Mechanism Influencing the Drying Behavior of Bitumen Emulsion

materials

Article Mechanism Inﬂuencing the Drying Behavior of Bitumen Emulsion

Chun Li 1, Jian Ouyang 2,*, Fangjie Dou 3 and Jingtao Shi 1

1 PetroChina Fuel Oil Limited Company Research Institute, Beijing 100195, China; [email protected] (C.L.); [email protected] (J.S.) 2 School of Transportation and Logistics, Dalian University of Technology, Dalian 116024, China 3 Foshan Gaofu Petrochina Fule & Asphalt Co., Ltd., Foshan 528531, China; [email protected] * Correspondence: [email protected]

Abstract: The drying process of bitumen emulsion largely dominates the strength development of emulsion-based mixtures for pavement structure, thus it can be used to judge the quality of bitumen emulsion. However, the drying behaviour of bitumen emulsions was seldom considered. The emulsion drying and film formation theory are employed to study the drying process of different bitumen emulsions with a thin layer. Results indicated the drying process of bitumen emulsion can be divided into three stages: (a) an initial high evaporation rate stage; (b) an intermediate stage with a rapidly decreasing evaporation rate; (c) a final stage with a very small evaporation rate. The boundaries among the three stages can be identified by studying the water evaporation rate. Three drying parameters, i.e., the critical volume fractions of bitumen defining the boundaries among the three stages and the maximum packing fraction of bitumen droplets, are proposed to quantitatively characterize the drying behavior of bitumen emulsion. High values of these parameters indicate a bitumen emulsion that has rapid drying behavior. Therefore, these parameters are independent of the emulsifier type, but they are highly dependent on the bitumen’s droplet size. These drying parameters increase with a decrease in bitumen droplet size. Therefore, bitumen emulsion with Citation: Li, C.; Ouyang, J.; Dou, F.; a smaller size distribution of bitumen droplets can have a more rapid drying behavior, which is Shi, J. Mechanism Influencing the recommended in real engineering. Drying Behavior of Bitumen Emulsion. Materials 2021, 14, 3878. Keywords: bitumen emulsion; drying behavior; emulsion quality; maximum packing fraction of https://doi.org/10.3390/ma14143878 bitumen droplets; bitumen droplets size

Academic Editor: Marek Iwa´nski

Received: 23 May 2021 1. Introduction Accepted: 9 July 2021 Published: 12 July 2021 As traditional pavement materials, bitumen emulsion-based materials, which can be paved at ambient temperature, are widely used in pavement maintenance, preservation,

Publisher’s Note: MDPI stays neutral and rehabilitation. With the growing environmental awareness worldwide in recent years, with regard to jurisdictional claims in bitumen emulsion-based materials are gradually more preferred due to their merits of published maps and institutional afﬁl- low energy consumption and carbon emission compared to traditional hot bitumen mix- iations. ture [1,2]. Although the beneﬁts of bitumen emulsion-based materials are evident, their application has great constraints because of long-time curing. As a typical feature, the precondition for recovering the adhesive ability of bitumen emulsion is that most of the water should be evaporated or consumed. In this regard, it is of great importance to deeply understand the drying behavior of bitumen emulsion as well as its corresponding mixtures. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. Because of the constraint of long-time curing, to promote the technologies of bitumen This article is an open access article emulsion-based materials, many scholars focused on how to accelerate the strength de- distributed under the terms and velopment of bitumen emulsion mixture [3–7]. Generally, the water consumption rate of conditions of the Creative Commons the bitumen emulsion mixture is related to the curing condition. Low humidity and high Attribution (CC BY) license (https:// temperature conditions can greatly increase the water evaporation rate, thus shortening creativecommons.org/licenses/by/ the curing time to reach the desired strength [3–6]. However, this curing condition cannot 4.0/). be realized in real engineering. Therefore, cement hydration can also consume some water

Materials 2021, 14, 3878. https://doi.org/10.3390/ma14143878 https://www.mdpi.com/journal/materials Materials 2021, 14, 3878 2 of 13

in the mixture, thus adding cement, even rapid-hardening cement, can be greatly beneficial to the early strength of the bitumen emulsion mixture [3,4]. However, the rapid-hardening cement can be definitely harmful to the chemical stability of bitumen emulsion so that the workable time of the bitumen emulsion mixture may be greatly shortened [8]. Emulsifier content can affect the chemical stability of bitumen emulsion, thus it can also affect the strength development of bitumen emulsion mixture. At lower emulsifier contents, the premature breaking of emulsion easily occurs so that the bitumen emulsion mixture can have a certain strength soon, however, it’s mechanical performance in the later curing stage may be inferior due to poor adhesion of bitumen at the interface to the aggregate [7]. Therefore, bitumen type, bitumen and water content can affect the strength of the mixture as well as its development process [2,7,9]. For instance, a bitumen emulsion mixture with the harder bitumen can have a higher indirect tensile strength [9]. However, the bitumen type is chosen according to the climate and traffic grade in engineering, and the optimum bitumen and water content should be used to ensure the best mechanical performance of the bitumen emulsion mixture [2]. Therefore, bitumen type, bitumen and water content are seldom adjusted in the application after knowing the optimum bitumen and water content. Overall, no efficient methods can be currently used in engineering to accelerate the strength development of bitumen emulsion mixture. Essentially, the adhesive ability of bitumen emulsion is highly related to its residual water content. Thus, the drying behavior of bitumen emulsion as well as its corresponding mixtures, which can greatly dominate the strength development of bitumen emulsion mixture, is being gained concerns recently. In the aspect of the drying of bitumen emulsion mixture, Saadoon et al. investigated the water evaporation process of bitumen emulsion mixtures with different types of cement [10,11]. They found that the Marshall stability of the bitumen emulsion mixture had a good relationship with its water evaporation. Therefore, although the addition of cement (especially for rapid hardening cement) can greatly increase the early Marshall stability of the bitumen emulsion mixture, it cannot shorten the drying process of the mixture [11]. Thus they believed that bitumen emulsion was the main component influencing the drying process of the mixture [10]. In the aspect of pure emulsion drying, Goavec et al. investigated the water concentration distribution in bitumen emulsion during drying through Magnetic Resonance Imaging [12]. Results indicated that non-uniform distribution of bitumen droplets in the vertical direction could be observed during drying. The concentration of bitumen droplets near the surface can be higher than that near the bottom. When the concentration of bitumen droplets can reach the maximum packing fraction of bitumen droplets, skin phenomenon can occur during drying [13]. Ouyang et al. discussed the reasons for the non-uniform distribution of bitumen droplets during drying and proposed a drying condition to ensure the uniform distribution of bitumen droplets during drying [14]. Therefore, they also recommended that the quality of bitumen emulsion can be evaluated by its drying behavior [14]. The above studies are very meaningful to understand the drying behavior of pure bitumen emulsion and bitumen emulsion mixture as well as their effects on the strength development of bitumen emulsion mixture. However, these studies are all based on a bitumen emulsion. No studies focused on the drying behavior of different emulsions. Therefore, the mechanism influencing the drying behavior of different bitumen emulsions is unclear. As a result, it is unclear how to produce fast-drying bitumen emulsion. Therefore, the drying of bitumen emulsion is seldom considered in real engineering. According to the current specifications [15,16], the types of bitumen emulsion are mainly classified by the breaking grade, such as rapid, medium and slow setting. Meanwhile, the quality of bitumen emulsion is mainly evaluated by the bitumen content in emulsion and the basic technical properties of the residual bitumen in the engineering. Because of no requirements for the drying process of bitumen emulsion, mixtures with different bitumen emulsions qualified by current requirements may have significantly different strength development processes, especially for the early strength. Based on the above consideration, the objectives of this research are as follows: Materials 2021, 14, 3878 3 of 13

