Experimental Study of Moisture Content Effect on Geotechnical Properties of Solidified Municipal Sludge

State Key Laboratory of Geomechanics and Geotechnical Engineering, Institute of Rock and Soil Mechanics, Chinese Academy of Sciences, Wuhan 430071, China IRSM-CAS/HK PolyU Joint Laboratory on Solid Waste Science, Wuhan 430071, China Hubei Province Key Laboratory of Contaminated Sludge & Soil Science and Engineering, Wuhan 430071, China University of Chinese Academy of Sciences, Beijing 100049, China


Introduction
e output of municipal sewage sludge has risen dramatically with the continuous increase in municipal sewage production in China. e country's municipal sewage sludge output is expected to exceed 60 million tons by 2020. e huge amount of sludge will cause substantial environmental problems [1][2][3], and thus, sludge stock absorption through various disposal methods should be urgently considered. e municipal sludge is o en solidi ed with materials and then used as land ll cover materials [4,5], subgrade llers [6], garden soil [7], and brick-making materials [8] or land lled [9].
Some scholars have conducted theoretical and experimental studies on sludge solidi cation with di erent solidifying agent. Yang et al. [10] used coal gangue, cement, clay, and ber to solidify municipal sewage sludge, and they found that the solidi ed sludge can be used as land ll cover materials with high strength, strong crack resistance, and low permeability coe cient characteristics. Vu et al. [11] solidi ed the sludge with y ash geopolymer and found that the solidi ed sludge can reach the maximum compressive strength a er 18 hours. e studies mostly focused on sludge with high moisture content [12,13], however, only few research has been conducted on the in uencing mechanism of di erent initial moisture contents on the geotechnical properties of the solidi ed sludge.
Research has shown that moisture content is an important index a ecting a series of engineering properties, such as shear strength [14,15], uncon ned compressive strength [16,17], and permeability coe cient [18,19] of solidi ed sludge. Boutouil and Levacher [12] studied the in uence of initial water content on the mechanical behaviour of solidi ed dredged sludge, and found that there was a good linear relationship between compressive strength and the inverse of water/cement ratio. Lin et al. [20] also found that an increase of the initial water content of the sewage sludge reduced the compressive strength of the solidi ed sludge largely. Horpibulsuk et al. [21] researched the compression behavior of solidi ed so soil with di erent initial water content, showing that the specimens with higher water content are stable at higher void ratios and provide higher compression indices beyond the transition stress. Wang et al. [22] investigated the shrinkage properties of cement solidi ed sludge in uenced by the initial moisture content. But on the whole, the e ect of initial moisture content on the permeability of solidi ed sludge is rarely studied, especially combined with microscopic analysis method. Most previous studies have addressed the issue of whether the solidi ed sludge meets land ll requirements. However, few people consider the solidi ed sludge as the impermeable layer material of land ll, which will be further explored in this study.
In this study, the sludge with di erent initial moisture contents was prepared and then solidi ed by the self-developed cement-based CERSM solidifying agent. en, the shear strength, compressive strength, permeability coe cient, and other engineering characteristic parameters of the CERSMsolidi ed sludge with di erent initial moisture contents and drying-saturated (DS) condition were studied by conducting direct shear test (DST), uncon ned compression test (UCT), and permeability test (PT). e microtopography of the solidi ed sludge pore structure was studied by scanning electron microscopy (SEM) and mercury intrusion porosimetry (MIP), and the relationship of the testing results with the engineering properties was analyzed. e research results can greatly support the modi cation and solidi cation of municipal sewage sludge and its utilization in China.

