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This work describes a novel approach to improve the dispersion of a nanoparticle MgCr2O4 additive in the matrix of magnesia-chrome refractories by functionalizing the nanoparticle surfaces with silanol groups. The effect of the silanol groups on the nanoparticles within the refractory matrix was shown by FTIR spectroscopy, zeta potential measurements, UV–visible spectroscopy, Dynamic Light Scattering (DLS) and TEM to result in a decrease of the average particle size of the MgCr2O4 additive by about 50% due to de-agglomeration of the nano particles by the silanol groups. Silanol functionalization of the nano-additives in the magnesia-chrome refractories fired at 1600 °C resulted in a 65 Kgf/cm2 increase in their compressive strength, while the hot modulus of rupture and corrosion resistance values of these refractories fired at 1400 °C were similar to those containing unfunctionalized nano-additives fired at 1600 °C. XRD and SEM results suggest the improvement in corrosion resistance is related to the formation of spinel phases in the refractories containing silanol-modified nano-additives.

Semiconducting nanocomposites were formed from layers of reduced graphene oxide (RGO) and ZnO and nanoparticle or mesoporous forms of TiO2 by dip-coating on fluorine-doped tin oxide (FTO) substrates. The effect of nanoparticle or mesoporous TiO2 on the electrical conductivity was compared. The crystalline phases and microstructures of the nanocomposites were investigated by XRD, DRS-UV, FTIR and FE-SEM, showing the layers to be homogeneous and of consistent thickness, with the TiO2 in the anatase form. The bandgaps and electrical conductivities of the nanocomposites, determined by Tauc plots and electrochemical impedance spectroscopy (EIS), show that under UV irradiation, the nanocomposites containing nanoparticle TiO2 have higher electrical conductivities than those containing mesoporous TiO2. A mechanism is suggested involving the formation of nano-capacitors on the surface of the ZnO layer, the pores in the mesoporous TiO2 acting as the dielectric and the conducting electrodes formed by UV irradiation of the TiO2.

Silicon carbide synthesis by a magnesiothermal method was investigated using MCM-48 as the silica source mechanically mixed with carbon nanotubes (CNTs) as the carbon source, and nanocomposites of MCM-48/functionalized CNTs (CNTF). SiC syntheses were carried out with different molar ratios of MCM-48, carbon and magnesium at 700 °C in argon. The MCM-48 and carbon nanotube starting materials and the SiC products were characterized by BET, XRD, FESEM, EDX and TEM. The effect of the carbon content and the type of CNTs (either functionalized or unfunctionalized) on the SiC synthesis was studied. The results show that an improved yield of SiC is obtained when the carbon nanotubes are functionalized, producing a better contact with the MCM-48. This improved contact between the reactants ensures a good degree of reaction in a stoichiometric mixture of silicon and carbon, with no improvement in product formation being achieved by the use of additional carbon. These findings suggest that the degree of contact between reactants is an important factor in the magnesiothermal synthesis of SiC. The SiC products from magnesiothermal synthesis of the functionalized nanocomposite precursors were shown by TEM and FESEM to have unusual nanofiber morphologies mimicking the morphology of the CNTF nanotubes.

A microstructural study of Coode Island Silt (CIS), a soft silty clay from the Melbourne area of Australia, stabilised with slag lime is reported. Slag lime is a blend of 80–85 wt% slag, 15 wt% hydrated lime, Ca(OH)2 and 3–8 wt% gypsum CaSO4.2H2O, and is typically used for soil stabilisation in roading applications. The morphologies of several homogeneous mixtures of slag lime and CIS were studied by Scanning Electron Microscopy (SEM) and X-Ray Diffraction (XRD), which indicated the formation of the cementitious phases calcium carbonate, calcium silicate hydrate, calcium aluminium silicate hydrate, and calcium aluminium silicate carbonate. The progress of the CIS-slag lime reactions was also investigated by pH measurements. Slag lime was found to very actively promote pozzolanic reactions with CIS, shown by SEM to form crystalline reticular calcium silicate hydrate and other cementitious products. The most effective pozzolanic reactions occurred in a composite containing 12.5 wt% slag lime with CIS and contained the cement mineral ettringite, contributing to its high strength. These composites constitute a new class of materials with excellent potential for construction applications.

