Alkaline Sodium Silicate Solution Research Articles

Formation and Dynamic Behavior of Macroscopic Aluminum-Based Silica Gardens.

Chemical gardens are self-assembled structures with intricate plant-like morphologies and consist of mineralized membranes, which form spontaneously at interfaces between compartments with dissimilar chemical composition, most typically acidic metal salt and alkaline sodium silicate solutions. While this phenomenon is thought to occur in a number of practical settings, it has also proven to be valuable for investigating transport characteristics in distinct applied systems. For example, coupled diffusion and precipitation processes were monitored in silica gardens based on calcium and iron salts, considered to be models for cement hydration and steel corrosion, respectively. Here we extend these studies to the case of aluminum-based silica gardens, one of the so far less frequently investigated examples of silica gardens. To this end, single macroscopic tubes were prepared in a reproducible way by the controlled addition of sodium silicate solution to a pellet of pressed aluminum nitrate. Continued sampling of the volumes enclosed by and surrounding the formed membraneous structure allowed the time-dependent development of ion concentration gradients to be tracked over extended periods of time, while both the pH and electrochemical potential differences across the membrane were recorded online by immersed probes. The dynamic behavior revealed in this way was finally complemented by ex-situ analyses of the composition of the formed tubes. The collected data shows that the as-prepared tubular structures consist of sodium aluminosilicate phases with certain similarities to zeolites and geopolymers. The emerging tube wall was further found to be permeable to all ionic species present in the system, allowing significant electrochemical potential to be sustained over tens of hours until diffusion had eventually diminished the initially generated gradients. The findings of this work may have important implications for the geochemical fate of natural aluminosilicate sources, the use of such geopolymers in construction applications, and the synthesis and properties of zeolites.

Development of reactive-transport models simulating the formation of a silica gel barrier under CO2 storage conditions

Geological storage of carbon dioxide (CO2) is a means of reducing greenhouse gas emissions. Key to the successful implementation of this technology is the ability to prevent CO2 leakage from the storage reservoir. One promising leakage abatement technology involves forming a geochemical barrier through reaction of an alkaline sodium silicate solution with CO2-enriched water or supercritical CO2. To investigate the feasibility of this technology at the field scale, three reactive-transport models were developed. The first simulations replicated previous laboratory studies (Castañeda-Herrera et al., 2018a), the second and third predicted the formation of a radial shield around the well and an impermeable silica gel layer serving as a flow diverter above the CO2 injection point. Good agreement with the experimental results was observed. Results also show the pH-dependent silica gel formation and its location deep in the reservoir strongly depend on the rate and the sequence of reagent injection as well as their efficient mixing in the reservoir. An alternative field application of the silica gel barrier is proposed to enhance capillary trapping beneath a lateral flow diverter. The application of silica gel barriers could make CO2 sequestration both safer and more efficient, enhancing further uptake of carbon capture and storage.

ИССЛЕДОВАНИЕ ВЛИЯНИЯ ТОЛЩИНЫ СЛОЯ КОМПОЗИЦИОННЫХ ПОКРЫТИЙ НА ОСНОВЕ СУЛЬФАТА БАРИЯ НА ОСЛАБЛЕНИЕ РЕНТГЕНОВСКОГО ИЗЛУЧЕНИЯ

Purpose: Defining the regularities in X-ray radiation attenuation, which characterize composite coatings based on barium sulfate and the aqueous alkaline sodium silicate solution, depending on the thickness of the layer of such coatings deposition. Material and methods: The experiment consisted in the manufacture of experimental samples of composite coatings based on barium sulfate and the aqueous alkaline solution of sodium silicate and testing of the manufactured experimental samples. Tests of the manufactured experimental samples of composite coatings were carried out using UPR-AT300 calibration X-ray unit, DKS-AT5350 precision dosimeter in combination with a TM23361 ionization chamber (PTW Freiburg), attorney at Mendeleev All-Russian Research Institute of Metrology (Russia) on the state standards of the Russian Federation. Results: It has been established that by increasing from 0.5 mm to 2.0 mm of the thickness of the deposition layer of these coatings, it is possible to ensure an increase of 2.0‒3.5 times of X-ray attenuation ratio values in the range of average energies 30.0‒60,0 keV. Conclusion: The obtained results are recommended for use in solving problems related to the shielding of premises (X-ray rooms) or areas of premises (inspection zones at airports) in which X-ray sources are located.

