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Effect of mineral fillers adsorption on rheological and chemical properties of asphalt binder
Construction and Building Materials 141 (2017) 152–159
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Effect of mineral fillers adsorption on rheological and chemical properties of asphalt binder Meng Guo a, Amit Bhasin b, Yiqiu Tan c,⇑ a
National Center for Materials Service Safety, University of Science and Technology Beijing, Beijing, China Department of Civil, Architectural, and Environmental Engineering, The University of Texas at Austin, Austin, TX, USA c School of Transportation Science and Engineering, Harbin Institute of Technology, Harbin, China b
h i g h l i g h t s Bitumen polar fractions preferentially adsorb on the surface of mineral fillers. Bitumen is 0.3–0.85 and 5–8 times stiffer close to and away from mineral interface. Magnitude of bitumen adsorption is dictated by specific surface area of fillers.
a r t i c l e
i n f o
Article history: Received 31 October 2016 Received in revised form 11 February 2017 Accepted 12 February 2017
Keywords: Bitumen-mineral interface Interfacial rheology Physicochemical interaction
a b s t r a c t An asphalt mastic imparts most of its characteristics to the asphalt concrete mixture and also dictates several forms of distresses in asphalt mixtures and pavements. Several studies have demonstrated that the interaction between asphalt binder and mineral fillers has a significant impact on the properties and performance of asphalt mastics and mixtures. The objective of this study was to investigate the nature of binder adsorption on mineral filler surface while simultaneously quantifying the influence of such adsorption on the properties of the binder in the immediate vicinity of the interface and bulk. An adsorption test using mineral fillers and binders was conducted to achieve this goal along with measurements of asphaltene content and rheology on the original binder and the residual binder from the adsorption tests. Results show that polar fractions preferentially adhered to the surface of the mineral filler. Such preferential adsorption resulted in a significant increase in the complex modulus of the adsorbed or fixed asphalt with a concomitant decrease in the complex modulus of the free or bulk asphalt binder. These changes in complex modulus varied only slightly as a function of the frequency. The magnitude of adsorption was dictated by the mineral nature of the surface and more importantly by the specific surface area of the particles. The findings from this study are useful to better understand and model the failure mechanisms in the micro structure of asphalt composites. Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction and Background Asphalt mastic is typically regarded as a matrix that binds aggregate particles together in an asphalt mixture. As such, the properties of the mastic have a strong influence on the properties of the asphalt mixture. In the context of this paper, asphalt mastic is defined as the mixture of asphalt binder and filler particles (particles finer than 75 lm or passing ASTM standard sieve number 200). In practice, the filler to binder ratio in a mastic is controlled based on weight or volumetric requirements. For example, the
original Superpave mix design method recommends the filler to binder ratio (by weight) to be anywhere from 0.6 to 1.2. European standards (EN 1097-4) call for measurement and control of the Rigden voids in the filler while selecting fillers and designing mixes. Interestingly, in contrast to practice, research studies have shown that in addition to volumetrics, the physicochemical interactions between the filler particles and the binder are equally important in dictating the performance of the mastic and consequently that of the mix [1–4]. Some of these studies are briefly described below. 1.1. Role of fillers in dictating the properties of mastic
⇑ Corresponding author. E-mail addresses: [email protected] (M. Guo), [email protected] (A. Bhasin), [email protected] (Y. Tan). http://dx.doi.org/10.1016/j.conbuildmat.2017.02.051 0950-0618/Ó 2017 Elsevier Ltd. All rights reserved.
