Salt crystallization dynamics in building rocks: a 4D study using laboratory X-ray micro-CT

We employ laboratory X-ray micro-computed tomography (μCT) during climate-controlled salt weathering experiments to acquire data on the kinetics of drying and salt precipitation and the distribution of crystals within the pore space of Mšené sandstone. For that purpose, a custom-designed setup was built at the UGCT’s scanners of the Ghent University Centre for X-ray Tomography (UGCT) that allows to acquire 4D scans while drying. Samples were initially capillary saturated with a saturated NaCl-solution and subsequently dried at 20% RH and at 50% RH, at room temperature. These RH-values are representative for winter and summer conditions for the salt NaCl, which is not temperature sensitive. Different salt precipitation dynamics result in different drying kinetics at the two RH’s. These crystallization and transport dynamics can be directly linked as revealed by the 4D X-ray μCT datasets

Effect of Salt Solutions on Compressibility of Bentonite

The main intention of the thesis is to evaluate the effect of pore fluid composition and stress state on volume change behaviour of bentonite. In this study compressibility behaviour of several saturated bentonite specimens were studied. The initial water content chosen is greater than the liquid limit for the remoulded soils, referred to herein as the initially saturated soil. The bentonite specimens were inundated with distilled water, 0.5 M and 1.0 M NaCl solutions. Consolidation tests were conducted for the range of pressure varying between 10 kPa to 1250 kPa. The results show that with increase in concentration of NaCl salt solution change in void ratio is increasing. The compressibility behaviour is increasing with salt solution concentration. The observed results similar to that has been reported by other researchers for compacted bentonite specimens subjected to NaCl salt solutions

Mathematical modelling of process of salt transfer under filtration and moisture transfer in saturated and non-saturated soil mass with available mobile free surface

Здійснено математичне моделювання процесу солепереносу при сумісній фільтрації підземних вод та вологопереносі в насичено-ненасиченому ґрунтовому середовищі. Розглянуто задачу безнапірної стаціонарної фільтрації сольових розчинів за наявності рухомої вільної поверхні ґрунтових вод, яка задана на межі областей повного та неповного насичення. Вважається, що сольові розчини находять у ґрунтове середовище з поверхні області неповного насичення і вимиваються через дрену (осушувальний режим). На основі побудованої математичної моделі даної крайової задачі проведено дослідження процесу солепереносу до дрени в області повного насичення та процесу вологопереносу з урахуванням солепереносу в області неповного насичення ґрунту; досліджено протікання даних процесів сумісно для обох областей водонасичення. Чисельні розв’язки відповідних крайових задач знайдено методом скінченних різниць із використанням чисельних конформних відображень. На основі програмної реалізації задачі проведено чисельні експерименти та здійснено їх аналіз.The mathematical modelling of the process of salt transfer under joint filtration of ground waters and moisture transfer in saturated-non-saturated soil medium has been carried out. The problem of the free flow stationary filtration of salt solutions under the available mobile free surface of ground waters which is set at the boundary of areas of complete and incomplete saturation has been considered. It is believed that salt solutions enter soil medium from the surface of incomplete saturation and are washed out via drain, i.e. the problem of salt transfer is considered in the case of drainage regime. On the basis of the built mathematical model of this boundary problem the research of the process of salt transfer to the drain in the area of complete saturation and of the process of moisture transfer taking into account salt transfer in the area of incomplete soil saturation has been conducted. The comprehensive study of the process of salt transfer in areas of complete and incomplete saturation is carried out. For this purpose these areas of water saturation are considered as one entity with common unknown boundary which is in the process of problem solution. It makes possible to find the interconnection between investigated processes and to carry out their prediction in the area of saturated-non-saturated soil medium. On the basis of the comprehensive solution of the problem the change of free surface under the influence of processes studied has been established. Numerical solutions of relevant boundary problems are determined by the method of finite differences using numerical conformal mapping. On the basis of programmed implementation of the problem the construction of the conformal mapping in areas of filtration flow, moisture flow and conformal differential network of the area of saturated-non-saturated soil medium has been done; there have been received numerical results and graphical representation of the distribution of the field of piezometric pressure heads, of moisture pressure heads and of concentration of salts in areas of complete and incomplete saturation and also of the distribution of the field of salt solutions concentration in saturated-non-saturated soil mass. A number of numerical experiments has been conducted which gave the possibility to carry out the analysis of results obtained with different initial data of the problem and to research in detail the nature of the action of the process of salt transfer under filtration and of moisture transfer in saturated-non-saturated soil medium

