Research on closed-loop utilization of engineering waste mud in engineering sites

The engineering waste mud used in this article comes from the construction site, with a moisture content of about 87.5% (the proportion of water to the entire mud mixture) and a pH value of 6. The appearance and particle size distribution of the original mud are shown in Figure 1, and the solid particles are mainly mudstone soil. Polyaluminum chloride (PAC), with a content of 28%, produced by Sichuan Wanjiajing Environmental Protection Technology Co., Ltd and Anionic polyacrylamide (APAM) with a molecular weight of 12 million, produced by Xinqi Polymer Co., Ltd. The cement is ordinary Portland cement (P.O 42.5) produced by Sichuan Jinding (group) Emei Cement Plant Co., Ltd.

Fig. 1.

(1) The optimal concentration of flocculant: First, PAC was configured into aqueous solutions with mass ratios of 0.5%, 1.0%, 1.5%, 2.0%, 2.5%, and 3.0%, while APAM was configured into aqueous solutions with mass ratios of 0.05%, 0.1%, 0.2%, 0.3%, 0.4%, and 0.5%. Then, 10ml of PAC and APAM solutions were measured separately and poured into a measuring cylinder containing 170ml of original mud while stirring. We stirred for 10s and let it stand. The volume change of the supernatant (the water after flocculation) in the measuring cylinder, as shown in Figure 2, was observed. The results indicate that the optimal concentrations of flocculants used for the waste mud in this experiment are 1.0% PAC and 0.3% APAM, respectively.

Fig. 2.

(2) Preparation of comparative samples for mud-water separation test: Based on the optimal concentrations of the two flocculants determined above, we added the optimal concentration of flocculants to the measuring cylinder containing 170ml of original mud and observed and tested the volume change and turbidity of the supernatant to determine which method has the best mud-water separation effect. The detailed addition methods are shown in Table 1.

(3) Preparation of fluidized solidified soil sample: This article studies the influence of flocculants on the performance of fluidized solidified soil by fixing cement and total water content and changing the types of flocculants in soil and water. Before the experiment, the sediment after flocculation and sedimentation with different flocculants was dried to a semi-dry soil with a moisture content of 20%. Then, we weighed 10kg of semi-dry soil and placed it in a mortar mixer, adding 5% cement and 30% water by weight to the soil, mixing for 5 minutes, obtaining pre-mixed fluidized solidified soil, and testing its slump expansion. To test the strength and water stability coefficient of solidified soil, pre-mixed fluidized solidified soil was poured into a 100mm × 100mm × 100mm cubic mold and covered with plastic film. After being placed in the cubic mold in an environment with a temperature of 20°C for 24 hours, the cube test block was placed in a standard curing room (temperature 20 ± 2°C, relative humidity 95%) for 7 and 28 days. The detailed mix proportion of fluidized solidified soil is shown in Table 2.

Laser particle size analyzer (Mastersizer 2000): tests the particle size of solid particles in mud. Turbidimeter (Hach 2100 N): tests the turbidity of the supernatant after mud-water separation. Slump cone (100mm × 200mm × 300mm): tests the slump flow of pre-mixed fluidized solidified soil, the value is the average diameter of the selfflowing spreading surface of pre-mixed fluidized solidified soil after extracting the slump cone. Electro-hydraulic pressure testing machine (DYE-2000): tests the unconfined compression strength of the fluidized solidified soil, and the loading rate is 5 kN/min. The softening coefficient related to the test is the ratio of the strength of the soaked and nonsoaked test blocks, with a soaking time of 48 hours (starting after 26 days of curing the test blocks). Field emission electron scanning microscope (SEM, JSM-5900LV JEOL): tests the internal microstructure of fluidized solidified soil specimens. X-ray diffraction (XRD, Ultima IV, Rigaku Corporation, Japan): tests the components related to fluidized solidified soil, with a testing range of 5°to 40°and a testing speed of 10°/min.

The efficiency of mud-water separation is not only related to the content of flocculants but also closely related to the types of flocculants. To study the effect of the types of flocculants on the speed of mud-water separation, the change in volume of the supernatant with settling time after the addition of flocculants was observed, as shown in Figure 3. Although the original mud still has a certain settling rate, it is much slower than the sample added with flocculant. The sample containing the original mud only showed a small volume of supernatant at 10 minutes, but the sample with added flocculant showed a large amount of supernatant at 1 minute. By analyzing the influence of the types of flocculants on the speed of mud-water separation, it was found that after adding PAC flocculants, the mud-water separation speed was slower, and there were no obvious flocs produced in the bottom mud. However, when APAM and composite PAC+APAM flocculants were added, there was a rapid separation of mud and water, and obvious flocs appeared in the bottom mud. In addition, after removing the supernatant, the water content of the sediment in groups B and C was measured by the drying method and found to be similar, and it decreased to about 54% with a flocculation dehydration efficiency of about 78%. This indicates that the flocculant containing APAM has an effective separation effect on mud and water.

Fig. 3.

