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The effect of dry grinding on the leaching behaviour of pyrophyllite has been studied by XRD, FT-IR, thermal analysis and N 2 adsorption techniques. Pyrophyllite samples were ground for 3, 6, 12 and 18 h. Grinding initially causes the surface area of the pyrophyllite to increase to ∼73 m 2 g −1 , decreasing with longer grinding because of particle agglomeration. Leaching the ground pyrophyllite with 4 M HCl at 80 °C for 2 h causes depletion of Al 3+ and increases the surface area. The highest surface area was recorded in the leached sample ground for 3 h (156.9 m 2 g −1 with a pore volume of 0.36 ml g). Structural breakdown by grinding is thought to be the main reason for the enhanced leaching behaviour of the pyrophyllite, but grinding for extended periods decreases the leaching rate without enhancement of the porous properties.

The uptake of various ions by amorphous and crystalline BaAl 2 Si 2 O 8 was investigated. The BaAl 2 Si 2 O 8 samples were prepared by solid-state reaction of kaolin ground with BaCO 3 for periods from 1 to 24 h followed by firing at temperatures of 800–1200 °C for 24 h. Uptake experiments were performed at room temperature (25 °C) using a solid/solution ratio of 0.1 g/50 ml, with a cation concentration of 10 −3 M and reaction time of 24 h. The cations used for the uptake experiments were alkaline earth ions (Mg 2+ , Ca 2+ and Sr 2+ ) and transition metal ions (Ni 2+ , Co 2+ , Cu 2+ and Zn 2+ ). The uptake of alkaline earth ions was low in all the samples while the transition metal ion uptakes were higher in the amorphous BaAl 2 Si 2 O 8 sample than crystalline BaAl 2 Si 2 O 8 samples. The high uptake of transition metal ions by the amorphous sample was further enhanced by prolonging the grinding time. Since the amorphous sample appears from the 29 Si and 27 Al NMR spectra to have a similar local structure to crystalline hexacelsian (double-layered structure consisted of alternatively corner shared AlO 4 and SiO 4 tetrahedra), the uptake of transition metal ions is suggested to occur by release of interlayer Ba ions in the vicinity of edge sites with the adsorption of transition metal ions from the solution.

A comparison was made of the solid-state 29Si, 27Al and 43Ca MAS NMR spectra of the outer casing stone from Snefru's Bent Pyramid in Dahshour, Egypt, with two quarry limestones from the area. The NMR results suggest that the casing stones consist of limestone grains from the Tura quarry, cemented with an amorphous calcium-silicate gel formed by human intervention, by the addition of extra silica, possibly diatomaceous earth, from the Fayium area.

Inorganic polymers based on alumina and silica polysialate units were synthesised from dehydroxylated aluminosilicate clay (metakaolinite) condensed with sodium silicate in a highly alkaline environment. Reaction of the aluminosilicate with alkali polysilicates yields polymeric Si–O–Al three-dimensional structures with charge-balancing positive ions such as hydrated Na+ in the framework cavities. A statistical study of the effect on the polymerisation process of the molar ratio of the component oxides and the water content of the mixture showed the latter to be a critical parameter. The polymerisation mechanism and structures of the products were investigated using NMR, XRD and FTIR spectroscopy. 29Si liquid-state NMR shows that some compositions do not cure properly because of incomplete reaction of the sodium silicate with the metakaolinite. FTIR confirms that during drying of the incompletely cured samples, Na migrates to the surface where it undergoes atmospheric carbonation. The cured polymers were found to be essentially X-ray amorphous, with bulk densities of 1.3–1.9. During polymerisation the coordination of Al in the metakaolinite reactant (IV, V and VI) changes almost completely to IV in all the polymer compositions. The environment of the Na is unchanged irrespective of the polymer composition. The solid-state 29Si NMR spectra indicate a range of Si–O–Al environments. Typical mechanical properties of the best polymers were: Mohs hardness >7, Vickers hardness about 54, and compressive strength (after drying for 1 h at 65°C) 48.1 MPa.

