Calcite Content Research Articles

Diagenetic development of rhizoliths in the Ironshore Formation (Pleistocene) of the Cayman Islands

Rhizoliths in the Ironshore Formation on the Cayman Islands, up to 12 cm in diameter and 1 m long, developed in skeletal grainstones that accumulated on sand flats above high tide level. Internally, these well-preserved rhizoliths are characterized by well-defined concentric zones, centered around the central root opening, that reflect radial variations in calcite–aragonite content, decreases in micrite content, increases in porosity, and radial variations in trace element (e.g., Ca, Ni, S, Si, and Sr) concentrations. The stable isotopes, δ13C and δ18O, which vary in accord with the internal zones are more variable than the δ13C and δ18O of the host sediments. For the rhizoliths there is a high correlation between the δ13C and δ18O values and the calcite content. The contrast between the rhizoliths and their host sediments is largely a reflection of the more intense diagenetic processes that were largely mediated by the microbial populations that were concentrated in the rhizosphere that developed around each plant root. Microbial activity, which mediated micritization of the skeletal sands and micrite precipitation in the intergranular pores, was largely responsible for development of the internal zonation in the rhizoliths. Use of the δ13C values to determine if the rhizoliths developed in association with C3 or C4 vegetation failed because the δ13C values correlate with the calcite content that developed largely as diagenetic products.

Shale permeability and microstructural alternation during CO2 pre-fracturing: A mechanistic study

CO2 pre-fracturing technology represents a novel approach to CO2 fracturing, effectively enhancing carbon capture efficiency in shale reservoirs while increasing the recovery of hydrocarbon resources. This study systematically quantifies the impact of water injection on the pore structure and permeability of shale samples saturated with pre-injected CO2. Based on X-ray diffraction (XRD) and low-temperature nitrogen adsorption (LT-NA) results, the dominant mechanism of the swelling-chemical coupling effect in shale property is clarified. Pre-injected CO2 can effectively mitigate the impact of water absorption. After a reaction time of 7 days, the permeability of the CO2-involved hydrated shale samples is four times that of samples without CO2. Nevertheless, the influence of subsequently injected water on shale permeability persists, resulting in an 80% reduction in shale permeability. XRD and LT-NA results indicate that the swelling-chemical coupling effect is the dominant factor in shale property variation during CO2 pre-fracturing. In the initial stage, the decline in calcite and clay mineral content is significant, and chemical dissolution dominates the change in pore structure. As the reaction progresses, the intensity of chemical reactions weakens, and clay mineral swelling becomes the primary factor affecting the shale properties. In this stage, K+ generated from original minerals effectively inhibits clay mineral swelling. Therefore, the swelling-chemical coupling effects should be comprehensively considered during the CO2 pre-fracturing process, and an appropriate soaking time should be selected to maximize CO2 storage efficiency and oil and gas production.

Genesis of the Hujiazhuang apatite-rich carbonatite complex in North China and its implication for the REE fertility of the carbonatite system

Apatite is a common REE-compatible mineral in carbonatites, so its formation can affect the REE ore-forming potential of carbonatite systems. In nature, there are numerous apatite-rich carbonatite complexes, but the petrogenesis of these complexes has long been debated. To resolve this controversy, this study conducts elemental and isotopic analyses on the Mesozoic (∼120 Ma) Hujiazhuang carbonatite complex in North China. The Hujiazhuang complex contains magnetite-calcite-apatite carbonatites and calcite carbonatites. Calcite grains from these two rock phases exhibit identical initial 87Sr/86Sr ratios (0.71027–0.71038), indicating a common parental magma derived from a metasomatized lithosphere mantle. Then, from magnetite-calcite-apatite carbonatites to calcite carbonatites, the Mg, Fe, and Mn contents of calcite continuously increase while the Sr and REE contents decrease, consistent with elemental variations of calcite during fractional crystallization of carbonatitic magma. Therefore, these two lithologies are proposed to be formed through fractional crystallization of a common magma. Notably, the Hujiazhuang carbonatite complex was derived from a metasomatized lithosphere mantle and experienced intensive fractionation, which potentially facilitate the formation of REE deposits. However, this complex cannot meet REE's economic standard. The early-stage magnetite-calcite-apatite carbonatites have high REE contents (3,675–11,075 ppm), and a positive correlation can be observed between REE and P contents in this lithology. It indicates that early, extensive crystallization of apatite led to REE dispersion, largely hindering REE enrichment in later stages. In general, this study provides important insights into the genesis of the apatite-rich carbonatite complex and advances our understanding of the REE dispersion or enrichment behaviors in carbonatite system.

