Slag Blends Research Articles

Leaching of blended copper slag in microwave oven

Leaching of blended slag (BS) was investigated in a microwave oven using hydrogen peroxide and acetic acid. The BS was a mixture of converter and flash furnace slag containing 51% Fe2O3, 3.8% CuO, and 3.2% ZnO. The important variables that influence the metal extraction yield were leaching time, liquid-solid ratio, H2O2 and CH3COOH concentrations. The preferred leaching conditions were as follows: CH3COOH concentration 4 mol/L; H2O2 concentration 4 mol/L; microwave power 900 W; leaching time 30 min; liquid-solid ratio 25 mL/g BS; leaching temperature 100 °C. Under these conditions, the metal extractions of 95% Cu, 1.6% Fe, and 30% Zn were obtained. The results were compared with the traditional leaching results. It is evident that microwave heating causes a reduction in the leaching time. Also, the extraction yield results indicate that selective leaching of BS can be achieved under the preferred conditions. The dissolution kinetic of BS in hydrogen peroxide with acetic acid is controlled by a shrinking unreacted core model equation. The apparent activation energy and reaction order were found to be 16.64 kJ/mol and 1.09, respectively.

Chloride binding and diffusion in slag blends: Influence of slag composition and temperature

This study has investigated the impact of a change in GGBS chemical composition on the chloride ingress resistance of slag blended cements under different temperature regimes. Two slags, having alumina contents of 12.23 and 7.77% respectively, were combined with a CEM I 52.5 R at 30wt% replacement. Chloride binding and diffusion tests were conducted on paste and mortar samples respectively. All tests were carried out at temperatures of 20°C and 38°C. The higher temperature resulted in an increase in chloride binding; attributed to greater degrees of slag hydration. Despite this, chloride ingress was greater at 38°C; attributed to changes in the pore structure and the chloride binding capacities of the slag blends. The more reactive, aluminium-rich slag performed better in terms of chloride binding and resistance to chloride penetration, especially at the high temperature and this was attributed to its higher alumina content and greater degree of reaction at 38°C.

Effect of Limestone Powder and Gypsum on the Compressive Strength Mixture Design of Blast Furnace Slag Blended Cement Mortar

Effect of Limestone Powder and Gypsum on the Compressive Strength Mixture Design of Blast Furnace Slag Blended Cement Mortar

Carbonation of a low-calcium fly ash geopolymer concrete

The carbonation resistance of a blended slag and low-calcium fly ash (FA) geopolymer concrete was investigated. The geopolymer binder studied was composed of 90% low-calcium FA and 10% ground granulated blast-furnace slag. The alkalinity of the pore solution plays a pivotal role in carbonation progression and subsequent corrosion initiation. pH profiles were measured to assess the pore solution alkalinity. Phenolphthalein indicator was used to measure the carbonation depth. X-ray diffraction (XRD) and quantification were carried out to identify and quantify the carbonation products. The obtained pH profiles illustrated a wider semi-carbonation zone in the geopolymer specimens, although the pH drop was insignificant in most cases. XRD analysis revealed that nahcolite mainly formed at 3% carbon dioxide concentration and led to a significant drop in pH values. The results further demonstrated that 1% accelerated carbonation replicated the natural carbonation process well, with only natron identified as a carbonation product. This work contributes to the assessment of the risk of carbonation-induced reinforcement corrosion in low-calcium FA geopolymer concrete.