Amorphous aluminosilicate xerogels of 9.4 mol% (15 mass%) Al 2 O 3 composition were prepared by coprecipitation under various preparation conditions to investigate their effect on the porous properties of the xerogels. The precipitates were synthesized by coprecipitation of an ethanol solution of aluminium nitrate nonahydrate (ANN) and tetraethylorthosilicate (TEOS) using conc. NH 4 OH as the precipitant. The xerogels were obtained by drying and calcining the precipitate at 300°C for 4 h. Various synthesis conditions were tried, such as changes in the precipitation method, concentration of total (ANN+TEOS), H 2 O/TEOS ratio, the mixing time of the ANN and TEOS solutions, and the amount of NH 4 OH added. Of these experimental parameters, the H 2 O/TEOS ratio and mixing time of the ANN and TEOS solutions exerted the greatest influence on the porous properties. The specific surface area, total pore volume and pore radius of the xerogels ranged from 123 to 455 m 2 g −1 , from 0.31 to 1.17 ml g −1 and from 1.8 to 20 nm, respectively. These porous properties could readily be controlled by controlling the preparation conditions of the xerogels.

The thermal transformation of montmorillonite clay compacts has been studied in water vapour (0·4 atm) and dry atmospheres by thermal analysis, XRD, and solid state MAS NMR. The water vapour atmosphere decreased the thermal transformation temperature by about 200°C and produced a threefold improvement in the mechanical properties of the fired clay compacts (crushing strength and softening coefficient). These effects are attributed to the increased strength at the contact points between the grains due to polycondensation of silanol groups and may be a general phenomenon in a range of clay mineral types.

The dehydroxylation of kaolinite and its subsequent transformation to cubic Al-spinel and mullite has been studied under water vapour atmospheres up to 0·8 atm pressure. Dehydroxylation, as determined by XRD, FTIR and solid state MAS NMR, can be accomplished at low temperatures (ca 500°C) under water vapour atmospheres, the efficiency increasing with increasing water vapour pressure. The subsequent thermal transformations to crystalline products are also facilitated by water vapour atmospheres, which also enhance the mechanical properties of the fired kaolinite (crushing strength and softening coefficient) by 3–4 times. These effects are discussed in terms of proton attack on the Si–O bonds followed by polycondensation of the resulting silanol groups to siloxane, facilitating the nucleation of mullite from amorphous aluminosilicate and strengthening the chemical bonds between the grains.

The influence of mechanochemical treatment on the homogeneity of gibbsite–silicic acid mixtures was investigated by 29 Si and 27 Al MAS NMR, DTA–TG and XRD. Increased homogeneity of mullite precursors prepared by grinding mixtures of the appropriate stoichiometry for 20 h is indicated by the appearance of metakaolinite-like 4-coordinated Al resonances in the 27 Al NMR spectra and new resonances at −93 and −101 ppm in the 29 Si spectra. The influence of hydration water on the grinding reactions is also discussed.

The products of mechanochemical activation of mixtures in the magnesia–silica system ground in a planetary pot mill for up to 20 h. were found to depend on the water content of the siliceous starting materials. Mixtures containing silica gel produced no new products detectable by XRD and 29 Si MAS NMR, but their thermal reaction to form forsterite at 890°C was slightly enhanced by grinding. In contrast, mixtures containing more highly hydrated silicic acid produce a layer-lattice magnesium silicate with an 8.4-A basal spacing and a 29 Si MAS NMR spectrum similar to that of the product obtained when the same mixture is subjected to hydrothermal reaction conditions. A possible explanation in terms of a mechanochemically induced hydrothermal reaction mechanism is discussed.

The evolution of mullite from organo-metal gel precursors above 700 °C is found to be strongly influenced in both gel pieces and powdered samples by the thermal pretreatment at lower temperatures. Under the present conditions, the optimum preheating temperature was found to be 350 °C, at which temperature an anomalously high concentration was found of an Al species with a characteristic 27Al magic-angle spinning NMR resonance at about 30 ppm. Such Al sites are often described as pentaco-ordinated, but an alternative assignment is considered. The optimum temperature for the formation of this Al site is also optimal for the catalytic formation of aromatic molecules from the residual organic fragments and/or solvent present. Mass spectrometry shows that under the present reaction conditions, these aromatics are thermally stable up to at least 900 °C in air, and the prolonged presence of their decomposition products (CO and water) could facilitate the transformation of the gel to crystalline mullite. The 29Si NMR spectra indicate at least three different Si environments, including one which may arise from the formation of silicon oxycarbide glasses in these gels.

