Classification and Zoning of Improved Materials of Weathered Redbed Soil in China Based on the Integrity of Mud Skin

Abstract

Natural redbeds are widely distributed throughout China. Ecological restoration entails implementing measures according to the local conditions and obtaining local materials, ensuring ecological environment conservation and restoration in a scientific manner. The mud skin of weathered redbed soil is often used to control soil desertification and repair slope, and its integrity is important to measure the repairing effect. However, most of the materials used for the improvement of weathered redbed soil contain harmful chemicals, bear high costs, and aggravate environmental pollution. At present, the knowledge about different kinds of improvement materials for Chinese different zones is unclear. To solve this problem, we considered naturally weathered redbed soil as the raw material, selected 77 kinds of improved materials, and tested the integrity of the natural redbed weathered soil after adding the improved material; then, we compared it with the natural redbed weathered soil with no added materials. Then, we put forth a classification standard for the materials, discussed the driving environmental factors, formulated the suitable zoning of the materials, and defined the improvement effects of different materials on the weathered redbed soil in different regions of China. The results suggested that, for weathered redbed soil, nano water-based adhesives were most suitable for the south-western, north-western, south-eastern, north-eastern, and northern regions of China and can be widely used in other regions as well. Starch was the least suitable material for the north-western, north-eastern, and northern regions of China. The most unsuitable material for South-West China was larch tannin extract; wormwood straw was the most unsuitable for South-East China. The modified material that was not suitable for use in most zones was starch. Thus, our study provides a concrete scientific basis regarding the effectiveness of different materials in addressing natural hazards caused by weathered redbed soil in China.

1. Introduction

Plastic film and non-woven fabric are commonly used ecological restoration materials in northern and southern China, respectively, playing a role in water and soil conservation. Currently, plastic mulch and non-woven fabrics are mainly made of man-made materials prone to environmental pollution. Due to the inability to take into account the environmental characteristics of ecological restoration in northern and southern China, plastic mulch and non-woven fabrics cannot be widely used nationwide and their suitability for national ecological restoration is not high. Therefore, it is not possible to further form a national universal system, resulting in high-cost engineering restoration. The redbeds are widely distributed in China, covering an area of 1,887,500 km² [1] (Figure 1), being environmentally friendly and low cost, and are beginning to be applied in the engineering field. Adapting measures to local conditions, using materials from nearby areas, and scientifically carrying out ecological environment restoration are also methods advocated by engineering ecological restoration. Therefore, red weathered soil can be selected locally to produce a mud film to replace polluting mulch and non-woven fabrics. Ecological restoration entails the implementation of measures according to local conditions and the use of local materials; the measures must be carried out in a scientific manner [2]. The mud skin of weathered redbed soil is often used to control soil desertification and repair slope; its integrity is an important indicator to measure the repairing effect of the mud [3]. Integrity includes skid, acid, and alkali resistance and compression resistance [4,5,6,7,8]. Weather conditions in various regions of China are inconsistent, and thus, the requirements for the integrity of weathered natural redbed soil mud in different regions vary [9]. Therefore, it is necessary to formulate zoning and grading standards for the materials used to improve the integrity of the weathered redbed soil surface and determine the driving environmental factors to support optimal ecological restoration [10].

