Pore Saturation Research Articles

Dye tracing of upward brine migration in snow

Abstract Salt is often present in the snow overlying seasonal sea ice, and has profound thermodynamic and electromagnetic effects. However, its provenance and behaviour within the snow remain uncertain. We describe two investigations tracing upward brine movement in snow: one conducted in the laboratory and one in the field. The laboratory experiments involved the addition of dyed brine to the base of terrestrial snow samples, with subsequent wicking being measured. Our field experiment involved dye being added directly (without brine) to bare sea-ice and lake ice surfaces, with snow then accumulating on top over several days. On the sea ice, the dye migrated upwards into the snow by up to 5 cm as the snow's basal layer became more salty, whereas no migration occurred in our control experiment over non-saline lake ice. This occurred in relatively dry snowpacks where brine took up $< 6\%$ of the snow's calculated pore volume, suggesting pore saturation is not required for upward salt transport. Our results highlight the potential role of microstructural parameters beyond those currently retrievable with penetrometry, and the potential value of longitudinal, process-based field studies of young snowpacks.

Revealing the mechanisms of water permeability enhancement of urban loess subgrades due to vibration application

Urban loess subgrades are affected by considerable vibrations from traffic, especially when the underground pipelines leak, and seepage under vibrations often causes road damage. However, the influence of vibrations on the water permeability of loess subgrades remains elusive. Here we address this issue by performing vibration-assisted permeability tests and scanning electron microscopy, mercury intrusion porosimetry, and suction–nuclear magnetic resonance measurements. This allowed the evolution of the saturated hydraulic conductivity (Ks), water-air migration, soil microstructure, and pore water forms to be evaluated. The water permeability of the loess subgrade is promoted by vibrations due to the increase in Ks, the acceleration of wet front migration, and the escape of entrapped air. Moreover, the value of Ks under vibration is 3–14 times greater than that without vibration, and the maximum increase occurs at a vibration frequency near the natural frequency of the loess. Furthermore, a theoretical framework of loess vibration permeability is proposed, and the mechanisms by which vibration accelerates the permeability behavior of the loess subgrade are revealed. Vibration promotes the expansion of soil pores, a decrease in the binding capacity of pore water, the mobilization of fine particles, and the formation of local low-permeability layer. Moreover, it accelerates the opening of entrapped air bubbles and the displacement of water-air, the reduction in seepage resistance. Thus, seepage water flows rapidly along infiltration channels. These findings are highly important for the road safety performance and the sustainable development of the traffic environment in loess regions.

Flow unit classification and characterization with emphasis on the clustering methods: a case study in a highly heterogeneous carbonate reservoir, eastern margin of Dezful Embayment, SW Iran

Previous attempts to classify flow units in Iranian carbonate reservoirs, based on porosity and permeability, have faced challenges in correlating the rock's pore size distribution with the capillary pressure profile. The innovation of this study highlights the role of clustering techniques, such as Discrete Rock Type, Probability, Global Hydraulic Element, and Winland's Standard Chart in enhancing the reservoir's rock categorization. These techniques are integrated with established flow unit classification methods. They include Lucia, FZI, FZI*, Winland R35, and the improved stratigraphic modified Lorenz plot. The research accurately links diverse pore geometries to characteristic capillary pressure profiles, addressing heterogeneity in intricate reservoirs. The findings indicate that clustering methods can identify specific flow units, but do not significantly improve their classification. The effectiveness of these techniques varies depending on the flow unit classification method employed. For instance, probability-based methods yield surpassing results for low-porosity rocks when utilizing the FZI* approach. The discrete technique generates the highest number of flow unit classes but provides the worst result. Not all clustering techniques reveal discernible advantages when integrated with the FZI method. In the second part, the study creatively suggests that rock classification can be achieved by concurrently clustering irreducible water saturation (SWIR) and porosity in unsuccessful flow unit delineation cases. The SWIR log was estimated by establishing a smart correlation between porosity and SWIR in the pay zone, where water saturation and SWIR match. Then, the estimated saturation was dispersed throughout the reservoir. Subsequently, the neural network technique was employed to cluster and propagate the three finalized flow units. This methodology is an effective recommendation when conventional flow unit methods fail. The study also investigates influential factors causing the failure of flow unit classification methods, including pore geometry, oil wettability, and saturation in heterogeneous reservoirs.

