Alkali Silicate Research Articles

Mechanical performance of metakaolin-based geopolymer mortar blended with multi-walled carbon nanotubes

In recent decades, the cement industry has been viewed as a major environmental problem mainly due to carbon dioxide CO2 emissions during the production of Portland Cement (PC). Among the solutions to this problem is to replace Portland cement-based materials with alkali-activated materials. Metakaolin-based geopolymer mortars are currently considered a serious alternative to ordinary Portland cement mortar due to its various advantages. The present experimental study aims to develop metakaolin-based geopolymer mortars reinforced with multi-walled carbon nanotubes (MWCNTs). The content of MWCNTs incorporated was 0.1; 0.2; 0.3, and 0.4 wt%, respectively, with respect to the mass of metakaolin. A mixture without the addition of carbon nanotubes was also prepared and used as a reference (control sample). The alkaline activation of metakaolin was carried out using aqueous sodium silicate solution having a molar ratio (SiO2/Na2O) equal to 1.77. Mechanical characterization of the hardened samples was performed after curing for 3, 7 and 28 days. The results indicate that the incorporation of multi-walled carbon nanotubes in the geopolymer matrices improves compressive strength. Microscopic analysis has shown that these carbon nanotubes contribute to the densification of the geopolymer matrix and give rise to crack bridging mechanisms. The highest value of the compressive strength was recorded using an alkali silicate content equal to 8 (expressed as a percentage of Na2O) combined with 0.2% of multi-walled carbon nanotubes in the geopolymer mortar. The integral absolute error (IAE) calculated on the compressive strength values at different curing ages are all within an acceptable range (0–10%).

Mechanical and Thermal Evaluation of Diatomite Doped Fly Ash Based Geopolymers

In this work, the properties of alkali silicate geopolymer type materials and diatomite as additive to fly coal ash was investigated using thermic coal plant’s fly ash in alkaline solution. The reacted products of alumino-silicate geopolymers using fly ash plus diatomite were pseudo-amorphous aluminosilicate gel and calcite and their mechanical and thermal properties were evaluated by the addition of diatomite. The compressive strength of this geopolymer is similar to that of the Portland cement mortar of PC20 (± 20 %) when cured for 28 days while the density and thermal properties are much lower that indicates the insulator properties. Alkaline solution was produced by NaOH in different concentrations to determine the least alkaline solution molarity in the range of 1 M, 3 M and 5 M. The characterization of geopolymers were done by using XRD for phase analysis, SEM for surface and morphological evaluation, compression tests for mechanical properties and transient plane source thermal analysis for thermal insulation properties. The results showed that, 1 M of NaOH alkaline solution and 10wt% diatomite addition can provide enough strength of 18 MPa which is a good candidate for constructional materials. The thermal conductivity coefficient of 10 wt.% diatomite added geopolymer was evaluated as 0.0018 W/m×K which can also be a good candidate for insulator materials to be used in green lateral wall production.

Preparation of Titanium-Containing Layered Alkali Silicates

Preparation of Titanium-Containing Layered Alkali Silicates

Alkali-activated fly ash-blast furnace slag blend rheology: Evaluation of yield and Maxwell responses

The rheological behavior of pastes made with fly ash-slag blends in alkali silicate activating solutions is evaluated. The link between the activating solution composition and the fundamental rheological behavior of alkali-activated pastes are studied for different fly ash-slag blends. The transient response of alkali-activated pastes prior to initiation of flow under an applied strain rate varies between yield-type or Maxwell-flow behaviors depending on the silica content in the activating solution. Adding dissolved silica in the alkaline activating solution initially produces a decrease in the yield stress of the paste, but with further increase the transient response transitions to a Maxwell-flow type behavior. Maxwell flow behavior is a fluid-dominated response with an apparent yield stress in the transient response under constant applied strain rate. The apparent yield stress in the Maxwell flow response increases sensitively with increasing silica content in the activating solution. On increasing the silica content in the activating solution, there is an increase in the plastic viscosity of the pastes. The thixotropy of the paste is influenced by the silica content in the activating solution and the slag content in the blend. Blends with yield-type behavior exhibit a rapid yield stress recovery after shearing. The yield stress and the structural rebuilding energy (SRE) increase very rapidly with age in activated mixtures containing dissolved silica. The pastes which exhibit Maxwell flow response have a significantly slower structural buildup indicated by a lower rate of increase in the SRE with age compared to material with yield type of behavior. The slag content in the blend contributes to a rapid increase in the SRE. The requirements of rheology control of the paste for different processing requirements including pumping and 3D concrete printing are evaluated.