Test Materials.
e sludge was collected from the Wuhan Sewage Treatment Plant in Hubei, China. e basic properties of the sludge were tested immediately a er collection. e initial moisture content was 82.98%, the density was 1.14 kg/m 3 , the pH value was 7.07, the organic matter content was 38%, and the COD was 2868 mg/L. e composition of the chemical elements in the sludge was tested with X-ray uorescence. e results are shown in Table 1.
e CERSM solidifying agent used in the test was optimized by a previous research on sludge modi cation [23]. e solidifying agent was composed of a sulphoaluminate-based cement, ordinary Portland cement, quicklime, gypsum, and lithium salt, by which the corresponding mass ratio was 0.30 : 0.60 : 0.05 : 0.049 : 0.001. Calcium oxide accounted for 88.3% of the total weight of quicklime used in the test, and HCl dissolution content was less than 1%. More than 95% of the quicklime particles can pass through a 200-mesh sieve. e basic solidifying agent of sulphoaluminate cement (SAC) was labeled 42.5, the speci c surface area was 430 m 2 /kg, the initial setting time was more than 25 min, the nal setting time was less than 180 min, the pH value of 1 h was less than 10.5, and the free swelling ratio of 28 d (curing age) was less than 0.15%. e ordinary Portland cement was labeled 42.5 and produced by the Huaxin Cement Plant of Wuhan city. e density of the CERSM solidifying agent was 1.83 kg/m 3 , and the speci c surface area of Blaine was 360 m 2 /kg.  [24], in this study, the sludge samples with target w 0 of 85%, 81%, 77%, 74%, 71%, 63%, and 56% were prepared and then labeled as Samples 1-7, just as shown in Table 2. e self-developed strong dehydration equipment for the sludge (Figure 1) was used to thoroughly dewater the sludge with moisture content of 80%. e dehydration process was conducted as follows. First, the moisture content of the sludge was adjusted to 90%, and a part of the free water with pressure of 0.8 MPa was dehydrated in the feeding process. Moisture content was dewatered to 70-80%. en, the appropriate pressure was applied for the secondary extrusion to dehydrate the sludge to a certain moisture content (nearly 55%). Finally, the appropriate amount of water was calculated and added into the sludge for target moisture content adjustment. e solidi ed sludge was obtained by adding the CERSM solidifying agent according to 20% wet weight of the sludge to the prepared sludge mentioned above (de ne Aw as the addition ratio of solidifying agent, and Aw = 20% in this study). e CERSM materials were weighed according to their design proportions and then placed in the mixer. en, 2 min of slow stirring followed by 2 min of quick stirring were conducted to produce a homogenous mixture. e mixture was subsequently placed into an uncon ned compression mold (Φ50 mm × 100 mm), a shear test mold (Φ61.8 mm × 20 mm), and a penetration test mold (Φ50 mm × 50 mm). e lling process was divided into three layers, and the lling in the last layer was 1 cm higher than the mold (or ring knife). en, the mold lled with solidi ed sludge was placed on a shaking table to vibrate for 2 min and then shaved up and down with a scraper. Subsequently, the molded samples were wrapped with a plastic lm and placed into a curing box at 20°C with relative humidity of 95% for standard curing. e solidi ed sludge samples were demolded a er 1 day of standard curing. Finally, the solidi ed samples were sealed with a lm to avoid the evaporation of moisture.

Drying-Saturated (DS) Treatment.
A er curing for 28 d, all the solidi ed samples were DS treated, and then tested for permeability coe cient. In the drying process, the samples were dried in the oven at 50°C for 72 h to ensure complete dry. en, the samples were saturated with the pump-out method by using a superimposed saturator and a vacuum pumping device. e vacuum cylinder was connected to the pump, and the pump was operated for nearly 1 h. Once the pressure gauge reading of the vacuum was similar to the local atmospheric pressure value, water was slowly injected into the vacuum cylinder to enable the samples to be soaked for 48 h and achieve full saturation.

Permeability Test (PT).
e solidi ed samples with standard curing ages of 1, 7, and 28 d and the DS samples were selected for the PT with deionized water. e permeability test was conducted according to ASTM D5084-03 [25] standards and by using the PN3230M (Geoequip Corporation, USA) exible wall permeameter. Before the PT, the samples were saturated in a vacuum. During the test, the con ning pressure was retained at 100 kPa, the lower osmotic pressure of the samples was 80 kPa, the upper osmotic pressure was 0 kPa, the e ective osmotic pressure was 80 kPa, and the room temperature was controlled at 25°C. e scale of the water pipe of the permeameter was recorded every hour. e permeability coe cient of the samples at di erent times were calculated according to Darcy's law. e relative stable value of permeability coe cient was adopted as the permeability coe cient of the samples.

Uncon ned Compression Test (UCT) and Direct Shear Test (DST).
e solidi ed sludge samples with di erent moisture contents cured for 7 and 28 d were selected for the DST and the UCT, according to the standards ASTM D2166 [26] and ASTM D3080 [27] respectively. e UCT was conducted with a universal testing machine. e loading rate was controlled at 2 mm/min. ree parallel experiments were conducted in each group, and the average value was adopted. e error of the parallel samples was less than 5%.