Thermal effects of activators on the setting time and rate of workability loss of geopolymers

A number of parameters influence the properties of both fresh and harden geopolymer concretes. Similar to the case of OPC concretes, the fresh geopolymer concrete must also exhibit desirable rheology and setting behaviours in order to be useful for field applications. These properties of geopolymer concrete cannot be controlled using traditional super plasticisers, water reducer or set retarders. Therefore, the significance of understanding alternative methods to adjust the properties of fresh geopolymer concrete is very high.Use of solid alkali activators in geopolymer mixes is less hazardous than the commonly used liquid activators and yields a “cement like” mix, where only water is added for its activation and hydration. The alkali activators used in solid form instead of a liquid form changes the kinetics of the reaction, as its dissolution occurs during the mixing process. This paper presents a study of the setting time and rate of slump loss of geopolymer binders based on fly ash and slag blends and activated with pentahydrate and anhydrous sodium metasilicate. Fly ash and slag were mixed at different ratios and activated with the two solid alkali activators mentioned above. Vicat needle test and calorimetric measurements were employed to determine the setting parameters and heat generation and sink effect of geopolymer mixes. The workability was measured using a flow table to examine the rate of slump loss. The calorimetric measurements showed that activation of the fly ash and slag blend by the pentahydrate activator is accompanied by negative heat flow (heat absorption) whereas anhydrous alkali activator dissolution is accompanied by release of heat. The additional heat released during the dissolution process accelerates the setting in anhydrous alkali activated geopolymers compared to hydrated alkali activated geopolymers. It was also observed that the strength of the geopolymers in later stage changes with changing calorimetric heat flow characteristics.

Utilisation of steel furnace slag coarse aggregate in a low calcium fly ash geopolymer concrete

This paper evaluates the performance of steel furnace slag (SFS) coarse aggregate in blended slag and low calcium fly ash geopolymer concrete (GPC). The geopolymer binder is composed of 90% of low calcium fly ash and 10% of ground granulated blast furnace slag (GGBFS). Mechanical and physical properties, shrinkage, and detailed microstructure analysis were carried out. The results showed that geopolymer concrete with SFS aggregate offered higher compressive strength, surface resistivity and pulse velocity than that of GPC with traditional aggregate. The shrinkage results showed no expansion or swelling due to delayed calcium oxide (CaO) hydration after 320days. No traditional porous interfacial transition zone (ITZ) was detected using scanning electron microscopy, indicating a better bond between SFS aggregate and geopolymer matrix. Energy dispersive spectroscopy results further revealed calcium (Ca) diffusion at the vicinity of ITZ. Raman spectroscopy results showed no new crystalline phase formed due to Ca diffusion. X-ray fluorescence result showed Mg diffusion from SFS aggregate towards geopolymer matrix. The incorporation of Ca and Mg into the geopolymer structure and better bond between SFS aggregate and geopolymer matrix are the most likely reasons for the higher compressive strength observed in GPC with SFS aggregate.

Application of a recently developed geotechnical carbon calculator in Europe

Recent research regarding soil stabilization has been increasingly concerned with environmental performance. The present paper applies the newly developed CO2(eq) geotechnical calculator, created by the European Federation of Foundation Contractors and the The Deep Foundations Institute, to assess the behavior of three different binders used in the construction of cutter-soil mixing columns, which formed an embankment/bridge transition wedge. The binders were based on ordinary Portland cement, on a blast furnace slag and cement blend and alkali activated fly ash. Results show that the last two binders are significabtly more effective, in terms of environmental performance, than the more traditional cement-based binder. Although the blast furnace slag cement, at this early stage of the geopolymeric binders, appears as the most interesting option, the fact that the CO2/cost ratio of both options is very similar stands out.

Development of greener alkali-activated cement: utilisation of sodium carbonate for activating slag and fly ash mixtures

Alkali activated fly ash/slag (AAFS), a newly evolved type of alkali-activated cements (AACs), is here studied with the aim of developing a more sustainable alternative to Portland cement (PC), known for its adverse environmental impact. In this study, sodium carbonate (Na2CO3) was used as the alkali activator for the fly ash (FA)/slag blends. The effects of different factors on the strength, reaction rate, hydration products and microstructure were examined; these factors include the activator dosage, FA/slag ratio, and curing regime. It was found that increasing the Na2CO3 dosage significantly increased the compressive strength. The inclusion of up to 25 wt% fly ash marginally decreased the compressive strength up to 28 days while the inclusion beyond 25 wt% can lead to a remarkable reduction in strength, particularly for water-cured specimens. Sealed curing in general was found to be beneficial to the strength development of AAFS paste especially at a 50/50 ratio of FA/slag. Both the activator dosage and FA/slag ratio were found to have notable influences on reaction rate and reaction products and microstructure. Increasing the activator dosage accelerates the kinetics of the reaction while increasing the FA/slag ratio slows the reaction rates. The main binding phase is C-(N)-A-S-H with varying Ca/Si ratios ranging between 0.6 and 1.0 depending mainly on FA/slag ratio. The results indicated the possibility of production of greener cementing materials by utilizing appropriate ratios of FA/slag, dosage of sodium carbonate, and curing regimes.

