Sodium Aluminate Research Articles

Reaction Behavior of Kaolinite in Sulfur-Bearing Sodium Aluminate Solution under the Simulated Bayer Process

Over a billion tons of high-sulfur bauxite has not been utilized effectively currently in China, because the pyrite existing in the bauxite poses a range of hazards during the Bayer process. A novel idea was proposed to remove sulfur by the silicon-containing minerals in bauxite reacting with sulfur species in sodium aluminate solution to form sulfur-bearing desilication products (SDSP) for discharge with the red mud in the Bayer process. This study investigated the reaction behavior between kaolinite and different sulfur-containing ions under the simulated Bayer process conditions, elucidating the desulfurization rate variation and formation mechanism of SDSPs. The thermodynamic calculations suggest that the reaction between kaolinite and sulfur-bearing sodium aluminate solution to form SDSPs can occur spontaneously. The experimental results demonstrated that various SDSPs can be produced through the reaction of kaolinite and sulfur-containing ions in sodium aluminate solution during the simulated Bayer process, resulting in various desulfurization efficiencies, while the desulfurization process will not result in additional alkali consumption. Increasing the kaolinite dosage, extending the reaction time, and elevating the reaction temperature all contribute positively to enhancing desulfurization efficiency. Kaolinite reacted with S2O32− in sodium aluminate solution to generate Na8Al6Si6O24S2O3·2H2O, achieving a desulfurization rate exceeding 90% under optimized conditions. Under the simulated Bayer digestion process conditions at elevated temperature, the desulfurization rates of kaolinite ranked in ascending order as S2− < SO32− < SO42− < S2O32−. Kaolinite reacted with SO42− and S2O32− to form cancrinite type SDSPs, and a superior desulfurization rate can be achieved. This work can provide a theoretical foundation and technological support for the efficient utilization of high-sulfur bauxite by the Bayer process.

Four-stage chemical treatment of spent pot lining carbon for titanium oxide doped high performance supercapacitor electrode material

Obtaining carbon materials in a sustainable way and applying them in energy storage systems is attracting more attention. Spent potlining (SPL) which is a carbon-rich waste produced in electrolytic cells of aluminium smelters can be a potential source of carbon for engineering applications. However, SPL is hazardous in nature and needs to undergo chemical treatment for most engineering applications. In this study, a four-step procedure was adopted to remove sodium fluoride (NaF), cryolite (Na3AlF6), alumina (Al2O3), calcium fluoride (CaF2), sodium aluminate (NaAl11O17) and silica (SiO2) from the SPL. The adopted treatment processes proved effective as Energy Dispersive X-ray (EDX) and X-ray Diffraction (XRD) results showed no traces of the major impurities after the treatment. The treated carbon-rich SPL was activated using NaOH to obtain activated SPL (ASPL). The ASPL was doped with TiO2 to form ASPL-TiO2 (1:1), ASPL-TiO2 (3:1) and ASPL-TiO2 (2:1) electrodes. The electrode materials were tested using cyclic voltammetry, galvanostatic charge-discharge, and electrochemical impedance spectroscopy. The ASPL, ASPL-TiO2 (1:1), ASPL-TiO2 (3:1) and ASPL-TiO2 (2:1) demonstrated specific capacitance of 180.2 F/g, 279.1 F/g, 536.7 F/g, and 203.2 F/g at 10 A/g respectively with high stability over 1000 cycles. The present study was designed to determine the suitability of using chemically treated SPL as an alternative source of carbon for supercapacitor applications and this was achieved as the material exhibited an improved charge storage capacity.

