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Durability of silty soil stabilized with recycled lignin for sustainable engineering materials
Journal of Cleaner Production xxx (xxxx) xxx
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Durability of silty soil stabilized with recycled lignin for sustainable engineering materials Tao Zhang a, b, *, Songyu Liu c, Hongbin Zhan d, Chong Ma e, Guojun Cai c a
Faculty of Engineering, China University of Geosciences, Wuhan, 430074, China University College London, Department of Civil, Environmental and Geomatic Engineering, Gower Street, London, WC1E 6BT, UK c Institute of Geotechnical Engineering, Southeast University, Nanjing, 210096, China d Department of Geology and Geophysics, Texas A&M University, College Station, TX, 77843-3115, USA e School of Mathematics and Physics, China University of Geosciences, Wuhan, 430074, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 June 2019 Received in revised form 18 September 2019 Accepted 12 November 2019 Available online xxx
Lignin is an industrial waste derived from the paper plant, which poses a potential threat to the surrounding environment without proper disposal. The present study aims to evaluate the durability performance of recycled lignin stabilized silty soil subjected to different adverse environments. A series of laboratory experiments were conducted to determine unconfined compressive strength (qu), moisture stability coefficient (Kr), mass loss (Sm) and pH value of the stabilized silt exposed to water soaking and wetting-drying cycles. The results showed that the strength and durability of natural silt were obviously improved with the inclusion of lignin. Lignin cementation and matrix suction played important roles in the strength behavior of the unsaturated stabilized silt. The compressive strength suffered a dramatic drop after 1 day of soaking due to the dissolution of cementing materials and the loss of suction. Lignin stabilized silt possessed a higher Kr than that of quicklime stabilized one, indicating its superiority in durability. After 28 days of standard curing, 12% lignin stabilized silt had an ability to resist 4 cycles of wetting-drying, resulting in the cumulative mass loss (Cm) of approximately 20%. Soil pH value presented a decreasing trend with wetting-drying cycles, and an exponential correlation of pH with Cm was found by data fitting. Scanning electron microscope analysis revealed that the lignin-based cementing materials are gradually lost during the process of wetting-drying cycles. Additional research is encouraged to explore an effective method to prevent the deterioration of mechanical properties of lignin stabilized soils in practical projects. © 2019 Elsevier Ltd. All rights reserved.
Handling editor: Zhen Leng Keywords: Stabilization Recycled materials Strength Durability Wetting-drying cycle
1. Introduction Chemical stabilization, using deep mixing (Liu et al., 2011), grouting or jet-grouting methods (Shen et al. 2013, 2017; Horpibulsuk et al., 2003), has been widely used for soft, problematic and contaminated soils (Yin et al., 2006; Latifi et al., 2018a). Traditional calcium-based additives, e.g., Portland cement (Horpibulsuk et al., 2010), quicklime (Bell, 1996) and gypsum (Puppala et al., 2005; Ghosh and Subbarao, 2006) are of the primary concern to the engineering constructors in the past decades.
* Corresponding author. Faculty of Engineering, China University of Geosciences, Wuhan, 430074, China. E-mail addresses: [email protected] (T. Zhang), [email protected] (S. Liu), [email protected] (H. Zhan), [email protected] (C. Ma), focuscai@ 163.com (G. Cai).
Utilization of these traditional stabilizers brings the acknowledged benefits on strength, permeability, and durability of various soils (Horpibulsuk et al. 2004, 2010). Meanwhile, some unsettled issues, such as, high cost of raw materials, huge energy consumption of production, CO2 and nitrogen oxide emission, and alkaline attack on the surround environment (Puppala et al., 2004; Mitchell and Soga, 2005; Higgins, 2007) are left, which are contrary to the requirements of sustainable development. Therefore, usage of industrial by-products or nontraditional additives attracts strong attentions from a view of engineering construction, owing to their potential of saving construction cost and recycling natural resources (Horpibulsuk et al., 2009; Arulrajah et al., 2019; Latifi et al., 2018b; Yaowarat et al., 2019). Earlier researchers discovered many nontraditional chemicals for soil stabilization, such as biomass silica (Hassan et al., 2019), xanthan (Dehghan et al., 2019), gypsum (Latifi et al., 2018b), and calcium carbide residue (Latifi et al.,
https://doi.org/10.1016/j.jclepro.2019.119293 0959-6526/© 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: Zhang, T et al., Durability of silty soil stabilized with recycled lignin for sustainable engineering materials, Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2019.119293
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2018a). These chemicals had good performance in improving soil strength and retarding the migration of contaminants. The previous studies revealed that nontraditional soil additives have great potential to address these issues in an environmentally friendly way. Lignin, an organic by-product of biomass-based industries, has been recognized as an environmentally friendly soil stabilizer that has an ability of strength enhancement for clay (Puppala and Hanchanloet, 1999; Tingle and Santoni, 2003; Kim et al., 2011), silt (Zhang et al., 2018a, 2018b), and silty sand (Santoni et al. 2002, 2005; Chen et al., 2014). Moreover, after lignin stabilization, the other engineering properties of problematic soils in terms of ductility, expansibility, and erosion resistance are dramatically improved (Pengelly et al., 1997; Indraratna et al., 2012; Vakili et al., 2018). Ceylan et al. (2010) selected two types of lignin to stabilize sandy lean clay in Iowa State as a highway subgrade filling. The unconfined compressive strength test results show that lignin stabilized clay with cured 7 days meets the strength requirements specified in the local design specifications. Additionally, they pointed out the important influence of the prepared moisture content on the strength development of specimens, which is consistent with the field test results reported by the authors (Zhang et al., 2017) for lignin stabilized silt. Indraratna et al. (2012) reported that 0.4% lignin (lignosulfonate mixtures) stabilized silty sand possesses a similar erosion resistance as compared to 2% Portland cement stabilized one. The stress-strain behavior of silty sand was dramatically changed after the addition of lignin, and then Chen et al. (2014) proposed a bounding surface plasticity model to capture this special mechanical response. Earlier studies by the authors confirmed that soil particles are drawn closely together with lignin-based cementing materials, and a more stable microstructure was clearly observed via microscopic analysis for silty soil stabilized with lignin (Zhang et al., 2015, 2018a; 2018b). Meanwhile, when rainfall infiltrated into the subgrade consisting of lignin stabilized silty soil, its road performance was marred with respect to the decrease in resilience modulus and the increase in deflection (Zhang et al., 2017). Similar deterioration in mechanical properties is commonly reported for other stabilized soils, such as cemented clay, fly ash treated silty clay (Walker, 1995; Zhang and Tao, 2008). These phenomena revealed that moisture intrusion has a great influence on the stability of engineering properties for the stabilized soils. Before the stabilized soils were used in constructions, a clear understanding of their durability performances should be made. However, the majority of previous researchers have focused on the variation in mechanical properties of several soils with lignin stabilization. The very limited information can be found in the earlier studies regarding the resistance of lignin stabilized soils against the intrusion of adverse environmental conditions (Santoni et al., 2002; He et al., 2017). Additionally, few studies have discussed the deterioration mechanism of lignin stabilized soils, incorporating the influence of soil suction. Based on the above-mentioned literature review, this paper aims to fill the key gaps in the literature by studying the durability of lignin stabilized silty soil. Quicklime, a traditional chemical additive, is selected here as a comparison purpose. An array of laboratory experiments including unconfined compressive strength test (UCS), moisture stability test, wetting-drying cycle test, and pH test were performed to obtain insights into the evolution of engineering performances of the stabilized silt under different conditions. The effects of additive content as well as curing time on the unconfined compressive strength qu, moisture stability coefficient Kr and mass loss Sm were analyzed. The role of suction on qu and the breakage of particle bondings were discussed. The variation in soil pH value with wetting-drying was monitored, and an empirical correlation of pH value with accumulative mass loss Cm was developed. The changes in the microstructure of lignin stabilized silt after suffering
wetting-drying cycles were qualitatively evaluated from scanning electron microscopy (SEM) images. Based on these results, an effort has been made to explore the deterioration mechanism of lignin stabilized soils subjected to the adverse conditions. 2. Materials and methods 2.1. Materials Tested soil was obtained from a highway subgrade construction site at approximate 2.0 m depth in Jiangsu province, China, and its basic engineering property indices are listed in Table 1. This soil is classified as low plastic silt according to the Unified Soil Classification System (ASTM D2487 (2011a)). Chemical compositions of the tested soil determined from X-ray fluorescence (XRF) analysis are shown in Table 2. Lignin used in this study was collected from a paper plant in Henan province, China, which is considered to be a low-valued industrial waste and usually stockpiled in land, resulting in a potential threat to the environmental safety. It is a fine powder at dry state with a yellow-brown color and fragrant smell and is completely soluble in water. After toxicity testing conducted by the Center of Modern Analysis in Nanjing University, the used lignin is verified to be a non-toxic organic material with some cellulose and impurities. In addition, lignin has a good chemical activity due to the presence of both hydrophilic and hydrophobic groups (Zhang et al., 2018a). The pH value of used lignin is 10.2 because the alkaline paper-making process is utilized in this plant. Quicklime, collected from Changzhou city, China with a specific gravity Gs of 2.98, was selected as a traditional soil stabilizer, and its chemical compositions are presented in Table 2. This quicklime is classified as high-calcium lime according to ASTM C51-11 (ASTM, 2011b), which is widely used in the treatment of problematic soils. Deionized water (pH ¼ 6.71, electrical conductivity < 1 mS/m) bought from the market was used in this study to minimize the effect of other chemicals on the test results. 2.2. Sample preparation The collected silt was air-dried and then passed through a sieve with 2 mm opening size. The predetermined weights (dry weight basis) of additive powders (i.e., lignin and quicklime) were added into the prepared silt and mixed thoroughly for about 5 min by resorting to an electrical agitator with a speed of 120 rpm. The soiladditive mixtures were then mixed twice with a designed weight of deionized water (optimum moisture content wopt) for 5 min to obtain homogeneous soil-additive-water mixtures. The homogenized mixtures were poured into a steel mold (inner diameter of
Table 1 Basic engineering properties of tested silty soil. Property
Value
Natural moisture content, wn (%) Specific gravity, Gs Grain size distribution (%)a Clay (<0.002 mm) Silt (0.002e0.074 mm) Sand (0.074e2 mm) Average diameter, d50 (mm) Liquid limit, wL (%)b Plasticity limit, wP (%)b Optimum moisture content, wopt (%) Maximum dry density, rdmax (g/cm3) pH (mass ratio: water/soil ¼ 1)c
28.5 2.71 e 9.7 81.2 9.1 38.7 32.4 23.6 16.1 1.72 8.14
a b c
Measured using a laser particle size analyzer Mastersize 2000. Measured as per China MOT JTG E40-2007. Measured as per ASTM D4972 (2013).
