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  . 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  . This method can reduce noise and environmental pollution and meet the requirements of green construction.
Wang Dongxing  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  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  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.  also proposed various strength prediction formulas. More scholars have studied the solidified materials. Shao Wei  conducted an experimental study on cement-pulverized ash-reinforced organic soil. Miura, N.  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  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  . Gu Huanda  and others summarized the liquidity measurement method and liquidity index. Ding Jianwen, Zhang Shuai et al.  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”  . 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. RCS = 0.09 - 0.16; Cement fly ash: cement quality 35% - 80%, i.e. RFC = 0.35 - 0.8; Cement phosphogypsum: 10 g. The specific experimental material dosage scheme is shown in Table 1.
Table 1. Material content ratio scheme.
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,
Figure 1. Relationship between curing agent dosage and fluidity.
Figure 2. The relationship between time and liquidity.
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 RCS in solidified soil when RFC = 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 (RCS = 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 RFC = 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: RCS = 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 RCS = 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 RCS = 0.16 and the proportion of fly ash RFC 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
Figure 3. Variation of unconfined compressive strength h at 14 d at different temperatures. (a) Cement content RCS (RFC = 0.65); (b) Fly ash content RFc (RCS = 0.14).
the curing time are in an increasing relationship. When the ratio of cement is RCS = 0.11 - 0.16 and the amount of fly ash is RFC = 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.
Figure 4. qu-7d/kPa.
Figure 5. qu-14d/kPa.
Figure 6. qu-28d/kPa.
Figure 7. Relationship between f7d - f14d.
Figure 8. Relationship between f14d - f28.
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.
1) When the cement content (RCS = 0.09 - 0.16) and the fly ash content (RFC = 35% - 80%), the fluidity of the sludge solidified soil decreases linearly with time, and the fluidity decreases with the amount of solidified material. When RCS = 0.09 - 0.16 and RFC = 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 RCS = 0.14, RFC = 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 RCS = 0.14 and the amount of fly ash is RFC = 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: RCS = 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 RCS = 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 RCS = 0.16, the proportion of fly ash RFC increases from 35% to 65%, and the strength can increase by 1.72 times.
4) When RCS = 0.09 - 0.16 and RFC = 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.
 Zhou, Y., Pu, Y. and Li, Z. (2019) Horizontal Drainage Board—Vacuum Preloading Combined Treatment of High Water Content Dredging Sludge Model Test. Journal of Rock Mechanics and Engineering, 35, 3246-3251.