From dolomite waste to katoite-based binder: Synthesis, performance and characterization

Calcium aluminate cement, in which katoite (KT) is the main binder, has garnered significant attention for its resistivity against aggressive environments. Despite its impressive durability, the production of this cement is costly and generates a substantial CO2 emission. Therefore, the objective of this study was to develop an eco-friendly KT-based binder using exchange reaction between sodium aluminate (NaAlO2) solution and dolomite waste powder. The performance of KT-binder was evaluated by measuring compressive strength, drying shrinkage, bulk density, and porosity. The phase composition and microstructural changes were also investigated by XRD, TG/DTG, FT-IR, and SEM/EDS techniques. The resulting hardened materials exhibited 28-day compressive strengths ranging from 5 MPa to 50 MPa and contained primarily katoite Ca3Al2(OH)12, along with pirssonite Na2CO3·CaCO3·2H2O, thermonatrite Na2CO3·H2O, and aluminum hydroxide Al(OH)3 phases. EDS mapping indicated that only a small amount of Mg2+ was present in the katoite phase, suggesting that the alkalinity and atomic radius difference between Ca2+ and Mg2+ were the main factors. Altering the content of concrete waste (CoW) in the cementitious system within the range from 10 to 50 wt% leads to an improvement in compressive strength and a reduction in both pore volume and drying shrinkage of NaAlO2-DW mixture. Conversely, perlite waste (PW) shows an opposite trend despite its high amorphicity. The presence of highly water-soluble thermonatrite, which is a byproduct of the cationic exchange reaction, is responsible for the low water resistivity of the prepared binder. As a result, more research is needed to tackle this problem.

Isothermal foaming-nucleation-crystallization of glass ceramic foams with hierarchical pore structure: A sustainable approach for disposal of secondary aluminum ash

Secondary aluminum ash (SAA) is a hazardous solid waste generated in the process of recycling aluminum. The preparation of glass ceramic foams from high content of SAA, used as nucleating and foaming agent, was proposed to achieve green and comprehensive utilization of SAA. With the help of thermodynamic phase diagram and viscosity calculation, a mixture of SAA and waste glass (WG) were transformed, at 1200 °C for 30 min, into glass ceramic foams with porosity of 45.49%–59.45%, respectively. The mean pore size of the samples decreased from 2.89 to 1.53 mm with the decrease of SAA addition. Through the adjustment of temperature, it was found that the mixture had a microporous region at 1100 °C. The expansion region was observed at 1180 °C and 1200 °C, because of the oxidation of AlN in SAA. The porosity of the samples could be raised to 64.51%, with 5–9% B2O3 as a supplement, which contributed to the development of the liquid phase and a decrease in the melt's viscosity. The mechanism of in-situ isothermal foaming, nucleation, and crystallization of SAA and WG was clarified. The crystal intensity dropped and the amorphous phase became the primary phase at 800 °C. Al2O3, sodium silicate and sodium aluminum silicate precipitated at 1000 °C. Al2O3 and spinel with high melting point were wrapped by silicate. Silicate turned into the melt again as the temperature continued to rise. With the further reduction of viscosity, the gas diffused in crystalline and amorphous phases, and finally forms glass ceramic foams with hierarchical pore structure.

Recovery of iron and aluminum from iron-rich bauxite residue by an integrated phase reconstruction approach

The comprehensive recovery of iron and aluminum from iron-rich bauxite residue (IRBR) is of critical importance both in terms of resource utilization and environment protection, which, however, is challenging due to the intertwined phases between Iron and aluminum. In this study, an integrated phase reconstruction approach, consisting of alkali roasting, two-stage column leaching, and carbonation decomposition, was proposed for Fe/Al recovery from IRBR. The results demonstrated that aluminum and sodium were fused into soluble substances such as sodium aluminate (Na7Al3O8, NaAlO2, and Na2O (Al2O3)11) in the alkali roasting process, allowing for the separation of Al and Fe in the subsequent leaching process. Following water/FeCl3 solution leaching, the removal efficiency of aluminum reached 84.66%, and Fe content in the residue could be enriched to 55.56%. Fe can be recycled as iron concentrate, and Al in the leaching solution with 75.95 g/L can be recovered in the form of Al(OH)3 through carbonation decomposition. This work provides an alternative strategy for the recovery of resources from IRBR, with potential implications for the sustainable development of the aluminum industry.

