Clay Aggregates Research Articles

Phase change materials embedded in expanded clay aggregates to develop energy storage concrete: A review

Thermal energy storage (TES) based on phase change materials (PCM) is an effective strategy to reduce energy consumption in buildings. The efficient implementation of TES in building through PCMs, requires modification of their thermal performance, appropriate design and evaluation of their thermal and economic efficiency. One major challenge in the application of PCMs is the selection of high-performance and environmental-friendly supporting materials to overcome the most critical drawback of PCMs (solid-to-liquid transition and resulting leakage). This review focuses on expanded clay (EC) lightweight aggregate (LWA), as one of the promising supporting materials for development of form-stabilized PCM (FSPCM) in the lightweight concrete (LWC) industry. The large specific surface area and porous structure of expanded clay can maintain considerable amount of PCM. In addition, mechanical properties and compatibility with various PCMs make EC a suitable carrier. A detailed review of available researches is presented to elucidate the properties of PCMs embedded in EC for thermally efficient cement-based concrete. Various combinations of EC with the most used PCMs, thermal and mechanical properties of matrices containing PCM-EC, strategies to improve the efficiency of PCM performance, and finally the impact of PCM-EC based concretes on the thermal comfort of buildings are critically summarized.

Hydro-mechanical behavior and microstructure evolution of red clay-bentonite backfills

Soil-bentonite (SB) cutoff walls have been extensively used as vertical engineering barriers to control groundwater flow and subsurface contaminant migration. As the pollution containment capability of the SB backfills is intimately related to the hydro-mechanical behavior, consolidation tests and hydraulic conductivity tests were carried out to investigate the effects of bentonite content on the hydro-mechanical behavior of red clay-bentonite backfills, and the microstructure evolution of backfills under vertical loading was evaluated based on mercury intrusion porosimetry (MIP) tests and field emission scanning electron microscope (FESEM) observations. The results of microstructure analysis also evaluated the influence of bentonite on the clay fabric which changed the relationship between hydraulic conductivity and void ratio. The results indicated that the void ratio and hydraulic conductivity decreased linearly with the increase of the effective consolidation stresses, and the compression index decreased with the increase of bentonite content. Meanwhile, the pore size distribution (PSD) curve of the backfill specimen almost turned into unimodal characteristics with only micropores distributed after consolidation. Additionally, the flow paths became more tortuous under compression which decreased the hydraulic conductivity. The montmorillonite aggregates in red clay-bentonite backfill formed into tortuous network structure which bound the clay aggregates together and effectively increased the tortuosity of flow paths.

Analyzing the Role of Fe0 and Fe3+ in the Formation of Expanded Clay Aggregates.

The effect of the addition of Fe0 and Fe3+ on the formation of expanded clay aggregates was studied using iron-free kaolin as an aluminosilicates source. Likewise, the incorporation of cork powder as a source of organic carbon and Na2CO3 as a flux in the mixtures was investigated in order to assess its effect in combination with the iron phases. An experimental protocol, statistically supported by a mixture experiments/design of experiments approach, was applied to model and optimize the bloating index, density, absorption capacity, and mechanical strength. The process of expansion and pore generation and the associated decrease in density required the addition of iron, such that the optimum mixtures of these properties presented between 25 and 40 wt.% of Fe0 or Fe3+, as well as the incorporation of 3.5-5 wt.% of organic carbon. The addition of Fe3+ produced a greater volumetric expansion (max. 53%) than Fe0 (max. 8%), suggesting that the formation of the FeO leading to this phenomenon would require reducing and oxidizing conditions in the former and the latter, respectively. The experimental and model-estimated results are in good agreement, especially in the aggregates containing Fe0. This reinforces the application of statistical methods for future investigations.