Experimental simulation of water-coal interactions during liquid CO2 injection into coal beds

Injection of liquid CO2 (LCO2) into coal reservoirs is an extremely effective way to enhance coalbed methane recovery and storage. This study aimed to investigate the changes in mineral composition and content during the chemical reaction between coal samples and weakly acidic fluids dissolved in LCO2. Therefore, in this study, simulating the formation temperature (30 °C), three types of coal samples with different metamorphic degrees (lignite, anthracite, and bituminous coal) were selected for CO2–H2O-coal interaction leaching experiments under various pressure conditions (1 MPa, 3 MPa, 5 MPa). X-ray diffraction analysis (XRD) was used to test coal samples' mineral composition and content before and after LCO2 injection under different pressure conditions. Scanning Electron Microscope (SEM) was used to observe the evolution pattern of surface characteristics before and after the dissolution of the coal body by LCO2. The results showed that minerals such as kaolinite, illite, quartz, and potassium feldspar in the three coal samples underwent mineral transformation during the reaction process at different dissolution pressures, and the content changed significantly in the early stage of the reaction. The initial content of kaolinite in the bituminous coal (16.5%) increased to 23.8%, 26.7%, and 25.2% respectively under different leaching pressures of 1 MPa, 3 MPa, and 5 MPa. Carbonate minerals dissolved rapidly in the initial stages of the reaction, the initial content of calcite, a carbonate mineral in the anthracite (9.8%), decreased to 4.2%, 3.8%, and 2.4% respectively under different leaching pressures of 1 MPa, 3 MPa, and 5 MPa. Similarly, the initial content of siderite decreased from 6.8% to 2.8%, 2.3%, and 1.6% respectively. LCO2 pressure had a significant influence on the reaction. These results advance our understanding of LCO2 dissolution of minerals in coal.

Development of an environmental foamed concrete incorporating recycled cement concrete powder with carbonation

From the perspective of efficient resource utilization and environmental protection, it is an effective way to reduce waste by carbonizing recycled cement concrete powder to replace part of cement. In this study, carbonated and non-carbonated recycled cement concrete powder were selected as supplementary cementitious materials to investigate the effects of different cement substitutes (0%, 20%, 40%, 60%) on the workability, microstructure, and thermal insulation performance of foamed concrete. The experimental results show that carbonation treatment can improve the workability of foamed concrete with waste cement concrete powder, with an amplitude ranging from 1.08% ∼ 5.61%. The influence of different carbonated powder substitution on the compressive strength of foamed concrete shows an initial increase followed by a decrease, with a range of variation from 2.27 MPa ∼ 4.47 MPa at 28 d, which is attributed to the effective acceleration of hydration due to a slight increase in the content of calcite. Mechanical strength is positively correlated with the density of the matrix, and the change of the two leads to an opposite trend in thermal insulation performance, with the maximum reduction in thermal conductivity being 9.98% at a substitution rate of 60%. These research findings indicate significant potential applications of waste cement concrete powder at various substitution rates, and are of great importance for achieving the dual-carbon strategy.