• To quantitatively study the drying behavior of different bitumen emulsions; • To evaluate the quality of bitumen emulsions from the drying aspect; • To reveal the key properties of bitumen emulsion dominating its drying behavior.

2. Materials and Specimens Preparation 2.1. Materials Self-made cationic bitumen emulsions were tested to achieve the objectives of this study. Five slow-setting bitumen emulsions were fabricated in the laboratory by the same basic formula in which emulsifier, water and bitumen contents are 4%, 36% and 60%, respectively. Paving grade base bitumen 60/80 is widely used in the production of bitumen emulsion in pavement engineering in China, which was also used in this study, with its general properties as well as its chemical components listed in Table1. The first two emulsions were produced by an emulsifier (coded as KZW) from Tianjin Kangzewei Co., Ltd. in Tianjin, China. The other three emulsions were produced by an emulsifier (coded as LF) from Shanghai Focusen Pavement Engineering Co., Ltd. in Shanghai, China. By adding small doses of different stabilizers, the five emulsions can have different particle size distributions of bitumen droplets, which can be shown in Figure1. The bitumen particle size distribution was measured by a laser particle size analyzer (Mastersize 2000, Malvern Instruments Ltd., Malvern, UK). Based on Figure1b, the bitumen droplet size r50 (droplet diameter at the 50th cumulative volume percentile) can be obtained in Table2. Therefore, the main difference of different emulsions with the same emulsifier is the droplets size distribution. The five emulsions can be used to study the effect of the emulsifier and bitumen droplet size distribution on the drying behavior of bitumen emulsion.

Table 1. Properties of the base bitumen.

Property Test Speciﬁcations Value General properties Penetration at 25 ◦C (0.1 mm) ASTM D 5 72 Softening point R and B (◦C) ASTM D 36 47.0 Ductility at 15 ◦C (cm) ASTM D 113 >100 Kinetic viscosity at 60 ◦C (Pa s) ASTM D 2171 227 Flash point (◦C) ASTM D 92 276 SARA composition ASTM D 4124 Saturates (mass fraction, %) 13.9 Aromatics (mass fraction, %) 41.5 Resins (mass fraction, %) 32.4 Asphaltenes (mass fraction, %) 12.4

2.2. Drying Test The drying process of the bitumen emulsion was investigated by the gravimetric measurement of the mass loss due to water evaporation. As shown in Figure2, cylindrical plastic containers of 52 mm in diameter and 15 mm in height were used in the drying test. According to the drying theory of latex, the concentration distribution of droplets may be non-uniform in the latex, especially for the thicker layer. In that case, skin formation easily occurs during drying [17]. According to previous studies [14,18], an ideal, typical drying curve of non-skin emulsion, drying can be obtained for the emulsion specimens with the thickness of 3 mm and the initial bitumen content of 40%, thus these specimen conditions were also employed in this study. Bitumen emulsions were diluted to emulsions with bitumen content at 40% by deionized water before the test. To guarantee the accurate layer thickness of specimens, the masses of specimens were calculated according to the theoretical volume and density of bitumen emulsion [14]. The drying test was performed Materials 2021, 14, x FOR PEER REVIEW 4 of 14

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in a temperature-controlled environmental chamber at 25 ◦C. Meanwhile, the airﬂow speed and relative humidity around the place of test specimens were around 0.4 m/s and 38%. Materials 2021, 14, x FOR PEER REVIEWThe water loss of specimens was measured in regular time intervals ranging from4 0.4 of 14 h

to 1 h by a balance with the precision of 10−4 g. The drying test for every specimen was performed with three repetitions. (a) (b) 20 100 KZW-1 Figure 1. The particle size distribution of bitumen emulsions, (a) Differ ential distribution, (b) Cumulative percentile. KZW-2 KZW-1 LF-1 80 KZW-2 15 2.2. Drying Test LF-2 LF-1 LF-3 LF-2 The drying process of the bitumen60 emulsion was investigated by theLF-3 gravimetric 10 measurement of the mass loss due to water evaporation. As shown50th cumulative in Figure percentile 2, cylindrical plastic containers of 52 mm in diameter40 and 15 mm in height were used in the drying test. According to the drying theory of latex, the concentration distribution of droplets may be 5 non-uniform in the latex, especially for20 the thicker layer. In that case, skin formation easily

