Influence of salt on the microstructure of loess under various drying conditions

Lime treatment of loess in foundation engineering modifies the soil structure, leading to changes in mechanical and hydraulic properties of soil, which in turn will affect the flow of water and transport of contaminants in the loess. In light of this, it is essential to identify the dominant effects of different lime treatments on hydraulic conductivity, and to ascertain the optimum lime treatment. For this purpose, we investigated the effects of dry density and lime content on changes in hydraulic conductivity and microstructure of loess in Yan’an City, China. The results indicate that hydraulic conductivity has a log negative correlation with dry density, and lime addition can result in a decrease of hydraulic conductivity of loess at the same dry density. Under a given degree of compaction, however, lime addition can lead to a decrease in dry density due to an increase in flocculation and aggregations. The significant decrease of dry density leads to an increase in hydraulic conductivity when lime content (in mass percentage) is lower than 3%. Nevertheless, when lime content is higher than 3%, the reactions between loess particles and lime will be intensified with an increase in lime content, and become the primary factors affecting pore characteristics. These reactions can further decrease the hydraulic conductivity of lime-treated loess, and the lowest hydraulic conductivity was obtained for lime-treated loess with 9% lime content. The excess lime (above 9% lime content) dramatically increased pore size, leading to a significant increase in hydraulic conductivity. Therefore, 9% is the optimum lime content for loess treatment, and the degree of compaction in engineering should be higher than 95%. In addition, statistical analysis of microstructure of lime-treated loess shows that the distribution trends of macro- and meso-pores coincided with that of saturated hydraulic conductivity, which indicates that lime content affects saturated hydraulic conductivity of lime-treated loess by changing the soil structure, especially the properties of pores larger than 8 µm.

Whether soil desiccation causes a fall in peak population abundances of Pratylenchus neglectus after wheat growth was investigated by drying a vertisol through a range of moisture contents (MC) by both fast and slow drying methods. Live nematodes were then extracted for 2 and 7 days and counted in four life stages (adult and juvenile stages J2, J3 and J4). The population of P. neglectus declined more steeply with decreasing soil moisture content from fast drying than from slow drying. These trends were modelled by regression equations. Upon desiccation of the soil from an initial 64.4% MC to the plant permanent wilting point (31.5% MC), 48% of the nematodes survived after fast drying, but 86% survived after slow drying. With further desiccation to 21% MC only 23% of the P. neglectus population survived after fast drying, whereas 81% of the population survived after slow drying. On continued drying, the fast dried soil was at 11.9% MC after 7 days with virtually no surviving P. neglectus, whereas the slowly dried soil reached 16.5% MC after 8 days with 79% surviving. The P. neglectus population declined by 2.2 and 0.5% per 1% loss of soil moisture for fast and slow drying, respectively. With fast drying, all life stages declined; however, with slow drying, only the J2 stage declined with 44% surviving at 16.5% MC. This information can be applied to crop growth modelling, to extraction techniques for soil from the glasshouse and field, and to consideration of whether molecular methods measure DNA from recently dead nematodes killed by soil desiccation in the field giving an overestimation of the number of live nematodes.

Salt crystallization in the pore spaces of building stones can produce significant deterioration. The properties of the salt solution, the salt phases and the climatic conditions, as well as the rock fabric, significantly influence the state of rock weathering. To examine the influences of rock fabric and salt type on salt weathering, detailed investigations were performed on three sandstones. The fabric (mineralogical composition, grain size, etc.) and the petrophysical properties (porosity, pore-size distribution and hygric dilatation) of the sandstones were analysed and correlated with length changes during cyclic salt loading. The salt tests were carried out with two different salt types: (i) sodium sulphate and (ii) sodium chloride. The observed length changes differ for the investigated sandstones. Contractions of the samples, as well as a pronounced residual strain after the applied salt cycles, were observed. Specific deterioration features can be determined for the sandstones independent from the salt types used. However, the decay mechanisms, which lead to a significant deterioration, are different for sodium sulphate and sodium chloride. For sodium sulphate, a strong expansion occurs during the solution uptake cycles. This expansion can be attributed to hydration pressure during the transition from the water-free thenardite to the hydrate phase mirabilite. In contrast, the samples in the sodium chloride test show the main expansion in the drying stage. This can be related to the crystallization pressure caused by the growth of halite.