Green Synthesis of Sodium Aluminate Nanophosphor and Study of Its Photoluminescence Properties

Sodium Aluminate (NaAlO2) nanophosphor was prepared from analytical grade Sodium Nitrate (NaNO3) and Aluminium Nitrate (Al(NO3)3.9H2O) by solution combustion method at 450° C using honey as a fuel. The prepared sample was characterised by X-ray diffraction technique (XRD), UV-Visible and Photoluminescence (PL). The X-ray diffraction analysis was carried out using Bruker AXS D8 Advance X-ray diffractometer with Cu-Target (Cu/Kα radiation of wavelength λ =1.54060Å). Data have been collected by step scanning 2θ from 10° to 90° at room temperature. The crystalline structure of the resulting sample was investigated.

Zeolite-Like Warm-Mix Asphalt Additive: Dosage Optimization Based on Asphalt Binder Characterization

Abstract The major objective of this research was to augment the temperature reduction characteristics of the recently developed sodium aluminate tetraethyl ammonium hydroxide montmorillonite (STEAM) warm-mix asphalt (WMA) additive by optimizing 8 dosages of 20 STEAM variants mixed with one viscosity graded-30 base asphalt binder. As the first step in the optimization process, a series of molar combinations of sodium aluminate and tetraethyl ammonium hydroxide were blended with constant concentrations of weights of montmorillonite (MMT) and water. The Fourier transform infrared (FTIR) spectroscopy test was performed on all the 20 STEAM variants to derive the hydroxyl functional groups and understand if they have potential in reducing the mixing and compaction temperatures of the STEAM-modified asphalt binder. The FTIR results indeed indicated that hydroxyl bonds increased with increasing MMT content, as a consequence of MMT releasing itself when mixed with the base asphalt binder. Next, the STEAM additives were mixed with the base binder at 8 dosages with 2 molar ratios covering 20 STEAM-modified asphalt binders on which penetration, softening point, and rotational viscosity tests were performed mainly to establish the ASTM Ai-VTSi relationships. Depending on the combination of molar ratio and dosages, the STEAM-modified asphalt binders demonstrated a decrease of about 3°C–8°C of mixing and compaction temperatures. Furthermore, the STEAM additive that exhibited highest reduction in mixing temperature was blended with dense-graded aggregate gradation to produce a STEAM-modified asphalt mix, which reduced the production temperature by 10°C based on the equivalent mixture performance criterion while also demonstrating stiffness performance similar to that of a control mix. Overall, the optimized indigenous STEAM WMA additives proved to be water-containing, zeolite-like products that have immense capacity in minimizing the working temperatures, albeit further studies should focus on tapping the full potentiality of several STEAM combinations to render the products as globally acceptable “green” paving materials.

Characteristics of product layer and effects of sulfur-bearing ions on pyrite leaching in sodium aluminate solutions

During the utilization of high-sulfur bauxite by the Bayer process, the pyrite will react with sodium aluminate solution and generate various sulfur-bearing ions, however, the effect of these ions on pyrite leaching is seldom studied. This work clarifies the reaction mechanism of pyrite in various sulfur-bearing sodium aluminate solutions by characterizing electrochemistry and the product layers. The results show that the sulfur leaching efficiency are 48.0, 56.2, 23.1, 90.6 and 58.3 % in sulfur-free, S2-, S2O32-, SO32- and SO42--bearing sodium aluminate solutions at 260 °C for 40 min, respectively. With the disproportionation of S2O32- and accumulation of products, a dense sulfur-rich layer formed on the surface of pyrite retards the diffusion of sulfur into solution. However, the presence of SO32- can catalyze the transformation of pyrite into hematite plates, forming a loose layer which accelerates the diffusion of S2- and S2O32- into solution. In the presence of S2- and SO42-, the formation of easily removed products from surface slightly promote the reaction between pyrite and sodium aluminate solution.