Anomalous diffusion in an electrolyte saturated paper matrix

Diffusion of colored dye on water saturated paper substrates has been traditionally exploited with great skill by renowned water color artists. The same physics finds more recent practical applications in paper based diagnostic devices deploying chemicals that react with a bodily fluid yielding colorimetric signals for disease detection. During spontaneous imbibition through the tortuous pathways of a porous electrolyte saturated paper matrix, a dye molecule undergoes diffusion in a complex network of pores. The advancing front forms a strongly correlated interface that propagates diffusively but with an enhanced effective diffusivity. We measure this effective diffusivity and show that it is several orders of magnitude greater than the free solution diffusivity and has a significant dependence on the solution pH and salt concentration in the background electrolyte. We attribute this to electrically mediated interfacial interactions between the ionic species in the liquid dye and spontaneous surface charges developed at porous interfaces, and introduce a simple theory to explain this phenomenon.Comment: 1

Paradoxical drying due to salt crystallization

We investigated how salt crystallization inside a porous building material influences the formation of a receding drying front. Nuclear Magnetic Resonance (NMR) is used to measure non-destructively both hydrogen and dissolved sodium ions simultaneously during drying experiments. In this study we focused on the influence of NaCl on the drying. The results show that salt changes the drying process. At low relative humidities (RH ~ 0%), the drying rate of a brick saturated with NaCl solution is much lower than the drying rate of water saturated brick. Moreover, the presence of salt suppresses the development of a receding front. In this case homogenous drying of the material continues till very low saturation values. This is due to salt crystallization near the surface of the brick that causes blockage of the pores. This blockage reduces evaporation rate at the surface and allows maintaining a continuous hydraulic connection between the surface of the porous medium and the liquid present inside the material till low saturation values. Increasing the relative humidity at 55% and 70% RH with the brick saturated with the salt solution leads to a paradoxical situation with evaporation rates greater than for 0% RH. The paradox is explained by the impact of evaporation rate on the efflorescence microstructure, leading to the formation of a blocking crust for sufficiently high evaporation rates and non-blocking efflorescence for sufficiently low evaporation rates. The fundamental difference between the two types of efflorescence is demonstrated from a simple imbibition experiment. Using a simple continuum scale model of drying, critical moisture content was determined and all the essential features of the experimental results are validated. It is shown that critical moisture content can be very low in case of fired-clay brick due to the low threshold of the pore space, which is consistent with the relatively large pore size distribution of the fired-clay brick

Instability of the salinity profile during the evaporation of saline groundwater

We investigate salt transport during the evaporation and upflow of saline groundwater. We describe a model in which a sharp evaporation-precipitation front separates regions of soil saturated with an air-vapour mixture and with saline water. We then consider two idealised problems. We first investigate equilibrium configurations of the fresh-water system when the depth of the soil layer is finite, obtaining results for the location of the front and for the upflow of water induced by the evaporation. Motivated by these results, we develop a solution for a propagating front in a soil layer of infinite depth, and we investigate the gravitational stability of the salinity profile which develops below the front, obtaining marginal linear stability conditions in terms of a Rayleigh number and a dimensionless salt saturation parameter. Applying our findings to realistic parameter regimes, we predict that salt fingering is unlikely to occur in low-permeability soils, but is likely in high-permeability (sandy) soils under conditions of relatively low evaporative upflow

Proses Pembuatan Serta Analisa Benton-38 Dari Bentonit Sebagai Bahan Aditif Pada Cat

The production of bentone for paint additive from Cabentonite was carried out by ion exchange process. The process has two steps, i.e. Ca++ ion in Ca-bentonite is exchanged by Na+ ion in saturated solution of NaCl to produce Na-bentonite. Then, Na+ ion in Na-bentonite is exchanged into {(R1)2N(R2)2 + by using alkyl quaternary ammonium salt to produce bentone. lnterms of swelling characteristic the ion exchange using 140 ml of saturated solution of NaCl for 100 g Ca-bentonite produced best product with a swelling characteristic of 15.56. The amount of dimethyl distearyl ammonium salt (DAM) used for production of bentone-38 was 60 g/100 g bentonite. This is based on the ion exchange capacity of bentone-38, i.e. 69 meq for 100 g bentonite. Swelling characteristics of the above bentone-TS-R (reference) and bentone-38-H (experiment) were 24.5 and 39 in Spirit Mineral Terplne (SMT), 55 and 54 in SMT + ethanol (17:3 v/v), and the viscosity of bentone-38-R, and bentone-38-H are 7.2 cps and 6.5 cps respectively