To study more clearly and accurately the effect of different flocculants on the separation efficiency of mud and water, the volume and turbidity of the supernatant after mud-water separation were tested, and the results are shown in Figure 4. By comparing the volume of the supernatant of the original mud in groups A, B, and C, it was found that the volume of the original mud’s supernatant showed a linear curve over time after being left standing for 10 minutes with an average sedimentation rate of about 1.23ml/min, so it reached 37ml after 30 minutes. After adding PAC to the mud and letting it stand for 30 minutes, the volume of the supernatant reached 104ml. After adding APAM or PAC+APAM to the mud, the volume of the supernatant can reach nearly 130ml after standing for 1 minute, indicating once again that APAM has a better mud-water separation effect than PAC. In addition, the turbidity analysis of the supernatant found that the turbidity of the supernatant in the samples added with PAC, APAM, and PAC+APAM decreased sequentially, and in the C group samples added with PAC+APAM, the turbidity of the supernatant was as low as 12.8 NTU. Explanation: In this experiment, the combination of PAC and APAM has the best mud-water separation effect. The separated supernatant can be recycled on the project site due to its low turbidity, which reduces sewage discharge.

Fig. 4.

To study the effect of the addition order of PAC and APAM in the PAC+APAM composite flocculant on the mud-water separation effect, PAC and APAM were added to the mud in different orders, and the mud-water separation situation was observed after standing for 4 minutes, as shown in Figure 5(a). Sample (D), with an even mixture of PAC and APAM before adding mud, had the worst mud-water separation effect with high turbidity in the supernatant. This was mainly due to the reaction between PAC and APAM during mixing, forming a complex or colloid that cannot perform normal flocculation function. The mudwater separation effect of sample (E), which had APAM added first and then PAC, was also poor, indicating that PAC flocculant had no synergistic effect on the APAM flocculant and even had an antagonistic effect, significantly weakening the effectiveness of APAM flocculant and resulting in a poor mud-water separation effect. For sample (C), with PAC added first and then APAM, the mudwater separation effect was best with low turbidity of the supernatant and larger floc particles in the sediment. The main reason was that the clay particles in the mud preferentially complex with PAC to form small particles, which are then entangled by the large molecular chains of APAM to form larger particles, thereby improving the settling speed and achieving an excellent mud-water separation effect.

Fig. 5.

To investigate whether the PAC+APAM composite flocculant with the optimal mixing order mentioned above is suitable for other types of muds, mudstone soil muds, clay soil muds, and silt soil muds with similar solid content and particle distribution were selected. The results after adding the composite flocculant are shown in Figure 5(b). The composite flocculant had a good mud-water separation effect on all three types of mud, and it was found that the turbidity of the supernatant of mud 2 (clay soil) could reach 8.9 NTU, and the flocculent particles in the sediment were the largest. The main reason was that the cohesion of clay was higher than that of mudstone and silt, making it easier for particles to aggregate and form larger particles. The gaps formed by the overlapping of large particles were larger, and the particle stacking was looser, resulting in a larger volume after flocculation settlement.

In summary, to achieve efficient mud-water separation of engineering waste mud within 10 minutes or even less, save mud storage space, and accelerate construction progress, the method of adding PAC flocculant first and then APAM flocculant should be chosen to treat the mud.

The surface of clay particles in mud usually carries electric charges, causing charge repulsion between particles and making it difficult for particles to aggregate and form agglomerate settlement. This results in a poor natural settlement effect. After adding the flocculant, the mechanism of action of the flocculant can be seen in Figure 6. When dissolved in water, the inorganic PAC flocculants will produce complexes and various polynuclear hydroxyl complex ions. After the clay particles in the mud adsorb the flocculant, the flocculant will promote the aggregation of small particles into clusters through potential neutralization, compression of the double layer, and reduction of Zeta potential. However, the formed particles are very small due to the short molecular chain of inorganic flocculants, resulting in slow sedimentation and weak flocculation.

Fig. 6.

For organic APAM flocculants, the flocculation effect is mainly achieved through the bridging effect of long molecular chains. Due to the longer molecular chain of APAM compared to PAC, it is easier for APAM to form a flocculent molecular network structure in solution mud that can capture small particles and colloidal substances in the mud to form larger particles and accelerate flocculation settlement. In addition, there are many active groups (such as hydrogen bonds, carboxyl groups, amino groups, etc.) on the long chain of APAM molecules that can attach to the surface of soil particles and further accelerate the settlement of flocs. However, as the concentration of the flocculant increases to a certain value, the flocculation effect will decrease since the long chains of its molecules intertwine and cannot be stretched to form flocs. The active groups on the long chains cannot continue to adsorb particles, resulting in a significant decrease in the bridging effect of the links.

For the PAC+APAM composite flocculant, adding the PAC flocculant first can neutralize the surface charge of solid particles in the mud and reduce the repulsion forces between particles. At the same time, it exerts compression on the double electron layer structure on the surface of mud solid particles, allowing them to bond to form small flocculent particles. Then, the APAM flocculant, whose long chain molecular network structure can bridge, wrap, and sweep the small flocculent particles formed by PAC flocculation, causes the small particles to condense and form large particle flocs. These large particle flocs can quickly settle after destabilization, significantly improving the mudwater separation effect on waste mud.

