Characteristic Tests for Waste Mud and Drilling Sludge and Reuse of Solidification from Plateau Ecological Reserve

Abstract

The global community has recently increased its efforts for sustainability and environmental protection. “Green Building” and “Green Construction” are the two methods of incorporating these environmental concerns into the construction and maintenance of mega infrastructures and highways with the intention of reducing any environmental impact that the infrastructures may have on the local ecosystems and ecology. This paper delves into one such study based on the Qinghai–Jiaxi Highway, which is currently under construction in the Qinghai–Tibet Plateau, China. A water treatment method for construction-related mud and sludge is presented as a solution to prevent environmental contamination in the Qinghai–Tibet Plateau. Several tests are performed on samples collected from the Qinghai–Jiaxi Highway to check the feasibility of this method. Several tests, such as X-ray diffraction, a chemical composition analysis, pH tests, mud density, and quality assessments, are performed for this purpose. Following these tests, mud sample mixtures are strengthened with concrete, and their suitability as a filler for road bases is evaluated using compression tests, in accordance with the GB50107 standard. The test results show that the concrete blocks have a compression strength of 35 MPA, which is well within the required standards. The proposed method helps to reduce the harmful discharges from construction sites into the local ecosystems and can potentially reduce the effluent matter in the Qinghai–Tibet Plateau and Ecological Reserve.

1. Introduction

Environmental pollution has become a grave concern for local and global communities. It severely affects human health, local wildlife (fauna and flora), and their intricate relationships within ecosystems. Despite initiatives to reduce pollution, the environment continues to deteriorate, with predictions that global temperatures will rise above 1.5 degrees Celsius by the year 2050 if emissions are not reduced [1]. This condition will make life on Earth inhospitable and cause the extinction of 30% of all known living species. The global concern over environmental pollution has made it difficult for communities (local or otherwise) to develop mega infrastructures, such as highways and railroads, which could help develop their economies and regional connectivities. Environmental pollution is particularly severe in countries such as China that have unique topographies, such as hilly plateaus and extensive river channels. China’s plateaus have a fragile ecological environment. Infrastructure development, such as highways and railways, is challenging due to their large carbon footprint and the untreated contamination of the local ecology by mud and sludge from construction sites [2]. However, it is imperative to develop such infrastructure to connect different regions for economic and social welfare and development. The Chinese government and scholars have been working towards sustainable development since the 1960s, with initiatives such as the “Green Wall of China”, focusing on reforestation around the Gobi Desert [3,4].

Environmental pollution is especially severe in countries such as China that have unique land topographies, i.e., hilly plateaus and extensive river channels. China’s plateaus have a fragile ecological environment. Their economic structures are mostly underdeveloped because these regions are located at extremely high and cold altitudes. Additionally, they are sparsely populated due to the harsh weather and environment. China relies heavily on a network of railways and highways to connect different regions for economic and social welfare and development [5,6,7,8,9]. It is, therefore, imperative that the development of highways and other connectivity infrastructures is not curtailed in favor of environmental pollution controls. Still, a more general focus must be exerted to facilitate the construction in lieu of the overarching environmental concerns. This has been achieved through the combined efforts of the Chinese government and Chinese and foreign scholars [10,11,12].

The Chinese government has made great efforts for sustainable development since the 1960s with ecological architecture. An aggressive reclamation drive in the 1970s focused on reforestation around the Gobi Desert as part of a national initiative called the “Green Wall of China” [3,4]. The 18th National Congress of the Communist Party of China also made a comprehensive plan to develop the “Five in One” layout in the new period. Green rivers and green mountains are regarded as “Jinshan” and “Yinshan” in China, as life itself and the “Five ecological principles” are strictly implemented [6,7].

