Experimental investigations on permeability and damage mechanism of cemented soil in acidic and alkali environments

2.1. Mixing proportion of cemented soil

Portland slag cement 32.5 was used in the test. Cement and silt were placed in a blender and mixed evenly for content of 7 %, 10 %, and 13 %. The mixture was poured into molds with an upper diameter of 80 mm, bottom diameter of 70 mm, and height of 40 mm. The cemented soil specimens were well compacted by vibration, numbered, demolded after 24 hours, and placed in preconfigured solutions of different pH values for curing. The laboratory temperature was between 22 °C and 28 °C. The specimens under curing are shown in Fig. 1(a).

2.2. Permeability tests of cemented soil in different pH environments

(1) Hydrochloric acid and sodium hydroxide were used to configure different pH solutions.

(2) After demolding, cemented soil specimens were cured in different pH solutions (pH = 2, pH = 4, pH = 9, pH = 10, pH = 13). The pH value is monitored daily during curing. When it fluctuated more than 0.5, hydrochloric acid or sodium hydroxide is immediately added to the solution to maintain a constant pH value.

(3) According to Cemented Soil Mixing Proportion Design Procedures (JGJT 233-2011) the experimental requirements, permeability tests of cemented soil specimens were carried out. The main equipment was a penetration apparatus as shown in Fig. 1(b).

Fig. 1Curing of cemented soil specimens and penetration apparatus

a) Curing of specimens

b) Penetration apparatus

3.1. Calculation of permeability coefficient of cemented soil

The equation for the permeability coefficient of cemented soil is:

where $k_T$ is the permeability coefficient of the cemented soil (cm/s) at water temperature $T$ °C, $V$ is the exudation water volume (ml) for time interval $t$, $h$ is the height of the specimen (cm), $p$ is the applied seepage pressure (MPa), $A$ is the cross-sectional area of the specimen (cm²), $t$ is the time interval (s), and ${\gamma }_w$ is the water gravity (N/cm³).

The permeability test was conducted under the standard 20 °C water temperature. The permeability coefficient at the standard temperature is:

where $k_{20}$ is the permeability coefficient of the specimen (cm/s) at the standard temperature, is the dynamic viscosity coefficient at a water temperature of $T$ °C (kPa·s), and ${\eta }_T$ is the dynamic viscosity coefficient at 20 °C (kPa·s),These specific values were given by JGJT 233-2011 cemented soil mixing proportion design procedures.

3.2. Effect of different curing times on permeability of cemented soil

Permeability tests of cemented soil specimens were respectively conducted after curing periods of 7 d, 15 d, 30 d, and 60 d. In an acidic environment (pH = 2), the permeability coefficient of cemented soil with 7 % cement content decreased gradually over the initial curing time (7-30 d). Its permeability coefficient reached its minimum in 30 d, decreasing by 67.42 % from its value after 7 d curing. After that, the permeability coefficient increased gradually, showing a U-shaped change, increasing by 14.25 % in 60 d relative to 7 d. The results indicate that in an acidic environment, cemented soil with low cement content cannot resist acidic corrosion. In contrast, the permeability coefficient of cemented soil with 10 % or 13 % cement content decreased gradually with curing time, relative to 7 d by 32.05 % and 85.31 %, respectively, at 60 d.

In acidic environment (pH = 4), the permeability of cemented soil presented the same trend as for pH = 2. As shown in Fig. 3, the permeability coefficient of cemented soil with 7 % cement content reached a minimum in 30 d, decreasing by 72.88 % relative to 7 d. The permeability coefficient increased gradually after that, showing a V-shaped change. In contrast, the permeability coefficients of cemented soil with 10 % and 13 % cement content decreased gradually, by 71.48 % and 94.65 %, respectively, at 60 d.

Fig. 2Variation of permeability coefficient in the environment of pH = 2

Fig. 3Variation of permeability coefficient in the environment of pH = 4

The variation of the permeability coefficient of cemented soil with different cement content in different alkaline environments (pH = 9, pH = 10, and pH = 13) is shown in Figs. 4, 5, and 6. In the alkaline environment of pH = 9, the permeability coefficient of cemented soil with 7 %, 10 %, and 13 % cement content decreased by 87.67 %, 89.81 %, and 97.73 % respectively, in 60 d. In the alkaline environment of pH = 10, the permeability coefficient decreased by 96.98 %, 94.30 %, and 95.35 %, respectively, over the same time. In the alkaline environment of pH = 13, it decreased by 96.80 %, 93.58 %, and 98.50 %, respectively in that time. These results show that the impermeability of cemented soil improves with increasing curing time in an alkaline environment.

