Effects of Acidic/Alkaline Contamination on the Physical and Mechanical Properties of Silty Clay

Abstract

Contaminated soil management and renovation is one of the major environmental geotechnical issues in China. Due to their special strength and stiffness properties, contaminated soil has attracted extensive attention in foundation and slope stability design. For the differentiated influence of acidic/alkaline contamination on the geotechnical physical and mechanical characteristics of soil, this study mainly introduced a remodeled silty clay sample contaminated by acidic and alkaline solutions in the laboratory and conducted research into its basic physical properties, compressive properties, shear strength, and microstructure. It was shown that when the hydrochloric acid concentration increased from 1% to 7%, the density and specific gravity decreased by 7.07% and 3.11%, respectively. The void ratio showed a descending trend with increasing concentration of acid. The acidic solution concentration was negatively correlated with the cohesion of the soil, but the internal friction angle remained constant with a concentration of 1–5%. Alternatively, when the sodium hydroxide concentration increased from 1% to 7%, the density and specific gravity increased by 1.88% and 2.67%, respectively. The void ratio decreased linearly with the increase in consolidation pressure. Alkaline concentration could affect the internal friction angle and cohesion in a positive correlation. Through the observation of microstructure, the surface of acidified soil particles was smooth and flat, while the surface of alkalized soil particles was rough and uneven. The results can provide reference for the evaluation of the mechanical properties of soil contaminated by acid and alkali.

1. Introduction

With the development of cities and the rapid growth of urban population, the production of municipal solid waste continually soars, which necessitates extensive construction of landfills. The stability of landfills has captured the attention of researchers as a prominent issue for the environment and nearby residents, owing to the threat to the properties and lives of residents caused by landfill failure [1,2,3,4,5,6,7,8]. Besides solid waste, the strength of landfills also depends on the soil cushion. Clay has been widely used in landfill liner system, landfill unit division, and waste storage due to its strong ability to intercept pollutants, little permeability, and low cost. Waste in the landfill process will produce a lot of gas and liquid pollutants and, if not properly treated, it will invade the cushion and cause serious secondary pollution. In addition, soil can also be contaminated by other factors, such as chemical leakage and acid rain. This is likely to cause serious safety accidents related to the stability of houses, which is mainly attributable to changes in the nature of the foundation soil of the houses. Therefore, an immediate necessity is to study the physical and mechanical properties of soil contaminated by various pollutants.

Frequent occurrences of water and soil contamination have long prompted a large number of studies in which the effects of the chemical composition of a pore fluid on the geotechnical properties of soils were scrutinized. Quite a few researchers had conducted several relevant experiments to investigate the stress–strain behavior of soils [9,10,11] and the shear strength of soils [12,13,14]. Nguyen et al. [15] found that the effect of pore water’s chemical composition is clear only in the low–stress range where the hydro–mechanical behavior is dominated by the physico–chemical effect. Koupai et al. [12] and Alibeiki et al. [16] found that the engineering properties of soil are greatly dependent on the pore water’s pH.

There are many causes of acidic/alkaline contamination of soil, which can be roughly divided into natural processes (weathering of pyrite in mudstone), human activities (municipal waste storage and factory accidental spills), and natural disasters [17]. Kamon et al. [18] analytically studied the rules of the physical and chemical features and engineering properties of soil samples that are contaminated by an acidic solution, which is prepared by an infiltration and soak method in a series of simulation experiments. Hu et al. [19,20] found that an increase in free hydrogen ion is due to the ionization of acetic acid, which corrodes the calcium and magnesium compounds in loess and further reduces the shear strength. Olphen [21] and Mitchell and Soga [22] found that a reduction in the diffuse double–layer thickness would occur when the concentration of hydrogen ion greatly increases, and this changes the geotechnical properties of the soil, such as its hydraulic conductivity and compressibility. Olphen [21] also noted that the edge–to–face (E–F) associations (flocculation) tend to prevail at low pH conditions (pH < 5.5), while the face-to-face (F–F) associations (dispersion) dominate at high pH conditions (pH > 7.5), which leads to changes between microscopic soil layers. The results of previous studies indicated that kaolin, with an open-flocculated structure, typically exhibits greater liquid limits, permeability, and strength [22,23,24]. Anandarajah and Zhao [9] suggested that the higher strength of the flocculated structure in kaolin was due to an increase in the forces of attraction between touching particles (van der Waals forces), which will bring some additional strength to the clay structure. Gratchev et al. [17,25,26,27] and Tang et al. [28] implemented the compression index and compressibility of soil obtained from a series of experiments on the geotechnical properties of soil with acidic contamination. Zhang and Wang [29] discussed the reasons for volume deformation and permeability changes caused by a sodium chloride solution through experiments and SEM observations, proving that the influences of salinity on the mechanical behavior of clays is mainly attributable to the interaction between diffused double layers. Han et al. [30] introduced Electrochemical Impedance Spectroscopy (EIS) to test sulfuric acid and sodium hydroxide contaminated silty soil; it turned out that, with an increase in the amount of acid–base pollutants, the shear strength decreases. Yang et al. [31] started with the change in the ion density of the solution after pollution and the particle gradation of the soil sample, studied the effect of calcium hydroxide on laterite clay, and noted that the economic life of earth dam is inconsequently shortened by alkaline materials that reinforce the laterite area. Seepage failure and collapse can befall once some engineering indices are less than their critical values.