Nanocomposites of MCM-48 mesoporous silica and hydroxyapatite (HA) are investigated as possible alternatives for the commonly used Mineral Trioxide Aggregate (MTA) dental cement, and the effect of their Ca/P ratio on their structure, properties and setting times is reported. XRD, FTIR and EDS/SEM analysis of the nanocomposites indicate that crystalline HA is formed within the mesoporous silica structure. High Ca/P ratios lead to the formation of free CaO within the nanocomposite, decreasing its setting time; the shortest setting time (~ 40 min), which occurs in the MCM-48/HA nanocomposite with a Ca/P ratio of 5, in physiological saline solution, is considerably shorter than the setting time of MTA (~ 140 min under identical conditions). Thus, the shorter setting time of the MCM-48/HA nanocomposites, coupled with their lack of toxicity, high degree of bioactivity and their stability in tissue fluids, suggest that they could serve as a useful replacement for MTA.

New photoactive composites to efficiently remove organic dyes from water are reported. These consist of Cu2O/TiO2 nanoparticles in a novel inorganic geopolymer matrix modified by a large tertiary ammonium species (cetyltrimethylammonium bromide, CTAB) whose presence in the matrix is demonstrated by FTIR spectroscopy. The CTAB does not disrupt the tetrahedral geopolymer structural silica and alumina units as demonstrated by (29)Si and (27)Al MAS NMR spectroscopy. SEM/EDS, TEM and BET measurements suggest that the Cu2O/TiO2 nanoparticles are homogenously distributed on the surface and within the geopolymer pores. The mechanism of removal of methylene blue (MB) dye from solution consists of a combination of adsorption (under dark conditions) and photodegradation (under UV radiation). MB adsorption in the dark follows pseudo second-order kinetics and is described by Freundlich-Langmuir type isotherms. The performance of the CTAB-modified geopolymer based composites is superior to composites based on unmodified geopolymer hosts, the most effective composite containing 5wt% Cu2O/TiO2 in a CTAB-modified geopolymer host. These composites constitute a new class of materials with excellent potential in environmental protection applications.

The photocatalytic activity of mesoporous TiO 2 modified by the addition of carbon nanotubes (CNTs) and Cu is reported. Nanocomposites of carbon nanotubes (CNTs) containing varying amounts of Cu were formed by treatment with Cu 2+ then reduced to Cu 0 using NaBH 4 as the reducing agent. The mesoporous TiO 2 , synthesized by a sol-gel method from titanium isopropoxide, was combined with the CNT/Cu nanocomposites to form the photocatalysts which were characterized by XRD, SEM, TEM, FTIR, XPS and BET surface area analysis. The photocatalytic properties of the mesoporous TiO 2 composites were studied by measuring the degradation of methyl orange (MO) which was optimal in the sample containing 20 wt% of the Cu-CNT nanocomposite. The degradation efficiency for MO was a synergistic effect of photo-degradation of TiO 2 and may be due to improvement of the electrical conductivity of the system by the presence of the CNT/Cu networks, since the photodegradation of MO and the photocatalytic activity of the photoactive systems increased with increasing copper content.

A new synthesis of β-Sialon/TiN composites by reaction bonding of aluminium powder coated with nanosized titania is reported. A sol–gel method was used to coat the aluminium particles with titania and the resulting powder was calcined in an inert atmosphere to produce a Ti x Al y –Al 2 O 3 composite which was heat-treated in nitrogen to form an AlN–Al 2 O 3 –TiN composite. After addition of a suitable silicon phase and yttrium oxide as a nitridation and sintering aid, a β-Sialon/TiN composite was formed by reaction bonding under nitrogen. Young׳s modulus, hardness, fracture toughness, tensile strength and thermal shock resistance of the new composites were measured and compared with a standard β-Sialon fabricated by reaction bonding of Al 2 O 3 , AlN and Si under similar conditions.

