ABSTRACT

Every year, a large amount of silt is generated by river dredging. For the characteristics of dredging silt, such as high moisture content, low strength and high compressibility, the traditional solidification method can no longer better solve this kind of silt problem. This paper mainly studies the fluidized solidification treatment of high water content sludge, not only makes the silt soil achieve a good solidification effect, but also the project cost is lower, the construction method is more environmentally friendly and green. The influencing factors of the solidified soil are mainly investigated by the unconfined compression test and the fluidity test. The experiment result shows 1) When the cement to sludge mass ratio (R_CS) is 0.09 - 0.16 and the fly ash to cement mass ratio (R_FC) is 0.35 - 0.80, as the amount of R_FC increases, the flow rate of the solidified soil gradually decreases. With the increase of time, the liquidity is significantly reduced, and the influence of cement on the fluidity is greater than that of fly ash. 2) When R_CS = 0.09 - 0.16, the strength of the sludge solidified soil at 28 d age increased by 4.5 - 6 times. 3) When R_CS = 0.09 - 0.16 and R_FC = 35% - 80%, the intensity of 14 d increased by 1.23 times than that of 7 d, and the intensity of 28 days increased by 1.29 times than that of 14 d. This experiment can provide the mix ratio design of solidified materials for different needs of the project, which can better provide a basis for engineering application and strength prediction.

Keywords:

Flowing Solidified Sludge, Influencing Factors, Fluidity, Unconfined Compressive Strength

1. Introduction

China is a country with a large water area, and thousands of lakes. The large amount of silt produced every year is a major problem that we face. According to statistics, the annual dredging volume of large and medium-sized dredging enterprises nationwide reached 1.3 billion cubic meters. The dredged soil has high initial moisture content, high compressibility, low strength, long self-weight consolidation time, and is difficult to directly use and occupies a large amount of yardland [1] . The flow curing treatment method is more suitable for this high water content dredged sludge. Compared with the previous principle of water cut rate, the fluidization treatment is to increase the moisture content of the silt and add a curing agent to make it flow, which greatly reduces the engineering cost and improves the engineering quality. No need for pre-pressing treatment, it can take advantage of its fluidity and self-hardening characteristics, without compression molding, and can directly adopt the pumping construction method [2] . This method can reduce noise and environmental pollution and meet the requirements of green construction.

Wang Dongxing [3] used MgO as a solidification material for sludge to analyze the appearance quality stress-strain relationship and unconfined compressive strength of samples under different immersion time. It was found that MgO can significantly change the water stability of solidified sludge. Wang Yuhua [4] eliminated the influence of organic matter by adding alkaline oxidant to the solidified sludge. It was found that the 7 d unconfined compressive strength of the solidified sludge reached 1.536 MPa, and the strength was 121.6% higher than that of cement-solidified sludge. Zhang Rongjun [5] and others used cement as the solidified material of sea mud, and found that the curing temperature has a significant impact on the strength development. The high curing temperature not only significantly increases the early strength of the solidified sludge, but also significantly increases the late strength of the solidified sludge. Boutoui et al. [6] also proposed various strength prediction formulas. More scholars have studied the solidified materials. Shao Wei [7] conducted an experimental study on cement-pulverized ash-reinforced organic soil. Miura, N. [8] and others have shown through experiments that the clay-water-cement ratio is the main parameter for analyzing the strength and deformation behavior of cement soil induced by high water content. The cement bond strength increases as the clay water to cement ratio decreases. Yong Yong [9] added cement lime and other solidified materials to the sludge to form a physical and chemical reaction with the sludge and pore water to increase the strength of the sludge.

Fluidity is the main indicator for describing flow-solidified sludge [10] . Gu Huanda [11] and others summarized the liquidity measurement method and liquidity index. Ding Jianwen, Zhang Shuai et al. [12] and other experiments through the high-water content dredging sludge flow curing treatment, found that the initial water content, the amount of curing agent and the time after preparation have a certain impact on the flowability of the flowing soil. And after the addition of gypsum in the curing agent, the curing strength far exceeds the strength of the cement only. The fluidity test is first seen in the determination of the workability of the concrete to evaluate the pumpability of the concrete mixture. This article is aimed at high moisture content sludge, starting from the concept of “disposing waste with waste” [2] . In this paper, composite cement and fly ash were used as curing agent materials to study and analyze sludge solidified soil and the factors affecting fluidity and strength were analyzed.

2. Experimental Material and Material Content Ratio

2.1. Experimental Material

The sludge used in this test was taken from the Baiyangdian marshland, and the sampling position was 5 - 15 cm below the surface layer. The initial moisture content was 52%, and water was added to increase the moisture content to 160% of the slurry. The liquid limit is 38.2% and the plastic limit is 26.2%. It belongs to the clay soil. The solidified material is made of cement and fly ash. The cement is made of P·O42.5 ordinary Portland cement. Fly ash is used as the primary auxiliary material. The industrial waste used in the experiment was a silica-alumina low-calcium fly ash.

2.2. Material Content Ratio

The experiment uses 1 L of mud as a unit amount, Cement content: 110 g - 200 g, i.e. R_CS = 0.09 - 0.16; Cement fly ash: cement quality 35% - 80%, i.e. R_FC = 0.35 - 0.8; Cement phosphogypsum: 10 g. The specific experimental material dosage scheme is shown in Table 1.

3. Introduction to Duncan-Chang Model

Figure 1 shows the relationship between the fluidity of the sludge and the amount of curing agent. The effect of cement content and fly ash content on the fluidity of sludge solidified soil was studied. As can be seen from the figure, the amount of the curing agent has a significant influence on the fluidity, and the fluidity decreases as the amount of the curing agent increases. The fluidity index of the solidified soil is between 27 and 36 cm. Since the curing agent does not completely react with the water in the slurry at the beginning, the initial fluidity is large.

