Mechanical properties of sodium and potassium activated metakaolin-based geopolymers

In this study, a set of mechanical properties of geopolymers, synthesized by alkali (NaOH or KOH) activation of metakaolin and SiO₂ mixture, were characterized at ambient temperature. Samples with K/Al or Na/Al atomic ratios equal to 1, Si/Al atomic ratios in the 1.25–2.5 range and H₂O/Al₂O₃ molar ratios of 11 or 13 are cured at 80 °C for 24 and 48 h before characterization, to determine effect of Si/Al ratio and curing time on the structure and mechanical properties of geopolymers. The structure of synthesized geopolymers characterized using XRD, NMR, SEM, and density measurements was correlated to their mechanical properties, including compressive strength, Young’s modulus, hardness, and fracture toughness. The results of this study suggest a strong effect of Si/Al ratios (in the 1.5–2 range), density, and microstructure on the maximum strength, Young’s modulus, and hardness of geopolymers. There were also notable differences in strength between samples cured for 24 and 48 h, suggesting that the degree of geopolymerization reaction also plays important role in mechanical properties of this new class of inorganic polymers.     

Effects of activator characteristics on the reaction product formation in slag binders activated using alkali silicate powder and NaOH

The influence of different levels of alkalinity, expressed using the Na₂O-to-source material ratio (n) and activator SiO₂-to-Na₂O ratio (M_s), on the compressive strength development of, and reaction product formation in sodium silicate and NaOH powder activated slag binder systems is discussed. Higher n value mixtures are found to exhibit higher early and later age compressive strengths. An increase in M_s results in reduced early age and slightly increased later age strengths. Compositional coefficients, which are functions of n and M_s are proposed, that relate to the early and later age strengths of the activated slag binders as well as to the shift in the FTIR spectra. The reaction product formation in these systems as a function of the total alkalinity is explained using the shifts of the dominant peak in the FTIR spectra. Fundamental changes in reaction products of powder activated binders as a function of alkalinity is observed. The deductions from the peak shifts are substantiated using the FTIR spectra of the pastes before and after salicylic acid–methanol (SAM) attack.     

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).     

Microstructural changes in alkali activated fly ash/slag geopolymers with sulfate exposure

Sulfate attack is recognized as a significant threat to many concrete structures, and often takes place in soil or marine environments. However, the understanding of the behavior of alkali-activated and geopolymer materials in sulfate-rich environments is limited. Therefore, the aim of this study is to investigate the performance of alkali silicate-activated fly ash/slag geopolymer binders subjected to different forms of sulfate exposure, specifically, immersion in 5 wt% magnesium sulfate or 5 wt% sodium sulfate solutions, for 3 months. Extensive physical deterioration of the pastes is observed during immersion in MgSO₄ solution, but not in Na₂SO₄ solution. Calcium sulfate dihydrate (gypsum) forms in pastes immersed in MgSO₄, and its expansive effects are identified as being particularly damaging to the material, but it is not observed in Na₂SO₄ environments. A lower water/binder (w/b) ratio leads to a greatly enhanced resistance to degradation by sulfate attack. Infrared spectroscopy shows some significant changes in the silicate gel bonding environment of geopolymers immersed in MgSO₄, attributed mostly to decalcification processes, but less changes upon exposure to sodium sulfate. It appears that the process of ‘sulfate attack’ on geopolymer binders is strongly dependent on the cation accompanying the sulfate, and it is suggested that a distinction should be drawn between ‘magnesium sulfate attack’ (where both Mg²⁺ and SO₄ ^2? are capable of inducing damage in the structure), and general processes related to the presence of sulfate accompanied by other, non-damaging cations. The alkali-activated fly ash/slag binders tested here are susceptible to the first of these modes of attack, but not the second.     

A multiscale investigation of reaction kinetics,phase formation,and mechanical properties of metakaolin geopolymers

A multiscale study is presented of the reaction kinetics, phase formation, mechanical properties of metakaolin-based geopolymers by varying Si/Al ratios of 1.2–2.2 and Na/Al ratios of 0.6–1.2. Macro- and nano-mechanical properties of geopolymer samples were determined by unconfined compression testing and grid nanoindentation technique, respectively. The latter, in combination with statistical deconvolution, also enables the extraction of generally 4 distinct phases together with their nanomechanical properties and volumetric fraction within the synthesized geopolymers. Moreover, the reaction kinetics, phase formation (particularly geopolymer gel development), and mechanical property development were investigated by characterizing geopolymers cured at the final setting time, 7, and 28 days. Phase formation was characterized by Fourier transform infrared spectroscopy (FTIR) via monitoring the evolution of the Si-O-T (T: Si or Al) and Al-O bonds. Results illustrate that the fraction of geopolymer gels dominantly governs the mechanical behavior, both of which increase with the Si/Al and Na/Al molar ratios, while the final setting time increases with the Si/Al ratio, but decreases with the Na/Al ratio. The chemical composition for the best mechanical performance of the studied geopolymers is a Si/Al ratio of 1.7 and Na/Al ratio of 0.9. The relationships among geopolymer chemical compositions, geopolymer gel formation rate, and macromechanical properties are also discussed.     

