content. The presence of RFA is highly restricted in ordinary Portland cement (OPC) concrete production because of the poor durability and mechanical performance. This paper presents a study of the effect of RFA on the mechanical and durability of alkali activated slag concrete (AASC). Results indicated that RFA can be used as fine aggregate in AASC with superior early age compressive strength compared to river sand in AASC. It is caused by the presence of non-hydrated cement paste attached to the RFA with alkali activate property, which is equivalent to increase the quantity of alkali activator, and accelerate the hydration of ground granulated blast furnace slag (GGBFS) especially in the early age. In later age, the mechanical properties of AASC incorporating RFA are slightly lower than conventional AASC due to the larger porosity in the concrete.

Analysis of carbonation and hydration of BOF steel slag for application as railway ballast

The reutilization of waste is a valuable alternative for the preservation of natural resources and the environment. In this sense, Basic Oxygen Furnace (BOF) steel slag is one of the most residues generated in steel the industry. It is estimated that for each ton of steel produced, about 150 kg of steel slag is generated in the BOF process. Generally, the newly produced BOF steel slag is volumetrically unstable, presenting expansive characteristics, and for this reason, its application should be based on stabilization studies. The main objective of this research was to analyze the chemical stabilization process of BOF steel slag by means of carbonation of the material. For this purpose, an experimental method based on hydration and carbonation in 2-meter high slag piles, moistened with sprinklers and instrumented with leaf wetting, temperature, precipitation and carbon dioxide content sensors was used. A control stack was also set up to accompany the stabilization process through the conventional procedure, in which the material is disposed of in large stacks and subjected to the weather conditions for long periods. The laboratory campaigns were developed in the laboratories of the Federal University of Ouro Preto. Thus, through the results obtained, it was possible to conclude that the stabilization method based on hydration and carbonation for the BOF steel slag was efficient, reducing the free lime content to values well below those prescribed internationally.

Hydration of potassium citrate-activated BOF slag

Basic Oxygen Furnace (BOF) slag is currently utilized with low-grade applications or landfilled. Here, we investigate a novel route to upgrade BOF slag to a high-performance binder by chemical activation with tri-potassium citrate. The impact of tri-potassium citrate on hydration and phase assemblage of BOF slag is analyzed with a multi-technique approach. Results reveal that the addition of tri-potassium citrate considerably enhances the reactivity of brownmillerite and accelerates the hydration of belite at early ages. The majority of brownmillerite hydrates within 24 h, and the reaction kinetics is controlled by the activator dosage. The main products of BOF slag hydration are siliceous hydrogarnet and C-S-H gel. Acting as a strong water reducer, tri-potassium citrate enables the manufacture of slag pastes with high compressive strength (up to 75 MPa at 28 days) and low porosity. Leaching of heavy metals from the slag pastes fulfills the Dutch Soil Quality Decree limits.

Prediction of the basic creep of concrete with high substitution of Portland cement by mineral additions at early age

AbstractThe present work analyses the basic creep of concrete compositions for which 75% of Portland cement is substituted by limestone filler and/or blast furnace slag. Compressive creep tests are performed during and after the peak of hydration of the slag in order to track the effect of the slag on the development of the creep properties. An adapted version of the Model Code 2010 (MC2010) is used to model basic creep. It is observed that the high substitution rate of cement by limestone filler and/or slag leads to a significant change in the evolution of the viscoelastic properties of concrete till a significant degree of reaction of slag is reached. It corresponds to the end of the second peak of hydration due to slag. After the slag peak of hydration, the basic creep is similar to ordinary Portland cement concrete with equivalent water‐to‐binder ratio. For very early age, a new model is developed and is composed of two terms, which consider the hydration of (a) Portland cement and (b) slag in the aging of the specific creep function.

Preliminary Interpretation of the Induction Period in Hydration of Sodium Hydroxide/Silicate Activated Slag.

