Stabilization of electric arc furnace dust by the use of cementitious materials: Ionic competition and long-term leachability

Ionic competition in stabilization of major heavy metals from electric arc furnace dust (EAFD) was investigated. The cementitious materials tested (ground granulated blast furnace slag (GGBFS) and ordinary Portland cement (OPC)) were put in contact with solutions made of various combinations of Cr(VI), Pb(II), Zn(II), Ni(II) and Mo(VI) ions. The presence of Ni, Zn or Mo did not influence the Cr fixation by GGBFS and OPC. The ionic competition phenomenon between Cr and Pb was observed for GGBFS in particular.Long-term leaching tests with OPC-EAFD and GGBFS-EAFD showed that OPC is more effective in fixing Cr at 7 days (4.7 mg/L in solution) than GGBFS (79.4 mg/L in solution). GGBFS becomes effective with time and offers a better performance from 56 to 365 days (under 1 mg/L in solution) than OPC (until 11 mg/L in solution). GGBFS and OPC decreased Zn, Pb and Ni concentrations in leaching solutions under 2.5 mg/L.     

Immobilization of chromium (VI) evaluated by binding isotherms for ground granulated blast furnace slag and ordinary Portland cement

The interaction mechanisms of the Cr(VI) ion in presence of GGBFS and OPC were evaluated by chromium binding isotherms and by pore solution analysis. The chromium in the final leaching solution was measured and the solid samples were investigated by SEM and by XRD. GGBFS was more efficient than OPC in fixing Cr ions at lower initial concentrations. However, from an initial Cr(VI) concentration of 2000 and 5000 mg/L, OPC was more efficient.

For an initial Cr(VI) concentration of 50 000 mg/L, around 145 mg Cr/g was fixed by OPC and only 8 and 55 mg Cr/g were fixed by GGBFS in alkaline and water-based immersion solutions, respectively. The Cr-bearing phases identified by XRD and SEM are: CaCrO₄ and CaCrO₄·2H₂O, C–S–H and calcium aluminate phases. The pore solution chemistry indicates that a value around 92% of chromium was retained by GGBFS and 87% by OPC.     

Investigation of stabilization/solidification for treatment of electric arc furnace dust: Dynamic leaching of monolithic specimens

Diffusion-controlled leaching of heavy metals (Cr, Ni, Pb and Zn) from electric arc furnace dust treated with ground granulated blast furnace slag (GGBFS) and with ordinary Portland cement (OPC) was evaluated. Monolithic specimens were evaluated under dynamic leaching conditions for 84 days with periodic leachant renewal. The influence of leaching time, nature of the leachant, binder type and the water/solid ratio of the monoliths were investigated. Results obtained showed both binders can immobilize heavy metals in the monoliths under dynamic leaching conditions, with cumulative quantity of leached metal under 0.138 mg (Cr). Alkaline leachant increased metal release from specimens and reducing the water/solid ratio of the monolith allowed for a decrease in the cumulative mass of metals leached. Chemical and mineralogical characterizations indicated that the metals were evenly distributed throughout the specimens for both binders. Decalcification was observed on the OPC monolith border following leaching. This decrease in Ca corresponded to an altered zone (20 μm), identified by scanning electron microscopy. The GGBFS sample did not show an altered zone.     

Investigation of hydraulic activity of ground granulated blast furnace slag in concrete

Ground granulated blast furnace slag (GGBFS), a by-product of the steel manufacturing industry, being used as an effective partial cement replacement material, has already been proven to improve several performance characteristics of concrete. The reactivity of GGBFS has been found to depend on the properties of slag, which vary with the source of slag, type of raw material used, method and the rate of cooling. The present work aims at bringing out a novel relationship between the Hydraulic Index (HI) of slag at 7 and 28 days (HI7 and HI28) and the influencing properties of slag, namely, glass content, fineness and chemical composition by employing multiple regression analysis on 37 slag samples from various sources. HI7 and HI28, thus obtained, have been mapped onto a Slag Activity Index (SAI) plot, giving an indication of the ranges of strength of slag.     

