Hydrolytic stability of sodium silicate gels in the presence of aluminum

Polycondensation in alkali silicate solutions comprises a fundamental process of the geopolymerization technology. Previous works had shown that the hydrolytic stability of sodium silicate gels depends on the SiO₂/Na₂O ratio. Sodium silicate gels totally insoluble in water can be produced at SiO₂/Na₂O molar ratios higher than 4.4. This article aims at elucidating the effect of tetra-coordinated aluminum addition on the hydrolytic stability of sodium silicate gels. According to the results, the aluminum addition stabilizes the sodium silicate gels in an aqueous environment. A sodium silicate gel with SiO₂/Na₂O molar ratio 3.48, which is totally soluble in deionized water at ambient temperature, can be transformed to insoluble sodium hydroaluminosilicates with the addition of tetrahedral aluminum at Al/Si molar ratios higher than 0.08. In addition, this article studies the structure of prepared sodium hydroaluminosilicates and draws very useful conclusions for the geopolymerization technology.     

Evaluation of sodium content and sodium hydroxide molarity on compressive strength of alkali activated low-calcium fly ash

Activation of low calcium fly ash is investigated using activating solutions of sodium hydroxide (NaOH) and sodium silicate (Na₂SiO₃). The oxide ratio of Na₂O relative to the total reactive silica in the activated mix provides consistent results in achieving the highest ultimate strength. The total reactive silica used for calculating the ratio consists of the reactive silica contributed by fly ash and the silica from the activating solution. There is an increase in the ultimate compressive strength on increasing the total sodium content relative to the total reactive silica content in the activated system. Increasing the sodium content beyond a certain limit does not provide additional gain in the ultimate compressive strength. The ratio of total reactive SiO₂ to Na₂O in the activated system equal to 4.72 is shown to provide the highest compressive strength and there is no further increase in the ultimate strength on increasing the sodium content. A N-A-S-H type gel with reaction products containing Si, Al and Na, is formed in the activated system. The ultimate strength achieved is directly related to the reaction product content in the system and is dependent on the extent of glassy phase dissolution from fly ash. The extent of glassy phase dissolution and the quantity of reaction product formed in the system increases with an increase in the molarity of NaOH, which also contributes to an increase in the sodium content in the activating solution. The decrease in the unreacted glassy phase content of fly ash is sensitive to temperature at a lower molarity of NaOH. The Al/Na ratio in the reaction product approaches a value of 0.9 on increasing the sodium content in the activated system. The Si/Al ratio in the reaction product varies within a range of 2.3–2.8.     

Polymerization in sodium silicate solutions: a fundamental process in geopolymerization technology

Geopolymerization is an innovative technology that can transform several solid aluminosilicate materials into useful products called geopolymers or inorganic polymers. Although the geopolymerization mechanism is not well understood, the most proposed mechanism includes four parallel stages: (a) dissolution of solid aluminosilicate materials in alkaline sodium silicate solution, (b) oligomerization of Si and/or Si–Al in aqueous phase, (c) polymerization of the oligomeric species, and (d) bonding of undissolved solid particles in the polymer. It is obvious that polymerization in sodium silicate solutions comprises a fundamental process in geopolymerization technology. Therefore, this article aims at studying experimentally the polymerization stage in synthetic pure sodium silicate solutions. The structure of sodium silicate gels as a function of the SiO₂/Na₂O molar ratio is examined and their hardness as well as hydrolytic stability are determined. In addition, the effect of aluminum incorporation in the hydrolytic stability of these gels is also examined. Finally, the structure of sodium silicate and aluminosilicate gels is correlated to the measured properties drawing very useful conclusions that could be applied on geopolymerization technology.     