Cumulative percentile (%) percentile Cumulative Probability by volume (%) occurs during drying [17]. According to previous studies [14,18], an ideal, typical drying curve of non-skin emulsion, drying can be obtained for the emulsion specimens with the 0 0 thickness of 3 mm and the initial bitumen0.1 content of1 40%, thus10 these specimen100 conditions1000 0.1 1 10 100 1000 Particle size (μm) Particlewere size also (μm )employed in this study. Bitumen emulsions were diluted to emulsions with bi- (tumena) content at 40% by deionized water before the test. To(b guarantee) the accurate layer thickness of specimens, the masses of specimens were calculated according to the theoret- Figure 1. The particle size distribution of bitumen emulsions emulsions,, ( a) Differ ential distribution, (b) Cumulative percentile.percentile. ical volume and density of bitumen emulsion [14]. The drying test was performed in a temperature-controlled environmental chamber at 25 °C. Meanwhile, the airflow speed 2.2.Table Drying 2. Bitumen Test droplet size at the 50th cumulative volume percentile. and relative humidity around the place of test specimens were around 0.4 m/s and 38%. The drying process of the bitumen emulsion was investigated by the gravimetric The water loss of specimens was measured in regular time intervals ranging from 0.4 h to measurementEmulsion Code of the mass KZW-1 loss due to water KZW-2 evaporation. LF-1 As shown LF-2 in Figure 2, cylindrical LF-3 1 h by a balance with the precision of 10−4 g. The drying test for every specimen was per- plastic containersr50(µm) of 52 mm 1.837 in diameter 1.874and 15 mm in 1.718 height were 2.926 used in the drying 4.170 test. formed with three repetitions. According to the drying theory of latex, the concentration distribution of droplets may be non-uniform in the latex, especially for the thicker layer. In that case, skin formation easily occurs during drying [17]. According to previous studies [14,18], an ideal, typical drying curve of non-skin emulsion, drying can be obtained for the emulsion specimens with the thickness of 3 mm and the initial bitumen content of 40%, thus these specimen conditions were also employed in this study. Bitumen emulsions were diluted to emulsions with bitumen content at 40% by deionized water before the test. To guarantee the accurate layer thickness of specimens, the masses of specimens were calculated according to the theoretical volume and density of bitumen emulsion [14]. The drying test was performed in a temperature-controlled environmental chamber at 25 °C. Meanwhile, the airflow speed and relative humidity around the place of test specimens were around 0.4 m/s and 38%. The water loss of specimens was measured in regular time intervals ranging from 0.4 h to 1 h by a balance with the precision of 10−4 g. The drying test for every specimen was per- (aformed) with three repetitions. (b)

Figure 2. DryingDrying test of asphalt emulsion and emulsifier emulsiﬁer solution, (a) Asphalt emulsion, (b) EmulsiﬁerEmulsifier solution.solution.

2 The mass per unit area (expressed in g/m ) was used to express the emulsion drying process. The bitumen volume fraction (φ) gradually changes during drying, which can be calculated by Equation (1):

V m /ρ φ = B = B,unit B (1) VB + VW mB,unit/ρB + (mW,unit − mloss)/ρW

where VB and VW are the volumes of bitumen and residual water, respectively; mB,unit and mW,unit are the mass of the bitumen and initial water by unit area, respectively; mloss is 2 the loss of the water by unit area (g/m ); ρB and ρW are the density of bitumen and water,

(a) (b) Figure 2. Drying test of asphalt emulsion and emulsifier solution, (a) Asphalt emulsion, (b) Emulsifier solution.

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The mass per unit area (expressed in g/m2) was used to express the emulsion drying process. The bitumen volume fraction (ϕ) gradually changes during drying, which can be calculated by Equation (1):

VB mB, unit B   (1) VVB,W m B unit B() m W , unit m loss W

where VB and VW are the volumes of bitumen and residual water, respectively; mB,unit and mW,unit are the mass of the bitumen and initial water by unit area, respectively; mloss is the loss of the water by unit area (g/m2); ρB and ρW are the density of bitumen and water, respectively; ϕ is the bitumen droplets volume fraction. Therefore, the average evaporation rate between two adjacent test points can be calculated by Equation (2). Materials 2021, 14, 3878 5 of 13

mmloss,1 i  loss , i E  (2) ii~1 ti(1)() ti respectively; φ is the bitumen droplets volume fraction. Therefore, the average evaporation rate between two adjacent test points can be calculated by Equation (2). where Ėi~i+1 is the average evaporation rate between test points of i and i+1; mloss,i and mloss,i+1 . m + − m are the corresponding lossE of= theloss,i water1 loss,iby unit area, respectively;(2) t(i+1) and t(i) are the i∼i+1 ( + ) − ( ) corresponding time of the test pointst i 1 of ti iand i+1, respectively.

where E˙ i~i+1 is the average evaporation rate between test points of i and i+1; mloss,i and m2.3.loss,i+1 Observationare the corresponding Test of lossBitumen of the waterDroplets by unit Packing area, respectively; during Dryingt(i+1) and t(i) are the corresponding time of the test points of i and i+1, respectively. A three-dimensional digital microscope with maximum magnification at 5000× 2.3.shown Observation in Figure Test of Bitumen 3 was Droplets employed Packing to during record Drying the bitumen droplets packing state during A three-dimensional digital microscope with maximum magnification at 5000× shown indrying. Figure3 wasAs an employed optical to recordmicroscope, the bitumen the droplets three-dimensional packing state during digital drying. microscope can directly Asobtain an optical the microscope,digital image the three-dimensional of specimens withou digital microscopet any treatment. can directly The obtain employed magnification thewas digital 700× image in this of specimens study. A without very anythin treatment. film of Thediluted employed bitumen magnification emulsion was was tested to clearly 700× in this study. A very thin film of diluted bitumen emulsion was tested to clearly observeobserve the the bitumen bitumen droplets droplets packing state packing in this test.state in this test.

FigureFigure 3. Three-dimensional 3. Three-dimensional digital microscope. digital microscope. 3. Methodology to Evaluate the Emulsion Drying 3.1.3. Methodology Theory and Qualitative to DescriptionEvaluate of the the EmulsionEmulsion Drying Drying 3.1.A Theory typical dryingand Qualitative process of bitumen Description emulsion of the is shown Emulsion in Figure Drying5. It can be seen from Figure5 that the drying process of bitumen emulsion can be roughly divided into three stages.A typical According drying to the process basic theory of ofbitumen the drying emulsion and film formation is shown process in Figure of 4. It can be seen emulsionfrom Figure [17–20], 4 the that drying the behavior drying of bitumenprocess emulsion of bitumen is highly emulsion related to the can bitumen be roughly divided into dropletsthree stages. packing state,According which can to be the described basic as theory followed: of the drying and film formation process of 1.emulsionThe first [17–20], stage corresponds the drying to the be dropletshavior in of the bitumen semi-diluted emulsion regime whenis highly the related to the bitu- concentration of the droplets is far from the maximum particle packing fraction in menFigure droplets4a. The packing bitumen emulsion state, which has a free can evaporation be described surface thusas followed: its evaporation rate can be equal to that of pure water or dilute emulsifier solution. This regime ranges from the initial state, φ0, to the critical volume fraction of droplets, φ1, at which bitumen droplets approach and the irreversible coalescence of bitumen droplets may begin as Figure4b.

