SiO2 H2O Research Articles

Physicochemical processes during solidification and the peculiarities of structure formation in aerated concretes using metallic silicon as a gas generator

The results of research on the structure and phase composition of non-autoclaved aerated concrete with a density of 550–750 kg/m3 using metallic silicon as a gas generator are presented. The peculiarities of the structure formation of aerated concrete products and the mineralogical composition of their hydration products were investigated. It was established that increasing the content of metallic silicon in aerated concrete leads to an increase in the pore space of the compositions. The results of diffractometric and thermal analysis methods for establishing the phase composition of aerated concrete compositions with metallic silicon as a gas generator are also presented. Analysis of XRD patterns and derivatograms showed that the aerated concrete samples under investigation contain a binder component, obermorite (5CaO6SiO25.5H2O); xonotlite (6CaO6SiO2H2O); -dicalcium silicate hydrate (2CaOSiO2H2O); and hillebrandite (2CaOSiO21.17H2O). It was established that increasing the amount of metallic silicon as a gas generator stimulates an increase in the content of hydrated phases in aerated concrete compositions.

Physicochemical processes during solidification and the peculiarities of structure formation in aerated concretes using metallic silicon as a gas generator

The results of research on the structure and phase composition of non-autoclaved aerated concrete with a density of 550–750 kg/m3 using metallic silicon as a gas generator are presented. The peculiarities of the structure formation of aerated concrete products and the mineralogical composition of their hydration products were investigated. It was established that increasing the content of metallic silicon in aerated concrete leads to an increase in the pore space of the compositions. The results of diffractometric and thermal analysis methods for establishing the phase composition of aerated concrete compositions with metallic silicon as a gas generator are also presented. Analysis of XRD patterns and derivatograms showed that the aerated concrete samples under investigation contain a binder component, obermorite (5CaO6SiO25.5H2O); xonotlite (6CaO6SiO2H2O); -dicalcium silicate hydrate (2CaOSiO2H2O); and hillebrandite (2CaOSiO21.17H2O). It was established that increasing the amount of metallic silicon as a gas generator stimulates an increase in the content of hydrated phases in aerated concrete compositions.

Corundum-quartz metastability: the role of silicon diffusion in corundum

The synthesis of the Al2SiO5 polymorphs kyanite, sillimanite and andalusite in a pure Al2O3–SiO2–H2O (ASH) system has long been known to be impeded. In order to decipher individual aspects of the reaction: corundum + SiO2aq, which repeatedly fails to produce thermodynamically stable Al2SiO5, we conducted experiments within the stability fields of kyanite and sillimanite (500–800 ℃; 0.2–1 GPa) with the aim of forming reaction coronas on corundum. Results showed that metastable corundum + quartz assemblages form persistently in pure ASH, even in Al2SiO5 seeded experiments, despite the presence of catalyzing fluid and evidence of fast reaction kinetics. Coronas on corundum spontaneously formed when additional components (Na, K, N, and Mg) were added to the experiment. In a similar experiment with baddeleyite (ZrO2) instead of corundum in silica saturated water, a zircon corona formed readily. This implies that nucleation and growth of Al2SiO5 is obstructed under conditions of Al and Si saturation in aqueous fluid, while both corundum and quartz saturated aqueous fluid are willing participants in other reactions towards stable corona formation. Instead of Al2SiO5 precipitation, an unexpected fluid-aided silica diffusion process into corundum was documented. The latter included the formation of nanometer wide hydrous silicate layers along the basal plane of the corundum host, which enhanced the silica diffusion rate drastically, leading to silica supersaturation in the host mineral, and ultimately to precipitation of quartz inside corundum. We conclude that the natural metastable assemblage of quartz and corundum is not necessarily the result of dry or fluid absent conditions, given that the aqueous fluid in experiments does not promote Al2SiO5 formation, but rather seems to support the formation and preservation of a metastable assemblage.