Effects of waste glass on alkali-activated tungsten mining waste: composition and mechanical properties

Increasingly more research is being directed towards the valorisation of waste materials as precursors for synthesising alkali-activated binders (AABs). For this study, varying blends of tungsten mining waste (TMW) and waste glass (WG) are activated using a combined sodium hydroxide (SH) and sodium silicate (SS) alkali solution. The activating solution itself is also varied with respect to the quantities of SS and SH to determine their effect on reactant formation and mechanical strength of TMW-based AABs. The results show that an increased WG content can effectively provide an additional source of reactive silica, contribute to the formation of (C, N)–A–S–H gel products and thus significantly improve the mechanical strength. High strength TMW–WG AABs were attributed to a faster TMW dissolution rate and dense microstructure. Such structures were characteristic of formulations with low alkali modulus (SiO₂/Na₂O < 2) combined with a SS/SH weight ratio of 2.8. For the latter, not only was a characteristic slower strength development with increasing alkali content observed, but there was also a limit of alkali metal concentration (Na₂O ~ 3.1%) beyond which the strength deteriorated. Furthermore, SEM micrographs disclose that unreacted particles of WG reinforced the matrix by acting as a filler.     

Performance of blended metakaolin/blastfurnace slag alkali-activated mortars

This paper studies the effect of silicate content on the mechanical and durability-related properties of metakaolin (MK) and metakaolin/blastfurnace slag (BFS) alkaline activated mortars. A reference mortar based on the alkaline activated MK was compared to 60/40 MK/BFS mortars containing different SiO₂/Na₂O molar ratios in the activator. The properties assessed were compressive strength, porosity (water saturation), porosity and pore size distribution by Mercury Intrusion Porosimetry (MIP) and water capillary sorption. The microstructure was assessed using SEM and x-ray computerized micro-tomography (μ-CT). Results show that the addition of BFS significantly alters the microstructure of alkali-activated mortars, promoting a reduction of porosity and capillary sorption. In addition, an optimum SiO₂/Na₂O molar ratio in the activator is required to produce better durability mortars, which however do not necessarily present the highest mechanical strength.     

Investigation of a synthetic aluminosilicate inorganic polymer

A melt-quenched of mixture of alumina and silica (46 wt% Al₂O₃ or Al₂O₃(SiO₂)₂) was found to react with an alkaline silicate solution (Na₂O)(SiO₂)_1.2(H₂O)_9.5) at low-temperatures to form a synthetic aluminosilicate inorganic polymer. The as-quenched material consisted of a mixture of amorphous and crystalline phases with a range of aluminium coordination environments. Upon reaction with the alkaline silicate solution, solid-state aluminium and silicon magic-angle spinning nuclear magnetic resonance (SS ²⁷Al and ²⁹Si MAS NMR) indicated that a conversion to four-fold aluminium coordination environments occurred, consistent with the formation of a three-dimensional cross-linked inorganic polymer comprised of NaAlO₄ and SiO₄ tetrahedra. Mechanical testing showed the compressive strength of the inorganic polymer increased as the Na₂O/Al₂O₃ molar ratio decreased. Solution studies indicated that 73% of the aluminosilicate starting material was reactive. Scanning electron microscopy (SEM) showed the inorganic polymers had a porous nanoscale grain structure. Open porosity was confirmed by relatively high specific surface area values. Energy dispersive spectroscopy (EDS) and elemental x-ray composition mapping showed that the high-strength specimens had a composite microstructure consisting of 40% unreacted Al₂O₃(SiO₂)₂ and an inorganic polymer binder Na₂O · Al₂O₃(SiO₂)_3.4. The high compressive strengths have been rationalized by this in-situ particle reinforced composite structure, consisting of 10 m agglomerates of unreacted starting material bonded within a sub-micron aluminosilicate/inorganic polymer matrix.     