The study of the microstructure and mechanical properties of contaminated soil is a new research direction involving traditional geotechnical engineering, environmental engineering, soil science, chemistry and chemical engineering, ecological engineering, evaluation of soil survey, geology, and many other disciplines, and it belongs to a cross–disciplinary approach involving these areas. Classical soil mechanics is based on the concept of effective stress, which states that strength behaviors depend on the stress borne by the structure of solid particles with water as the second phase [22]. However, considering it from a meso–mechanism point of view, the variation of ion composition and concentration in porous water solution has an obvious influence on the deformation and strength characteristics of clay [32,33]. Meng and Li [34] pointed out that loess undergoes cementation when it is soaked with acetic acid, which is accompanied by bond failure. Sun et al. [35] found that the newly formed insoluble (slightly soluble) substances increase the density of the Xiashu loess through alkalization, while the minerals of the Xiashu loess through acidification mainly dissolve, resulting in a decrease in soil density. The strength and compressibility of the soil are inseparably related to the microstructure and soil particles. The relationships and substances of soil particles will all be affected by the contaminated solution. These influencing factors work together and ultimately affect the compressibility and strength of the soil. In summary, researching the acid and alkali pollution of soil’s physical and mechanical properties, discussing the mechanism of the acidic and alkaline contamination of soil, evaluating foundation soil in engineering, and forecasting the engineering properties of foundation soil are of great significance.

From the viewpoint of the current desk research, the study on the weakening mechanical properties of remolded silty clay with acidic and alkaline pollution is still in a stage of exploration. Regarding the soil sample type, study on the pollution of loess has drawn people’s attention, but silty clay has been less discussed. In addition, the majority of research on soil–water chemical interaction has focused on organic liquids and inorganic fluids, while little research has revolved around the effects of pH, such as an acidic/alkaline solution, even though acidic/alkaline contamination has become an increasing concern around the world.

The objective of this study is to investigate the physical and mechanical characteristics of silty clay in the conditions of acidic and alkaline pollution with basic physical property test, consolidation test, direct shear test, and scanning electron microscopy test. A quantitative analysis of the mechanical properties of the specimens corresponding to different acidic and alkali solution concentrations is performed, and then the ultimate principles affecting the properties of the specimens and their law is discussed.

2. Materials and Methods

The soil sample, taken from an engineering site in Beijing, is silty clay located five meters deep from the ground. The chemical compositions presented in

Table 1. The chemical and mineral compositions of the test soil sample.

Chemical Composition(%)	SiO2	Al2O3	MgO	CaO	K2O	ZnO	MnO	Fe2O3	Others
--	58.86	17.33	12.63	4.17	2.06	1.96	1.12	0.98	0.89
Mineral Composition(%)	Quartz	Plagioclase		Microcline	Calcite	Pyrite	Dolomite		Total Clay
--	39	11		4	23	2	5		56