Fig. 4Variation of permeability coefficient in the environment of pH = 9

Fig. 5Variation of permeability coefficient in the environment of pH = 10

3.3. Effect of different pH values on permeability of cemented soil

As shown in Figs. 7, 8, and 9, the permeability coefficients of cemented soil with 7 %, 10 %, and 13 % cement content decreased gradually as the pH value increased. After curing periods of 7 d, 15 d, 30 d, and 60 d, from an acidic environment (pH = 2) to an alkaline environment (pH = 13), the permeability coefficient decreased gradually. In an alkaline environment (pH = 13), the permeability coefficients showed that $K_{7d}>K_{15d}>K_{30d}>K_{60d}$, which indicates that the alkaline conditions are conducive to the hydration reaction and promote the inner compaction against pores, resulting in improvement of cemented soil’s impermeability. Therefore, the acidic environment has a corrosive effect on the cemented soil. The lower the pH, the larger the permeability coefficient of cemented soil is.

Fig. 6Variation of permeability coefficient in the environment of pH = 13

Fig. 7Variation of permeability coefficient with 7 % cement content with pH

Fig. 8Variation of permeability coefficient with 10 % cement content with pH

Fig. 9Variation of permeability coefficient with 13 % cement content with pH

3.4. Effect of different cement content on permeability of cemented soil

As shown in Figs. 10 and 11, in an acidic environment (pH = 2 and pH = 4), the permeability coefficient of cemented soil with different curing ages decreased gradually with increasing cement content. In acidic environment (pH = 2), at the curing age of 7 d, 15 d, 30 d, and 60 d, the permeability coefficient of cemented soil with 13 % cement content decreased by 92.36 %, 81.41 %, 94.45 %, and 98.90 % respectively, compared to cemented soil with 7% cement content. Similarly, in acidic environment (pH = 4), it decreased by 81.92 %, 91.57 %, 93.49 %, and 97.50 %, which shows that in an acidic environment, increasing the cement content increases cemented soil’s resistance to acidic corrosion.

As shown in Figs. 12, 13, and 14, in an alkaline environment (pH = 9, pH = 10, and pH = 13), the permeability coefficient of cemented soil with different curing ages decreased gradually with increasing cement content. In an alkaline environment (pH = 9), at curing ages of 7 d, 15 d, 30 d, and 60 d, the permeability coefficient of cemented soil with 13 % cement content decreased by 83.54 %, 89.50 %, 92.84 %, and 96.97 %, respectively, compared to cemented soil with 7 % cement content. Similarly, in alkaline environments with pH = 10 and pH = 13, the permeability coefficient decreased respectively by 68.74 %, 58.04 %, 93.33 %, and 51.91 % for pH = 10 and 68.27 %, 75.15 %, 94.98 %, and 85.18 % for pH = 13. This shows that in an alkaline environment, an increase in cement content is advantageous to hydration reaction and impermeability of cemented soil.

Fig. 10Variation of permeability coefficient with cement content in the environment of pH = 2

Fig. 11Variation of permeability coefficient with cement content in the environment of pH = 4

Fig. 12Variation of permeability coefficient with cement content in the environment of pH = 9

Fig. 13Variation of permeability coefficient with cement content in the environment of pH = 10

Fig. 14Variation of permeability coefficient with cement content in the environment of pH = 13

4.1. Macro damage analysis of pH effect on cemented soil

The surface damage of cemented soil specimens is shown in Fig. 15 for curing periods of 60 d in pH = 2, pH = 4, pH = 9, pH = 10, and pH = 13. Among these, in the acidic environment, the cemented soil specimens with 7 % cement content were seriously damaged compared to others, and their surfaces appeared loose. The surface of the cemented soil specimens with 13 % cement content changed very little. These observations indicate that acidic environment has an effect on cemented soil specimens with lower cement content. In the alkaline environment, the surface of the specimens did not appear broken or loose, and the cemented soil specimens with different cement content changed very little. These observations indicate that the alkaline environment does not cause damage to the specimens.