Scholars in China and abroad have used various experimental methods to study environmental pollution and proposed a range of green technologies to mitigate harmful externalities [13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28]. Much previous work has been conducted on mud treatments to reduce the ecological impact. Wu Zhihong [14] analyzed the solidification mechanism of drilling waste mud using samples taken from the Jiangsu Oilfield. According to the composition and climate characteristics of oilfield mud, they compared and studied the solidification formula of drilling waste mud. Bai [18] used cement as a curing agent to cure waste mud as a subgrade filler. They studied the engineering performance of solidified mud using the unconfined compressive strength, moisture content, compaction performance, and the California bearing ratio as the evaluation indices. The results show that cement has a good curing effect on waste mud. Walsh [29] focused their research on the waste mud produced at a tunnel construction site in Xiamen. They used a chemical curing technology to dispose of the mud. They tested the effects of different influencing factors (the curing agent, the mixing ratio of the curing agent, the initial water content of the mud) on the compressive strength, pH value, water content, and other characteristics of the modified cured mud. However, most of these studies were conducted in the eastern coastal areas. Therefore, applying these methods to construction in the western part of China is difficult because of the special geological conditions due to the rough and elevated terrains of the plateau’s ecological regions [30,31].

A construction project currently underway in western China that has garnered much attention regarding green construction is the Qinghai–Jiaxi Highway project. It is hailed as a game-changer by the Chinese Communist Party because it will connect the previously undeveloped regions of the Qinghai–Tibet Plateau. A landmass of one-fourth of the whole Chinese territory will be connected to the rest of developed China, presenting itself as a gateway to Central and South Asia as a mass transit corridor [11,12]. The project has great significance for the people of the Qinghai–Tibet Plateau and China and, therefore, must be completed with due haste. However, the implications of constructing a highway across the Tibetan Ecological Reserve could disrupt the local environment if implemented without remedial steps [32,33,34].

This paper proposes a novel solution to address the environmental impact of construction projects, specifically the Qinghai–Jiaxi Highway project, which is currently underway in western China. The project is a game-changer as it will connect previously undeveloped regions in the Qinghai–Tibet Plateau, presenting itself as a gateway to Central and South Asia as a mass transit corridor [18]. However, constructing a highway across the Tibet Ecological Reserve could disrupt the local environment if remedial steps are not taken. Therefore, this paper proposes a treatment of drill slag from a construction site to prevent the environmental contamination of the Qinghai–Tibet Plateau by separating the solid particles from the reusable water and implementing green technology in the project. Tests, such as X-ray diffraction, a chemical composition analysis, mud density, and quality assessments, are performed on the samples collected from the Qinghai–Jiaxi Highway to check the feasibility of this method. Additionally, the effectiveness of the solidified mud as a road base reinforcing agent is verified through compression tests performed on cement-solidified mud. The paper highlights the novelty of this research and aims to contribute to the ongoing efforts toward sustainable infrastructure development.

2. Materials and Methods

Characteristics and Reuse Tests on Drill Slag:

2.1. The Qinghai–Jiaxi Highway

The highway is being constructed by the China Construction Second Engineering Bureau. The starting and ending chainage of the project section is k43 + 755–k62 + 560. The total length of the construction section is 18.805 km. The project is in the northeast of the Qinghai–Tibet Plateau, with an altitude of 2800 to 3100 m (Figure 1). It belongs to a plateau/mountain climate where the average temperature is 5.8 °C and the extreme minimum temperature is −26.9 °C. The frost-free period is only 114 days. The project’s starting point is the National Forest Park Nature Reserve.

There are three secondary water source protection sites in the actual bid section of the Qinghai–Jiaxi construction project. To protect the National Forest Park Nature Reserves, green construction is needed for treating the drilling slags with high water content and mud pollution components, as there are no waste slag sites in these reserves. According to the strict requirements for environmental protection in the environmental impact report of the Jiading (Qingganjie) Haiyan (Xihai) Highway in the Qinghai Province (QHF [2017] no. 281), it is necessary to study and develop a waste slag treatment and application technology.

The characteristics of the drill slag/mud are studied to check whether reusing the solidified slag as a road base filler would harm the local ecology in the Qinghai–Tibet Plateau. Afterward, the mud is solidified, and its capabilities as a filler are tested through a series of compression tests.