Many calorimetric studies have been carried out to investigate the reaction process of alkali-activated slag paste. However, the origin of the induction period and action mechanism of soluble Si in the dissolution of slag are still not clear. Moreover, the mechanisms behind different reaction periods are not well described. In this study, the reaction kinetics of alkali-activated slag paste was monitored by isothermal calorimetry and the effect of soluble Si was investigated through a dissolution test. The results showed that occurrence of the induction period in hydration of alkali-activated slag paste depended on the presence of soluble Si in alkaline activator and the soluble Si slowed down the dissolution of slag. A dissolution theory-based mechanism was introduced and applied to the dissolution of slag, showing good interpretation of the action mechanism of soluble Si. With this dissolution theory-based mechanism, origin of the induction period in hydration of alkali-activated slag was explicitly interpreted.

Reactivity of slag in 15 years old self-compacting concrete

The paper summarizes changes in slag microchemical composition and its hydration products in Portland cement self-compacting concrete after 15 years of exposure in water under laboratory conditions. Scanning electron microscopy (SEM) and X-ray microanalysis (EDX) techniques show the results of the degree of hydration of blast furnace slag particles. The experiment showed a connection between the slag content and the hydration process. It was verified that the average values of the base-forming oxides ratio (MgO + Al2O3)/CaO are inversely proportional to the slag content in the mixture. The results of these procedures provide valuable information not only in the long-term predictions of the effect of slag and other additives in cement concrete, but also in the evaluation of slag hydration in historical concrete structures.

Hydration and Microstructure of Steel Slag as Cementitious Material and Fine Aggregate in Mortar.

Due to the low hydration activity and poor volume stability, extensive steel slag utilization is restricted. In this paper, the hydration process and microstructure of alkali-activated materials with steel slag as a cementitious material and fine aggregate were studied. The phase composition and micro-morphology of hydration products were measured using XRD, NMR and SEM. The response relationship between microstructure and mechanical properties during hydration was revealed. The results show that the main hydration products of the alkali-activated steel slag powder-granulated blast furnace slag powder cementitious system are Ca(OH)2 and calcium aluminosilicate hydrate (C-A-S-H) gel. With the progress of hydration, the amount of calcium silicate hydrate (C-S-H) gel and the average molecular chain length increase, Al[4]/Si decreases, while C/S increases first and then decreases, and the structure of cement paste becomes much more compact. The interface between steel slag sand and cement paste is denser than that of river sand, since the hydration occurs on the surface of steel slag sand, which leads to the formation of C-A-S-H gel and Ca(OH)2. As a result, the compressive strength of concrete prepared by steel slag sand is higher than that of river sand with the same mix proportion.

Blast Furnace Slag Hydration in an Alkaline Medium: Influence of Sodium Content and Sodium Hydroxide Molarity

AbstractThe reaction of blast furnace slag in sodium hydroxide (NaOH) solutions of different molarities is evaluated. The compressive strength of the activated slag does not scale with the molarity...

Towards Understanding the Rheological Properties of Slag-Cemented Paste Backfill

ABSTRACT Ground Granulated Blast Furnace Slag (GGBS) has been increasingly used as replacement of Portland cement (PC) at different proportions in cemented paste backfill (CPB) material due to its relative low cost and sustainable utilization of the metallurgical slag. However, our understanding of the effect of adding GGBS to CPB on its flowability (defined by the rheological properties) is limited. Therefore, the aim of this study is to clarify and quantify the rheological properties of Slag-CPB and the key factors (e.g., transportation distance or time, temperature, mixing water quality, GGBS proportions) that affect them. Results show that curing time and temperature have a significant impact on the rheology of the Slag-CPB samples. Furthermore, the yield stress and viscosity of Slag-CPB are a function of its initial sodium chloride, GGBS and sulphate contents. It was also noticed that sulphate can inhibit, as well as activate the hydration of slag, depending on the slag’s initial content. Additionally, the rheological properties of Slag-CPB decreased when the concentration of sodium chloride increased. The results presented in this manuscript will provide a better understanding of the rheological properties of Slag-CPB and thus contribute to the better designing of Slag-CPB with optimal flowability and suitable mechanical stability.