Please cite this article as: Zhang, T et al., Durability of silty soil stabilized with recycled lignin for sustainable engineering materials, Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2019.119293
T. Zhang et al. / Journal of Cleaner Production xxx (xxxx) xxx Table 2 Oxide chemistry of tested soil and quicklime. Oxide chemistrya
Tested soil (%)
Quicklime (%)
Silicon oxide (SiO2) Aluminium oxide (Al2O3) Calcium oxide (CaO) Ferric oxide (Fe2O3) Potassium oxide (K2O) Magnesium oxide (MgO) Sodium oxide (Na2O) Others Loss on ignitionb
63.20 12.53 6.41 3.12 2.46 2.39 2.30 7.59 5.81
2.62 1.16 71.23 0.74 0.18 0.46 0.20 23.41 24.36
a Mineral composition was analyzed by X-ray flourescence method using ARL9800XP þ XRF spectrometry. b Value of loss on ignition is referenced to 950 C.
Table 3 Values of compaction parameters for lignin and quicklime stabilized silt (from Zhang et al., 2018a). Additive content
2 5 8 12 15
Lignin stabilized silt
as chemically treated soil. The UCS test of such stabilized soil was conducted by using strain-controlled application of test load. The load was applied continuously and without shock to produce an approximately constant rate of deformation. Cylindrical soil samples with a height/diameter ratio of 2 were selected for the UCS test by fixing the strain rate as 1%/min. The testing procedure followed as per ASTM D4219 (ASTM, 2008) to determine the crushing strength of the stabilized soils. The moisture stability test of the stabilized silt was performed as per China MOHURD (2015) CJ/T 486e2015. The samples with standard curing of 6 days and 27 days were selected and then soaked in the deionized water for 1 day, after that, the qu was measured as followed the same procedures previously described for UCS test. The beaker with a volume capacity of 1.2 L was used to make sure that the sample is completely submerged into the water, as shown in Fig. 1. Under the same curing time (i.e., 7 days or 28 days), the strength ratio of 1 day soaking to standard curing is defined here as moisture stability coefficient Kr:
Quicklime stabilized silt
wopt (%)
rmax (g/cm3)
wopt (%)
rmax (g/cm3)
11.52 11.57 11.62 14.03 12.85
1.81 1.82 1.83 1.82 1.82
16.52 16.71 17.87 18.23 18.79
1.68 1.65 1.60 1.58 1.51
50 mm; height of 100 mm) with three layers and compacted to the design degree of compaction (96% of the maximum dry density rmax) by a hydraulic jack at a constant lifting rate. The values of wopt and rmax of the stabilized silt were determined from the results of the standard Proctor compaction test that were reported in earlier studies (Zhang et al., 2018a), as shown in Table 3. The cylindrical specimen was carefully extruded from the mold and wrapped by a vinyl bag to minimize moisture evaporation, and then placed in an ambient condition controlled room (22 C and relative humidity of 95%) to the target curing time (viz., standard curing). A soil block of approximately 1 cm3 was collected from the hand-broken identical specimens for SEM analysis. The SEM specimens were quickly frozen using the liquid nitrogen, after which the specimens were placed into a vacuum chamber with a temperature of 80 C for 24 h. The freeze-dried specimens were coated with gold first, and then subjected to the SEM analysis to capture the microstructural images. 2.3. Testing methods Lignin stabilized silt investigated in this study can be classified
2% Lignin
3
5% Lignin
Kr ¼
qut qu0
(1)
where qu0 is the compressive strength of standard curing (kPa); and qut is the compressive strength of specimen with 1 day soaking (kPa). The higher Kr value, the stronger resistance against the moisture intrusion. The wetting-drying cycle test followed the recommendations specified by ASTM D559 (ASTM, 2015), while the drying temperature was set as 30 C and the wetting process was performed firstly in this study. Selecting 30 C is to protect the lignin-based materials that present in the specimens from destruction. The authors previously used 60 C (recommended temperature for chemical cemented soils) to dry lignin stabilized specimens (lasted 24 h) and found that the color of specimen surface became darken, accompanied with a pungent smell. This is probably due to the destruction of lignin under this high temperature. After turned down the temperature as 30 C, these negative phenomena vanished. The temperature modification in this study was also consistent with that reported by Kamon et al. (1993) for cement-/slag-stabilized soils. The identical specimens of UCS test were used for the wetting-drying cycle test. These samples firstly were immersed in the deionized water for 24 h with normal room temperature (approximately 20 ± 2 C), and then placed in the oven (30 C) for 48 h. Therefore, such one wetting-drying cycle needs 3 days to be completed, and the Sm after each drying process was recorded. The terminal criterion of the wetting-drying cycle test is either specimen fractured into at least two parts or Cm exceeds 30%. The mass loss Smi value after the ith cycle is computed by Equation (2):
8% Lignin
12% Lignin
15% Lignin
Fig. 1. Photo of lignin stabilized specimens during moisture stability test.
Please cite this article as: Zhang, T et al., Durability of silty soil stabilized with recycled lignin for sustainable engineering materials, Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2019.119293
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Table 4 Additive content, curing time, and soaking condition for various tests in this study. Test type
Lignin content (%)
Lime content (%)
Soaking time (d)
Curing time (d)
UCS test Moisture stability test W-D cycle test pH test SEM
0, 2, 5, 8, 12, 15 2, 5, 8, 12, 15 12 2, 5, 8, 12, 15 12
0, 2, 5, 8, 12, 15 2, 5, 8, 12, 15 8 e e
e 1 1 per cycle e e
1, 7, 28, 60 7, 28 7, 28 7, 28 28
Note: UCS test is unconfined compressive strength test; W-D is wetting-drying.