Observations on the role of internal sand moisture dynamics in wave-driven dune face erosion

Understanding and predicting dune erosion is crucial for coastal hazards mitigation, ecosystem preservation, as well as the protection of human settlements and infrastructure along sandy coastlines. However, our knowledge regarding the influence and potential importance of internal sand moisture content on wave-driven dune face erosion processes remains limited. This paper presents the findings from an extensive series of controlled wave flume experiments of eroding, unvegetated dunes, combining a range of wave conditions and water levels. A further variable that is studied directly for the first time is the internal moisture content of the dune. The initial moisture content and its evolving dynamics within the eroding dune face are quantified, revealing this to be a determining factor of the observed erosion rates, the final horizontal erosion distance, and the dune face failure type. Importantly, infiltration into the dune with each wave was not a driving mechanism of observed failures, but rather the rapid increase and then decrease of the phreatic surface within the dune, resulting in excess pore pressures and destabilization of the sediment matrix. Based on these observations two distinct mechanisms of shear failure and resulting dune face slumping are identified for unsaturated dunes corresponding to ‘minimum’ and ‘field capacity’ internal moisture content: Type 1 is associated with a circular failure surface in the absence of notching at the base of the dune, while for Type 2, notching is present and the failure surface is vertical. Under fully saturated conditions, the phreatic surface is observed to decouple from the wave runup, with excess water continuously exiting the dune face near the base. This results in rapid shear failure with no notching (Type 3). Significantly, dunes with saturated initial pore moisture content above the capillary fringe are observed to have up to 35 % greater erosion potential, consistently receding further landward than the two unsaturated counterparts as well as undergoing slumping in the absence of wave impact. A new conceptual framework is presented, comprising of a three-phase erosion sequence that directly links dune face failure mechanisms to wave runup and prevailing groundwater conditions.

Quantifying the water saturation degree of cement-based materials by hydrogen nuclear magnetic resonance (1H NMR)

This study investigated the T2 spectrum of hydrogen nuclear magnetic resonance (1H NMR) signals in cement paste with water-to-cement ratios (w/c) of 0.2, 0.35, and 0.5 under three different saturation methods (natural soak, vacuum saturation, and vacuum saturation with high pressure). The saturation degree and pore structure of the cement paste under different saturation methods has been calculated through the linear relationship between water absorption and increment of NMR amplitude intensity. The results showed that the saturation degree of the cement paste increased progressively with the duration of saturation until it reached a state of equilibrium. It can increase saturation degree and reduce the saturation time by increasing the saturation pressure, and vacuum saturation with high pressure is necessary for cement paste with lower w/c. It is important to noted that conditions such as unsaturation, natural soak, and vacuum saturation may result in incomplete water saturation, leading to a partial loss of the 1H NMR signal amplitude intensity and underestimation of porosity measurements. Considering the duration of saturation and the degree of saturation, it is recommended a saturation duration of at least 4 h with a vacuum pressure of 8 MPa for cement paste with w/c ratio of 0.2 and 0.35. For cement paste with w/c of 0.5, a saturation duration of at least 0.5 h with a vacuum pressure of 8 MPa is suggested.

Characterization and damage of the historic brick masonry of the Zákupy castle stables

Abstract A chemical-mineralogical analysis and mechanical tests of historical bricks were carried out as part of the construction-technical survey. The objective was to identify the causes of significant masonry decay. The ruined agricultural yard is a national cultural monument. Conservators believe that the historical importance exceeds that of the neighbouring castle. Stables were designed in 1650 by prominent Italian builders Giulio Broggio and Giovanni Domenico Orsi. The building has not been maintained since the conclusion of World War I. After removal from the masonry, the analyzed samples of bricks reached a very high, almost total degree of water saturation - 87-94%. Some bricks are heavily contaminated with nitrates and sulfates, infrared spectrometry analysis showed that calcium nitrate tetrahydrate is the most abundant of the nitrates. The determined average values of physical properties (bulk weight 1764 kg/m3, capillary absorption 13.6% by weight and open porosity 24% by volume) from samples under atmospheric pressure are comparable to the values of masonry bricks produced today. The results of the tests on the bricks from the Zákupy Castle stables construction demonstrate the high manufacturing quality of the masonry elements utilized. Based on the results of the analyses, it can be assumed that three degradation factors contribute to the observed damage to the bricks: high wetting of the masonry (almost total saturation of pores with water), which is most likely long-term lasting, frost cycles - repeated freezing and thawing of water in the bricks during the winter months, and repeated crystallization of water-soluble salts present in pores near the surface.