Quantitative Evaluation of Crack Resistance Mechanism of Blast Furnace Slag Blended Cement Concrete via Restrained Shrinkage Stress Analysis

Quantitative Evaluation of Crack Resistance Mechanism of Blast Furnace Slag Blended Cement Concrete via Restrained Shrinkage Stress Analysis

Compressive strength development and shrinkage of alkali-activated fly ash–slag blends associated with efflorescence

This study reports the changes in the compressive strength and shrinkage property of alkali-activated fly ash and slag (AAFS) blends associated with efflorescence. The AAFS specimens cured in air for different periods were further aged under the following three conditions: bottom-contact with water, fully immersed in water and in air. The second condition, i.e., bottom in contact with water, was used to accelerate efflorescence of AAFS. The efflorescence of the AAFS binders was dependent on the reaction extent of the solid precursors. A higher reaction extent present in the binder by extending initial-curing age can reduce efflorescence rate. The incorporation of low percentage of slag (lower than 50 wt%) in the AAFS binders has little influence on the efflorescence potential, while with the increased contents of slag higher than 50 %, the binders exhibit more rapid efflorescence than the 100 % fly ash binders. The efflorescence led to the strength loss because of the formation of carbonate crystals in the binder and the damage of the microstructure. The linear shrinkage of the binders was also related to efflorescence and alkali leaching. The slag addition can reduce the negative effects of efflorescence and alkali leaching on the absolute compressive strength and shrinkage of the binders.

Bond strength between blended slag and Class F fly ash geopolymer concrete with steel reinforcement

Abstract In this paper, geopolymer concrete bond with both deformed and smooth reinforcing steel bars is investigated using the standard RILEM pull-out test. The geopolymer binder is composed of 85.2% of low calcium fly ash and 14.8% of ground granulated blast furnace slag (GGBFS). The tests were aimed to assess the development of the bond strength from 24 h to 28 days after casting, with different heat curing conditions. The results show that 48 h of heat curing at 80 °C is required in order to obtain similar or better performances to those of the reference 45 MPa OPC concrete. The 28-day bond strength and the overall bond stress–slip behaviour of the geopolymer concrete were similar to those previously reported for OPC-based concretes. Providing intensive heat curing, high early bond strength can be achieved showing that Class F fly ash geopolymer concrete is well suited for precast applications.

Potential Use of Silica Fume Coupled with Slag in HVFA Concrete Exposed to Elevated Temperatures

AbstractIn this study, cement was partially replaced with a Class-F fly ash (FA) at a level of 70% to produce high-volume fly ash (HVFA) concrete (F70). The F70 was modified by partially replacing the FA with an equal combination of silica fume (SF) and slag at levels of 10 and 20% by weight. All HVFA concrete types were compared to plain portland cement (PC) concrete. After curing, the specimens were exposed to elevated temperatures ranging from 400 to 1,000°C with an interval of 200°C. Weight and compressive strength before and after firing were thoroughly explored. The various decomposition phases were identified using X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results indicated a higher relative strength of all HVFA concrete types. The F70 exhibiting the highest relative strength. The 10% SF+10% slag blend exhibited good fire performance followed by the 5% SF+5% slag blend up to 600°C; then severe degradation in residual strength was observed at 800 and 1,000°C.