Surface treatment of inorganic modifier for improving carbonation resistance of geopolymers

Geopolymer (GP) is a low-carbon binder with the potential to replace Portland cement in reinforced concrete, however, the risk of corrosion to reinforced steel bars due to its low carbonation resistance must be addressed. This study proposed the use of an inorganic surface modifier, sodium aluminate solution (AN), to improve the carbonation resistance of GP. The effects of AN surface treatment on carbonation depths were investigated for GP mortars using fly ash (FA), ground granulated blast furnace slag (BFS), or their blend as active fillers. Rational AN surface treatment conditions and microstructural changes of treated GPs were also discussed. The obtained results show that AN surface treatment can significantly reduce carbonation rates of FA/BFS blend-based GP, and largely delay the onset of neutralization of heat-cured GP through the reduction of cracks and the formation of Na-gmelinite and gismondine, which refine the surface layer of GP. The surface modification using 40 wt%-AN solution reduced carbonation rate coefficient by about 70% for the FA/BFS blend-based GP cured at 80 ℃, and by above 30% for that cured at 20℃. The larger the FA blending ratio or the smaller the liquid-active filler ratio, the greater the improvement in carbonation resistance of FA/BFS blend-based GPs, caused by the AN surface modification. For the ambient-cured GP, repeated surface treatment is preferred, while for the heat-cured GP, once treatment is enough, and seal curing after the AN surface treatment is recommended regardless of the curing method.

Development of a selective nanoporous Na-zeolite X from Moroccan coal fly ash for anionic dye adsorption and removal

ABSTRACT The hydrothermal treatment of Moroccan coal fly ash (CFA) in a basic medium leads to the selective formation of Na-X zeolite. By fusion with sodium hydroxide in the presence of CFA, the particles are transformed into sodium silicate and sodium aluminate, from which hydrothermal reaction favourably results in the production of a pure Na-X zeolite. A good crystallinity of Na-X zeolite was attained at NaOH:fly ash ratio of 1.2. Their high surface area and pore characteristics make the Na-X zeolite useful for the removal of the model anionic dye RY145 from water. The adsorption efficiency of the synthesised Na-X zeolite was evaluated and compared to raw CFA. Dye adsorption is more significant at acid pH, suggesting that the zeolite surface is positively charged. Kinetic and isotherm models applied to fit the experimental data reveal that the adsorption process is physisorption. It results that zeolite X is an efficient adsorbent for the extraction of dyes.

Preparation of Mesoporous γ-Alumina from Red Mud Waste

In this paper, a novel green and cost-effective synthesis method in this field is presented. Mesoporous γ-alumina was obtained by inorganic in situ crystallization, and sodium aluminate as an aluminum origin was abstracted from red mud. The influence mechanism of the reactant ratio of NaHCO3 to NaAlO2, the reaction temperature and the concentration of the template was systematically investigated. As indicated by the results, the obtained mesoporous γ-alumina possesses extensive specific surface areas, narrow pore size distribution, nearly spherical particle morphology and wormlike structure pores. This green and economical synthesis method can be used for the preparation of alternative mesoporous or various porous materials as well.

Adsorption of methylene blue and metachromasy over analcime zeolites synthesized by using different Al precursors

Pure crystalline Analcime (ANA) zeolite samples were successfully synthesized using various aluminum precursors such as aluminum nitrate (ANA-nit), aluminum sulfate (ANA-sul), aluminum isopropoxide (ANA-isop), sodium aluminate (ANA-sodalu) and aluminum chloride (ANA-chl) to study the influence of various anions on the physicochemical properties of ANA zeolite structure. The physicochemical properties of the samples have been studied using elemental analysis, X-ray diffraction, Fourier-transformed infrared spectroscopy, scanning electron microscope, N2-physisorption, X-ray photon spectroscopy, and diffuse reflectance UV–vis measurements. The synthesized analcime samples exhibited highly crystalline ANA structure; however, the anions present in the Al precursor influenced the crystallite and particle sizes. All the samples possessed a band gap energy of around 3.8 eV. The metachromasy phenomenon of methylene blue (MB) over synthesized Analcime samples was investigated under UV irradiation. The UV–vis absorption spectra of MB solution show a transformation from mono-MB to H-aggregated MB species due to the metachromasy phenomenon occurring over the surface of synthesized analcime samples. The formation of H-aggregated MB species differs for the synthesized ANA samples, which depends on the presence of active species on the ANA surface. The ANA-nit sample exhibited a fast metachromatic phenomenon due to more surface basic sites (e.g., O2−, O−) distributed over different (micro-, meso- and macro-) pores.