were obtained via X-ray fluorescence spectrum (XRF) analysis and they suggest that the silty clay is composed of silicon dioxide, alumina, magnesium oxide, and a small number of other chemical compounds. After unravelling its mineral composition with the aid of X-ray diffraction patterns, the analytical results are reported in Table 1. The physical properties, such as liquid limits (ω_L) and plastic limits (ω_p), were determined using a grain distribution analytical test following the China National Standards for Geotechnical Testing Method GB/T 50123–2019 [36]. The ω_L and ω_p of the silty clay is 31.2% and 18.4%, respectively. The soil particle size distribution curve is shown in Figure 1. Compared to reconstituted soil, the most remarkable characteristics of undisturbed soil is that it maintains its natural structure, which is related to the grain composition of the soil in addition to the stress history and sedimentary environment. In order to avoid the influence of naturally-occurring structural differences among the soil samples on the test results, the silty clay was remolded [37,38]. Considering that a soil sample soaked in an acidic solution could be damaged, a large dry density standard was adopted in the remolding process. The sample preparation process was as follows: (1) The soil passed through a 0.02 mm sieve, in order to remove clusters, agglomerated particles, and other impurities. (2) All the moisture was removed with a dry oven set at a temperature of 105 °C for 24 h. (3) Water was sprayed on a predetermined quantity of the dried soil, and then the mixture was manually blended. (4) The wet soil was put into the cutting rings for tamping to a defined dry density, and the filter paper and permeable stone were fixed on both sides of the cutting rings. (5) The prepared samples were placed into a pure water vacuum to saturate for 24 h. (6) The prepared samples were put into the prepared solution and immersed in it completely, keeping the samples saturated. (7) To ensure the soil samples reacted fully with the solution, the samples were left in a cool place for 7 days. The mass concentrations of the hydrochloric acid solution (hydrochloric acid) and sodium hydroxide alkaline solution (sodium hydroxide) were configured in the experiment to be 1%, 3%, 5%, 7%, and 9%, respectively, for different tests.

3. Physical Experiment

3.1. Apparent Feature Changes

When the prepared samples were put into the prepared solution, a number of bubbles were produced, which indicated that the material in the soil reacted significantly with the solution. The color of the samples was unchanged after being immersed in the acidic or alkaline solution for seven days. However, the color of the acidic solution had obviously been changed, as shown in Figure 2a. This changed the color of the acidic solution to a light yellowish green, which indicated the soil samples originally contained ferrous iron and ferric ion, according to the basic elements of the soil. The alkaline solution turned yellowish brown. The soil samples immersed in the alkaline solution expanded obviously. Many cracks appeared on the surface of the soil samples contaminated by acid, accompanied by significant volume shrinkage. Nevertheless, obvious swelling in the soil samples contaminated by alkali was observed.

3.2. Weight and Density Change

The soil samples were all weighed after saturation before being immersed in the solution. The density of the soil samples was measured with the cut rings. After the seven days of immersion, the soil samples and the cut rings were weighed again and the densities calculated. The changed quality and density of the contaminated soil can be seen in

Table 2. The changed quality and density of the contaminated soil.

Concentration	Change in Weight (g)	Density (g/cm3)	Concentration	Change in Weight (g)	Density (g/cm3)
pure water	−0.820	1.88825	—	—	—
acid 1%	−10.291	1.71673	alkali 1%	−0.366	1.88215
acid 3%	−9.635	1.72767	alkali 3%	+0.450	1.98575
acid 5%	−13.087	1.67013	alkali 5%	+1.665	1.91600
acid 7%	−17.574	1.59535	alkali 7%	+1.762	1.91762
acid 9%	−34.161	1.31890	alkali 9%	+2.804	1.93498

. Figure 2b shows the changes in density. As the pH increases from being acidic to alkaline, the density shows an ascending trend. Compared to the most alkaline state (acid 9%), the density of the soil with 9% acidity decreases 31.83%. When the concentration of hydrochloric acid changes from 1% to 7%, the density decreases by 7.07%. The density of sodium hydroxide increases from 1% to 7%, and the density increases by 1.88%.

3.3. Specific Gravity Changes

According to Code for China National Standards GB/T 50123–2019 [36], the specific gravity of soil particles, G_s, is used to reflect the change in the quality of soil particles. It is necessary to study the effect of acidic or alkaline solution on soil particles and to study the quality change after the chemical reaction of soil particles. A pycnometer was used to determine the specific gravity. The test data are listed in

Table 3. Change in the specific gravity of the contaminated soil.

Concentration	Specific Gravity	Concentration	Specific Gravity
pure water	2.6772	—	—
acid 1%	2.6541	alkali 1%	2.6764
acid 3%	2.5914	alkali 3%	2.7020
acid 5%	1.9268 1	alkali 5%	2.7028
acid 7%	2.5716	alkali 7%	2.7479
acid 9%	2.5283	alkali 9%	2.7500

¹ Error data, eliminate.

. When the concentration of hydrochloric acid changes from 1% to 7%, the specific gravity decreases by 3.11%. The specific gravity of sodium hydroxide increases from 1% to 7%, and the density increases by 2.67%.

Specific gravity depends on the content of original mineral, clay mineral, organic matter, free oxide, and composite colloid in the soil body. In actual engineering, clay usually contains a lot of organic matter, which reacts more easily with acidic and alkaline solutions. Therefore, the content of organic matter influences specific gravity the most. On the other hand, clay minerals contain silicon dioxide, alumina, ferric oxide, potassium oxide, and other substances which can react with acidic or alkaline solutions, forming soluble and insoluble matters. This is also the cause of the change in soil particle specific gravity.