Factors Affecting Alkali Activation of Laterite Acid Leaching Residues

In this experimental study, the alkali activation of acid leaching residues using a mixture of sodium hydroxide (NaOH) and alkaline sodium silicate solution (Na2SiO3) as activators is investigated. The residues were also calcined at 800 and 1000 °C for 2 h or mixed with metakaolin (MK) in order to increase their reactivity. The effect of several parameters, namely the H2O/Na2O and SiO2/Na2O ratios present in the activating solution, the pre–curing time (4–24 h), the curing temperature (40–80 °C), the curing time (24 or 48 h), and the ageing period (7–28 days) on the properties of the produced alkali activated materials (AAMs), including compressive strength, porosity, water absorption, and density, was explored. Analytical techniques, namely X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), and elemental mapping analysis were used for the identification of the morphology and structure of the final products. The experimental results show that the laterite acid leaching residues cannot be alkali activated in an unaltered state, and the compressive strength of the produced AAMs barely reaches 1.4 MPa, while the mixing of the residues with 10 wt% metakaolin results in noticeably higher compressive strength (41 MPa). Moreover, the calcination of residues at 800 and 1000 °C has practically no beneficial effect on alkali activation. Alkali activated materials produced under the optimum synthesis conditions were subjected to high temperature firing for 2 h and immersed in distilled water or acidic solution (1 mol L−1 HCl) for 7 and 30 days in order to assess their structural integrity under different environmental conditions. This study explores the potential of alkali activation of laterite leaching residues amended with the addition of metakaolin for the production of AAMS that can be used as binders or in several construction applications in order to enable their valorization and also improve the environmental sustainability of the metallurgical sector.

Formation of an amorphous silica gel barrier under CO2 storage conditions

Risk assessments of Carbon Capture and Storage (CCS) have identified that in some instances carbon dioxide (CO2) leakage through the caprock cannot be entirely precluded, e.g. through high permeability undetected zones. This study recommends the use of a geochemical barrier that forms upon reaction with CO2, as a means to mitigate and remediate CO2 leakage. The proposed technology is based on the injection of an alkaline sodium silicate solution that reacts with the leaking CO2, leading to silica gel formation. Laboratory studies undertaken to evaluate this technology, included flow-through and core-flood experiments at ambient and reservoir conditions, respectively. The tests aim was to assess the barrier formation performance by changes in pressure as an indicator of permeability reduction. Results show that the formation of the silica barrier was controlled by the mixing gradient of the two reactants, where the reaction resulted in a permeability reduction between one and three orders of magnitude under reservoir conditions. Thus, using sodium silicate as a reagent for forming a barrier is a promising technology to abate CO2 leakage for CCS purposes. Further research using reactive transport modelling to investigate barrier formation in a reservoir model is needed before applying this technology at the field scale.

Inorganic polymers from laterite using activation with phosphoric acid and alkaline sodium silicate solution: Mechanical and microstructural properties

Geopolymers from laterite, an iron-rich soil available in developing countries, have great potential as building materials. In this work, laterite from Togo (Africa) was used to prepare geopolymers using both phosphoric acid and alkaline sodium silicate solution. Microstructural properties were investigated by scanning electron microscopy, X-ray powder diffraction and mercury porosimetry, whereas thermal properties were evaluated by thermal analyses. The local environment of iron was studied by X-ray Absorption Spectroscopy (XANES region). The mechanical properties were determined. Modulus of Rupture and Young's modulus fell in the ranges 3.3–4.5MPa and 12–33GPa, respectively, rendering the materials good candidates for construction purposes. Heating above 900°C results in weight-gain, presumably due to iron redox reactions. X-ray Absorption Spectroscopy data evidence changes in the chemical and structural environments of iron following thermal treatment of geopolymers. These changes indicate interaction between the geopolymer structure and iron during heating, possibly leading to redox properties.

Silica/alkali ratio dependence of the microscopic structure of sodium silicate solutions