Multi-property response and optimisation of lightweight self-consolidating mortar containing silica fume and nano silica

This article aims to investigate and optimise the properties of lightweight self-consolidating mortar (LSCM) by using the response surface method (RSM). Silica fume (SF), nano silica (NS), water-cement ratio (W/C) and superplasticiser (SP) were chosen as variables. Also, part of the aggregates was replaced with lightweight expanded clay aggregate (LECA). Due to the advantages of RSM in accommodating multi-response optimisation of LSCM properties, the investigation and analysis were carried out for the stability, segregation and compressive strength. The optimised results recommended acceptable ranges of the required rheological and hardened criteria for LSCM. The mixtures achieved in these ranges include an LSCM with sufficient stability and low segregation. In particular, the results showed that the ultimate segregation must be < 8% for structural concrete. In addition, the results proved that increasing the fraction of NS and SF improves the properties of the LSCM by reducing the segregation and increasing the stability and compressive strength. It was found that adding 6% of NS at a high W/C ratio decreased the segregation by at least 15%. Adding 6% NS and %SF to LSCM within 0.5 W/C increased the compressive strength up to 15.1 MPa for the former and 3.8 MPa for the latter.

Modelling the creep behaviour and induced failure of clay rock from microscale viscosity to large-scale time-dependant gallery convergences using a multiscale numerical approach

The large-scale creep behaviour of Callovo-Oxfordian (COx) clay rock is modelled from small-scale viscous mechanisms of the rock using a multiscale numerical approach in the context of radioactive waste repositories. At the mesoscale, the saturated non-homogeneous rock is represented in digital 2D Representative Elementary Areas (REAs) as a cracked composite material consisting of rigid elastic mineral inclusions (quartz, calcite, and pyrite) embedded in a clay matrix. The modelling of these materials is considered within double-scale finite element framework, in which the homogenised responses of the REA to the enforced global kinematics serve as a numerical constitutive relation at the macroscale. To reproduce the rock creeping, two viscous mechanisms have been introduced to consider the creep of the clay matrix: either the viscoplasticity of the clay aggregates or the viscoelasticity of their contacts, or both. Firstly, laboratory biaxial creep tests of clay rock samples are simulated. A three-stage creep stage is reproduced and the creep failure process is discussed. Then, large-scale underground engineering structures are modelled to reproduce its time-dependent behaviour. It is found that the developed multiscale model is able to provide some valuable insights into the large-scale creep behaviour of clay rocks through the morphological and material small-scale characterization of REA.

Durability of concrete mixtures containing Iranian electric arc furnace slag (EAFS) aggregates and lightweight expanded clay aggregates (LECA)

This article reports the results of an experimental investigation on novel concrete mixtures containing electric arc furnace slag aggregates (EAFS) and lightweight expanded clay aggregates (LECA), focusing on the durability properties. Six concrete mixtures consist of EAFS-natural-LECA (M-Group) and five mixtures of EAFS-LECA (N-Group) compared with 100% EAFS as reference. Workability and density of the mixtures were studied as fresh properties and compressive strength, as well as durability indexes including the electrical resistivity (RCPT and four-point Wenner test), salt-scaling under freeze and thaw cycles, abrasion resistance (abrasive groove length and wear depth), durability under wet and dry cycles, and durability factor under freeze/thaw cycles. Results showed that application of both LECA and natural aggregates, by maintaining the structural-level strength, leads to density adjustment while increasing the freeze/thaw durability factor, improving dimensional stability and controlling the level of cyclic wet/dry-stains. An average increase of durability factor of M-Group and N-Group comparing to the reference were observed up to about 124% and 63% respectively. The results indicated the positive effects of using natural aggregate and LECA on modifications of reference mixture. Measured abrasive groove length ranged between 20 and 23 mm and the depth of wear ranging from 0.1 to 0.3 mm confirming the improving effects of physical structure of EAFS aggregates. The studied mixtures, namely M6 and N5 with 0.744 kg/m2 and 1.204 kg/m2 exhibited the lowest salt-scaling mass-loss with 31% and 112% increase compared with the reference.

