MAS-NMR studies of geopolymers heat-treated for applications in biomaterials field

In the field of biomaterials applied in bony restoration, systems based on amorphous silicate network present the ability to link to bone matrix. Amorphous geopolymers of the potassium-poly(sialate)-nanopolymer type with a mole ratio Si:Al = 31 were studied for their use as potential biomaterials. This implied a heat treatment at 500 °C in order to reduce the alkalinity of the geopolymer matrix from pH 11.5 to pH 7.1 and to provide high porosity for biological compatibility. ²⁷Al and ²⁹Si MAS NMR spectroscopy has been used to characterise the foamed geopolymers obtained after different thermal treatments at 250 and 500 °C, and with three different mole ratios K₂O/SiO₂ for the potassium silicate: 0.54; 0.67 and 0.84. Best results in terms of biological compatibility were obtained with a geopolymer matrix resulting from a mole ratio K₂O/SiO₂. of 0.54, heat-treated at 500 °C.     

Magnesium analogues of aluminosilicate inorganic polymers (geopolymers) from magnesium minerals

Attempts to synthesise magnesium-containing analogues of aluminosilicate geopolymers from the 1:1 and 2:1 layer magnesiosilicate minerals chrysotile and talc, as well as the magnesium mineral sepiolite are reported. The effect of pre-treating these starting minerals by grinding and/or dehydroxylation was also investigated by XRD, ²⁹Si and natural-abundance ²⁵Mg solid-state magic angle spinning (MAS) NMR spectroscopy. The products from sepiolite most closely resembled an aluminosilicate geopolymer, setting at 40 °C to an X-ray amorphous product containing a broad characteristic ²⁹Si MAS NMR resonance at ?90 ppm. The ²⁵Mg MAS NMR spectrum of this product also showed evidence that some of the Mg was located in tetrahedral sites, as expected for a conventional geopolymer. A similar ²⁵Mg MAS NMR result was obtained for chrysotile, but talc proved to be extremely resistant to geopolymer synthesis, requiring treatment at 120 °C for 3 days to set to a friable material retaining the XRD and NMR characteristics of the original talc or its crystalline dehydroxylation products. This lack of reactivity may be related to the 2:1 layer-lattice talc structure, or to the fact that a suitably reactive amorphous product is not formed upon dehydroxylation.     

Preparation of geopolymer precursors by sol–gel method and their characterization

Pure Al₂O₃–2SiO₂ precursors (powders) for a geopolymer were prepared by a sol–gel method. The alkali-activated products derived from the precursors meet the general criteria for a geopolymer. The structure of the powders was investigated by NMR, XRD, and FTIR analysis, and their alkali-activation properties were studied. The data show that the powders when heat treated at 200 °C begin to contain 5-coordinated Al, those heat treated at 300 °C can begin to undergo alkali-activation, and those heat treated at 300–800 °C possess a number of structure characteristics similar to metakaolin, and the properties of their alkali-activated products are similar to those of the metakaolin geopolymers.     

Reaction mechanism,kinetics and high temperature transformations of geopolymers

The reaction kinetics and mechanism of geopolymers are studied. The dissolved silicate concentration decreases from the beginning of the reaction. A characteristic time ‘t _0,vit’ for the setting of the reaction mixture is derived from isothermal Dynamic Mechanical Analysis experiments. ‘t _0,vit’ increases with SiO₂/R₂O but goes through a minimum for increasing water content. The reaction is slower for K compared to Na-silicate based systems. ²⁹Si and ²⁷Al solution NMR are used to probe the molecular changes. ²⁷Al NMR and FTIR reveal that an ‘intermediate aluminosilicate species’ (IAS) is formed from the start of the reaction. The concentration decrease of OH⁻ during low-temperature reaction is related to the formation of IAS. The rate law of this process seems to be obeyed by a total reaction order of 5/3, with a partial order of 1 for OH⁻ and 0 for Na⁺ in the silicate solution. During first heating after polymerization water is lost leading to a distortion of the Al environment. According to XRD, no crystallization occurs below 900 °C. However, between 950 and 1100 °C a crystallization exotherm of nepheline is observed with DSC for a geopolymer with SiO₂/Na₂O = 1.4. Neither T _g of the amorphous geopolymer, nor the shrinkage and expansion around T _g during first heating, cause a measurable heat effect.     

