Alkali activation of MSWI bottom ash: Effects of the SiO2/Na2O ratio

Due to its high mineral content, the valorization of bottom ash from municipal solid waste incineration (MSWI) as potential precursor in the application of alkali activated materials is attracting attention. In literature there is a large variation on using of the activator solutions to activate MSWI bottom ash. In most studies, the bulk composition rather than reactive fraction of MSWI bottom ash is considered in the alkali activation design. However a large part of the Si present in MSWI bottom ash is in the form of non-reactive quartz. In this study, mainly slag fraction was considered, the glass, ceramic and natural stony materials were removed before MSWI bottom ash was used as precursor. An efficient activator solution was developed by considering the reactive silica content of MSWI bottom ash determined by a dissolution test. Alkali activator was made of NaOH solution with concentration varying from 4M to 8M and Na2SiO3 solution with moduli of 0.75 to 1.5. The effects of SiO2/Na2O ratio, where the oxide ratio for SiO2 consisting of the reactive Si contributed by MSWI bottom ash slag and by the Na2SiO3 in the activator solution, on the compressive strength of alkali activated MSWI bottom ash were studied. XRD was used to determine the reaction products. SEM was used to observe the morphology of synthesized binder phase and EDX will be used to determine the binder chemistry

Thermal Treatment on MSWI Bottom Ash for the Utilisation in Alkali Activated Materials

At present, most municipal solid waste incineration (MSWI) bottom ash is directly landfilled, raising concerns about environmental issues and loss of resources. Due to its high mineral content, MSWI bottom ash is now being considered as a raw material to prepare alkali-activated materials (AAMs). However, the mineral fraction unavoidably contains metallic aluminium (Al) and zinc (Zn) scraps (<1 wt.%), which easily oxidise and generate H2gas under alkaline conditions. As a result, when using MSWI bottom ash to prepare AAMs, the formation of a porous structure, as well as expansive cracks (both detrimental to strength development) can be observed. In this research, thermal treatment of MSWI bottom ash, at temperatures of 500 and 1000 °C, was performed to deal with the issue caused by metallic Al/Zn. A series of tests, including Quantitative X-ray diffraction (QXRD) analysis, fineness measurements (particle size and surface area), and the dissolution test, were conducted to examine the effects of thermal treatment on as-received bottom ash. The results indicate that it is difficult to oxidise metallic Al/Zn at 500°C, but heating up to 1000 °C can realize the complete oxidation of Al/Zn, which in turn allows the wide utilisation of bottom ash in AAMs. Keywords: MSWI bottom ash, thermal treatment, alkali-activated materials

Leaching of monolithic and granular alkali activated slag-fly ash materials, as a function of the mixture design

\u3cp\u3eThis study explores the leaching of oxyanionic metalloid species (As, Mo, Se, V and Cr) from alkali activated slag-fly ash materials (AAM), dependent on various mixture parameters i.e., activator molarity, slag-fly ash precursor/binder compositions, liquid to binder ratio, curing time and strength. The analyses focusses on the leaching of potentially hazardous elements in a monolithic and granular material state. For monolithic state AAMs (concrete) overall leaching is within comparable range with traditional Portland cement and in both systems their leaching is far below the regulatory leaching limit values even though AAM strongly differs in mixture composition. For granular state AAMs (aggregate) the parameters, activator alkalinity and the slag-fly ash precursor/binder composition, significantly influence the leaching. The release of As and V strongly increases with a higher activator molarity as an effect of changes in the system alkalinity and related material pH. The release of As, Mo, Se and V strongly increase with a higher fly ash content within the precursor/binder composition. Overall, the leaching of aggregate state AAMs meets the Dutch leaching limits for open application of granular building materials, when the fly ash content within the binder composition ≤ ≈30 wt%. Typically, the pH dependent leaching data show oxyanionic metalloid species have a relatively high leaching potential, being less effectively bound as a result of the amorphous AAM microstructure. However, the leachable concentrations of a AAM system are within the bandwidth with that of blended (slag and or fly ash) Portland cement system.\u3c/p\u3

Waste glass as partial mineral precursor in alkali-activated slag/fly ash system

\u3cp\u3eThe feasibility of a waste glass powder residue (GP) from glass recycling as partial mineral precursor to produce alkali-activated materials is investigated. GP served as powder coal fly ash (PCFA) replacement within a reference system composed of 50% PCFA and 50% ground granulated blast furnace slag (GGBS). Compared with PCFA, GP was better involved in the alkali activation process by having a higher silica and Ca dissolution. Furthermore, increasing GP replacement up to 30% prolonged the induction period, facilitated the gel formation and yielded a 35% higher 28-day compressive strength. These observations are similar to the effect of using both sodium hydroxide and sodium silicate as alkali activator in alkali-activated slag/fly ash systems. A higher polymerization of the gel network was also observed. Microstructure analysis indicated that the main reaction product is a calcium silicate hydrate type gel substituted with Al and Na (C-(N)-A-S-H type gel). This work largely contributes to the understanding of the reactivity and potential of GP and promotes its practical utilization as a mineral precursor in the production of alkaline cements.\u3c/p\u3

