Study on the Stabilization Mechanisms of Clayey Slope Surfaces Treated by Spraying with a New Soil Additive

applied sciences

Article Study on the Stabilization Mechanisms of Clayey Slope Surfaces Treated by Spraying with a New Soil Additive

Cuiying Zhou 1,2,3, Shanshan Zhao 1,2,3, Wei Huang 1,2,3, Dexian Li 1,2,3 and Zhen Liu 1,2,3,*

1 School of Civil Engineering, Sun Yat-sen University, No.135 XinGangXiLu, Guangzhou 510275, China; [email protected] (C.Z.); [email protected] (S.Z.); [email protected] (W.H.); [email protected] (D.L.) 2 Guangdong Engineering Research Centre for Major Infrastructure Safety, School of Civil Engineering, Sun Yat-sen University, Guangzhou 510275, China 3 Research Center for Geotechnical Engineering and Information Technology, Sun Yat-sen University, No.135 XinGangXiLu, Guangzhou 510275, China * Correspondence: [email protected]

Received: 17 February 2019; Accepted: 22 March 2019; Published: 25 March 2019

Abstract: The topsoil of a clayey slope is easily washed off by rain due to its loose structure. To protect the slope surface, in recent years, several types of non-traditional soil additives have been used by means of mixing with soil. In this work, a new organic polymer soil stabilizer, named aqua-dispersing-nano-binder (ADNB), was sprayed on the soil surface to stabilize the topsoil of a clayey slope. To understand the interaction between the polymer and soil particles during the infiltration process as well as the stabilization mechanism, infiltration tests, water stability tests and scanning electron microscopy (SEM) analyses were performed with different polymer contents. The infiltration tests showed that the infiltration rate of the polymer stabilizer in the soil was slower than that of water due to its characteristics of easy adhesion to soil particles, poor fluidity and large molecular volume. The maximum effective infiltration depth was achieved in the specimen treated with 2% ADNB, and the minimum was achieved in the specimen treated with 5% ADNB. The water stability of the soil increased with the content of the soil stabilizer in the soil aggregates with diameters of either 5–10 mm or 10–20 mm. The SEM analysis showed that the quantity of polymer decreased with infiltration depth; a polymer membrane was formed on the surface of the topsoil and chains were formed inside. The amelioration of the soil water stability may have been due to the bonding between soil particles and polymers generated after evaporation of water in the emulsion. The polymer stabilizer could be applied to improve the erosion resistance of the slope topsoil and reduce soil loss.

Keywords: new soil additives; effective inﬁltration depth; water stability; silty clay; stabilization mechanisms

1. Introduction Clayey slopes are extensively distributed in southern China. The topsoil of a clayey slope is easily washed off by rain due to its loose structure and weak water stability [1]. The loss of topsoil of clayey slopes causes great damage to the environment. With the loss of soil, the clayey slope becomes unstable due to the reduction in soil strength and the lack of reinforcement of plant roots. Traditional methods to protect the slope surface include physical methods and inorganic chemical stabilization methods. Physical methods, such as geotextiles, support the external forces without modifying the soil properties [2–5]. Although inorganic chemical stabilization methods can greatly improve the

Appl. Sci. 2019, 9, 1245; doi:10.3390/app9061245 www.mdpi.com/journal/applsci Appl. Sci. 2019, 9, 1245 2 of 13 engineering properties of soils and have been widely applied [6–13], the inorganic materials in soils will inhibit plant growth, which is unfavorable to the ecological development of slopes [14,15]. In recent decades, non-traditional chemical additives such as polymers, enzymes, ionic substances, lignosulfonates, salts and petroleum resins have been studied [16–27]. Non-traditional soil stabilization additives consist of a variety of chemical agents that are diverse in their composition and in the ways they interact with the soil. Many researchers have studied stabilization additives to improve the physical and engineering properties of soil to meet the requirements of different engineering practices [28,29]. Calcium carbide residue (CCR), a by-product of the acetylene production process, was investigated for its ability to improve the engineering properties of clays [30]. A new calcium-based powder additive named SH-85 was used for the stabilization of a tropical residual laterite soil [31]. Xanthan gum was proposed as an environmentally friendly stabilizer that could improve the engineering properties of both low- and high-swelling clays [32]. SS299 was a liquid polymer stabilizer used as a compaction aid or a stabilizer for soil improvement that could increase the unconfined compressive strength of kaolin [20]. Biosolids have been reported to modify the aggregate stability [33]. Polyacrylamide could improve the water stability of clay aggregates [34,35]. However, these studies were all based on the method of mixing stabilizers and soils. An effective way to protect the topsoil of a slope is to spray the stabilization additive onto the surface of the slope without any disturbance. In this case, the stabilization additive should be able to dissolve or disperse in water, unlike organic solvents, which are harmful to the environment. Therefore, it was important to conduct a survey to fully understand the effective infiltration depths of stabilization additives. A new soil additive named aqua-dispersing-nano-binder (ADNB) was applied in this work. Efforts were made to study the mechanism of the water stability of silty soil that had been modified by the polymer emulsion. Various tests were performed with different polymer contents including infiltration tests to obtain the effective depth that the polymer could reach, water stability tests to estimate the water stability of the modified silty clay and scanning electron microscopy (SEM) to analyse the possible mechanism that contributed to the stabilization process. Furthermore, the infiltration mode of the polymer emulsion was identified, and the possible stabilization mechanism was discussed based on the infiltration process and SEM results.

2. Materials and Testing Programs

2.1. Materials

2.1.1. Silty Clay The silty clay examined in this study was obtained from Guangzhou city of Guangdong Province, China. The basic properties were determined according to the methods in GB/T 50123-1999, a standard of soil testing methods in China. Table1 summarizes the physical properties of the soil.

Table 1. Characteristics of the natural soil.