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1. The first stage corresponds to the droplets in the semi-diluted regime when the concentration of the droplets is far from the maximum particle packing fraction in Figure 5a. The bitumen emulsion has a free evaporation surface thus its evaporation rate can Materials 2021, 14, x FOR PEER REVIEW 6 of 14 be equal to that of pure water or dilute emulsifier solution. This regime ranges from the initial state, ϕ0, to the critical volume fraction of droplets, ϕ1, at which bitumen droplets approach and the irreversible coalescence of bitumen droplets may begin as 1. The first stage corresponds to the droplets in the semi-diluted regime when the con- Figure 5b. centration of the droplets is far from the maximum particle packing fraction in Figure 2. 5a.The The second bitumen stage emulsion corresponds has a free to evaporat the compaction surface regime thus itswhen evaporation the increase rate can in the volume befraction equal to of that bitumen of pure bywater evaporation or dilute em beyondulsifier solution. ϕ1 takes This place regime with ranges the furtherfrom packing, Materials 2021, 14, 3878 thedeformation initial state, andϕ0, to irreversible the critical volume coalescence fraction of of bitumen droplets, droplets.ϕ1, at which The bitumen evaporation6 of 13 rate dropletssharply approach decreases and in the this irreversible stage because coalescence the areaof bitumen of the droplets evaporation may begin surface as is getting Figuresmaller. 5b. This process is determined by the flow out of the water from the gap among 2. The second stage corresponds to the compact regime when the increase in the volume 2.the compactThe second droplets stage corresponds until they to thecoalesce compact to regime a roughly when continuous the increase in film the volumeas Figure 5d. fraction of bitumen by evaporation beyond ϕ1 takes place with the further packing, fraction of bitumen by evaporation beyond φ takes place with the further packing, deformationIn this stage, and there irreversible is a moment coalescence such of as bitumen that demonstrated1 droplets. The inevaporation Figure 5c rate when bitumen deformation and irreversible coalescence of bitumen droplets. The evaporation rate sharplydroplets decreases reach thein this ma stageximum because packing the area state. of the evaporation surface is getting sharply decreases in this stage because the area of the evaporation surface is getting 3. smaller.The final This stage process corresponds is determined to by the th diffuse flow outed regimeof the water when from the the bitumen gap among molecules are smaller. This process is determined by the flow out of the water from the gap among the compact droplets until they coalesce to a roughly continuous film as Figure 5d. inter-diffusedthe compact among droplets droplets, until they coalescethus beco to aming roughly the continuous continuous film asbitumen Figure4 d.phase In with In this stage, there is a moment such as that demonstrated in Figure 5c when bitumen the inclusionsthis stage, there of the is aaqueous moment suchphase as thatin Figure demonstrated 5d. The in residual Figure4c whenwater bitumen in the film es- dropletscapesdroplets either reach reachbythe diffusionma theximum maximum throughpacking packing state. the capill state. ary channels between the coalescing drop- 3. The final stage corresponds to the diffused regime when the bitumen molecules are 3.lets orThe extrusion final stage due corresponds to the reconfiguration to the diffused regime of the when bitumen the bitumen phase. molecules In this arestage, the inter-diffused among droplets, thus becoming the continuous bitumen phase with evaporationinter-diffused rate amongis much droplets, smaller thus and becoming tends to the zero. continuous bitumen phase with the inclusionsthe inclusions of the ofaqueous the aqueous phase in phase Figure in Figure5d. The4d. residual The residual water in water the film inthe es- film capes escapeseither by either diffusion by diffusion through the through capillary the channels capillary between channels the between coalescing the drop- coalescing 600 lets ordroplets extrusion or due extrusion to the duereconfiguration to the reconfiguration of the bitumen of the phase. bitumen In phase.this stage, In this the stage, (1) Stage one (2) Stage two (3) Stage three 2000evaporationthe evaporation rate is much rate smaller is much and smaller tends and to zero. tends to zero. 500 /h) y=5.4374x+1745.3600 2 (1) Stage one (2) Stage two (3) Stage three 2 20001600 R =0.904 ) 2 At the maximum particle 400 500 /h) packing fraction y=5.4374x+1745.3 2 16001200 R2=0.904 )

2 At the Fastestmaximum drop particle for drying rate 400 300 packing fraction 1200

800 Fastest drop for drying rate 300 200 Mass loss (g/m Data of mass loss 800 y=413.36x Fitted line for the first stage 2 200 Mass loss (g/m 400 R =0.9971 Data of massFitted loss line for the third stage 100

y=413.36x Fitted line Data for theof dryingfirst stage rate (g/m rate evaporation Water 2 400 R =0.9971 Fitted line for the third stage 100 0 Data of drying rate (g/m rate evaporation Water 0 024681012 0 Time (h) 0 024681012 Time (h) Figure 4. Drying process of bitumen emulsion (LF-1). FigureFigure 4. Drying 4. Schematic process representationof bitumen emulsion of the three-stage(LF-1). drying process.

Figure 5. Schematic representation of the three-stage drying process . Figure 5. Drying process of bitumen emulsion (LF-1). Figure 5. Schematic representation of the three-stage drying process . 3.2. Quantitative Analysis of the Drying Process According to the typical drying process of bitumen emulsion mentioned previously, it can be easily inferred that the key parameters dominating the drying process are the average evaporation rate of the ﬁrst stage (E˙0), the critical droplet volume fraction between the ﬁrst and second stage (φ1), the maximum particle packing state (φm), and the critical droplet volume fraction between the second and third stage (φ2). Bitumen emulsion can have a high evaporation rate from the initial droplet volume fraction (φ ) to φ and φ , 0 1 m thus higher values of φ1 and φm indicate that bitumen emulsion can be dried to a denser state at a high evaporation rate equal to E˙ 0. Accordingly, bitumen emulsion has a very

low evaporation rate in the third stage, thus the third drying stage can be shortened if φ2 is high. Materials 2021, 14, 3878 7 of 13