The synthesis of novel composites of spherical Cu2O nanoparticles and P25 TiO2 with aluminosilicate inorganic polymers (geopolymers) is described and the morphology of the homogeneous oxide nanoparticle dispersion within the geopolymer matrix demonstrated by SEM/EDS and TEM. XRD and FTIR confirmed the formation of a well-reacted geopolymer matrix that was unaffected by the insertion of the Cu2O/TiO2 nanoparticles. Experiments in dark conditions and under UV irradiation showed that these materials efficiently remove a model organic pollutant (methylene blue dye) from solution by a dual process of adsorption on the geopolymer matrix, and photodecomposition of the dye without destroying the geopolymer structure. The adsorption kinetics are described by a pseudo-first-order model and Freundlich-type isotherms. Incorporation of 10 wt% Cu2O/TiO2 nanoparticles into the geopolymer composites decreases their adsorption ability by blocking the active adsorption, sites, but at higher Cu2O/TiO2 concentrations, dye adsorption is facilitated, probably by expanding the geopolymer matrix structure. Under UV irradiation, the composites remove the MB dye by a combination of adsorption and photodegradation without destroying the geopolymer structure. The combination of nanosized Cu2O particles and photoreactive P25 titania in an aluminosilicate inorganic polymer matrix is a more efficient photocatalyst under UV irradiation than geopolymer composites containing either of the individual oxides alone. These new geopolymer composites show promise as ecologically-friendly materials which can efficiently remove organic pollutants from water or the atmosphere.

Inorganic polymers (geopolymers) containing carbon are shown to be viable precursors for CRN synthesis of SiAlONs at 1400 °C in nitrogen. The reaction mechanism and rate depends on the form and fineness of the carbon, the formation of β-SiAlON being faster and more complete with fine carbon black. The presence of the charge-compensating alkali cations in the geopolymer leads to the formation of a mixture of β-SiAlONs of low and high z -value, the former being formed via O-SiAlON resulting from the presence of silica-rich eutectics, and the latter forming by CRN reaction from an alumina-rich amorphous component. In ammonium-exchanged geopolymers lacking the alkali, SiAlON formation occurs primarily by conventional CRN of crystalline mullite and cristobalite intermediates. The potential advantages of geopolymer CRN precursors include their ability to be tailored to specific SiAlON compositions and their formation and curing at ambient temperatures to required shapes prior to the CRN reaction.

Magnesiothermal synthesis of nanosized SiC has been successfully achieved from a mixture of the natural zeolite clinoptilolite and a high-surface area mesoporous carbon CMK-1, synthesized by impregnating a mesoporous silica template MCM-48 with sucrose, followed by carbonization in argon and removal of the silica template. Magnesium powder was used to initiate the self-combustion reaction. Removal of the alkaline and alkaline earth exchangeable cations from the clinoptilolite by ion exchange with NH 4 + was essential for a good yield of product, which is also dependent on the use of excess of carbon. TEM confirmed the nanostructure and size of the 15–25 nm SiC product crystallites.

This paper reports the synthesis and characterization of a microporous geopolymer body based on metakaolin. Two kaolinites were used as the starting materials in conjunction with sodium silicate and sodium hydroxide and three sizes of polylactic acid (PLA) fibers (12, 20 and 29 mm diameter) were used as the pore-forming media. The microporous bodies were formed from geopolymer pastes of optimized composition by extrusion, which yielded bodies with reasonably well-aligned pore-forming fibers which were decomposed by the alkaline conditions during curing and drying of the bodies. XRD, FTIR, 27 Al MAS NMR, and SEM were used to characterize the chemistry and microstructure of the porous products and their compressive strengths were also measured. The molar composition SiO2/Al2O3¼4 was found to give the best geopolymer characteristics, while the optimal ratio Na2O/Al2O3 which affected the leachability of the PLA fibers and thus the size of the resulting pores, was optimized in the range 1.5–1.75 depending on the kaolinite. Water permeability measurements indicate that the use of PLA fibers as pore-formers significantly increases the permeability of the samples, while the maximum pore size of the porous bodies, determined by bubble point measurements, is related to the size of the particulate matter that could be retained, and indicates that their performance could be tailored for particular filtering applications by adjusting their synthesis parameters. & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Nanostructured SiC was synthesized by magnesiothermal reduction of silica derived from zeolite ZSM-5 with a Si/Al ratio of about 80 and four carbons from different sources. The carbons were carbon black and three carbons synthesized by impregnating furfuryl alcohol or sucrose into three porous templates (ZSM-5, clinoptilolite and MCM-48). Magnesiothermal reduction was carried out by reacting mixtures of silica, carbon and magnesium powder at 600–800 °C in a flowing argon atmosphere. The starting materials and final products were characterized by X-Ray diffraction, thermogravimetric analysis, scanning electron microscopy and BET analysis. The results indicated that the synthesis of nanostructured SiC is influenced by the chemical nature of the carbon and its surface area.