Smi ¼
mi1 mi 100% m0
(2)
where m0 is the initial mass of dry sample prior to the wettingdrying cycle test (g); mi-1 and mi are the mass values measured immediately after (i-1)th and ith cycle, respectively (g), and the subscript i represents the number of wetting-drying cycles. The cumulative mass loss Cm at the ith cycle is calculated by:
Cmi ¼
X Smi
(3)
i¼1
The soil pH measurement was conducted according to ASTM D4972 (ASTM, 2013), where 10 g of the sieved soil powder was added into 10 mL deionized water in a beaker, and then mixed thoroughly by using a glass rod and left to the stationary state for 1 h. After that, the pH meter (HORIBA D-54, Japan) was placed into the supernatant and the pH value was measured. The microstructural characterizations of lignin stabilized silt before and after the wetting-drying cycle test were observed using LEO 1530VP (LEO Instruments, Germany) device. The small fragments (approximately 1 cm3) from the core of specimens were dehydrated as the following steps: (i) make the fragments immerse in the liquid nitrogen (boiling point of 195 C) for a quick freezing; (ii) place the frozen fragments into a freezing unit (temperature of 80 C) with a vacuum chamber for 24 h for sublimation; (iii) coat the freeze-drying fragments with gold before SEM analysis. Triplicate samples were prepared and tested for qu, Kr, Sm, and pH, and the average values were shown in this study. For the sake of clearness, additive content, curing condition and curing time of the stabilized specimens are summarized in Table 4. According to the UCS test results, it is observed that 12% lignin stabilized specimen shows the highest qu value, and therefore, only silt stabilized with 12% lignin and 8% quicklime (for comparison) were selected for the wetting-drying cycle and the test results were presented hereafter in the paper. Fig. 2. Evolution of unconfined compressive strength of the stabilized soils with curing time: (a) lignin stabilized silt and (b) quicklime stabilized silt.
3. Results and discussion 3.1. Unconfined compressive strength The evolution of unconfined compressive strength qu of lignin stabilized and quicklime stabilized silt with curing time is shown in Fig. 2. It can be observed that an increasing trend of qu with curing time is easily observed regardless of the additive type and dosage. For lignin stabilized specimens, the majority increment in qu is generated within 28 days, and with curing time further increases to 60 days, the qu value keeps almost a constant for each additive content (see Fig. 2(a)). This reveals that approximately one month curing is a necessary requirement to ensure the completion of lignin stabilization, and further increment of curing time has little benefit to strength enhancement. The qu value increases with increasing of lignin content from 2% to 12%,
but when lignin content exceeds 12%, a slight decrease is found. Therefore, the optimum lignin content for tested silt is about 12% in terms of compressive strength. Chen et al. (2014) reported that 2% lignin (lignosulfonate mixtures) treated specimen has the highest qu value, and the qu value decreases slightly once lignin content exceeds 2%. This evolution trend of qu is consistent with that presented in this study. The reason for the qu reduction in lignin stabilized soils probably is the presence of some local weak spots, resulting from the excessive free lignin (Zhang et al., 2018a). Quicklime stabilized silt shows a similar increasing trend of qu to that of lignin case, while some different characteristics also need to be indicated. Firstly, quicklime presents a superiority on strength enhancement with respect to the qu value as compared to lignin.
Please cite this article as: Zhang, T et al., Durability of silty soil stabilized with recycled lignin for sustainable engineering materials, Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2019.119293
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Within the curing time of 28 dayse60 days, an obvious increment in the qu value is still presented, as shown in Fig. 2 (b). Increase in quicklime content has a positive effect on the strength improvement of the stabilized silt, and the highest qu value is found in 15% quicklime case. Based on the aforementioned results, it could be deduced that all the discrepancy in strength evolution is mainly attributed to the difference of stabilization mechanism between lignin and quicklime (Vinod et al., 2010). It is noteworthy that suction has an influence on the compressive strength of unsaturated specimens. Although the soil suction measurements have not been conducted in this study, this cannot prevent us from making a preliminary evaluation of the mechanical response of lignin stabilized silt. The suction value of the compacted silt without lignin (qu is 131 kPa) can be approximately estimated to be 110 kPa at an effective friction angle of 21. It is acknowledged that addition lignin into silt would inevitably
5
change the soil suction of the compacted specimens. For simplicity, we selected the suction value of the compacted natural silt (i.e., 110 kPa) as a reference scale to the stabilized specimens. After 60 days of curing, all the lignin stabilized specimens have a dramatic strength increment, and the magnitude of this increment is much greater than the reference suction value. Taking 12% lignin stabilized specimen as an example, the ultimate increment of qu is 582 kPa that is approximately 5 times as much as the reference suction value. This great discrepancy implies that qu of lignin stabilized silt after curing is mainly generated from lignin cementation, followed by suction. At the initial stages of curing (less than 7 days), qu of the stabilized silt is mostly controlled by suction. These conclusions are also applicable to the case of quicklime stabilized silt. Additional matric suction testing of the stabilized silt with different lignin contents is warranted to further assess the influence of suction on the strength. This
(a)
(b)
(c)
(d)
(e)
Fig. 3. Photos of lignin stabilized specimens with additional 1 day soaking in moisture stability test: (a) 2% lignin; (b) 5% lignin; (c) 8% lignin; (d) 12% lignin; (e) 15% lignin.
Please cite this article as: Zhang, T et al., Durability of silty soil stabilized with recycled lignin for sustainable engineering materials, Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2019.119293
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recommended work would be beneficial to further explore the deterioration mechanism of lignin stabilized soils in adverse environments. 3.2. Moisture stability After soaking in the deionized water for 1 day, the photos of lignin stabilized specimens with cured 28 days were captured and presented in Fig. 3 to obtain macroscopic insights into the moisture stability. For 15% lignin stabilized specimen, surface spalling as well as some tiny fragments at the bottom of beaker are found in Fig. 3(e). Additionally, the color of soaking water in 15% lignin case has changed from transparent to light yellow, which probably results from the dissolution of excessive free lignin. This interesting phenomenon provides evidence to verify the reduction in qu once lignin content exceeds 12%, as discussed on the UCS test results. For other lignin content cases, the intensity of both surface spalling and the color change is much less as compared to that of 15% lignin case (see Fig. 3(a)e3(d)). To provide a quantitative evaluation of the effect of moisture intrusion on the strength, the qu values for both standard and additional 1 day soaking conditions are shown in Fig. 4. Compared to the standard curing, an additional 1 day soaking makes qu of lignin stabilized silt having an obvious decrease. The maximum decrement in qu is 269.6 kPa, which is found in 12% lignin stabilized silt with cured 28 days, and 15% lignin case has a very similar value of qu decrement (268.9 kPa). Referring to the studies of Rotta et al. (2003) for the cemented silty sand, soil suction is expected to have reduced significantly after the soaking process, and so nonsaturation is considered to have only a marginal influence on unconfined compressive strength. Therefore, it can be concluded that the deterioration of strength in a soaking test for lignin stabilized silt is mainly due to the following two reasons: reduction of suction and dissolution of lignin-based cementing materials. Moreover, it should be indicated that the dissolution of lignin/lignin-based cementing materials poses a negligible threat to environmental safety. This conclusion has been confirmed by the field test results (Zhang et al., 2017), where lignin stabilized silt is used as the subgrade filling. A parameter of moisture stability coefficient Kr was employed to compare the resistance against moisture intrusion between lignin stabilized and quicklime stabilized silt, and its variation with
Fig. 4. Comparison of unconfined compressive strength for lignin stabilized silt between standard and additional 1 day soaking conditions.
additive contents was shown in Fig. 5. The error bars marked in the figure indicate the size of data variability. It can be observed that the magnitude of Kr value for silt stabilized with lignin is higher than that of quicklime stabilized one. There is no clear correlation between Kr value and lignin content at either 7 days or 28 days curing. The average values of Kr for the specimens with different quicklime contents are around 0.4, and furthermore, with curing time increasing from 7 days to 28 days, a slight decrease in Kr is observed. 3.3. Mass loss After the first cycle of the wetting-drying process, some photos of lignin stabilized specimens with 7 days of curing were collected and shown in Fig. 6. It can be observed that the surface of each specimen has different degrees of damage, and two specimens with low lignin contents (i.e., 2% and 5%) were destroyed into two parts, while the other specimens of high lignin contents (i.e., 8%, 12%, and 15%) can keep a good integrity after one cycle. According to the aforementioned discussion on moisture intrusion, soil suction would be greatly reduced after soaking, and meanwhile, lignin-based cementing materials were gradually dissolved in water. Once the specimens were placed in
Fig. 5. Variation of moisture stability coefficient with additive content: (a) 7 days curing and (b) 28 days curing.