To obtain the key drying parameters for quantitatively analyzing the drying process of bitumen emulsion, the boundaries among different stages should firstly be identified. According to previous studies [14,18], the boundaries could be identified by studying the water evaporation rate. As shown in Figure5, the water evaporation rate firstly undulates and then decreases sharply with the drying time, and finally slowly tends to zero. The first boundary, in which the later evaporation rate decreases sharply compared to the average evaporation rate before this point, can be observed. Similarly, the second boundary, in which the later evaporation rate decreases very slowly, can be found. After identifying the two boundaries, the corresponding water loss can be obtained. Accordingly, the values of φ1 and φ2 can be calculated by Equation (1). Meanwhile, the initial evaporation rate, which is the average evaporation rate of the first stage, can be also calculated. Determination of φm is more difficult than φ1 and φ2. Theoretically, there is no obvious droplets deformation and coalescence about bitumen droplets before the drying time of φm, but a significant droplets deformation and coalescence are occurred after this moment [17,20]. Therefore, the area of evaporation surface and the connection channels of water in emulsion are significantly changed in the drying time of φm. It can be inferred that the change in the evaporation rate at the point of φm should be maximum in the drying curve. Since the data of water loss is recorded in regular time intervals ranging from 0.4 h to 1 h, the accurate curve of the drying rate is unknown. Therefore, as shown in Figure5, the cross point of the fitted lines of the first and third stages is used to estimate the drying time of φm according to our previous studies [14,18]. Then, the corresponding water loss can be accurately estimated from the curve of mass loss versus drying time. After knowing the water loss, the maximum packing fraction of bitumen droplets can be calculated according to Equation (1). Based on the above quantitative calculation process, the key drying parameters of the specimen in Figure5 can be calculated in Table3.

Table 3. Key drying parameters of bitumen emulsion (LF-1).

2 Parameters E˙ 0 (g/m /h) Time at φ1 (h) Time at φm (h) Time at φ2 (h) φ1 (%) φm (%) φ2 (%) Value 413.36 3.76 4.29 6.23 82.48 88.55 97.64

4. Results about the Drying Process of Different Emulsions The drying processes of different bitumen emulsions are shown in Figure6. It can be seen from Figure6 that all five bitumen emulsions have a typical three-stage drying process. The mass loss of the five emulsions differs little at the first stage, however, it differs greatly at the third stage. To better clearly show the drying behavior of different emulsions, the residual water content of emulsions during drying is calculated in Figure7. It can be seen from Figure6 that the residual water content of different emulsions differs greatly at the third stage. The residual water content of emulsions is ranked as: LF-2 > LF-3 > KZW-2 > LF-1 ≈ KZW-1. Since the water evaporation rate is very slow in the third stage, the difference in the residual water content indicates that the time for recovering the adhesive ability differs greatly for the five emulsions. For instance, LF-2 and LF-3 have very large values of residual water content. As a result, it can be a long process for recovering their adhesive ability. On the contrary, KZW-1 and LF-1 can quickly recover their adhesive ability. Therefore, even for different emulsions with the same emulsifier, the drying process can be also different. It should be stated here that the five emulsions can have the same breaking grade and residual properties if they are evaluated by the current specifications. However, it can be inferred according to Figure7 that they may have significantly different drying and adhesion forming properties if they are applied in engineering. Therefore, it is necessary to evaluate the quality of bitumen emulsion by its drying behavior. Materials 2021, 14, x FOR PEER REVIEW 8 of 14

the adhesive ability differs greatly for the five emulsions. For instance, LF-2 and LF-3 have very large values of residual water content. As a result, it can be a long process for recovering their adhesive ability. On the contrary, KZW-1 and LF-1 can quickly recover their adhesive ability. Therefore, even for different emulsions with the same emulsifier, the drying process can be also different. It should be stated here that the five emulsions can have the same breaking grade and residual properties if they are evaluated by the current specifications. However, it can be inferred according to Figure 7 that they may have significantly different drying and adhesion forming properties if they are applied in engineering. Therefore, it is necessary to evaluate the quality of bitumen emulsion by its drying behavior. Based on the above quantitative analysis method of the emulsion drying process, the drying parameters of different emulsions can be calculated in Table 4. It can be seen from Table 4 that the initial evaporation rate of five emulsions ranges from 403 g/m2/h to 438 g/m2/h. The small relative difference (the maximum value is 8.7%) nearly has no effect on the drying behavior of the first stage. However, the three key drying parameters (ϕ1, ϕm and ϕ2) differ greatly for different emulsions. For instance, the value of ϕ2 can range from 91.22% to 97.91% for the five emulsions. As a result, the residual water content ranges from 2.09% to 8.78% for the five emulsions at the beginning of the third drying stage. Since the evaporation rate is very low in the third drying stage. A little difference in ϕ2 can significantly affect the drying process of emulsions. LF-1 and KZW-1 have high values of ϕ1, ϕm and ϕ2, thus their drying process is relatively short compared to other emulsions. LF-2 and LF-3 have low values of ϕ1, ϕm and ϕ2, thus a very long time is required to reach their desirable adhesive ability. Therefore, the drying parameters of ϕ1, ϕm and ϕ2 can effectively and quantitatively reflect the drying process of bitumen emulsion. A good quality bitumen emulsion should be dried quickly to recover its adhesive ability after placement. A previous study indicated that the ϕm for a given bitumen emulsion was a constant parameter that differed little with the initial bitumen concentration and drying Materials 2021, 14, 3878 8 of 13 conditions [12]. Therefore, the drying parameters of ϕ1, ϕm and ϕ2 can be reasonable indexes for evaluating the quality of bitumen emulsion.

2000 1800 1600 1400 ) 2 1200 1000 800

Mass loss (g/m Mass 600 KZW-1 KZW-2 400 LF-1 200 LF-2 LF-3 0 024681012 Materials 2021, 14, x FOR PEER REVIEW Time (h) 9 of 14

FigureFigure 6.6. DryingDrying processprocess ofof differentdifferent emulsions.emulsions.

70 14 KZW-1 12 KZW-2 60 10 LF-1 LF-2 8 50 LF-3 6 40 4 2 Residual water content (%) content water Residual 0 30 6789101112 Time (h) KZW-1 20 KZW-2

Residual water content (%) LF-1 10 LF-2 LF-3 0 024681012 Time (h)

Figure 7. Residual water content of different emulsionsemulsions duringduring drying.drying.