The literature on adding materials to improve the integrity and stability of weathered redbed soil mud mainly focuses on inorganic and organic improved materials. Commonly used organic improved materials include air-entraining, drought strength, expansive agents [11], thickeners, water reducers, and sodium- and potassium-containing minerals. For example, Shirazi [12] and Kaniraj and Havanagi [13] indicated that initiators and thickeners can improve the integrity of weathered redbed soil. To further explore the improvement effects of more types of building materials on weathered redbed soil, Miller and Azad [14] compared the improvement effects of common air-entraining, water reducing, drought strength, and expansion agents and thickeners on the compression resistance of weathered redbed soil. Previous studies have proved that putty and environmentally friendly glue have similar properties to air-entraining, water-reducing [15], drought strength, and expansion agents and thickeners [16]. Nalbantoglu et al. [17] and Rajasekaran and Rao [18] used putty and environmentally friendly glue to improve the compression resistance of weathered redbed soil. Al-Rawas et al. [19] further tested the acid and alkaline resistance of weathered redbed soil after adding putty and environmentally friendly glue. However, building materials have a certain number of pollutants. To explore more environmentally friendly inorganic improved materials, Basha et al. [20] and Kolias et al. [21] studied the improvement in weathered redbed soil after the addition of sodium-containing mineral materials. Additionally, Edil et al. [22] and Jauberthie et al. [23] expanded the range of minerals and introduced potassium- and silicon-containing materials into weathered redbed soil for improvement. To further evaluate the improvement effects of mineral materials on weathered redbed soil, Horpibulsuk et al. [24] and Phetchuay et al. [25] studied the changes in mechanical properties of weathered redbed soil after adding albite. However, inorganic-improved materials have high requirements for mixing, and the mixing affects the quality of improvement while compromising environmental remediation. In terms of organic-improved materials, the commonly used organic materials are plant straw, nano-aqueous adhesives, and cellulose. Onyejekwe and Ghataora [26] used wheat, rice, and corn straw to improve the integrity of weathered redbed soil. Plants are environmentally friendly but their improvement effect is weak [27]. To explore more efficient organic improvement materials, Al-Khanbashi and Abdalla [28] and Naeini and Ghorbanalizadeh [29] studied the improvement effect of HHR cellulose on weathered redbed soil; however, the price of HHR cellulose is high. To further determine more economical and practical improved materials, Jin et al. [30] analysed the effectiveness of a nano-aqueous binder in improving weathered redbed soil. Additionally, Dong et al. [31] and Aehnelt et al. [32] compared the improvement in the skid resistance of weathered redbed soil after the addition of nano-water-based adhesive. However, in general, the price of organic improved materials is expensive, and the processes are complex. Notably, in the current literature, no study combines inorganic- and organic-improved materials for comparative analysis, systematically carries out tests and property analysis for a variety of improved materials, or develops grading standards for improved materials according to the differences in the property requirements of weathered redbed soil in different zones. In particular, no existing study has determined the kinds of improved materials that can be used in different zones for the best improvement effect.

In this study, we selected the widely distributed weathered natural redbed soil in China, analysed the effects of 77 kinds of improved materials, and designed and carried out a mud-skin-forming test for the soil mud after the addition of improved materials. Furthermore, we tested the skid, pressure, acid, and alkali resistance of the redbed soil mud skin after adding the improved materials and compared it with the mud skin formed without adding any improved material. Based on the test results, we put forward the classification standard of improved materials, determined the important driving environmental factors, and developed suitable zoning for the improved materials.

2. Materials and Methods

2.1. Preparation of Mud Improvement Materials

In this study, we selected the weathered redbed soil commonly found in China as the slurry improvement material. The redbeds in China are mainly composed of red continental clastic rocks and their weathered materials deposited in the Mesozoic and Cenozoic eras. The redbeds in this era have the same sedimentary background and therefore have similar properties [33,34]. The test samples were sampled 12 m underground in the Pearl River Delta in South China, which is a typical Mesozoic–Cenozoic redbed with the same properties as the redbeds in other provinces of China. The sample is representative [35]. In general, the redbed samples had interacted with air, water, carbon dioxide, and other substances for a long time, resulting in complex chemical reactions, and were influenced by temperature, water, and biology. Notably, the redbed on the surface or close to the surface disintegrated easily, forming fragments or sand particles of different sizes, referred to as ‘weathered redbed soil’ in this study, as shown in Figure 2.

The parameters of the redbed weathered soil used for the test [35] are shown in

Table 1. The parameters of the redbed weathered soil.

Less than 1 mm, and the Particle Size Content	The Non-Uniformity Coefficient (Cu)	Curvature Coefficient
29–85%	6–16	0.4–1.2

.

Calculate the relative content of each component of the redbed weathered soil sample through powder diffraction analysis [35], as shown in

Table 2. The relative content of each component of the redbed weathered soil.

Silicon Dioxide	Kaolinite	Illite	Chlorite	Montmorillonite	Anorthosite	Hematite
66.32–81.07%	2.48–13.36%	5.09–12.11%	1.32–4.94%	0.38–1.83%	0.5–7.28%	1.86–7.16%

.