High intakes of Cr, Ni, and Zn in clinker: Part II. Influence on the hydration properties

This work presents the results of the hydration of cements with high intakes of Cr, Ni, and Zn. The cements were produced from clinkers that were doped with 200 to 25,000 ppm of heavy metal. Investigations on the clinkers were presented in Part I. In this paper the rate of heat generation of the cements in the first 2 days was analysed by differential scanning calorimetry. The hydration products were investigated by scanning electron microscopy combined with energy-dispersive X-ray spectrometer and also by X-ray powder diffraction. The initial setting of some samples was tested, as well as the strength. The results show that heavy metals only have an influence on the hydration properties of the cements if the dosage is much higher than in ordinary Portland cement.     

Effect of magnesium and sulfate ions on durability of silica fume blended mixes exposed to the seawater tidal zone

The effect of silica fume on deterioration resistance to sulfate attack in seawater within tidal zone and simulated wetting-drying condition has been studied in Portland cement concretes and pastes containing silica fume (SF) with/without ground granulated blast furnace slag (GGBS). Changes in the compressive strength and capillary water absorption of specimens as a function of SF content have been investigated combined with phases determination by means of scanning electron microscopy and X-ray energy dispersion analysis. The strength change factors (SCFs) of specimens with SF (the more SF content, the higher strength loss) were greater than that of the mixes without SF or cured under tap water. Mg²⁺ ion originated attack found to be the dominating deterioration mechanism as confirmed by X-ray and chemical analyses.Further, the incorporation of GGBS with SF mixes in different exposure conditions led to the worst performance in all of the test environments. Lower cement content and hydration rate accompanied with particular chemical composition of GGBS made concrete and paste specimens to be more susceptible to deleterious seawater environment.     

Activation of blast furnace slag by a new method

Blast furnace slag is used as supplementary cementing material for the production of blended cement and slag cement. Its latently hydraulic properties can be activated by several methods. Most applications employ the use of high pH values in the pore solution (> 13.0) to accelerate the corrosion of the glass network of the slag.It is shown in this work that activation is also possible by lowering the pH to a range between 11.8 and 12.2 by the addition of calcium hydroxide and soluble calcium salts. Among the salts investigated in this study are calcium chloride, calcium bromide, calcium nitrate, calcium formate, and calcium acetate. Other salts can be used alternatively as long as they are able to increase the calcium ion concentration and thus reduce the pH in the pore solution via the calcium hydroxide equilibrium. Complex formation of organic anions with calcium ions in the pore solution is a serious handicap when using organic calcium salts.This concept was tested on a particular slag improving its early compressive strength. It was possible to increase the strength of mortar bars produced from the pure slag from 3 MPa to 25 MPa after seven days by adding calcium hydroxide, calcium carbonate and calcium acetate. The early strength of slag cement containing 80% slag was increased from 6 to 16 MPa after two days by adding calcium chloride. The final strength was increased from 36 to 53 MPa after 28 days (water/cement-ratio = 0.40, 20 °C).Analytical data is included to demonstrate that application of the aforementioned concept is able to increase heat liberation and degree of slag consumption.     

Immobilization of Cr, Cd, Zn and Pb in alkali-activated slag binders

Granulated blast furnace slag is the main component of alkali-activated slag cementitious materials (AASCs). Calcium silicate hydrates with a low Ca/Si ratio, hydrotalcite-type phase, some amounts of hydrogarnets and sodium zeolites form as main AASC hydration products. The microstructure of alkali-activated slag pastes shows a higher amount of gel pore content compared to OPC pastes and, simultaneously, significantly lower amount of capillary pores. The microstructure and phase composition of hydrated slag indicate that they can play an essential role in the immobilization of heavy metals. The properties of alkali-activated slag pastes in the presence of Zn, Cd, Cr and Pb ions were studied. The leaching TANK test was used to evaluate the level of immobilization of particular elements in mortars made containing these elements. It was found that the degree of Cd, Zn and Pb ion immobilization was very high (exceeding 99.9%). The values for Cr⁶⁺ were lower (ca. 99.0%). The strength development as well as microstructure observations are presented in the paper.     

The role of sulfide in the immobilization of Cr(VI) in fly ash geopolymers

The use of fly ash-based geopolymer binders to immobilize chromium is investigated in detail, with particular regard to the role of the sulfide ion as a reductant for Cr(VI) treatment. In the absence of sulfide, Cr added as Cr(VI) is highly leachable. However, addition of a small quantity of Na₂S reduces the Cr to Cr(III), and enables leaching efficiencies in excess of 99.9% to be reached after 90 days' exposure to deionized water, Na₂CO₃ or MgSO₄ solutions. Leaching in H₂SO₄ is somewhat greater than this, due most probably to the oxidation of the Cr(III) present. Addition of the Cr(VI) as a highly soluble salt is preferable to its addition as a sparingly soluble salt, because a higher salt solubility means the Cr(VI) is more available for reduction prior to geopolymeric setting. The potential value of geopolymer technology as an immobilization process for problematic heavy metal waste streams is highlighted by these results, and the need for a full understanding of binder chemistry in any immobilization system outlined.     