Unveiling the Essential Parameters Driving Mineral Reactions during CO2 Storage in Carbonate Aquifers through Proxy Models

Numerical simulation is a commonly employed technique for studying carbon dioxide (CO2) storage processes in porous media, particularly saline aquifers. It enables the representation of diverse trapping mechanisms and the assessment of CO2 retention capacity within the subsurface. The intricate physicochemical phenomena involved necessitate the incorporation of multiphase flow, accurate depiction of fluid and rock properties, and their interactions. Among these factors, geochemical reaction rates and mechanisms are pivotal for successful CO2 trapping in carbonate reactive rocks. However, research on kinetic parameters and the influence of lithology on CO2 storage remains limited. This limitation is partly due to the challenges faced in laboratory experiments, where the time scale of the reactions and the lack of in situ conditions hinder accurate measurement of mineral reaction rates. This study employs proxy models constructed using response surfaces calibrated with simulation results to address uncertainties associated with geochemical reactions. Monte Carlo simulation is utilized to explore a broader range of parameters and identify influential factors affecting CO2 mineralization. The findings indicate that an open database containing kinetic parameters can support uncertainty assessment. Additionally, the proxy models effectively represent objective functions related to CO2 injectivity and mineralization, with calcite dissolution playing a predominant role. pH, calcite concentration, and CO2 injection rate significantly impact dolomite precipitation, while quartz content remains unaffected.

The Role of the Mineralogical Composition on Wettability via Flotation Test and Surface Complexation Modeling (SCM)

Minerals are the chief constituents of rocks and have varied properties, such as the surface area, surface charge, site density, etc. Hence, numerous interactions are bound to occur in a reservoir during rock–fluid (i.e., rock, crude oil and brine) interactions. This study seeks to assess the role of the mineralogical composition in the wettability of sandstone rocks (SRs) and mineral mixture (MM) using both surface complexation modeling (SCM) and a flotation test. From the considered sandstone rocks, both the experimental results and the simulated counterparts revealed that the SRs were preferentially hydrophilic. For the MM, when the mass fraction of the hydrophobic mineral was increased, the affinity of the MM became slightly hydrophobic, and vice versa. For the dominant sandstone reservoir rock minerals with predominantly negatively charged surfaces, negligible oil adsorption took place due to the interfacial repulsive forces at the oil–brine and mineral–brine interfaces. For the MM with low calcite content, the wetting preference was influenced by the mineral with a prominent surface area. Our developed model portrayed that the main mechanism of oil adhesion onto sandstone minerals was divalent cation bridging. Nonetheless, adhesion of carboxylate (>COO−) onto the illite, montmorillonite and calcite sites also took place, with the latter being more pronounced.

Pore Structure and Migration Ability of Deep Shale Reservoirs in the Southern Sichuan Basin

The migration phenomenon of deep shale gas is a subject that has yet to be fully comprehended, specifically regarding the migration ability of deep shale gas. This study focuses on the Longmaxi Formation in the southern Sichuan Basin, utilizing it as an example. Various experimental techniques, such as temperature-driven nitrogen and carbon dioxide adsorption, high-pressure mercury intrusion, XRD, and TOC analysis, are employed. The goal is to analyze the pore structure and fractal characteristics of the Longmaxi Formation shale. Additionally, the study aims to calculate its Knudsen number based on parameters like temperature gradient and pressure coefficient. The migration ability of the Longmaxi Formation shale in southern Sichuan Basin is also discussed. The results show the following: (1) The pore volume distribution of the Longmaxi Formation shale in the study area ranges from 0.0131 to 0.0364 cm3/g. Mesopores contribute approximately 56% of the pore volume, followed by micropores with a contribution rate of about 26%, and macropores contributing approximately 18%. Additionally, the Longmaxi Formation shale exhibits strong heterogeneity, with the fractal dimension (D1) of mesopores ranging from 2.452 to 2.8548, with an average of 2.6833, and the fractal dimension (D2) of macropores ranging from 2.9626 to 2.9786, averaging 2.9707. (2) The fractal dimensions of shale were significantly influenced by organic matter, inorganic minerals, and pore structure parameters. D1 and D2 were positively correlated with TOC, clay mineral content, and specific surface area, while exhibiting negative correlation with quartz. However, the correlations with calcite content, pore volume, and average pore size were not significant. (3) The proportion of pores satisfying Darcy flow in the Longmaxi Formation shale was approximately 3.7%–11.8%, with an average of 6.6%. Consequently, the migration capability of shale gas can be calculated using Darcy’s law. Furthermore, the migration capability of shale gas is controlled by D2, specifically the surface area, and the connectivity of macropores.