Please cite this article as: Zhang, T et al., Durability of silty soil stabilized with recycled lignin for sustainable engineering materials, Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2019.119293
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Spalling Spalling
Connected crack
Spalling
Crack Fracture
7
Spalling
Fracture Spalling
2% Lignin
5% Lignin
8% Lignin
12% Lignin
15% Lignin
Fig. 6. Images of lignin stabilized specimens with cured 7days after first wettingdrying cycle.
the oven (30 C) for drying, soil suction increases dramatically. A very high soil suction would be produced at the dry state, resulting in shrinkage and cracks in the specimens. For silt stabilized with low lignin content (i.e., 2% and 5%), the shrinkage force produced breaks the bonding among soil particles, which is the reason for the surface spalling and fracture. The particle bondings are reinforced as an increase in lignin content, which has a good resistance against the shrinkage. The connected crack in 15% lignin stabilized specimen is probably attributed to the dissolution of excessive free lignin. In summary, lignin-based cementation is marred by the dissolution of cementing materials in the first wetting stage, and the durability of the stabilized silt in the next drying stage depends on the balance between particle bonding and soil suction. We selected 12% lignin stabilized specimens to continue the next wetting-drying cycles until the termination conditions (Cm >30% or fracture) were reached. The results of silt stabilized with 8% quicklime are also presented here for comparison purpose. Fig. 7(a) and (b) show the evolution of mass loss Sm and cumulative mass loss Cm with wetting-drying cycles, respectively. The lignin stabilized specimens with cured 7 days and 28 days are able to suffer 3 cycles and 4 cycles, respectively, while specimens of quicklime stabilization only have 2 cycles. An increasing trend of Sm with the cycles is observed for lignin stabilized silt, however, quicklime stabilized silt presents an opposite trend in Fig. 7(a). Nevertheless, they have similar increasing trends of Cm with an increase in the wetting-drying cycle (see Fig. 7(b)). In addition, the curves of 7 days curing locate above those of 28 days. These observations reveal that: (1) lignin stabilization has superiority on improving the resistance of silt against the wetting-drying intrusion as compared to quicklime stabilization; (2) extension curing time from 7 days to 28 days is beneficial to soil durability, which is consistent with the enhancement of compressive strength.
Fig. 7. Results of wetting-drying test for 12% lignin stabilized silt: (a) mass loss versus dry-wetting cycle and (b) cumulative mass loss versus dry-wetting cycle.
3.4. pH value and microstructure characteristics Fig. 8 shows the variation of soil pH value with the wettingdrying cycle. The fitted lines represent a decreasing trend of pH value with increasing of the wetting-drying cycle, and the exponential function can be used to depict this trend well with a good coefficient of correlation R2 > 0.90. Before the wetting-drying cycle starts, the soil pH values with cured 7 days and 28 days are 9.54 and 9.82, respectively, which are much higher than that of natural silt (pH ¼ 8.14). Furthermore, at the end of the wetting-drying test, the fractured specimens still have a higher pH value. This increment in pH value is attributed to the addition of lignin that has a high pH value of 10.2. Earlier researchers have utilized pH value to characterize the mechanical properties of the stabilized soils and proposed many empirical correlations of pH with compressive strength (Miller and Azad, 2000; Chrysochoou et al., 2009). The authors also found a positive correlation between pH value and qu for lignin stabilized silt in a previous study (Zhang et al., 2018a). The
Fig. 8. Variation of soil pH value with wetting-drying cycle for 12% lignin stabilized silt.
Please cite this article as: Zhang, T et al., Durability of silty soil stabilized with recycled lignin for sustainable engineering materials, Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2019.119293
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Fig. 9. Relationship between soil pH value and cumulative mass loss Cm.
measurement work of qu for lignin stabilized silt is not been done in the wetting-drying program, but we could choose parameter Cm instead of a mechanical parameter to develop a correlation with soil pH value. Furthermore, a negative relationship between these two parameters is expected to be obtained. Because the magnitude of Cm represents the integrity level of specimens, deducing that the higher of Cm, the lower value in qu. The relationship between pH value and Cm for lignin stabilized
(a)
silt is presented in Fig. 9. The expected decreasing trend of pH value with an increase in Cm is observed, and a fitted line of exponential function provides a good description of this relationship (R2 ¼ 0.94). It is easy to understand that more destruction in particle bondings would produce a higher value of Cm, corresponding to a lower value in qu as well as a higher pH value (Horpibulsuk et al., 2016). A similar trend of mass loss with an increase in pH value was reported in the literature (Hewayde et al., 2007) for concrete attacked with sulfuric acid. Therefore, this empirical relationship would be useful to provide a preliminary assessment of the durability of such stabilized soil using pH value index. The SEM images of lignin stabilized specimens before and after wetting-drying cycles are shown in Fig. 10. It is evident that after 28 days of standard curing, soil particles are coated and bonded with precipitated cementing materials, and additionally, the pores among the particles are partially filled, as shown in Fig. 10(a) and (c). This stable soil structure is beneficial to the improvement of mechanical property and durability performance. A similar microstructure was observed on silty sand stabilized with lignin, which was used to resist water erosion in embankment dams (Indraratna et al., 2008). After suffering 4 cycles of wetting-drying intrusion, the clear boundaries among the solid particles are observed in Fig. 10(b) and (d). In addition, the particle surface becomes smooth, and the strong connections among particles are hard to be observed in the images. The loss of cementing materials should be responsible for the abovementioned changes in soil microstructure after wetting-drying cycles. These microstructural changes would result in the deterioration of both strength and durability of stabilized soils. The stabilized soils are commonly used as filling materials in
(b)
Cementing materials
5 m
5 m
(d)
(c)
Cementing materials Cementing materials
5 m
5 m
Fig. 10. Microstructure behavior of lignin stabilized silt with 28 days of curing: (a) 8% lignin, standard curing; (b) 8% lignin, after 4 cycles of wetting-drying; (c) 12% lignin, standard curing; (d) 12% lignin, after 4 cycles of wetting-drying.
Please cite this article as: Zhang, T et al., Durability of silty soil stabilized with recycled lignin for sustainable engineering materials, Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2019.119293
T. Zhang et al. / Journal of Cleaner Production xxx (xxxx) xxx
infrastructure projects, such as building foundation and highway subgrade. The durability of such stabilized soils is closely related to the service performance of the superstructures. According to the results presented in this paper, it can be confirmed that under the attack of the adverse natural environment, the dissolution of ligninbased cementing materials and deterioration of engineering properties of lignin stabilized silt are found. The similar phenomena were observed in the field construction of highway subgrade, which was conducted by the authors in the earlier time (Zhang et al., 2017). Several engineering protective measures were performed to protect lignin stabilized silt from the attacking of rainfall, for instance, covering waterproof film on the surface of the compacted soil during the curing period, and setting the compacted clay line of low hydraulic conductivity on the filling. These measures provide strong support to the long-term durability of lignin stabilized soil as subgrade filling. Additional protective technology is warranted to improve the durability performance of such stabilized soils in practical projects. 4. Conclusions A laboratory testing program was conducted to investigate the durability of lignin stabilized silt under moisture intrusion and wetting-drying cycle conditions. Quicklime, as a traditional chemical additive, was selected in this study for a comparison purpose. Inclusion of lignin is able to enhance unconfined compressive strength qu of silt, and nearly one month curing is required for the completion of strength improvement. The mechanical response of silt stabilized with lignin is mainly controlled by cementation as well as suction at the unsaturated state. The discrepancy in terms of compressive strength evolution implies that lignin and quicklime have different stabilization mechanisms. A significant decrease in qu was observed after suffering moisture intrusion because of the combined influences of suction loss with the dissolution of lignin-based cementing materials. From the results of moisture stability coefficient Kr, a higher average Kr value was found in silt stabilized with lignin as compared to that of quicklime stabilized silt. After 28 days of curing, lignin stabilized silt is able to resist 4 cycles of wetting-drying intrusion and with cumulative mass loss Cm of approximately 20%. The large suction produced from the drying process has a negative influence on soil particle bonding, which could result in surface spalling and fracture to the specimens of weak lignin cementation. Extension curing time is beneficial to improve the durability of stabilized silt. Soil pH value presents a decreasing trend in the wetting-drying cycles, and an exponential correlation of pH value with Cm is found in this study. The changes in microstructure, including the presence of clear boundaries and the smooth surface of soil particles, provide evidence to the dissolution of lignin-based cementing materials. It is anticipated that a clear understanding of the deterioration mechanism of lignin stabilized silt would be gained with additional micro-chemical analyses. Acknowledgements The funding provided by National Natural Science Foundation of China (Grant No. 41807260) and the Fundamental Research Funds for the Central Universities, China University of Geosciences (Beijing) (CUG170636, CUGL170807) are appreciated. References Arulrajah, A., Mohammadinia, A., Maghool, F., Horpibulsuk, S., 2019. Tire derived aggregates as a supplementary material with recycled demolition concrete for pavement applications. J. Clean. Prod. 230, 129e136.