Physical Properties Values Speciﬁc gravity 2.70 Liquid limit (%) 25.76 Plastic limit (%) 14.10 Plasticity index (%) 11.66 BS classiﬁcation MLS Maximum dry density (g/cm3) 1.864 Optimum moisture content (%) 15.10

2.1.2. Organic Polymer The organic polymer emulsion named ADNB applied in this work as a soil additive was a white viscous liquid, as shown in Figure1a. As a new type of soil additive, ADNB could be dispersed in Appl. Sci. 2019, 9, 1245 3 of 13 water as nanoscale particles, and its suitable anti-hydrolysis properties resulted in ADNB always Appl. Sci. 2018, 8, x FOR PEER REVIEW 3 of 13 retaining its polymer chains, even when dispersed in water. Upon reducing the ADNB solution viaretaining water evaporation, its polymer ADNB chains, would even when turn intodispersed its membrane in water. form, Upon which reducing had the a suitable ADNB elasticitysolution via (Figurewater1b). evapora Whention, the ADNBADNB membranewould turn wasinto immersedits membra inne water, form, itwhich became had soft a suitable but remained elasticity intact (Figure and1b). permeable. When the The ADNB ﬁnal degradationmembrane was products immersed of ADNB in wa wereter, it CO became2 and Hsoft2O, but which remained are friendly intact to and thepermeable. environment The [36 final,37]. degrada The physicaltion products properties of of ADNB ADNB were are providedCO2 and inH2 TableO, which2. are friendly to the

(a) (b)

FigureFigure 1. Different 1. Different states states of the of aqua-dispersing-nano-binder the aqua-dispersing-nano-binder (ADNB) polymer(ADNB) emulsion:polymer emulsion: (a) initial (a) stateinitial (left), state dispersed (left), dispersed in water (right); in water (b )(right); after water (b) after evaporation, water evaporation, an elastic ﬁlman elastic is generated. film is generated.

Table 2. Characteristics of the organic polymer emulsion. Table 2. Characteristics of the organic polymer emulsion. Physical Properties Values Physical properties Values 3 SpecificSpecific gravity gravity (g/cm (g/cm) 3) 1.011.01 pH 6–7 pH 6–7 Solid content (%) 41 WaterSolid solublecontent (%) No 41 Water soluble No 2.2. Sample Preparation 2.2. Sample Preparation 2.2.1. Test of the Effective Infiltration Depth 2.2.1. Test of the Effective Infiltration Depth To ensure the uniformity of the specimens, the silty clay was oven dried and sieved through a 2 mm mesh.To ensure Six samples the uniformity were prepared of the atspecimens, the same time.the silty For clay each was sample, oven 15dried g of and soil sieved was first through placed a 2 in amm tube, mesh. which Six was samples 15 mm were in diameter prepared and at 80the mm same in time height. For and each closed sample, off at 15 one g of end soil by was a permeable first placed film.in Then,a tube, the which tubes was were 15 mm vibrated in diameter until all and the 80 soil mm samples in height had and a height closed of off 50 at mm. one end by a permeable film.To ensureThen, the that tubes the soilwere samples vibrated could until be all completely the soil samples infiltrated, had a a height certain of quantity 50 mm. of water was measuredTo andensure filled that in athe container soil samples to reach could a moisture be complete contently infiltrated, of 20%, which a certai wasn betweenquantity theof water plastic was andmeasured liquid limits and of filled soil. in Polymer a container emulsions to reach with a moisture weights ofcontent 0.5%, of 1.0%, 20%, 2.0%, which 3.0% was and between 5.0% of the that plastic of theand dry soilliquid were limits dispersed of soil. in Polymer prepared emulsions water to obtainwith we polymerights of solutions 0.5%, 1.0%, with 2.0%, different 3.0% concentrations. and 5.0% of that Finally,of the the dry five soil solutions, were dispersed and pure waterin prepared as a reference, water to were obtain sprayed polymer slowly solutions onto the soilwith surface different in theconcentrations. six prepared Finally, test tubes. the five The solutions, evolution and of thepure infiltration water as a depths reference, of the were solutions sprayed in slowly the soil onto wasthe recorded. soil surface in the six prepared test tubes. The evolution of the infiltration depths of the solutions inAccording the soil was to recorded. engineering practices, the part of the soil in which the polymer emulsion achieved thoroughAccording infiltration to should engineering have a practices, high degree the of part water of the stability. soil in Hence, which thethe effectivepolymer infiltration emulsion achieved depth of thethorough polymer infiltration emulsion should could be have determined a high degree by immersing of water the stability. soil samples Hence, in waterthe effective and measuring infiltration thedepth remaining of the height, polymer H, ofemulsion the soil could samples be afterdetermined immersion. by immersing To perform the the soil experiments, samples in thewater test and tubesmeasuring were fixed the vertically remaining in height, a dry container H, of the withoutsoil samples contacting after immersion. the bottom. To Then, perform the containerthe experiments, was slowlythe filledtest tubes with waterwere fromfixed thevertically bottom untilin a thedrysoil container was completely without undercontacting water the (Figure bottom.2). The Then, time the container was slowly filled with water from the bottom until the soil was completely under water (Figure 2). The time of immersion until collapse occurred was measured as 10 min. Three specimens were tested for each experiment, and their average was calculated as the result of the experiment. Appl. Sci. 2019, 9, 1245 4 of 13 of immersion until collapse occurred was measured as 10 min. Three specimens were tested for each Appl.experiment, Sci. 2018, 8 and, x FOR their PEER average REVIEW was calculated as the result of the experiment. 4 of 13