Mechanism of pore expansion and fracturing effect of high-temperature ScCO2 on shale

To elucidate the mechanisms underlying the alteration of shale pore fractures by high-temperature supercritical carbon dioxide (ScCO2), this study employed medium–low maturity shale samples from a specific region as research subjects. The samples were subjected to high-temperature ScCO2 reactions in a corrosion-resistant high-pressure reaction vessel at various temperatures. A suite of analytical techniques, including X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), ImageJ image processing software, N2 and CO2 adsorption experiments, thermal gravimetric-differential thermal analysis (TG-DTG) of CO2 pyrolyzed shale oil, and gas chromatography (GC) analysis, were utilized to investigate and analyze the microstructural changes in the shale samples before and after ScCO2 treatment.The results reveal a subtle increase in the content of clay minerals and quartz, accompanied by a decrease in the content of carbonates, potassium feldspar, and calcite. These mineral transformations, encompassing dissolution, precipitation, and swelling processes, exert a substantial impact on the shale surface microstructure and modify the internal pore structure. Consequently, a shift in the mechanical properties of the rock is observed, which bears significance for its fracturing behavior.The experimental results reveal a steady growth in fracture area following high-temperature ScCO2 treatment. Importantly, the extent of initial fracture development plays a crucial role in determining the efficacy of hydraulic fracturing, indicating a positive correlation. Moreover, the connection between fracture area and temperature exhibits a clear trend marked by an initial rise, a subsequent decline, and finally, a resurgence. In particular, the thermal fracturing threshold temperature for the shale samples was found to be around 400 °C.From the analysis, it can be inferred that ScCO2, when subjected to an elevated temperature range of 100-200℃, exhibits mutual interactions with the shale. This results in effective extraction of organic matter and dissolution of mineral constituents within the shale matrix. Consequently, this treatment process leads to a significant reduction in the number of micropores (pore sizes less than 2 nm), while the quantities of mesopores (pore sizes between 2 and 50 nm) and macropores (pore sizes greater than 50 nm) display a noticeable increase.Upon exposure to a temperature range of 200-300℃, a remarkable affinity of the shale for CO2 adsorption becomes evident. The adsorption process contributes to the expansion of the matrix material and deposition of minerals, effectively filling the pore spaces. As a result, the contraction of mesopores and macropores is mitigated, with the predominant change being an increase in the number of micropores.When temperatures reach 300-500℃, the intensified heat initiates thermal decomposition of the kerogen, culminating in the release of pore spaces. This phenomenon prompts an increase in the number of micropores and mesopores, thereby augmenting the pore volume and specific surface area of the shale.

Machine learning - based shale wettability prediction: Implications for H2, CH4 and CO2 geo-storage

Shale wettability determines shale gas productivities and gas (H2, CH4 and CO2) geo-storage efficiencies. However, shale wettability is a complex parameter which depends on multiple influencing factors, thus very time-consuming and costly to measure experimentally. Herein, we combined the eXtreme gradient boosting (XGBoost) and Shapley additive explanation (SHAP) machine learning methods to accurately predict brine advancing (θA) and receding (θR) contact angles and estimate shale wettability. The XGBoost model demonstrated much higher prediction accuracies than the commonly-used multiple linear regression and partial least squares regression models, e.g., R2 was 0.946–0.999, 0.794–0.821, and 0.635–0.674, respectively for these three models. The SHAP sensitivity analyses showed that total organic carbon content and gas molecular weight (MG) were the two most significant factors influencing shale wettability. In addition, shale hydrophobicity positively correlated with MG, calcite content, pressure and brine ionic strength, while negatively correlated with temperature and quartz content. This work provides an efficient approach for shale wettability estimation, thus aiding in the implementation of improved gas recovery and gas geo-storage processes, to further guarantee energy security and mitigate climate change.