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using a chemical admixture. Proc. Inst. Civ. Eng.-Ground Improv. 163 (1), 43e51. Walker, P.J., 1995. Strength, durability and shrinkage characteristics of cement stabilised soil blocks. Cement Concr. Compos. 17 (4), 301e310. Yaowarat, T., Horpibulsuk, S., Arulrajah, A., Mohammadinia, A., Chinkulkijniwat, A., 2019. Recycled concrete aggregate modified with polyvinyl alcohol and fly ash for concrete pavement applications. J. Mater. Civ. Eng. 31 (7), 04019103. Yin, C.Y., Mahmud, H.B., Shaaban, M.G., 2006. Stabilization/solidification of leadcontaminated soil using cement and rice husk ash. J. Hazard Mater. 137 (3), 1758e1764. Zhang, T., Cai, G., Liu, S., 2017. Application of lignin-based by-product stabilized silty soil in highway subgrade: a field investigation. J. Clean. Prod. 142, 4243e4257. Zhang, T., Cai, G., Liu, S., 2018a. Application of lignin-stabilized silty soil in highway subgrade: a macroscale laboratory study. J. Mater. Civ. Eng. 30 (4), 04018034. Zhang, T., Cai, G., Liu, S., 2018b. Reclaimed lignin-stabilized silty soil: undrained shear strength, atterberg limits, and microstructure characteristics. J. Mater. Civ. Eng. 30 (11), 04018277. Zhang, T., Liu, S., Cai, G., Puppala, A.J., 2015. Experimental investigation of thermal and mechanical properties of lignin treated silt. Eng. Geol. 196, 1e11. Zhang, Z., Tao, M., 2008. Durability of cement stabilized low plasticity soils. J. Geotech. Geoenviron. Eng. 134 (2), 203e213.
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Durability of silty soil stabilized with recycled lignin for sustainable engineering materials Tao Zhang a, b, *, Songyu Liu c, Hongbin Zhan d, Chong Ma e, Guojun Cai c a
Faculty of Engineering, China University of Geosciences, Wuhan, 430074, China University College London, Department of Civil, Environmental and Geomatic Engineering, Gower Street, London, WC1E 6BT, UK c Institute of Geotechnical Engineering, Southeast University, Nanjing, 210096, China d Department of Geology and Geophysics, Texas A&M University, College Station, TX, 77843-3115, USA e School of Mathematics and Physics, China University of Geosciences, Wuhan, 430074, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 June 2019 Received in revised form 18 September 2019 Accepted 12 November 2019 Available online xxx
Lignin is an industrial waste derived from the paper plant, which poses a potential threat to the surrounding environment without proper disposal. The present study aims to evaluate the durability performance of recycled lignin stabilized silty soil subjected to different adverse environments. A series of laboratory experiments were conducted to determine unconfined compressive strength (qu), moisture stability coefficient (Kr), mass loss (Sm) and pH value of the stabilized silt exposed to water soaking and wetting-drying cycles. The results showed that the strength and durability of natural silt were obviously improved with the inclusion of lignin. Lignin cementation and matrix suction played important roles in the strength behavior of the unsaturated stabilized silt. The compressive strength suffered a dramatic drop after 1 day of soaking due to the dissolution of cementing materials and the loss of suction. Lignin stabilized silt possessed a higher Kr than that of quicklime stabilized one, indicating its superiority in durability. After 28 days of standard curing, 12% lignin stabilized silt had an ability to resist 4 cycles of wetting-drying, resulting in the cumulative mass loss (Cm) of approximately 20%. Soil pH value presented a decreasing trend with wetting-drying cycles, and an exponential correlation of pH with Cm was found by data fitting. Scanning electron microscope analysis revealed that the lignin-based cementing materials are gradually lost during the process of wetting-drying cycles. Additional research is encouraged to explore an effective method to prevent the deterioration of mechanical properties of lignin stabilized soils in practical projects. © 2019 Elsevier Ltd. All rights reserved.
Handling editor: Zhen Leng Keywords: Stabilization Recycled materials Strength Durability Wetting-drying cycle
1. Introduction Chemical stabilization, using deep mixing (Liu et al., 2011), grouting or jet-grouting methods (Shen et al. 2013, 2017; Horpibulsuk et al., 2003), has been widely used for soft, problematic and contaminated soils (Yin et al., 2006; Latifi et al., 2018a). Traditional calcium-based additives, e.g., Portland cement (Horpibulsuk et al., 2010), quicklime (Bell, 1996) and gypsum (Puppala et al., 2005; Ghosh and Subbarao, 2006) are of the primary concern to the engineering constructors in the past decades.
* Corresponding author. Faculty of Engineering, China University of Geosciences, Wuhan, 430074, China. E-mail addresses: [email protected] (T. Zhang), [email protected] (S. Liu), [email protected] (H. Zhan), [email protected] (C. Ma), focuscai@ 163.com (G. Cai).
Utilization of these traditional stabilizers brings the acknowledged benefits on strength, permeability, and durability of various soils (Horpibulsuk et al. 2004, 2010). Meanwhile, some unsettled issues, such as, high cost of raw materials, huge energy consumption of production, CO2 and nitrogen oxide emission, and alkaline attack on the surround environment (Puppala et al., 2004; Mitchell and Soga, 2005; Higgins, 2007) are left, which are contrary to the requirements of sustainable development. Therefore, usage of industrial by-products or nontraditional additives attracts strong attentions from a view of engineering construction, owing to their potential of saving construction cost and recycling natural resources (Horpibulsuk et al., 2009; Arulrajah et al., 2019; Latifi et al., 2018b; Yaowarat et al., 2019). Earlier researchers discovered many nontraditional chemicals for soil stabilization, such as biomass silica (Hassan et al., 2019), xanthan (Dehghan et al., 2019), gypsum (Latifi et al., 2018b), and calcium carbide residue (Latifi et al.,
https://doi.org/10.1016/j.jclepro.2019.119293 0959-6526/© 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: Zhang, T et al., Durability of silty soil stabilized with recycled lignin for sustainable engineering materials, Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2019.119293
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2018a). These chemicals had good performance in improving soil strength and retarding the migration of contaminants. The previous studies revealed that nontraditional soil additives have great potential to address these issues in an environmentally friendly way. Lignin, an organic by-product of biomass-based industries, has been recognized as an environmentally friendly soil stabilizer that has an ability of strength enhancement for clay (Puppala and Hanchanloet, 1999; Tingle and Santoni, 2003; Kim et al., 2011), silt (Zhang et al., 2018a, 2018b), and silty sand (Santoni et al. 2002, 2005; Chen et al., 2014). Moreover, after lignin stabilization, the other engineering properties of problematic soils in terms of ductility, expansibility, and erosion resistance are dramatically improved (Pengelly et al., 1997; Indraratna et al., 2012; Vakili et al., 2018). Ceylan et al. (2010) selected two types of lignin to stabilize sandy lean clay in Iowa State as a highway subgrade filling. The unconfined compressive strength test results show that lignin stabilized clay with cured 7 days meets the strength requirements specified in the local design specifications. Additionally, they pointed out the important influence of the prepared moisture content on the strength development of specimens, which is consistent with the field test results reported by the authors (Zhang et al., 2017) for lignin stabilized silt. Indraratna et al. (2012) reported that 0.4% lignin (lignosulfonate mixtures) stabilized silty sand possesses a similar erosion resistance as compared to 2% Portland cement stabilized one. The stress-strain behavior of silty sand was dramatically changed after the addition of lignin, and then Chen et al. (2014) proposed a bounding surface plasticity model to capture this special mechanical response. Earlier studies by the authors confirmed that soil particles are drawn closely together with lignin-based cementing materials, and a more stable microstructure was clearly observed via microscopic analysis for silty soil stabilized with lignin (Zhang et al., 2015, 2018a; 2018b). Meanwhile, when rainfall infiltrated into the subgrade consisting of lignin stabilized silty soil, its road performance was marred with respect to the decrease in resilience modulus and the increase in deflection (Zhang et al., 2017). Similar deterioration in mechanical properties is commonly reported for other stabilized soils, such as cemented clay, fly ash treated silty clay (Walker, 1995; Zhang and Tao, 2008). These phenomena revealed that moisture intrusion has a great influence on the stability of engineering properties for the stabilized soils. Before the stabilized soils were used in constructions, a clear understanding of their durability performances should be made. However, the majority of previous researchers have focused on the variation in mechanical properties of several soils with lignin stabilization. The very limited information can be found in the earlier studies regarding the resistance of lignin stabilized soils against the intrusion of adverse environmental conditions (Santoni et al., 2002; He et al., 2017). Additionally, few studies have discussed the deterioration mechanism of lignin stabilized soils, incorporating the influence of soil suction. Based on the above-mentioned literature review, this paper aims to fill the key gaps in the literature by studying the durability of lignin stabilized silty soil. Quicklime, a traditional chemical additive, is selected here as a comparison purpose. An array of laboratory experiments including unconfined compressive strength test (UCS), moisture stability test, wetting-drying cycle test, and pH test were performed to obtain insights into the evolution of engineering performances of the stabilized silt under different conditions. The effects of additive content as well as curing time on the unconfined compressive strength qu, moisture stability coefficient Kr and mass loss Sm were analyzed. The role of suction on qu and the breakage of particle bondings were discussed. The variation in soil pH value with wetting-drying was monitored, and an empirical correlation of pH value with accumulative mass loss Cm was developed. The changes in the microstructure of lignin stabilized silt after suffering