Long-term performances of hydroceramic systems as a potential cementing material at 240 °C

The performance of the Ca(OH)2–Al2O3–SiO2–H2O hydroceramic system (with and without Al2O3) as a potential well cementing material was investigated from both macroscopic and microscopic perspectives. To simulate the harsh conditions typical of deep wells, marked by high temperature and pressure, the material was cured at 240 °C and 50 MPa. Several hydroceramic systems with varying compositions were designed and a retarder was used to optimize the thickening time of all systems. Finally, the optimized hydroceramic systems were cured for different periods ranging from 2 to 90 days to investigate their long-term stabilities. It is found that the raw materials with finer particle sizes and higher reactivity (such as silica fume and nano-activated alumina) could improve the performance of the hydroceramic system, evidenced by increased compressive strengths and decreased permeability. Changing the Ca/Si molar ratio within the range from 2:1 to 1:1 had little effect on the physical and mechanical properties of the hydroceramic system, despite significant variations in the composition of hydration products. The hydroceramic systems exhibited good stability over a curing period from 2 to 30 days, but strength retrogression phenomenon was observed during longer curing periods from 30 to 90 days. Microscopic evaluations revealed that the strength retrogression could be due to the formation of reyerite in high calcium systems, while it was the conversion of amorphous C–S–H to xonotlite that led to the strength retrogression in low calcium systems.

Mechanochemical activation of waste glass in alkaline composite cements with blastfurnace slag: A sustainable approach to recycling

This study investigates alkali-activated cements incorporating blast furnace slag and urban waste glass across varying slag:glass ratios (100:0, 25:75, 75:25, 0:100). Activation with 4 % and 12%Na2Oeq, coupled with two curing regimes (24h at 60 °C, followed by either 60 °C or 20 °C for 28 days), was explored. Prior to each formulation, part of the waste glass underwent mechanochemical ball milling with the required water and alkalis, to enhance the glass reactivity and to pre-dissolve SiO2 and other species to obtain a waterglass-like material in an environmentally friendly manner. Optimal strengths were achieved for slag binders (27 MPa) with 4%Na2O and 20 °C curing, while waste glass binders (48 MPa) required 12%Na2O and 60 °C. Composite formed cementitious products of CaO-(Al2O3)–SiO2–H2O gel, calcite, and hydrotalcite as reaction products, while 100 % waste glass binders displayed cementitious products of CaO–SiO2–H2O intermixed with silica gel. This approach holds promise for reducing environmental impact and promoting the efficient widespread and straightforward reuse of urban waste glass in construction materials.

Nanoscale insight into the reconstruction reaction mechanism of alkali-activated silicate materials

As a green alternative to cement-based materials, alkali-activated silicate materials (AASM) have garnered attention for their ability to reduce greenhouse gas emissions and efficiently utilize industrial waste. During AASM preparation, Si monomers such as [SiO2(OH)2]2-, [SiO(OH)3]-, Si(OH)4 play a crucial role in governing the formation rate and unique gel structure of AASM. Despite their widespread use in construction, the microscopic mechanism of Si monomer formation from reactive SiO2 groups via reconstruction reactions (RR) remains unclear. This study comprehensively explored the structural transformations, formation pathways, and electron transfer mechanisms of Si monomers in AASM gels during RR using molecular dynamics (MD) simulations and first-principles calculations. We identified six distinct formation pathways for three Si monomers, with SiO2 → SiO2H2O → SiO2(H2O)2 → SiO(OH)2H2O exhibiting the highest energy barrier of 16.61kJ/mol. Further analysis revealed a correlation between energy barriers and orbital hybridization strength, providing new insights into Si monomer formation mechanisms in AASM. This lays a solid foundation for optimizing AASM's preparation and enhancing its performance, further promoting the environmental friendliness and efficient utilization of AASM.

Phase diagram and density of SiO2–H2O fluid across critical conditions

The SiO2–H2O binary system serves as a basis for understanding complex silicate-water systems. In this study, based on limited existing experimental data of solubility, we propose a new thermodynamic model for SiO2–H2O fluid by modifying the traditional non-random two-liquid model with a simplified polymerization reaction. This model is applicable from 773 K to the anhydrous quartz melting temperature and from 0.5 GPa to at least 2 GPa across the critical conditions. It can predict solid–liquid equilibrium and vapor–liquid equilibrium in good agreement with available experiments. The upper critical endpoint of the SiO2–H2O system is predicted to be at ~ 1.14 ± 0.18 GPa and 1344 ± 87 K. With the new model, we obtain a quantitative three-dimensional pressure–temperature–composition phase diagram of the SiO2–H2O fluid, which greatly facilitates the understanding of the complex phase behavior of this binary around the upper critical endpoint. In addition, since the model is based on the Gibbs free energy foundation, we further discuss the derived density variations of SiO2–H2O fluid along with its complex phase changes in typical geochemical processes.