Combination formation in the reinforcement of metakaolin geopolymers with quartz sand

Although quartz sand is widely used as filler material in construction, a few studies investigated the incorporation of quartz sand in geopolymers. To study the incorporation of quartz sand in the reinforcement of metakaolin geopolymer not only fills this gap, but also gives a clue on using non-calcinated aluminosilicates (e.g., mine tailings) in the synthesis of geopolymers. In the presence of sodium silicate, metakaolin geopolymers were synthesized with quartz sand of various size ranges as filler material. XRD, FTIR, SEM and NMR characterizations on the geopolymers indicate the dissolution, precipitation, and the formation of combination on quartz particles that associates them into the geopolymeric gel, so as to reinforce the mechanical strength of geopolymers. The compressive strength of metakaolin geopolymers with only silicate, silicate plus quartz sand and silicate plus rutile sand is 31.2, 52.2 and 41.5 MPa, respectively. In geopolymer with silicate and quartz sand, a decreasing Si/Al ratio as increasing distance from the quartz particle is observed through an energy dispersive X-ray (EDX) mapping. The SEM images and NMR spectra suggest that the formed combination is of several micrometers with main species of polysialates (-Si-O-) such as Q⁴(2Al), Q⁴(1Al).     

Influence of curing schedule on the integrity of geopolymers

The curing at ambient and controlled relative humidity (RH) with mild heating (40–60°C) of a metakaolinite-based geopolymer of molar ratios Si/Al and Na/Al of 2 and 1 respectively was studied. To obtain these geopolymers in a crack-free state, rapid drying during curing should be avoided. Curing at a lower RH (e.g. 30%) is preferable to that at a higher RH (e.g. 70%). Curing in an RH oven does not offer any advantage over curing at ambient followed by mild heating (40–60°C) in sealed containers. The compositions of the geopolymers were then varied somewhat to see the effect on open porosity for the same curing schedule. The compositions with Si/Al = 1.86 and Na/Al < 0.8 had an increased tendency to crack, probably due to the larger water loss during curing. The lowest open porosity of <1% was obtained for a geopolymer of composition Si/Al = 2.14 and Na/Al = 0.87, but this developed some cracks after 1 year. However, the geopolymer of composition, Si/Al = 2, Na/Al = 1, on which most of the work was carried out when cured correctly did not crack even after 1 year.     

Characterisation of class F fly ash geopolymer pastes immersed in acid and alkaline solutions

Acid and alkaline resistance of class F fly ash based geopolymer pastes has been investigated. As prepared geopolymers showed high solubility in both strong alkali and acid solutions. Calcination of the fly ash based geopolymers at 600 °C resulted in a decrease of amorphous component from 63.4 to 61.6 wt.%. However, the solubility of the Al, Si and Fe ions in 14 M NaOH and 18% HCl after 5 days immersion decreased from 1.3 to 16-fold in comparison to as prepared geopolymer samples. Calcination of the geopolymers also resulted in a 30% reduction in compressive strength. Acid and alkali resistance of the geopolymers investigated strongly depends on mineralogical composition change of the calcined geopolymer. Partial crystallisation of non-reacted fly ash particles in the geopolymer decreases its solubility in acid and alkali solutions.     

Formation of aluminosilicate geopolymers from 1:1 layer-lattice minerals pre-treated by various methods: a comparative study

Materials resembling aluminosilicate geopolymers have been prepared from the kaolinitic 1:1 layer lattice aluminosilicate clay mineral halloysite by reaction with sodium silicate solution under alkaline conditions. The effect on geopolymer formation of pretreating the clay mineral reactant by heating, high-energy grinding or exposure to acid or alkali was monitored by the ability of the samples to cure and harden, and by XRD, ²⁷Al and ²⁹Si solid-state MAS NMR spectroscopy. Only samples prepared from the fully thermally dehydroxylated clay showed the typical XRD and NMR geopolymer characteristics. Less complete reaction was found in samples pretreated by highly energetic grinding, whereas samples exposed to chemical pretreatment with dilute acid did not react to form viable geopolymers. Pretreatment with dilute alkali produced a zeolite which reacted with sodium silicate, but the hardened sample was not X-ray amorphous and showed subtle differences in its NMR spectra. These results are discussed in terms of the vital role played in the early stages of the reaction sequence by the presence of labile aluminium. The efficacy of the various pretreatment methods is related to their ability to render the aluminium source (the solid aluminosilicate clay) sufficiently soluble in alkali.     