wetting-drying cycles were qualitatively evaluated from scanning electron microscopy (SEM) images. Based on these results, an effort has been made to explore the deterioration mechanism of lignin stabilized soils subjected to the adverse conditions. 2. Materials and methods 2.1. Materials Tested soil was obtained from a highway subgrade construction site at approximate 2.0 m depth in Jiangsu province, China, and its basic engineering property indices are listed in Table 1. This soil is classified as low plastic silt according to the Unified Soil Classification System (ASTM D2487 (2011a)). Chemical compositions of the tested soil determined from X-ray fluorescence (XRF) analysis are shown in Table 2. Lignin used in this study was collected from a paper plant in Henan province, China, which is considered to be a low-valued industrial waste and usually stockpiled in land, resulting in a potential threat to the environmental safety. It is a fine powder at dry state with a yellow-brown color and fragrant smell and is completely soluble in water. After toxicity testing conducted by the Center of Modern Analysis in Nanjing University, the used lignin is verified to be a non-toxic organic material with some cellulose and impurities. In addition, lignin has a good chemical activity due to the presence of both hydrophilic and hydrophobic groups (Zhang et al., 2018a). The pH value of used lignin is 10.2 because the alkaline paper-making process is utilized in this plant. Quicklime, collected from Changzhou city, China with a specific gravity Gs of 2.98, was selected as a traditional soil stabilizer, and its chemical compositions are presented in Table 2. This quicklime is classified as high-calcium lime according to ASTM C51-11 (ASTM, 2011b), which is widely used in the treatment of problematic soils. Deionized water (pH ¼ 6.71, electrical conductivity < 1 mS/m) bought from the market was used in this study to minimize the effect of other chemicals on the test results. 2.2. Sample preparation The collected silt was air-dried and then passed through a sieve with 2 mm opening size. The predetermined weights (dry weight basis) of additive powders (i.e., lignin and quicklime) were added into the prepared silt and mixed thoroughly for about 5 min by resorting to an electrical agitator with a speed of 120 rpm. The soiladditive mixtures were then mixed twice with a designed weight of deionized water (optimum moisture content wopt) for 5 min to obtain homogeneous soil-additive-water mixtures. The homogenized mixtures were poured into a steel mold (inner diameter of
Table 1 Basic engineering properties of tested silty soil. Property
Value
Natural moisture content, wn (%) Specific gravity, Gs Grain size distribution (%)a Clay (<0.002 mm) Silt (0.002e0.074 mm) Sand (0.074e2 mm) Average diameter, d50 (mm) Liquid limit, wL (%)b Plasticity limit, wP (%)b Optimum moisture content, wopt (%) Maximum dry density, rdmax (g/cm3) pH (mass ratio: water/soil ¼ 1)c
28.5 2.71 e 9.7 81.2 9.1 38.7 32.4 23.6 16.1 1.72 8.14
a b c
Measured using a laser particle size analyzer Mastersize 2000. Measured as per China MOT JTG E40-2007. Measured as per ASTM D4972 (2013).
Please cite this article as: Zhang, T et al., Durability of silty soil stabilized with recycled lignin for sustainable engineering materials, Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2019.119293
T. Zhang et al. / Journal of Cleaner Production xxx (xxxx) xxx Table 2 Oxide chemistry of tested soil and quicklime. Oxide chemistrya
Tested soil (%)
Quicklime (%)
Silicon oxide (SiO2) Aluminium oxide (Al2O3) Calcium oxide (CaO) Ferric oxide (Fe2O3) Potassium oxide (K2O) Magnesium oxide (MgO) Sodium oxide (Na2O) Others Loss on ignitionb
63.20 12.53 6.41 3.12 2.46 2.39 2.30 7.59 5.81
2.62 1.16 71.23 0.74 0.18 0.46 0.20 23.41 24.36
a Mineral composition was analyzed by X-ray flourescence method using ARL9800XP þ XRF spectrometry. b Value of loss on ignition is referenced to 950 C.
Table 3 Values of compaction parameters for lignin and quicklime stabilized silt (from Zhang et al., 2018a). Additive content
2 5 8 12 15
Lignin stabilized silt
as chemically treated soil. The UCS test of such stabilized soil was conducted by using strain-controlled application of test load. The load was applied continuously and without shock to produce an approximately constant rate of deformation. Cylindrical soil samples with a height/diameter ratio of 2 were selected for the UCS test by fixing the strain rate as 1%/min. The testing procedure followed as per ASTM D4219 (ASTM, 2008) to determine the crushing strength of the stabilized soils. The moisture stability test of the stabilized silt was performed as per China MOHURD (2015) CJ/T 486e2015. The samples with standard curing of 6 days and 27 days were selected and then soaked in the deionized water for 1 day, after that, the qu was measured as followed the same procedures previously described for UCS test. The beaker with a volume capacity of 1.2 L was used to make sure that the sample is completely submerged into the water, as shown in Fig. 1. Under the same curing time (i.e., 7 days or 28 days), the strength ratio of 1 day soaking to standard curing is defined here as moisture stability coefficient Kr:
Quicklime stabilized silt
wopt (%)
rmax (g/cm3)
wopt (%)
rmax (g/cm3)
11.52 11.57 11.62 14.03 12.85
1.81 1.82 1.83 1.82 1.82
16.52 16.71 17.87 18.23 18.79
1.68 1.65 1.60 1.58 1.51
50 mm; height of 100 mm) with three layers and compacted to the design degree of compaction (96% of the maximum dry density rmax) by a hydraulic jack at a constant lifting rate. The values of wopt and rmax of the stabilized silt were determined from the results of the standard Proctor compaction test that were reported in earlier studies (Zhang et al., 2018a), as shown in Table 3. The cylindrical specimen was carefully extruded from the mold and wrapped by a vinyl bag to minimize moisture evaporation, and then placed in an ambient condition controlled room (22 C and relative humidity of 95%) to the target curing time (viz., standard curing). A soil block of approximately 1 cm3 was collected from the hand-broken identical specimens for SEM analysis. The SEM specimens were quickly frozen using the liquid nitrogen, after which the specimens were placed into a vacuum chamber with a temperature of 80 C for 24 h. The freeze-dried specimens were coated with gold first, and then subjected to the SEM analysis to capture the microstructural images. 2.3. Testing methods Lignin stabilized silt investigated in this study can be classified
2% Lignin
3
5% Lignin
Kr ¼
qut qu0
(1)
where qu0 is the compressive strength of standard curing (kPa); and qut is the compressive strength of specimen with 1 day soaking (kPa). The higher Kr value, the stronger resistance against the moisture intrusion. The wetting-drying cycle test followed the recommendations specified by ASTM D559 (ASTM, 2015), while the drying temperature was set as 30 C and the wetting process was performed firstly in this study. Selecting 30 C is to protect the lignin-based materials that present in the specimens from destruction. The authors previously used 60 C (recommended temperature for chemical cemented soils) to dry lignin stabilized specimens (lasted 24 h) and found that the color of specimen surface became darken, accompanied with a pungent smell. This is probably due to the destruction of lignin under this high temperature. After turned down the temperature as 30 C, these negative phenomena vanished. The temperature modification in this study was also consistent with that reported by Kamon et al. (1993) for cement-/slag-stabilized soils. The identical specimens of UCS test were used for the wetting-drying cycle test. These samples firstly were immersed in the deionized water for 24 h with normal room temperature (approximately 20 ± 2 C), and then placed in the oven (30 C) for 48 h. Therefore, such one wetting-drying cycle needs 3 days to be completed, and the Sm after each drying process was recorded. The terminal criterion of the wetting-drying cycle test is either specimen fractured into at least two parts or Cm exceeds 30%. The mass loss Smi value after the ith cycle is computed by Equation (2):
8% Lignin
12% Lignin
15% Lignin
Fig. 1. Photo of lignin stabilized specimens during moisture stability test.
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Table 4 Additive content, curing time, and soaking condition for various tests in this study. Test type
Lignin content (%)
Lime content (%)
Soaking time (d)
Curing time (d)
UCS test Moisture stability test W-D cycle test pH test SEM
0, 2, 5, 8, 12, 15 2, 5, 8, 12, 15 12 2, 5, 8, 12, 15 12
0, 2, 5, 8, 12, 15 2, 5, 8, 12, 15 8 e e
e 1 1 per cycle e e
1, 7, 28, 60 7, 28 7, 28 7, 28 28
Note: UCS test is unconfined compressive strength test; W-D is wetting-drying.