There is a wide divergence in the reaction of soil to acidic and alkaline pollution. When reacting with acid, some oxides, insoluble salt, and organic matter in the soil body resolve and dissolve in the solution. When reacting with alkali, some oxides react with it to form some flocculated or precipitated material. Soil particles contain metallic oxides, water-insoluble salts, organic substances, and other substances which react with acids, decompose into water-soluble compounds, and separate from the soil particles, leading to the decrease in specific gravity. In contrast, the reaction between the matters in the soil particles and the alkali creates insoluble hydroxides that stick to the soil particles, which increases the specific gravity.

4. Mechanical Experiment

4.1. Compressive Property Research

The compressibility of the clean and contaminated soil samples during one–dimensional loading was studied using a series of consolidation tests according to China National Standards GB/T 50123–2019 [36]. The soil samples were kept in compression for 24 h (even more) under each level of loading to ensure the consolidation was complete. To simulate a real situation, the saturated solution used in each experimental stage was the contaminated solution. The test data are shown in Figure 3.

The e–log p curves always appear like a straight line in the range of high consolidation pressure, considering the preconsolidation pressure (The soil samples had already been compacted and consolidated). The slope of the line is the compressibility coefficient. Fitting curves are drawn with the fitting formula as follows:

e = −λ ln p + b

(1)

where e is the void ratio, p is the vertical pressure, and λ is the slope of the line. Specific values and fitting images are shown in Figure 2. The fitting parameters are list in

Table 4. The fitting parameters of the p–e curves.

Soil Samples	λ	b	R2	Soil Samples	λ	b	R2
acid 1%	0.019	0.6734	0.9923	acid 1% curved segment	—	—	0.9981
acid 5%	0.034	0.7384	0.9470	acid 1% line segment	0.019	0.6701	0.9652
acid 7%	0.047	0.7931	0.9677	acid 5% curved segment	—	—	0.9975
alkali 1%	0.017	0.6673	0.9855	acid 5% line segment	0.049	0.8230	0.9871
alkali 5%	0.019	0.6712	0.9894	acid 7% curved segment	—	—	0.9999
alkali 7%	0.023	0.6750	0.9820	acid 7% line segment	0.059	0.8659	0.9748

.

As can be seen in Figure 3, the soil samples contaminated by acid with a higher concentration have larger slopes, which means their compressibility is better. With increasing consolidation pressure from 350 kPa to 950 kPa, the void ratio of acid 1%, acid 5%, and acid 7% decreases by 0.085, 0.163, and 0.212 times, respectively. For samples with the same initial dry density, it means more pores and the solid matters are decomposed in the soil. In the alkaline contamination test, the first two curves have the same change rule, but the compressibility of the contaminated soil is obviously improved in a high–concentration alkaline environment. With increasing consolidation pressure from 50 kPa to 800 kPa, the void ratio of alkaline 1%, alkaline 5%, and alkaline 7% decreases by 0.075, 0.084, and 0.104 times, respectively. When the alkali concentration is relatively low, the main reaction is the reaction between the soil particles and the alkaline solution. According to the aforementioned studies, an increase in alkaline concentration destroys the cementation among the skeletons of the macroporous soil and, thus, the connecting strength among the macroporous soil reduces and the compressibility of the soil rises.

When the samples are subjected to acid pollution, the compressibility is positively correlated with acid concentration. In the previous section, we explain that hydrogen ions will destroy the cementation among the macroporous soil and weaken the contact ability of the soil. On the other hand, hydrogen ions enter the layer of bound water that envelops soil particles and reduce the thickness of the bound water layer, thus reducing the distance between the soil particles and increasing the compressibility of the soil. There are obvious straight lines in the e–log p curve of the acid contaminated soils under higher pressures, as illustrated in Figure 3c, while curve fitting can be considered in the lower pressure range. Accordingly, the test points are fitted by a mixture of curves and straight lines, and the benefit is that statistical analysis is performed to find the best–fitting curve. It indicates that the soil samples contaminated by acid have an elastic phase and a plastic phase. In high–stress intervals, as the material among the particles is dissolved, the soil pores increase and the compressibility increases.

