Preparation of Mullite-Silica Composites Using Silica-Rich Monophasic Precursor Obtained as a Byproduct of Mineral Carbonation of Blast-Furnace Slag

Abstract

Previously, mineral carbonation of blast-furnace slag was carried out to sequestrate CO 2 and attain pure CaCO 3 crystals. In this process, amorphous silica-alumina nanoparticles were obtained as a byproduct. In this study, the crystallization of these nanoparticles on calcination at various temperatures in air was examined using TGA-DTA, XRD, MAS-NMR spectroscopy, and FT-IR spectroscopy. The precursor nanoparticles (Si:Al = 78:22 mol %) were prepared using the solution extracted from blast-furnace slag (BFS) with acetic acid at room temperature. The XRD analysis showed that the initial amorphous state was retained up to 800 • C, and decomposition to amorphous silica and mullite started after calcination at 950 • C. At temperatures between 1150 • C and 1250 • C, amorphous silica crystalized to cristobalite, which eventually melted to glassy silica at 1500 • C. The mullite crystals initially adopted a metastable tetragonal phase and transformed to a stable, needle-like orthorhombic phase at higher temperatures. 27 Al MAS-NMR spectroscopy revealed that octahedrally coordinated Al was favored up to a temperature of 800 • C as a result of the dehydration process and transformed into tetrahedrally coordinated Al at higher temperatures. A microstructural examination revealed that the initially randomly-oriented mullite developed into stable, needle-like grains owing to anisotropic grain growth in the presence of a glass phase at high temperatures. This study suggests that the recycling of BFS can be exploited for the procurement of a mullite-type ceramic material.

Physicochemical processes in the zone of local laser heating of porous aluminosilicate ceramics

“…The prevalence of Si content over Al in the surface layer of the channel indicates that the composition corresponds to the SiO 2 ‐Al 2 O 3 compound, which is close to cristobalite‐mullite (SiO 2 ‐ 3Al 2 O 3 • 2SiO 2 ) 40‐42 . Taking into account the XRD data, mullite is mainly in the amorphous state 43‐45 . Some weak peaks in the halo can be attributed to mullite nuclei in the glass phase.…”

“…[40][41][42] Taking into account the XRD data, mullite is mainly in the amorphous state. [43][44][45] Some weak peaks in the halo can be attributed to mullite nuclei in the glass phase. As follows from, 5 at a low Al 2 O 3 content in a SiO 2 -Al 2 O 3 mixture, the mechanical strength of the ceramics increases as a result of the rearrangement of initially randomly oriented mullite nano-grains into stable acicular nano-grains in hightemperature glass melt.…”

Cold sintering of diatomaceous earth

“…No clear differences can be identified with respect to the raw powder, this pointing out the presence of compounds containing Si–O and Al–O bonds. Only a slight blue‐shift of the Si–O stretching signal (≈1050 cm −1 ) is present and can be attributed to the decomposition of clay minerals 42 /crystallization of mullite 61 …”

“…Only a slight blue-shift of the Si-O stretching signal (≈1050 cm −1 ) is present and can be attributed to the decomposition of clay minerals 42 /crystallization of mullite. 61 The influence of pressure on the bulk density of cold sintered samples is shown in Figure 5, with the effect of temperature and solvent concentration being also included. Bulk density increases with the applied pressure in all the tested conditions, because pressure provides an extra driving force for densification.…”