It is widely acknowledged that the mechanical behavior of a soil is significantly influenced by the soil microstructure; and the microstructure can evolve as a result of any mechanical, hydraulic, chemical or thermal change taken place on soil sample. The present study aimed to investigate the microstructures of intact loess and remolded loess and to explore the evolution of the microstructure and PSD (pore-size distribution) due to consolidation for both intact and remolded loess. A loess from the Loess Plateau of China was used as test material. A series of intact loess specimens were consolidated to various vertical pressures in oedometer cells. The same loess was remolded at the optimum water content state to various compaction degrees using static compaction method. It is equivalent to consolidate the remolded loess with the least compaction degree under various stress levels. The microstructures of intact loess specimens after consolidation tests and remolded loess specimens are characterized using the SEM (scanning electron microscope) and MIP (mercury intrusion porosimetry) techniques. The micrographs and PSDs under various consolidation pressures (or compaction energies) were compared to investigate the evolution of the microstructure and PSD induced by mechanical loading for each kind of soil. The microstructure of intact loess is more homogeneous and is less dependent on consolidation stress than remolded loess. In both soils, the pores with entrance diameter smaller than 6 μm are almost not influenced by consolidation. In intact loess, inter-aggregate pores with entrance diameter greater than 6 μm are compressed randomly due to loading. However, in remolded loess, the pores are compressed until all larger pores have been compressed. The cumulative pore volume curve of remolded loess of any compaction degree can be divided into two segments, large-pore segment (6–50 μm) and small-pore segment (< 6 μm). The large-pore segment is simplified into a horizontal line and a straight line which slope is independent on the compactness. Remolded loess has very different microstructure and PSD from intact loess even though they may have the same GSD (grain-size distribution), mineralogical composition, and some other physical properties. The microstructural evolution induced by mechanical loading also varies in both kinds of soils. Based upon the measured PSDs, a method is proposed for predicting the PSD of remolded loess of any compaction degree using a reference PSD.

The Loess Plateau of China, located to the west of the Liupan Mountains and north of the Qinling Mountains stretching across the Yellow River, is the main loess deposit area in Northwest China. The loess in the northwest of China has deposits with large thickness and extensive distribution. Scanning electron microscopy (SEM) and energy spectrum analysis of loess have been performed to study the relationship between the loess microstructures and the forming climates Era. The relevant indexes were evaluated including the sand‐dropping speed (Vn), certain sedimentary depths (hn sand particle volume (Vd) and element ratios of Ca/Fe, K/Al, Si/Al and Ca/Mg. The microstructure indexes of loess accumulation and evolution reflect paleoclimate conditions and time scales to a certain extent. The important discovery is the microscopic sand‐dropping speed (Vn) and the sedimentary depth hn) calculation method of different sedimentary ages. These indices are compared with the record of major aeolian‐forming climates from the Guliya ice core, and provide a reliable benchmark for studying climate change It also can be used as important indicators of monsoonal change and environmental evolution reconstruction. The index of sand sedimentation speed (Vn) got from loess microstructure could reflect sand‐dropping speed and loess deposition course. According the article can serve as new indicators of climatic changes of different forming loess layers. It can also be concluded that the climatic indexes obtained from loess microstructure can reflect climate conditions of loess forming. The loess forming climatic parameters are synchronous correspond to Tengger Desert and Guliya ice core for studying climate change, then microscopic parameters can also be used for preliminary analysis of loess climate formation and has be found corresponding evidence, and the loess climatic parameters correspond to the other two indexes. The analysis of loess microstructure indexes is very useful in researching climate change. Loess microstructure indexes can find new indicators and information about the monsoon climate evolution and paleoclimate changes.

A laboratory salt weathering experiment was performed using five salts to attack eight types of rocks to determine the relative significance of rock durability and salt aggressivity to salt crystallization damage. The influence of individual rock properties on the salt susceptibility of the rocks was also evaluated. To study the relation between pore characteristics, salt uptake, and damage, the pre‐ and post‐experiment pore size distributions of the rocks were also examined. It is observed that both salt type and rock properties influenced the damage pattern. The durability ranking of the rocks became significantly altered with the salt type while the variation in salt efficacy ranking with rock type was less pronounced. Of the five salts used, sodium chloride and aluminium sulfate were invariably ineffective with all rock types while sodium carbonate, sodium sulfate, and magnesium sulfate, were markedly more effective in damaging most types of rock used. Of the rock properties investigated, the microporosity (of pores smaller than 0·05 or 0·1 µm) showed the most significant influence on deterioration of the rocks associated with salt crystallization, whereas microporosity of pores smaller than 5 µm played a more important role in salt uptake. Pore size distribution was thus the key factor controlling salt uptake and damage. Rocks with a large number of pores (&lt;5 µm) and a high proportion of pores (&lt;0·05 or 0·1 µm) were particularly susceptible to salt crystallization damage. However, anomalies arose that could not be explained in terms of rock properties or salt efficacy alone. Overall, the relative influences of salt type/efficacy and rock type/properties on salt damage propensity were not clear enough to draw a reasonable conclusion. Salt crystallization damage appears to be influenced by the individual interactions between salts and rocks, which could explain the anomalous results. Copyright © 2009 John Wiley &amp; Sons, Ltd.