Advances in alternative cementitious binders

There is a burgeoning interest in the development, characterization, and implementation of alternatives to Portland cement as a binder in concrete. The construction materials industry is under increasing pressure to reduce the energy used in production of Portland cement clinker and the associated greenhouse gas emissions. Further, Portland cement is not the ideal binder for all construction applications, as it suffers from durability problems in particularly aggressive environments. Several alternative binders have been available for almost as long as Portland cement, yet have not been extensively used, and new ones are being developed. In this paper, four promising binders available as alternatives to Portland cement are discussed, namely calcium aluminate cement, calcium sulfoaluminate cement, alkali-activated binders, and supersulfated cements. The history of the binders, their compositions and reaction mechanisms, benefits and drawbacks, unanswered questions, and primary challenges are described.     

A study on the relationship between porosity of the cement paste with mineral additives and compressive strength of mortar based on this paste

Mercury intrusion porosimetry study was carried out on ordinary Portland cement (OPC) pastes with 10% to 40% mineral additives, such as steel-making slag, granulated blast furnace slag and fly ash. For all samples, the porosity of paste and compressive strength of mortar based on this paste were determined at 3, 7, 28, 90 and 180 days. Relationship between the porosity and strength was investigated and some equations for the strength-porosity relationship were presented according to Balshin multiplicative model. Results show that mineral additives delayed process that micropore structure of OPC paste developed and strength development of sample with mineral additives was faster than that of OPC sample. Balshin equation fits the results of strength and porosity of all samples and there is a strongly quantitative relationship between strength and porosity. After being mixed with mineral additives, the intrinsic strength σ₀ and power n both increased and the sequence of σ₀ and n for different mineral additives was fly ash>steel-making slag>blast furnace slag.     

铬渣-水泥体系物理性能及其Cr(Ⅵ)的控制研究

探讨了铬渣的胶凝活性,研究了不同掺量铬渣对水泥浆体性能的影响。同时,通过掺入沸石、粉煤灰及矿渣微粉等矿物外加剂,探讨了这3种矿物外加剂对铬渣-水泥体系物理性能的影响及其对Cr(Ⅵ)的控制作用。结果表明,铬渣具有良好的胶凝活性,矿渣微粉的加入对Cr(Ⅵ)的浸出有良好的控制作用。     

炉渣基透水材料对Cr^6＋的吸附性能研究

利用炉渣的性质制成炉渣基透水材料,研究对六价铬离子的去除效果。在试验条件下,分析骨料配比和水泥的比例对样品的影响,确定在骨料配比（细粒：中粒=2∶1）和对水泥的比例为2∶1时,样品对重铬酸根离子的去除效果较好,即1 cm厚炉渣基透水砖对六价铬离子三次透过后的去除率高达78.50%,而对于10 cm透水填料柱,一次性对高浓度重铬酸根离子的去除率就可达到99.32%。     

Preparation and performance of slag-based binders for the cementation of fine tailings

To achieve effective cementation of fine tailings, slag-based binders were prepared using Portland cement clinker stimulation, early strength activator (ESA, mixture of anhydrite and triethanolamine at 97:3 (w/w)) activation and slag pulverization methods. The compressive strength, hydration products, slag reaction degree and non-evaporable water content of the consolidated samples under different curing times were analyzed to clarify the application performance and early strength action mechanisms of this slag-based binder. The results showed that clinker alone was able to effectively stimulate the slag’s cementitious property, but the cementation strength was relatively low. The addition of ESA in the clinker activated slag promoted the conversion of C₄AH₁₃ into ettringite (AFt) and accelerated the consumption of Ca(OH)₂, all of which significantly improved the early cementation strength of fine tailings. Slag pulverization promoted the slag reaction degree and increased the yield of hydrated products, which led to a further increase in the early strength of the slag-based binder. Eventually, a more efficient and higher early strength slag-based binder was prepared with the composition of 27% clinker, 10% ESA and 63% pulverized slag, and the cementation strength at 3 curing days for the fine tailings sample was 231% more than that of P.O 42.5 Portland cement.     