We introduced the dry weathered redbed soil particles into a standard sieve, as shown in Figure 3; the samples were then vibrated mechanically for more than 20 min and we weighed the mass of the soil particles for different particle sizes to obtain the particle gradation of the sample. After screening the soil, we found that the content of the redbed weathered soil sample with a particle size of less than 1 mm is 5–85%. Therefore, with a gradient of 10%, a total of 9 experimental groups were set up, with sample particle sizes smaller than 1 mm and contents of 5%, 15%, 25%, 35%, 45%, 55%, 65%, 75%, 85%. The 9 experimental groups cover the particle grading range of redbed weathered soil samples, which can further obtain the most suitable particle grading for film formation. On the basis of this particle grading, 77 materials were added to conduct a material-based film-formation test [35]. The grading of soil particles was conducted according to the Standard for Geotechnical Test Methods, and the thickness of the mud film was measured with a ruler [36].

2.2. Preparation of Modifier

We carried out a comparative test for the formation of the mud skin of the weathered soil slurry samples and analysed the thickness of the mud skin after the addition of improved materials on different slopes to evaluate the effects of mud-skin formation on the integrity of the slope. The soil sample was soaked and stirred with water to obtain a slurry. The slurry had a high viscosity, good entrainment, certain plasticity, and was not easily dispersed in water. According to the type, we divided the improved materials into five categories: adhesives, minerals, building materials, plants, and cellulose. Notably, we used 77 test groups added with improved materials and 1 control group with no added improved materials, as shown in Figure 4.

2.3. Mud Scaling and Property Tests

For the same improved material, starting from 10 g, we gradually increased the content to 100 g, according to the mass gradient of 10 g; then, this amount was mixed with 200 g of the weathered soil sample and 1000 g of water, to study the optimal solution of the influence of the improved material on the soil. Notably, in all these samples, 10 g of the soil sample was added to the control group each time. Then, we mixed different improved materials with the soil sample and water to create mud skin, as shown in Figure 5.

After the mud skin developed, the properties of each group of mud skin were tested. The integrity of the mud skin was reflected by its skid, acid, alkali, and compression resistance, which are explained in detail below:

1. Skid resistance: We measured the thickness of the mud skin in each group. The thicker the mud skin, the stronger the skid resistance of the improved material to the weathered soil;

2. Acid and alkaline resistance: We used droppers to continuously add hydrochloric acid and sodium hydroxide solutions to different groups. The concentrations of hydrochloric acid solution and sodium hydroxide solution were 0.01 mol/L, 0.1 mol/L, and 1 mol/L. The solutions were added every 5 s, until the mud skin tore. We recorded the pH value at that time, which indicated the acid and alkali resistance of the mud skin;

3. Compression resistance: We applied pressure to different groups of mud, starting at 100 N and increasing by 10 N each time until the mud skin tore. We recorded the pressure value at that time, which indicated the compression resistance of mud skin.

3. Results and Discussion

3.1. Influence Law of Various Improved Materials on Mud Skin Integrity and Its Optimal Ratio

We simulated the mud skin of different groups to obtain the thickness range of the mud crust, as shown in Figure 6.

The influence of the improved material on the mud-skin thickness was then summarised into linear and quadratic function types, as shown in Figure 7. The yellow curve represents the control group. When no other improved materials were added, the increase in the use of the soil had no effect on the thickness of the mud crust. Compared with the control group, adhesives, minerals, cellulose, building materials, and plants all improved the thickness of the mud skin.

The amount of adhesive used had a linear effect on the thickness of mud skin, with the thickness, ranging from 60 mm to 176 mm, and there were seven kinds of improved materials. The amount of cellulose used had a linear effect on the thickness of the mud skin, with a thickness of 50–158 mm. There were four kinds of improved materials. The influence of mineral dosage on the thickness of the mud skin could be explained using a quadratic function, with the thickness ranging from 53.2 mm to 135 mm. The influence was greatest when the dosage of the improved materials was 40 g. Notably, the influence of the amount of building improvement materials on the thickness of the mud skin could be expressed using a quadratic function, with the thickness ranging from 36.3 mm to 148 mm. The influence was greatest when the amount of the improved materials was 60 g; the influence of plant dosage on the thickness of the mud skin could be expressed as a quadratic function, with the thickness ranging from 26.3 mm to 119 mm. For 56 improved materials, the influence was the greatest when the dosage was 50 g.