TableBased 4. Drying on theparameters above of quantitative different bitumen analysis emulsions. method of the emulsion drying process, the drying parameters of different emulsions can be calculated in Table4. It can be seen from Table4 that the initialTime evaporation at Time rateat ofTime five emulsionsat ranges from 403 g/m 2/h Sample Ė0(g/m2/h) ϕ1(%) ϕm(%) ϕ2(%) to 438 g/m2/h. The smallϕ relative1 (h) differenceϕm (h) (theϕ2 maximum(h) value is 8.7%) nearly has no effectKZW-1 on the drying431.00 behavior3.32 of the first4.14 stage. However,6.45 the77.13 three key88.42 drying parameters97.91 (φKZW-21, φm and φ405.592) differ greatly3.75 for different4.37 emulsions.6.53 For77.16 instance, the85.36 value of95.47φ2 can rangeLF-1 from 91.22%413.36 to 97.91%3.76 for the five4.29 emulsions. 6.23 As a result,82.48 the residual88.55 water97.64 content rangesLF-2 from 2.09%438.11 to 8.78%2.98 for the five3.80 emulsions 7.09 at the beginning70.67 of the79.64 third drying91.22 stage. Since the evaporation rate is very low in the third drying stage. A little difference in φ can LF-3 403.2 3.34 4.05 6.69 73.31 81.61 91.462 significantly affect the drying process of emulsions. LF-1 and KZW-1 have high values of φ , φ and φ , thus their drying process is relatively short compared to other emulsions. 5.1 Discussionm 2 LF-2 and LF-3 have low values of φ1, φm and φ2, thus a very long time is required to reach As mentioned above, the drying behavior of bitumen emulsion can reflect its quality. their desirable adhesive ability. Therefore, the drying parameters of φ1, φm and φ2 can effectivelyQuick-drying and bitumen quantitatively emulsion reflect is more the dryingpreferred process in pavement of bitumen engineering. emulsion. In Athis good re- qualitygard, factors bitumen dominating emulsion the should drying be behavior dried quickly of bitumen to recover emulsion its adhesivewill be studied ability based after placement.on the five emulsions A previous in study the following indicated section. that the φm for a given bitumen emulsion was a constant parameter that differed little with the initial bitumen concentration and drying conditions5.1. Effect of [Emulsifier12]. Therefore, on the theDrying drying Behaviour parameters of Bitumen of φ Emulsion1, φm and φ2 can be reasonable indexesThe for first evaluating drying stage the qualityof bitumen of bitumen emulsion emulsion.s and their emulsifier solutions are shown in Figure 8. The emulsifier content in the emulsifier solution is 4.4%, which is almost equal to the emulsifier concentration in the diluted bitumen emulsion. Based on Figure 8, the initial evaporation rates of bitumen emulsions and their emulsifier solutions are calculated in Figure 9. It can be seen from Figure 9 that the evaporation rate of different emulsifier solutions and emulsions differs slightly. Their relative difference can be lower than 5%. Therefore, the emulsifier types do not affect the evaporation rate of bitumen emulsion. Actually, the evaporation rate of emulsifier solution is nearly equal to that of water, which mainly depends on the drying condition, i.e., temperature, humidity and air speed. Emulsifier not only cannot affect the initial evaporation rate of bitumen emulsion, but also cannot affect its drying parameters of ϕ1, ϕm and ϕ2. It can be seen from Table 4 that KZW-1 and LF-1 can have high values of ϕ1, ϕm and ϕ2, but parameters of ϕ1, ϕm and ϕ2 are relatively low for KZW-2, LF-2 and LF-3. Therefore, emulsions produced by both emulsifiers may be of good or bad quality in drying. Overall, the drying behavior of bitumen emulsion is independent on the emulsifier type.

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Table 4. Drying parameters of different bitumen emulsions.

2 Time at Time at Time at Sample E˙ 0(g/m /h) φ1(%) φm(%) φ2(%) φ1 (h) φm (h) φ2 (h) KZW-1 431.00 3.32 4.14 6.45 77.13 88.42 97.91 KZW-2 405.59 3.75 4.37 6.53 77.16 85.36 95.47 LF-1 413.36 3.76 4.29 6.23 82.48 88.55 97.64 LF-2 438.11 2.98 3.80 7.09 70.67 79.64 91.22 LF-3 403.2 3.34 4.05 6.69 73.31 81.61 91.46

5. Discussion As mentioned above, the drying behavior of bitumen emulsion can reﬂect its quality. Quick-drying bitumen emulsion is more preferred in pavement engineering. In this regard, factors dominating the drying behavior of bitumen emulsion will be studied based on the ﬁve emulsions in the following section.

5.1. Effect of Emulsifier on the Drying Behaviour of Bitumen Emulsion The first drying stage of bitumen emulsions and their emulsifier solutions are shown in Figure8. The emulsifier content in the emulsifier solution is 4.4%, which is almost equal to the emulsifier concentration in the diluted bitumen emulsion. Based on Figure8, the initial evaporation rates of bitumen emulsions and their emulsifier solutions are calculated in Figure9. It can be seen from Figure9 that the evaporation rate of different emulsifier solutions and emulsions differs slightly. Their relative difference can be lower than 5%. Materials 2021, 14, x FOR PEER REVIEWTherefore, the emulsifier types do not affect the evaporation rate of bitumen emulsion.10 of 14 Actually, the evaporation rate of emulsifier solution is nearly equal to that of water, which mainly depends on the drying condition, i.e., temperature, humidity and air speed.

2800

2400

2000 ) 2 1600 KZW-1 1200 KZW-2 LF-1 Mass loss (g/m 800 LF-2 LF-3 400 Emulsifier-KZW Emulsifier-LF 0 0123456 Time (h) FigureFigure 8.8. TheThe ﬁrstfirst dryingdrying stagestage ofof emulsionsemulsions andand theirtheir emulsiﬁeremulsifier solutions.solutions.

500Emulsifier not only cannot affect the initial evaporation rate of bitumen emulsion, but also cannot affect its drying parametersEmulsifier of solutionsφ , φ and φ . It can be seen from Table4 Emulsions 1 m 2 that KZW-1 and LF-1 can have high values of φ1, φm and φ2, but parameters of φ1, φm /h) and2 400φ2 are relatively low for KZW-2, LF-2 and LF-3. Therefore, emulsions produced by both emulsifiers may be of good or bad quality in drying. Overall, the drying behavior of bitumen emulsion is independent on the emulsifier type. 300

200

100 Initial evaporation rate (g/m rate evaporation Initial

0 KZW-1KZW-2 LF-1 LF-2 LF-3 KZW LF Emulsions and emulsifier solutions

Figure 9. Initial evaporation rate of emulsions and their emulsifier solutions.

5.2. Effect of Droplet Size on the Drying Behavior of Bitumen Emulsion As mentioned previously, the initial evaporation rate differs little for different bitumen emulsions, but the three drying parameters (ϕ1, ϕm and ϕ2) differ greatly. Therefore, the drying parameters (ϕ1, ϕm and ϕ2) dominate the drying process of bitumen emulsion. The relations of the drying parameters (ϕ1, ϕm and ϕ2) with bitumen droplets size are shown in Figure 10. Since bitumen droplets in emulsion have a wide size distribution. Droplet size r50 (droplet diameter for the 50th cumulative volume percentile) is employed as a representative particle size in Figure 10. It can be seen from Figure 10 that all of the three drying parameters are decreased with the increasing bitumen droplets size. Bitumen emulsion with a smaller droplet size can be dried more quickly. Therefore, decreasing the bitumen droplets' size can improve the drying quality of bitumen emulsion.