In addition to its importance as a high-technology ceramic, mullite is also an intermediate phase in the formation of non-oxide ceramics (sialons) by carbothermal reduction of clay minerals. This reaction involved the formation of mullite under non-oxidizing atmospheres and in the presence of carbon. The effect of eight different reaction atmospheres on mullite formation from kaolinite in the presence and absence of carbon was studied by X-ray diffraction, 27 Al and 29 Si magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy. Generally, a greater amount of mullite is formed at 1200 °C under vacuum and reducing atmospheres, which also produce mullites that are slightly more silica-rich (as deduced from X-ray lattice parameter measurements) but contain higher proportions of tetrahedral 27 Al NMR resonance at ≈ 46 ppm, probably associated with Al ∗ sites. Reaction systems containing either nitrogen or ammonia in the presence of carbon show NMR evidence of previously unreported early stage formation of SiON bonds. Thermodynamic calculations are presented which suggest that the formation of SiO is an important factor in the complex interactions between the various solid and gaseous reactants.

Phase formation in kaolinite heated at 1200°C in eight different reaction atmospheres in the presence and absence of carbon has been studied by solid-state magic angle spinning nuclear magnetic resonance (MAS NMR) and X-ray powder diffraction. Mullite (3Al2O3-2SiO2) and amorphous SiO2 are the principal products in all atmospheres. The amount of mullite formed is generally greater under vacuum and in reducing atmospheres, but the precise effect of the atmosphere is also modified by the presence of carbon. Vacuum and reducing atmospheres generally produce mullites of alumina: silica composition nearer 3∶2 than 2∶1 (estimated from unit cell measurements) and containing a higher proportion of Al*, i.e. tetrahedral aluminium associated with an oxygen defect (estimated by 27AlNMR measurements). The result of most significance to sialon formation is the detection by 29Si NMR of silicon oxynitride formation at 1200°C in systems containing either nitrogen or ammonia, in the presence of carbon. The preferential formation of Si-O-N bonds at such an early stage of the reaction under carbothermal conditions was confirmed by thermodynamic calculations, which also clarify other details of the complex interactions between the aluminosilicate, carbon, and the various gas atmospheres.

The room-temperature setting process in compacts of various silicate and non-silicate mineral particles bonded with sodium silicate was found to be markedly accelerated by treatment with the acidic gases CO2, S02 and H2S, but was unaffected by neutral or alkaline gases. Strength development increases with gassing time up to a maximum value which depends on the Si to Na ratio of the sodium silicate, the nature of the mineral matter and the gas used. Longer gassing times are needed to achieve ultimate strength with sodium silicates of higher pH and gases of lower solubility in water. The chemical species formed by reaction of CO2, S02 and H2S with sodium silicate were investigated by IR spectroscopy, X-ray diffraction and 29-Si solid state NMR spectroscopy.

Nanoporous materials have attracted great interest because of their excellent porous properties, which are expected to lead to a variety of applications exploiting their ordered nanospace structures. In general they are prepared using chemical reagents as the inorganic sources, structurally directed by the use of organic templates. Thus, an ordered nanospace structure is formed by the polymerization of inorganic monomers assisted by organic templating molecules. By contrast, many studies have been reported using minerals as the inorganic sources to prepare unique ordered nanoporous materials which can display different properties from materials prepared using chemical reagents. In these cases, the mineral starting materials play an important role in building up ordered nanospace structures originating from their unique initial structures, i.e., these reactions are self-templating. Thus, the resulting nanoporous materials constitute a new family of ordered nanospace structures. In this chapter, the preparation and properties of nanoporous materials from mineral and organic templates are outlined and compared with nanoporous materials derived from chemical reagents by the use of organic templates.

Aluminium site occupancies deduced from 27 Al NMR measurements of several aluminosilicate glasses of composition 35–60 mol% Al 2 O 3 were used to calculate the mean coordination number, assuming two polyhedral models. In the models the 30 ppm Al NMR resonance was assigned to fivefold coordinated Al or to distorted tetrahedral units, respectively. Comparison of these mean coordination numbers with those derived from pair distribution functions (PDF) from X-ray scattering data of these glasses support the model in which the 30 ppm Al NMR peak is assigned to distorted tetrahedral units. This conclusion is also supported by simulations of the PDF line profiles using the NMR site occupancies and mean polyhedral bond lengths.