Effect of sodium aluminate on chromium spinels oxidation in chromite lime-free roasting process

The reaction between Na2O·Al2O3 and chromium spinels or chromite in the roasting process was systemically investigated to reveal the effects of Na2O·Al2O3 on chromium oxidation. The results of Na2O·Al2O3 roasted with MgO·Cr2O3 illustrate that only about 50% of the chromium in MgO·Cr2O3 can convert into Na2CrO4, while the remaining chromium converts into an Al-bearing spinel Mg(CrAl)O4. Mg(CrAl)O4 is found to be difficult to further react with Na2O·Al2O3. The oxidative roasting of Na2O·Al2O3 with chromite further confirms that Na2O·Al2O3 cannot fully react with some chromium spinels, which reveals the cause of the low chromium oxidation rate in the traditional chromite lime-free roasting process. Based on the experiment results, a strategy for eliminating the impacts of Na2O·Al2O3 in the lime-free roasting process was proposed, in which adding more Na2CO3 for forming Na2O·Al2O3 is considered. The chromium oxidation rate can rise close to 100% with the new strategy. This study contributes to better understanding the chromite lime-free roasting process and provides a new idea for developing the chromate salts manufacturing process more efficiently and cleanly.

Pengolahan Air Sadah Menggunakan Zeolit Buatan Dari Batu Apung Lampung Dan Daun Kerai Payung

This research was conducted to determine the composition and crystal structure of the artificial zeolite produced, as well as the ability of artificial zeolite to reduce calcium and magnesium content in hard water. The zeolite was prepared using the hydrothermal method by mixing sodium silicate from Lampung pumice silica, sodium aluminate from aluminum can waste and charcoal from sunshade umbrellas. The artificial zeolite products were characterized using XRF, XRD, UV-Vis spectrophotometric and ICP-EOS. The results showed that the composition of the artificial zeolite was aluminum, silica, phosphorus, potassium, calcium, titanium, vanadium, chromium, manganum and ferum. The crystal structure of the optimum sample from the UV-Vis spectrophotometric characterization obtained the crystal structure of a natural zeolite type, namely Chabazite. And the artificial zeolite which was tested for adsorption on hard water could reduce the calcium content by 36.57% but could not reduce the magnesium content.

A Review: Progress in Molecular Dynamics Simulation of Portland Cement (Geopolymer)—Based Composites and the Interface between These Matrices and Reinforced Material

Molecular dynamics (MD) is an important method for studying the molecular and atomic scale of cement (geopolymer)-based composites which provides an effective method for the optimal design of cementitious materials. In this paper, the research progress of MD simulation in Portland cement and geopolymer-based materials is discussed in detail, including molecular structure models of calcium silicate hydrate, calcium aluminosilicate hydrate, sodium aluminum silicate hydrate gel, and auxiliary experimental techniques. The basic mechanical properties of calcium silicate hydrate, calcium aluminosilicate hydrate and sodium aluminum silicate hydrate in Portland cement-based materials (CBM) and geopolymer-based materials are reviewed. In addition, the dynamic simulation of the interface between CBM and reinforcement materials such as rebar, synthetic fibers, plant fibers and nanoparticles is also discussed. Through the macroscopic experimental results of cement (geopolymer)-based materials and the performance analysis of an MD microscopic model, MD helps to better explain the macroscopic properties of materials, and can quickly and conveniently analyze the mechanical properties, transport properties and interface properties of composite materials, so as to improve the fine design of cement (geopolymer)-based materials. Existing structural models and force fields are affected by environment and time, and MD simulation shows great differences in application range and characterization ability. It is necessary to further study and reveal the internal mechanism for improving concrete performance through a large number of experiments and MD simulation, and lay a theoretical foundation for preparing the next generation of (super) high-performance concrete.