Fly ash from the 4th thermal power station of Ulaanbaatar city has two drawbacks to its use as a raw material for producing geopolymers suitable for construction applications; these are its high radium equivalent radioactivity resulting from the radioactive Mongolian coal burnt in this power station (314–343 Bq/kg), and its high CaO content (14–30 wt%). Two fly ashes, from the Baganuur and Shivee Ovoo coal deposits were activated either by sodium hydroxide solution or mixtures of sodium hydroxide and sodium silicate solutions and cured at 70 °C for 22 h. Flash setting of these high-calcium fly ash systems occurs where the activating solutions contain sodium silicate in addition to NaOH, but the fly ashes can be activated by 8 M NaOH solution alone, producing pastes with usefully long setting times (up to 10 h) and good 7-day compressive strengths (up to 32 MPa in the Baganuur ash and 23 MPa in the Shivee Ovoo fly ash). The addition of 1–5 mm river gravels produced geopolymer concretes with 7-day compressive strengths of 26 MPa and good freeze-thaw resistance. The radioactivity of the concretes prepared from these fly ashes was 130–152 Bq/kg, well within the standard safe limits for construction of dwellings. The durability of the concrete is influenced by the mineralogy of the calcium component of the fly ash; the presence of crystalline calcium components such as lime, anhydrite and amorphous calcium compounds in the higher-Ca Shivee Ovoo fly ash negatively influencing the freeze-thaw resistance. By contrast, Baganuur fly ash, which contains less calcium in the form of crystalline components, produces a geopolymer concrete with high durability.

A novel in situ synthesis is presented of a hydroxyapatite composite with the mesoporous silicate MCM-48 with potential applications as a drug delivery agent. The structure and properties of this nanocomposite, investigated by a number of techniques including XRD, SEM and TEM, show the intergrowth of the hydroxyapatite particles with the silica structure. The bioactivity of the new material was determined by measuring the decrease in the calcium ion concentration of simulated body fluid (SBF) after soaking the nanocomposite, while the uptake of ibuprofen on MCM-48 and MCM-48/HAp composite from ethanol was monitored by UV spectroscopy at 222 nm which was also used to monitor the release of ibuprofen into SBF, MTT assay was used to assess effect of the sample on MG68 cell live proliferation. The results confirm the successful synthesis of a MCM-48/hydroxyapatite nanocomposite and its potential biomedical applications as a bioactive ceramic and as a drug delivery agent with a higher rate of ibuprofen release compared to MCM-48.

The aim of this paper was to improve the magnetic properties of magnetoplumbite-type (M-type) strontium hexaferrite substituted with Co 2+ –La 3+ produced by conventional ceramic forming techniques. The effect on the magnetic properties of varying the composition of the target compound Sr 1− x La x Fe 12− y Co y O 19 and the primary and secondary firing temperatures was investigated. Microstructure studies and XRD phase analysis indicated that optimum values of the remanent magnetization B r and coercive field H cj were obtained with a primary firing temperature of 1240 °C and a final firing temperature of 1180 °C, where ( x = y ) th 1 =0.15, ( y / x ) exp 2 =0.75 and the molar ratio of ferric oxide to strontium oxide=5.8. The optimized magnetic properties obtained under these conditions were B r =4070 Gs, H cj =4710 Oe, ( H k / H cj )=82.