Effect of V2O5 on structural and elastic properties of alkali silicate SiO2–Na2O–NaF glasses

A series of V2O5–doped alkali silicate SiO2–Na2O–NaF glasses were prepared by the melt quenching technique. Amorphousness of the samples was characterized using powder X-Ray diffraction (XRD) analysis; and the structural role of V2O5, Na2O and NaF in the silicate network was explored using Fourier Transform Infrared (FTIR) spectroscopy. Elastic properties, determined by using ultrasonic technique, were correlated with the compositional and structural parameters as well as the excess molar volume. The substitution of SiO2 by Na2O and NaF in the base SiO2–Na2O–NaF glass resulted in the breaking of Si–O–Si linkages and creation of NBOs, whereas the addition of V2O5 resulted in formation of VO5 structural units and weak V–O–V and Si–O–V linkages. Makishima-Mackenzie's theory is valid for pure SiO2 and V2O5–free glasses. The discrepancy between theoretical and observed elastic moduli and micro-hardness of V2O5–doped glasses was discussed in details.

Phase separation in alkali silicate glasses detected through inverse Laplace transform of29Si nuclear magnetic resonance echo train decay

The ratio of Si-29 nuclear magnetic resonance coherence lifetimes for Q4and Q3sites under magic-angle spinning and a π-pulse train in silicate glasses can detect phase separation, even at small scales where the glass appears optically homogenous.

The structure of CaO–SiO2 and PbO–SiO2 glasses

Whereas the cybotactic theory and thermodynamic modelling have proved invaluable tools in understanding the structures and properties of alkali silicate glasses, questions have been raised as to their validity for alkaline earth and related glasses such as PbO–SiO2. The present paper discusses the specific cases of the CaO–SiO2 and PbO–SiO2 systems, and it is shown that the presence of Si[3] units (silicate tetrahedra with three bridging and one nonbridging oxygen atoms) can easily be explained in terms of the thermodynamic equilibria that underlie the model of associated solutions and the cybotactic theory. Similarly, the much more random distribution of silicate tetrahedral species in PbO–SiO2 glasses derives from the amphoteric nature of PbO. A related question concerns the relevance of atomistic structural modelling/simulation to the evaluation of structural theories of glasses, but to date all of the models of binary silicate glasses have been generated using periodic boundary conditions, which means that they are incapable of reproducing the long range disorder that characterises the vitreous state. Furthermore, it is demonstrated that, in their present form, the RMC and related computer codes, such as EPSR, are fundamentally flawed, in that they merely involve the fitting of an early crystallite model to experimental diffraction data, albeit one where the average internal structure of the crystallites is based on a large highly disordered unit cell, but the crystallites themselves have an entirely unphysical shape. It is also concluded that, to fully interpret the structure of binary and multicomponent glasses, it is essential to study the relevant phase diagram, together with the structures of the thermodynamically-stable and metastable crystalline phases that occur in that particular glass-forming system, and to understand that, since the supercooled liquid is only transiently metastable, the cybotactic/chemical grouping species present in the final glass may not necessarily be determined by equilibrium thermodynamics, but may be greatly influenced by the quench rate. The temperature range over which these crystalline phases/polymorphs are stable is also important, as is the temperature dependence of the glass transition temperature, Tg, and its relationship to the solidus temperature, Ts, at the same composition. Only in this way is it possible to derive the maximum information concerning the structure of a given glass and, much more importantly, to explain why this glass has its particular structure. It is therefore concluded that the key to developing a comprehensive theory of the formation and structure of the vitreous state lies not with ever more precise determinations of the short range order (i.e. diffraction studies), but rather in understanding the role of the thermodynamic equilibria that drive the characteristic long wavelength fluctuations in both number density and composition that distinguish the vitreous from the crystalline state.

