Synthesis and characterisation of materials based on inorganic polymers of alumina and silica: sodium polysialate polymers

The effect of composition and temperature on the properties of fly ash- and kaolinite -based geopolymers

Properties and applications of halloysite nanotubes: recent research advances and future prospects

Porcelain represents the foundation of the ceramics discipline and one of the most complex ceramic materials. Composed primarily of clay, feldspar, and quartz, porcelains are heat-treated to form a mixture of glass and crystalline phases. This review focuses on raw materials, processing, heat treatment, and mechanical behavior. Because of the complexities of the porcelain system and despite the substantial amount of research already conducted within the field, there remain significant opportunities for research and study, particularly in the areas of raw material understanding, processing science, and phase and microstructure evolution.

Porcelain—Raw Materials, Processing, Phase Evolution, and Mechanical Behavior

The origin of the pozzolanic activity of calcined clay minerals: A comparison between kaolinite, illite and montmorillonite

Structural models previously proposed for metakaolinite are examined in light of the most recent published experimental data and new information obtained by solid-state high resolution 29Si and 27AI NMR. A new model for metakaolinite is proposed, consisting of anhydrous regions of distorted AI-0 tetrahedra containing randomly distributed isolated residual hydroxyls associated with A1-0 configurations of regular octahedral and tetrahedpal symmetry. Such a structure, which can readily be formed from kaolinite by the removal of hydroxyls in certain sequences, accounts for the lack of a well-defined X-ray pattern and the persistence of ∼10% residual hydroxyls in metakaolinite and is consistent with the most recent density data, bond lengths, and the new 29Si and 27AI NMR data.

Outstanding Problems in the Kaolinite-Mullite Reaction Sequence Investigated by 29Si and 27Al Solid-state Nuclear Magnetic Resonance: I, Metakaolinite

Solid-state 29Si and 27Al NMR spectra of kaolinite fired at 800° to 1450°C, interpreted in light of a newly proposed metakaolinite structure and complementary X-ray diffraction results, lead to the following conclusions about the hightemperature reactions: (1) Removal of the final residual hydroxyl radicals of metakaolinite at ∼9707deg;C triggers the separation of a considerable amount of amorphous free silica and the formation of poorly crystalline mullite and a spinel phase. (2) Mullite and spinel form in tandem, the former originating in the vicinity of AI-0 units of regular octahedral and tetrahedral symmetry randomly distributed throughout the metakaolinite structure. (3) The initially formed mullite is alumina-rich but at higher temperatures progressively gains silica, approaching the conventional 3Al2O3· 2SiO2 composition. (4) The spinel phase contains insufficient Si to be detected by 29Si NMR but has a 27Al NMR spectrum consistent with γ-Al2O3. On further heating, the spinel is converted to mullite by reaction with some of the amorpholls silica, the balance of which eventually becomes cristobalite.

Outstanding Problems in the Kaolinite‐Mullite Reaction Sequence Investigated by 29Si and 27Al Solid‐state Nuclear Magnetic Resonance: 11, High‐Temperature Transformations of Metakaolinite

Water is lost in two overlapping steps from well-crystallized pyrophyllite from Coromandel, New Zealand. The pyrophyllite structure survives the loss of the first 30% of the total water content, but the loss of a further 60% water in the second step results in the formation of pyrophyllite dehydroxylate, with corresponding changes in both the 29Si and 27Al solid-state NMR spectra. Detailed analysis of the 29Si chemical shift of the dehydroxylate has allowed the silicate layer structure of this phase to be refined. A similarly detailed interpretation of the 27A1 spectra is not possible because of electric field gradient effects which result in the loss of ∼90% of the A1 spectral intensity due to the formation of five-coordinate A1 on dehydroxylation. The loss of further water from the dehydroxylate on further heating results in the formation of mullite and cristobalite and is accompanied by changes in the 29Si and 27Al spectra which can be accounted for in terms of coordination changes in the structural regions which contained the residual hydroxyls.

Thermal Reactions of Pyrophyllite Studied by High‐Resolution Solid‐state 27Al and 29Si Nuclear Magnetic Resonance Spectroscopy

Thermal reactions of kaolinite studied by solid state 27-Al and 29-Si NMR

Studies of two muscovites of different iron contents, using solid-state NMR with magic-angle-spinning (MAS) combined with X-ray powder diffraction, thermal analysis and57Fe Mossbauer spectroscopy, suggest that dehydroxylation occurs by a homogeneous rather than an inhomogeneous mechanism, forming a dehydroxylate in which the aluminium is predominantly 5-coordinate. On further decomposition at about 1100° C, the tetrahedral layer and interlayer K+ form a feldspar-like phase similar to leucite (KAISi2O6), the remainder forming a spinel, which, contrary to previous suggestions, appears to contain little silicon. Further heating induces the formation of mullite (AI6Si2OP13), and, in the higher-iron sample, corundum (α-Al2O3), in addition to the feldspar-like phase. The presence of the iron impurity enhances the recrystallization reactions and promotes the conversion of mullite to corundum, which eventually becomes the sole aluminous product in the high-iron sample. In samples fired to higher temperatures, only the tetrahedral aluminium resonance is detectable by27AI NMR, probably because most of the iron is located in either the mullite or corundum phases, in which it broadens the octahedral aluminium resonance beyond detection.

R. H. Meinhold

Papers

Outstanding Problems in the Kaolinite-Mullite Reaction Sequence Investigated by 29Si and 27Al Solid-state Nuclear Magnetic Resonance: I, Metakaolinite

Outstanding Problems in the Kaolinite‐Mullite Reaction Sequence Investigated by 29Si and 27Al Solid‐state Nuclear Magnetic Resonance: 11, High‐Temperature Transformations of Metakaolinite

Thermal Reactions of Pyrophyllite Studied by High‐Resolution Solid‐state 27Al and 29Si Nuclear Magnetic Resonance Spectroscopy

Thermal reactions of kaolinite studied by solid state 27-Al and 29-Si NMR

The thermal reactions of muscovite studied by high-resolution solid-state 29-Si and 27-AI NMR