Figure 2. Apparatus of the effective infiltration depth test. Figure 2. Apparatus of the effective infiltration depth test. 2.2.2. Test of the Water Stability 2.2.2. Test of the Water Stability The water stability of aggregates has a significant impact on the erosion resistance of the slope topsoil.The Aggregates water stability with of good aggregates water stability has a significant will not be impact washed on offthe easily, erosion thereby resistance reducing of the the slope loss topsoil.of soils andAggregates nutrients with on thegood slope. water Combined stability withwill not vegetation be washed growth off easily, promoted thereby by the reducing good structure the loss ofbetween soils and soil nutrients aggregates, on the the slope. erosion Combined resistance with of the vegetation slope topsoil growth can promoted be further by improved the good understructure the betweenjoint action soil of aggregates, self-stability the of erosion aggregates resistance and reinforcement of the slope topsoil of soils can by be vegetation. further improved First, the under soils were the jointoven action dried of and self-stability sieved through of aggregates a 2 mm mesh and reinfo to ensurercement homogeneity of soils by ofvegetati the soil.on. The First, dried the soils were were oventhen mixeddried and well sieved with waterthrough to a the 2 mm optimum mesh to moisture ensure homogeneity content (OMC), of the which soil. was The determineddried soils were by a thenstandard mixed Proctor well with compaction water to test the (GB/T optimum 50123-1999). moisture Thereafter, content (OMC), the humid which soils was were determined placed in sieves by a standardwith mesh Proctor sizes of compaction approximately test 20,(GB/T 10 and 50123-1999) 5 mm. The. Thereafter, sieves were the shaken humid to dividesoils were the aggregatesplaced in sievesinto two with groups: mesh thesizes aggregates of approximately that remained 20, 10 onand the 5 10mm. mm The (the sieves diameter were ofshaken the aggregates to divide wasthe aggregatesbetween 10 andinto 20two mm, groups: which wasthe notedaggregates as an averagethat remained diameter on of the 15 mm)10 mm and 5(the mm diameter (the diameter of the of aggregatesthe aggregates was was between between 10 and 5 and 20 10 mm, mm, which which was was noted noted as as an an average average diameter of 7.515 mm) sieves.and 5 mm (theThe preparationdiameter of ofthe the aggregates polymer solution was between was the 5same and as10 thatmm, used which totest was the noted effective as an infiltration average diameterdepth. The of different7.5 mm) sieves. polymer solutions were then sprayed onto the surfaces of the different groups of aggregatesThe preparation and kept of atthe a polymer constant solution temperature was the of 25same◦C as for that three used days. to test The the results effective are collectedinfiltration in depth.Table3 ,The where different the notation polymer of thesolutions test group were included then sprayed the average onto the diameter surfaces of of the the aggregates different andgroups the ofapplied aggregates solution and (UNT kept indicatesat a constant water temperature without ADNB). of 25 °C for three days. The results are collected in TableThe 3, where water the stability notation test of required the test immersinggroup included the aggregates the average in di staticameter water. of the Fifty aggregates aggregates and were the appliedrandomly solution placed (UNT on a sieveindicates covered water by without filter paper. ADNB). To avoid the sudden disintegration of aggregates, immersionThe water was stability begun with test required capillarity; immersing water was the added aggregates to a level in static at which water. the Fifty filter aggregates paper was were just randomlyimmersed. placed Thereafter, on a sieve water covered was slowly by filter added paper. until To all avoid aggregates the sudden were disintegration submerged. The of aggregates, immersion immersiontime was set was as 10begun min, with and the capillarity; cumulative water number was added of collapsed to a level aggregates at which was the recorded filter paper every was minute just immersed.for 10 min. Thereafter, The water stabilitywater was index slowly K was added calculated until all with aggregates the following were submerged. formula: The immersion time was set as 10 min, and the cumulative number of collapsed aggregates was recorded every (a × 5 + a × 15 + a × 25 + ··· + a × 95 + a × 100) minute for 10 min. TheK = water1 stability2 index K3 was calculated10 with the following∞ formula: （a ×5+ a ×15+ a ×2550+⋅⋅⋅+ a ×95+ a ×100） K = 1 2 3 10 ∞ where a1, a2, a3 ... a10 represent the number of collapses50 at an immersion time of 1 min, 2 min, 3 min, where... 10 mina1， anda2，a∞a3is thea10 number represent of aggregates the number that of remainedcollapses at intact an immersion after 10 min. time The of coefficients 1 min, 2 min, of 5,3 min, …10 min and a∞ is the number of aggregates that remained intact after 10 min. The coefficients of 5, 15, 25 and 100 represented the water stability of the soil aggregates during each minute. That means that if an aggregate collapsed within 1 min, its coefficient was defined as 5, which was calculated by (0 + 10)/2. If an aggregate collapsed between 1 and 2 min, its coefficient was defined as Appl. Sci. 2019, 9, 1245 5 of 13

15, 25 and 100 represented the water stability of the soil aggregates during each minute. That means Appl.that Sci. if an 2018 aggregate, 8, x FOR PEER collapsed REVIEW within 1 min, its coefficient was defined as 5, which was calculated5 of by13 (0 + 10)/2. If an aggregate collapsed between 1 and 2 min, its coefficient was defined as 15, which was15, which calculated was calculated by (10 + 20)/2. by (10 If + the 20)/2. aggregate If the aggregate never collapsed never collapsed during the during immersion the immersion time of 10 time min, ofits 10 coefficient min, its wascoefficien definedt was as 100.defined The asK-value 100. The had K a-value range had of 5 a to range 100. Aof higher5 to 100.K-value A higher represented K-value representedhigher water higher stability water of the stabil soility aggregate. of the soil aggregate.

3. Results and and Discussion Discussion

3.1. The Effective Infiltration Infiltration Depth Figure 33 shows the the evolution evolution of of the the infiltration infiltration depth depth of the of polymer the polymer solution solution or water. or water.Water Waterinfiltrated infiltrated the soil the faster soil fasterthan the than polymer the polymer solution. solution. Approximately Approximately three three hours hours were were required required for waterfor water to completely to completely infiltrate infiltrate the the entire entire sample sample (50 (50 mm). mm). Within Within the the same same period, period, the the 0.5% 0.5% ADNB solution infiltrated infiltrated 30 mm, which was 3/5 of of the the whole whole length. length.

50 UNT 0.5% ADNB 1.0% ADNB 40 2.0% ADNB 3.0% ADNB 5.0% ADNB

） 30 mm （

20 Depth

0.1 1 10 100 200 Time（min） FigureFigure 3. Evolution 3. Evolution of ofthe the infiltration inﬁltration depth depth ofof thethe polymer polymer solution solution or water.or water.

However, thethe observed observed infiltration infiltration depth depth of the of solution the solution may not may be exactlynot be the exactly effective the infiltration effective infiltrationdepth of the depth polymer, of the which polymer, may bewhich characterized may be characterized by the remaining by the length remaining of the soillength samples of the after soil 10samples min of after immersion 10 min of (shown immersion in Figure (shown4). The in Figure soil sample 4). The treated soil sample with water treated completely with water disintegrated completely withindisintegrated 1 min. Thewithin sample 1 min. treated The sample with 2% treated ADNB wi hadth a2% maximum ADNB had effective a maximum infiltration effective depth infiltration of 5.5 mm depth(Figure of5). 5.5 mm (Figure 5). Appl. Sci. 2019, 9, 1245 6 of 13 Appl. Sci. 2018,, 8,, xx FORFOR PEERPEER REVIEWREVIEW 6 of 13

3 Effective depth (mm) depth Effective Effective depth (mm) depth Effective Effective depth (mm) depth Effective 2

0.0 1.0 2.0 3.0 4.0 5.0 Polymer content (%) Polymer content (%) FigureFigure 4. 4.TheThe effective effective infiltration inﬁltration depth depth of of ADNB. ADNB.

0.5 1.0 2.0 3.0 5.0

FigureFigure 5. R 5.esidualsResiduals of ofthe the different different soil soil samples after after immersion. immersion.