The formation of an X-sialon/mullite nanocomposite by a combination of mechanical activation and silicothermal reduction and nitridation is reported. The starting materials (andalusite, Si and various aluminium sources) were activated in a planetary mill and fired in N2 at 1450 °C. Microstructure and phase analysis was carried out on fired and unfired samples using X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). Taguchi statistical experiments were carried out to optimize the formation and crystallinity of the X-sialon component of the sialon–mullite composite. The milling time is the factor of greatest influence on the formation and crystallite size of X-sialon; milling for 15 h increases the amount of X-sialon formed but longer milling times decrease the crystallinity of the products and the sialon/mullite ratio. High-energy milling of an andalusite/Si/alumina mixture produces a mullite-sialon nanocomposite powder. TiO2 additive stabilises the mullite component whereas Y2O3 enhances the formation of X-sialon and Al(OH)3 increases the crystallinity. The optimized conditions for the production of X-sialon/mullite nanocomposite powders are the use of Al2O3 as the Al source, milling time 15 h, avoiding the use of TiO2 as an additive.

Nanocomposites of Al–SBA-15 with the organic polymers poly-4-vinyl pyridine (P4VP) and polyacrylamide (PAM) were synthesized to produce SiAlON precursors containing various carbon contents. These CRN precursors, and their products after firing under N 2 at 1450 °C for 3 h were investigated by XRD, SEM/EDS and surface area analysis (BET) to compare the influence of the organic polymer carbon source. The results were also compared with those from precursors containing carbon black as the carbon source. XRD and SEM/EDS analysis of the Al–SBA-15 confirmed the formation of mesoporous structures, while BET and pore size distribution measurements indicated that the entry of P4VP into the Al–SBA-15 nanocomposite precursor is significantly more efficient than that of PAM. Firing these precursors in nitrogen produced β-SiAlON and β-cristobalite. At all carbon concentrations the P4VP nanocomposite precursors formed significantly more SiAlON than those of the PAM and carbon black precursors, due to the proximity of a greater amount of the organic carbon source to the silica template in the P4VP composite, and possibly also to the lack of oxygen in the P4VP monomer, in contrast with PAM. At least twice the stoichiometric amount of carbon was required in all cases, with optimal SiAlON formation occurring in the P4VP precursor containing 6 times the stoichiometric amount of carbon. Increasing the carbon content further suppresses SiAlON formation, possibly due to the build-up of back pressure of CO by-products in the pores of the nanocomposite.

Highly charged swelling sodium micas (Na-2-, Na-3-, and Na-4-micas) with different layer charge densities were synthesized and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and solid-state 29Si and 27Al magic-angle spinning nuclear magnetic resonance (MASNMR) spectroscopy. Their ion exchange properties were examined for Sr2+ and Ba2+ ions using isotherms and Kielland plots. XRD patterns after 2Na+ → Sr2+ or Ba2+ exchange were investigated to check for change in ‘d’ values of 0 0 1 reflections of Na-micas. The ‘d’ values of 0 0 1 reflections were shifted from two-layer hydrate to one-layer hydrate depending on layer charge density and the extent of ion exchange by divalent cations. A comparison of shifting in ‘d’ values of 0 0 1 reflections indicates that there are two different hydration states of Sr2+ and Ba2+ in the interlayer space of Na-micas. Na-2-mica showed the highest selectivity for Sr2+ and Ba2+ among Na-micas because of easier expansibility of its interlayers and thus accessibility to exchanging sites. Therefore, Na-2-mica is a good candidate for decontamination of Sr2+ and Ba2+ from drinking water and contaminated soils or nuclear waste solutions.

Two different microporous Na-titanosilicates with high cation exchange capacities were synthesized and characterized by X-ray diffraction, scanning electron microscopy (SEM) and solid-state 29 Si and 27 Al magic-angle spinning nuclear magnetic resonance (MASNMR) spectroscopy. The ion exchange properties of Na-ETS-4 (ETS = Engelhard TitanoSilicate) and Na-titanosilicate were investigated for Sr 2+ and Ba 2+ using isotherms and Kielland plots in order to check the potential for the decontamination of 90 Sr and 133 Ba from nuclear waste solutions. Na-ETS-4 and Na-titanosilicate show high affinity for Sr 2+ to high equivalent fraction of Sr 2+ on solid phase. In case of Ba 2+ the selectivity coefficient of both titanosilicate exchangers rapidly decreased due to the steric hindrance at exchange sites at X ¯ Ba > ∼0.5. However, Na-ETS-4 was still selective for Ba 2+ even after the above rapid decrease in selectivity, unlike Na-titanosilicate. Therefore, Na-ETS-4 is one of the best candidates for the removal of both Sr 2+ and Ba 2+ from nuclear waste solutions.