It has long been recognized that the compressibility of clay is governed by mechanical and physicochemical factors. However, the test results obtained in this study indicate that the physicochemical factors may have more pronounced impact on the consolidation properties of clay when the soil is contaminated by acidic and alkaline solutions. As the specimens are assumed to have the same micro fabric, under the same stress path, it can be concluded that changes in compressibility and strength are mainly due to the soil–water chemical interaction.

4.2. Shear Strength Research

Since the saturated solution of this test has a certain corrosiveness, it can cause damage to the instrument, so the triaxial shear test cannot be performed on the samples. According to the China National Standards GB/T 50123–2019 [36], the effect of the contaminated soil samples on shear strength characteristics was measured via a total of 16 direct shear tests. During the experiment, the sample was placed in a shear box and consolidated for 24 h to ensure that the applied pressure was consistent with the effective stress. The shear deformation rate was set to 1 mm/min. According to the results of the consolidation test, when the vertical pressure is 50 kPa and 100 kPa, the soil sample contaminated by the acidic solution is super–consolidated soil, which will be a shear dilatancy failure, and the stable value of shear stress is selected as the shear strength. Other soils under other vertical pressures are normally consolidated soils, and shear shrinkage will occur in the direct shear test. According to the geotechnical test standard, the shear stress corresponding to a shear displacement of 4 mm was selected as the shear strength. The test data are shown in Figure 4. In the specimens exposed to hydrochloric acid, the values of shear strength show a descending trend. In the presence of sodium hydroxide, the strength values increase with increasing concentration. For example, 11.3%, 22.6%, and 34.7% increases in strength are observed for the concentrations of 3%, 5%, and 7%, respectively, compared to the alkali 1% specimens in the case that the vertical pressure is 400 kPa. On the whole, 16.4%, 25.9%, and 54.2% increases in shear strength are observed for the concentrations of 3%, 5%, and 7%, respectively, compared to the alkali 1% specimens.

The slope of the fitting line is the tangent value of internal friction angle and the intercept is the cohesion value. According to the Mohr–Coulomb criterion, the fitting formula is as follows:

$$\tau =c+\sigma \tan \phi$$

(2)

where τ is shear strength, and σ(p) is vertical pressure. According to Mohr–Coulomb’s law, parameter c is the cohesion value and $\phi$ is the internal friction angle value.

The fitting parameters are shown in

Table 5. The fitting parameters of the τ–σ (p) curves.

Soil Samples	c (kPa)	$\mathit{\phi }$ (°)	R2	Soil Samples	c (kPa)	$\mathit{\phi }$ (°)	R2
acid 1%	48.04	18.00	0.9918	alkali 1%	9.49	19.75	0.9985
acid 3%	20.30	15.22	0.9620	alkali 3%	20.24	20.41	0.9880
acid 5%	5.15	15.54	0.9886	alkali 5%	22.79	21.70	0.9350
acid 7%	0.75	19.65	0.9939	alkali 7%	29.25	25.41	0.9630

. Under the condition of acid pollution, the cohesion sinks as the acid concentration increases, which also reflects that the cohesion among soil particles (colloid cohesion) is destroyed by the acidic solution from the aspect of mechanical strength. Acid 7% completely destroys the cohesion between soil particles, causing the internal friction angle to be zero. The friction between soil particles can be divided into occlusion friction and sliding friction. At a low concentration level, hydrogen ions mainly destroy the cementation between soil particles and do not affect the friction between soil particles. With a reduced concentration, sliding friction gradually decreases due to lubrication, showing the macroscopic characteristics of smaller friction angle. However, when the concentration of acidic solution is too high, hydrogen ions enter in large quantities. The cohesive water layer of soil particles and the compact state of soil particles caused by the external loads lead to diminishing inter friction, which results in an increase in friction angle. The cohesive force mainly includes the following factors: electrostatic attraction force, van der Waals force, cementation, chemical bond of indirect contact of particles, apparent cohesion (suction force, inter force, and freezing force), and other factors. Therefore, the initial cohesive values of the soil samples contaminated by different solutions can be explained.

4.3. Microstructure Observation

The engineering behavior of a soil element significantly depends upon its existing structure. In general, an element of flocculated soil has higher strength, lower compressibility, and higher permeability than the same element of soil put in a dispersed state at the same void ratio [39]. To further explain the physical and mechanical properties, a scanning electron microscopy test (SEM) was carried out to observe the microstructure of the samples after contamination according to General Rules for Measurement of Length in Micron Scale by SEM GB/T16594–2008 [40]. The microstructure of soil is the essential factor affecting its engineering properties. To ensure the representation of the photo, the photo area is randomly selected, as shown in Figure 5.