As can be seen from Figure 2, as time increases, the liquidity decreases significantly,

The liquidity decreases slowly during the period of 0 - 40 min, and the variation range is between 32.5 - 27 cm. The fluidity decreased significantly during the 40 - 60 min period, and the variation ranged from 30 to 23.5 cm. The flow rate decreases slowly during the period of 60 - 180 min, and the variation range is between 27 - 18 cm. After being placed for 3 hours, the fluidity is as low as 18 cm, which meets the requirements of pumping and construction of the flowing soil.

4. Analysis of Factors Affecting Strength

4.1. Impact of Environmental Conditions on the Strength of Curing

Adjust the temperature of the Constant temperature and humidity box to 20˚C, 30˚C and outdoor natural conditions, the outdoor time is 14 d, and the time period is from early May to mid-May. The highest average daytime temperature in the outdoor is 23˚C, and the lowest average temperature in the night is 6˚C. The average temperature difference between day and night is 12˚C, and there is rainfall for 2 and half days. Figure 3(a) shows the relationship between unconfined compressive strength and R_CS in solidified soil when R_FC = 0.65. It can be seen from Figure 3(a) that the strength of the solidified soil under outdoor natural conditions is higher in three different environments. And as the amount of cement and fly ash is increased, the highest strength of solidified sludge at 30˚C. The lowest strength is the solidified sludge at 20˚C, the sludge at 30˚C is 1.23 times the strength of the sludge at 20˚C.

It can be seen from Figure 3(b) that when the amount of cement is certain (R_CS = 0.14), the strength of the solidified soil at 30˚C is not much different from the strength under outdoor natural conditions, but both are stronger than the strength of the solidified soil at 20˚C. When the amount of fly ash is R_FC = 0.5, the strength of the solidified soil under outdoor conditions is 1.36 times the strength of 20˚C

4.2. Effect of Curing Age and Dosage on Strength

As can be seen from Figures 4-6, When the cement content is: R_CS = 0.09 - 0.16, the strength of the sludge solidified soil increases with the increase of cement content, and the strength can be increased by 4.5 - 6 times, indicating that the cement content plays an important role in strength. When the cement content is R_CS = 0.09 - 0.14, the amount of fly ash is increased, and the strength of the solidified soil is not obvious. However, when the amount of cement is R_CS = 0.16 and the proportion of fly ash R_FC increases from 35% to 65%, the strength can be increased by 1.72 times. However, the amount of fly ash continued to increase, and the strength of the solidified soil decreased. This also shows that when the cement content is 0.16, the optimum dosage of fly ash is 65%, so the excessive addition of fly ash has little effect on the strength increase. When the cement content is low, too much fly ash is added, which will reduce the strength of the solidified soil.

As can be seen from Figures 4-6, the strength of the sludge solidified soil and

the curing time are in an increasing relationship. When the ratio of cement is R_CS = 0.11 - 0.16 and the amount of fly ash is R_FC = 65% - 80%, the strength at 28d curing age is 1.25 - 1.6 times the intensity of 14 d. However, when the amount of cement and fly ash is relatively small, the effect of curing time on strength is not obvious. When the curing time is short, the effect of cement on the strength of the solidified soil is greater than that of the fly ash. This also means that the solidifying agent of fly ash must function after reaching a certain value at the age.

According to the fitting results of Figure 7 & Figure 8, Equations (1)-(2) give

the relationship between curing time and strength. It can better judge the strength and provide valuable reference for the project. It can be seen that the intensity of 14 d is 1.23 times higher than that of 7 d, and the intensity of 28 days is 1.29 times higher than that of 14 d.

$f_{\text{14d}}=1.23f_{\text{7d}}$ (1)

$f_{\text{28d}}=1.23f_{\text{14d}}$ (2)

5. Conclusions

1) When the cement content (R_CS = 0.09 - 0.16) and the fly ash content (R_FC = 35% - 80%), the fluidity of the sludge solidified soil decreases linearly with time, and the fluidity decreases with the amount of solidified material. When R_CS = 0.09 - 0.16 and R_FC = 0.60, the flow value changes in each time period are different, and the fluidity decreases slowly in the 0 - 40 min time period

2) When R_CS = 0.14, R_FC = 0.65, and the curing time is 14 d, the sludge at 30˚C is 1.23 times the strength of the sludge at 20˚C. When the amount of cement is R_CS = 0.14 and the amount of fly ash is R_FC = 0.5, the strength of the solidified soil under outdoor conditions is 1.36 times the strength of 20˚C.

3) When the cement content is: R_CS = 0.14 - 0.16 and the curing time is 28 d, the strength of the sludge solidified soil increases with the cement content, and the strength of the solidified soil can increase by 4.5 - 6 times. When the cement content is R_CS = 0.09 - 0.14, the amount of fly ash increased, and the strength of the solidified soil not significantly increased. However, when the amount of cement is R_CS = 0.16, the proportion of fly ash R_FC increases from 35% to 65%, and the strength can increase by 1.72 times.

4) When R_CS = 0.09 - 0.16 and R_FC = 35% - 80%, the intensity of 14 d increased by 1.23 times than that of 7 d, and the intensity of 28 days increased by 1.29 times than that of 14 d.

Conflicts of Interest

The authors declare no conflicts of interest regarding the publication of this paper.

Cite this paper

Ding, J.H., Feng, Z.M., Sun, D.X., You, K.K., Yan, M.H. and Xun, Y.L. (2019) Analysis of Influencing Factors of Silt Solidified Soil in Flowing State. World Journal of Engineering and Technology, 7, 455-464. https://doi.org/10.4236/wjet.2019.73033

References