Relationships between composition,structure and strength of inorganic polymers

Inorganic polymers, or geopolymers, are novel synthetic binders produced by reactions between alkali silicate solutions and solid aluminosilicates. In Part 1 of this study, 12 metakaolin-derived inorganic polymers were produced with various compositions. The effect of the concentration of each of the four most important oxide components of inorganic polymers (Na₂O, SiO₂, Al₂O₃ and H₂O) was assessed by electron microscopy and by strength testing. Additionally, the effect of the type of alkali cation was determined. In general, the results followed expected trends and there were clear correlations between composition, microstructure and strength. It was found that high strength was related to low porosity and a dense, fine grained microstructure. Such a structure was found in inorganic polymers with high alkali contents (Na₂O/Al₂O₃ = 1.2) and low water contents (H₂O/Al₂O₃ = 12). High silica and low alumina contents (SiO₂/Al₂O₃ = 3.5–3.8) also produced this structure, however, there was a limit beyond which the strength deteriorated. In relation to the effect of alkali cations, sodium was found to give higher resin strength than potassium. The results of the study further confirm that the selection of precursor raw materials remains a critical factor to initial strength development. The relationship between different resin formulations and resulting microstructures are discussed.     

Compressive strength and microstructural characteristics of class C fly ash geopolymer

Geopolymers prepared from a class C fly ash (CFA) and a mixed alkali activator of sodium hydroxide and sodium silicate solution were investigated. A high compressive strength was obtained when the modulus of the activator viz., molar ratio of SiO₂/Na₂O was 1.5, and the proper content of this activator as evaluated by the mass proportion of Na₂O to CFA was 10%. The compressive strength of these samples was 63.4 MPa when they were cured at 75 °C for 8 h followed by curing at 23 °C for 28 d. In FTIR spectroscopy, the main peaks at 1036 and 1400 cm^?1 have been attributed to asymmetric stretching of Al–O/Si–O bonds, while those at 747 cm^?1 are due to the Si–O–Si/Si–O–Al bending band. The main geopolymeric gel and calcium silicate hydrate (C–S–H) gel co-exist and bond some remaining unreacted CFA spheres as observed in XRD and SEM–EXDA. The presence of gismondine (zeolite) was also observed in the XRD pattern.     

Preparation of porous supports in the SiO2-ZrO2-Na2O system from microspherical silica gels

Preparation of porous supports in the SiO₂-ZrO₂-Na₂O system was investigated using a commercially available silica gel as a starting material. The microspherical silica gel, impregnated with ZrOCl₂ and NaCl, was heated and subsequently washed with water. Porous supports, composed with sponge-like skeletons on the surface of the particles, were obtained owing to suppression of crystallization of the supports as well as their sintering. Similar supports were formed using a silica gel prepared from sodium silicate solution by the same procedure. In contrast, crystallization of silica proceeded in supports prepared by heating the mixture of silica gel and NaCl in the absence of ZrOCl₂. A new method for preparing the analogous supports was also investigated by heating a mixture of silica gel impregnated with ZrOCl₂ and Na₂CO₃ powder.     

Relationships between composition,structure and strength of inorganic polymers

This article is the second in a two-part series and discusses inorganic polymers derived from fly ash. Part 1 [1] concerns inorganic polymers derived from a metakaolin precursor. For this study, 15 fly ash-derived inorganic polymers were produced with various compositions. The effect of the concentration of each of the four component oxides (Na₂O, SiO₂, Al₂O₃ and H₂O) and two alkali cations (Na and K) on the microstructure and compressive strengths were assessed. Similar to metakaolin-derived inorganic polymers, it was observed that high-strength fly ash inorganic polymers were related to low porosity and a dense, fine-grained microstructure. Such structures were characteristic of formulations with high silica mole fractions (SiO₂/Al₂O₃ ∼ 3.9) and low water contents, as well as those with high alkali and low alumina contents. For the latter, not only was a characteristic slower strength development with increasing alkali content observed, but there was also a limit of alkali concentration (Na₂O/Al₂O₃ ∼1) beyond which the strength deteriorated. Furthermore, SEM micrographs disclose that the fly ash precursor dissolves more readily in the sodium-based system compared to the potassium equivalent. The interrelation between microstructures of the respective formulations and their strength development are discussed. It is observed that the charge-balancing role of the alkali cations in the fly ash formulations may be dominant compared to initial alkali dissolution reaction of the aluminosilicate fly ash particles, which is partly responsible for initial strength development.     