The Adsorption Characteristics of Heavy Metals in Acid Mine Drainage from Abandoned Tin Mines on Lightweight Expanded Clay Aggregate (LECA)

Abandoned tin mines produce acid mine drainage in their water basin. If not treated well, it will damage environmental ecosystem by entering rivers or other water bodies. This acid mine drainage is attempted to be remediated by adsorption technique. The adsorbent used in this study is Lightweight Expanded Clay Aggregate (LECA) because its base material is natural clay. LECA is commonly used for hydroponics and constructions. LECA is made from natural clay that being heated at temperature over 1100°C. This study aims to determine how significant LECA in adsorbing metals in acid tin mine drainage. This research used two materials namely LECA and tin acid mine drainage. Both materials were contacted for two days in shaker bath. The results of this study were the adsorbing Fe(II) and Cu(II) on LECA could be approached by the Langmuir-Freundlich (LF) combined model where the Cµ,max are 0.406 and 0.020 mg/g of adsorbent, respectively. Unlike the other two metals, Sn(II) was more likely to experience precipitation instead of adsorption because of increasing of pH value. The conclusion, heavy metals in tin acid mine drainage could be remediated well by using LECA.

Collagen-nanoclay hydrogels for stiffness enhancement of the extracellular matrix: the effect on spheroid growth

The extracellular matrix (ECM) is crucial for regulating cellular functions and maintaining tissue integrity. Alterations in ECM stiffness are associated with various pathological conditions, including cancer and cardiovascular disease. Therefore, being able to tailor and customize ECM stiffness could revolutionize personalized research and therapies in tissue engineering. Clay nanomaterials are an emerging class of biomaterials with unique properties that make them promising candidates for biomedical applications.&#x0D; In this study, we investigated the potential of laponite nanoclays (LapNC) to enhance the mechanical properties of collagen hydrogels without increasing the collagen amount. Collagen hydrogels of different concentrations (2.5, 4, and 6 mg/ml) were supplemented with LapNC, and the resulting hydrogels were thoroughly characterized using rheology, scanning electron microscopy (SEM) and other assays to quantify additional physical parameters, such as the permeability and swelling potential. In this study, we investigated the potential application of the customized hydrogels as 3D cancer culture systems for tumor spheroid growth. To this end, we quantified the spheroid area over time by culturing three cancer cell lines in the supplemented and non-supplemented matrices. Moreover, we assessed the migration capacity of the spheroids with respect to the initial cluster formation.&#x0D; Our results confirmed that LapNC solution could be successfully incorporated into the culture media without clay aggregation. The supplemented hydrogels exhibited enhanced mechanical properties without compromising cellular viability or causing collagen collapse. Regardless of the collagen concentration, LapNC-supplemented hydrogels showed higher storage moduli and lower permeability than control hydrogels, which was qualitatively corroborated by SEM visualization of the porosity. Furthermore, the addition of LapNC resulted in the formation of larger spheroids at every collagen concentration. These findings have significant implications for tissue engineering applications, particularly in cancer spheroid analysis.&#x0D; The successful incorporation of LapNC into collagen hydrogels as nanofillers to modify the rheological and structural properties highlights their potential for tissue engineering applications. Our results demonstrate that the mechanical properties of collagen hydrogels can be enhanced by the addition of LapNC without increasing collagen density. This may provide a powerful method for personalized research and therapies in multiple fields of tissue engineering. In particular, our study suggests that LapNC-supplemented hydrogels can serve as 3D cancer culture systems for stiffness-dependent tumor spheroid growth, opening up new avenues for cancer research and therapy.

Glass Fiber for Improved Behavior of Light Expanded Clay Aggregate Concrete Beams: An Experimental Study

Concrete developed from light expanded clay aggregate (LECA) and glass fiber has good performance, durability, and sustainability. Towards this, the experimental investigation was designed to study cubes, cylinders, and simply supported beams. Four mixtures had LECA volume of 0%, 75%, 85%, and 95% as coarse aggregate replacement and glass fiber content volume of 2% (N, L75, L85, and L95), and the other two mixtures had 75% LECA and glass fiber content of 1% and 1.5% (L75-F1 and L75-F1.5). Results compared to normal concrete showed the weight reduction of samples while adding more glass fiber caused slump reduction in contrast to LECA. Increasing glass fiber volume in the mixture had a negative influence on tensile strength while causing compressive strength enhancement. Moment resistance and energy absorption capacity of L85 were enhanced by 7.5% and 10.3%, respectively. For L75-F1 specimens, the beam stiffness and ductility were enhanced by 14.8% and 14.3%, respectively.