MIP and SEM
Tests. Samples 1, 4, and 7 (initial moisture contents of 85%, 74%, and 56%), which represent the solidi ed sludge cured for 28 d before drying-saturated process, were selected for the MIP and SEM tests. During MIP testing, the solidi ed bodies were broken o carefully. e small sample blocks measuring approximately 1 cm 3 were taken from the fresh section and treated by vacuum freezedrying technology. e MIP test was conducted by using the PoreMaster-33 (Quantachrome Company, USA) automatic mercury injection.
During SEM testing, the solidi ed bodies were carefully broken o , and the small test blocks measuring approximately 1 cm 3 were taken from 1 cm from the outer surface of the samples. en, the broken-o samples were soaked in ethanol at normal temperature for 96 h. Finally, the sample blocks were freeze-dried and evacuated for 12 h. e SEM was conducted with Quanta 250 microscope. Advances in Polymer Technology 4 sludge samples decreased with the reduction of initial moisture content. e derived values, which were less than 10 −7 cm/s and all between the magnitude of 10 −8 and 10 −10 cm/s, can meet the requirements of permeability coe cient and China's standards for managing the impermeable layers of land lls. e permeability coe cient decreased with the increase of curing age, and the decrease was in 0.5 orders of magnitude.
As shown in Figure 5, the permeability coe cient of the solidi ed sludge sample with the standard curing for 28 d was the lowest when the initial moisture content was 63%. en, the permeability coe cient increased signi cantly for the 28 d of curing a er the drying-saturated process. e permeability coe cient of the solidi ed sludge with di erent initial contents a er the drying-saturated treatment increased in varied extents, and the maximum increase was 4 orders of magnitude. When the initial moisture content of the sludge was 74%, the

Shear Strength and Uncon ned Compressive Strength
Analysis.
e results of the DST of the solidi ed sludge samples with di erent initial moisture contents and curing ages of 7 and 28 days are shown in Figure 2. As shown in Figure 2(a), the cohesion of the solidi ed sludge sample decreases almost linearly as the initial moisture content increases, but is a ected inversely by the curing ages. When the curing age increases from 7 days to 28 days, the increasing amplitude of the solidi ed sludge cohesion ranges from 11 to 37 kPa.
As shown in Figure 2(b), the internal friction angle increases with the decrease in initial moisture content. In particular, the internal friction angle initially increased slowly (moisture content: 85-75%), then increased rapidly (moisture content: 75-60%), and nally increased slowly (moisture content: less than 60%). e internal friction angle of the solidi ed sludge samples varied minimally with age. Figure 3 shows the uncon ned compressive strength of saturated samples with standard curing for 7 and 28 days. e uncon ned compressive strength of the solidi ed sludge samples increased with the decrease in the initial moisture content of the sludge. e strength of the solidi ed sludge increased the fastest in the early stage. e increase in the uncon ned compressive strength of the samples with standard curing for 28 days was approximately 15% higher than that of the samples cured for 7 days. e uncon ned compressive strength of the DS samples with curing for 28 days increased to a certain extent. e main reason may be that the pores of the solidi ed sludge will largely shrink during the drying process, making the structure denser and then the strength increase. A er resaturation, the strength of the solidi ed body will not decrease due to the good water stability of the solidifying agent.

Results of the Permeability Test.
e changes in the permeability coe cient of the solidi ed sludge samples with di erent initial moisture contents and curing ages are shown in Figure 4. Under the modi cation conditions for 1, 7, and 28 days, all of the permeability coe cient of the solidi ed  1 μm), and all the upper limit were included in the range. e di erential pore size distribution, cumulative pore volume, and cumulative pore percentage of the solidi ed samples with di erent moisture contents are shown in Figures 6-8.
As shown in Figures 7 and 8, the higher the initial moisture content of the solidi ed sample, the larger the total pore volume of the solidi ed sludge will be, that is why the permeability coe cient of the solidi ed sludge sample a er the drying-saturated was the smallest (2.46 × 10 −8 cm/s), and the corresponding increase rate was the smallest. is result was consistent with the change of pore diameter in Figure 6.