Comparative Evaluation of the Repair Bond Strength of Dental Resin Composite after Sodium Bicarbonate or Aluminum Oxide Air-Abrasion.

The dental prophylactic cleaning of a damaged resin-based composite (RBC) restoration with sodium bicarbonate can change the surface characteristics and influence the repair bond strength. The purpose of this study was to compare the effect of sodium bicarbonate (SB) and aluminum oxide (AO) surface treatments on the microtensile bond strength (µTBS) of repaired, aged RBC. Bar specimens were prepared from microhybrid RBC and aged in deionized water for 8 weeks. Different surface treatments (AO air-abrasion; SB air-polishing), as well as cleaning (phosphoric acid, PA; ethylene-diamine-tetraacetic-acid, EDTA) and adhesive applications (single bottle etch-and-rinse, ER; universal adhesive, UA), were used prior to the application of the repair RBC. Not aged and aged but not surface treated RBCs were used as positive and negative controls, respectively. The repaired blocks were cut into sticks using a precision grinding machine. The specimens were tested for tensile fracture and the µTBS values were calculated. Surface characteristics were assessed using scanning electron microscopy. AO-PA-UA (62.6 MPa) showed a 20% increase in µTBS compared to the NC (50.2 MPa), which proved to be the most significant. This was followed by SB-EDTA-UA (58.9 MPa) with an increase of 15%. In addition to AO-PA-UA, SB-EDTA-UA could also be a viable alternative in the RBC repair protocol.

PH-dependent Cs leaching from nitrate bearing geopolymers: A combined experimental and model analysis

Geopolymers are considered as promising wasteforms for the immobilization of low-level radioactive waste streams containing 137Cs. This work presents an experimental study and modelling of the pH-dependent leaching of Cs from nitrate bearing geopolymers. Longer curing periods and higher curing temperatures induce the formation of nitrate bearing zeolitic phases (NO3-sodalite and NO3-cancrinite), that exhibit higher Cs/Na selectivity relative to nitrate-free zeolitic phases (zeolite A and zeolite X). Such phase formation was found to have a significant effect on the pH-dependent leaching of Cs, as well as on the leaching of other major elements in the geopolymeric system. An innovative, semi-empirical geochemical thermodynamic model was developed for the sodium aluminum silicate hydrate (NASH) solid solution, based on the pH-dependent leaching experimental data. The model succeeded to describe most of the studied system, with emphasis on their interaction with Cs.

TiO2- geopolymer based novel corrosion protective micro-coatings to emaciate mild steel oxidation in severe environments

Now-a-days, cementitious geopolymer coatings are one of the most studied and adaptable coating systems to combat corrosion on low-carbon steel/structural steel. Due to its green and sustainable nature, it has enormous potential to partially replace traditional organic coatings in terms of corrosion resistant properties and economic aspects. Herein, corrosion protection and antibacterial efficiency of titanium oxide (micro-sized, 5/10/15%) containing sodium aluminosilicate (N-A-S-H) geopolymer coating for mild steel is reported to enhance the lifetime of structural steel. Developed coatings were characterized using XRD, FTIR and FESEM. Characterization results indicated the presence of titanium containing new inorganic phase (Sodium titanium oxide hydrate) along with sodium aluminium hydrate phases in XRD, supported by Ti-O stretching vibrations in FTIR along with Si-O-Al stretching vibrations. Microstructural studies using FESEM indicated the uniform distribution of TiO2 in NA-S-H gel matrix. Results revealed that developed coatings depicted corrosion protective characteristics and exhibited adhesion strength between 16.14 and 17.3 MPa. The scratch resistance test revealed that the coating could bear a point load of greater than 5 kg. Minimum percentage weight change and adhesion loss were observed for coating after immersion in NaCl/H2O for 119 days. An accelerated corrosion test (ASTM B117) was performed in the salt environment for 500 h and the best corrosion protective performance was observed for coating formulation containing 10 % TiO2 with maximum rust grading 5. Electrochemical studies showed better corrosion resistance performance for coating formulation containing 10 % TiO2 as compared to the other two formulations containing 5 and 15% TiO2. The antibacterial efficiency of coatings was evaluated by colony count assay (viability method) against the most commonly found gram-positive bacteria Staphylococcus aureus and one gram-negative bacteria Escherichia coli. All three TiO2-incorporated geopolymer coating formulations revealed a good bacterial reduction in percentage with the best performance observed for coating formulation containing 15% TiO2. The accelerated weathering test (ASTM G154-500 h) was performed to ensure the durability and stability of the developed formation. This study developed a novel, environment-friendly, and lucrative cementitious coating material which is potentially suitable for preventing early oxidation and bacterial fouling of mild steel structures.