Mechanistic study on the significant difference in flotation collector consumption of different coal gasification fine slag

ABSTRACT Froth flotation, a prevalent technique for the separation of fine materials, is employed in the decarburization process of coal gasification fine slag (CGFS). This process is known to consume significant quantities of reagents. This study investigates the pronounced variance in collector consumption during the flotation decarbonization of two distinct CGFS samples, where a tenfold difference was observed when the tailings ash content surpassed 95%. A comprehensive set of analyses, including XPS, BET, SEM, wettability tests, LF-NMR and IGC-SEA, was conducted on the residual charcoal from CGFS to elucidate the mechanism behind reagent consumption. The findings reveal that the reagent’s ability to alter the surface hydrophobicity of residual charcoal is contingent upon the adsorption saturation of micropores and transition pores. Furthermore, variations in porosity and surface energy significantly impact collector consumption, independent of differences in the type and quantity of oxygen-containing functional groups. This investigation aims to provide a theoretical framework for reducing the dosage of reagents in the flotation decarburization of CGFS.

Radon deficit technique applied to the study of the ageing of a spilled LNAPL in a shallow aquifer

A recent diesel spill (dated January 2019 ± 1 month) in a refilling station is investigated by the Radon deficit technique. The primary focus was on quantifying the LNAPL pore saturation as a function of duration of ageing, and on proposing a predictive model for on-site natural attenuation. A biennial monitoring of the local fluctuating shallow aquifer has involved the saturated zone nine times, and the vadose zone only once. Rn background generally measured in external and upstream wells is elaborated further due to the site characteristics, using drilling logs and phreatic oscillations. Notably, this study marks the first application of the Rn deficit method to produce a detailed Rn background mapping throughout the soil depth. Simultaneously, tests are performed on LNAPL surnatant samples to study diesel ageing. In particular, they are focused on temporal variations of LNAPL viscosity (from an initial 3.90 cP to 8.99 cP, measured at 25 °C, after 34 months), and Rn partition coefficient between the pollutant and water (from 47.7 to 80.2, measured at 25 °C, after 14 months). Rn diffusion is also measured in different fluids (0.092 cm2 s−1, 1.14 × 10−5 cm2 s−1, and 2.53 × 10−6 cm2 s−1 at 25 °C for air, water and LNAPL, respectively) directly. All parameters and equations utilized during this study are introduced, discussing their influence on Radon deficit technique from a theoretical point of view. Experimental findings are used to mitigate the effect of LNAPL ageing and of phreatic oscillations on determination of LNAPL saturation index (S.I.LNAPL). Finally, S.I.LNAPL dataset is discussed and elaborated to show the pollutant attenuation across subsurface over time, induced by natural processes primarily. The proposed predictive model for on-site natural attenuation suggests a half-removal time of one year and six months. The significance of such models lies in their capability to assess site-specific reactions to pollutants, thereby enhancing the effectiveness of remediation efforts over time. These experimental findings may offer a novel approach to application of Rn deficit technique and to environmental remediation of persistent organic compounds.