Microstructure Analysis of Pores.
Microstructure is an important factor for the change of geotechnical properties. e peak value of the di erential curve of a pore size distribution is de ned as the most-probable pore size. is most-probable pore size de nition has a physical meaning, i.e., pores with sizes less than the most-probable pore size cannot easily generate connected pore channels. As shown in Figure 6, the most-probable pore sizes of Samples 1, 4, and 7 (solidi ed sludge samples) cured for 28 days are 6.30, 1.46, and 0.05 μm, Advances in Polymer Technology 6 samples, the contents, forms, and distribution of the hydrated products of the samples also di ered from one another. A large number of pores can be seen in the solidi ed sample (Figure 10(a)) and the DS samples (Figure 10(b)), resulting in poor density. e initial moisture content of Sample 1 was the highest (85%), and the sludge and the solidi ed bodies were lled with large amounts of water. A substantial number of pores were formed a er drying. In addition, the contents of A , calcium hydroxide, and other hydrated products in Figure 10(b) were higher than those of Figure 10(a), and the quantities of the solidi ed bodies composed of sludge particles and reaction products were higher. e di erence permeability coe cient decreased with the reduction of initial moisture content. With the decrease of the initial moisture content, the pores transformed from small pores to mesopores and micropores. For example, the total porosity of the solidi ed sludge (Sample 1) reached 0.953 mL/g when the initial moisture content was 85%. e pores were mainly concentrated as medium and small pores (1.0-10.0 μm), in which the small ones accounted for 80% and the middle ones for 16%. e mesopores was 4% of total porosity, and no micropores were visible. When the initial moisture content was 56% (Sample 7), the total porosity was 0.66 mL/g, and the pores were mainly concentrated in the small pores, mesopores, and micropores (1.0-10.0 μm), accounting for approximately 80% of the total porosity. A er drying, the volume of the solidi ed sludge sample shrunk, thus leading to the reduction of total porosity. en, the pores moved toward the smaller pore sizes. Figure 9 presents the varied volumes of the solidi ed sludge with di erent moisture contents a er drying. e samples shrunk a er drying, and the volume shrinkage rate increased initially and then decreased with the decrease in moisture content. When the moisture content was 74%, the volume shrinkage of the sample was the largest at 39.29%.