Characterization of heterogeneities in the sandy facies of the Opalinus Clay (Mont Terri underground rock laboratory, Switzerland)

SUMMARY This contribution is presenting a multidisciplinary investigation of heterogeneities in a clay rock formation, based on seismic tomography, logging and core analysis, as a reconnaissance study for a diffusion experiment. Diffusion experiments in clay rock formations provide crucial experimental data on diffusive transport of radionuclides (RN) in extremely low hydraulic conductivity media. Previous diffusion experiments, conducted, for example, in the Mont Terri underground rock laboratory within the relatively homogeneous shaly facies of Opalinus Clay, and modelling studies of these experiments have demonstrated that the clay rock could sufficiently well be described as a homogeneous anisotropic medium. For other lithofacies, characterized by larger heterogeneity, such simplification may be unsuitable, and the description of heterogeneity over a range of scales will be important. The sandy facies of the Opalinus Clay exhibits a significantly more pronounced heterogeneity compared to the shaly facies, and a combined characterization and RN diffusion study has been initiated to investigate various approaches of heterogeneity characterization and subsequent diffusion in a heterogeneous environment. As an initial step, two inclined exploratory boreholes have been drilled to access the margins of the experiment location. These boreholes have been used to acquire a cross-hole tomographic seismic data set. Optical, natural gamma and backscattering logging were applied and rock cores were analysed. The integrated results of these investigations allowed the identification of an anomalous brighter layer within the investigated area of the sandy facies of approximately 1 m thickness and with its upper bound at roughly 10 m depth within the inclined exploratory wells. Mineralogical analyses revealed only slight variations throughout the rock cores and indicated that the anomalous layer exhibited a slightly higher quartz content, and locally significantly higher calcite contents, accompanied by a lower content of clay minerals. The anomalous layer was characterized by reduced natural gamma emissions, due to the lower clay content, and increased neutron backscattering likely indicating an increased porosity. Seismic P-wave velocities, derived from anisotropic tomography, exhibited a maximal gradient near the top of this layer. The transition from the overlaying darker rock matrix into this layer has been identified as an appropriate location for the setup of a tracer diffusion experiment in a heterogeneous environment.

РЕОЛОГИЧЕСКИЕ СВОЙСТВА ИЗВЕСТКОВЫХ СОСТАВОВ С ДОБАВКАМИ ПОЛИСАХАРИДОВ

Lime compositions with polysaccharide additives are being studied. The results are aimed at creating a new lime plaster composition with improved performance properties. Materials and methods. Standard methods for studying rheological and technological properties were used. Results. The influence of polysaccharides on changes in the rheological properties of lime compositions has been established. It has been established that the introduction of a polysaccharide additive (hydroxyethylcellulose) helps to accelerate the development of plastic strength. The results of infrared spectroscopy indicate a higher content of calcite in the sample with the addition of polysaccharide. The absence of chemical interaction between lime and polysaccharides has been established. Conclusions. The introduction of polysaccharide additives promotes the formation of a structure that improves the performance properties of the lime composite.

Habitat sensitivity in the West African coastal area: inferences and implications for regional adaptations to climate change and ocean acidification.