By dripping water and the hydrochloric acid and sodium hydroxide solutions on the naturally weathered redbed soil film to evaluate acid-base resistance, we could estimate the influence of different improved materials on the acid-base resistance of the mud skin. The influence of the improved materials on the acid-base resistance of the mud skin was only linear, as shown in Figure 8. The yellow curve represents the control group. When no other improved materials were added, the increase in the use of the weathered redbed soil had no effect on the acid and alkali resistance of the mud skin. Compared with the control group, adhesives, minerals, cellulose, building materials, and plant straw all improved the acid-base resistance of the mud skin.

The amount of adhesive used had a linear effect on the acid resistance of the mud skin, with a pH of 0.5–4.8. The amount of cellulose used had a linear effect on the acid resistance of the mud skin, with a pH range of 0.8–5.2. The amount of building improvement materials had a linear effect on the enhancement of the mud skin’s acid resistance. Additionally, the amount of mineral-modified materials had a linear effect on the acid resistance of the mud skin, with a pH of 1.9–6.2. Notably, the amount of plant-improved materials had a linear effect on the acid resistance of mud skin, with a pH range of 2.7–6.8. There are 56 kinds of improved materials that exhibit the best results.

The amount of adhesive used had a linear effect on the alkaline resistance of the mud skin, with the pH being 9.2–13.5. The use of cellulose had a linear effect on the alkaline resistance of the mud skin, with the pH being 8.8–13.2. The usage of building improvement materials had a linear effect on the alkaline resistance of the mud skin, with a pH range of 7.6–11.6. Furthermore, the use of mineral-modified materials had a linear effect on the alkaline resistance of the mud skin, with the pH being 7.8–12.1. Notably, the amount of plant-improved materials had a linear effect on the alkaline resistance of the mud skin, with the pH being 7.2–11.3.

Starting from 100 N for different groups of mud, we increased the pressure, with a gradient of 10 N, until the mud skin tore, and recorded the pressure value to determine the influence of different improved materials on the compression resistance of the mud. The influence of the improved materials on the compression resistance of mud was linear, as shown in Figure 9. The yellow curve was the control group. When no other improved materials were added, the increase in the use of the weathered redbed soil had no effect on the compressive strength of the mud skin. Compared with the control group, in the other groups, adhesives, minerals, cellulose, building improvement materials, and plants improved the compression resistance of the mud skin.

The amount of adhesive had a linear effect on the compressive strength of mud skin, with the pressure being 1800–3000 N. The use of cellulose had a linear effect on the compressive strength of the mud skin, with the pressure being 1200–2000 N. Additionally, the usage of building improvement materials had a linear effect on the compressive strength of the mud skin, with the pressure being 1600–2600 N. The usage of mineral-modified materials had a linear effect on the compressive strength of the mud skin, with the pressure being 1400–2300 N. Plant-improved materials also had a linear effect on the compressive strength of the mud skin, with the pressure being 1000–1700 N.

3.2. Comparison of Mud-Skin Integrity for Different Improved Materials Based on the Optimal Ratio

Combined with the influence of improved materials, we also determined those improved materials that had a greater impact on the thickness of the mud skin. The influence of plants and building improvement materials on the skid resistance of the mud skin could be explained as a quadratic function and the peak of the function corresponding to the amounts of the improved materials (40, 50, and 60 g). The influence of binder and cellulose on the skid resistance of the mud skin was linear, and we used 100 g of the improved material as the dosage, as shown in Figure 10. The thickness of the mud skin without modified material was 21 mm and with adhesive was 176 mm, indicating an increase of 738%. The thickness of the mud skin after the addition of cellulose was 158 mm, indicating an increase of 652%. The thickness of the mud crust after the addition of minerals was 135.8 mm, indicating an increase of 547%, and that after the addition of plants was 119 mm, indicating an increase of 467%. Additionally, the mud-skin thickness after the addition of building improvement materials was 148 mm, indicating an increase of 605%. The thicker the mud skin, the stronger the skid resistance; the increase in the skid resistance of the mud skin was significant after the addition of adhesives, cellulose, building improvement materials, minerals, and plants. Among all the improved materials tested, the No. 6 adhesive had the greatest effect on increasing the skid resistance of the mud skin, with the thickness of the mud skin being 176 mm, indicating an increase of 652%. The three-centimetre wormwood straw increased the mud-skin thickness to 26.3 mm, indicating an increase of 24%, and the seed germination rate was 85–95%.