Fig. 15The surface damage of specimens with different cement content in different environment

pH = 2

b) pH = 4

c) pH = 9

d) pH = 13

4.2. Analysis of micro-mechanism of pH effect on cemented soil

Using scanning electronic microscopy (SEM) and energy spectrum analysis, specimens magnified 3,000 times are presented in Figs. 16, 17, and 18. The pictures show that cement gel existed between the particle surface and intergranular when cured in acidic environment. Sliced surfaces of specimens with 7 % and 10 % cement content showed honeycomb hydrate crystallization, which was attached to a fibrous gel material surface generated by the hydration reaction of cemented particles. When cured in acidic environment, specimen sections with 7 % cement content showed a larger loose region. Specimen sections with 10 % cement content appeared relatively more compact in the loose zone with reduced pore size. Specimen sections with 13 % cement content was full of small particles, in which small particles and the overall slice are bonded with each other. The sliced surface had a small number of needle crystals. The cemented soil had a granular-mosaic-gel structure. The skeleton was in the shape of a particle. There were many tiny pores in the connection position between the skeleton particles and gel, most with a long, flat shape and uneven size, and they were connected to each other. The pore sizes in cemented soil sections with a cement content of 13% decreased relative to those of 7 % and 10 % cement content. This indicates that an increase in cement content can improve the resistance to acidic corrosion. When cemented soil was cured in acidic environment (pH = 2 and pH = 4), the cemented hydration products were decomposed by H⁺, which caused the decomposition of the internal fillers. Specimens of Fe₂O₃, Al₂O₃, K₂O, and Na₂O oxide were also decomposed by H⁺, leading to internal looseness and the appearance of a large number of pores inside the specimens. These macroscopic effects reflect that the surface of the specimens becomes loose (as shown in Fig. 15), the compressive strength diminishes, and the permeability increases.

As shown in Figs. 19, 20, and 21, in the alkaline environment of pH = 10, sections of soil with 7 % cement content magnified 3,000 times show that these cemented soil sections have no obviously loose regions compared to those in a strongly acidic curing environment. Particles were clearly distinct, and many needle crystals were attached to the particles. These particles showed solid cementation by needle crystals constituting a skeleton embedded in a contact structure. The strength of specimens improved. This shows that in contrast to an acidic environment, the internal cement hydration reaction of specimens was sufficient in the alkaline environment. Under the same curing condition, sections with 10 % and 13 % cement content were relatively denser, hydrate crystallization increased significantly, and the crystalline structure was bulkier. Small particles and needle crystals consisted of large particles in a rod-like system structure distributed in the soil. Sections with 13 % cement content were relatively denser. Therefore, hydration was increasing, which intensified cementation. Consequently, a granular-mosaic-gel structure is formed.

Fig. 16The scanning electronic microscopy and energy spectrum of 7 % cement content in the environment of pH = 2

a) The scanning electronic microscopy

b) The energy spectrum

Fig. 17The s canning electronic microscopy and energy spectrum of 10 % cement content in the environment of pH = 2

a) The scanning electronic microscopy

b) The energy spectrum

Fig. 18The scanning electronic microscopy and energy spectrum of 13 % cement content in the environment of pH = 2

a) The scanning electronic microscopy

b) The energy spectrum

Fig. 19The scanning electronic microscopy and energy spectrum of 7 % cement content in the environment of pH = 10

a) The scanning electronic microscopy

b) The energy spectrum

Fig. 20The scanning electronic microscopy and energy spectrum of 10 % cement content in the environment of pH = 10

a) The scanning electronic microscopy

b) The energy spectrum

As shown in Figs. 22, 23, and 24, in the alkaline environment of pH = 13, sections with 7 % cement content magnified 3,000 times showed that the degree of density follows 13 % > 10 % > 7 %. Compared to other environmental conditions, the degree of density of the cement content sections increased with different size of particles in mutual cementation. In sections with 7 % cement content, the large and small particles glued to each other and the porosity decreased significantly. The sections with 10 % and 13 % cement content did not show obvious porosity, indicating that the increase in pH was advantageous to cement hydration. The corresponding macroscopic manifestation was the enhancement of both the unconfined compressive strength and impermeability of specimens.

Fig. 21The scanning electronic microscopy and energy spectrum of 13 % cement content in the environment of pH = 10

a) The scanning electronic microscopy

b) The energy spectrum

Fig. 22The scanning electronic microscopy and energy spectrum of 7 % cement content in the environment of pH = 13

a) The scanning electronic microscopy

b) The energy spectrum

Fig. 23The scanning electronic microscopy and energy spectrum of 10 % cement content in the environment of pH = 13

a) The scanning electronic microscopy

b) The energy spectrum

Fig. 24The scanning electronic microscopy and energy spectrum of 13 % cement content in the environment of pH = 13

a) The scanning electronic microscopy

b) The energy spectrum