Understanding the filling effect of ice on the mechanical properties of calcium silicate hydrate gel

Ice exhibits good mechanical performance at low temperatures, and when it replaces the weakest pores in cement-based materials, it can noticeably enhance the overall mechanical properties of the material. However, the specific micro-mechanisms that contribute to the improvement of tensile mechanical properties in gel pores at low temperatures are still not fully understood. Based on the results of molecular dynamics simulations, significant roles have been attributed to pore size and temperature in the mechanical performance of gel pores at low temperatures. A thorough investigation was conducted on the process of tensile failure in saturated gel pores, as well as the changes in bonding interactions between calcium silicate hydrate (CSH)-ice and the unfrozen water film (UWF) at different pore sizes and temperatures. The results indicate that increasing pore size and decreasing temperature can improve the mechanical properties of saturated gel pores. With the decrease in temperature, the mechanical properties of the three gel pore systems with heights of 100 Å, 68 Å, and 54 Å were significantly enhanced. Under an engineering strain rate of 8×10−4/ps, the tensile strength and elastic modulus of gel pores increased from 0.57 GPa and 17.81 GPa at 250 K, to 0.97 GPa and 20.87 GPa at 150 K, respectively. To explain this phenomenon, further studies were conducted on the radial distribution function and dynamic hydrogen bonding behavior. The critical contribution of the hydrogen bond network to the mechanical performance of gel pores was discovered. The hydrogen bond structure which experienced the earliest damage, was investigated. As the strain increased, the hydrogen bonds continuously broke and reformed. This research provides a theoretical understanding of the mechanical properties of gel pores under low-temperature conditions.

Stochastic Periodic Microstructures for Multiscale Modelling of Heterogeneous Materials

Plurigaussian simulation is a method of discrete random field generation that can be used to generate many complex geometries depicting real world structures. Whilst it is commonly applied at larger scales to represent geological phenomena, the highly flexible approach is suitable for generating structures at all scales. Here, an extension of plurigaussian simulation to periodic plurigaussian simulation (P-PGS) is presented, such that the resulting fields are periodic in nature. By using periodic Gaussian random fields as components of the method, periodicity is enforced in the generated structures. To substantiate the use of P-PGS in capturing complex heterogeneities in a physically meaningful way, the pore-scale microstructure of cement paste was represented such that its effective properties can be calculated through a computational homogenisation approach. The finite element method is employed to model the diffusion of heat through the medium under dry and saturated pore conditions, where numerical homogenisation is conducted to calculate the effective thermal conductivity of the medium. Comparison of the calculated values with experimental observations indicated that the generated microstructures are suitable for pore-scale representation, given their close match. A maximal error of 1.38% was observed in relation to the numerically determined effective thermal conductivity of mortar paste with air filled pores, and 0.41% when considering water filled pores. As the assumption of a periodic domain is often an underlying feature of numerical homogenisation, this extension of plurigaussian simulation enables a path for its integration into such computational schemes.Article HighlightsIntegrating P-PGS into numerical homogenisation frameworks enhances complex heterogeneous material representationThe flexibility of P-PGS enables a wide range of material microstructures to be represented accuratelyUse of the generated structures allows material properties to be estimated accurately through numerical homogenisation

Engineered bioclogging in sands: comparison of microbially induced and enzyme-induced biopolymer formation

This study presents two methods of producing an insoluble biopolymer − microbially induced biopolymer formation (MIBF) and enzyme-induced biopolymer formation (EIBF) − and explores their ability to reduce hydraulic conductivity and cause bioclogging in soil from pore to column scales. The batch experiments confirm that insoluble polysaccharidic biopolymers, dextran, are successfully produced either by the model bacteria or by the extracted cell-free enzyme. The results show that the EIBF method is more efficient in producing biopolymer and reducing hydraulic conductivity compared to the MIBF method. This study also uses microfluidic chips, which reveals the pore-filling behaviour of biopolymers produced by both methods. EIBF produces larger dextran flocs than MIBF, and hence EIBF lowers the hydraulic conductivity more than MIBF for a given pore occupancy of dextran. Column experiments demonstrate that both MIBF and EIBF can significantly lower the hydraulic conductivity of coarse sands by two orders of magnitude with only 3% biopolymer pore saturation. The presented results suggest that both methods have the potential to induce well-controlled, engineered bioclogging in coarse-grained soils, and have applications in various geotechnical practices, such as sealing leakage in water-front structures, installing hydraulic barriers and mitigating soil erosion.