Optimisation of coal wash–slag blend as a structural fill

Coal wash (CW) and basic oxygen steel slag fines (BOS) are by-products of the coal mining and steel industries, respectively. Their effective reuse and recycling through large-scale geotechnical projects, such as port reclamation, is economically beneficial and environmentally sustainable. In this study, CW and BOS were blended in order to explore the possibility to obtain synthetic fills having geotechnical properties similar or superior to conventional fills, therefore suitable as a structural fill for the Port Kembla Outer Harbour reclamation near Wollongong City, Australia. A framework with four levels of acceptance is proposed in this paper to select granular waste as structural fill materials. This framework was used for optimising the CW-BOS blend. It was found that for the Port Kembla Outer Harbour reclamation, a CW-BOS blend with a BOS content between 30 and 45% can meet most geotechnical specifications (i.e. high shear strength and bearing capacity, low swelling and particle breakage levels, and adequate permeability) required for a suitable structural fill above the high tidal level.

Experimental Research for the Application of Mining Waste in the Trench Cutting Remixing Deep Wall Method

This paper focuses on the use of fly ash (FA) or ground granulated blast slag (GGBS) and reactive lime blends for cement-stabilized Nanjing clay, comparing them with Portland cement (PC) for enhanced technical performance. A range of tests were conducted to investigate the properties of stabilized soils, including macrostrength (UCS), permeability, and microstructure analyses by scanning electron microscopy (SEM). The influence of PC : (FA + lime) ratio, PC : (GGBS + lime) ratio and curing time was addressed. The UCS and permeability results revealed that PC-FA-lime was more efficient than PC-GGBS-lime as a binder for soil stabilization, with an optimum proportion of PC : (FA + lime) = 3 : 7 at 25% binder content, varying with curing time. The microstructure analysis reveals that fly ash mainly changes the pore volume distribution, which ranges between 0.01 μm and 1 μm, and produces more CSH/CASH bonding and fissures due to the secondary hydration and pozzolanic reactions. Based on the favourable results obtained, it can be concluded that the soft soils can be successfully stabilized by the combined action of cement, fly ash, and lime. Since fly ash is much cheaper than cement, the addition of fly ash and lime in cement-soil mix may particularly become attractive and can result in cost reduction of construction.

Drying Shrinkage of Slag Blended Fly Ash Geopolymer Concrete Cured at Room Temperature

Recent studies have shown that blending of ground granulated blast furnace slag (GGBFS) with low-calcium fly ash can have significant effects on the setting and early strength development of geopolymers cured at room temperature. This paper presents the shrinkage behaviour of geopolymer concrete mixtures in which class F fly ash was replaced with 10% or 20% GGBFS and the sodium silicate to sodium hydroxide (SS/SH) ratio was either 1.5 or 2.5. Shrinkage of 4 geopolymer and 1 ordinary Portland cement (OPC) concrete mixtures cured at room temperature were studied. Comparisons are made between the shrinkage behaviours of geopolymer concretes with different mixture proportions and those of the OPC concrete. It was found that shrinkage decreased with the increase of slag content and decrease of SS/SH ratio in geopolymer concrete cured at room temperature. The shrinkage of geopolymer concrete up to the age of 180 days was found to be comparable to that of OPC concrete of similar compressive strength. Thus, shrinkage of geopolymer concrete could be reduced to values within the limit recommended in the Australian Standards for normal OPC concrete.

Comparison of Two Alkali-activated Systems: Mechanically Activated Fly Ash and fly Ash-blast Furnace Slag Blends

Poor reactivity of fly ash (FA) in alkali activation process can be improved either by mechanical activation of fly ash or by blending fly ash with more reactive material, such as blast furnace slag (BFS). Both options were explored in this paper and comparison was performed. Mechanical activation of fly ash was conducted in a planetary ball mill, while blends of fly ash and blast furnace slag were prepared with different ratios (FA/(FA+BFS) = 1; 0.75; 0.50; 0.25; 0). Alkali activation was carried out at 95°C by use of sodium silicate solution as an activator. In both cases significant increase of geopolymer strength was observed in respect to the geopolymer based on the initial fly ash. Optimal geopolymer strength was correlated with the chemical composition of the binding gel. Empirical values of optimal gel composition could serve as a basis for tailoring properties of alkali-activated binders based on different precursors. Both alkali-activated systems represent promissing routes for geopolymer technology development.