Materials 2021, 14, x FOR PEER REVIEW 10 of 14

2800

2400

2000 ) 2 1600 KZW-1 1200 KZW-2 LF-1 Mass loss (g/m 800 LF-2 LF-3 400 Emulsifier-KZW Emulsifier-LF 0 0123456 Materials 2021, 14, 3878 Time (h) 10 of 13 Figure 8. The first drying stage of emulsions and their emulsifier solutions.

500 Emulsifier solutions Emulsions /h)

2 400

300

200

100 Initial evaporation rate (g/m rate evaporation Initial

0 Materials 2021, 14, x FOR PEER REVIEW KZW-1KZW-2 LF-1 LF-2 LF-3 KZW LF 11 of 14 Emulsions and emulsifier solutions

FigureFigure 9.9. InitialInitial evaporationevaporation raterate ofof emulsionsemulsions andand theirtheir emulsiﬁeremulsifier solutions.solutions. Essentially, the three parameters (ϕ1, ϕm and ϕ2) are all indexes about bitumen drop- 5.2.lets5.2. EffectEffectpacking ofof DropletDroplet state during SizeSize onon drying. thethe DryingDrying Thus, BehaviorBehavior it is reasonable ofof BitumenBitumen EmulsionthatEmulsion the three parameters are all relatedAsAs to mentionedmentioned bitumen previously,droplets' previously, size. the the Equation initial initial evaporation evaporation (3) is used rate to rate fit differs the differs relation little little for of differentfor the different three bitumen param- bitu- emulsions, but the three drying parameters (φ , φm and φ ) differ greatly. Therefore, the etersmen emulsions,with r50. It can but bethe seen three from drying Figure parameters 10 that1 the (ϕ 1three, ϕm andparameters2 ϕ2) differ (ϕ greatly.1, ϕm and Therefore, ϕ2) have drying parameters (φ1, φm and φ2) dominate the drying process of bitumen emulsion. athe positive drying correlation parameters with (ϕ1, 1/ϕrm50 and. Especially ϕ2) dominate for ϕm the, the drying correlation process coefficient of bitumen can emulsion.be higher The relations of the drying parameters (φ1, φm and φ2) with bitumen droplets size are thanThe relations0.9. Therefore, of the the drying bitumen parameters droplet size(ϕ1, canϕm greatlyand ϕ2) dominate with bitumen the drying droplets behavior size are of shown in Figure 10. Since bitumen droplets in emulsion have a wide size distribution. bitumenshown in emulsion. Figure 10. Since bitumen droplets in emulsion have a wide size distribution. Droplet size r50 (droplet diameter for the 50th cumulative volume percentile) is employed Droplet size r50 (droplet diameter for the 50th cumulative volume percentile) is employed as a representative particle size in Figure 10. Ita can be seen from Figure 10 that all of the as a representative particle size in Figurem 10. It canb be seen from Figure 10 that all of the(3) three drying parameters are decreased with the increasing bitumen droplets size. Bitumen three drying parameters are decreased with thre50 increasing bitumen droplets size. Bitumen emulsion with a smaller droplet size can be dried more quickly. Therefore, decreasing the emulsion with a smaller droplet size can be dried more quickly. Therefore, decreasing the bitumenwhere a and droplets’ b are sizefitting can parameters. improve the drying quality of bitumen emulsion. bitumen droplets' size can improve the drying quality of bitumen emulsion. 100 KZW-1 95 y=20.61/x+85.5 LF-1 R2=0.8985 KZW-2 LF-3 90 LF-2 85 y=25.94/x+73.09 R2=0.9334 80

75 y=27.69/x+63.73 2 1 R =0.8543

Bitumen volume fraction (%) fraction volume Bitumen 70 m 2 65 1.01.52.02.53.03.54.04.55.05.5 r50(μm)

Figure 10. Relation of drying parameters (ϕ1, ϕm and ϕ2) with r50 for the five emulsions. Figure 10. Relation of drying parameters (φ1, φm and φ2) with r50 for the ﬁve emulsions.

5.3. MechanismEssentially, Analysis the three about parameters the Drying (φ1, Parametersφm and φ2) are all indexes about bitumen droplets packingAs mentioned state during above, drying. the Thus, drying it is parameters reasonable (ϕ that1, ϕ them and three ϕ2), parameters which dominate are all the related dry- toing bitumen process droplets’of bitumen size. emulsion, Equation are (3) highly is used related to ﬁt to the the relation bitumen of droplets' the three size. parameters To ana- withlyze ther50. possible It can be mechanism seen from Figurefor this 10 phenom that theenon, three the parameters evolution (processφ1, φm andof theφ2 )bitumen have a positivedroplet’s correlation packing during with 1/ dryingr50. Especially is studied for byφm ,a the three-dimensional correlation coefﬁcient digital can microscope, be higher thanwhich 0.9. is Therefore,shown in Figure the bitumen 11. It can droplet be seen size from can greatlythe four dominate images that the the drying motion behavior of small of bitumen emulsion. bitumen droplets is very active so that they cana be densely packed. However, large bitumen droplets almost stay in the same φpositim = on during+ b the whole droplets packing process(3) r50 due to their weak motion ability. As a result, large bitumen droplets are acted as obstacles during the packing process of small bitumen droplets. Because of this obstacle effect, as shown in Figures 11c,d, pores are normally formed around large bitumen particles at the end of the particles packing process. Therefore, large bitumen droplets are harmful to the bitumen droplets packing state. It is reasonable that the three drying parameters (ϕ1, ϕm and ϕ2) decrease with the increasing bitumen droplet’s size in Figure 10.

Materials 2021, 14, 3878 11 of 13

where a and b are ﬁtting parameters.

5.3. Mechanism Analysis about the Drying Parameters

As mentioned above, the drying parameters (φ1, φm and φ2), which dominate the drying process of bitumen emulsion, are highly related to the bitumen droplets’ size. To analyze the possible mechanism for this phenomenon, the evolution process of the bitumen droplet’s packing during drying is studied by a three-dimensional digital microscope, which is shown in Figure 11. It can be seen from the four images that the motion of small bitumen droplets is very active so that they can be densely packed. However, large bitumen droplets almost stay in the same position during the whole droplets packing process due to their weak motion ability. As a result, large bitumen droplets are acted as obstacles during the packing process of small bitumen droplets. Because of this obstacle effect, as shown in Figure 11c,d, pores are normally formed around large bitumen particles at the end of the particles packing process. Therefore, large bitumen droplets are harmful to the bitumen Materials 2021, 14, x FOR PEER REVIEW 12 of 14 droplets packing state. It is reasonable that the three drying parameters (φ1, φm and φ2) decrease with the increasing bitumen droplet’s size in Figure 10.

