Ground granulated blast furnace slag (GGBS) is a glassy by-product of pig iron production and is commonly used in concrete industry to replace cement and thereby lower the carbon footprint of the material. Large variations in reactivity exist depending on the GGBS physical and chemical features. Here we investigate the ability of three rapid calorimetric methods to evaluate the reactivity of GGBS. On a set of 16 industrial GGBS, we show that 24h heat release, using the R3protocol, correlates well with 2d compressive strength of standard mortars using 75 wt.-% GGBS. The correlation of R3-test results (R²= 0.87) is better than for traditional reactivity indices calculated from chemical composition. Furthermore, we present data on the repeatability of the test protocol and show that the R3 protocol is very sensitive to sample fineness. Finally, XRD patterns show that slight differences in phase assemblage exist between the most and least reactive GGBS.
The aim of this work is to describe the inorganic phase transformation of Miscanthus × giganteus ash at a high temperature taking into account the effect of oxidizing/reducing atmospheres and different harvest periods. A double approach was used: thermodynamic calculations and experimental validation in laboratory devices. The samples were characterized by scanning electron microscopy−energy-dispersive X-ray spectroscopy (SEM−EDX) and X-ray diffraction (XRD). Below 800 °C, the inorganics are mainly present as salts [KCl, K 2 SO 4 , and Ca 3 (PO 4 ) 2 ], carbonates (CaCO 3 ), and SiO 2 , while at higher temperatures (>900 °C), the solid phase is enriched in CaSiO 3 and Ca 3 (PO 4 ) 2 . The ash starts to melt around 750 °C, and the liquid phase is composed of SiO 2 , K 2 O, and CaO. The main difference between oxidizing and reducing atmospheres is the form of sulfur. In an oxidizing atmosphere, K 2 SO 4 is formed, while in a reducing atmosphere, only traces of organically attached S were detected by SEM−EDX and most of the S has volatilized. The thermodynamic calculations were performed using the software package FactSage 6.4 with two different oxide databases: FToxid and GToxid. GToxid database predicts a higher solidus temperature (T s > 900 °C) but greater liquid/solid ratio than FToxid (T s ≈ 740 °C). Both databases predict the formation of K silicates or K−Ca (Mg) silicates below 800 °C, while the main solid phase observed by XRD was SiO 2 . Reasons are linked to uncertainties in the initial mass balance of ashes, database uncertainties, and limitations of the diffusion and chemical reaction of elements. The composition of the liquid phase is well-described in both databases, although GToxid gives somewhat closer results to the SEM−EDX analysis with the incorporation of Ca and Mg into the liquid phase.
One of the main concerns about biomass fluidized bed gasification and combustion is the risk of bed particle agglomeration due to ash melting. Although many studies have been conducted about the agglomeration mechanism using silica sand, olivine is mostly mentioned as an alternative bed material for tar decomposition, and its interaction with biomass ash has not been yet fully understood. The aim of this work is to investigate the agglomeration of miscanthus ashes, focusing on thermophysical and thermochemical aspects. Three different bed materials (silica sand (SiO 2 ), raw and calcined olivine ((Mg,Fe) 2 SiO 4 )) and an additive to prevent agglomeration (dolomite (CaMg(CO 3 ) 2 ) were tested. The effects of atmosphere and miscanthus harvest time were also investigated. It was found that the key parameter of agglomeration is the wettability of bed particles by molten ashes. In contact with ashes all three bed material showed good wetting tendencies, while dolomite had non-wetting properties. The adhesion between bed materials and molten ashes increases in the order of silica, olivine and calcined olivine. While in the case of silica sand only physical adhesion occurred, the diffusion of iron oxide into the molten ash was observed using olivine. Calcined olivine has a roughened surface which further increased the adhesion. The atmosphere did not influence the mechanism of ash/bed material interaction. On the other hand, miscanthus harvest time had a significant effect on ash reactivity and interaction with raw and calcined olivine.
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