Observation of the soil samples with a magnification of 4000 times shows that the undisturbed soil is like flocculated or aggregated microstructure. The soil contaminated by acid behaves like an undulating filmy microstructure or a flake–like microstructure (in Figure 5d). The particle surface has also become smoother (in Figure 5c). It indicates that the flocculated soil particles are dispersed to form lamellar structure in acidic environments. Therefore, the compressibility of the soil sample is improved, and the cohesive force among the soil particles decreases. In an acidic environment, magnesium oxide, aluminum oxide, and other substances in the soil particles react with hydrogen ion and cause the specific gravity of the soil particles to decrease. Soil contaminated by alkali behaves like flocculated or aggregated microstructure (in Figure 5d). The particle surface has also become rougher (in Figure 5c). In the alkalized silty clay, silicon dioxide reacts with sodium hydroxide to form sodium silicate, which leads to a decrease in silicon dioxide content (Equation (6)). The produced sodium silicate plays a key role in binding soil particles together. The microstructure images also further illustrate the surface–surface bonding properties of sodium silicate [20]. It indicates that the relationship between soil particles is closer in an alkaline environment. Therefore, the compressibility of the soil sample is reduced, and the cohesive force between the soil particles decreases. The involved chemical reactions include the following:

$$2\mathrm{HCl}+\mathrm{MgO}={\mathrm{MgCl}}_2+{\mathrm{H}}_2\mathrm{O}$$

(3)

$$6\mathrm{HCl}+{\mathrm{Al}}_2{\mathrm{O}}_3=2{\mathrm{AlCl}}_3+3{\mathrm{H}}_2\mathrm{O}$$

(4)

$${\mathrm{MgO}+\mathrm{H}}_2{\mathrm{O}=\mathrm{Mg}\left(\mathrm{OH}\right)}_2\downarrow$$

(5)

$${2\mathrm{NaOH}+\mathrm{SiO}}_2{=\mathrm{Na}}_2{\mathrm{SiO}}_3{+\mathrm{H}}_2\mathrm{O}$$

(6)

$$2\mathrm{NaOH}+{\mathrm{Al}}_2{\mathrm{O}}_3{=2\mathrm{NaAlO}}_2{+\mathrm{H}}_2\mathrm{O}$$

(7)

The magnification of SEM has an influence on the microstructure of the observed soil. During observation, the magnification is not the higher the better. If the magnification is too large, it will not reflect the overall situation of the soil, which will make it unrepresentative. It can be seen from the photo that the silty clay used in the test can better reflect the surface of the soil particles at the magnifications of 2000 and 3000 times, which is representative to a certain extent.

When coupled with the electron image information, it leads to some possible conclusions that there is precipitation on the surface of soil particles, which further promotes agglomeration. It is obvious that the soil particles contaminated by the acidic environment become less rough and the surface is smoother; the links of the agglomerates become weaker or even disappear; and the pores increase significantly. The surface of the soil particles contaminated by the alkaline environment is rougher, which further validates the conclusions obtained earlier in this paper.

5. Discussion

5.1. Application and Limitation

A multi–scale analysis, not only on the macroscopic mechanical properties of silty clays, but also the microstructural characteristics of soil, explains the physical and mechanical properties of soil under complex conditions. The two methods are complementary and both approaches are of significance. This paper focuses on the influence of inorganic acid and alkali contaminated solution on the mechanical properties of silty clay, in which the applied inorganic solution does not involve heavy metals, and hence, it is more suitable for the evaluation of the mechanical properties of silty clay contaminated by inorganic acidic/alkaline solution.

While the observation of the experimental phenomena is at an advantageous position, this study still has some limitations, but we can learn much from them. The limitations are as follows:

(1) All the results are obtained by measuring the mechanical properties of the soil after pollution. Therefore, it is impossible to discuss the accumulation process of pollutants and the interaction process with the silty clay.

(2) Chemical pollutants are roughly divided into inorganic pollutants and organic pollutants. The former also contains heavy metals, such as mercury, cadmium, lead, and arsenic, while the latter is represented by various chemical pesticides. According to the issue of the extremely rich diversity in terms of soil pollution and the ways pollutants entering the soil, there exist many differences in the mechanical properties of contaminated soil. Therefore, it will be a systematic and complex project to explore the influence of different chemical pollutants on the mechanical properties of contaminated soil. This is also a direction that needs to be supplemented and perfected in the future.

(3) In practical engineering, the long–term effects of environmental factors, such as weathering, rainfall, and freeze-thaw cycles, will change the physical and mechanical properties of soil. However, the interaction of environmental factors is so complex that a brief study cannot do justice to it. This paper only focuses on the change in the soil’s mechanical properties in an acidic/alkaline solution under normal conditions. With further in–depth research, the influence of external environmental factors will be gradually considered in a follow–up study.