The pore structure characteristics of soil are closely related to soil engineering properties. For saline soil distributed in seasonally frozen areas, existing studies have focused on the influence of freeze–thaw cycles on pore structure, while the influence of soluble salt in the soil is not well understood. This study aims to explore the influence of salt content and salt type on the pore structure of freeze-thawed soil. Soil samples with different salt contents (0–2%) and types (bicarbonate salt and sulfate salt) were subjected to 10 freeze–thaw tests, and their pore size distributions (PSDs) were obtained by mercury intrusion porosimetry tests. In addition, the PSDs were quantitatively analyzed by fractal theory. For both salts, the PSDs of the tested soil samples were bimodal after the freeze–thaw cycles, and the porosity of saline soil samples increased with increasing salt content overall. However, the contents of various types of pores in soil samples with two salt types were quite different. The variation in bicarbonate salt content mainly affected the mesopore and macropore contents in the soil samples, and their change trends were opposite to each other. For soil samples with sulfate salt, the porosity and macropore content increased significantly when the salt content exceeded 1%. In addition, the pore structures in saline soil presented fractal characteristics after the freeze–thaw cycles, and the fractal dimension was positively correlated with macropore content. This study may provide references for understanding the engineering properties of saline soil in seasonally frozen areas at the microscale.

Volcanic tuff is widely used in Anatolian architecture and represents building stones of cultural heritage. The present study included a relevant assessment of the effect of four salt types and their four different concentrations on the durability of the Doger tuff used as building stones of cultural heritages, which is subjected to salt crystallization tests. For this purpose, an investigation was conducted on their chemical, mineralogical-petrographical (polarized microscopy, XRD, SEM analyses, pore size distribution), and physical and mechanical properties as well as their deterioration behavior in the salt crystallization test by different salt solutions (and their different concentrations). In salt crystallization tests, sodium sulfate (Na2SO4·10H2O), magnesium sulfate (MgSO4·7H2O), sodium chloride (NaCl), and potassium chloride (KCI) solutions were used, and 2, 5, 10, and 14% concentration ratios were used for each salt solution. In this work, it was observed some physical and mechanical values decreased along with weight loss in samples. The ultrasound pulse velocity (UPV) and uniaxial compression strength (UCS) values of the Doger tuff samples in all the salt solutions and concentrations decreased after the 15 test cycles. The tested Doger tuff samples completely broke down in the 10% and 14% concentrations in the sodium sulfate, magnesium sulfate, and potassium chloride solutions.

To scientifically reveal the characteristics of the setting and long-term performance of PS-(C+F) slurry which is applied in the anchoring conservation of earthen sites, the slurry with the optimum mix ratio (0.39) is studied by means of aging strength, aging moisture, aging wave velocity and shrinkage tests, and its durability is investigated by using the temperature-humidity cycling, frozen-thaw cycling, water stabilizing, soundness and alkali resistivity tests. The test results indicate that the aging strength can firmly meet the requirements of the anchoring craft, and that the elastic-fragile mechanical performance of the slurry is compatible to the earthen sites on the appearance, density, void ratio and strength. Furthermore, the very low shrinkage guarantees the interfacial mechanical property. Besides them, the consolidation has a strong durability to resist the arid and semi-arid climate in Northwest China. However, the slurry is not suitable for the earthen sites with high-content sodium sulphate. In conclusion, with strong compatibility and durability, the PS-F slurry with mix ratio of (CF=1, PS( C+F) =0.39) can satisfy the need of the anchoring conservation of earthen sites in Northwest China. ©, 2015, Chinese Society of Civil Engineering. All right reserved.

Loess is widely deposited in arid and semi-arid areas and is characterized by low dry density, developed pore space, and loose structure, which is not commensurate with high structural strength and shear strength in the dry state. Many natural phenomena and experimental results show that intact loess is very sensitive to the change of water content, and a slight increase in water content can cause a rapid reduction in strength. Abundant information is available in the literature for the collapsibility of loess. However, research on the evolution of loess compressibility during wetting, which is very helpful in understanding loess collapsible deformation caused by long-term irrigation, remains minimal. To this end, in this article, the evolution of the compressibility of intact loess at different wetting stages is investigated by oedometer tests, and microstructure and pore size distribution (PSD) were characterized on intact loess specimens at different water contents before and after oedometer tests by scanning electron microscope (SEM) and mercury intrusion porosimetry (MIP) methods. The results show that the compression index (Cc) and secondary compression index (Cα) of intact loess depend on water content and vertical stress and change abruptly when vertical stress exceeds yield stress. Cα/Cc values of intact loess are not constant, which increases with vertical stress to a peak and then gradually decreases to 0.025. Wetting and loading can cause damage to the microstructure of intact loess; specifically, loading leads to the collapse of the overhead structure and the transformation from a bimodal PSD into a unimodal PSD, and wetting intensifies the collapse of the microstructure to form a compacted interlocking structure and promotes the transformation of medium pores into small pores.