SEM Image Analysis.
e microstructures of the solidi ed sludge samples with di erent moisture contents before and a er the DS process are shown in Figure 10. Hydrated products were observed in the solidi ed sludge samples. A er DS treatment, the hydrated products of the solidi ed bodies were distributed relatively more densely. is nding can be attributed to the larger amount of hydration products caused by the volume shrinkage of the dried samples. Moreover, given the variations in the initial moisture contents of the di erent  3.5. Discussion. Municipal sewage sludge is currently a major problem in China's urbanization process, and its improper disposal may result in high levels of environmental pollution [28,29]. Agricultural application, composting, and incineration cannot be widely adopted due to economic or technical reasons, which renders the existing problem of municipal sewage sludge di cult to solve [30,31]. e application of geotechnical engineering, such as subgrade lling, land ll cover layering, and road base development, to modify municipal sewage sludge provides important ways to solve the sludge disposal problem in China. In geotechnical engineering, the emphasis of the application is solving the geotechnical properties of solidi ed municipal sewage sludge from di erent sources and determining the e ect of moisture can be explained by volume shrinkage during the drying process. Furthermore, the products became concentrated and subsequently formed additional stents to connect particles and generate connective hydrophobic pores. As a result, the samples a er DS treatment had relatively high strengths, and the permeability coe cient markedly increased. e above mentioned ndings are consistent with the results of SEM for the solidi ed sludge samples and the DS samples (Samples 4 and 7). Compared with those of Sample 1, the pores of Samples 4 and 7 were smaller due to their low moisture contents, and thus, microcracks were produced during the drying process. A er the DS treatment, the total porosity of the samples decreased but the permeability coe cient increased. skeletal structure can e ectively resist the matrix suction and thus resist deformation. With the decrease of the initial moisture content, the pore structure of the formed solidi ed sludge sample changes, and the pores move toward mesopores and micropores. In the drying process, the water losses of the mesopores and micropores lead to the increase in matrix suction. At this time, the ability of the skeletal structure to resist deformation is weakened, and the deformation of the solidi ed sludge sample increases. When the moisture content decreases at a certain value, the pore structure further changes, the number of mesopores and micropores further increases, the matrix suction in the process of water loss further increases, and the compressive ability of the skeletal structure of the solidi ed sludge sample increases greatly. e drying shrinkage is reduced relative to the heightened ability to resist deformation.
A er the DS treatment, the most-probable pore size of the solidi ed sludge sample reduces to a certain extent, and the total porosity decreases. e larger the volume shrinkage of the solidi ed sludge sample is, the greater the decrease in total porosity and the smaller the permeability coe cient will be. However, the permeability coe cient of the solidi ed sludge sample will eventually increase by orders of magnitude. e ndings imply that the huge change in the permeability of the solidi ed sludge samples is mainly caused by the microcracks produced in the solidi ed bodies a er drying and the altered properties of some organic matters in the sludge during drying, thus leading to the production of highly connective hydrophobic pores. A er drying, the volume shrinks to form additional stents to connect the particle, and thus, the structure becomes denser. More importantly, the compression resistance ability of the solidi ed sludge sample is enhanced greatly, which is an important factor in increasing the compressive strength of the solidi ed sludge sample. content on the geotechnical properties of solidi ed sludge.
us, geotechnical engineering technologies can greatly contribute to the disposal of solidi ed sludge in China.
e results of the test analysis of this study show that the pore distribution of the solidi ed sludge varies with di erent moisture contents. e sludge with high moisture content has many pores. Moreover, the contact area between the products of the solidifying agent reaction and the sludge particles is relatively small, and the frictional resistance is also small. With the decrease in moisture content, the total porosity of the solidi ed sludge sample is reduced. en, with the increase in contact area between particles, frictional resistance is also increased. Under nondrying condition, the hydrated products produced by cement and other materials in the solidifying agent have lled the pores of the sludge sample. As a result, all of the permeability coe cient of the solidi ed sludge samples with different moisture contents have decreased with the increase of curing age. At the optimum moisture content, the permeability coe cient of the solidi ed sludge sample is at the lowest. e volume of the solidi ed sludge sample shrinks in the drying process, and the moisture content with the largest shrinkage is 74%. Previous studies [32] showed that when the water-binder ratio is greater than 0.5, the autogenous shrinkage is negligible compared with the drying shrinkage. erefore, the drying shrinkage of the solidi ed sludge is mainly determined by the interaction of pore structure, matrix suction, and mechanical properties of the solidi ed sludge sample. e pore structure determines matrix suction, and the matrix suction and mechanical properties of the solidi ed sludge sample determine volume shrinkage. e solidi ed sludge sample formed by the solidi ed sludge with high initial moisture content has substantial middle and small pores. In the drying process, the matrix suction is small, and the bearing capacity of the pore In order to clarify the better performance of solidifying agent used in this study, the permeability coefficient and cohesion of solidified sludge samples were compared with the results of Yang et al. [10] e solidifying agents used in the study of Yang et al. [10] were composed of one or more materials including ordinary Portland cement, gangue, clay, and fiber, and the initial moisture content of municipal sludge was 68.75%, which is lower than Sample 3 (i.e., 71%) in this study. e addition ratio of solidifying agent by wet sludge in Yang et al. [10] was 43.27%, 43.47%, 54.32%, and 66.94% respectively, which is much larger than the ratio in this study (i.e., 20%). However, as Figure 11(a) shown, the permeability coefficient of Sample 3 in this study was much lower than that in Yang et al. [10], and the cohesion of the solidified sludge samples (Sample 3, 4, and 5) were obviously larger than that of Yang et al. [10] (Figure 11(b)).
is indicates that the performance of the solidifying agent in this study is significantly better than that of other compound solidifying agents composed of cement, clay, gangue, etc.

Conclusions
e effects of initial moisture content on the geotechnical properties of the solidified sludge were investigated. Typical geotechnical experiments and microscopic tests for the treated sludge were conducted to determine the relationship between initial moisture content variations and the hydraulic and mechanic characteristics of the solidified municipal sludge before and a er the drying-saturated treatment. e main conclusions are as follows: (1) e unconfined compressive strength and the cohesion of the solidified sludge decreased linearly with the increase in moisture content. With the increase in curing ages, the hydration reaction was gradually completed and the cohesion and strength of the samples with different moisture contents increased.
(2) A er CERSM modification, the permeability coefficient of the solidified sludge were all below 10 −7 cm/s. However, a er drying-saturated process, the permeability coefficient of solidified sludge can be increased up to 4 times, mainly due to the formation of a considerable number of microconnected pores and microcracks in the process of drying. (3) Moisture content can remarkably affect the internal pore volume and the pore distribution of the solidified sludge, with the decrease in the initial moisture content, the pores transformed from small pores to mesopores and micropores. Consequently, the altered microstructure influences the engineering properties of the solidified sludge on the applied environment.
Data Availability e data used to support the findings of this study are included within the article.