Heat transfer enhancement of nanofluids in concentric cylindrical heat exchanger

Two models of heat transfer channels for concentric cylindrical heat exchangers with porous ribs are developed herein to optimize the transfer efficiency of the heat storage reactor channels. Rings and spiral ribs are added to the smooth circular tube inside the reactor. Spoiler holes are arranged in the ribs to reduce the stagnation area around the ribs. The effects of coolant (SiO2–H2O nanofluids), rib shapes, and the number of holes in the rib on the overall performance of the heat storage system were numerically investigated. Results revealed that the 0.5% mass fraction nanofluids are the most suitable working fluid for channel thermal transport in the simulated Reynolds number range. Both ring ribs and spiral ribs reinforce the heat exchange of the pipe to varying degrees. Spiral perforated ribs have superior heat transfer and drag reduction performance compared with ring ribs. The comprehensive performances of spiral ribs are always higher than those of ring ribs under all conditions.

Effects of the addition of slaked lime to alkali-activated pastes based on volcanic ashes from Mt. Etna volcano (Italy)

This study provides experimental insights into the optimization of alkali-activated materials (AAMs) carried out in situ at the restoration site of Monreale Cathedral, Italy. Volcanic ash from Mt. Etna volcano (Italy) was used as a precursor to obtain AAMs. Standard and modified pastes with a small amount of slaked lime paste were compared in terms of setting time, rheology, mineralogy, chemical morphology, and mechanical performance. The modified pastes exhibited a faster setting time, superior mechanical strength and rheological properties. Mineralogically, no difference was observed between the two pastes for the small amount of slaked lime paste added very close to the detectable limit (<2 wt%). Moreover, its addition causes slight differences in the morphology of the gel without changing the chemical composition of the Na2O–CaO–Al2O3–SiO2–H2O ((N,C)-A-S-H)) type. The results obtained may be useful for the development and optimization of building materials (especially for restoration purposes) where appropriate physico-mechanical performance is required.

Phase stabilities and thermodynamic properties of crystalline phases in CaO–SiO2–H2O above 100 °C

The current study revises the phase diagram of the CaO–SiO2–H2O system at saturated steam pressure between 100 and 250 °C by combining experimentally observed phase relations, solubility data and thermodynamic restrictions. A new self-consistent thermodynamic dataset for crystalline C-S-H phases is reported. The dataset revises known thermodynamic properties and derives values for phases for which no data have been published up to now. Chemical compositions and molar volumes of the phases were updated based on recent crystal structure refinements. Additionally, thermodynamic properties are deduced for hydroxylellestadite, scawtite and spurrite as they are relevant for autoclaved concrete and metamorphism/metasomatism of rocks. Portlandite dominates together with hillebrandite, xonotlite and low-quartz the equilibrium phase assemblage up to 250 °C. If the formation of the latter three phases is delayed by kinetic reasons, numerous metastable phases can occur.

Rhenium solubility and speciation in aqueous fluids at high temperature and pressure