Porous ceramics with unidirectionally oriented pores have been prepared by various methods such as anodic oxidation, templating using wood, unidirectional solidification, extrusion, etc. The templating method directly replicates the porous microstructure of wood to prepare porous ceramics, whereas the extrusion method mimics the microstructures of tracheids and xylems in trees. These two methods are therefore the main focus of this review as they provide good examples of the preparation of functional porous ceramics with properties replicating nature. The well-oriented cylindrical through-hole pores prepared by the extrusion method using fibers as the pore formers provide excellent permeability together with high mechanical strength. Examples of applications of these porous ceramics are given, including their excellent capillary lift of over 1 m height which could be used to counteract urban heat island phenomena, and other interesting properties arising from anisotropic unidirectional porous structures.

A simple ion-exchange method is reported for replacing the charge-balancing cations in aluminosilicate inorganic polymers with a range of other cations. Complete exchange of the Na+ in a conventional Na-inorganic polymer by K+, Ag+, NH4+ and Pb2+ was achieved by this method, with a lower degree of exchange by Li+ (82%), Cd2+ (78%) and Mg2+ (57%). The NH4+-compound can be completely exchanged by Ag+, but the exchange of NH4+ for other cations is less efficient in this compound than when using the Na-compound as the starting material. The X-ray-amorphous nature and Al and Si environment of the original Na inorganic polymer are unchanged by the ion exchange process. The K+, Li+ and NH4+-compounds prepared by ion exchange are suitable precursors for crystalline ceramics, forming leucite, spodumene and mullite respectively, upon heating at 1100 °C. The ammonium ion in the NH4+-exchanged compound was shown by FTIR and solid-state MAS NMR to decompose gradually on heating up to 600 °C, at which temperature, a significant proportion of the Al is in 5-fold coordination with oxygen. This suggests that the NH4+-exchanged compound heated at 600 °C may be an efficient catalyst for organic reactions such as the Friedel–Crafts alkylation. The efficient exchange capability of Na aluminosilicate inorganic polymer for Pb2+ and Cd2+ (but not Hg2+) suggests a potential application for this material for the cost-effective removal of heavy metal ions from wastewater streams, while the Ag+-exchanged compound was shown in this work to be a powerful antimicrobial agent against Staphylococcus aureus.

Only very slight changes are observed in the29Si and27Al solid-sate magic-angle spinning NMR spectra of a montmorillonite containing almost equal numbers of octahedral aluminium and magnesium ions when its interlayer water is driven off by heating. The29Si NMR spectra are unaffected by dehydroxylation which begins at ∼ 450° C, but the27Al spectra show a decrease in total intensity, possibly due to the formation of 5-coordinated aluminium, with a slight increase in the intensity of the tetrahedral aluminium resonance. On the basis of these results, a structural model is proposed for the dehydroxylate phase and its formation mechanism is discussed. The destruction of the dehydroxylate X-ray pattern at ∼ 850° C and the subsequent recrystallization of the high-temperature products (β-quartz, enstatite and high-cordierite at ∼ 1100° C;β-cristobalite, enstatite and sapphirine at ∼ 1200° C) is accompanied by changes in the silicon and aluminium NMR spectra and in the57Fe Mbssbauer spectra which are fully consistent with the known structural features of these phases.

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.

Augmentation lineaire du parametre cristallin a des solutions solides Fe 2 O 3 /Al 2 O 3 avec le taux de Fe 2 O 3 . Les spectres Mossbauer de ces solutions solides montrent deux principaux doublets quadrupoles: l'intensite de l'un croit avec le taux de Fe 2 O 3 tandis que l'intensite de l'autre decroit

Gibbsite (AlOH)3 was mechanically activated by grinding for 20 h, and the changes in its structure were studied by thermal analysis, X-ray powder diffraction and 27 Al MAS NMR. Grinding caused the rupture of a significant proportion of the Al–OH bonds, but the resulting molecular water was immediately adsorbed on to the activated surfaces from which it could be endothermically desorbed at 125°C. Grinding causes the gibbsite to become X-ray amorphous, containing octahedral and tetrahedral sites, and another site at about 34 ppm sometimes ascribed to Al in 5-fold coordination; the relative site occupancies suggest that this phase is similar to ρ-Al2O3. The amorphous phase converts via γ-Al2O3 to α-Al2O3 (corundum) at 900°C, by contrast with unactivated gibbsite which transforms to corundum via γ and θ-Al2O3 at least 400°C higher.