Residual strength properties of sodium silicate alkali activated slag paste exposed to elevated temperatures

The residual compressive strength behavior of alkali activated slag paste (AASP) after temperature exposures up to 1,200°C was investigated. Strength loss of approximately 60% occurred between 100 and 200°C and a further strength loss in the order of 30% at 800°C. Total loss of strength occurred at 1,200°C. Thermogravimetric studies (TGA/DTG) verified AASP contained no Ca(OH)₂ which governs the chemical mechanism of strength loss for ordinary Portland cement (OPC) and blended slag cement pastes. However, the TGA results showed that AASP had a higher water loss than the other binders between 100 and 200°C and higher thermal shrinkage as indicated by the dilatometry studies. The high thermal shrinkage led to a differential thermal shrinkage gradient within the AASP and induced micro stresses and cracking which was more prominent for larger samples. Differential thermal shrinkage caused by the higher thermal shrinkage of the AAS material was concluded as the mechanism which gives lower residual strength in AASP compared to OPCP.     

Thermal behaviour of inorganic geopolymers and composites derived from sodium polysialate

Inorganic polymers based on alumina and silica polysialate units were synthesised at room temperature from metakaolinite and sodium silicate in a highly alkaline medium, followed by curing and drying at 65 °C. When properly cured, these polymers exhibit remarkable thermal stability; after losing their hydration water at about 200 °C, they retain their X-ray-amorphous tetrahedral Al and Si network up to the onset of melting at 1300 °C. A small amount of mullite and corundum formed at 1200-1300 °C may result from the presence of a trace of unreacted metakaolinite. Similar experiments with poorly-curing formulations containing higher Na and Si contents show that their unpolymerised components form crystalline nepheline (NaAlSiO₄) at 800 °C, prior to melting at about 1100 °C.A series of geopolymer composites were prepared containing 10-20 vol.% of various granular inorganic fillers ranging from waste demolition materials through mineral tailings to engineering ceramics. The physical and thermal properties (bulk density, compressive strength and thermal expansion) of these composites were measured. The thermal expansion is influenced by the properties of the filler, but all the samples showed only slight expansion up to ∼800 °C on the first heating cycle. Microcracking of the composite bodies during drying can be minimised by the addition of a small amount of glycerol.     

Effects of halloysite in kaolin on the formation and properties of geopolymers

A kaolin containing 31 wt.% halloysite and a relatively pure kaolin were selected to study the effects of halloysite on the dissolution behavior of precursors and the formation of geopolymers. The Al and Si concentrations in the leached solutions were studied by inductively coupled plasma-optical emission spectrometry (ICP-OES). The reaction process of metakaolin–activator mixtures was monitored by isothermal conduction calorimetry (ICC) while the reaction products were examined by X-ray diffractometry (XRD), scanning electron microscopy (SEM) and mercury intrusion porosity (MIP). Results showed that the halloysite containing kaolin and its metakaolin possessed higher Si and Al dissolution rate than the purer kaolin and its metakaolin. When mixed with sodium silicate activator at 20 °C, the presence of halloysite in kaolin led to a higher geopolymerization rate of metakaolin as reflected by the heat evolution rate. The presence of halloysite improved the reactivity of metakaolin but did not change the geopolymerization pathway under 20 °C air curing and 80 °C steam curing conditions. The products from the two metakaolins had a similar XRD characteristic (i.e. a typically amorphous diffraction). Only a minor difference in the pore distribution and the porosity was found between those products from the two sources of metakaolins.     

Some electrical properties of sodium silicate glasses modified with titania

A number of glass samples were prepared from mixtures of SiO₂, TiO₂ and Na₂CO₃ (for Na₂O) and their electrical characteristics were measured and interpreted in terms of an electrical energy gap somewhat smaller than the optical gaps previously reported for these glasses. Capacitance measurements showed that one effect of increasing the TiO₂ content was to increase the permittivity. Traditional sodium silicate glasses conduct electricity by ions, but the admixture of TiO₂ brings a significant electronic contribution. Some measurements at high electric fields were made and evidence of the applicability of small polaron theory to the results is presented.     