Acid-base treatment of lightweight expanded clay aggregate (LECA) for removal of paraquat from aqueous media

This study focuses on the modification of lightweight expanded clay aggregate (LECA) with sulfuric acid, potassium hydroxide, and sulfuric acid-potassium hydroxide to enhance the removal efficiency of the highly hazardous herbicide (paraquat) from the aqueous solution. The modification results showed that samples activated with sulfuric acid before KOH yielded higher removal efficiency (70.8%) than those activated with acid or base (25.6% and 35.8%, respectively). The paraquat uptake mechanism by LECA was investigated using XRD, XRF, FTIR, FESEM, BET, and zeta potential measurements. Batch adsorption tests were conducted under different conditions using LECA and acid-base-modified LECA. The results indicated the maximum removal efficiency for PQ at 240 min contact time, adsorbent dose of 3.5 g L−1, and pH 9 was achieved with the acid-base treated sample. Experimental data were evaluated with different isothermal, thermodynamic, and kinetic models. The pseudo-second-order model with a correlation coefficient of R2 > 0.99 was congruent with experimental results for LECA and acid-base-activated LECA through kinetic investigations. The highest adsorption capacities for the LECA and acid-base-activated LECA samples, respectively, were 6.12 mg g−1 and 21.23 mg g−1 according to the Langmuir equilibrium isotherm, which offered greater agreement with the experimental results. A negative standard enthalpy change (−21.09 kJ mol−1) and Gibbs free energy change for PQ uptake at 15, 25, and 35 °C indicated an exothermic and spontaneous process. Reusability studies confirmed that the treated adsorbent could successfully remove paraquat even after four regeneration cycles.

Factors affecting bat occupancy of boxes in managed coniferous forests

The gradual conversion of forest stands to single-aged and single-species stands is resulting in the loss of natural roosts for many animal groups. The installation of bat boxes is one solution to compensate for the lack of natural roosting opportunities. The box models differ in their design and material, which to some extent can determine their suitability for bats. We investigated the occupancy of 187 boxes made of wood, ceramic and two sizes concrete with styrofoam; and the intra-seasonal (spring migration, breeding, mating/migration) thermal profile for each type of box. The environment of the boxes was defined by parameters that could directly affect the box’s thermal conditions (solar energy availability), or be related to sociality (distances, obstacles) or food availability (edge of forest, water). The box occupancy depended on the box type and the season: styrofoam-concrete boxes were preferred, with a higher occupancy during the mating and migration period (>75%), whereas the highest species richness occurred in ceramic boxes. Box types also differed significantly in their diurnal thermal profiles: the wooden boxes had inside temperatures similar to outside the box, whereas the styrofoam-concrete and with expanded clay aggregate (ceramic type) averaged 2°C higher. Overall, we found 6 bat species, although we concentrated our analysis on the most common species: Pipistrellus nathusii (88%). For this species, none of the parameters that could affect the box thermal condition had an impact on the occupancy, whereas the presence of obstacles and the distance to a water body, proved to be important. To ascertain that our results may be valid and be the result of differences in the box thermal properties, we tested the relationship of the box occupancy to the latitude, using data available in the literature. The occupancy of wooden boxes from the latter dataset significantly decreases with latitude, whereas for the concrete (with addition of sawdust or styrofoam) it increases, although this relationship is not significant and requires a larger sample.