2.2. Mud Samples Characteristics and Quality Tests

2.2.1. X-ray Diffraction Experiment

The X-ray diffraction test is used due to its non-destructive nature to analyze the chemical makeup of different samples. The mud was poured into a large iron box and dried in an oven. The dried soil sample was pounded and passed through a 0.075 mm sieve, and the sieved soil sample was subjected to an X-ray diffraction experiment. The sieved sample was filled evenly on a glass sheet and covered with a glass lid, and the sample holder was placed on the X-ray diffractometer. The results are shown in Figure 2.

2.2.2. Chemical Composition Analysis

The chemical composition analysis of the above figures reveals that the mud samples under analysis contain Na(AlSi³O₈), K(Al₄Si₂O₉(OH)₃), SiO₂, Al₄(OH)₈(SiO₁₀), and Ca (CO₃). There are no harmful substances in this chemical composition. Thus, after a drying technique to obtain solidified mud, the mud samples can be reused as road base fillers without requiring bleaching or additional treatments.

2.2.3. Slurry Performance Analysis

According to the Environmental Impact Report on the Qinghai–Ganjiang Highway, clay is typically used to create the slurry during the on-site drilling. When drilling in thicker sand layers, the mud is prepared by swelling the clay, or the clay is put into a hole to make the mud and to make the mud have better technical performance. Dispersants, such as sodium carbonate, are added appropriately, starting at 0.5% of the added water. The slurry performance indicators are shown in

Table 1. Slurry performance indicators.

Pore Forming Method	Stratum Condition	Mud Performance Index
		Relative Density	Viscosity (s)	Sand Content (%)	Colloid Ratio (%)	Water Loss Rate (mL/30 min)	Mud Skin Thickness (mm/30 min)	Static Shear Force (PA)	pH
Impact	Silty Clay	1.05–1.20	16–22	8–4	≥96	≤25	≤2	1.0–2.5	8–10
	Sand Layer	1.2–1.5	22–28	≤4	≥95	≤20	≤3	3–5	8–11

.

According to the “HTG/T F50-2011 Highway Bridge and Culvert Construction Specification”, a mud’s relative density meter determines the relative density of mud. The density of the mud samples is an important parameter because it indicates the overall mud quality of the samples. It is found in Equation (1), which is as follows:

$$p=\frac{m_3-m_1}{m_2-m_1}$$

(1)

where m₁ is the mass of the empty beaker, m₂ is the mass of the beaker filled with clear water, and m₃ is the same beaker filled with the mud sample. Considering that the mud samples were taken from the site’s mud pond and some of the mud in the mud pond had settled, the densities of the two buckets of mud were measured separately. Twelve samples were tested for mud, solids, and water quality, with average values of 30.98, 8.29, and 22.68, respectively. The relative density of the mud samples (the ratio of the water and mud quality) was found to be 72% of the total amount of mud by water on average.

2.2.4. pH Analysis

Another important parameter to consider is the pH value of the mud samples. Since the zeta potential on the surface of the waste slurry particles is greatly influenced by the pH value of the waste slurry, the pH value is also directly related to the stability of the waste slurry. Several experimental groups were obtained from the collected samples. Groups 1, 2, and 3 were taken from the first bucket of mud, while Groups 4, 5, and 6 were taken from the second bucket of mud. Their average values were calculated, resulting in a mud density of 1.30 g/cm³. The pH value of the waste slurry was measured with a pH meter, and it was pH = 8.4 (Figure 3).

2.2.5. The Density of Drill Spoil and Slag Ratio

The density of the samples was measured next, and the sealed wax method was employed for this purpose. A known mass of soil was immersed in melted paraffin so that the specimen had a wax casing to maintain its complete shape. By weighing the mass of the specimen with the wax casing in air and water, respectively, the volume of the specimen and the density of the soil were calculated according to the principle of buoyancy (

Table 2. Wax seal method density experiment on six samples.

Sample No.	Quality of the Sample	Beeswax Sample Body	Water Temperature	Density of Water	Beeswax Specimen Volume	Wax Volume	Sample Volume	Density (g/cm3)
1	81.75	83.97	25	1	39	2.47	36.53	2.37
2	17.64	18.36	25	1	10	0.8	9.2	1.91
3	33.97	35.14	25	1	18	1.3	16.7	2.03
4	36.31	17.05	25	1	20	0.82	19.18	1.89
5	32.05	33.76	25	1	17	1.9	15.1	2.12
6	27.32	28.89	25	1	14	1.74	12.26	2.24

The average density of the mixture of mud and slag is ρ = 2.09 g/cm³, calculated by taking the average value of the results obtained.