Understanding the early reaction and structural evolution of alkali activated slag optimized using K-A-S-H nanoparticles with varying K2O/SiO2 ratios

So far, little is known about the manner the synthesized K-A-S-H (K2O–Al2O3–SiO2–H2O) nanoparticles affects the hydration of alkali-activated slag (AAS). To understand the effects of adding K-A-S-H nanoparticles with two distinct K2O/SiO2 ratios on reaction and microstructural development of AAS, heat evolution, TGA, MIP and SEM-BSE analyses were measured. Results revealed that incorporation of synthesized K-A-S-H nanoparticles can not only be used as seeding to induce nucleation, thereby significantly promoting the hydration of slag and manipulating the chemical composition of the hydrate phases, but also as fillers to increase the density of alkali-activated slag pastes; then microstructure analysis found that incorporation of synthetic K-A-S-H nanoparticles in AAS could optimize the pore size distribution, leading to a dense matrix structure. Furthermore, the effect of K-A-S-H particles with varying K2O/SiO2 ratios was mainly related to their specific surface area. These important findings provide possible practical implications for controlling the late-stage performance of alkali-activated binders.

Influence of whitening additives on the properties of decorative slag-alkaline cements

Traditional decorative cements are made on the basis of white cements and the demand for them is constantly growing. But such cements have all the disadvantages of clinker cements, namely, high energy intensity and high price. An alternative may be slag-alkali cements having higher physical and mechanical characteristics and lower price. Therefore, these studies were aimed at choosing such additives that would have the maximum whitening properties and would enhance the stabilization of bleaching processes during hydration and hardening of slag in the presence of alkalis, to determine the simultaneous effect of three additives on the whiteness and physical-mechanical characteristics, as well as to determine the optimal the content of these additives in slag-alkali decorative compositions. In this work, as a result of the studies performed, the possibility of obtaining effective slag-alkali white cements as the basis for the production of decorative concrete with acceptable economic characteristics is proved. The influence of each of the bleaches on such characteristics of decorative slag-alkali cements as strength, setting time, whiteness is established. Methods for optimizing compositions are identified.

Effect of steel slag on the hydration and strength development of calcium sulfoaluminate cement

In this study, effects of steel slag on the hydration and strength development of calcium sulfoaluminate (CSA) cement were investigated. Pastes with 0%, 10%, 20%, and 40% (by mass) of steel slag replacements for CSA cement were tested for setting time, chemical shrinkage, electrical resistivity, and compressive strength. The hydration products, bound water content, pore structure, and pH value of the pore solutions of these pastes were also analyzed. The results indicate that at early ages (from 2 h to 28 d), steel slag worked like a filler and had a dilution effect on CSA cement hydration. Therefore, setting time increased and chemical shrinkage, electrical resistivity, chemically bound water content, and compressive strength of the pastes reduced with increase of steel slag replacements. But, at late ages (≥90 days), these properties in pastes with steel slag replacements surpassed those of the plain CSA cement paste due to hydration of steel slag, which increased pH values of pore solutions and subsequently stabilized ettringite in the pastes. Steel slag replacement for CSA cement also accelerated strätlingite formation. Both ettringite stabilization and strätlingite formation appeared to be the primary mechanisms for strength compensation of CSA cement paste at late ages.