Smi ¼
mi1 mi 100% m0
(2)
where m0 is the initial mass of dry sample prior to the wettingdrying cycle test (g); mi-1 and mi are the mass values measured immediately after (i-1)th and ith cycle, respectively (g), and the subscript i represents the number of wetting-drying cycles. The cumulative mass loss Cm at the ith cycle is calculated by:
Cmi ¼
X Smi
(3)
i¼1
The soil pH measurement was conducted according to ASTM D4972 (ASTM, 2013), where 10 g of the sieved soil powder was added into 10 mL deionized water in a beaker, and then mixed thoroughly by using a glass rod and left to the stationary state for 1 h. After that, the pH meter (HORIBA D-54, Japan) was placed into the supernatant and the pH value was measured. The microstructural characterizations of lignin stabilized silt before and after the wetting-drying cycle test were observed using LEO 1530VP (LEO Instruments, Germany) device. The small fragments (approximately 1 cm3) from the core of specimens were dehydrated as the following steps: (i) make the fragments immerse in the liquid nitrogen (boiling point of 195 C) for a quick freezing; (ii) place the frozen fragments into a freezing unit (temperature of 80 C) with a vacuum chamber for 24 h for sublimation; (iii) coat the freeze-drying fragments with gold before SEM analysis. Triplicate samples were prepared and tested for qu, Kr, Sm, and pH, and the average values were shown in this study. For the sake of clearness, additive content, curing condition and curing time of the stabilized specimens are summarized in Table 4. According to the UCS test results, it is observed that 12% lignin stabilized specimen shows the highest qu value, and therefore, only silt stabilized with 12% lignin and 8% quicklime (for comparison) were selected for the wetting-drying cycle and the test results were presented hereafter in the paper. Fig. 2. Evolution of unconfined compressive strength of the stabilized soils with curing time: (a) lignin stabilized silt and (b) quicklime stabilized silt.
3. Results and discussion 3.1. Unconfined compressive strength The evolution of unconfined compressive strength qu of lignin stabilized and quicklime stabilized silt with curing time is shown in Fig. 2. It can be observed that an increasing trend of qu with curing time is easily observed regardless of the additive type and dosage. For lignin stabilized specimens, the majority increment in qu is generated within 28 days, and with curing time further increases to 60 days, the qu value keeps almost a constant for each additive content (see Fig. 2(a)). This reveals that approximately one month curing is a necessary requirement to ensure the completion of lignin stabilization, and further increment of curing time has little benefit to strength enhancement. The qu value increases with increasing of lignin content from 2% to 12%,
but when lignin content exceeds 12%, a slight decrease is found. Therefore, the optimum lignin content for tested silt is about 12% in terms of compressive strength. Chen et al. (2014) reported that 2% lignin (lignosulfonate mixtures) treated specimen has the highest qu value, and the qu value decreases slightly once lignin content exceeds 2%. This evolution trend of qu is consistent with that presented in this study. The reason for the qu reduction in lignin stabilized soils probably is the presence of some local weak spots, resulting from the excessive free lignin (Zhang et al., 2018a). Quicklime stabilized silt shows a similar increasing trend of qu to that of lignin case, while some different characteristics also need to be indicated. Firstly, quicklime presents a superiority on strength enhancement with respect to the qu value as compared to lignin.
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Within the curing time of 28 dayse60 days, an obvious increment in the qu value is still presented, as shown in Fig. 2 (b). Increase in quicklime content has a positive effect on the strength improvement of the stabilized silt, and the highest qu value is found in 15% quicklime case. Based on the aforementioned results, it could be deduced that all the discrepancy in strength evolution is mainly attributed to the difference of stabilization mechanism between lignin and quicklime (Vinod et al., 2010). It is noteworthy that suction has an influence on the compressive strength of unsaturated specimens. Although the soil suction measurements have not been conducted in this study, this cannot prevent us from making a preliminary evaluation of the mechanical response of lignin stabilized silt. The suction value of the compacted silt without lignin (qu is 131 kPa) can be approximately estimated to be 110 kPa at an effective friction angle of 21. It is acknowledged that addition lignin into silt would inevitably
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change the soil suction of the compacted specimens. For simplicity, we selected the suction value of the compacted natural silt (i.e., 110 kPa) as a reference scale to the stabilized specimens. After 60 days of curing, all the lignin stabilized specimens have a dramatic strength increment, and the magnitude of this increment is much greater than the reference suction value. Taking 12% lignin stabilized specimen as an example, the ultimate increment of qu is 582 kPa that is approximately 5 times as much as the reference suction value. This great discrepancy implies that qu of lignin stabilized silt after curing is mainly generated from lignin cementation, followed by suction. At the initial stages of curing (less than 7 days), qu of the stabilized silt is mostly controlled by suction. These conclusions are also applicable to the case of quicklime stabilized silt. Additional matric suction testing of the stabilized silt with different lignin contents is warranted to further assess the influence of suction on the strength. This
(a)
(b)
(c)
(d)
(e)
Fig. 3. Photos of lignin stabilized specimens with additional 1 day soaking in moisture stability test: (a) 2% lignin; (b) 5% lignin; (c) 8% lignin; (d) 12% lignin; (e) 15% lignin.
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recommended work would be beneficial to further explore the deterioration mechanism of lignin stabilized soils in adverse environments. 3.2. Moisture stability After soaking in the deionized water for 1 day, the photos of lignin stabilized specimens with cured 28 days were captured and presented in Fig. 3 to obtain macroscopic insights into the moisture stability. For 15% lignin stabilized specimen, surface spalling as well as some tiny fragments at the bottom of beaker are found in Fig. 3(e). Additionally, the color of soaking water in 15% lignin case has changed from transparent to light yellow, which probably results from the dissolution of excessive free lignin. This interesting phenomenon provides evidence to verify the reduction in qu once lignin content exceeds 12%, as discussed on the UCS test results. For other lignin content cases, the intensity of both surface spalling and the color change is much less as compared to that of 15% lignin case (see Fig. 3(a)e3(d)). To provide a quantitative evaluation of the effect of moisture intrusion on the strength, the qu values for both standard and additional 1 day soaking conditions are shown in Fig. 4. Compared to the standard curing, an additional 1 day soaking makes qu of lignin stabilized silt having an obvious decrease. The maximum decrement in qu is 269.6 kPa, which is found in 12% lignin stabilized silt with cured 28 days, and 15% lignin case has a very similar value of qu decrement (268.9 kPa). Referring to the studies of Rotta et al. (2003) for the cemented silty sand, soil suction is expected to have reduced significantly after the soaking process, and so nonsaturation is considered to have only a marginal influence on unconfined compressive strength. Therefore, it can be concluded that the deterioration of strength in a soaking test for lignin stabilized silt is mainly due to the following two reasons: reduction of suction and dissolution of lignin-based cementing materials. Moreover, it should be indicated that the dissolution of lignin/lignin-based cementing materials poses a negligible threat to environmental safety. This conclusion has been confirmed by the field test results (Zhang et al., 2017), where lignin stabilized silt is used as the subgrade filling. A parameter of moisture stability coefficient Kr was employed to compare the resistance against moisture intrusion between lignin stabilized and quicklime stabilized silt, and its variation with
Fig. 4. Comparison of unconfined compressive strength for lignin stabilized silt between standard and additional 1 day soaking conditions.
additive contents was shown in Fig. 5. The error bars marked in the figure indicate the size of data variability. It can be observed that the magnitude of Kr value for silt stabilized with lignin is higher than that of quicklime stabilized one. There is no clear correlation between Kr value and lignin content at either 7 days or 28 days curing. The average values of Kr for the specimens with different quicklime contents are around 0.4, and furthermore, with curing time increasing from 7 days to 28 days, a slight decrease in Kr is observed. 3.3. Mass loss After the first cycle of the wetting-drying process, some photos of lignin stabilized specimens with 7 days of curing were collected and shown in Fig. 6. It can be observed that the surface of each specimen has different degrees of damage, and two specimens with low lignin contents (i.e., 2% and 5%) were destroyed into two parts, while the other specimens of high lignin contents (i.e., 8%, 12%, and 15%) can keep a good integrity after one cycle. According to the aforementioned discussion on moisture intrusion, soil suction would be greatly reduced after soaking, and meanwhile, lignin-based cementing materials were gradually dissolved in water. Once the specimens were placed in
Fig. 5. Variation of moisture stability coefficient with additive content: (a) 7 days curing and (b) 28 days curing.