Influence of starting material on the early age hydration kinetics,microstructure and composition of binding gel in alkali activated binder systems

The influence of starting material on the hydration kinetics and composition of binding gel in alkali activated binder systems was evaluated. The starting materials used were ground granulated blast furnace slag, class C fly ash and class F fly ash. All starting materials were activated using alkaline solution with a SiO₂/Na₂O ratio of 1.5. The hydration kinetics were monitored using in situ isothermal conduction calorimetry and the chemical compositions of the binder gels were determined by energy dispersive X-ray spectroscopy. In the fly ash systems, the calorimetric curves had only one peak, which occurred in the first 30 min of reaction, and lacked an induction period. Two peaks were distinguishable in slag systems, though the induction period was much shorter than that of a typical OPC system. The gel composition ratios, including Ca/Si, Na/Si, Na/Al and Al/Si, were different in each of the systems and are discussed in detail.     

Effects of activator type/concentration and curing temperature on alkali-activated binder based on copper mine tailings

This article investigates the effects of activator type/concentration and curing temperature on alkali-activated binder based on copper mine tailings (MT). Different alkaline activators including sodium hydroxide (NaOH), sodium silicate (SS), and sodium aluminate (SA) at different compositions and concentrations were used and four different curing temperatures, 60, 75, 90, and 120?°C, were considered. Scanning electron microscopy/energy-dispersive X-ray spectroscopy (SEM/EDX), and X-ray diffraction (XRD) were conducted to investigate the effect of these factors on the unconfined compressive strength (UCS), microstructure, and phase composition of the binder. The results indicate that NaOH concentration and curing temperature are two important factors that affect the UCS and micro-structural properties of the alkali-activated MT binder. The optimum curing temperature, i.e., the curing temperature at the maximum UCS, depends on the NaOH concentration, lower optimum curing temperature at smaller NaOH concentration. Addition of aqueous SS to the NaOH solution can lead to strength improvement, with the highest UCS obtained at a SiO₂/Na₂O ratio of 1.0–1.26. Addition of powder SA to the NaOH solution profoundly delays the setting at 60?°C but improves the UCS at 90?°C. The SEM/EDX results show highly heterogeneous microstructure for the alkali-activated MT binder as evidenced by the variable Si/Al ratios in different phases. The XRD patterns indicate a newly formed crystalline phase, zeolite, in the 90?°C-cured specimens. The results of this study provide useful information for recycling and utilization of copper MT as construction material through the geopolymerization technology.     

Do properties of bioactive glasses exhibit mixed alkali behavior?

The effect of substituting K₂O for Na₂O on the physical and chemical properties of 15 glasses in the system Na₂O–K₂O–CaO–P₂O₅–SiO₂ was studied for three series: low (52 mol% SiO₂), medium (60 mol% SiO₂) and high (66 mol% SiO₂) silica. The SiO₂ content expressed as weight-% varied from 46 to 64 wt%, thus suggesting that the compositions were either bioactive or biocompatible. The crystallization tendency and sintering behavior were studied using differential thermal analysis and hot stage microscopy. Formation of silica- and hydroxy-apatite-rich layers were studied for glass plates immersed in static simulated body fluid. The release of inorganic ions into Tris buffer solution was analyzed using inductively coupled plasma optical emission spectrometer in dynamic and static conditions. Substitution of K₂O for Na₂O suggested mixed alkali effect (MAE) for the thermal properties with a minimum value around 25% substitution. With increased share of K₂O in total alkali oxides, the hot working window markedly expanded in each series. Silica and hydroxyapatite layers were seen only on the low silica glasses, while a thin silica-rich layer formed on the other glasses. In each series, greater dissolution of alkali and alkali earth ions was seen from K-rich glasses. Clear MAE and preferential ion dissolution were recorded for medium and high silica series, while for low silica glasses, the initial MAE dissolution trends become rapidly covered by other simultaneous surface reactions. MAE enables designing especially low silica bioactive glasses for improved hot working properties and medium and high silica glasses for controlled dissolution.     