Influence of main components of accelerators on mechanical property and hydration of Portland cement in a dry-hot geothermal environment

The behavior of accelerators in the shotcrete of tunnel structure is significantly affected by the high geothermal tunnel environment. The objective of this study is to investigate the influence of main components of alkali-free accelerators (aluminum sulfate, AS) and alkaline accelerators (sodium aluminate, NA) on mechanical property and hydration of Portland cement in a dry-hot geothermal environment of 80 ℃ (DH). Setting time test, compressive strength test, XRD, TGA, MIP and SEM were performed on cement systems with AS and NA. Results show that relative to standard curing environment, the cement hydration was promoted and the pore structure at early stage was optimized in DH, thereby significantly accelerating the setting time of cement pastes and improving the early compressive strength development of mortars. In contrast, the later cement hydration was retarded and pore structure was detrimental in DH, resulting in abnormally slow development of later strength and even reduction. Additionally, the stability of hydration products such as ettringite (AFt) and monosulfoaluminate hydrate (AFm) would decline in DH. DH could accelerate the dehydration and decomposition of AFt, and promote the transformation process from AFt to AFm at early stage. After 28 days, AFm would convert into hemicarboaluminate due to carbonation. The addition of AS elevated the hydration degree and optimized the pore structure, especially at later stage, thus exhibiting more stability in long-term strength. Nevertheless, the incorporation of NA would bring about serious deterioration phenomenon in DH, and the compressive strength almost stagnated after 1 day. This work provides a guidance for the real application of accelerators in the shotcrete of tunnel structure in a dry-hot geothermal environment.

Influence mechanisms of CaCO3/NaAlO2 ratios in carbonaluminate cementitious materials

This study investigated the carbonaluminate cementitious materials (i.e., sodium aluminate (NaAlO2)-activated limestone pastes) with gradient calcite (CaCO3)/NaAlO2 molar ratios ranging from 1:1 to 6:1 (samples M1 to M6). NaAlO2 dosage affected the reaction process, hydration product, and cementitious property of the formed pastes. The thermonatrite (Na2CO3·H2O) and AH3 phase as hydration products exist in all the samples. Microcrystalline AH3 phase (crystallite size is near 22 nm) held a superior cementitious property, which was evidenced by diffraction spots scattered on concentric rings in transmission electron microscopy (TEM) results. Other hydrates varied with the CaCO3/NaAlO2 ratio. In case of lower CaCO3/NaAlO2 ratio (i.e., M1), NaAlO2 was incompletely consumed. The residual NaAlO2 promoted the formation of cubic-shaped katoite (3CaO·Al2O3·6H2O). Such hydrate barely contributed to the cementitious property of the formed matrix. When the CaCO3/NaAlO2 ratio reached 1.5:1 (e.g., M1.5-M6), NaAlO2 was entirely consumed, and monocarboaluminate (Mc, 3CaO·Al2O3·CaCO3·11H2O) instead of katoite was preferentially generated. The amounts of total hydrates decreased with the increasing CaCO3/NaAlO2 ratio, leading to the degraded properties of formed matrix. Thus, the 28-day compressive strength of M1.5 was the highest among these samples, reaching 35.55 MPa.