When the concentration of the polymer emulsion was was lower lower than than 2% 2% ADNB,ADADNB, the water-stable lengthlength ofof thethe the soilsoil soil samplesample sample increasedincreased increased withwith with thethe the coco concentrationncentration of of the the polymer polymer emulsion. emulsion. However, However, a polymera polymer emulsionemulsion concentration concentration higher higher than than 2% re 2%sulted resulted in an in adverse an adverse effect: effect: the effective the effective water- stablewater-stable length lengthof the ofsoil the sample soil sample treated treated with 5% with ADNB 5% ADNBwas only was 2.5 only mm, 2.5 which mm, whichwas 45% was of 45%the maximum.of the maximum. Since the Since polymer the polymer emulsion emulsion was viscous was viscousand easily and adhered easily adhered to the soil to theparticles, soil particles, during theduring infiltration the infiltration and adherence and adherence of the polymer of the polymer emulsion emulsion to the tosoil, the small soil, small particles particles agglomerated agglomerated and theand number the number of pores of pores in the in soil the decreased. soil decreased. The po Thelymerlymer polymer solutionsolution solution withwith aa with highhigha concentrationconcentration high concentration easilyeasily reachedeasily reached the infiltration the infiltration limit where limit wherethe pores the poresof the ofsurface the surface soil layer soil layerwere werecompletely completely filled filledwith polymerwith polymer solution solution and an and impermeable an impermeable layer layerhad formed had formed through through which which polymers polymers could couldnot pass. not Thus,pass. Thus,the additional the additional polymer polymer emulsi emulsionon was prevented was prevented from infiltrating from infiltrating and accumulated and accumulated to a certain to a thicknesscertain thickness on the onsurface the surface (Figure (Figure 6). The6). higher The higher the concentration the concentration of the of polymer the polymer solution solution was, was, the quickerthe quicker the impermeable the impermeable layer layer formed, formed, and and the theless less the thepolymer polymer solution solution could could infiltrate infiltrate the thesoil; soil; as aas result, a result, a decrease a decrease in the in the effectiv effectivee water-stable water-stable depth depth was was observed. observed.

FigureFigure 6. Accumulation 6. Accumulation of of the the polymer polymer emulsionemulsion on on the the surface. surface.

3.2. Water Stability of the Aggregates Appl. Sci. 2019, 9, 1245 7 of 13

3.2. Water Stability of the Aggregates The cumulative number of disintegrated aggregates was recorded every minute for 10 min and the stability percent values, noted as K, are shown in Table3. A signiﬁcant increase in the water stability of the aggregates was observed due to the ADNB treatment. For example, in the test group of 7.5 mm aggregates, K increased to over 90% when treated with 3% ADNB, which was approximately 17 times higher than that of the untreated aggregates. Similarly, K increased to 98.5% when treated with 5% ADNB, which was approximately 19 times higher than that of the untreated aggregates. Furthermore, less soil was washed off and/or fewer aggregate structures were destroyed by water due to the signiﬁcant increase in the water stability of the aggregates. As a result, the erosion resistance of slope topsoil was improved.

Table 3. Static water stability of the different aggregates.

Immersion Time (min) Test Group Stability K (%) 1 2 3 4 5 6 7 8 9 10 7.5-UNT 50 50 50 50 50 50 50 50 50 50 5 15-UNT 50 50 50 50 50 50 50 50 50 50 5 7.5-ADNB 0.5% 8 35 50 50 50 50 50 50 50 50 16.4 15-ADNB 0.5% 10 20 21 34 35 45 50 50 50 50 32 7.5-ADNB 1.0% 0 5 13 20 30 38 40 45 48 50 47.2 15-ADNB 1.0% 0 0 0 5 11 25 30 41 45 45 64.1 7.5-ADNB 2.0% 5 5 8 13 16 21 27 30 32 35 65.1 15-ADNB 2.0% 0 0 0 0 0 0 0 5 5 10 97 7.5-ADNB 3.0% 0 0 0 3 3 5 5 8 10 13 91.9 15-ADNB 3.0% 0 0 0 0 0 0 0 0 0 0 100 7.5-ADNB 5.0% 0 0 0 0 0 0 0 3 3 3 98.5 15-ADNB 5.0% 0 0 0 0 0 0 0 0 0 0 100

It was also observed that even when treated with the same concentration of polymer solution, the K values of the 15 mm aggregates were higher than those of the 7.5 mm aggregates. For example, when treated with 2% ADNB, the K value of of the 15 mm aggregate was 97%, which was 1.5 times that of the 7.5 mm aggregate. Additionally, when treated with 3.0% ADNB and 5.0% ADNB, the K values of the samples increased to 100%, which indicated that the aggregates maintained their integrity during the immersion period of 10 min. This result could be explained by the fact that the aggregates with larger diameters had smaller specific surface areas, on which there were larger amounts of effective polymer emulsions than on the aggregates with smaller diameters. Figure7 shows the comparison of the K values of the aggregates with average diameters of 7.5 mm and 15 mm. Almost no difference in the K value was observed when the aggregates were treated with a high-concentration polymer solution (higher than 2% ADNB). This result may have been due to the amount of polymer that exceeded the amount required for bonding in the samples. Based on the analysis results obtained from the effective infiltration test, the polymer solution with a concentration of 3%/5% ADNB could rapidly fill the pores of the surface soil layer due to its high proportion of polymer material, which led to the halting of infiltration of the additional polymer emulsion and the formation of a thick film on the surface. The film could then prevent the aggregates from disintegration during the 10 min immersion (Figure8). Appl. Sci. 2018, 8, x FOR PEER REVIEW 8 of 13

100 7.5 mm 15 mm

80 ）

% 60 （

Appl. Sci. 2019, 9, 1245 8 of 13 Appl. Sci. 2018, 8, x FOR PEER40 REVIEW 8 of 13 Stability K Stability 100 7.5 mm 20 15 mm

B ）

% 60 UNT DNB ADN A ADNB

（ % .0 0.5% 1 2.0% ADNB 3.0% ADNB 5.0% Figure40 7. K values of the different aggregates treated with ADNB. Stability K Stability Figure 7 shows the comparison of the K values of the aggregates with average diameters of 7.5 mm and 15 mm. Almost20 no difference in the K value was observed when the aggregates were treated with a high-concentration polymer solution (higher than 2% ADNB). This result may have been due to the amount of polymer that exceeded the amount required for bonding in the samples. Based on the analysis results obtained from the effective infiltration test, the polymer solution with a concentration of 3%/5% ADNB could rapidlyB fill the pores of the surface soil layer due to its high UNT DNB ADN A ADNB proportion of polymer material, which led to% the halting of infiltration of the additional polymer .0 emulsion and the formation of a thick0.5% film on1 the surface.2.0% ADNB The 3.0%film ADNB could5.0% then prevent the aggregates from disintegration duringFigure 7.th Ke valuesvalues10 min of of immersion the different (Figure aggregates 8). treated with ADNB.