Preparation of lamellar-stacked TS-1 and its catalytic performance for the ammoximation of butanone with H<Subscript>2</Subscript>O<Subscript>2</Subscript>

A SiO₂ particle was prepared with different alkali sources, and then lamellar-stacked TS-1 catalysts were hydrothermally synthesized using the SiO₂ particle as a silica source. Powder X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared (FT-IR) spectra, nitrogen adsorption–desorption and UV–vis absorption spectra were used to characterize the TS-1 catalysts. The effect of the alkali source during the preparation of the SiO₂ particle on the textural properties and catalytic performance of the TS-1 catalyst was thoroughly investigated. The TS-1 catalyst that was prepared with a SiO₂ particle using tetrapropylammonium hydroxide (TPAOH) as an alkali source (TS-1-TPAOH) possessed more meso- and macro-pores and a higher framework Ti content than the catalyst that was prepared with a SiO₂ particle using NH₃·H₂O as an alkali source (TS-1-NH₃·H₂O). As a result, the TS-1-TPAOH catalyst had a better catalytic performance for butanone ammoximation with H₂O₂ than conventional TS-1 and TS-1-NH₃·H₂O catalysts. Furthermore, the influences of reaction conditions, including reaction temperature, reaction time, the amount of catalyst and the molar ratio between H₂O₂ and butyl ketone oxime on the catalytic performance of the TS-1-TPAOH catalyst were evaluated. The unique structure of the lamellar-stacked TS-1 catalyst can effectively avoid the diffusing of large reactant molecules into zeolite channels and has potential applications in other oxidation reactions.     

Room temperature synthesis of Si-MCM-41 using polymeric version of ethyl silicate as a source of silica

Synthesis of mesoporous MCM-41 materials at room temperature using less expensive polymeric version of ethyl silicate (40 wt% SiO₂) as a source of silica was established. The influence of crucial synthesis parameters such as molar ratios of H₂O/NH₄OH, NH₄OH/SiO₂ and CTMABr/SiO₂ in gel on the quality of the phase formed was investigated. Powder X-ray diffraction (XRD), scanning electron microscopy (SEM) and low temperature N₂ adsorption-desorption isotherms have been employed to characterize the products. The magnitude of orderness, textural properties and thermal stability of the Si-MCM-41 samples prepared under identical judiciously pre-controlled synthesis conditions using ethyl silicate and conventional tetraethyl orthosilicate (TEOS) were assessed. Even though, ethyl silicate has proved to be suitable source for the preparation of MCM-41 at room temperature, there exists an optimum value of H₂O/NH₄OH for different NH₄OH/SiO₂ molar ratios in the gel. Changes in the morphology were observed when NH₄OH/SiO₂, H₂O/NH₄OH molar ratios in the gels were changed.     

Crystalline phase formation in metakaolinite geopolymers activated with NaOH and sodium silicate

The effect of the NaOH content and the presence of sodium silicate activators on the formation of crystalline phases from metakaolinite-based geopolymers were studied by X-ray powder diffraction (XRD), Rietveld quantitative XRD, solid-state MAS NMR and SEM in samples synthesized with varying NaOH contents and different curing times at 40 °C. Geopolymers activated with NaOH alone with Si/Na ratios of 4/4 or less formed the crystalline zeolite Na–A (Na₉₆Al₉₆Si₉₆O₃₈₄·216H₂O), but at ratios >4/4 nanosized crystals of another zeolite (Na₆[AlSiO₄]₆·4H₂O) were formed. The Si/Na ratio of 4/4 produces a product of greatest crystallinity. The addition of sodium silicate in addition to NaOH significantly reduces crystallite formation. The network units of all the materials containing NaOH and sodium silicate are essentially the same, namely, tetrahedral [SiO₄] units coordinated through four bridging oxygens to four aluminium atoms [denoted as Q⁴ Si(4Al) units]. A templating function of the various silicate units of the sodium silicate molecules is suggested to occur in geopolymerization, which differs from the reaction route operating when NaOH alone is used as the activator. This templating function is responsible for the suppression of crystallization and the increase in strength of the geopolymers activated with sodium silicate.     