PHYSICAL AND CHEMICAL PROCESSES OCCURRING IN THE GRANULATED PHOSPHORUS SLAG DUMPS

One of the most dangerous man-made mineral formations located in the territory of Kazakhstan is disposal areas for granulated phosphorus slag, since its shells contain initially the dangerous gases such as PH3, H2S, HF, etc. Permanent sources of such gases are also Ca3P2 (intrinsically poisonous gas), CaS and CaF2. In the structures of phosphorous ash disposal areas, the chemical and physical-chemical processes contain initially gases, which may be in different modifications. For example: • Phosphene (PH3) in the form: PH3→P2O5→HPO3→H3PO4; • Hydrogen sulfide (H2S): H2S→H2SO3→ H2SO4; • Hydrogen fluoride and silicon tetrafluoride: HF→SiF4→H2SiF6→H2SiO3; • Carbon oxide (СО): СО→СО2→СОСl2. Such chemical processes intensify greatly the anthropogenic effect of phosphorous ash disposal areas on the environment – biogeocoenosis. In addition to the processes related to gas components, in the phosphorous ash disposal areas, the hydration processes occur as a result of interaction of slag with water. Thereat, sub-microcrystalline CSH(I) and calcium are generated. Depth of slag hydration reaches 1…3 μm. Such processes form a thin layer on the surface of slag particles resulting in linking of separate grains into a whole. Therefore, the surface of phosphorous ash disposal areas has the certain strength with cracks, through which water, air, snow, etc. are penetrating into the disposal area. It is evident that granulated phosphorus slag in the form of disposal areas has very adverse impact on the biogeocoenosis, causing the pressing need for their disposal; moreover, such slags are valuable raw material for the construction industry and agriculture only after their decontamination from dangerous gases. Keywords. Granular phosphorus slag, dumps, hazardous gases, chemical reactions.

Effect of gypsum on the properties of composite binder containing high-volume slag and iron tailing powder

• Composite binder with high-volume slag and ITP shows low early compressive strength. • The addition of gypsum compensates for the poor early properties of composite binder. • The addition of gypsum negatively affects later-age properties of composite binder. • Adding gypsum increases Ca/Si ratio and S/Si ratio but doesn’t affect Al/Si ratio. • High-volume slag and ITP can be used to produce high performance green concrete. The effect of gypsum on the properties of composite binder containing high-volume slag and iron tailing powder was studied by measuring compressive strength, hydration heat, bound water content, hydration products, pore structure and morphology. The addition of gypsum increases early-age compressive strength and bound water content, but the opposite trend is observed at later ages. A higher quantity of gypsum leads to a higher third exothermic peak and a higher total hydration heat. Although incorporating gypsum promotes early hydration of cement and slag, it limits their further hydration at later ages. Adding gypsum forms a large amount of ettringite and densifies early-age pore structure, but it coarsens later-age pore structure. The addition of gypsum increases Ca/Si and S/Si ratios of C-S-H, but it doesn’t affect Al/Si ratio. The addition of gypsum compensates for the poor early-age properties of composite binder containing high-volume slag and iron tailing powder, and the later-age properties are acceptable.

Effect of the Formation of Amorphous Networks on the Structure and Hydration Characteristics of Granulated Blast Furnace Slag

The slag obtained in the process of pig iron smelting has been widely used, but the variational hydration activity always is a significant factor affecting its quality. In this experiment, the laboratory simulated slag was prepared by adjusting the chemical composition and cooling method. The experiment primary characterized the structure and hydration process with different types of slag by using MAS NMR, XRD, compressive strength, ICP, SEM, and hydration heat, then obtained the influence of the composition of the network former S/A (the mass ratio of SiO2 and Al2O3 in chemical composition) and amorphous phase content on its structure and hydration activity. The result shows that lowering the S/A value can reduce the degree of vitreous polymerization in the slag; reducing the S/A value of the slag can make the slag hydration time advance, and consequently, the cumulative exotherm increases, the liquid phase Ca/Si and Al/Si ionic ratio increases, and the hydration product changes from C–S–H gel to C–A–S–H gel, which ultimately leads to an increase in compressive strength. In the high S/A value slag, the formation of the trace crystal phase of gehlenite is beneficial to reduce the degree of polymerization of the amorphous.