Comparative Study of the Lightening of Concrete Through the Use of Recycled Tire Rubber and Expanded Clay Aggregate

Abstract The waste of tire rubber is characterized by the generation of huge amounts, which is annually discarded. This waste causes problems for human health and the environment. Thus, the valorization of recycled tire rubber in concrete production is considered as an alternative and sustainable solution. This experimental work is a comparative study, which aims to investigate the possibility of using recycled tire rubber as a fine aggregate in the production of lightweight concrete. To achieve this objective, two types of lightweight concrete containing (35, 70 and 100%) of expanded clay and (5, 10 and 15%) of recycled tire rubber as a fine aggregate were produced. Their fresh properties were investigated using slump, density and air content tests, while the hardened properties were investigated by compressive strength, flexural strength, ultrasonic pulse velocity, and water absorption tests. The results obtained showed that the utilization of both expanded clay aggregate and recycled tire rubber leads to a reduction in density and mechanical strength but increases the porosity of lightweight concrete. However, the lightweight concrete containing the expanded clay aggregate had a higher efficiency factor than those containing recycled tire rubber.

Optimization of dye-contaminated wastewater treatment by fungal Mycelial-light expanded clay aggregate composite

Dye-contaminated wastewaters from the printing batik industry are hazardous if discharged into the environment without any treatment. Finding an optimization and reusability assessment of a new fungal-material composite for dye-contaminated wastewater treatment is important for efficiency. The study purposes to optimize fungal mycelia Trametes hirsuta EDN 082 – light expanded clay aggregate (myco-LECA) composite for real priting batik dye wastewater treatment by using Response Surface Methodology with Central Composite Design (RSM-CCD). The factors included myco-LECA weight (2–6 g), wastewater volume (20–80 mL), and glucose concentration (0–10%) were applied for 144 h of incubation time. The result showed that the optimum condition was achieved at 5.1 g myco-LECA, at 20 mL wastewater, and at 9.1% glucose, respectively. In this condition, the decolorization values with an incubation time of 144 h were 90, 93, and 95%, at wavelengths 570, 620, and 670 nm, respectively. A reusability assessment was conducted for 19 cycles and the result showed that decolorization effectiveness was still above 96%. GCMS analysis showed the degradation of most compounds in the wastewater and the degradation products of the wastewater demonstrated detoxification against Vigna radiata and Artemia salina. The study suggests that myco-LECA composite has a good performance and therefore is a promising method for the treatment of printing batik wastewater.

Conventional and lightweight aggregate one-way reinforced concrete slabs subjected to fire and repeated loads: comparative experimental study

ABSTRACT This study presents experiments on nine one-way, reinforced concrete slabs with a slab weight reduction approach utilising lightweight expanded clay aggregate (LECA). All slabs’ sizes, steel reinforcing, and concrete mix composition were equal. They were subjected to the same repeated linear load and high-temperature conditions. The slabs were classified into three groups based on the presence of LECA. The main variables were the LECA ratio (0, 20, 40) % of coarse aggregate and the exposure temperatures (ambient, 400, 700) ℃. The repeated load was applied according to the ACI-19 code, 50% of the ultimate load for seven cycles. The results showed that the presence of LECA reduced flexural strength and fire resistance. The deflection of the slabs increased with the LECA. The most significant reduction in ultimate load capacity was approximately 35.1%, with a maximum increase in slab deflection of 37.2%. However, the stiffness decay was significant, about 50%.

Efficiency of wastewater purification in medium sand with a lightweight expanded clay aggregate assisting layer

The objective of this experimental study was to examine whether an assisting layer of lightweight expanded clay aggregate (LECA) of the granulation 1–4 mm, introduced into a subsoil, is able to improve an efficiency of removal of total nitrogen and total phosphorus from domestic wastewater. In the investigations, an assisting 0.10 and 0.20 m thick LECA layer was applied. It has been observed that the effectiveness of removal of total suspended solids (TSS), total nitrogen and total phosphorus from wastewater as well as the level of biochemical oxygen demand ( BOD 5) and chemical oxygen demand ( COD) is in accordance with the Polish standards on wastewater disposal into grounds and surface water. The performed experiments showed that the effectiveness of raw wastewater purification for the medium sand soil bed with the 0.20 m thick assisting LECA layer is higher than for the 0.10 m thick assisting layer. In the medium sand soil bed with the 0.20 m thick assisting LECA layer, the removal efficiency regarding total nitrogen increased by 20.6%, total phosphorus by 5.2%, ammonium nitrogen by 8.8% and TSS by 5.3%, and reduction efficiency regarding BOD 5 increased by 1.7% and COD by 2.3% with relation to the 0.10 m thick assisting LECA layer (all percentages – in average). The results of the experiment showed that the LECA with the granulation 1–4 mm can be used to assist in removal of total nitrogen and total phosphorus from wastewater with application of infiltration drainage.