Low-temperature sintered pollucite ceramic from geopolymer precursor using synthetic metakaolin

In this article, pollucite ceramic with high relative density and low coefficient of thermal expansion (CTE) was prepared from Cs-based geopolymer using synthetic metakaolin. Crystallization and sintering behavior of the Cs-based geopolymer together with thermal expansion behavior of the resulted pollucite ceramic were investigated. On heating at 1200 °C for 2 h, the amorphous Cs-based geopolymer completely crystallized into pollucite based on crystal nucleation and growth mechanism. Selected area diffraction analysis and XRD results confirmed the resulted pollucite ceramic at room temperature was pseudo-cubic phase with superlattice structure. Compared with Cs-based geopolymer using natural metakaolin, geopolymer using synthetic metakaolin in this article showed a much lower viscous sintering temperature range, which started at 800 °C, reached a maximum value of ?7.47 × 10^?4/°C at 1121.9 °C, and ended at 1200 °C. Cesium volatilization appeared only when temperature was above 1250 °C. Therefore, densified pollucite ceramic can be prepared from Cs-based geopolymer using synthetic metakaolin without cesium volatilization. Abnormal thermal shrinkage of pollucite ceramic was observed at temperature range from 25.3 to 54.6 °C because of pseudo-cubic to cubic phase transition, and its average CTE was 2.8 × 10^?6/°C from 25 to 1200 °C.     

Geopolymerization reaction to consolidate incoherent pozzolanic soil

Pozzolanic material-based geopolymer has been proposed as a solving methodology to the geohazards, due to pozzolanic collapsible soils widely present in the South Italy. The geopolymer was synthesized from pozzolana material under activation of NaOH 10 M or slurry of NaAlO₂ in NaOH 10 M solution. The specimens were cured at 25 °C and 100% RH for different ageing times. The effect of the two activation methods on the properties of the geopolymer was investigated by means of X-ray diffraction, scanning electron microscopy (SEM), FTIR spectroscopy, nuclear magnetic resonance (²⁷Al and ²⁹Si NMR) and uniaxial compression tests. XRD, NMR and IR analysis indicate the geopolymer is generated by the dissolution of the silico-aluminate phases present in the pozzolana and the successive re-organization in amorphous and crystalline neo-formed phases. The spectroscopic evidences confirm that the 4-coordinated Al atoms present in the neat pozzolana and in the NaAlO₂ change their coordination state splitting between 6- and 4-coordinated atoms, modifying the traditional chemistry of polysialate formation. SEM results show the synthesized geo-polymer maintained the granular morphology of the pozzolana and the geo-polymeric reactions occurred mainly at the surface of pozzolana particulates. Furthermore, uniaxial strength data increase gradually upon the curing time, until 40 MPa for the specimens activated with the slurry system.     

Enhanced thermal stability in K<Subscript>2</Subscript>O-metakaolin-based geopolymer concretes by Al<Subscript>2</Subscript>O<Subscript>3</Subscript> and SiO<Subscript>2</Subscript> fillers addition

Based on the principle of stability of geopolymer gel as refractory binder, a geopolymeric paste in the K₂O–Al₂O₃–SiO₂ system was developed and used to produce refractory concretes by adding various amount of α-quartz sand (grain size in the range 0.1 μm to 1 mm) and fine powder alumina (grain size in the range 0.1–100 μm). The consolidated samples were characterized before and after sintering using optical dilatometer, DSC, XRD and SEM. The total shrinkage in the range of 25–900 °C was less than 3%, reduced with respect to the most diffused potassium or sodium based geopolymer systems, which generally records a >5% shrinkage. The maximum shrinkage of the basic geopolymer composition was recorded at 1000 °C with a 17% shrinkage which is reduced to 12% by alumina addition. The temperature of maximum densification was shifted from 1000 °C to 1150 or 1200 °C by adding 75 wt% α-quartz sand or fine powder alumina respectively. The sequences of sintering of geopolymer concretes could be resumed as dehydration, dehydroxylation, densification and finally plastic deformation due to the importance of liquid phase. The geopolymer formulations developed in this study appeared as promising candidates for high-temperature applications: refractory, fire resistant or insulating materials.     