Study of changes in the induced wave field in mining waste storage facilities

An integrated approach to analyzing changes in the induced wave field in mining waste storage facilities for mineral waste is considered to localize potentially dangerous zones in the body of bulk structures using georadar sounding. It has been established that the zone of possible water saturation of rocks is perceived by the georadar signal as a transition to a medium with a higher dielectric constant and is reflected by the maximum amplitude of the signal. The boundaries of water saturation zones are not always sharply defined, but change smoothly depending on the degree of water saturation from a natural state unsaturated with moisture to complete saturation of soil pores. During GPR sounding, below the expected zone of water saturation, the soil structure is noticeably smoothed out, where, due to increasing attenuation, the signal amplitude sharply drops, which manifests itself in a noticeable decrease in the contrast of the wave pattern of the radargram. The use of intra-method integration by profiling on a permanent base and a common depth point with simultaneous synchronization of groundwater level measurements in a piezometer made it possible to identify and identify zones of increased water saturation in the dam body.

Synthesis, characterization, and application of N-CNT/1-(2-Hydroxyethyl)-3-methylimidazolium dicyanamide as a green nanocatalyst for the sulfur removal from light oils

Adsorptive desulfurization of light fuels is sustainable due to its ambient operation and reusability of exhausted adsorbents. In this study, 1-(2-hydroxyethyl)-3-methylimidazolium dicyanamide [HEMIM][DCA] IL was synthesized and utilized to modify N-doped carbon nanotubes (CNTs) to produce N-CNT/[HEMIM][DCA] as a green hybrid adsorbent. The adsorbent was characterized using XRD, FE-SEM, FTIR, BET, and TGA. It was indicated that the N-CNT treatment with [HEMIM][DCA] IL resulted in decreased crystallinity with the cubic and rod-shaped morphology and harsh surfaces and curved edges. The absence of shifts or variations in FTIR peaks of starting materials and N-CNT/[HEMIM][DCA] suggested that neither component was affected by chemical interactions. The adsorption capacity of N-CNT and N-CNT/[HEMIM][DCA] was 54.3 mg/g and for 83.6 mg/g for 50 ppm BT, respectively. Saturated with BT, the adsorbent's performance was decreased at high BT concentrations. The adsorption isotherms provided an understanding of interactions of BT with sorbent surface which follows the Langmuir model for N-CNT/[HEMIM][DCA] and N-CNT. The kinetics of BT adsorption on N-CNT/[HEMIM][DCA] was fitted with second-order kinetic model with the decreased adsorption ratio over time due to pore saturation. 25 % reduction of the adsorption capacity was obtained after two recycling cycles of the adsorbent (62.5 mg/g). N-CNT/[HEMIM][DCA] showed good recyclability and potential as a promising BT adsorbent.

A novel lattice model to predict chloride diffusion coefficient of unsaturated cementitious materials based on multi-typed pore structure characteristics

This paper develops a novel lattice diffusive model to quantitatively study the chloride diffusion coefficient in unsaturated cementitious materials, in which the pore voxels are redistributed to make a better representation of a real microstructure of hardened cement paste. Considering the hierarchical microstructure and different drying-wetting cycles, water distributions in multiscale pore structures are modelled and the structure characteristics of water-filled pores, including water connectivity, water tortuosity and effective porosity, are computationally extracted based on that. A lattice diffusion network is established to predict relative chloride diffusion coefficient by combining the effect of both water saturation degree and pore structure characteristics. The predicted results are validated against experimental data, and a concise analytical equation is proposed to predict the relative chloride diffusion coefficient. The equation indicated that the relative chloride diffusion coefficient is proportional to water connectivity but inversely proportional to the square of water tortuosity. Besides, the lattice model's quantitative results reveal that the water connectivity and water tortuosity are highly related to pre-water loading processes, and influenced by the gel pore fraction, which in turn will affect the relative chloride diffusion coefficient. Compared with existing equations and non-redistributed models, the present model could improve the prediction accuracy significantly.