Figure 7 shows the comparison of the K values of the aggregates with average diameters of 7.5 mm and 15 mm. Almost no difference in the K value was observed when the aggregates were treated with a high-concentration polymer solution (higher than 2% ADNB). This result may have been due to the amount of polymer that exceeded the amount required for bonding in the samples. Based on the analysis results obtained from the effective infiltration test, the polymer solution with a concentration of 3%/5% ADNB could rapidly fill the pores of the surface soil layer due to its high proportion of polymer material, which led to the halting of infiltration of the additional polymer emulsion and the formation of a thick film on the surface. The film could then prevent the aggregates from disintegration during the 10 min immersion (Figure 8). Figure 8. Aggregates treated with 0.5% ADNB (left) and 5.0% ADNB (right).

4. Infiltration Infiltration Mode and Stabilization MechanismMechanism The results of the tests performed in this work showed that the ADNB polymer was an effective soil stabilization material which infiltratedinfiltrated the soil and formed a protective shell of a certain thickness on the soil surface. The process of infiltrationinfiltration and the stabilization mechanism of the polymer emulsion are shown in Figure9 9.. TheThe untreateduntreated aggregatesaggregates werewere composedcomposed ofof numerousnumerous particlesparticles andand containedcontained porespores ofof varying sizes benefitingbenefiting the circulation of air andand waterwater (Figure(Figure9 9a).a). InIn contrastcontrast toto thethe penetrationpenetration behavior of water in soil, viscou viscouss polymer emulsions enclosed soil particles through bonding during infiltration.infiltration. TheTheFigure thickness thickness 8. Aggregates of of the the polymer polymertreated emulsion with emulsi 0.5% on onADNB the on outer (left)the aggregate andouter 5.0% aggregate ADNB particles (right). particles was more was important more thanimportant that of than emulsion that of on emulsion the inner on particles the inner (Figure particles9b). (Figure 9b). 4. Infiltration Mode and Stabilization Mechanism The results of the tests performed in this work showed that the ADNB polymer was an effective soil stabilization material which infiltrated the soil and formed a protective shell of a certain thickness on the soil surface. The process of infiltration and the stabilization mechanism of the polymer emulsion are shown in Figure 9. The untreated aggregates were composed of numerous particles and contained pores of varying sizes benefiting the circulation of air and water (Figure 9a). In contrast to the penetration behavior of water in soil, viscous polymer emulsions enclosed soil particles through bonding during infiltration. The thickness of the polymer emulsion on the outer aggregate particles was more important than that of emulsion on the inner particles (Figure 9b). Appl. Sci. 2019, 9, 1245 9 of 13 Appl.Appl. Sci. Sci. 2018 2018, , 88,, xx FOR PEERPEER REVIEW REVIEW 9 of 13 9 of 13

FigureFigure 9.9. IllustrationIllustration of ofthe the infiltration infiltration and stabilizat and stabilizationion mechanism mechanism of the polymer of the emulsion polymer on emulsion the on Figurethesurface surface 9. of Illustration the of soil the aggregates: soil of aggregates: the infiltration (a) untreated (a) untreated and aggregate; stabilizat aggregate; (bion) polymer mechanism (b) polymeremulsion of the infiltrated emulsion polymer the infiltrated emulsion soil; (c) the on soil;the polymers gathered on the surface; (d) soil-polymer matrix formed after water evaporating. surface(c) polymers of the gathered soil aggregates: on the surface; (a) untreated (d) soil-polymer aggregate; matrix (b) polymer formed emulsion after water infiltrated evaporating. the soil; (c) polymersThe polymer gathered emulsion on the first surface; adhered (d) tosoil-polymer the external matrix soil particles, formed afterwhich water reduced evaporating. the number of The polymer emulsion first adhered to the external soil particles, which reduced the number pores in the surface soil layer. On the other hand, water, which has a low viscosity, first reached the of pores in the surface soil layer. On the other hand, water, which has a low viscosity, first reached innerThe aggregates polymer and emulsion filled the first pores, adhered which to prevented the external the soilcontinuous particles, infiltration which reduced of the polymer the number of porestheemulsion. inner in the aggregates Therefore, surface soil andthe quantitylayer. filled On the of the pores,infiltrated other which hand, polymer prevented water in the, which interior the continuous has of athe low aggregate viscosity, infiltration was first greatly of thereached polymer the inneremulsion.decreased. aggregates Therefore, As a result, and thefilled an quantityincreasing the pores, of amount infiltrated which of preventedpolymer polymer emulsion inthe the continuous interiorgathered of oninfiltration the the aggregate surface of of the wasthe polymer greatly emulsion.decreased.aggregate Therefore,(Figure As a result, 9c). theInside an qua increasingthentity aggregate, of infiltrated amount the wa ofterpolymer polymerwould inprevent emulsionthe interior the polymer gathered of the from aggregate on adhering the surface was to greatly of the decreaaggregatethe innersed. soil (FigureAs particles.a result,9c). Inside When an increasing the aggregate aggregate, amount had the beof wateren polymer completely would emulsion preventinfiltrated gathered the by polymer water, on the the from polymer surface adhering of the to aggregatetheemulsion inner soil(Figurecould particles. not 9c). be Insidedriven When theby the wateraggregate, aggregate to continue the had wa beentheter infiltrationwould completely prevent due infiltrated to the the polymer lack by of water,water from flow theadheringpolymer to theemulsionpotential. inner soil couldWithout particles. not ADNB, be When driven the internalthe by aggregate water soil partic to continuehadles becoulden the completelynot infiltration be stabilized infiltrated due (Figure to theby 9b). water, lack When of the the water polymer flow water in the solution evaporated, polymer coated the soil particles and formed a soil-polymer matrix emulsionpotential. could Without not ADNB, be driven the by internal water soil to particlescontinue couldthe infiltration not be stabilized due to the (Figure lack9 b).of water When flow the that acted as a stabilized shell of a certain thickness (Figure 9d). water in the solution evaporated, polymer coated the soil particles and formed a soil-polymer matrix potential.SEM Withoutimages of ADNB, the aggregates the internal showed soil two partic waysles for could ADNB not to stabilizebe stabilized particles: (Figure a membrane 9b). When the that acted as a stabilized shell of a certain thickness (Figure9d). waterstructure in the and solution chain bonding. evaporated, The surface polymer of the coated aggregate the soil treated particles with 2% and ADNB formed is shown a soil-polymer in Figure matrix that10a, actedSEM which imagesas indicateda stabilized of the that aggregatesshell a smooth of a certain showedsurface thickness twomorphology ways (Figure for had ADNB 9d). formed to stabilizeresulting particles:from polymer a membrane structuremoleculesSEM andimages filling chain theof thepores bonding. aggrega and the Thetes bonding showed surface between two of the wa particles. aggregateys for ADNBFigure treated 10bto stabilize shows with the 2% particles: surface ADNB of a is themembrane shown in structureFigureaggregate 10 a,and treated which chain indicatedwith bonding. 5% ADNB,that The asurface smoothwhere ofa surface membranethe aggr morphologyegate could treated be observed had with formed 2% that ADNB resultinghad iscompletely shown from in polymer Figure 1molecules0a,adhered which to filling theindica particles theted pores thatand filled anda smooth thethe bondingpores. surface between morphology particles. had Figure formed 10b resulting shows the from surface polymer of the moleculesaggregate treatedfilling the with pores 5% ADNB, and the where bondingSoil stabilizer a membrane between couldparticles. be observed Figure 10b that shows had completely the surface adhered of the aggregateto the particles treated and with filled 5% the ADNB, pores. where a membrane could be observed that had