Thermo-mechanical and microstructural characterisation of sodium-poly(sialate-siloxo) (Na-PSS) geopolymers

The thermo-mechanical and microstructural character of sodium-poly(sialate-siloxo) (Na-PSS) geopolymers synthesised from dehydroxylated kaolinite (metakaolinite) have been investigated. Thermal analysis by means of TG–DTA showed a single endothermic peak at 135–140 °C due to dehydration (water evolution) from the geopolymer framework. Thermal expansion measurements show that geopolymer suffers 2% shrinkage below 250 °C and is then dimensionally stable up to 800 °C. The inclusion of aggregate (α-quartz or granite) was found to reduce the shrinkage by 1% although the presence of the quartz limits the working temperature range of the composite due a to disruptive phase change. Thermal conductivity and compressive strength of Na-PSS geopolymers varied with change in chemical composition of the geopolymer as well as the amount and type of aggregate. Investigation of the microstructure by electron microscopy showed that the ratio of the starting materials influences the homogeneity of the geopolymer microstructure, which in turn leads to differences in thermal conductivity and compressive strength.     

Synthesis,characterisation and thermal behaviour of lithium aluminosilicate inorganic polymers

Lithium aluminosilicate inorganic polymers were synthesised from dehydroxylated kaolin-type clay (halloysite) by the conventional method under highly alkaline conditions with lithium hydroxide or lithium silicate solutions of two different Li₂O/SiO₂ molar ratios. Variants were also developed of a solid-state synthesis method involving the thermal reaction of dehydroxylated halloysite with LiOH followed by hydration of the product. The molar compositions of the materials prepared by all three methods (SiO₂/Al₂O₃ = 2.41–3.27, Li₂O/SiO₂ = 0.30–0.61, and H₂O/Li₂O = 9.33–10.40) fall within the range of compositions previously reported to produce viable geopolymers. Curing at 40 °C produces solid samples of varying viability depending on the amount of synthesis water. The cured materials are not characteristically X-ray amorphous, but contain the lithium zeolites Li-ABW and fibrous Li-EDI, the latter in the materials synthesised by solid-state reaction. The ²⁷Al and ²⁹Si MAS NMR spectra of the cured materials contain narrow resonances more characteristic of zeolites than of inorganic polymers. Heating the synthesised products at <800 °C produces β-eucryptite, LiAlSiO₄. In a further series of thermal reactions, β-spodumene, LiAlSi₂O₆ is formed at 900 °C, decomposing at 1100 °C to form additional β-eucryptite. At 1275 °C, β-spodumene reappears in the samples of higher silica content. Judicious manipulation of the composition and thermal treatment of the Li-zeolites formed in these lithium aluminosilicate syntheses could make them useful precursors to β-eucryptite and β-spodumene ceramics.     

Influence of calcination temperature of kaolin on the structure and properties of final geopolymer

In this paper, the structure of two types of metakaolins from kaolin calcined at 800 and 900 °C, respectively, and the obtained geopolymer were systematically characterized. It was found that calcination temperature had little effect on the environment of silicon atoms but had great effect on that of aluminum ones. ²⁷Al NMR analysis showed that tetrahedral aluminums in the metakaolin from kaolin calcined at 800 and 900 °C were in different environment, of the type AlQ₃(3Si) and AlQ₄(4Si), respectively, leading to different environment of aluminum atoms in the resulted geopolymer. Aluminum atoms in the geopolymer based on metakaolin from kaolin calcined at 800 °C were in the types of tetrahedral and octahedral, and silicon atoms were in the types of tetrahedral Q₄(3Al) together with a small amount of Q₄(0Al). However, geopolymer based on metakaolin from kaolin calcined at 900 °C consisted of Q₄(4Si) unit aluminum and Q₄(3Al) unit silicon. The results revealed that the calcination temperature had a great effect on environment of the aluminum atoms of the metakaolin, thus led to the different structure and properties including mechanical strength and thermal conductivity of the post obtained geopolymer.     