Low-temperature synthesized aluminosilicate glasses: Part III Influence of the composition of the silicate solution on production, structure and properties

The low-temperature reaction between an aqueous sodium or potassium silicate solution and metakaolinite yields a solid aluminosilicate. The influence of the molar ratios H₂O/R₂O (between 6.6 and 21.0) and SiO₂/R₂O (between 0.0 and 2.3) of the silicate solution (R=Na or K) on the aluminosilicate's production, on the reaction stoichiometry and on the aluminosilicate's molecular structure is studied with differential scanning calorimetry, ²⁷Al and ²⁹Si magic angle spinning nuclear magnetic resonance (MAS NMR), cross-polarization MAS NMR, Fourier transform infrared spectroscopy and X-ray diffractometry. The reaction stoichiometry is determined by a one to one ratio for R/Al. H₂O/R₂O has no influence on the molecular structure of the aluminosilicate. Aluminium in the aluminosilicate is four-fold coordinated for the whole range of silicate solutions investigated. Moreover, Si and Al are homogeneously distributed and the ratio Al/Si in the aluminosilicate is the same as in the reaction mixture if the stoichiometric one-to-one ratio for R/Al is used. If SiO₂/R₂O in the Na-silicate solution is equal to or higher than 0.8, the low-temperature reaction yields an amorphous aluminosilicate or “inorganic polymer glass”. For smaller values of SiO2/R2O the Na-aluminosilicates are partially crystalline. Thermomechanical analysis and dynamic mechanical analysis indicate that a variation in the composition of the amorphous aluminosilicates can shift the glass transition over a few hundreds of degrees, with a minimum value of 650°C. This revised version was published online in November 2006 with corrections to the Cover Date.     

Geopolymers based on a coarse low-purity kaolin mineral: Mechanical strength as a function of the chemical composition and temperature

A coarse mineral with 70% kaolinite and 30% quartz was calcined and chemically activated by alkaline solutions of Na₂SiO₃ and NaOH. The compressive strength evolution was investigated as a function of the curing temperature at 20 and 80 °C, and the molar ratios of SiO₂/Al₂O₃ (2.64-4.04) and Na₂O/Al₂O₃ (0.62-1.54). For curing at 20 °C, the best composition was SiO₂/Al₂O₃ = 2.96 and NaO/Al₂O₃ = 0.62, reaching 85 MPa at 28 days. Curing at 80 °C had a positive effect on the strength development only in the first 3 days. X-ray diffraction of the geopolymeric formulations showed the formation of amorphous silicoaluminates of similar nature. The microstructure consisted of unreacted quartz and metakaolinite particles in a matrix of silicoaluminate polymer and condensed silica gel from the unreacted sodium silicate.     

Raman spectroscopic investigation of sodium borosilicate glass structure

Raman spectra of sodium borosilicate glasses with a wide range of Na₂O/B₂O₃ ratios were systematically measured. Variations of the spectra with glass composition were studied to interpret the implied distribution of Na⁺ ions between silicate and borate units. When Na₂O/B₂O₃ is less than 1, all Na⁺ ions are associated with borate units as indicated by the absence of the 1100 cm⁻¹ band of Si-O⁻ non-bridging bond stretching. For the (1−x)Na₂O · SiO₂ ·xB₂O₃ glass withx≦0.4 the peak-height ratio of the 950 cm⁻¹ band to the 1080 cm⁻¹ band was used to analyse semiquantitatively the distribution of the Na⁺ ions between silicate and borate units. Sodium ions are divided between silicate and borate units approximately in proportion to the amount of SiO₂ and B₂O₃ present in these glasses. Some of the high sodium content glasses were crystallized and their spectra were compared with the bulk glass spectra. The distribution of Na⁺ ions in the glass was quite different from their distribution after crystallization. Spectra of high silica glasses that had been heat-treated for phase separation indicated exclusion of borate units from the silica network and the formation of borate groups. For high boron content glasses, no change was observed on heat treatment. Raman bands due to borate groups seem to be little affected by their environments. Also affiliated with the Department of Geosciences.