This study focuses on assessing coastal vulnerability and habitat sensitivity along the West African coast by delineating hotspots based on surface temperature, pH, chlorophyll-a, particulate organic carbon, and carbonate concentrations between 2018 and 2023 depending on data availability. Initial exploration of these variables revealedtwo distinct focal pointsi.e.,the Togo-Nigerian coastal stretch and the stretch from Sierra Leone to Mauritania.Lower pH trends (acidification) in surface waters were observed off the West African coast, particularly in areas around the south-south Niger Delta in Nigeria and the coastal regions of Guinea and Guinea Bissau. Sea surface temperature analysisrevealed highest temperatures (27-30°C)within Nigeria to Guineacoastal stretch, intermediate temperatures (24-27°C) within the Guinea Bissau and Senegalcoastal stretch, and the lowest temperatures off the coast ofMauritania. Furthermore, correlation analysis between sea surface temperature and calcite concentration in the Mauritania-Senegal hotspot, as well as between overland runoff and particulate organic carbon in the Togo-Nigeria hotspot, revealed strong positive associations (r>0.60) and considerable predictive variability (R2 ≈ 0.40). From the habitat sensitivity analysis, certain regions, including Cape Verde, Côte d'Ivoire, Nigeria, Senegal, and Sierra Leone, exhibited high sensitivity due to environmental challenges and strong human dependence on coastal resources. Conversely, Gambia, Guinea, Guinea-Bissau, Liberia, and Togo displayed lower sensitivity, influenced by geographical-related factors(e.g.coastal layout, topography, etc.) and current levels ofeconomic development(relatively lowerindustrialization levels). Regional pH variations in West African coastal waters have profound implications for ecosystems, fisheries, and communities. Addressing these challenges requires collaborative regional policies to safeguard shared marine resources. These findings underscore the link between ecosystem health, socioeconomics, and the need for integrated coastal management and ongoing research to support effective conservation.

Preservation and hydrocarbon generation potential of microalgal organic matter in product aggregates induced by Chlorella sp. MASCC-0008

Microalgae are one of the important sources of organic matter which, if preserved well in sediments, can generate hydrocarbons during burial diagenesis. However, the preservation mode of microalgal organic matter within product aggregates induced by marine microalgae has rarely been examined. Herein, a strain of eukaryotic marine microalgae, Chlorella sp. MASCC-0008, was used to conduct biomineralization experiments in an f/2 medium with a 0.02 mol/L Ca2+ ion concentration and various Mg2+ ion concentrations of 0.05, 0.06, 0.07 and 0.10 mol/L. It was found that the soluble organic matter of the microalgae contains amino acids and fulvic acids which can attract and absorb cations. With an increase in Mg2+ ion concentration in the experimental group with Chlorella sp., the aragonite content of the mineral precipitates increased whereas the calcite content decreased. In contrast, calcite was the sole mineral precipitated in the control experiments with no Chlorella sp.. Due to the porous structure of minerals, their organic matters were preferred to be preserved in aragonite aggregates compared to calcite aggregates. The TOC content of the minerals aggregates is the highest (0.45%) and the hydrocarbon generation potential (S1 + S2) of organic matter is the greatest (3.66 mg/g) in those precipitated in the medium with a Mg2+ ion concentration of 0.10 mol/L. Microalgal organic matter has significant hydrocarbon generation potential due to its effective preservation in the product aggregates formed by the microalgal biomineralization. Squalene is the most abundant alkene detected in the pyrolytic products. These results suggest that microalgae can induce precipitation of carbonate minerals, a process that facilitates the sequestration of organic matter within the product aggregates precipitated, thus potentially preserving this microalgal organic matter into the burial diagenetic realm where, if conditions are appropriate, hydrocarbons may be developed.

On the resilience of bio-cemented silica sands in chemically reactive environment

Exposure of geomaterials to an acidic environment is frequently encountered in modern-day geo-energy and geo-environmental engineering activities, in e.g. incorporation of chemical stimulation for unconventional shale gas exploitation, enhanced geothermal systems, geological carbon sequestration, and the long-term regional stability in carbonate-rich coastal areas. The Multiphysics-involved process for each application is complex and an optimised control calls for a better understanding on the coupling mechanism of the chemical, hydraulic and mechanical fields. This laboratory-based study aims to provide a quantitative calibration and derivation of the key coupling parameters accommodating our recently proposed framework of reactive chemo-mechanics, using a bio-cemented rock-like material as a representative for dissolvable rocks. The advantage of bio-cemented specimens (here by microbially induced carbonate precipitation) over natural rocks lies in their more uniform grain-bond structure and laboratory tunable calcite content. An experimental setup is introduced for investigating the role of calcite content on the mechanical and hydraulic properties of bio-cemented silica sands, followed by uniaxial tests on the bio-cemented specimens immersed in acidic environment to allow a reactive chemo-mechanical setting. Our results show that bio-cemented samples appear to be more “resilient” to an acidified aqueous environment in terms of less strength degradation compared to natural carbonate-rich rocks. Ductile failure mode is observed in the bio-cemented specimens within a certain range of the calcium carbonate content and a brittle-to-ductile transition in the failure mode occurs when the calcite content in the specimen decreases. With the calibrated model and the derived coupling parameters, we further illustrate an example of numerical prediction on the mechanical response of bio-cemented specimens under varying acidic environments and loading rates.