Factors affecting the performance of metakaolin geopolymers exposed to elevated temperatures

The effects of geopolymer binder systems exposed to elevated temperatures are examined. Geopolymers investigated were synthesized from metakaolin, activated by combinations of sodium/potassium silicate and sodium/potassium hydroxide. The specimens were then exposed to temperatures of 800 °C. The factors studied were: (1) calcining temperatures of kaolin; (2) Si/Al ratio of the geopolymer; (3) activator/metakaolin ratio; (4) curing temperature; and (5) alkali cation type. Altogether 30 geopolymer formulations were studied. The samples were subjected to compressive strength, thermogravimetry, and scanning electron microscopy tests. Results showed that Si/Al ratio has a significant influence on elevated temperature exposure deterioration. Lesser strength loss due to elevated temperature exposures were observed in geopolymer with high Si/Al ratios (>1.5). The geopolymer binders activated by potassium-based activators showed an enhanced post-elevated temperature exposure performance compared to sodium-based systems. The optimum calcining temperature of kaolin and curing temperatures for improved temperature performance are also reported.     

Electrical and mechanical properties of aluminosilicate inorganic polymer composites with carbon nanotubes

The DC electrical conductance of potassium aluminosilicate inorganic polymers (geopolymers) containing up to 6 wt% single-wall carbon nanotubes has been determined as a function of temperature up to 340 °C. After removal of the processing water during the first heating cycle, the conductance in subsequent heating cycles increases as a function of carbon nanotube content and temperature from 9.75 × 10⁻⁴ to 1.87 × 10⁻³ S m⁻¹ in the composites containing 0 and 0.2 wt% carbon nanotubes, respectively, at 290 °C. By comparison, the electrical conductance of potassium inorganic polymer composites containing graphite was generally lower. The conductance activation energies of the carbon nanotube and graphite composites were similar, and decreased from about 55 to 5 kJ mole⁻¹ with increasing carbon content. The tensile strengths of carbon nanotube and graphite-containing potassium geopolymer composites, determined by the Brazil method on 10–12 replicates, were about 2 MPa, and showed little change with increasing carbon nanotube content up to 0.3 wt%. By contrast, the tensile strengths of an analogous set of sodium composites were up to four times greater, possibly reflecting the necessity for less processing water in the synthesis of the sodium samples.     

Influence of elevated temperatures on the mechanical properties of foamed geopolymer materials

Current research focuses heavily on geopolymer concrete as possible applications for insulation materials. The aim of the research is to test the strength properties of lightweight geopolymer concrete after exposure to high temperatures. Waste material from the Wieczorek mine (Poland) was used to produce the foamed geopolymers. Alkaline activation took place by mixing the mine powder with an aqueous solution of sodium hydroxide combined with an aqueous sodium silicate with a concentration of 10 M. Prepared geopolymer samples after temperature curing at 75 °C for 24 hours in a laboratory dryer, they were seasoned for 28 days, after which the strength properties were determined. Mechanical tests: compressive strength and bending strength were carried out at temperatures: 20 °C, 200 °C, 600 °C, 800 °C, 1100 °C. Research has shown the precursor activation with the presence of hydrogen peroxide enabled the manufacturing of foamed geopolymers. Heating in the temperature range up to 1100 °C influenced, to some extent, the total porosity of the tested foams. The geopolymer foams based on coal gangue present stable mechanical properties in the range up to 800 °C. No sharp mechanical performances decrease or material chipping was observed. Only colour change of heated samples occurred.     