Alkaline sodium silicate solutions with SiO2:Na2O molar ratios in the range 4–10 are known to be colloidally unstable manifested in phase separation or gelation. The mechanistic understanding of this instability is generally poor. To improve this situation the microscopic structure of a series of solutions with ratios in the range 3.3–8.9 has been characterised using small-angle X-ray scattering, Dynamic light scattering, Fourier transformed infrared spectroscopy, and 29Si Nuclear magnetic resonance spectroscopy to cover the relevant length scales related to silica clusters, aggregates, and particles present. In the starting solution, with ratio 3.3, there are silica present in three fractions. The main part is present as small silica clusters with a radius of 0.7nm. There are also a significant portion of monomers/small oligomeric silica species as well as a minute amount of larger colloidal silica particles. At a higher SiO2:Na2O ratio, above approximately 4, smaller spherical colloidal particles are formed due to condensation reactions. However, as a result of a too high ionic strength the suspension is not stable and the particles aggregate to fractal structures with a size that depends on ratio and ageing time. At the highest SiO2:Na2O ratio, fractals are not formed because of the lower ionic strength and the smaller colloidal particles are stable in the solution. By carefully adding small amounts of NaCl to the high ratio solution it is possible to induce gelation of the solution confirming the hypothesis that the instability region is due to too high electrolyte concentration for the silica species present under those conditions.

Concentration- and pH-dependence of highly alkaline sodium silicate solutions

In this study two routes for the gelation of water glass have been investigated; the destabilization by a change in pH and by an increase in concentration through evaporation. Both methods produce optically transparent, highly viscous, homogeneous solutions. The structure and dynamics of the solutions along the two routes have been investigated with dynamic light scattering, 29Si-Nuclear Magnetic Resonance spectroscopy, viscosity measurements and infrared spectroscopy. We find that the two routes are fundamentally different. Increasing the concentration of the sodium silicate system leaves the silica speciation apparently unchanged. Lowering the pH leads to condensation reactions, thus a change in the silica speciation.

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.

Geopolymers from Algerian metakaolin. Influence of secondary minerals

The influence of secondary phases (illite, quartz) on the geopolymerization reaction of metakaolin has been investigated by comparing two metakaolins, one prepared from a pure kaolinite and the other from illite- and quartz-containing Algerian kaolin from the Tamazert region, respectively. Geopolymerization was achieved by mixing the metakaolins with an alkaline sodium silicate solution at room temperature and curing at 50 °C. The products were characterized by X-ray diffraction and 29Si and 27Al MAS-NMR. The results show that the secondary phases, at the concentration used in this work, do not prevent the geopolymerization reaction.

Low-temperature synthesized aluminosilicate glasses

The reaction below 100 °C of a dehydroxylated clay (metakaolinite: (Al2O3)(SiO2)2(H2O)0.05) suspended in an alkaline sodium silicate solution ((Na2O)(SiO2)1.4(H2O)x) leads to an amorphous glassy aluminosilicate, called in this work “low-temperature inorganic polymer glass” (LTIPG or IPG).

Low-temperature synthesized aluminosilicate glasses

The reaction below 100 °C of a dehydroxylated clay (metakaolinite) suspended in an alkaline sodium silicate solution leads to an amorphous aluminosilicate, called low-temperature inorganic polymer glass (LTIPG or IPG). Some rheological transformations during the isothermal hardening process are followed with dynamic mechanical analysis (DMA) and compared with differential scanning calorimetry (DSC) and modulated differential scanning calorimetry (MDSC). It can be concluded that the change in storage modulus (DMA) during the formation of the inorganic network can be characterized quantitatively with the evolution of the heat capacity (MDSC), and that the reaction rate is not decreased by the vitrification process. During the first heating after polymerization up to 1000°C, the material shrinks due to the evaporation of residual water from the reaction mixture as illustrated by thermogravimetric analysis (TGA) and thermomechanical analysis (TMA). The low-temperature synthesized inorganic polymer glass is thermomechanically stable up to a temperature of at least 650°C. In that temperature zone, the glass transition can be detected with TMA and DMA.

Quantitative silicon‐29 NMR investigations of highly concentrated high‐ratio sodium silicate solutions

AbstractIn concentrated alkaline sodium silicate solutions a consistent decrease in the amount of silicate monomer Q0, end‐groups Q1, cyclic trimer Q groups and middle groups Q2, and also branching groups in ring formations Q, is observed with increasing molar ratio of SiO2 to Na2O (RM), whereas for branching groups Q3 and cross‐linking groups Q4 a consistent increase is found. The interest of this work is particularly focused on the Q4 region, which may indicate the formation of colloidal silicate particles. For this region the 29Si background signal from the NMR probe and the glass tube can cause problems, so the subtraction technique was used. Spin‐lattice relaxation times, not previously reported for silicate solutions of concentrations and molar ratios as high as those studied here, are given for all structural units, including Q4. It was found that sodium silicate solutions of RM = 2.0 do contain a Q4 region when they are extremely highly concentrated in SiO2 (38 wt% SiO2) or aged.