Self-Floating Adsorbent Derived from a Layered Alkali Silicate Modified with a Silane Coupling Reagent by Mechanochemical Reaction for Water Purification

Self-Floating Adsorbent Derived from a Layered Alkali Silicate Modified with a Silane Coupling Reagent by Mechanochemical Reaction for Water Purification

High pressure effect in the near-infrared emission of Nd3+-doped alkali silicate glasses

ABSTRACT The effect of high pressure (7.7 GPa) densification on the absorption and near-infrared emission of Nd-doped lithium, sodium and potassium disilicate glasses was investigated. Judd-Ofelt parameters Ωλ (λ = 2, 4 and 6) were calculated from the absorption spectra, before and after densification of glasses, indicating that the irreversible structural changes induced by pressure increased the symmetry of the Nd-O bonding. Density and refractive index of the glasses also increased, probably related to the close packing of SiO4 tetrahedra during densification. The magnitude of the splitting due to the Stark effect was larger for potassium silicate, before and after densification. The radiative transition probabilities of the hypersensitive transition for sodium and potassium silicate increased after densification while the radiative lifetime decreased. For lithium silicate, the opposite behavior was observed. The experimental results corroborate the influence of alkali ion and densification on the luminescence properties of Nd-doped alkali silicate glasses.

Physical Characteristics and Compressive Strength of Na-Geopolymer Paste Designed by a Taguchi Method

Geopolymer paste is a revolutionary building material that the chemical activity of inorganic molecules will create. It is an alternative to traditional Portland cement and is more Eco-friendly. This analysis aimed to classify the mixtures and their process parameters suitable for the development of Geo-polymer paste with one of the ultimate compressive powers, the highest-lowest porosity, and the lowest-lowest final and initial setting time. In the experimental design of the Geo-polymer-based-metakaolin, a Taguchi methodology has been utilized. Five variables parameters were chosen that are mostly to influence the properties of the geopolymer. These are the quantity of Si, alkali, the proportion of alkali reagents, duration of blending, and water amount. These variables’ influence has been calculated at 7 and 28 days on compressive strengths, porosity, density, and setting time. The analysis indicates that the strong compressive strength (115MPa) of Geopolymer paste could be achieved with the formula (1Na2O. Al2O3. 3.8SiO2.xH2O) utilizing suitable processing conditions under which the molar ratio of alkali silicate to alkali hydroxide must be held within the range of 3.25-3.02.

Silicon-29 echo train coherence lifetimes and geminal [formula omitted]-couplings in network modified silicate glasses

The natural abundance 29Si echo-train coherence lifetimes in network-modified silicate glasses were examined under static and magic-angle spinning (MAS) conditions. The nuclear magnetic properties of modifier cations were found to play a major role in determining 29Si coherence lifetimes, leading to differences as large as three orders of magnitude. In compositions with abundant NMR active nuclei, such as alkali silicates, the 29Si coherence lifetimes are dominated by coherent dephasing due to residual heteronuclear dipolar couplings, whereas in compositions dilute in NMR active nuclei, such as alkaline earth silicates, the 29Si coherence lifetimes are dominated by incoherent dephasing due to paramagnetic impurities. Expressing the inverse of the coherence lifetime as a residual full width at half maximum (FWHM), we found that increasing rates of both MAS and a π-pulse train are effective in removing the residual 29Si heteronuclear broadenings, with a near-linear relationship between FWHM and MAS rotor period and π-pulse spacing. Based on these results, we conclude that accurate 29Si J coupling measurements will be the most challenging in lithium silicate glasses due to strong homonuclear dipolar couplings among 7Li nuclei, requiring MAS speeds up to 100 kHz, and be the least challenging in the alkaline earth silicate glasses. At a modest MAS speed of 14kHz, distributions of geminal J couplings across Si-O-Si linkages were measured in alkali and alkaline earth silicate glasses giving mean values of 4.2Hz and 5.1Hz in 0.4 CaO·0.6 SiO2 and 0.33 Ba2O·0.67 SiO2 glasses, respectively, and 5.2Hz and 5.3Hz in 0.33 Na2O·0.67 SiO2 and 0.33 K2O·0.67 SiO2 glasses, respectively. We also observe greater variance in the J distributions of alkaline earth silicate glasses consistent with greater structural disorder due to increased modifier cation potential, i.e., the charge-to-radius ratio, Z/r of the cation.