The influence of mineral-modified materials, plants, building improvement materials, adhesives, and cellulose on the acid and alkaline resistance of the mud skin was linear, with the maximum dosage of modified materials being 100 g, as shown in Figure 11. After the addition of hydrochloric acid solution, the pH value of the mud skin without any added improved material was 6. After adding adhesives, the pH of the mud was 0.5. For added cellulose, the pH was 0.8, indicating a decrease of 5.2. The pH value of the mud skin with added minerals was 0.9, indicating a decrease of 5.1. For added plants, the pH was 1.7, indicating a decrease of 4.3. For added building improvement materials, the pH was 1.4, indicating a decrease of 4.6. From strong to weak, the anti-acid enhancement effect of the mud skin was as follows: additions of adhesives, cellulose, minerals, building improvement materials, and plants. Among all the improved materials tested, the No. 6 adhesive had the greatest effect on increasing the acid resistance of the mud skin; the pH of the mud skin was 0.5, indicating a decrease of 5.5. The least significant effect was that of starch, with the pH of the mud skin being 5.8 and the seed germination rate being 90–95%.

After adding the sodium hydroxide solution dropwise, the pH value of the mud skin without any modified material was 8.1. After adding adhesives, the pH was 13.5, indicating an increase of 5.4. After the addition of cellulose, the pH of the mud skin was 13.2, indicating an increase of 5.1. For the group with added minerals, the pH of the mud skin was 13.1, indicating an increase of 5. For added plants, the pH of the mud skin was 12.3, indicating an increase of 4.2. For the added building improvement materials, the pH of the mud skin was 11.8, indicating an increase of 3.7. Among all the improved materials tested, the No. 6 adhesive had the greatest effect on increasing the alkaline resistance of the mud skin; the pH value of the mud skin was 13.5, indicating an increase of 5.4. The effect of starch was the least, with the pH of the mud skin being 8.2, and the seed germination rate being 80–95%.

The influence of mineral-modified materials, plants, building materials, adhesives, and cellulose on the compressive strength of the mud skin was linear, with the maximum dosage of the modified materials being 100 g (Figure 12).

After pressure was applied, the mud skin developed without the addition of any modified material tore at 800 N. For the group with added adhesives, the pressure at which the mud skin tore was 3000 N (an increase of 275% in the pressure). For the group with added cellulose, the pressure at which the mud skin tore was 2000 N (an increase of ~150%). For the group with added minerals, the pressure at which the mud crust tore was 2300 N (an increase of 186%). For added plants, the pressure at which the mud skin tore was 1700 N (an increase of 113%). For added building improvement materials, the pressure at which the mud skin tore was 2600 N, (an increase of 113%). The strengthening effect of the materials on the compression resistance of the mud skin from strong to weak was in the following order: adhesives, building improvement materials, minerals, cellulose, and plants. Among all the improved materials tested, the No. 6 adhesive had the greatest effect on increasing the compressive strength of the mud skin, with the pressure of the mud skin being 3000 N, indicating an increase of 275%. Larch tannin extract had the least impact on the compressive strength of the mud skin, with the pressure being 1000 N and the seed germination rate being 80–93%.

Materials will enhance the internal cohesion of the redbeds weathered soil mud film, so materials closely combine with the redbed weathered soil particles. As the strength of the redbed weathered soil mud film increases the shear capacity also increases.