The water content of the engineering waste mud can be significantly reduced after being flocculated, reducing the transportation volume of the waste mud, but the outward transportation of sediment still causes environmental pollution and waste of land resources. If it is reused as a resource on the construction site, it will contribute to the green construction development of the project. Therefore, this article recommends adding cement to the sediment treated with flocculants to prepare fluidized solidified soil and achieve resource utilization of the mud. Although there have been many studies on fluidized solidified soil [28–30], almost no reports have been made on fluidized solidified soil containing flocculants in the original soil, and the impact of the presence of flocculants on the performance of fluidized solidified soil is still unclear. It is urgent to carry out such research to promote the resource utilization of engineering waste mud.

First, the most important aspect of the application process of fluidized solidified soil is its construction performance. Therefore, this article studied the effect of flocculants on the slump flow of pre-mixed fluidized solidified soil, as shown in Figure 7. When the raw material (water or soil) used for preparing fluidized solidified soil contains flocculants, the slump flow of pre-mixed fluidized solidified soil will decrease, and the degree of reduction in slump flow of solidified soil by APAM flocculants (4) is higher than that of PAC flocculants (3). If both PAC and APAM are present in the soil (5), the slump flow is the smallest. For PAC, its molecular chain is short, the particles formed by flocculation are small, and the frictional resistance between particles is small, which has a small impact on the flow performance. For APAM, its long molecular chain will form a network structure, adsorbing and entwining more soil particles to form larger flocs, increasing the resistance to sliding between flocs, which has a significant impact on the slump flow of pre-mixed fluidized solidified soil. When PAC and APAM coexist, the long-chain network of APAM will trap and entangle small particles that have already been flocculated by PAC, forming larger particle flocs. The resistance between the flocs will further increase, resulting in lower flow performance of pre-mixed fluidized solidified soil.

Fig. 7.

In addition, the strength and water stability of fluidized solidified soil are important performance indicators, so it is crucial to study the influence of the presence of flocculants in the soil on its strength and softening coefficient, as shown in Figure 8. For sample 2 as only the added water contains trace amounts of flocculants, the impact on strength and softening coefficient was relatively small. For samples 3, 4, and 5, flocculants had an enhancing effect on the strength and water stability of the fluidized solidified soil (the enhancing effect of flocculants in descending order is PAC+APAM > APAM > PAC) due to the high content of flocculants in the soil. The main reasons are that PAC can neutralize the surface charge of soil particles, promote the aggregation of clay particles, improve engineering plasticity, and improve the stability of soil. The formed flocs can improve the strength of soil by filling the pores in the soil and bonding the soil particles. In addition, APAM has long polymer chains and adsorption bridging ability. Its molecular chains are intertwined with the soil particle skeleton, and the molecular chains are cross-linked to form a network structure that runs through the soil skeleton, improving the integrity of the skeleton and greatly improving the strength of the soil. Furthermore, the molecular structure of APAM contains amide groups, which hydrolyze to produce hydroxyl groups and react with cement hydration products (Ca²⁺, Al³⁺, etc.) to form stable complexes, which are tightly connected to the cement through chemical bonds, further enhancing the strength and water stability of the soil. If PAC and APAM are used together, they will have a positive superposition effect on the performance of solidified soil. For example, the unconfined compressive strength and softening coefficient of the fluidized solidified soil in sample 5 can be increased by 25% and 34%, respectively, compared to sample 1. The corresponding data indicators meet the requirements of most foundation pit backfilling indicators, so they can be applied to backfilling projects on the project site.

Fig. 8.

To further demonstrate the strengthening effect of the combined flocculants on fluidized solidified soil from the microstructure, SEM and XRD were used to observe the fluidized solidified soil without flocculants (1) and the fluidized solidified soil containing the combined flocculants (5). The results are shown in Figure 9. From Figure 9(a, b), it could be seen that the clay particles in sample 1 were dispersed, with many pores between the particles, and there was very little needlelike ettringite (AFt). In Figure 9(c, d), sample 5 showed dense and large flocs, with wrinkles on the surface of the flocs, indicating that organic APAM flocculants adhere to the surface of the particles, and there were many needle-like ettringite filled between the interface gaps of the large flocs. In addition, it could be observed in the XRD pattern that the diffraction peak of ettringite in sample 5 was significantly stronger than that in sample 1. The main reason for the occurrence of a large amount of ettringite in sample 5 is that PAC reacts with the hydration product Ca(OH)² of cement, rapidly generating Al(OH)₃ precipitates. Then, the colloidal particles take Al(OH)₃ as the reaction core in water and adsorb SO4²⁻ and Ca²⁺ into the floc to form ettringite. At the same time, APAM has a synergistic effect, which can further promote the formation of a large amount of ettringite [31]. Therefore, flocculants can promote the formation of ettringite, facilitate the integrity of soil structure, and improve the strength and stability of solidified soil.

Fig. 9.