Evolution of Carbonatite Magmas in the Upper Mantle and Crust

Carbonatites are the most silica-poor magmas known and are amongst Earth’s most enigmatic igneous rocks. They crystallise to rocks dominated by the carbonate minerals calcite and dolomite. We review models for carbonatite petrogenesis, including direct partial melting of mantle lithologies, exsolution from silica-undersaturated alkali silicate melts, or direct fractionation of carbonated silicate melts to carbonate-rich residual melts. We also briefly discuss carbonatite–mantle wall-rock reactions and other processes at mid-to upper crustal depths, including fenitisation, overprinting by carbohydrothermal fluids, and reaction between carbonatite melt and crustal lithologies.

Highly improved mechanical and thermal properties of alkali silicate and graphene nanoplatelet composite adhesives

Inorganic adhesives have received increased attention due to their high temperature resistance, non-toxicity, and environment friendliness. However, poor adhesion strength of inorganic adhesives hinders their application. Further, graphene improves the mechanical and thermal properties of polymer composites. Herein, graphene nanoplatelets (GNP) with different specific surface areas were added into a mixed silicate (MS) containing different ratios of alkali silicates. The results revealed that lap shear strengths increased from 7.4 MPa with the unmodified MS to 9.42 MPa with the MS/GNP 750_10% composite. In addition, the GNP powder with the highest surface area (MS/GNP 750_10% composite) formed maximum interfaces with the silicate matrix, leading to effective load transfer and reduction in stress concentration sites. Hardness and modulus increased as the GNP surface area increased; unmodified MS had the highest values because GNP changed MS from brittle to ductile. In addition, at room temperature, thermal conductivity increased by 60% in the vertical direction and 173% in the horizontal direction in the MS/GNP 750_10% composite compared to the unmodified MS. Furthermore, at a higher temperature (100 °C), the thermal conductivity of the MS/GNP 750_10% composite increased by 273% in the vertical direction and 412% in the horizontal direction. • This study investigated the effect of the surface area of GNP powder on the properties of Alkali silicate based adhesives. • The lap shear strength increased from 7.4 MPa in the bare MS to 9.42 MPa in the MS/GNP 750_10%. • The thermal conductivity increased by 60% in the vertical and 173% in the horizontal direction in the MS/GNP 750_10%.

Vacuum crack growth in alkali silicate glasses

Crack growth velocity in alkali silicate glasses was measured in vacuum across 10 orders of magnitude with double cantilever beam technique. Measured and literature crack growth data were compared with calculated intrinsic fracture toughness data obtained from Young's moduli and the theoretical fracture surface energy estimated from chemical bond energies. Data analysis reveals significant deviations from this intrinsic brittle fracture behavior. These deviations do not follow simple compositional trends. Two opposing processes may explain this finding: a decrease in the apparent fracture surface energy due to stress-induced chemical changes at the crack tip and its increase due to energy dissipation during fracture.

Durability Behavior of Mortars Containing Perlite Tailings: Alkali–Silicate Reaction Viewpoint

Tailing incorporation into mortars has been the subject of much research in recent years. Despite this, most of these studies did not investigate the harmful effects resulting from the exposure of such mortars to an environment containing aggressive agents. This work investigated the effects of perlite tailing addition into mortars containing cement CP V-ARI MAX and hydrated lime. The raw materials were subjected to chemical characterization (X-ray fluorescence (XRF)) and mineralogical (X-ray diffraction (XRD)), while the samples immersed in 1 N NaOH solution were characterized by XRD, thermogravimetric analysis (TGA), scanning electron microscopy (SEM), and compression strength (CS). The results showed the harmful effects of incorporating perlite tailings into the mortar investigated. Such a degradation was proven by linear expansion and compressive strength experiments accomplished in the samples after the test of resistance to an alkali–silicate reaction.