5.2. Contamination Mechanism

Attempts were made to further discuss the mechanism and effect of the pore solution on the soil by analyzing the above test results and the findings of previous studies. In the multiphase medium material, the particle composition and concentration changes of the liquid phase have a significant impact on the deformation and strength characteristics of soil [41,42]. The strength, seepage, and deformation characteristics of soil are not only related to the three phases of solid, liquid, and gas, but are also closely related to the interaction between the three phases and the complex structure of the composition. The complicated nature of the interaction mechanism in the soil, the multiplicity of factors, the diversity of internal interaction (the interaction force between particles, the chemical force and the surface tension between soil and water, and so on) are all finally reflected the changes in the strength, seepage, and deformation characteristics of the excavated soil on a macroscopic scale.

An analysis of the mechanism of the soil particles show that they are affected by acid and alkali. Due to the large radius of sand particles and small specific surface area, the surface is usually not charged. Relatively stable compounds (such as silica), as its main mineral components, are difficult to interact with acidic and alkaline solutions. In addition, the capillary action of sand is so small that there is almost no attraction between particles. Therefore, the friction among particles is determined by the roughness of the particles and has nothing to do with the nature and concentration of the solution. However, it is not ruled out that there are other processes that are determined by the colloidal interactions and the surface activity of the newly formed compounds. In view of the fact that the properties of sand are not subjected to a shifting in the internal solution, a few researchers have conducted sand pollution tests. The more common research objects are clay, silty clay, red clay, or silty clay with a certain amount of organic matter. In contrast, clay is composed of a variety of silicates and some oxides, which easily react with the pore solution. In addition, clay particles are small, and the specific surface area is usually large. The surface of the particles is charged and their capillary action is strong, hence one of the reasons why the chemical bond between the particles and the pore solution is more complicated. Many mineral components in clay and a large amount of cementation among the particles would more likely lead to the soil particles’ exchange and reaction with the ions in the solution. Various existing studies have shown that the strength, seepage, and deformation characteristics of clay are all affected by the pore solution and its pH value. Alibeiki et al. [16] and Sivapullaiah et al. [43] found that clay loses the most weight after being polluted by an alkaline solution, followed by silty clay, and silt is affected the least. This further proves that clay particles are more likely to react with pore solution than silt particles.

In the analysis of how the mechanism of soil strength is affected by acid and alkali, when combining existing test results and the results of this test, it shows that the initial cohesion of the soil samples in acidic solutions is relatively large. These soil samples usually have relatively large initial dry density and low acidity. The soil particles are dispersed in the film microstructure and sheet–like microstructure, owing mainly to hydrogen ion entering the double electron layer of the soil in a low–concentration acid environment, resulting in the expansion of each soil layer and higher strength. With an increase in acidity, the surface material of the particles and the colloidal material among the particles dissolve in the solution, which makes the surface of the soil particles smoother. Thus, the cohesive force accumulation decreases, which is consistent with the microscopic observation results. In a low–concentration alkaline environment, hydroxide ion destroys the colloidal structure among particles, resulting in fewer connections between particles and less cohesion. As the alkalinity increases, insoluble matter gradually adheres to the surface of the particles, which further increases the contact among the particles and increases cohesion.

In the analysis of how the mechanism of soil deformation is affected by acid and alkali, soil deformation is mainly determined by the pores. In an acidic solution, the acid will corrode the soil particles, thereby increasing the internal pores and smoothing the soil particles. Therefore, the silt samples are more likely to show a lower compressibility coefficient. In the presence of generous substance that can react with acids, clay with a high dry density will show higher compressibility. For alkali solutions, since alkali does not cause the erosion of soil particles, it will instead produce precipitates and reduce internal pores. When the water is filtered, the highly water-soluble compounds migrate, which causes the situation to change. Potential collapses of the soil skeleton can also be expected.

The influence of pore solution on soil can be mainly classified into the following three aspects:

1. Reaction with colloid and organic matter among particles. The pH change of the solution directly causes the colloid to dissolve and decompose, which causes the colloid to weaken or lose its cementing ability and, thus, reduces the connection between soil particles. When the soil particles react with hydrogen ions, some mineral components and oxides react as ions in the solution, creating pores and reducing the shear strength of the soil.