Effect of admixture on the pore structure refinement and enhanced performance of alkali-activated fly ash-slag concrete

\u3cp\u3eThis paper investigates the influence of a plasticizing admixture on the pore structure refinement of alkali-activated concrete and paste mixtures and the consequently enhanced performance. Alkali-activated fly ash-slag concrete and paste are designed using a polycarboxylate-based admixture with different dosages. The pore structure and porosity are analyzed using mercury intrusion porosimetry (MIP). The workability, compressive strength, chloride migration resistance and electrical resistivity of alkali-activated fly ash-slag concrete and paste are determined. The results show that significantly improved workability and strength development are obtained at an increased admixture content. The admixture improves the gel polymerization product layer most likely around the GGBS particles, densifying the matrix. The 28-day Cl-migration coefficient of admixture (1–2 kg/m \u3csup\u3e3\u3c/sup\u3e) modified concrete is equal to the reference mixture, while at the highest admixture content the Cl-ingress is increased. At the later ages (91-days), the Cl-migration coefficients of all concretes, non- and admixture-containing samples, are comparable and low (about 2.6 × 10 \u3csup\u3e−12\u3c/sup\u3e m \u3csup\u3e2\u3c/sup\u3e/s). The MIP analyses show a significant decrease of the total and effective capillary porosity over time at an increased admixture content. The relationships between the porosity and other properties are discussed, at varying admixture contents. \u3c/p\u3

Testing procedures for CO2 uptake assessment of accelerated carbonation products: Experimental application on basic oxygen furnace steel slag samples

Many CO2 reactive by-products from industrial processes can be valorised via carbonation to produce building materials. The amount of CO2 captured in mineralisation processes is environmentally and economically relevant, however results from the literature are often controversial owing to a lack of standardisation in the existing CO2 quantification techniques. In this study testing procedures for the assessment of carbonation efficiency are outlined and applied on basic oxygen steel slag samples providing CO2,uptake estimates ranging from 13.3% to 17.0%. Thermogravimetry, thermal decomposition and acid digestion, emerged as valuable techniques for carbonates quantification that can be adopted on samples of different nature, i.e. powders, pastes and mortars. Advantages and drawbacks of each method are discussed, and their field of application is defined. A comparison of the experimental results with similar studies from the literature is reported as well

Waste glass as partial mineral precursor in alkali-activated slag/fly ash system

The feasibility of a waste glass powder residue (GP) from glass recycling as partial mineral precursor to produce alkali-activated materials is investigated. GP served as powder coal fly ash (PCFA) replacement within a reference system composed of 50% PCFA and 50% ground granulated blast furnace slag (GGBS). Compared with PCFA, GP was better involved in the alkali activation process by having a higher silica and Ca dissolution. Furthermore, increasing GP replacement up to 30% prolonged the induction period, facilitated the gel formation and yielded a 35% higher 28-day compressive strength. These observations are similar to the effect of using both sodium hydroxide and sodium silicate as alkali activator in alkali-activated slag/fly ash systems. A higher polymerization of the gel network was also observed. Microstructure analysis indicated that the main reaction product is a calcium silicate hydrate type gel substituted with Al and Na (C-(N)-A-S-H type gel). This work largely contributes to the understanding of the reactivity and potential of GP and promotes its practical utilization as a mineral precursor in the production of alkaline cements.Materials and Environmen

Contaminated biomass fly ashes - characterization and treatment optimization for reuse as building materials

The incineration of treated waste wood generates more contaminated fly ashes than when forestry or agricultural waste is used as fuel. The characteristics of these biomass fly ashes depend on the type of waste wood and incineration process parameters, and their reuse is restricted by their physical, chemical and environmental properties. In this study, four different fly ash types produced by two different incineration plants were analysed and compared to Dutch and European standards on building materials. A combined treatment was designed for lowering the leaching of contaminants and the effect of each treatment step was quantified. A pilot test was performed in order to scale up the treatment. It was found that chlorides (which are the main contaminant in all studied cases) are partly related to the amount of unburnt carbon and can be successfully removed. Other contaminants (such as sulphates and chromium) could be lowered to non-hazardous levels. Other properties (such as particle size, LOI, oxide and mineralogical compositions) are also quantified before and after treatment