Efficient Concentration of PB From Water by Reactions With Layered Alkali Silicates, Magadiite and Octosilicate

AbstractHuman health problems are often related to contamination of the aqueous environment by toxic metal ions. In the present study, two layered alkali silicates (magadiite and octosilicate) were examined to assess removal of Pb2+ from aqueous solutions in terms of quantity and kinetics. The ion-exchange reaction between the silicates and aqueous solutions of lead(II) acetate at various concentrations was examined at room temperature for 24 h. The adsorption isotherms were H-type, showing the strong interactions between Pb2+ and the silicates. The amounts of Pb2+ adsorbed were as much as 1.23 mmol Pb/g magadiite and 2.32 mmol Pb/g octosilicate, which are larger than the reported values for various ion exchangers. They were larger than the theoretical cation exchange capacities (2.2 and 3.7 meq/g for magadiite and octosilicate, respectively), suggesting that the collection of Pb2+ included the precipitation as basic lead salts in addition to the ion exchange. The adsorption isotherms for magadiite and octosilicate fitted the Langmuir equation with correlation coefficients, R2, of 0.9991 and 0.9972, respectively. The adsorption of Pb2+ onto the layered alkali silicates from acidic aqueous solution was examined to obtain smaller amounts of adsorbed Pb2+ (0.32 mmol Pb/g magadiite and 0.34 mmol Pb/g octosilicate), confirming the important role of pH on the surface charge of the layered silicates in terms of ion exchange. The adsorption of Pb2+ reached equilibrium within 5 min for magadiite while it took 60 min for octosilicate. The difference was in the particle morphology; smoother diffusion of Pb2+ was possible through flower-shaped aggregates of particles of magadiite.

Hydrogen isotope fractionation inside silicate melts and glasses studied by 1H and 2H MAS NMR spectroscopy – Molecular insights into deuterium exchange at the melt-fluid interface

Hydrogen isotope ratios (D/H) measured on geological samples are used to trace the pathways of water in magmatic and hydrothermal systems. Interpreting respective isotope data relies thereby on the theoretical and empirical constraints on hydrogen isotope fractionation. This study revisits a recently discovered hydrogen isotope fractionation effect between alkali-rich and alkali-poor regions within hydrous alkali silicate melts (Wang et al., 2015). In a series of experiments conducted at variable P-T-XH2O, D2O conditions, we studied this intramolecular isotope effect by 1H and 2H solid-state nuclear magnetic resonance (NMR) spectroscopy on quenched hydrous sodium tetrasilicate melts (Na2O⋅4SiO2 with 1:1 H2O and D2O).The results indicate a marked difference in the reactivity of the protonated and deuterated hydrous species. It is observed that c. 70% of deuterium concentrates as deuterated silanols (Si-OD) in proximity to the sodium cations in the silicate melt structure, while the remaining 30% occupy the Na-poor regions in the silicate melt in the form of Si-OD and presumably molecular water (HDOm). This distribution remains nearly constant under all experimental conditions. In contrast, the concentration of protonated hydrous species (Si-OH, H2Om) in the Na-rich and -poor regions of the silicate melt varies as a response to changing experimental P-T-XH2O and cooling rate, which is linked to common water speciation reactions taking place within the melt (Si-O-Si + H2Om = 2Si-OH). Previously considered insignificant, 1H NMR indicates that the speciation equilibrium is sensitive to pressure and correlates positively with Si-OH concentration. The significantly different reactivity of the isotope substituted hydrous species results in large molecular variations in the D/H ratio inside the sodium silicate melt. Measured intramolecular hydrogen isotope fractionation yields a 2- to 9-fold (in average 3-fold) enrichment of deuterium in Na-rich depolymerized regions in the silicate network, which translates to intramolecular fractionations of 1000ln(α) = 400 to 2200. Generally, intramolecular fractionation tends to increase when the melt evolves towards fluid-saturated conditions. The pre-concentration of deuterium in the depolymerized Na-rich regions of the silicate melt results in a higher local D/H ratio that may carry over when these regions disintegrate into a silicate saturated aqueous fluid. In this way, intramolecular isotope fractionation poses a potent way of enriching silicate saturated aqueous fluids in deuterium, potentially explaining previous experimental in-situ observations of large hydrogen isotope fractionations in the hydrous-melt – silicate-saturated fluid system. The observed affinity of deuterium to combine with alkali-associated silanols could comprise an important molecular sieving mechanism for deuterium that enhances the D/H fractionation between silicate-rich fluids (and hydrous melts) and the surrounding host rock. Consequently, intramolecular hydrogen isotope fractionation in hydrous silicate melts and silicate-rich aqueous fluids may be a very effective deuterium filter acting in the deep geological water cycle.