The volcanic ash occurring as an abundant and readily accessible natural resource in the Central African country of Cameroon was used to synthesize aluminosilicate geopolymers using sodium hydroxide as the sole alkaline activator. Both the curing conditions and the Na 2 O/SiO 2 molar ratio were found to influence the development of compressive strength of the geopolymer cement paste, which achieved a maximum strength of 55 MPa at Na 2 O/SiO 2 = 0.3. The formation of a mortar by the addition of 40 wt% sand to the optimized geopolymer cement composition reduced the compressive strength to 30 MPa, still within the useful range for construction applications. The geopolymers consist largely of X-ray amorphous material with a small content of crystalline phases. Scanning electron microscopy showed a homogenously distributed mixture of lath-shaped and agglomerated morphologies, with a homogeneous distribution of Si, Al and O in the geopolymer matrix. The geopolymers are relatively stable to heat, shrinking only slowly and retaining about 60% of their as synthesized compressive strength on heating to 900 °C. The FTIR spectra of both the as synthesized and heated geopolymers show two broad absorbance bands, between 820–1250 cm −1 and 450–730 cm −1 assigned to the internal vibrations of Si–O–Si, and Si–O–Al respectively. The compressive strengths and the thermal stability of these materials suggest their suitability for building applications and low-grade refractories.

Ceramics based on Bi 3 NbTiO 9 (BNT) were investigated for their high-temperature pyroelectric properties. Doping BNT with 0.05 atoms of W 6+ donor ions decreased its DC conductivity, enhanced its dielectric strength and remnant polarization. Thick films of BNT produced by airflow deposition show preferential grain orientation and enhanced sinterability, giving improved polarizability and higher dielectric strength than the comparable ceramic. Further improvements in grain orientation and enhanced remnant polarization are derived by sintering under an applied electric field. Airflow deposition was also used to prepare graded films of Ba (1− x ) Sr x TiO 3 (BST), showing temperature dependent hysteresis offsets that originate from asymmetric leakage currents and are not related to the pyroelectric properties. These graded films show an enhanced conventional pyroelectric coefficient over a wide temperature range. These results predict that a graded structure of BaBi 2 Ta 2 O 9 and Bi 3 NbTi 0.95 W 0.05 O 9 should show enhanced high-temperature pyroelectric properties.

Bismuth forms double oxides with the oxides of aluminium, gallium, iron and manganese, all of which have the mullite structure. Since other bismuth-based complex oxides show useful functional properties including high-temperature pyroelectricity and piezoelectricity, the electronic and magnetic properties of the mullite-structured bismuth compounds are of potential interest. In the present study, all the above compounds were synthesised in pure form by solid-state reaction, and their thermal behaviour and structures characterised by thermal analysis, powder X-ray diffraction, solid-state MAS NMR and Mőssbauer spectroscopy as appropriate. The thermal analysis information allowed highly sintered polycrystalline samples to be produced. After the application of silver electrodes, the electronic properties of the pellet samples were determined as a function of frequency and temperature up to 900 °C. Frequency-dependent inflexions in the relative permittivity and loss angle curves of the ferrate and gallate may be due to losses due to dielectric relaxation, but the most notable and consistent electronic phenomenon in all samples is a steep rise in the dc conductivity at higher temperatures. The P – E hysteresis loops for the ferrate and manganate indicate that these are very poor ferroelectric materials, consistent with the centrosymmetric crystal structure of all these compounds.