).

In order to prepare the mud drill slag mixtures, the ratio of slurry to drilling slag is an essential quality constituent. The mass and volume of the slurry and the drilling slag are measured separately and then mixed to make four sets of the mixture. The mud drilling slag was mixed in the ratios of 4:6, 5:5, 6:4, and 7:3, and the consequently measured densities of the mixture came out to be 1.7080, 1.6892, 1.4855, and 1.2932 g/cm³, respectively. This relationship is shown in Figure 4.

The relationship curve between the density and mud-drilling slag ratio indicates that the density decreases as the mud drilling slag ratio increases.

2.2.6. Cement Block Mix Preparation

This section delves into the method and preparation technology of using waste mud to develop road base reinforcement grouting material. According to modern site construction and environmental requirements, the block strength should reach the C25 standard. To verify compliance with the C25 standard, several tests are performed. The experimental process is as follows: First, the production and adjustment of the standard C25 test piece’s initial fitting ratio are ensured, as shown in Equation (2).

Cement:Water:Stone:Sand = 1:0.55:1.76:3.3

(2)

To conform with the required standards, the solution ratio is modified to a drill slag to sand ratio of 7:3. After verifying the conformity to the C25 standard through a strength test on the fabricated specimen, other specimens were made and tested. The total weight of the specimens is given in

Table 3. Experiments with different scales.

Material	Cement	Water	Fine Aggregate (3:7)		Cobble	Water Reducing Agent	Air Entraining Agent
			Drilling Slag	Sand
Usage	3.633 kg	1.936 kg	1.488 kg	3.472 kg	9.064 kg	18 g	9.067 g
Material	Cement	Water	Fine Aggregate (9:1)		Cobble	Water Reducing Agent	Air Entraining Agent
			Drilling Slag	Sand
Usage	3.633 kg	1.936 kg	4.464 kg	0.496 kg	9.064 kg	18 g	9.067 g
Material	Cement	Water	Fine Aggregate (8:2)		Cobble	Water Reducing Agent	Air Entraining Agent
			Drilling Slag	Sand
Usage	3.633 kg	1.936 kg	3.968 kg	0.992 kg	9.064 kg	18 g	9.067 g
Material	Cement	Water	Fine Aggregate (7:3)		Cobble	Water Reducing Agent	Air Entraining Agent
			Drilling Slag	Sand
Usage	3.633 kg	1.936 kg	3.472 kg	1.488 kg	9.064 kg	18 g	9.067 g

.

In Table 3, the ratios are the ratio of drilling slag to the sand used in kg to make a fine aggregate.

2.2.7. Compression Tests

Compression tests are conducted to determine the compressive strength of a material. In the case of the study, the cement-solidified mud was tested for its compressive strength. The specimens were prepared by mixing the mud with different ratios of slag and sand, as shown in Figure 5. The prepared specimens were placed in a compression apparatus and subjected to increasing compressive loads until failure. The load was applied constantly, and the maximum load and corresponding deformation were recorded.

The apparatus and equipment used in the compression test included a consolidation apparatus (or compression apparatus), a percolation apparatus, rod and bar pressurization equipment, a micrometer, a stopwatch, a balance, a soil cutter, a large aluminum box, filter paper, and petroleum jelly. These tools and equipment were used to ensure accurate and precise measurements of the compressive strength of the cement-solidified mud.

(a)

Dry Density Test:

The experiment revealed that when the ratio is 8:2, the strength can meet the compressive strength requirements of the C25 concrete. It was planned to determine the optimum moisture content and maximum dry density of the mud soil and to mix the mud soil with different percentages of cement, respectively, as an inorganic binding material to seek the method of reusing the mud.