Effects of Pore Geometry and Saturation on the Behavior of Multiscale Waves in Tight Sandstone Layers

AbstractGeometric heterogeneities in tight reservoir rocks saturated with a fluid mixture may exhibit different scale distribution characteristics. Conventional models of rock physics based on poroelasticity, which usually consider single‐scale pore structure and fluid patches, are inadequate for describing elastic wave responses. A major challenge is to establish the relationship between the wave response at different spatial scales and frequencies. To address this problem, three sets of observational data over a wide frequency range were obtained from a tight oil reservoir in the Ordos Basin, China. Ultrasonic measurements were made on eight sandstone samples at partial oil‐water saturation at 0.55 MHz. Data from six borehole measurements and seismic profiles were acquired and analyzed at about 10 kHz and 30 Hz, respectively. Analysis of the cast thin sections shows that dissolution pores and microcracks generally develop, with fractal dimensions of the pores ranging from 2.45 to 2.67 for the samples with porosities between 5.1% and 10.2%. Compressional wave velocity and attenuation were estimated from the observed data. The results show that the velocity dispersion from seismic to ultrasonic frequencies is 10.02%, mostly occurring between sonic and ultrasonic frequencies. The attenuation is stronger at higher oil saturation. The relationships between velocity, attenuation, and wavelength were established and can be used for further forward modeling and seismic interpretation studies. A partial saturation model has been derived based on effective differential medium theory and a double double‐porosity model, assuming that the medium contains fractal cracks and fluid patches. The effects of scale and saturation on wave responses are prevalent. Modeling results consistent with observed data show that the radii of cracks and fluid patches range from 0.1 μm to 2.8 mm, affecting ultrasonic, acoustic, and seismic attenuation. The multiscale data and proposed model quantify the relationship between fracture and fluid distributions and attenuation and could be useful for upscaling to the reservoir scale. The study helps improve the understanding of seismic wave propagation in partially saturated rocks, which has potential applications in seismic exploration, hydrocarbon production in reservoirs, and CO2 sequestration in aquifers.

Comparison between mechanical and hydrological reinforcement effects of cultivated plants on shallow slope stability

Root reinforcement, provided by plants in soil, can be exerted by a mechanical effect, increasing soil shear strength for the presence of roots, or by a hydrological effect, induced by plant transpiration. No comparisons have been still carried out between mechanical and hydrological reinforcements on shallow slope stability in typical agroecosystems. This paper aims to compare these effects induced by sowed fields and vineyards and to assess their effects towards the shallow slope staibility. Root mechanical reinforcement has been assessed through Root Bundle Model-Weibull. Root hydrological reinforcement has been evaluated using an empirical relationship with monitored or modelled pore water pressure. Each reinforcement has been inserted in a stability model to quantify their impacts on susceptibility towards shallow landslides. Considering the same environment, corresponding to a typical agroecosystem of northern Italian Apennines, land use has significant effects on saturation degree and pore water pressure, influencing hydrological reinforcement. Root hydrological reinforcement effect is higher in summer, although rainfall-induced shallow landslides rarely occur in this period due to dry soil conditions. Instead, in wet and cold periods, when shallow landslides can develop more frequently, the stabilizing contribution of mechanical reinforcement is on average higher than the hydrological reinforcement. In vineyards, the hydrological reinforcement effect could be observed also during autumn, winter and spring periods, giving a contribution to slope stability also in these conditions. This situation occurs when plants uptake enough water from soil to reduce significantly pore water pressure, guaranteeing values of hydrological reinforcement of 1–3 kPa at 1 m from ground, in agreement with measured mechanical root reinforcement (up to 1.6 kPa). These results suggest that both hydrological and mechanical effects of vegetation deserve high regard in susceptibility towards shallow landslides, helping in selection of the best land uses to reduce probability of occurrence of these failures over large territories.

Photon Doppler velocimetry for resolving vertical penetration into sand targets

This paper presents details for using a photon Doppler velocimeter (PDV) to produce high fidelity velocity penetration data from vertical penetration into granular soil targets. The PDV theory used for collecting data is presented, along with details relevant to soil penetration testing. Experimental aspects related to specific equipment used, measurement configurations implemented, power losses, and probe alignment are presented in order to facilitate replication. The PDV equipment were mounted on a vertical gas gun and used to measure conical and blunt nose rod projectile velocities in penetration into loosely packed and densely packed soil targets. An in-depth discussion of data smoothing and filtering, data sampling and repeatability is presented. Penetration tests in several soil targets are used to reveal important phenomenology of penetration in granular media. The roles of soil target bulk density and pore saturation are investigated in terms of Poncelet drag and bearing resistance parameters. It is shown that bulk density plays a major role in penetration resistance while saturation has a secondary effect. The Poncelet drag coefficient was found to be projectile nose shape dependent. The drag coefficient was as high as 2.0 in penetration of blunt nose projectiles in densely packed sand, while a conical nose reduced the drag coefficient to 1.4. Saturation reduced the drag coefficient by 32 % and 28 % in densely packed and loosely packed soil targets, respectively.

