Partial replacement of Portland-composite cement by fluidized bed combustion fly ash

Abstract Fly ash from fluidized bed combustion differs greatly from that of pulverized coal firing. The most noticeable differences are in morphology, reactivity, and chemical composition. The use of biomass fly ash from fluidized bed combustion as a cement replacement material could be a promising method for both minimizing the amount of landfilled fly ash and reducing CO2 emissions in the concrete and cement industry. In this study, fly ash from fluidized bed combustion of peat and forest industry residue was used as a supplementary cementitious material for portland-composite cement (CEM II) containing clinker, blast furnace slag, and limestone. Even with a 40% cement replacement ratio, the compressive strengths of the mortar samples were still as high as 88% of the control sample’s strength. Comparison with unreactive replacement material revealed that moderate hydraulic properties of the studied fly ash explained the positive effects on strength rather than filler or nucleation effects

Comparison of standard methods for evaluating the metal concentrations in bio ash

Abstract The current growth strategy and environmental legislation of the European Union both aim to increase the amount of renewable energy and to improve the use of waste streams. These policies mean there will be an increasing need to utilise bio ash. Currently, Finland and Denmark are the only European countries with specific national legislation concerning bio ash use. Sweden has recommendations concerning the use of bio ash fertilisers. Besides having different limit values for harmful elements and nutrients in ash fertilisers, all these countries have different digestion methods that are allowed for element content determinations. This study compared the results of the five digestion methods (aqua regia, nitric acid, nitric/hydrochloric acids, nitric/hydrochloric/hydrogen fluoride acids, and lithium tetraborate fusion) established by Nordic authorities. Two Finnish peat-wood fly ash samples were studied. Our results indicate that the choice of digestion methods produces a significant difference in the obtained heavy metal or nutrient concentration of bio ash, especially regarding the potassium concentration

Combined granulation–alkali activation–direct foaming process:a novel route to porous geopolymer granules with enhanced adsorption properties

Abstract High-value applications, such as adsorbents, have drawn attention to geopolymers. In several of those applications, having the geopolymer as porous spherical particles is beneficial. This study presents a novel process for fabricating porous metakaolin-based geopolymer granules using a combination of direct foaming, one-part alkali activation, and granulation. In short, the precursor (e.g., metakaolin) and solid activator (e.g., sodium silicate) are loaded in a granulator, in which an aqueous blowing agent (e.g., H₂O₂) is added while the granulator is running, and the obtained granules are cured at 60 °C. Characterization of the granules for physico-chemical and morphological properties indicated an increase in overall porosity, especially in the µm-scale pores. Also specific surface area (+50%) and nanoscale pore volume (+102%) increased when using more concentrated H₂O₂ (20 or 30%) compared to nonporous granules. The use of porous granules was also demonstrated in dynamic adsorption experiments for ammonium (NH₄⁺) uptake, which showed up to ∼126% increase in cumulative adsorption amount compared to nonporous granules. The highest NH₄⁺ uptake was obtained with 10% H₂O₂ solution as the granulation fluid. The results confirmed the feasibility of the method for introducing porosity to geopolymer granules, which enhances the adsorption properties of the granules

Alkali activation as new option for gold mine tailings inertization

Abstract The mining industry produces a huge quantity of sulphidic mine tailings, which cause several short- and long-term environmental problems when disposed by landfilling in impounding lakes. The possibility of immobilizing several heavy metals from gold mine tailings by reactive geopolymerization technique has been investigated in the present study. The chemical stability of geopolymers synthetized by the alkali activation of metakaolin and blast furnace slag and the addition of 40–50 wt% gold mine tailings is demonstrated. The geopolymers were cured at room temperature, and the effects of different Si/Al and Na/Al molar ratios and curing times were investigated. The inertization effectiveness was evaluated by means of leaching tests carried out according to standard EN 12457 after 7 and 28 days and after 18 months. The samples were immersed into the water for 1 day, and the leachable metals in the test solution were determined by ICP-OES. The results show that various elements (Cr, Cu, Ni, Zn and Mn) from gold mine tailings are able to immobilize almost completely by alkali activation with proper co-binder material. The immobilization efficiency were highly improved with longer curing period also for the problematic elements As, V, Sb and B

The effect of fibrous reinforcement on the polycondensation degree of slag-based alkali activated composites

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

Incorporation of bioleached sulfidic mine tailings in one-part alkali-activated blast furnace slag mortar

Abstract Sulfidic mine tailings are potential waste materials from mining and mineral processing, and they can contain a high content of sulfur and metal(loid)s, even after bioleaching. Due to the large amount of tailings waste from historical mining, it is crucial to find alternative methods for utilizing such waste rather than permanent storage in tailings impoundments. One-part alkali-activated slag mortars are promising co-binder systems for the recycling of sulfidic mine tailings thanks to their practicability, easy transportation, and user-friendly production. In this work, up to 50 wt% mine tailings were incorporated into alkali-activated blast furnace slag mortars. C-(N)-A-S-H gels were formed in all final samples with hydrotalcite zeolites. Tailings hardly participate in alkali activation, but they do have a considerable influence on physical and chemical properties. The 20 wt% tailings-containing sample showed the highest compressive strength of 91.1 MPa after 90 days of curing. The results of isothermal calorimetry indicate that incrementally increasing the percentage of tailings promotes the pre-induction reaction but hinders the slag hydration process. In addition, the results of X-ray microcomputed tomography showed higher porosity when the mortar contains more tailings; thus, the sample with 10 wt% mine tailings showed the lowest porosity. According to the European Union batch leaching test, up to 20 wt% bioleached sulfidic mine tailings can be valorized in a co-binder system while remaining below the nonhazardous waste thresholds

On the hydration of synthetic aluminosilicate glass as a sole cement precursor

Abstract This paper reports on the synthesis and characteristics of a novel aluminosilicate glass-based cementitious binder. We investigated the hydration kinetics, degree of reaction, and phase assemblage of the glass via XRD, DTG, ²⁷Al and ²⁹Si MAS NMR, FTIR, SEM/EDS and thermodynamic modelling. The glass exhibits hydraulic properties in which the binder developed impressive compressive strength at early age. The main hydration products are an intermixed of Na and/or Al incorporated in C–S–H gel. Hydrotalcite precipitated with slower rate and thus may generate crystallization pressure on the binder at late stage. The glass reached a high degree of hydration (ca. 73 % based on quantitative ²⁹Si NMR analysis) without using any activators or co-binding systems. Therefore, the developed glass reported herein has high potential as a new low-carbon cementitious binder since it can be synthesised from naturally occurring carbonate free silicate minerals