In order to characterize rhenium transport via infiltration of fluids in the Earth's interior, the solubility and solution mechanisms of ReO2 in aqueous fluids were determined to 900 °C and about 1710 MPa by using an externally–heated hydrothermal diamond anvil cell. In order to shed light on how Re solubility and solution mechanisms in aqueous fluids can be affected by interaction of Re with other solutes, compositions ranged from the comparatively simple ReO2–H2O system to compositionally more complex Na2O–ReO2–SiO2–H2O fluids. Fluids in the ReO2–SiO2–H2O, SiO2–H2O, Na2O–SiO2–H2O, and Na2O–ReO2–H2O systems also were examined. The presence of Na2O enhances the ReO2 solubility so that in Na2O–ReO2–H2O fluids, for example, Re solubility is increased by a factor of 10–15 compared with the Re solubility in Na2O-free ReO2–H2O fluids. The SiO2 component in ReO2–SiO2–H2O causes reduction of ReO2 solubility compared with ReO2–H2O fluids. The ReO2 solubility in the Na-bearing Na2O–ReO2–SiO2–H2O fluids is greater than that in fluids in both the ReO2–H2O and ReO2–SiO2–H2O systems. Rhenium is dissolved in aqueous fluid as ReO4-complexes with Re in fourfold coordination with oxygen. Some, or all, of the oxygen in these complexes is replaced by OH-groups depending on whether Na2O also is present. It is proposed that during dehydration of hydrated subduction zone mineral assemblages in the upper mantle, the alkali/alkaline earth ratio of the source of the released aqueous fluid affects the extent to which Re (and other HFSE) can be transported into an overlying peridotite mantle wedge. The infiltration of such fluids will, in turn, affect the Re content (and Re/Os ratio) of magma formed by partial melting of this peridotite wedge.

Evolution of Contact Metamorphic Rocks in the Zhoukoudian Area: Evidence from Phase Equilibrium Modelling

The Yanshan intraplate tectonic belt is a tectonic-active area in the central part of the North China Craton that has undergone long-term orogenic evolution. Detailed studies on magmatic activity and metamorphism of this belt are significant for restoring its orogenic thermal evolution process. The Fangshan pluton in the Zhoukoudian area within this tectonic belt is a product of the late Mesozoic Yanshan event. However, there is a lack of detailed research on the metamorphic evolution history of the ancient terrane surrounding the Fangshan pluton subjected to contact thermal metamorphism. To further constrain the metamorphic P–T evolution of contact metamorphism associated with the Fangshan pluton, we collected rock samples in the andalusite–biotite contact metamorphic zone of the Fangshan pluton, and conducted petrographic investigations, geochemical and mineral composition analysis, and phase equilibrium modeling. The phase equilibrium modeling in the MnO–Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O system indicates that the peak mineral assemblages of andalusite–biotite schists are pl + q + mu + bi + and ± kfs + ilm + mt, formed at 550 to 610 °C, 1 to 3.5 kbar, and the peak mineral assemblage of garnet–andalusite–cordierite–biotite schists is gt + pl + q + bi + and + cord + ilm + mt, formed at 580 to 620 °C, 1.5 to 2.1 kbar. Therefore, we believe that the rocks in the andalusite biotite contact metamorphic zone of the Fangshan pluton underwent low pressure and medium temperature metamorphism, with the peak metamorphic conditions of about 550–610 °C, &lt;3.5 kbar. The results show that the rocks in contact with the thermal metamorphic zone were rapidly heated by the heat released by the Fangshan pluton, and after reaching the peak metamorphic temperature, they were cooled down simultaneously with the cooling of the rock mass, defined in a nearly isobaric P–T trajectory.

Growth of kyanite and Fe‐Mg chloritoid in Fe2O3‐rich high‐pressure–low‐temperature metapelites and metapsammites: A case study from the Massa Unit (Alpi Apuane, Italy)

AbstractChloritoid and kyanite coexist in metapelites from the high‐pressure/low‐temperature Massa Unit in the Alpi Apuane metamorphic complex (Northern Apennines, Italy). The composition of chloritoid is extremely variable throughout the Massa Unit. Fe‐chloritoid occurs in association with hematite‐free, graphite‐bearing schists, whereas strongly zoned Fe‐Mg chloritoid is found with hematite and kyanite. We investigated the effect of different bulk Fe2O3 contents in controlling chloritoid composition through phase equilibria modelling of four selected samples, representative of the different chloritoid‐bearing parageneses found in the Massa Unit. The ferric iron content, measured through wet chemical titration, ranges from 0 (graphite‐chloritoid schist) to 73% of the total iron (hematite‐chloritoid schist). We show that Mg‐rich chloritoid compositions and stability of kyanite at greenschist to blueschist facies conditions can be reproduced in the MnO–Na2O–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O (MnNKFMASHTO) chemical system only considering the presence of significant amounts of ferric iron as part of the bulk composition. The stabilization of kyanite at lower grade is directly linked to the presence of Fe2O3, which renders the reactive bulk rock composition effectively enriched in Al2O3 with respect to Fe and Mg. We also document that high Fe2O3 contents exacerbate the effect of chloritoid fractionation, producing strongly zoned Fe‐Mg‐chloritoid grains. Finally, the P–T modelling of the Massa Units performed in this study allows, for the first time, the recognition of a two‐stage evolution at peak conditions, with an earlier pressure peak (1.2–1.3 GPa at 350–400°C), and a later thermal peak (0.7–1.1 GPa at 440–480°C), compatible with subduction, underthrusting and exhumation of the Adria continental margin during growth of the Northern Apennine orogenic wedge.