Al-containing mesoporous silicas were synthesized by hydrothermal treatment of microporous silica prepared by selectively acid leached metakaolinites with Si/Al = 3.9–92.5 mixed with a surfactant of cetyltrimethylammonium bromide (CTABr). The specific surface area of the products increased with higher surfactant/microporous silica (surf/Si) ratio and Si/Al ratio of the microporous silica, reaching about 1400 m2/g at CTABr/Si ⩾ 0.1 and Si/Al ⩾ 40. The XRD patterns of these products show a hexagonal (100) peak with the lattice parameter a0=4.2–4.3nm and the N2 adsorption isotherms show steep increase of adsorption between relative pressure of 0.3 and 0.4. Hexagonal mesoporous microstructure is observed by high resolution TEM. The pore size distributions of the products show a sharp peak at 2.8 nm by the BJH method. The high specific surface area of the present mesoporous samples is attributed to the lower matrix density and surface roughness of mesopore wall. The highest specific surface area of the products reached up to 1420 m2/g and this value is apparently higher than those reported in hexagonal mesoporous silicas. A unique microporous structure of the starting material is thought to be related to achieve such a high specific surface area of the products.

Aluminosilicate geopolymers with SiO2/Al2O3 ratios ranging from 0.5 to 300 have been prepared from mixtures of dehydroxylated kaolinite with either ρ-Al2O3 or fine Aerosil SiO2, with the ratios Na2O/SiO2 and H2O/SiO2 kept constant throughout the series. All the compositions hardened at ambient temperature, but the high-alumina compositions were of low strength and did not display typical XRD and NMR geopolymer characteristics, by contrast with the compositions of S/A between 2 and 300 which showed typical amorphous geopolymer XRD traces and 27Al, 29Si and 23Na NMR spectra. The samples with increasing SiO2 content (S/A > 24) showed increasingly elastic behaviour, deforming rather than crushing in brittle fashion, and upon heating at 100–250 °C, their hydration water was expelled as bubbles, forming stable foamed materials at about 300 °C.

Scanning electron micrographs of a well-cured polysialate siloxo geopolymer (silica:alumina ratio 2:1) prepared from metakaolinite reveal a microstructure consisting of a glass-like matrix containing metakaolinite relicts and impurity quartz grains. This inhomogeneity probably results from viscosity increases during preparation militating against efficient mixing of the components, but does not appear to seriously degrade the physical properties of the product. The matrix composition, determined by EDAX, conforms closely to the expected molar ratio and is unchanged by heating at 1200 ° C, which however, brings about the crystallization of mullite needles in areas of the geopolymer matrix, the dissolution of the quartz impurities and the depletion of silica from the metakaolinite relicts, leaving behind fine alumina grains. These previously unsuspected thermal reactions help to explain the exceptional stability of the geopolymer matrix. Transmission electron micrographs confirm the essentially amorphous nature of both the as-prepared and heated geopolymer samples, but unexpectedly reveal in the latter nanometre-sized features resembling spinodal decomposition structures in glass.

Traditionally, X-ray powder diffraction has been a favoured method for studying chemical reactions in the solid state, but the increasing importance of energy-efficient synthesis methods for solids (e.g. sol–gel synthesis, mechanochemical synthesis) has led to the need for an analytical method not dependent on long-range structural periodicity. Multinuclear solid state nuclear magnetic resonance (NMR) represents a technique which is equally applicable to amorphous or crystalline solids, and is now used in increasing numbers of solid state studies. This paper briefly outlines the principles and practical details of this powerful technique and gives examples of its use in solid-state chemistry, particularly in very recent studies of mechanochemical synthesis of advanced sialon ceramics. The temperature at which these technically important silicon aluminium oxynitride compounds are formed can be significantly lowered by high-energy grinding of their components to produce X-ray amorphous precursors. Solid-state NMR has been used to provide detailed information which could not have been obtained by any other means about the chemical environment of the Si and Al atoms in these amorphous precursors, and the various atomic movements undergone as they crystallise to the final product.

The characteristics of an acid-leached montmorillonite-containing clay from Tuulant (Mongolia) were studied by XRD, DTA-TG, FTIR and N 2 adsorption techniques. Sample aliquots (2 g) were treated with 50 ml 2 M hydrochloric acid at 80 °C for various times. Leaching caused the surface area of the clay to increase 3-fold to 93.9 m 2 /g reflecting the formation in the porous silica product of 3–5 nm micropores and 6–10 nm mesopores, the latter resulting from a delaminated card-house structure rather than from the condensation of the micropores. Differences between the surface area of the present leached samples and those reported for other montmorillonite clays reflect the mineralogical composition of the clay, particularly the presence of non-clay constituents.

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 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.