Molecular Simulation Study on the Effect of Co-Associated Minerals on Methane Adsorption and Mechanical Properties of Coal

When rockbursts and coal and gas outbursts simultaneously occur in a coal mine, changes in gas adsorption (concentration of ambient methane) and displacement of coal and rock must occur. The co-associated minerals in coal reservoirs can affect the mechanical properties and methane adsorption capacity, which are commonly disregarded. It is important to construct compound molecular structure models of coal and rock and conduct molecular dynamic simulations to gain a microscopic understanding of underground disasters. In this work, the molecular structure models of anthracite and coking coal–rock compound models containing different contents of calcite and kaolinite were constructed, and the methane adsorption amount and mechanical properties considering temperature, pressure, and mineral contents were simulated and analysed. The results showed that the methane adsorption amount of the compound models increased rapidly, then increased moderately, and stabilized eventually with increasing adsorption pressure, and the Langmuir fitting findings were good. The saturation adsorption amount of methane in the coal models linearly decreased with increasing temperature, while the methane adsorption heat increased. The presence of minerals adsorbed a certain amount of methane, and the methane adsorption amount increased with increasing mineral contents. The mechanical properties of coal molecules changed when mineral molecules such as calcite and kaolinite were present, which had opposite contribution effects. The addition of kaolinite minerals to the coal molecular model always increased the bulk modulus and shear modulus, while the addition of calcite decreased the bulk modulus of the anthracite, causing an increase in the brittleness of the models. The results of the study further explain the adsorption behaviour and mechanical properties of methane in coal and minerals.

Effect of crystal morphology on cementability and micromechanical properties of calcium carbonate precipitate induced by crude soybean enzyme

Enzyme-induced carbonate precipitation (EICP) has emerged as an innovative soil stabilization technology to precipitate CaCO3 by catalyzing urea decomposition. Although extensive efforts have been made to increase the calcium carbonate content (CCC) formed in the EICP process for the better bio-cementation effect, the cementability and micromechanical properties of CaCO3 are rarely known. A study of the cementitious characteristics and micromechanical properties of CaCO3 precipitates with different mixing percentages of crystal morphology is essential for soil improvement. In the present study, ultrasonic oscillation tests and nanoindentation tests were performed to investigate the cementability and micromechanical properties of CaCO3 precipitate. The results show that the cementability and micromechanical properties of CaCO3 precipitate are related to the composition of the crystal morphology. A high content of calcite is beneficial to improve the adhesion of calcium carbonate precipitate. Calcite has better mechanical properties (elastic modulus, hardness and ductility) than vaterite, and the presence of vaterite can significantly affect the measured value of mechanical properties in nanoindentation tests. The ductility of CaCO3 precipitate induced by crude soybean urease (CSU) is higher than that of CaCO3 precipitate induced by commercially available pure enzyme, suggesting that commercially available pure enzyme can be replaced by CSU for cost-effective field-scale engineering applications. This work can provide insight into optimizing the properties of CaCO3 precipitate from the micro-scale.