Reliability of Different Nanofluids and Different Micro-Channel Configurations on the Heat Transfer Augmentation

Nanofluid, the fluid suspensions of a metallic nanoparticle, became a coolant fluid that is used when a promising enhancement in heat transfer is required. In the current study, the characteristics of fluid flow and heat transfer are numerically investigated using different nanofluids (Al2O3–H2O, TiO2–H2O, and SiO2–H2O) and different micro-channel heat sink (MCHS) configurations (rectangular, triangular, trapezoidal, and circular). In this numerical investigation, the effect of Re number ranged from 890 to 1500, and the effect of nanoparticle concentration ranged from 1% to 7% at constant heat flux q = 106 W/m2, and constant fluid inlet temperature of 288 K, were studied. The average heat transfer coefficient, h, and pressure drop, Δp, are used to quantify the fluid flow and heat transfer characteristics in each MCHS configuration and for each nanoparticle concentration. It is revealed that a better heat transfer coefficient is obtained for Al2O–H2O compared with other types of nanoparticles and pure water, such as 8.58% heat transfer coefficient improvement obtained at Re = 1500 and φ=7% more than that of pure water. It is also inferred that the maximum heat transfer coefficient is obtained by the triangular MCHS; however, it has the highest pressure drop because of the lowest hydraulic diameter.

The state of Fe3+ in the C–F–A–S–H system with varying Fe/Si and Ca/Si ratios

Synthetic CaO–Fe2O3–Al2O3–SiO2–H2O (C–F–A–S–H) gels with Fe/Si and Ca/Si ratios in the ranges 1/8–1/4 and 1.0–2.0, respectively, are investigated to reveal the coordination, location, and doping configuration of Fe3+.

Alkali-reinforced hydrothermal solidification of waste soil

To promote the resource utilization of waste soil, hydrothermal solidification technology was used to convert waste soil into a tough material. The effects of curing conditions (time and temperature), molding conditions and alkaline conditions on the physical-mechanical properties of hydrothermally solidified waste soil were investigated in the CaO–MgO–Al2O3–SiO2–H2O system. The solidification mechanism and microstructure evolution under different synthesis conditions were analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM) and low-field nuclear magnetic resonance (NMR). The strength of waste soil was effectively enhanced under hydrothermal conditions; in particular, the addition of NaOH solution could nearly double the strength. Calcium silicate hydrate (C–S–H) and hydrogarnet formed by the hydrothermal reaction filled the pores and cemented mineral particles to form a dense skeleton structure. The filling of hydrothermal products and the dissolution of mineral particles together affected the pore structure and compactness of the material. The synthesis conditions determined the type and quantity of hydrothermal products that affect the microstructure, consequently reflecting the changes in macroscopic physical-mechanical properties.

P–T–X (Fe–Mg) Relations of Cummingtonite-sillimanite-cordieritequartz-H2O Equilibrium: Implications on Evolution of Metamorphic Rocks

Abstract The P–T–X (Fe–Mg) phase relations of the cummingtonitesillimanite-cordierite-quartz-fluid assemblage have been constructed considering the model system FeO–MgO–Al2O3–SiO2–H2O (FMASH) and using available experimental and thermodynamic data. The assemblage represents a divariant reaction: cum + sil+ qtz= crd + H2O, the Fe- and Mg-end member reactions of which have positive slope in P–T space. Isobaric T–X (Fe–Mg) loop and isothermal P–X (Fe–Mg) loop show that,at constant pressure, both cummingtonite and cordierite become Fericher with increasing temperature, whereas, at constant temperature, this assemblage is displaced towards Mg-richer compositions with increasing pressure. The study of the P–T–X relationships of cummingtonite-sillimanite-cordierite-quartz-H2O equilibrium suggests that the cummingtonite–cordierite bearing assemblages are stable at the low-pressure metamorphic environment.