Soil stabilizer

(a) (b)

Figure 10. SEM images of the aggregate surfaces: (a) treated with 2% ADNB; (b) treated with 5% ADNB.

SEM images of the interior of an aggregate are shown in Figure 11. Only a small amount of polymer emulsion had reached(a) this zone, which was insufficient to form an integrated(b) membrane structureFigure and 10.10. instead SEMSEM imagesimages just formed of of the the aggregate chains. aggregate The surfaces: chainsurfaces: bonding (a) treated(a) treated was with weak with 2% compared ADNB; 2% ADNB; (b) with treated (b the) treated with membrane 5% with ADNB. 5% structure.ADNB. Because of the excellent elasticity of ADNB, the integrated membranes coated on particles and/orSEM filled images in soil of pores the interior played a of major an aggregate role in protecting are shown the aggregates in Figure 11from. Only collapsing a small when amount of polymerimmersedSEM emulsion images in water. of had the reached interior thisof an zone, aggregate which wasare shown insufﬁcient in Figure to form 11. anOnly integrated a small membraneamount of polymerstructure emulsion and instead had just reached formed this chains. zone, The which chain was bonding insufficient was weak to form compared an integrated with the membrane membrane structurestructure. and Because instead of thejust excellentformed chains. elasticity The of chain ADNB, bonding the integrated was weak membranes compared with coated the on membrane particles structure. Because of the excellent elasticity of ADNB, the integrated membranes coated on particles and/or filled in soil pores played a major role in protecting the aggregates from collapsing when immersed in water. Appl. Sci. 2019, 9, 1245 10 of 13

Appl. Sci. 2018, 8, x FOR PEER REVIEW 10 of 13 and/or ﬁlled in soil pores played a major role in protecting the aggregates from collapsing when Appl.immersed Sci. 2018 in, 8, water. x FOR PEER REVIEW 10 of 13 Chained bonding

Chained bonding

Figure 11. SEM image of the interior of the aggregates.

The particle-polymerparticle-polymerFigure matrix 11. SEM significantlysignificantly image of the improvedim interiorproved of thethe the wateraggregates.water stability.stability. When a modifiedmodified aggregate was immersed in water, thethe waterwater moleculesmolecules could notnot immediatelyimmediately penetrate into the The particle-polymer matrix significantly improved the water stability. When a modified interior becausebecause ofof thethe stabilizedstabilized shell.shell. The The wate waterr first first completely infiltratedinfiltrated the membranemembrane that aggregate was immersed in water, the water molecules could not immediately penetrate into the coated the soil particles and/or filled filled the the pores pores and and then then further further entered entered the the interior interior of of the the aggregate. aggregate. interior because of the stabilized shell. The water first completely infiltrated the membrane that During this process, the aggregate remained stable in the water.water. Thereafter, the aggregatesaggregates would coated the soil particles and/or filled the pores and then further entered the interior of the aggregate. become saturatedsaturated with with water, water, and and swelling swelling of the of soil the particles soil particles would thenwould generate then angenerate inner expansionan inner During this process, the aggregate remained stable in the water. Thereafter, the aggregates would pressureexpansion (Figure pressure 12 a).(Figure Due 12a). to its Due elasticity, to its elasticity, the membrane the membrane remained remained intact in intact the water in the until water it until was become saturated with water, and swelling of the soil particles would then generate an inner disturbedit was disturbed by forces. by forces. The membrane The membrane ruptured ruptur whened the when inner the expansion inner expansion pressure pressure exceeded exceeded the strength the expansion pressure (Figure 12a). Due to its elasticity, the membrane remained intact in the water until ofstrength the membrane of the membrane structure. structure. The membrane The membrane broke at thebroke location at the withlocati aon stress with concentration. a stress concentration. With the it was disturbed by forces. The membrane ruptured when the inner expansion pressure exceeded the outflowWith the of outflow internal of particles, internal particles, the outer protectivethe outer pr shellotective gradually shell gradually tore along tore the rupturealong the until rupture it reached until strength of the membrane structure. The membrane broke at the location with a stress concentration. ait stablereached state. a stable The innerstate. particlesThe inner flowed particles out flowe of thed aggregatesout of the inaggregates the form in of the discrete form particlesof discrete as With the outflow of internal particles, the outer protective shell gradually tore along the rupture until wellparticles as small as well aggregates as small dueaggregates to the weakdue to chain the weak bonding chain (Figure bonding 12b), (Figure while 12b), the outerwhile protective the outer it reached a stable state. The inner particles flowed out of the aggregates in the form of discrete shellprotective disintegrated. shell disintegrated. particles as well as small aggregates due to the weak chain bonding (Figure 12b), while the outer protective shell disintegrated.

Figure 12. Illustration of the mechanism of water stability of the aggregates: (a) inner expansion Figure 12. Illustration of the mechanism of water stability of the aggregates: (a) inner expansion pressure generated;generated; ((bb)) stabilizedstabilized shellshell rupturedruptured byby thethe innerinner expansionexpansion pressure.pressure. Figure 12. Illustration of the mechanism of water stability of the aggregates: (a) inner expansion 5. Conclusions pressure generated; (b) stabilized shell ruptured by the inner expansion pressure.