Properties of inorganic polymer (geopolymer) mortars made of glass cullet

This study presents the results of experiments aiming to produce geopolymers from glass cullet, a non-traditional material compared to those usually found in the manufacture of geopolymers (e.g., metakaolin and fly ash). The study gives the principal formulation parameters affecting the behavior of glass cullet geopolymers. The glass used comes from recycled glass bottles. The parameters studied are the fineness of the glass (Blaine of 1000 to 4000 cm²/g), the temperature of synthesis (20, 40 and 60 °C), and the nature and concentration of the activation product (KOH, NaOH). The properties are evaluated in terms of compressive strength and durability. The results show that cullet of soda-glass can be used as a base material for the production of geopolymers and, contrary to metakaolin-based geopolymers, no waterglass is necessary for its setting and hardening since cullet glass already contains a high proportion of alkalis. Thermal activation at 40 or 60 °C is necessary but sufficient to obtain strength of more than 50 MPa, especially for the finer glass (4000 cm²/g). The durability of glass cullet geopolymers is affected by water conservation.     

Structural studies of geopolymers by <Superscript>29</Superscript>Si and <Superscript>27</Superscript>Al MAS-NMR

A systematic study of geopolymers by ²⁹Si and ²⁷Al MAS NMR has been carried out in an attempt to understand polymer structural details. ²⁷Al MAS NMR data shows that transient aluminium species are formed during the reaction of metakaolin with NaOH. Interaction of silicate anions with the aluminium sites of metakaolin was evident during the synthesis of geopolymers as observed from low field shift of ²⁹Si MAS NMR resonance lines of silicate centres. As the reaction progresses, the coordination of aluminium (IV, V and VI) in metakaolin changes almost completely to IV. ²⁹Si MAS NMR of selected compositions of the ternary system of sodium silicate, metakaolin and aqueous alkali reveals that geopolymerisation occurs in a distinct compositional region. At high alkalinity [> 30% (mol/mol) overall Na²O content], connectivity of silicate anions is reduced, consistent with poor polymerisation. At low alkalinity [<10% (mol/mol) overall Na²O content], a clear ²⁹Si NMR resonance line due to unconverted metakaolin is observed. NMR spectra were recorded from a series of samples with a fixed Na²O content (20 mol%) and varied SiO²/Al²O³ ratio to observe aluminium substitution in the cross-linked silicon tetrahedra of polymer network. Aluminium insertion into the silicate network is confirmed from the observed ²⁹Si NMR shift as a function of Si/Al ratio. The identification of the presence or absence of metakaolin in the cured geopolymer product is not possible even by ²⁹Si NMR as the signal from metakaolin is indistinguishable from a broad ²⁹Si NMR peak consisting of many resonance lines from the network of cross-linked silicon/aluminium tetrahedra. In an attempt to identify metakaolin signal, we prepared geopolymers with higher SiO²/Al²O³ molar ratios. Since aluminium substitutions in the silicate tetrahedral network are decreased, this results in better-resolved ²⁹Si NMR lines. The ²⁹Si NMR signal due to metakaolin is then distinguishable in the spectra of cured products in a series of samples with 3 to 11 mol% metakaolin. These results indicate that a geopolymer structure is a network of silicon/aluminium tetrahedra with some presence of unreacted metakaolin. The silicon/aluminium tetrahedra might have connectivity ranging from 1 to 4.     

Effect of fire exposure on cracking,spalling and residual strength of fly ash geopolymer concrete

Fly ash based geopolymer is an emerging alternative binder to cement for making concrete. The cracking, spalling and residual strength behaviours of geopolymer concrete were studied in order to understand its fire endurance, which is essential for its use as a building material. Fly ash based geopolymer and ordinary portland cement (OPC) concrete cylinder specimens were exposed to fires at different temperatures up to 1000 °C, with a heating rate of that given in the International Standards Organization (ISO) 834 standard. Compressive strength of the concretes varied in the range of 39–58 MPa. After the fire exposures, the geopolymer concrete specimens were found to suffer less damage in terms of cracking than the OPC concrete specimens. The OPC concrete cylinders suffered severe spalling for 800 and 1000 °C exposures, while there was no spalling in the geopolymer concrete specimens. The geopolymer concrete specimens generally retained higher strength than the OPC concrete specimens. The Scanning Electron Microscope (SEM) images of geopolymer concrete showed continued densification of the microstructure with the increase of fire temperature. The strength loss in the geopolymer concrete specimens was mainly because of the difference between the thermal expansions of geopolymer matrix and the aggregates.