Thermodynamic constraints on the composition of orogenically thickened lower crust

Orogenically thickened lower crust is the key site of crustal differentiation, crustal deformation, and Moho modification. However, the composition of thickened lower crust is still highly debated. Here, we calculate a set of pseudosections with mafic lower crust compositions in the Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O2 (NCKFMASHTO) system. Our modeling results show that the maximum thickness of the mafic lower crust increases with the Moho temperature (TMoho). In addition, the lithologies of stable mafic crust are characterized by medium-pressure (MP) to high-pressure (HP) granulites at 40–50 km, HP granulites and garnet-omphacite granulites at 50–60 km, and garnet-omphacite granulites at 60–70 km. Under the Pamir geothermal conditions, mafic rocks with high SiO2 (>50.2 wt%), XMg (>0.70), XCa (>0.49), or low XAl (<0.11) could be stable at 70 km; however, only ~10% of global mafic granulite xenoliths lie within this compositional range. Further modeling indicates that if TMoho reaches 900–1000 °C, neither the lower crust nor the upper mantle has significant strength relative to the upper crust, and that only ~5–37% of mafic materials are gravitationally stable at 70 km. This implies that the base of doubly thickened (70 km) crust is dominated by intermediate-felsic rocks, consistent with the low Vp and Vp/Vs values seismically observed in young orogenic crustal roots. Thus, most mafic materials at >70 km could delaminate into the deep mantle. Our results provide insights on the formation of extremely thick crust with a predominantly intermediate-felsic base and the crustal thickness variation in continental collision zones.

Calculated phase equilibria for high‐pressure serpentinites and compositionally related rocks close to the MgO–Al2O3–SiO2–H2O (MASH) system

AbstractThe metamorphic evolution of serpentinite has a fundamental bearing on geochemical cycles, mechanical behaviour and seismic properties of the Earth. Phase equilibria in the system MgO–Al2O3–SiO2–H2O (MASH) provide a ‘backbone’ for phase equilibria in larger systems that are applicable to serpentinites and also many whiteschists. Phase equilibria for MASH, and departures from MASH, are calculated using the Holland and Powell data set, tc‐ds62, and new activity–composition relations for antigorite and talc. Phase relations are presented using PT projections and compatibility diagrams to highlight the role of P, T and composition on the equilibrium mineral assemblages stable in these chemical systems at high pressure. These phase relations provide the context for pseudosection and other modelling focused on individual rock compositions.

Aluminous hydrous magnesium silicate as a lower-mantle hydrogen reservoir: a role as an agent for material transport

The potential for storage of a large quantity of water/hydrogen in the lower mantle has important implications for the dynamics and evolution of the Earth. A dense hydrous magnesium silicate called phase D is a potential candidate for such a hydrogen reservoir. Its MgO–SiO2–H2O form has been believed to be stable at lower-mantle pressures but only in low-temperature regimes such as subducting slabs because of decomposition below mantle geotherm. Meanwhile, the presence of Al was reported to be a key to enhancing the thermal stability of phase D; however, the detailed Al-incorporation effect on its stability remains unclear. Here we report on Al-bearing phase D (Al-phase D) synthesized from a bridgmanite composition, with Al content expected in bridgmanite formed from a representative mantle composition, under over-saturation of water. We find that the incorporation of Al, despite smaller amounts, into phase D increases its hydrogen content and moreover extends its stability field not only to higher temperatures but also presumably to higher pressures. This leads to that Al-phase D can be one of the most potential reservoirs for a large quantity of hydrogen in the lower mantle. Further, Al-phase D formed by reaction between bridgmanite and water could play an important role in material transport in the lower mantle.