Preparation,structure and hydrothermal stability of alternative (sodium silicate-free) geopolymers

In this contribution, we present the preparation and structural characterization of a new type of alternative (sodium silicate-free) geopolymer system. A new procedure of geopolymer synthesis based on the preparation of a reactive geopolymer precursor by direct calcinations of low-quality kaolin with Na/K hydroxides is introduced. The subsequent formation of geopolymer matrix does not require activation by alkaline silicate solution. The compact and hardened material was prepared only by adding a small amount of water. Besides the introduction of a new synthetic procedure, we focused also on the systematic study of chemical structure, mineralogical composition and hydrothermal stability of the prepared geopolymer systems as seen by solid-state NMR spectroscopy and powder X-ray diffraction (XRD). An important part of our contribution is the demonstration of structural and mineralogical changes induced by hydrothermal treatment and long-term aging of the prepared geopolymers. It was found that redistribution of basic structural units (SiO₄⁴⁻ and AlO₄⁵⁻) and gradual formation of zeolite fractions can be related to the observed changes in mechanical properties. Up to a certain level, the presence of zeolites enhances the mechanical properties of the prepared geopolymer systems. However, the additional formation of a new generation of zeolite fractions, occurring over the long-term period causes an inversion of this trend and a dramatic reduction of mechanical strength. Nevertheless, formation of the geopolymer matrix by alkaline and thermal activation of low-quality kaolin has the potential to be used in ecological problems solving (solidification of powdered and dangerous waste materials).     

Structural study of sodium silicate glasses and melts

We report an X-ray diffraction study and modeling of the structure of sodium silicate glasses and melts that are used as binders in the production of the latest generation of ceramic welding fluxes. Experimental data and reverse Monte Carlo (RMC) simulations results are analyzed in comparison with results for a sodium silicate monolith and the water glass prepared from it. The RMC results agree well with structure factor and radial distribution function analysis data. The sodium silicate glasses and melts are shown to have a number of structural features, which have much in common with their crystalline counterparts. The high capacity of the silicate glasses and melts for water is due to the presence of nanopores formed (largely) by six-membered rings of silicon-oxygen tetrahedra and partially by sodium-oxygen octahedra.     

木质素活性炭的制备及工艺优化

以造纸黑液木质素为原料,NaOH为活化剂,通过化学活化制备活性炭(AC).考察了活化温度、活化时间和浸渍比对AC比表面积的影响.采用中心组合设计(Central composite design,CCD),运用响应面法(Response surface methodology,RSM)进行工艺参数优化.得出最佳工艺参数:活化温度为751℃,活化时间为57 min,浸渍比为2.06,可制得AC比表面积为l 437.22 m2/g.并对模型进行了检验,验证了数学模型的有效性.     

Bonelike apatite formation on niobium metal treated in aqueous NaOH

The essential condition for a biomaterial to bond to the living bone is the formation of a biologically active bonelike apatite on its surface. In the present work, it has been demonstrated that chemical treatment can be used to create a calcium phosphate (CaP) surface layer, which might provide the alkali treated Nb metal with bone-bonding capability. Soaking Nb samples in 0.5 M NaOH, at 25 degrees C for 24 h produced a nano-porous approximately 40 nm thick amorphous sodium niobate hydrogel layer on their surface. Immersion in a simulated body fluid (SBF) lead to the deposition of an amorphous calcium phosphate layer on the alkali treated Nb. The formation of calcium phosphate is assumed to be a result of the local pH increase caused by the cathodic reaction of oxygen reduction on the finely porous surface of the alkali-treated metal. The local rise in pH increased the ionic activity product of hydroxyapatite and lead to the precipitation of CaP from SBF that was already supersaturated with respect to the apatite. The formation of a similar CaP layer upon implantation of alkali treated Nb into the human body should promote the bonding of the implant to the surrounding bone. This bone bonding capability could make Nb metal an attractive material for hard tissue replacements.     

Characterization of gas components and deposits in bubbles in silicate glasses prepared with sodium sulphate

The gas contents and surface deposits in bubbles that were formed at various temperatures in sodium calcium silicate glasses using sodium sulphate as a starting material were investigated by means of Raman microprobe and scanning microscopic techniques. Either elemental sulphur or sodium sulphate could be readily detected in the deposits, depending on treatment conditions for the glass. Higher amounts of elemental sulphur were found as deposits on bubble surfaces for glasses prepared using both carbon and sodium sulphate. SO₂ or CO₂ could be detected as gas components in various bubbles.     