Modelling for determining the thermal conductivity of porous solid materials

The aim of this study was to develop a simple algebraic equation to calculate the effective thermal conductivities of porous solid materials, particularly concrete with porous aggregates. The components of the equation consisted of material porosity and density, and thermal conductivity coefficients of the matrix and pore gas. The equation was applied to ten samples of organic sediment stones, volcanic rocks and porous aggregates, including concrete with expanded clay, expanded polystyrene and pumice aggregates. The thermal conductivity coefficients given by the equation were higher than those measured experimentally (6.62–16.89%). The proposed equation was also compared with six algebraic equations. The results suggest that the proposed algebraic equation can be used to calculate the thermal conductivity coefficients of solid materials containing complex geometrical pores. The newly produced porous samples can be used as light concrete in building.

Influence of Calcined Clay Pozzolan and Aggregate Size on the Mechanical and Durability Properties of Pervious Concrete

Pervious concrete has been reported as a viable solution to reduce stormwater run-off, the heat-island effect, road noise, and pavement flooding. Previous researchers have focused on analysing the structural properties and functionality of pervious concrete. However, relatively few studies have been conducted into the addition of supplementary cementitious materials (SCMs), such as calcined clay, in pervious concrete and its effect on long-term durability. This paper has studied the effect of calcined clay pozzolan as a partial substitute for Portland cement in pervious concrete, together with the influence of coarse aggregate size. A water–binder ratio of 0.4 and aggregate–binder ratio of 4.0, as well as a superplasticiser content of 0.95%, were maintained for all mixes. Two sizes of coarse aggregates were used for this study: 9.5 mm and 20 mm. CEM-I cement was partly substituted with calcined clay in dosages of 0 to 30% in replacement intervals of 5%. The mechanical tests conducted included the split tensile test, compressive strength test, and flexural strength test. Durability measurements such as the rapid chloride permeability test (RCPT), thermal conductivity and sulphate resistance tests were also carried out. The mechanical properties of the pervious concrete followed a similar trend. The results showed that at 20% replacement with calcined clay, the compressive strength increased by 12.7% and 16% for 9.5 mm and 20 mm aggregates, respectively. The flexural strength improved by 13.5% and 11.5%, whereas the splitting tensile strength increased by 35.4% and 35.7%, respectively, as compared to the reference concrete. Beyond 20% replacement, the tested strengths declined. The optimum calcined clay replacement was found to be 20% by weight. Generally, pervious concrete prepared with 9.5 mm obtained improved mechanical and durability properties, as compared to those of 20 mm aggregates.

Bearing Capacity of Footing on Soft Clay Strengthened by Lightweight Expanded Clay Aggregate Raft

This research represents an investigation into the effectiveness of replacement methods to increase the bearing capacity of soft clays under footing load, where Light Expanded Clay aggregates (LECA) were used as a substitute for common aggregate fillers. The soil replacement technique is the easiest and cheapest way to improve soft soil compared to installing a raft footing or using a deep foundation such as piles. LECA is known to be light, strong and environmentally sustainable and is widely used in Geotechnical applications where weight is an issue. The bearing capacity of the footing on soft soil reinforced by LECA was analysed through finite element analysis using commercial software PLAXIS 3D (2020). The soft ground is represented by Hardening Soil (HS) constitutive model, while LECA has been modelled as Mohr-Coulomb (MC). Parametric studies were conducted to assess the effect of LECA raft thickness to bearing capacity improvement for various friction angles of LECA. The research found that the bearing capacity is directly proportional to the internal friction angle of LECA and the LECA raft thickness. Nevertheless, the bearing capacity appears to be almost linear when 2.5 m and 3.5 m thick LECA rafts are used, indicating that the depth of replacement ratio more than 25% give insignificant effect towards improvement ratio.