3.3. Zoning and Grading of Improved Materials Based on Ecological Restoration Environment of Weathered Redbed Soil

The climatic characteristics of dry and wet cycles in each division of China are inconsistent, and the requirements for the integrity of the mud skin of naturally weathered redbed soil are also different. The properties of the natural redbed soil in each region of China are similar (Cheng, 2004); therefore, it is necessary to formulate zoning and classification standards for improved materials, while determining the driving environmental factors. The north-west, north-east, and northern regions of China experience drought frequently, and the land is undergoing desertification; therefore, there is a great demand to improve the acid and alkali resistance of the mud skin in these regions. The construction and development efforts of South-West China are large; there are many engineering projects, and there is a great demand for the improvement of the compression resistance of mud skin in the region. Additionally, there are many slopes in South-East China, posing serious landslide risks; therefore, improving the skid resistance of the mud skin in this region can help reduce the risks to life and property [37]. Combined with the experimental study of the upper and lower limits of the nature of mud skin, we have suggested the restoration of the ecological environment in China based on the weathered redbed soil commonly found in the country; the classification of the properties of improved materials is shown in Figure 13.

Based on the characteristics of the zoning requirements for the properties of natural redbed weathered soil and mud film, a weight of 1.5 times the score is given for the integrity and skid resistance required by the zoning, and a weight of 1.25 times the score is given for acid and alkali resistance [38], the evaluation scores of the north-western, northern, and north-eastern zoning are explained below:

S₁ = M₁ + 1.25 M₂ + 1.25 M₃ + M₄

(1)

Evaluation score of the south-west division:

S₂ = 1.5 M₁ + M₂ + M₃ + M₄

(2)

Evaluation score of the south-east division:

S₃ = M₁ + M₂ + M₃ + 1.5 M₄

(3)

The equation that is generally suitable for the evaluation scores of different divisions is given below:

S₄ = M₁ + M₂ + M₃ + M₄

(4)

Then, we substitute the test results into Equations (1)–(4); the zoning results of the improved materials are shown in Figure 14 and Figure 15. The improved materials selected in the test process were representative of the ones having a high market share. Thus, we suggest that the improved materials most suitable for the south-western, north-western, south-eastern, north-eastern, and northern regions of China, which can also be widely used in other regions of the country, are No. 6 adhesives. Starch was the least suitable improved material for the north-western, north-eastern, and northern regions of China. The most unsuitable improved material for South-West China was larch tannin extract. Wormwood straw was the most unsuitable improved material in South-East China. The modified material that was not suitable for any region was starch.

The improved materials can improve the cohesive force of the weathered soil film of the natural redbeds. Adhesives, building materials, and plant materials improve the molecular structure of the redbed weathered soil membrane, while mineral materials and cellulose improve the ion distribution of the redbed weathered soil membrane [39,40].

4. Conclusions

(1) According to the test results, we put forward the classification standard of the 77 improved materials analysed in this study while considering the different environmental factors of the different regions of China. Additionally, we formulated the suitable zones for the application of the improved materials and defined the improvement effect of these materials on the weathered redbed soil in different regions in China. The suggestions for ecological restoration are as follows: natural redbeds soil acts as a nano-water-based adhesive and is the most suitable improved material for the south-west, north-west, south-east, north-east and northern regions of China and can be widely used in other regions as well. Starch is the least suitable improved material for the north-west, north-east, and northern regions of China. The most unsuitable improved material for South-West China is larch tannin extract. Wormwood straw is the most unsuitable improved material for South-East China. Notably, the modified material that is unsuitable for most regions is starch;

(2) Among all the improved materials tested, the nano-water-based adhesive had the greatest effect on the skid resistance of the mud skin. Nano-aqueous adhesives had the greatest effect on increasing the acid resistance of the mud skin, and starch had the least effect. Nano-water-based adhesive had the greatest effect on increasing the alkaline resistance of the mud skin, whereas starch had the least effect. Additionally, nano-water-based adhesive had the largest effect on increasing the compression resistance of the mud skin, whereas larch tannin extract had the smallest effect;

(3) The results of the addition of improved materials to the mud skin of the weathered redbed soil examined in this study cannot only be introduced into the field of ecological restoration but also the property laws regarding the use of improved materials. The improved materials can also be applied to other kinds of mud skins to explore whether they have similar properties and laws, so as to increase the scale of ecological restoration. The zoning and grading method proposed in this study can be used to not only evaluate more types of materials but also extend to more regions to further develop a more comprehensive zoning and grading evaluation system.