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Spalling Spalling
Connected crack
Spalling
Crack Fracture
7
Spalling
Fracture Spalling
2% Lignin
5% Lignin
8% Lignin
12% Lignin
15% Lignin
Fig. 6. Images of lignin stabilized specimens with cured 7days after first wettingdrying cycle.
the oven (30 C) for drying, soil suction increases dramatically. A very high soil suction would be produced at the dry state, resulting in shrinkage and cracks in the specimens. For silt stabilized with low lignin content (i.e., 2% and 5%), the shrinkage force produced breaks the bonding among soil particles, which is the reason for the surface spalling and fracture. The particle bondings are reinforced as an increase in lignin content, which has a good resistance against the shrinkage. The connected crack in 15% lignin stabilized specimen is probably attributed to the dissolution of excessive free lignin. In summary, lignin-based cementation is marred by the dissolution of cementing materials in the first wetting stage, and the durability of the stabilized silt in the next drying stage depends on the balance between particle bonding and soil suction. We selected 12% lignin stabilized specimens to continue the next wetting-drying cycles until the termination conditions (Cm >30% or fracture) were reached. The results of silt stabilized with 8% quicklime are also presented here for comparison purpose. Fig. 7(a) and (b) show the evolution of mass loss Sm and cumulative mass loss Cm with wetting-drying cycles, respectively. The lignin stabilized specimens with cured 7 days and 28 days are able to suffer 3 cycles and 4 cycles, respectively, while specimens of quicklime stabilization only have 2 cycles. An increasing trend of Sm with the cycles is observed for lignin stabilized silt, however, quicklime stabilized silt presents an opposite trend in Fig. 7(a). Nevertheless, they have similar increasing trends of Cm with an increase in the wetting-drying cycle (see Fig. 7(b)). In addition, the curves of 7 days curing locate above those of 28 days. These observations reveal that: (1) lignin stabilization has superiority on improving the resistance of silt against the wetting-drying intrusion as compared to quicklime stabilization; (2) extension curing time from 7 days to 28 days is beneficial to soil durability, which is consistent with the enhancement of compressive strength.
Fig. 7. Results of wetting-drying test for 12% lignin stabilized silt: (a) mass loss versus dry-wetting cycle and (b) cumulative mass loss versus dry-wetting cycle.
3.4. pH value and microstructure characteristics Fig. 8 shows the variation of soil pH value with the wettingdrying cycle. The fitted lines represent a decreasing trend of pH value with increasing of the wetting-drying cycle, and the exponential function can be used to depict this trend well with a good coefficient of correlation R2 > 0.90. Before the wetting-drying cycle starts, the soil pH values with cured 7 days and 28 days are 9.54 and 9.82, respectively, which are much higher than that of natural silt (pH ¼ 8.14). Furthermore, at the end of the wetting-drying test, the fractured specimens still have a higher pH value. This increment in pH value is attributed to the addition of lignin that has a high pH value of 10.2. Earlier researchers have utilized pH value to characterize the mechanical properties of the stabilized soils and proposed many empirical correlations of pH with compressive strength (Miller and Azad, 2000; Chrysochoou et al., 2009). The authors also found a positive correlation between pH value and qu for lignin stabilized silt in a previous study (Zhang et al., 2018a). The
Fig. 8. Variation of soil pH value with wetting-drying cycle for 12% lignin stabilized silt.
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Fig. 9. Relationship between soil pH value and cumulative mass loss Cm.
measurement work of qu for lignin stabilized silt is not been done in the wetting-drying program, but we could choose parameter Cm instead of a mechanical parameter to develop a correlation with soil pH value. Furthermore, a negative relationship between these two parameters is expected to be obtained. Because the magnitude of Cm represents the integrity level of specimens, deducing that the higher of Cm, the lower value in qu. The relationship between pH value and Cm for lignin stabilized
(a)
silt is presented in Fig. 9. The expected decreasing trend of pH value with an increase in Cm is observed, and a fitted line of exponential function provides a good description of this relationship (R2 ¼ 0.94). It is easy to understand that more destruction in particle bondings would produce a higher value of Cm, corresponding to a lower value in qu as well as a higher pH value (Horpibulsuk et al., 2016). A similar trend of mass loss with an increase in pH value was reported in the literature (Hewayde et al., 2007) for concrete attacked with sulfuric acid. Therefore, this empirical relationship would be useful to provide a preliminary assessment of the durability of such stabilized soil using pH value index. The SEM images of lignin stabilized specimens before and after wetting-drying cycles are shown in Fig. 10. It is evident that after 28 days of standard curing, soil particles are coated and bonded with precipitated cementing materials, and additionally, the pores among the particles are partially filled, as shown in Fig. 10(a) and (c). This stable soil structure is beneficial to the improvement of mechanical property and durability performance. A similar microstructure was observed on silty sand stabilized with lignin, which was used to resist water erosion in embankment dams (Indraratna et al., 2008). After suffering 4 cycles of wetting-drying intrusion, the clear boundaries among the solid particles are observed in Fig. 10(b) and (d). In addition, the particle surface becomes smooth, and the strong connections among particles are hard to be observed in the images. The loss of cementing materials should be responsible for the abovementioned changes in soil microstructure after wetting-drying cycles. These microstructural changes would result in the deterioration of both strength and durability of stabilized soils. The stabilized soils are commonly used as filling materials in
(b)
Cementing materials
5 m
5 m
(d)
(c)
Cementing materials Cementing materials
5 m
5 m
Fig. 10. Microstructure behavior of lignin stabilized silt with 28 days of curing: (a) 8% lignin, standard curing; (b) 8% lignin, after 4 cycles of wetting-drying; (c) 12% lignin, standard curing; (d) 12% lignin, after 4 cycles of wetting-drying.
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infrastructure projects, such as building foundation and highway subgrade. The durability of such stabilized soils is closely related to the service performance of the superstructures. According to the results presented in this paper, it can be confirmed that under the attack of the adverse natural environment, the dissolution of ligninbased cementing materials and deterioration of engineering properties of lignin stabilized silt are found. The similar phenomena were observed in the field construction of highway subgrade, which was conducted by the authors in the earlier time (Zhang et al., 2017). Several engineering protective measures were performed to protect lignin stabilized silt from the attacking of rainfall, for instance, covering waterproof film on the surface of the compacted soil during the curing period, and setting the compacted clay line of low hydraulic conductivity on the filling. These measures provide strong support to the long-term durability of lignin stabilized soil as subgrade filling. Additional protective technology is warranted to improve the durability performance of such stabilized soils in practical projects. 4. Conclusions A laboratory testing program was conducted to investigate the durability of lignin stabilized silt under moisture intrusion and wetting-drying cycle conditions. Quicklime, as a traditional chemical additive, was selected in this study for a comparison purpose. Inclusion of lignin is able to enhance unconfined compressive strength qu of silt, and nearly one month curing is required for the completion of strength improvement. The mechanical response of silt stabilized with lignin is mainly controlled by cementation as well as suction at the unsaturated state. The discrepancy in terms of compressive strength evolution implies that lignin and quicklime have different stabilization mechanisms. A significant decrease in qu was observed after suffering moisture intrusion because of the combined influences of suction loss with the dissolution of lignin-based cementing materials. From the results of moisture stability coefficient Kr, a higher average Kr value was found in silt stabilized with lignin as compared to that of quicklime stabilized silt. After 28 days of curing, lignin stabilized silt is able to resist 4 cycles of wetting-drying intrusion and with cumulative mass loss Cm of approximately 20%. The large suction produced from the drying process has a negative influence on soil particle bonding, which could result in surface spalling and fracture to the specimens of weak lignin cementation. Extension curing time is beneficial to improve the durability of stabilized silt. Soil pH value presents a decreasing trend in the wetting-drying cycles, and an exponential correlation of pH value with Cm is found in this study. The changes in microstructure, including the presence of clear boundaries and the smooth surface of soil particles, provide evidence to the dissolution of lignin-based cementing materials. It is anticipated that a clear understanding of the deterioration mechanism of lignin stabilized silt would be gained with additional micro-chemical analyses. Acknowledgements The funding provided by National Natural Science Foundation of China (Grant No. 41807260) and the Fundamental Research Funds for the Central Universities, China University of Geosciences (Beijing) (CUG170636, CUGL170807) are appreciated. References Arulrajah, A., Mohammadinia, A., Maghool, F., Horpibulsuk, S., 2019. Tire derived aggregates as a supplementary material with recycled demolition concrete for pavement applications. J. Clean. Prod. 230, 129e136.
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Please cite this article as: Zhang, T et al., Durability of silty soil stabilized with recycled lignin for sustainable engineering materials, Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2019.119293