Parallel and orthogonal tests are used to explore the influence law of the dosage and age of curing agent on the strength of solidified sludge. The test results show that: 1) The strength of solidified sludge is mainly related to the cement content and dry soil content, and presents a good linear relationship. The influence of gypsum content is not significant. As the age increases, the strength is greatly affected by the cement content. 2) At different ages, the unconfined compression strength of solidified sludge presents a linear relationship, and the change law of later strength can be predicted by early strength. 3) Degree of influence of curing agent dosage: cement dosage > gypsum dosage > dry soil dosage. The optimal mixture ratio is 8% of cement content, 30% of gypsum content (proportion of cement content), and 4 times of dry soil content (multiple of cement content).
Domestic treatment of sludge is mostly landfill methods. As a sludge product after sewage treatment, sludge has the characteristics of easy rancidity, high moisture content, low permeability coefficient and low strength. Therefore, nowadays, with insufficient urban development space, the sludge strength of sludge landfill site is difficult to meet the requirements of individual landfill or bear the upper load as foundation. The solidification treatment of sludge by some on-site solidification methods can make it meet the requirements of foundation filling and has important significance for eco-city construction [
The test uses the original sludge, which was taken from a sludge landfill in Shenzhen (as shown in
The sludge landfill soil belongs to peat silt, and the micro-structure image of sludge soil is shown in
sheet, which is related to the addition of a large amount of flocculant to the sludge during the sewage treatment. Therefore, it has the characteristics of strong flocculation and high combined water content, which leads to the conventional curing method such as vacuum pre-pressing method and stacking pre-pressing method being too low in efficiency to achieve the curing effect. In this paper, cement is the main curing agent, and gypsum and dry soil are used as curing agent.
In this test, the YYW-1 manual lime soil unconfined pressure gauge (as shown in
at which the sample is broken is called the unconfined compressive strength and is expressed by qu.
According to the experience of the previous attempts in this study and the in-situ curing test [
Parallel test, by controlling the amount of two additives unchanged, changing the amount of another additive, to achieve the principle of controlling a single variable, such as
Orthogonal test, uses orthogonal tables to find better influence conditions from a small number of trials to achieve the best experimental results [
Group | Cement content | Gypsum content | Dry soil content |
---|---|---|---|
1.1 | 4% | 0% | 0 |
1.2 | 6% | 0% | 0 |
1.3 | 8% | 0% | 0 |
1.4 | 10% | 0% | 0 |
2.1.1 | 8% | 20% | 0 |
2.1.2 | 8% | 30% | 0 |
2.1.3 | 8% | 40% | 0 |
2.1.4 | 8% | 50% | 0 |
2.2.1 | 8% | 0% | 1 Multiple |
2.2.2 | 8% | 0% | 2 Multiple |
2.2.3 | 8% | 0% | 3 Multiple |
2.2.4 | 8% | 0% | 4 Multiple |
Level | Cement | Plaster | Dry soil |
---|---|---|---|
1 | 4% | 10% | 2 Multiple |
2 | 6% | 20% | 3 Multiple |
3 | 8% | 30% | 4 Multiple |
Group | Cement content | Gypsum content | Dry soil content | Test results |
---|---|---|---|---|
3.1 | 4% | 10% | 2 Multiple | X1 |
3.2 | 4% | 20% | 3 Multiple | X2 |
3.3 | 4% | 30% | 4 Multiple | X3 |
3.4 | 6% | 20% | 2 Multiple | X4 |
3.5 | 6% | 30% | 3 Multiple | X5 |
3.6 | 6% | 10% | 4 Multiple | X6 |
3.7 | 8% | 30% | 2 Multiple | X7 |
3.8 | 8% | 10% | 3 Multiple | X8 |
3.9 | 8% | 20% | 4 Multiple | X9 |
the sludge increased linearly with the increase of cement content.