Prediction of compressive strength of slag concrete using a blended cement hydration model

Partial replacement of Portland cement by slag can reduce the energy consumption and CO2 emission therefore is beneficial to circular economy and sustainable development. Compressive strength is the most important engineering property of concrete. This paper presents a numerical procedure to predict the development of compressive strength of slag blended concrete. This numerical procedure starts with a kinetic hydration model for cement–slag blends by considering the production of calcium hydroxide in cement hydration and its consumption in slag reactions. Reaction degrees of cement slag are obtained as accompanied results from the hydration model. Gel-space ratio of hardening slag blended concrete is determined using reaction degrees of cement and slag, mixing proportions of concrete, and volume stoichiometries of cement hydration and slag reaction. Furthermore, the development of compressive strength is evaluated through Powers\'gel-space ratio theory considering the contributions of cement hydration and slag reaction. The proposed model is verified through experimental data on concrete with different water-to-binder ratios and slag substitution ratios.

Hybrid cement based on the alkali activation of by-products of coal

This study focuses on the production of an alternative cementitious material with low environmental impact through the valorization of two-coal combustion by-products. Hybrid cements based on the alkali activation of fly ash, (FA) and boiler slag (BS) blend with a proportion of Portland cement (OPC) up to 30% were produced. FA and BS contain an unburned material up to 16%. The response surface methodology was used to optimize the compressive strength. BS geopolymer achieved high compressive strength (>100 MPa at 28 days) and FA geopolymer reach 30 MPa with thermal curing. The addition of OPC modified the curing method. In the case of hybrid based on FA (HFA), there was additionally a significant increase in the compressive strength with levels ranging up to 65 MPa at 28 days without requiring a thermal curing.

Alkali-activated blends of calcium aluminate cement and slag/diatomite

Abstract The present study deals with the formulation of new cementitious materials via the alkaline activation of an industrial by-product (blast furnace slag) or a natural rock (diatomite) in the presence of reactive aluminium sourced from calcium aluminate cement (CAC). Two blends, one containing 20% CAC and 80% slag and the other 20% CAC and 80% diatomite, were prepared and activated with sodium sulphate or a sodium hydroxide solution. The hardened materials were characterised with X-ray diffraction (XRD) as well as 27 Al and 29 Si nuclear magnetic resonance (NMR) and tested for their 2-day mechanical strength. The main reaction product was a cementitious gel that precipitated with crystalline phases such as ettringite, U phase and katoite. While the slag blend reacted to generate a C–(A)–S–H-like gel under moderately alkaline conditions, diatomite reactivity proved to be very low under such conditions. The greater reactivity of both slag and diatomite at high pH (high alkalinity) favoured their interaction with CAC.

Isothermal reaction kinetics and temperature dependence of alkali activation of slag, fly ash and their blends

Alkali silicate activated Class F fly ash–slag blends containing fly ash as the major constituent (up to 70% by mass) are proportioned to develop sufficient mechanical properties under moist curing conditions. Isothermal calorimetric studies are carried out on OPC, activated slag and fly ash, and selected fly ash–slag blends at 25°C, 35°C, and 40°C to explore their reaction kinetics. OPC and activated slag pastes exhibit similar heat release response, especially at higher temperatures (40°C). For the fly ash–slag blends, the calorimetric responses at ambient and elevated temperatures are markedly different from those of OPC and activated pure slag pastes. Two simple kinetic models are used to extract the relevant parameters of the activation reaction. Increase in temperature within the studied range influences the total heat released by the fly ash rich pastes more than those of slag pastes. For the fly ash rich blends, it is shown using the Knudsen rate constant that higher reaction temperatures are required to extract realistic model-based kinetic parameters.