5. ConclusionsThe ADNB polymer was a suitable choice as a soil stabilizer due to its special characteristics, such asasuniform uniform nanoscale nanoscale particles particles when when dispersed disper insed water, in water, which permittedwhich permitted the polymer the topolymer penetrate to The ADNB polymer was a suitable choice as a soil stabilizer due to its special characteristics, penetrate into the very small pores of soil, and suitable cohesive properties, which permitted it to such as uniform nanoscale particles when dispersed in water, which permitted the polymer to play a better role in bonding soil particles together. To understand the penetration process of the soil penetrate into the very small pores of soil, and suitable cohesive properties, which permitted it to stabilizer and the stabilization mechanism, laboratory trials of infiltration tests and water stability play a better role in bonding soil particles together. To understand the penetration process of the soil stabilizer and the stabilization mechanism, laboratory trials of infiltration tests and water stability Appl. Sci. 2019, 9, 1245 11 of 13 into the very small pores of soil, and suitable cohesive properties, which permitted it to play a better role in bonding soil particles together. To understand the penetration process of the soil stabilizer and the stabilization mechanism, laboratory trials of infiltration tests and water stability tests were performed. The mode of infiltration of ADNB in the soil as well as the stabilization of soil were proposed. The mechanism of water stability was analysed based on SEM images. The main conclusions can be summarized as follows: (a) The ADNB polymer emulsion had the characteristics of easy adhesion to soil particles, poor fluidity and large molecular volume. The effective infiltration depth of soil reached a maximum for samples treated with 2% ADNB and decreased when the concentration of ADNB increased. (b) The water stability of aggregates was significantly ameliorated by the ADNB treatment, which generated a protective shell with a particle-polymer matrix after evaporation of the water in the solution. (c) Aggregates with an average diameter of 15 mm exhibited an improved water stability over those with an average diameter of 7.5 mm when treated with the same concentration of polymer emulsion. This result could be explained by the fact that larger aggregates had smaller specific surface areas, on which there were larger amounts of effective polymeric emulsions than on aggregates with smaller diameters. (d) Based on the SEM results, two ways for ADNB to stabilize particles were observed: formation of a membrane structure on the surface and chain bonding inside. The latter had a weak bonding effect. The membrane structure that coated the soil particles and/or filled the pores via bonding was believed to play the main role in generating water stability. (e) The higher the concentration of the polymer emulsion was, the larger the thickness of the polymer membranes and the higher the water stability of the aggregates. However, the most economical and effective concentration for engineering applications was 2% ADNB. When ADNB is sprayed on a slope, the protective shell on the soil surface and large aggregates in the subsoil can not only prevent the loss of topsoil but also increase the number of pores of the soil interior, thereby enhancing the permeability of water and air in the soil, which is very important to the growth of plant roots in soil. Furthermore, because the final degradation products (CO2 and H2O) do not pollute the environment, the application of ADNB can achieve the purpose of the ecological protection of the topsoil of clayey slopes.

Author Contributions: Conceptualization, C.Z. and Z.L.; Data curation, C.Z., S.Z. and Z.L.; Formal analysis, S.Z., W.H., D.L. and Z.L.; Funding acquisition, C.Z. and Z.L.; Investigation, S.Z. and Z.L.; Methodology, C.Z. and Z.L.; Project administration, C.Z. and Z.L.; Resources, C.Z. and Z.L.; Software, S.Z., W.H. and D.L.; Supervision, C.Z. and Z.L.; Validation, C.Z., Z.L. and S.Z.; Visualization, Z.L.; Writing—original draft, C.Z., S.Z. and Z.L.; Writing—review & editing, C.Z., S.Z. and Z.L. Funding: This research was supported by the Major Programs Special Funds of Applied Science and Technology Research and Development of Guangdong Province (No. 2015B090925016), the National Key Research and Development Project of China (No. 2017YFC1501201) and the National Key Research and Development Project of China (No. 2017YFC0804605). Conﬂicts of Interest: The authors declare no conﬂict of interest.

References

1. Petry, T.M.; Little, D.N. Review of Stabilization of Clays and Expansive Soils in Pavements and Lightly Loaded Structures-History, Practice, and Future. J. Mater. Civ. Eng. 2002, 14, 447–460. [CrossRef] 2. Lekha, K.R. Field instrumentation and monitoring of soil erosion in coir geotextile stabilised slopes—A case study. Geotext Geomembr. 2004, 22, 399–413. [CrossRef] 3. Poh, P.S.H.; Broms, B.B. Slope stabilization using old rubber tires and geotextiles. J. Perform. Constr. Fac. 1995, 9, 76–79. [CrossRef] 4. Shao, Q.; Gu, W.; Dai, Q.; Makoto, S.; Liu, Y. Effectiveness of geotextile mulches for slope restoration in semi-arid northern China. Catena 2014, 116, 1–9. [CrossRef] Appl. Sci. 2019, 9, 1245 12 of 13