Experimental Study on the Electrical Resistivity Characteristics of Uniformly Graded Calcareous Sand

The resistivity of calcareous sand is one of its inherent physical characteristic parameters, and the study of the resistivity characteristics of calcareous sand is of great significance in the subsequent study of the relationship between the resistivity of calcareous sand under different loads and its mechanical properties. This study uses alternating current (AC) electrical resistance testing equipment based on the two-electrode method to study the effects of pore water, temperature, relative density, saturation, and other factors on the resistivity of calcareous sand. We used the experimental results to analyze in detail the effects of various influencing factors on the resistivity of calcareous sand and established a relationship between them. Research has shown that as the temperature increases, the conductivity of the medium in the solution increases and the resistivity decreases. The resistivity of the calcareous sand of the three size fractions decreases with increasing saturation, and the relationship between them is a power function. The resistivity of saturated calcareous sand with three size fractions increases with increasing density, and the two show a good linear relationship. The results of this research can provide support for the study of the relationship between the resistivity and mechanical properties of calcareous sand under different loads.

Evaluation of slake durability for carbonaceous rocks exposed to water soaking using electrical resistivity

The present study investigated the electrical conduction and slake durability behaviors of carbonaceous rocks subjected to water soaking. Several series of electrical resistivity and slake durability tests were conducted on carbonaceous rocks with different mineralogical characteristics to explore the intrinsic connection between these two property indexes. Based on the experimental data, a new empirical model was developed to estimate slake durability index of carbonaceous rocks at varied soaking times. The results showed that the clay minerals content of investigated carbonaceous rocks ranged from approximately 20 % to 50 %. The clay minerals are beneficial in enhancing the electrical conduction of carbonaceous rocks, but impose a negative impact on the slake durability, attributing to their superior performance of hydrophilicity and swelling. Electrical resistivity decreased sharply with increasing soaking time within the first 8 days and then it achieved a stable state as soaking time further increased, which was mainly due to the completion of pore saturation and salts/minerals dissolution. The slake durability index exhibited similar evolution characteristics to those of electrical resistivity, which deteriorated sharply to a low or very low level after soaking for 16 days. The clay minerals experienced water absorption and mineral expansion during the first 8 days soaking, leading to the development of micro-cracks that facilitates water intrusion and accelerates the slake durability deterioration. These two processes were essentially complete as the soaking time further increased, and the clay minerals played an insignificant role at this stage. Liner increasing trends were observed in the plot of electrical resistivity decrease versus slake durability index decrease, and the slope values presented a power function relation with the clay minerals content of carbonaceous rocks. The proposed empirical model well captured the trends in the experimental data, with an absolute difference in percentage δ below30 % and a root mean square error RMSE value of 5.613 %. The outcomes of this study provided a viable answer to assess slake durability of carbonaceous rocks exposed to adverse external environments using non-destructive testing techniques.

Understanding the negative effects of alkalis on long-term strength of Portland cement

It has long been known that alkalis favor an increase in the early strength of cementitious materials but have a negative impact on long-term strengths. The reasons for the negative effect on later strengths are not clear. In this paper we investigated the influence of alkali addition on the properties of a white cement and for the first time propose a plausible mechanism for the late strength reduction. Quantitative XRD analysis and strength results showed that the addition of alkali generally limits the total degree of hydration (DoH) after 1 day. The evolution of internal relative humidity (RH) and saturated pore radius indicate that the more alkali added, the faster the decrease of internal RH and desaturation of the larger pores. The remaining solution in higher alkali system is concentrated in the smaller pores, where the needed supersaturation is higher. As a consequence, the hydration rate of cement is depressed by added alkali.