FigureFigure 11.11. EvolutionEvolution process of of bitumen bitumen droplets droplets packing packing during during drying. drying. (a) Most of droplets are well dispersed and water is enough; (b) Droplets packing begin in water- deficient region; (c) Large dropletsOverall, can affect the thepacking small state droplets of the packing bitumen during drop dropletslets is packing;related to (d )the Final motion droplets ability packing of statebitumen with littledroplets free water.in the emulsion. The motion ability of bitumen droplets in the emulsion is highly related to the bitumen droplet size. Smaller bitumen droplets can have higher motionOverall, ability the in packingthe emulsion. state ofEssentially, the bitumen the dropletsmotion ability is related of particles to the motion in a dilute ability sus- of bitumenpension dropletsis due to inthe the particle emulsion. Brownian The motionmotion. ability According of bitumen to the Stokes–Einstein droplets in the emulsionrelation is[18], highly the particle related toBrownian the bitumen motion droplet can be size. evaluated Smaller by bitumen the particle droplets diffusion can have coefficient higher motion(D) in Equation ability in (4). the emulsion. Essentially, the motion ability of particles in a dilute suspension is due to the particle Brownian motion.kT According to the Stokes–Einstein relation [18], the particle Brownian motionD  can be evaluated by the particle diffusion(4) coefficient (D) in Equation (4). 3r kT where kT is the thermal energy; r is the particleD = diameter; η is the medium viscosity. It can(4) 3πηr be seen from Equation (4) that the particle diffusion coefficient has a positive correlation wherewith 1/kTr. isSince the the thermal three energy; drying rparametersis the particle (ϕ1, diameter; ϕm and ϕ2η) isdepend the medium on the viscosity.motion ability It can beof seenthe bitumen from Equation droplets (4) (i.e, that the the diffusion particle coeffi diffusioncient of coefficient the bitumen has droplets), a positive it correlationis reason- withable 1/thatr. Sincethese theparameters three drying have parameters high correlations (φ1, φm withand 1/φ2r)50 depend in Figure on 10. the motion ability of

6. Conclusions In this paper, the drying behavior of different bitumen emulsions is studied. Quanti- tative drying parameters are proposed to characterize the drying process of bitumen emulsion according to the drying theory of emulsion, and factors influencing the drying process of bitumen emulsion are discussed. Based on the results and discussion, the following conclusions can be drawn: 1. The drying process of bitumen emulsion, as well as the corresponding bitumen droplets packing evaluation process, can be divided into three stages: (a) an initial high evaporation rate stage in which bitumen droplets are well dispersed; (b) an intermediate stage in which bitumen droplets are contacted, deformed and further coalesced so that the evaporation rate rapidly decreases; (c) a final stage with a continuous bitumen film in which the evaporation rate is near-constant but much smaller than that of the first stage. The boundaries among the three stages can be identified by studying the water evaporation rate.

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the bitumen droplets (i.e., the diffusion coefﬁcient of the bitumen droplets), it is reasonable that these parameters have high correlations with 1/r50 in Figure 10.

6. Conclusions In this paper, the drying behavior of different bitumen emulsions is studied. Quan- titative drying parameters are proposed to characterize the drying process of bitumen emulsion according to the drying theory of emulsion, and factors influencing the drying process of bitumen emulsion are discussed. Based on the results and discussion, the following conclusions can be drawn: 1. The drying process of bitumen emulsion, as well as the corresponding bitumen droplets packing evaluation process, can be divided into three stages: (a) an initial high evaporation rate stage in which bitumen droplets are well dispersed; (b) an intermediate stage in which bitumen droplets are contacted, deformed and further coalesced so that the evaporation rate rapidly decreases; (c) a final stage with a continuous bitumen film in which the evaporation rate is near-constant but much smaller than that of the first stage. The boundaries among the three stages can be identified by studying the water evaporation rate. 2. The initial evaporation rate differs little for different emulsions, thus the key parameters affecting the drying process of bitumen emulsion are the critical volume fractions of bitumen defining the boundaries among the three stages and the maximum packing fraction of bitumen droplets (i.e., φ1, φm and φ2). Bitumen emulsion with higher values of φ1 and φm can be dried to a denser state at a high evaporation rate. Bitumen emulsion with a higher value of φ2 has low residual water content when the third drying stage begins. Therefore, high values of the drying parameters (φ1, φm and φ2) indicate that bitumen emulsion has a good drying behavior. 3. The emulsifier type has little effect on the initial evaporation rate and the key drying parameters (φ1, φm and φ2) of bitumen emulsion. The key drying parameters (φ1, φm and φ2) of bitumen emulsion are highly related to the bitumen droplet size. They are all decreased with the increasing bitumen droplet size. The mechanism for this phenomenon is that the three drying parameters (φ1, φm and φ2) are essentially indexes about the bitumen droplets packing state during drying. Due to Brownian motion, small bitumen droplets can have high motion ability, thus they can easily be densely packed. Overall, decreasing the bitumen droplet size can improve the drying quality of bitumen emulsion.

7. Future Work The key drying parameters are proposed to evaluate the quality of bitumen emulsion, and the mechanism influencing the key drying parameters is analyzed. However, the direct index of the quality of bitumen emulsion is its film formation properties (e.g., the adhesive ability and its recovery rate, dynamic modulus and its recovery rate). Therefore, the film formation properties and their relationship with the key drying parameters should be investigated for better evaluating the quality of bitumen emulsion.

Author Contributions: Conceptualization, J.O.; methodology, C.L.; formal analysis, C.L.; investigation, C.L., F.D., and J.S.; data curation, F.D. and J.S.; writing—original draft preparation, C.L., F.D., and J.S.; writing—review and editing, J.O.; supervision, J.O.; funding acquisition, J.O. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the National Key R & D Program of China (2018YFB1600100), the Natural Science Foundation of Liaoning Province (2020-MS-116) and the National Natural Science Foundation of China (51608096). Data Availability Statement: Data is contained within the article. Conﬂicts of Interest: The authors declare that they have no conﬂict of interest. Materials 2021, 14, 3878 13 of 13

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