The use of ferro-silicate slag from ISF zinc production as a sand replacement in concrete

Technical studies have shown that ferro-silicate slag from the Imperial Smelting Furnace (ISF slag) production of zinc can be used as a replacement for sand in cementitious mixes. The ISF slag contains trace quantities of zinc and lead, which are known to cause retardation of concrete set. Testing of experimental concrete mixes proves this retardation affect, although the delay in set does not appear deleterious to the eventual concrete hydration. If a gelatinous layer containing lead and zinc ions is formed around the cement grains in the concrete mix, set begins when this layer is disrupted and then continues as it would as if no zinc or lead had been present. Leaching studies demonstrate that pulverized-fuel ash and ground granulated blastfurnace slag have the potential to reduce the leaching of lead and zinc ions from the ISF slag, even in highly alkaline solutions.     

几种激发剂对钢矿粉活性影响的研究

研究了几种单一激发剂不同掺量对钢矿粉活性的激发作用及机理,优选最佳激发剂种类及掺量.结果表明：对钢矿粉激发效果较好的激发剂为半水石膏、氧化钙、二水石膏、硫酸钠、氢氧化钠,其激发剂最优掺量分别为半水石膏0.6％、氧化钙0.15％、二水石膏0.2％、硫酸钠0.15％、氢氧化钠0.03％,其中半水石膏可提高钢矿粉各龄期活性指数8％以上.     

Direct tension test and tensile strain capacity of concrete at early age

The tensile strain capacity of concrete under uniaxial tension is investigated using the direct tension test method. The adopted method of testing improves the weak bond strength between the embedded bar and concrete and reduces the stress concentration at the end of the embedded bar. The method has overcome the difficulties in centralizing and aligning the two embedded bars in the specimens. Seven mixes of concrete were designed to study the effects of age, compressive strength and mineral admixture on the tensile strain capacity. The investigation shows that the tensile strain capacity of concrete is a relatively independent parameter. The average tensile strains at failure and at 90% failure load are 120 and 100 μ, respectively. The corresponding characteristic tensile strain values at failure and at 90% failure load are 86 and 78 μ, respectively.     

High intakes of Cr, Ni, and Zn in clinker: Part I. Influence on burning process and formation of phases

The influence of high intakes of Cr, Ni, and Zn on the burning process and the formation of clinker phases is described. Three different raw meals were used and were burned with the metal oxides in four concentrations from 200 to 25,000 ppm. The resulting clinker was analysed for the content of free lime and X-ray diffraction analysis was done. Some of the clinker material was crushed and polished, and quantitative point counting was performed to provide the content of the different clinker phases. These samples were also analysed by scanning electron microscope connected with an energy-dispersive X-ray-spectrometer to detect the composition of the clinker phase. The results show that only very high intakes of heavy metals have measurable effects on the formation and composition of the clinker.     

Hydration study of limestone blended cement in the presence of hazardous wastes containing Cr(VI)

Considering the increasing use of limestone cement manufacture, the present paper tends to characterize limestone behavior in the presence of Cr(VI). The research reported herein provides information regarding the effect of Cr(VI) from industrial wastes in the limestone cement hydration.The cementitious materials were ordinary Portland cement, as reference, and limestone blended cement.The hydration and physicomechanical properties of cementitious materials and the influence of chromium at an early age were studied with X-ray diffraction (XRD), infrared spectroscopy (FTIR), conductimetric and mechanical tests. Portland cement pastes with the addition of Cr(VI) were examined and leaching behavior with respect to water and acid solution were investigated.This study indicates that Cr(VI) modifies the rate and the components obtained during the cement hydration.     

The development of high-strength mortars with improved thermal and acid resistance

Granulated blast furnace slag (GBFS) cement, containing up to 60% slag, is sometimes used in repair materials applied at intermediate temperatures of 150-300 °C. Low rate of strength development, especially at early ages, is considered a common disadvantage of repair mortars based on slag cement. The present research was oriented to improving a GBFS-portland cement binder for application as a repair material in the chemical industry when high thermal or acid resistance is required. It was found that the enhancement of GBFS-portland cement-based materials can be achieved with the help of silica fume (SF) and a superplasticizer (SP). The effect of different SPs on the compressive and flexural strength of SF-blast furnace slag-portland cement mortars was investigated. These mortars, in addition to high strength, demonstrate high thermal and acid resistance.