Highly Luminescent Inorganic-Organic Hybrids with Molecularly Dispersed Perylene.

A highly photoluminescent material was obtained by the incorporation of perylene into an inorganic-organic hybrid film. Octosilicate, a layered alkali silicate, was modified with a cationic surfactant, dioleyldimethylammonium ion, to accommodate perylene molecularly and uniformly. The perylene-doped dioleyldimethylammonium octosilicate films were fabricated by simply casting the toluene solution of perylene with dispersed dioleyldimethylammonium octosilicate on substrates. Near-unity photoluminescence quantum efficiency was achieved for hybrids containing a high concentration of perylene.

Investigation of biomass alkali release in a dual circulating fluidized bed chemical looping combustion system

Chemical looping combustion (CLC) of biomass is a promising technology for power generation with integrated carbon capture. In CLC, alkali content of biomass poses potential issues of bed agglomeration, as well as heat exchanger fouling and corrosion. The fate of biomass alkalis was investigated in a dual-interconnected circulating fluidized bed CLC system. Experiments were conducted in oxygen carrier aided combustion (OCAC) and CLC modes. Ilmenite and braunite oxygen carriers and three biomass fuels (wood pellets, wood char, straw pellets) were tested. Flue gas alkali emissions in the air reactor (AR) and fuel reactor (FR) were measured with a surface ionization detector (SID). Results showed that CLC operation yields gas-phase alkali emissions that are up to 15 times higher than in comparable OCAC operation. Results analysis concluded that increased alkali emissions in CLC arise from the steam atmosphere in the FR, whereby steam accelerates the decomposition of alkali compounds in the biomass. Retention of alkalis in the condensed phase was found to be >97% for ilmenite and >92% for braunite CLC operation. Up to 60–80% of the retention was attributed to fuel ash formation. The residual retention was attributed to absorption of alkalis by the oxygen carriers. Absorption likely occurred mainly through formation of alkali manganates and silicates in braunite, and formation of alkali silicates, aluminosilicates, manganates, and titanates in ilmenite. Gas-phase alkali emissions in the AR, although less than in the FR, were found to occur due to combustion of unconverted fuel carried over from the FR to the AR.

Exfoliation of hexagonal boron nitride nanosheets in low-modulus concentrated alkali silicate pastes

We herein demonstrated the exfoliation of hexagonal boron nitride (h-BN) into nanosheets (BNNSs) by using concentrated low-modulus water glass (WG, SiO2/Na2O ratio = 1.6) as high viscosity exfoliating medium. The process led to BNNSs yield of nearly 30%. As the sheared suspension was neutralized with HCl, BNNSs with lateral size of approximately 250 nm and narrow size distribution, were found to align into perfectly parallel rows. High-resolution transmission electron microscopy (HR-TEM) analysis confirmed how WG preserved the integrity of the exfoliated BNNS in comparison with an analogous route with sole hydroxide, resulting in a high amount of BNNSs with undamaged structure.