Mixing ratio (gypsum and dry soil are relative cement content) | Unconfined compression strength (unit: kPa) | |||||
---|---|---|---|---|---|---|
Group | Cement content | Gypsum content | Dry soil content | 7 d | 14 d | 28 d |
1.1 | 4% | 0% | 0 | 21.55 | 25.20 | 31.92 |
1.2 | 6% | 0% | 0 | 58.87 | 68.47 | 82.17 |
1.3 | 8% | 0% | 0 | 106.12 | 111.55 | 131.96 |
1.4 | 10% | 0% | 0 | 171.17 | 212.97 | 281.44 |
2.1.1 | 8% | 20% | 0 | 108.45 | 120.59 | 140.26 |
2.1.2 | 8% | 30% | 0 | 123.54 | 143.99 | 160.35 |
2.1.3 | 8% | 40% | 0 | 110.05 | 126.26 | 149.41 |
2.1.4 | 8% | 50% | 0 | 108.70 | 124.23 | 147.73 |
2.2.1 | 8% | 0% | 1 Multiple | 106.76 | 112.38 | 132.62 |
2.2.2 | 8% | 0% | 2 Multiple | 109.33 | 114.25 | 133.38 |
2.2.3 | 8% | 0% | 3 Multiple | 112.24 | 124.91 | 142.70 |
2.2.4 | 8% | 0% | 4 Multiple | 123.54 | 133.62 | 157.32 |
without adding gypsum.
Age | Fitting of cement content and qu when cement is used alone | Fitting of dry soil content with qu when cement content is 8% | ||
---|---|---|---|---|
Fitting function | R2 | Fitting function | R2 | |
7 d | y = 24.81x − 84.209 | 0.97655 | y = 11.959x + 75.485 | 0.96096 |
14 d | y = 30.32x + 107.688 | 0.92141 | y = 7.437x + 102.698 | 0.91101 |
28 d | y = 39.92x + 102.698 | 0.90239 | y = 10.059x + 114.529 | 0.92986 |
As shown in Figures 9-11, the unconfined compression strength of the solidified sludge is enhanced with the increase of age under the three different curing agent dosages. From
Tables 6-8 are the calculation results of R values of different ages, where K1, K2, and K3 represent the average values of unconfined compression strength at each level of each factor. At the same time, the difference R of the average intensity in each horizontal direction of the same factor (extreme difference = the maximum value of the average intensity − the minimum value of the average intensity) was used to reflect the magnitude of the impact of changes in the horizontal direction of each factor on the test results (intensity). A very large extreme difference means that the change in the level of the factor has a large impact on the test results [
Cement | Plaster | Dry soil | |
---|---|---|---|
K1 | 35.26 | 84.48 | 90.20 |
K2 | 97.22 | 105.33 | 85.69 |
K3 | 138.96 | 81.63 | 95.54 |
max | 138.96 | 105.33 | 95.54 |
min | 35.26 | 81.63 | 85.69 |
R | 103.70 | 23.70 | 9.85 |
Cement | Plaster | Dry soil | |
---|---|---|---|
K1 | 41.12 | 99.59 | 105.93 |
K2 | 115.10 | 119.50 | 102.91 |
K3 | 161.21 | 98.34 | 108.60 |
max | 161.21 | 119.50 | 108.60 |
min | 41.12 | 98.34 | 102.91 |
R | 120.09 | 21.16 | 5.69 |
Cement | Plaster | Dry soil | |
---|---|---|---|
K1 | 47.09 | 116.46 | 120.03 |
K2 | 124.44 | 136.74 | 132.10 |
K3 | 209.21 | 127.53 | 128.61 |
max | 209.21 | 136.74 | 132.10 |
min | 47.09 | 116.46 | 120.03 |
R | 162.12 | 20.28 | 12.08 |
In Tables 6-8, the R values of cement are 103.7, 120.09, and 162.12 at three different ages, which are the maximum influencing factors and are 5-8 times of the R value of gypsum. The soil R value is 10 - 24 times, the cement content is the main factor, and the dry soil and gypsum content are secondary factors.
1) The cement content and the dry soil content have a good linear relationship with the strength of the solidified sludge y = Ax + B (A > 0), and the effect of cement content on the strength is higher than that of the dry soil. From the fitted function, the amount of curing agent can be estimated according to the strength requirements, and the actual engineering can be guided.
2) The age of the solidified sludge has a good linear relationship with the unconfined compression strength qu28 = 1.34qu7, qu14 = 1.12qu7. In this ratio, the strength of the late solidified sludge can be estimated from the strength value of 7 d in length.
3) It can be seen from the orthogonal test that the strength is affected by the three curing agents: cement content > gypsum content > dry soil content. Through experiments, it is determined that the optimal curing agent mix ratio was 8% of cement content, 30% of gypsum content (% of cement content), and 4 times of dry soil content (multiple of cement content).
The authors declare no conflicts of interest regarding the publication of this paper.
Ding, J.H., Zhang, T.Y., Huang, J. and Chen, W. (2018) Analysis of Factors Affecting Solidification Strength of Sludge in a Landfill Area of Shenzhen. World Journal of Engineering and Technology, 6, 794-805. https://doi.org/10.4236/wjet.2018.64052