The porous structure of geomaterials is of utmost importance for various industrial and natural processes. In this study, various conventional porous structure characterization techniques such as mercury intrusion porosimetry (MIP), nuclear magnetic resonance (NMR), micro-X-ray computed tomography (μCT) imaging, as well as gas injection have been employed to perform a systematic and critical evaluation of all such techniques for characterization of a carbonate rock sample porous structure. The porosity obtained from μCT (5 μm/voxel) (21.5%) is closer than the overall porosity obtained by MIP (17.23%) to the gas porosimetry result (23%). The 5% difference could be due to inaccessible pores to mercury, which can be accessible to nitrogen with much smaller molecules. The porosity obtained from NMR is 21.4%. It is lower than porosity values by μCT (5 μm/voxel) and by gas injection and higher than the prediction of MIP. The porosity is obtained by μCT, but the much lower resolution (27.5 μm/voxel) results in 8.19% underestimating the porosity by around 50%. Regarding permeability, the results of the NMR technique are highly dependent on the cutoff range used and very different from other techniques, whereas the permeability obtained by MIP is around 18.42 mD, close to that obtained by gas permeameter (20 mD). The μCT imaging provides the opportunity to measure pore and throat size distribution directly, to achieve open and closed porosity, the coordination number of pores and surface and volume characteristics of the porous medium, which can hardly be performed through other techniques. The resolution of images, however, fully controls the obtained pore and throat size distribution in CT analysis. The Kolmogorov–Smirnov distribution analysis reveals that the resulting pore size distribution from MIP is rather a rough estimation of the throat size distribution obtained from μCT (5 μm/voxel), while NMR prediction can provide a rather good approximation of the pore size distribution obtained from μCT (5 μm/voxel). The NMR prediction is however dependent on the choice made for the surface relaxivity coefficient, and changing it would significantly affect the resulting distribution. The results of this study provide further insight and elucidate the differences of the quantities such as porosity, permeability, and pore and throat size distribution obtained from various techniques which are essential either as an input to numerical models of flow and transport in porous media or as a building block of the theoretical models.

SummaryLoess features metastable microstructure and is deemed susceptible to chemical contaminant permeation. However, studies on the loess permeability evolution under water and chemical environments are remarkably limited. In this study, the response of the loess to the water and sodium sulfate seepages was analyzed using the temporal relationship of cations concentration, X-ray diffraction and fluorescence (XRD and XRF), mercury intrusion porosimetry (MIP), and scanning electron microscope (SEM) tests. The permeability evolution characteristics were identified, and its underlying mechanisms were revealed from aspects of the diffuse double layer (DDL) theory and physiochemical actions. The discharge of Mg2+ and precipitation of calcium carbonate, referred also to as the dedolomitization, degraded the macro permeability when subjected to the water seepage test. The salt-induced swelling, induced by the intrusion of Na+ into the DDL, caused an increase in the micropore fraction under the sodium sulfate seepage test, thereby increasing the macro permeability.

Abscisic acid-pretreated carrot (Daucus carota) somatic embryos survive dehydration upon slow drying, but fast drying leads to poor survival of the embryos. To determine whether the acquisition of desiccation tolerance is associated with changes in the physical stability of the cytoplasm, in situ Fourier transform infrared microspectroscopy was used. Although protein denaturation temperatures were similar in the embryos after slow or fast drying, the extent of the denaturation was greater after fast drying. Slowly dried embryos are in a glassy state at room temperature, and no clearly defined glassy matrix was observed in the rapidly dried embryos. At room temperature the average strength of hydrogen bonding was much weaker in the rapidly dried than in the slowly dried embryos. We interpreted the molecular packing to be "less tight" in the rapidly dried embryos. Whereas sucrose (Suc) is the major soluble carbohydrate after fast drying, upon slow drying the trisaccharide umbelliferose accumulates at the expense of Suc. The possibly protective role of umbelliferose was tested on protein and phospholipid model systems, using Suc as a reference. Both umbelliferose and Suc form a stable glass with drying: They depress the transition temperature of dry liposomal membranes equally well, they both prevent leakage from dry liposomes after rehydration, and they protect a polypeptide that is desiccation sensitive. The similar protection properties in model systems and the apparent interchangeability of both sugars in viable, dry somatic embryos suggest no special role of umbelliferose in the improved physical stability of the slowly dried embryos. Also, during slow drying LEA (late-embryogenesis abundant) transcripts are expressed. We suggest that LEA proteins embedded in the glassy matrix confer stability to these slowly dried embryos.

Effectiveness of crystallization inhibitors in preventing salt damage in building materials

Fractal dimension and its variation of intact and compacted loess