2. The solution and particles produce new substances. When soil particles react with hydroxide ions, some free ions will form precipitates and adhere to the surface of the particles. There are some colloids which are insoluble in alkali solutions, and those will also convert into precipitates and consolidate on the surface of the particles. If the foundation soil is contaminated, some new minerals, such as gypsum, alunite, and limonite, will be produced based on existing studies.

3. Ion exchange occurs between the solution and the particles or the electric double layer on the particle surface. According to previous studies [44,45], soil particles usually have a large number of negative charges, which will attract some cations or polar molecules. Therefore, when the type and concentration of ions in the solution change, some particles will exchange with the particles on the surface of the soil particles. According to the different adsorption capacities of cations on the surface of soil particles, hydrogen ions will replace some cations in an acidic solution, such as calcium ion, aluminum ion, and ferric ion. According to the study on clay colloid chemistry [21], hydrogen ions entering the electric double layer will increase the thickness of the electric double layer, weaken the connection force between particles, increase the distance among particles, and expand the soil. This expansion is relative to the dissolution of the substance. The impact is small, which often fails to be reflected in the macroscopic view, but this has a greater impact on soils with larger surface areas (montmorillonite) and soils with higher compaction, and will cause changes in soil permeability.

The above three aspects work together to have different macro performances on different types of soil. Therefore, when analyzing the effect of a solution on soil, it is necessary to comprehensively consider the mineral composition, compaction degree, specific surface area, ion species, and other factors of the soil.

5.3. Comparison and Promotion

In this study, the acidic solution concentration was negatively correlated with density, the specific gravity, and the cohesion value of the soil, while the internal friction angle value had no noticeable change. As for the effect of the alkali solution on the soil samples, alkali contamination could affect those properties in a positive correlation. Compared to silty soil polluted by acidic and alkaline solution [30], the trend of macroscopic mechanical properties after acidification was consistent. However, the shear strength of the contaminated silty soil decreased with increasing alkaline contaminants. According to the experimental results of the references [16,20], the shear strength of loess can be improved under an alkaline environment. The results of this paper are consistent with those of the literature [16,20]. The main influencing factors of this difference are the proportion of fine particles and the mineral composition of the soil.

6. Conclusions

A series of experimental studies were carried out to determine the geotechnical characteristics of the soil contaminated by acidic and alkaline solutions. The research included basic properties, consolidation, direct shear, and microstructure observation on the soil samples at the same initial dry density. Based on the obtained results, the following conclusions can be drawn:

(1) The quality of the soil samples contaminated by the acidic solution is reduced. Compared to the most alkaline state (alkali 9%), the density of the soil with 9% acidity decreases by 31.83%. The actual cause is some substances in the particles react with the acid to dissolve into a liquid. On the contrary, the quality of the soil samples in the alkaline solution is increased.

(2) Soil–water–acid interaction can lead to considerable changes in the compressibility of the soil samples. The effect of the acidic solution leads to an increase in the pore ratio of the soil and change the structure of the soil samples, therefore, the soil samples in the acidic solution show higher compressibility. When increasing the consolidation pressure from 350 kPa to 950 kPa, the void ratio of acid 1%, acid 5%, and acid 7% decreases by 0.085, 0.163, and 0.212 times, respectively. For the same reason, the soil samples after contamination by the alkaline solution result in higher compressibility. When increasing the consolidation pressure from 50 kPa to 800 kPa, the void ratio of alkali 1%, alkali 5%, and alkali 7% decreases by 0.075, 0.084, and 0.104 times, respectively.

(3) The cohesion value of the soil samples in the acidic solution reduces, however, the internal friction angle value has no noticeable change. As for the soil samples in the alkaline solution, both the cohesion value and the internal friction angle value gradually increase, and the shear strength increases to 16.4% in alkali 3% and to 54.2% in alkali 7% compared with the alkali 1% specimens, which means that the strength of the soil has increased.

(4) By observing the microscopic structure of the soil samples under different magnifications, the results indicate that the surface of acidified soil particles is smooth and flat, while the surface of alkalized soil particles is rough and uneven. This also reflects the influence on the mechanical properties of soil that the internal friction angle of alkalized silty clay is slightly above that of acidified silty clay.

(5) The declining soil strength under acidic conditions is caused by the decomposition of colloids among the particles and the reaction and decomposition of the particles themselves with the solution. The detailed implications are divided into two aspects that work together to change the nature of the soil. On the one hand, it decomposes the colloid between the particles, which leads to a decrease in soil cohesion and an increase in compressibility. On the other hand, the colloidal substances and ions between the particles react and adhere to the surface of the particles, making the surface of the particles rougher. The particle spacing increases as the friction angle and the compression coefficient increase.