5. Smets, T.; Poesen, J.; Fullen, M.A.; Booth, C.A. Effectiveness of palm and simulated geotextiles in reducing run-off and inter-rill erosion on medium and steep slopes. Soil Use Manag. 2007, 23, 306–316. [CrossRef] 6. Basha, E.A.; Hashim, R.; Mahmud, H.B.; Muntohar, A.S. Stabilization of residual soil with rice husk ash and cement. Constr. Build. Mater. 2005, 19, 448–453. [CrossRef] 7. Edil, T.B.; Acosta, H.A.; Benson, C.H. Stabilizing Soft Fine-Grained Soils with Fly Ash. J. Mater. Civ. Eng. 2006, 18, 283–294. [CrossRef] 8. Horpibulsuk, S.; Phetchuay, C.; Chinkulkijniwat, A. Soil Stabilization by Calcium Carbide Residue and Fly Ash. J. Mater. Civ. Eng. 2011, 24, 184–193. [CrossRef] 9. Kaniraj, S.R.; Havanagi, V.G. Compressive strength of cement stabilized fly ash-soil mixtures. Cem. Concr. Res. 1999, 29, 673–677. [CrossRef] 10. Kolias, S.; Kasselouri-Rigopoulou, V.; Karahalios, A. Stabilisation of clayey soils with high calcium fly ash and cement. Cem. Concr. Compos. 2005, 27, 301–313. [CrossRef] 11. Miller, G.A.; Azad, S. Influence of soil type on stabilization with cement kiln dust. Constr. Build. Mater. 2000, 14, 89–97. [CrossRef] 12. Tang, C.; Shi, B.; Gao, W.; Chen, F.; Cai, Y. Strength and mechanical behavior of short polypropylene fiber reinforced and cement stabilized clayey soil. Geotext Geomembr. 2007, 25, 194–202. [CrossRef] 13. Zhu, Z.; Liu, S. Utilization of a new soil stabilizer for silt subgrade. Eng. Geol. 2008, 97, 192–198. [CrossRef] 14. Yue, Z.; Watts, D. Current development of slope eco-engineering principle and application in Europe and America. J. Soil Eros. Soil Water Conserv. 1999, 5, 80–86. 15. Bian, P.; Zhang, J.; Zhang, C.; Huang, H.; Rong, Q.; Wu, H.; Li, X.; Xu, M.; Liu, Y.; Ren, S. Effects of Silk-worm Excrement Biochar Combined with Different Iron-Based Materials on the Speciation of Cadmium and Lead in Soil. Appl. Sci. 2018, 8, 1999. [CrossRef] 16. Chang, I.; Cho, G. Strengthening of Korean residual soil with β-1,3/1,6-glucan biopolymer. Constr. Build. Mater. 2012, 30, 30–35. [CrossRef] 17. Latifi, N.; Rashid, A.S.A.; Siddiqua, S.; Horpibulsuk, S. Micro-structural analysis of strength development in low- and high swelling clays stabilized with magnesium chloride solution—A green soil stabilizer. Appl. Clay Sci. 2015, 118, 195–206. [CrossRef] 18. Liu, J.; Shi, B.; Jiang, H.; Bae, S.; Huang, H. Improvement of water-stability of clay aggregates admixed with aqueous polymer soil stabilizers. Catena 2009, 77, 175–179. [CrossRef] 19. Ma, G.; Ran, F.; Feng, E.; Dong, Z.; Lei, Z. Effectiveness of an Eco-friendly Polymer Composite Sand-Fixing Agent on Sand Fixation. Water Air Soil Pollut. 2015, 226, 221. [CrossRef] 20. Marto, A.; Boss, S.; Makhtar, A.M.; Latifi, N. Strength Characteristic of Brown Kaolin Treated with Liquid Polymer Additives. J. Teknol. 2015, 76, 89–94. [CrossRef] 21. Marto, A.; Latifi, N.; Eisazadeh, A. Effect of Non-Traditional Additives on Engineering and Microstructural Characteristics of Laterite Soil. Arab. J. Sci. Eng. 2014, 39, 6949–6958. [CrossRef] 22. Mousavi, F.; Abdi, E.; Rahimi, H. Effect of polymer stabilizer on swelling potential and CBR of forest road material. Ksce J. Civ. Eng. 2014, 18, 2064–2071. [CrossRef] 23. Naeini, S.A.; Ghorbanalizadeh, M. Effect of wet and dry conditions on strength of silty sand soils stabilized with epoxy resin polymer. J. Appl. Sci. (Faisalabad) 2010, 10, 2839–2846. [CrossRef] 24. Naeini, S.A.; Naderinia, B.; Izadi, E. Unconfined compressive strength of clayey soils stabilized with waterborne polymer. Ksce J. Civ. Eng. 2012, 16, 943–949. [CrossRef] 25. Onyejekwe, S.; Ghataora, G.S. Soil stabilization using proprietary liquid chemical stabilizers: Sulphonated oil and a polymer. B Eng. Geol. Environ. 2015, 74, 651–665. [CrossRef] 26. Shrivastava, A.K.; Jain, D.; Vishwakarma, S. Frictional resistance of drilling fluids as a borehole stabilizers. Int. J. Geo-Eng. 2016, 7, 12. [CrossRef] 27. Zhang, M.; Guo, H.; El-Korchi, T.; Zhang, G.; Tao, M. Experimental feasibility study of geopolymer as the next-generation soil stabilizer. Constr. Build. Mater. 2013, 47, 1468–1478. [CrossRef] 28. Song, Z.; Liu, J.; Bai, Y.; Wei, J.; Li, D.; Wang, Q.; Chen, Z.; Kanungo, D.P.; Qian, W. Laboratory and Field Experiments on the Effect of Vinyl Acetate Polymer-Reinforced Soil. Appl. Sci. 2019, 9, 208. [CrossRef] 29. Escolano, F.; Sánchez, J.; Pacheco-Torres, R.; Cerro-Prada, E. Strategies on Reuse of Clayey Expansive Soils as Embankment Material in Urban Development Areas: A Case Study in New Urbanized Zones. Appl. Sci. 2018, 8, 764. [CrossRef] Appl. Sci. 2019, 9, 1245 13 of 13

30. Latifi, N.; Vahedifard, F.; Ghazanfari, E.; Safuan, A. Sustainable Usage of Calcium Carbide Residue for Stabilization of Clays. J. Mater. Civ. Eng. 2018, 30, 04018099. [CrossRef] 31. Latifi, N.; Eisazadeh, A.; Marto, A.; Meehan, C.L. Tropical residual soil stabilization: A powder form material for increasing soil strength. Constr. Build. Mater. 2017, 147, 827–836. [CrossRef] 32. Latifi, N.; Horpibulsuk, S.; Meehan, C.L.; Majid, M.Z.A.; Tahir, M.M. Improvement of Problematic Soils with Biopolymer—An Environmentally Friendly Soil Stabilizer. J. Mater. Civ. Eng. 2016, 29, 04016204. [CrossRef] 33. García-Orenes, F.; Guerrero, C.; Mataix-Solera, J.; Navarro-Pedreño, J.; Gómez, I.; Mataix-Beneyto, J. Factors controlling the aggregate stability and bulk density in two different degraded soils amended with biosolids. Soil Tillage Res. 2005, 82, 65–76. [CrossRef] 34. Inyang, H.I.; Bae, S. Polyacrylamide sorption opportunity on interlayer and external pore surfaces of contaminant barrier clays. Chemosphere 2005, 58, 19–31. [CrossRef][PubMed] 35. Nadler, A.; Perfect, E.; Kay, B.D. Effect of Polyacrylamide Application on the Stability of Dry and Wet Aggregates. Soil Sci. Soc. Am. J. 1996, 60, 555–561. [CrossRef] 36. Salehpour, S.; Jonoobi, M.; Ahmadzadeh, M.; Siracusa, V.; Rafieian, F.; Oksman, K. Biodegradation and ecotoxicological impact of cellulose nanocomposites in municipal solid waste composting. Int. J. Biol. Macromol. 2018, 111, 264–270. [CrossRef] 37. Zhou, Z.; Zeng, J. A Hydrolysis-Resistant Polyester, Hydrolysis-Resistant Waterborne Polyester Dispersion and Its Application. CN104629034 B, 29 September 2017.

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