Mineral composition of the Sumer Formation sediments near the village of Sumer (Northwestern Bulgaria)

Previous analyses of the Sumer Formation sediments have been limited to samples up to 400 m deep near the village of Varbitsa. This study extends the investigation, exploring the mineral composition and carbonate content of the sediments near the village of Sumer. Samples were collected from two outcrops at the Montana-Vratsa Road junction (C-1) and the Shugavitsa river basin (C-2). Both outcrops revealed a quartz and calcite content of approximately 75–80%, with a significant variance in calcite levels: 11.4% at C-1 and 31.3% at C-2. Illite (4.5%) and kaolinite (2%), these clay minerals essential for the marls’ sorption properties, were only detected in C-1 samples. The C-2 outcrop exhibits a higher carbonate content (17.65%) compared to C-1 (10.85%). These findings mirror the mineral composition trends observed near Varbitsa, where the content of illite and kaolinite increases with depth. The results provide valuable data for future geochemical and geotechnical evaluations pertinent to the development of geological repositories.

Coupled bio-chemo-hydro-mechanical modeling of microbially induced calcite precipitation process considering biomass encapsulation using a micro-scale relationship

Geomaterials with inferior hydraulic and strength characteristics often need improve ment to enhance their engineering behaviors. Traditional ground improvement techniques require enormous mechanical effort or synthetic chemicals. Sustainable stabilization technique such as microbially induced calcite precipitation (MICP) utilizes bacterial metabolic processes to precipitate cementitious calcium carbonate. The reactive transport of biochemical species in the soil mass initiates the precipitation of biocement during the MICP process. The precipitated biocement alters the hydro-mechanical performance of the soil mass. Usually, the flow, deformation, and transport phenomena regulate the biocementation technique via coupled bio-chemo-hydro-mechanical (BCHM) processes. Among all, one crucial phenomenon controlling the precipitation mechanism is the encapsulation of biomass by calcium carbonate. Biomass encapsulation can potentially reduce the biochemical reaction rate and decelerate biocementation. Laboratory examination of the encapsulation process demands a thorough analysis of associated coupled effects. Despite this, a numerical model can assist in capturing the coupled processes influencing encapsulation during the MICP treatment. However, most numerical models did not consider biochemical reaction rate kinetics accounting for the influence of bacterial encapsulation. Given this, the current study developed a coupled BCHM model to evaluate the effect of encapsulation on the precipitated calcite content using a micro-scale semiempirical relationship. Firstly, the developed BCHM model was verified and validated using numerical and experimental observations of soil column tests. Later, the encapsulation phenomenon was investigated in the soil columns of variable maximum calcite crystal sizes. The results depict altered reaction rates due to the encapsulation phenomenon and an observable change in the precipitated calcite content for each maximum crystal size. Furthermore, the permeability and deformation of the soil mass were affected by the simultaneous precipitation of calcium carbonate. Overall, the present study comprehended the influence of the encapsulation of bacteria on cement morphology-induced permeability, biocement-induced stresses and displacements.

Carbonation of a Synthetic CAF Compound by CO2 Absorption and Its Effect on Cement Matrix

In the field of construction materials, the development of fundamental technologies to reduce energy consumption and CO2 emissions, such as manufacturing process improvement and the expanded use of alternative materials, is required. Technologies for effectively reducing energy consumption and improving CO2 absorption and reduction that can meet domestic greenhouse gas reduction targets are also required. In this study, calcium–aluminate–ferrite (CAF), a ternary system of CaO·Al2O3·Fe2O3, was sintered at a low temperature (1100 °C) to examine the possibility of CO2 adsorption, and excellent CO2 absorption performance was confirmed, as the calcite content was found to be 11.01% after 3 h of the reaction between synthetic CAF (SCAF) and CO2. In addition, the physical and carbonation characteristics were investigated with respect to the SCAF substitution rate for cement (10%, 30%, 50%, 70%, and 100%). It was found that SCAF 10% developed a compressive strength similar to that of ordinary Portland cement (OPC 100%), but the compressive strength tended to decrease as the SCAF substitution rate increased. An increase in the SCAF substitution rate led to the rapid penetration of CO2, and carbonation was observed in all the specimens after 7 days. As carbonation time increased, the CO2 diffusion coefficient tended to decrease. This is because the diffusion of CO2 in the cement matrix follows the semi-infinite model of Fick’s second law. SCAF can contribute to reduced energy consumption and CO2 emissions because of the low-temperature sintering and can absorb and fix CO2 when a certain amount is substituted.