It was found that the compaction curves of the improved soils with different admixtures were relatively close to each other, and the compaction characteristics of the soils varied less. At the same time, the maximum dry density was proportional to the amount of cement admixture, and the optimum moisture content was inversely proportional to the amount of cement admixture. Therefore, the maximum dry density increases and the optimum water content decreases when the slurry is modified with the appropriate amount of cement.

From the compaction curves in Figure 6, it can be seen that the compaction curves of the soils with high liquid limits are steeper on both sides of the peak, and the control interval of the moisture content for the compaction is narrower. After mixing with the improved cement, the compaction curves become significantly slower. Furthermore, the curves become flatter as the number of mixes increases [31,32]. This indicates that the improved soil has a wider compaction moisture content range, which is conducive to large-scale on-site mixing.

The project is in the high-altitude ecological protection zone, so only a small amount of cement is considered for improvement. The unconfined compressive strength of cement-modified soil with different cement contents is tested to explore the influence of the cement content on the strength of the improved soil. It was shown in the above experiment that combining wet soil with a drying agent, such as cement, can increase its compressive strength; this compressive strength is measured through the following procedure:

(b)

Test procedure:

A ring knife was used to uniformly cut into the soil surface to obtain a soil sample. The weight of the sample, the total water content, and the specific gravity of the soil sample were determined. Then, the percolation ring was placed with the porous stone in the consolidator, and a moist filter paper was placed on the surface. The ring knife with the soil sample was then pressed into the percolation ring. The moist filter paper was placed on the sample’s surface before the permeable stone, the pressurizing piston, and the pressure transmitter were added.

A bar-type pressurizing device was then used to apply the load. After checking that the connections were flexible, the consolidator was placed in the frame. A micrometer was inserted into the transfer block’s connection to the crossbeam pressure cap’s center of the arc. Contact was established between the micrometer’s side rod and the piston rod’s top surface, and the carrying rod was leveled. Next, the load could be applied.

The first loading stage was applied directly to the soil sample, subjecting it to a pressure of 50 kPa. While the pressure was applied, the stopwatch was turned on, and the micrometer readings were noted at the 1st, 2nd, 3rd, 5th, 10th, and 15th-minute intervals. The deformation was measured until it became stable. Generally, a reading of 15 min assumes that the deformation has stabilized at one level. After the final level of the load had stabilized and the deformation had been read, the micrometer was released, the load was removed, the consolidator was dismantled, and the soil sample was removed. The net height of the soil particles in the specimen, H_s, is calculated in Equation (3), and the pore space of a specimen after compression stabilization at a given load is shown in Equation (4). The standard value of the compressive strength of the concrete cubes is expressed as f_cu,k, as shown in Equations (5)–(7).

$$H_s=\frac{H_0}{1+e_0}$$

(3)

$$e=\frac{H}{H_s}-1$$

(4)

$$f_{cu,0}\ge f_{cu,k}+1.645\sigma$$

(5)

$$f_{cu}=\frac{F}{S_0}$$

(6)

$$\overline{f_{cu}}=\sum f_{cu} / n$$

(7)

The GB50107 concrete strength testing and assessment standard specifies the standard value of the cube’s compressive strength for a batch of concrete, which is measured by the standard test method for cube specimens with a side length of 150 mm, made and maintained according to the standard method, at 28 days. The representative value of the strength of the concrete is determined based on certain provisions, such as the difference between the maximum and minimum strength values in a group of specimens. GB50204 provides the standard value of the concrete cube’s compressive strength based on the specimen’s size and the maximum particle size of the aggregate, which should be multiplied by a strength size conversion factor. C25 indicates the standard value of the cube’s compressive strength of the batch of concrete, measured by curing the concrete cube specimen with 150 mm sides at 20 ± 2 °C in a standard curing room with a relative humidity of 95% or more, or at 20 ± 2 °C in a non-flowing Ca (OH)² saturated solution for 28 days, guaranteed with 95% probability.

The next step was crushing the specimens and using their physical properties to determine their proportions. The main indices of compaction are shown in

Table 4. Main crushing indicators.