Variations in soil organic carbon storage and stability with vegetation restoration stages on the Loess Plateau of China

Soil organic carbon (SOC) plays the most significant role in climate change mitigation and ecosystem productivity. However, dynamic changes in the storage and stability of SOC during vegetation restoration and succession, especially the physical protection role of soil aggregates, are not well understood. This study, therefore, explored the variations in SOC storage and stability along with vegetation restoration by combining the physical protection of soil aggregates with the chemical structural stability of organic carbon (OC). OC fractions and carbon functional groups of bulk soil and soil aggregates were investigated using soil samples collected from abandoned farmland, grassland, miscellaneous shrubland, pioneer arbor forest, and coniferous forest plots representing typical vegetation restoration consequence on the Loess Plateau of China. Vegetation restoration enhanced clumps of clay particles with visible pores and cementing substances. The contents of OC fractions (total, inert, and active carbons) in bulk soil and aggregates significantly increased during vegetation restoration process. Total and active OC in macroaggregates (>0.25 mm) were significantly higher than those in other aggregate fractions, while inert OC had no significant difference among different aggregate fractions. The elevated absorption intensity, i.e., absolute content of OC functional groups, especially stable OC functional groups, enhanced SOC chemical stability during vegetation restoration. From the abandoned farmland to miscellaneous shrubland, the relative content of active OC functional groups significantly increased and was primarily distributed in macroaggregates (84.94–86.32%), while silt–clay aggregates (<0.053 mm) had higher relative content of stable functional groups (21.84–26.53%). A significant increase was observed in the relative content of the stable functional group from miscellaneous shrubland to pioneer arbor forest, primarily in silt–clay aggregates (∼27.15%). In the coniferous forest, stable functional groups mainly distributed in macroaggregates also showed the most pronounced increase (∼36.15%). We concluded that vegetation restoration improves the soil structure and storage and stability of SOC due to the increased accumulation of OC in macroaggregates and enhanced chemical stability of OC in silt–clay aggregates from the abandoned farmland to the pioneer arbor forest. However, SOC stability in the coniferous forest is significantly influenced by macroaggregates in the soil.

Effect of clay on methane hydrate formation and dissociation in sediment: Implications for energy recovery from clayey-sandy hydrate reservoirs

Natural gas hydrate (NGH) is an unconventional energy source with high energy density, huge reserves and worldwide distribution. Sand-dominated hydrate-bearing sediments (HBS) are believed to be the most economically and technically feasible category of HBS for exploitation. Clay is one major mineral component of the natural HBS apart from sands. Due to the inhibition effect of clay on thermodynamics and kinetics of hydrate formation, it is challenging to synthesize representative clayey-sandy HBS samples in laboratory, and thus the corresponding fluid production behavior remains poorly understood. In this study, a sample synthesis method dedicated to clayey-sandy HBS (20 wt% clay) was developed. Two synergistic strategies including pre-wetting and multiple-site water injection were employed to enhance hydrate formation kinetics, increasing the final hydrate saturation from 0.285 to 0.473. Water-rich clayey-sandy HBS samples were formed with no dry sediment zones and improved hydrate distribution homogeneity. This optimized HBS synthesis method fills the gap between the conventional HBS synthesis methods and the need to form more representative clayey-sandy HBS samples. Energy recovery and fluid production from sandy and clayey-sandy HBS samples were investigated by depressurizing to 3.0 MPa at 6.0 °C. Unique features were observed for hydrate dissociation in clayey-sandy HBS: immediate hydrate decomposition upon depressurization and a great pressure difference (∼1.0 MPa) between sediment and wellbore during the production period. This pressure gradient, likely caused by the aggregation and clogging of clays near the wellbore, slowed hydrate decomposition compared to sandy HBS. Additionally, the gas recovery increased from 81.7% to 88.5%, whereas the water recovery substantially decreased from 25.3% to 8.1%. The significant difference in fluid production behavior between clayey-sandy HBS and sandy HBS calls for more thorough investigations into clay-containing HBS to develop specific production strategies for energy recovery from this type of HBS.