Mullite formation from ethyl silicate and aluminium chlorides

The formation of mullite via gels prepared from technical ethyl silicate and aluminium chlorides has been studied. Normally, gels prepared specifically with the oxide stoichiometry of mullite (3Al₂O₃·2SiO₂) do not form the mineral mullite on firing to 1200° C in the absence of a mineralizer. However, when the stoichiometric gel is homogeneous (achieved by acidic or neutral catalysts during the gel preparation) firing at 1200° C can lead to an almost quantitative yield of mullite. For a homogeneous gel, the presence of strontium or caesium salts, or an organo-tin compound such as dibutyltin diacetate or dibutyltin oxide during the gel preparation promotes almost quantitative conversion to mullite at about 1000° C. There is a threshold concentration under which conversion to mullite is incomplete, some cristobalite being formed. For the organo-tin compounds, the type of aluminium chloride is unimportant and the way in which water for the hydrolysis step is added is also unimportant. When the gel is non-homogeneous, the product obtained on firing contains cristobalite and-alumina or-alumina, with little mullite, even if strontium or caesium salts, or organ-otin compounds are present. A ceramic bond is formed from alumina and some other refractory grains during firing.     

The formation and microstructure of dental silicate cements

The chemistry of the cement-forming reaction between phosphoric acid solutions and fluorine-containing aluminosilicate glasses has been studied using a variety of physicochemical methods. These materials are the dental silicate cements and the microstructures of a number of these cements have been examined by optical, electron and scanning electron microscopy. The element distribution between partially reacted glass particles and the gel matrix which binds them has been determined by electron probe microanalysis. The chemical stability and variations in the mechanical strength between different cements is discussed in terms of a new hypothesis on the constitution of the gel matrix. Consideration is given to the possible strengthening role of microcrystallinity which has been shown by X-ray and electron diffraction to develop in the matrix. Attempts to strengthen the cement by incorporation of reinforcing agents is described.     

Crystalline phase transition with a large conformational change in a thiodicarboxylic acid

A large conformational transition in a crystal of thiodipropionic acid is investigated by differential scanning calorimetry (DSC), X-ray diffraction, optical microscopy, and computer modeling. The room temperature phase (A-phase) was already known to be orthorhombic where the chains have largely zigzagged conformation with both S–C bonds in gauche conformation. A high temperature phase (C-phase) above 105°C is found by DSC. By X-ray analysis, the crystal structure of the C-phase is found to be monoclinic with molecules having nearly extended conformation. Nucleation and growth of the C-phase are clearly observed by polarizing microscope. It is found that each C-phase domain grows very fast in a preferred direction which is nearly parallel to the {110} planes of the C-phase. Molecular dynamics simulation of the transition reveals that the largely zigzagged molecules in the A-phase readily transforms to the extended all trans form at higher temperatures.     

The effect of organic and inorganic fibres on the mechanical and thermal properties of aluminate activated geopolymers

The addition of fibres to a brittle matrix is a well-known method to improve the flexural strength. However, the success of the reinforcements is dependent on the interaction between the fibre and the matrix. This paper presents the mechanical and microstructural properties of PVA and basalt fibre reinforced geopolymers. Moreover low density and thermal resistant materials used as insulating panels are known be susceptible to damage due to their poor flexural strength. As such the thermal and fire resistance properties of foamed geopolymers containing fibre reinforcement were also investigated.The results highlight that the presence of PVA fibres greatly increased the flexural strength and the toughness of the geopolymer composite, while the presence of basalt fibres improved the flexural behaviour of the composite after high temperature exposure.     

Synthesis and biodegradability of starch-g-ethyl methacrylate/sodium acrylate/sodium silicate superabsorbing composite

A new superabsorbent composite polymer (SAP) has been prepared by graft copolymerization reaction using starch, ethyl methacrylate (EMA), benzoyl peroxide (BPO) as initiator and sodium acrylate as crosslinking agent. Further, it is observed that the composite doped with sodium silicate exhibits higher waterabsorbency as the silicate can well disperse in the water and enter into the network structure of the composite, thereby increasing the porosity of the composite. Sodium acrylate plays an important role as the crosslinker forming a network structure of the superabsorbent composite. The composite was characterized by IR, TGA, and XRD methods. The biodegradability of these valuable novel materials was evaluated for their industrial and commercial importance. The surface morphology was compared by scanning electron microscopy (SEM) before and after biodegradation of the composite.