Group	Control Specimens	Test Specimen (7:3)	Test Specimen (8:2)	Test Specimen (9:1)
1	805 KN	640 KN	720 KN	770 KN
	800 KN	645 KN	725 KN	765 KN
	790 KN	635 KN	725 KN	755 KN
2	790 KN	640 KN	730 KN	755 KN
	780 KN	635 KN	725 KN	760 KN
	785 KN	640 KN	725 KN	750 KN
3	780 KN	650 KN	730 KN	765 KN
	780 KN	635 KN	735 KN	760 KN
	775 KN	640 KN	725 KN	760 KN

.

The strength of the specimens with different ratios was observed individually for each of the four specimens under the above stress conditions. These results are shown in Figure 7.

Table 5. Strength of specimens (in MPa) with different ratios.

Group	Control Specimens	Test Specimen (7:3)	Test Specimen (8:2)	Test Specimen (9:1)
1	35.48	28.44	32.18	34.07
2	34.88	28.37	32.29	33.56
3	34.59	28.51	32.44	33.85

shows the results of the compressive test. It can be seen that, on average, the compressive strengths of the specimens range from 28 to 35 MPa.

3. Comparison of Results to the Literature

The results of the compression tests are consistent with the previous literature on sludge cement. As roads are part of the highway in the Qinghai district and are expected to support a large volume and weight of freight, a high compression strength value was set as the criteria to determine whether the prepared admixture can be used as a road base filler. The acceptable range of compression tests for normal concrete used in buildings and roads is between 20 and 40 MPa, with stronger concrete having values higher than 40 MPa [31]. The performance of the mud sludge can depend on the proportion of added cement. The tests conducted in this study concluded a range of 28 to 35 MPa, which is within the acceptable range. In a study by [35], industrial sludge was combined with marine clay to form a new construction aggregate. Their results showed that adding 20% of clay to the sludge in an 8:2 ratio is similar to the sand to drill slag ratio used in this study. The compressive strength tests showed that the mixture strength lies in the range of 31 to 39 MPa. In another study by [36], a compressive test was performed on a cement, sludge, and 4% limestone mixture to verify its feasibility as a road pavement material. The test revealed that the prepared mixture had a compressive strength of up to 600 KPa. Although limestone increases the compressive strength of the mixture, these results do not meet the demand set by [31] for high-use highway road construction. This comparison highlights the validity of the tests performed in this research, and the mud sludge mixture can be used as a road base filler. However, it is important to acknowledge this study’s limitations and conduct further research to investigate the long-term performance of the proposed mixture in different weather conditions.

4. Conclusions

The proposed method helps reduce the harmful discharges in the Qinghai–Tibet Plateau and Ecological Reserve by reusing waste on site. The study examines the reuse of waste materials from the construction site, such as waste mud and drilling slag. A series of basic physical quantity tests were conducted to measure their physical and mechanical properties, which are as follows:

• X-ray diffraction was used to scan the waste for harmful compounds.
• The pH value of the mud was measured using a pH meter, and it was found to be 8.4, indicating an alkaline nature.
• The density of the drilling slag was measured using the wax sealing method, and it was found to be 2.09 g/cm³.
• The density of the mixture of mud and drilling slag was also measured, ranging from 1.4–1.7 g/cm³ at a mud to drilling slag ratio of 4:6 or 5:5.
• The fine aggregates, such as sand, were replaced with drilling slag to prepare the concrete blocks for an unconfined compressive strength measurement.

It was found that the compaction curves of the improved soils with different admixtures were relatively close to each other, and the compaction characteristics of the soils varied less. The maximum dry density was proportional to the amount of cement admixture, and the optimum moisture content was inversely proportional to the amount of cement admixture. Although a correlation study between the mud properties was not conducted due to limitations, the study identified the feasibility of reusing waste materials as road-based reinforcing agents. The compressive strength of the mixture was found to be in the acceptable range of 28–35 MPa, and adding cement to the slurry soil met the specification requirements. Moreover, reusing waste materials on site resulted in lower waste management costs. Future research could focus on studying the compressive strength of reusable mud under different weather conditions to ascertain the proposed mixture’s lifespan. There is potential for future research in investigating the correlations between the mud properties in construction projects to gain a better understanding of their interrelationships.