Anderson et al. [5] found that different fillers resulted in different amount of stiffening when added to asphalt. The extent
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of stiffening could not be explained solely on the basis of filler gradation. Other researchers also proposed that the effects of the physicochemical interactions must be studied carefully to better understand the influence of fillers on the performance of the mastic [6,7]. Petersen et al. [7] used steady state and dynamic test methods to quantify the stiffening effects of active fillers such as hydrated lime and showed that there were significant interactive effects between the asphalt binders and hydrated lime that can affect aging as well as rheological properties. More recently, Wang et al. [8] investigated the effect of mineral filler characteristics on the permanent deformation characteristics of asphalt mastics and mixtures. They found that asphalt mastic performance was significantly affected by the fractional voids in the filler and possibly by the CaO content and fineness modulus. This effect, however, varied with the type of binder indicating the possibility of binder-filler interactions. Hesami et al. [9] used a combination of numerical and experimental approaches to study the effects of interfacial interaction between asphalt binder and fillers on mastics. They arrived at a conclusion that was similar to the previous study, i.e. the shape and size of the filler particles, and the interfacial interaction between the filler and the binder had a significant influence on the properties of the composite. Several studies have tried to better understand and quantify the influence of such interfacial interaction on the performance of the mastic. For example, Tan and Guo [10,11] studied the mechanism of interfacial interaction between asphalt binder and mineral fillers by using physical and chemical test methods (e.g. dynamic mechanics analysis - DMA, differential scanning calorimetry – DSC, and Fourier transform infrared spectroscopy – FTIR) and micro/nano scale imaging techniques (e.g. atomic force microscope – AFM). They found that the impact of physicochemical interactions was significant and that such interactions were dictated by the specific surface area of fillers. In another similar study, Miljkovic and Radenberg demonstrated that the interfacial interaction between the binder and the sand in the localised contact regions influenced the fracture behavior of the bitumen emulsion mortar mixtures [12]. 1.2. Nature of interactions at the binder-filler interface While the aforementioned studies demonstrate the importance of binder-filler interaction on the overall or macroscopic properties of the mastic and consequently on the properties of the mixture, some studies have also investigated the nature of these interactions at the interface based on binder chemistry. For example, the adsorption of asphalt components on mineral fillers was investigated by many researchers. These studies showed that the polar components, such as asphaltenes, are preferentially adsorbed on the surface of the mineral filler [13–15]. Curtis et al. [13] studied the adsorption of asphalt model components (carboxylic acids, sulfoxides, phenol, pyridines, pyrrolics and ketones) representing the key asphaltic functional groups. They found that a competition between the functional groups existed towards mineral filler surface, and generally the polar functions adsorbed to a higher extent compared to nonpolar groups. Within the polar groups the sulfoxides, carboxylic acids, pyridines and phenols are the most adsorbed components. The nature of interactions at the interface of mineral aggregates (or particles) and organic molecules has also been closely studied by researchers working on flow of crude oil. For example, Acevedo et al. [16] reported that the asphaltenes dissolved in toluene were capable of creating a monolayer or a multilayer on the inorganic substrate. They also reported that the adsorption of resins dissolved in toluene on the silica was insignificant. Marczewski and Szymula [17] studied the adsorption of asphaltene dissolved in toluene on rock minerals, including natural Brazilian quartz,
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dolomite, calcite and kaolin, as well as pure oxides: Fe2O3 and TiO2. Their adsorption measurements showed that although very often adsorption of asphaltenes on minerals was described as ‘‘Langmuir type”, it was in fact quite distant from such a simple model. Saraji et al. [18] measured the asphaltene adsorption in porous media under flow conditions using the ultraviolet visible spectrophotometry. They reported that the adsorption depended on the mineral type; Calcite showed higher adsorbance compared to quartz and dolomite. Similar adsorption studies have also been conducted using asphalt binders. For example, [19] found that different functional groups have different affinities towards mineral filler, and the Langmuir and Freundlich isotherm can be used to describe adsorption. The Langmuir isotherms are based on the assumption that a monolayer forms at the interface while Freundlich isotherms are not constrained by any such assumption. Also the Langmuir adsorption model explains adsorption by assuming an adsorbate behaves as an ideal gas at isothermal conditions. Freundlich isotherms are typically used for multisite adsorption on rough surfaces. [20] studied the effect of asphaltenes/maltenes on adsorption. They found that the addition of the asphaltenes increased the amount of binder adsorbed, while the addition of maltenes reduced the amount of bitumen adsorbed. [10] conducted a net adsorption test to study the adsorption type of asphalt binder on mineral fillers. They found that the type of adsorption isotherm that they observed corresponded to multimolecular layer adsorption. Fig. 1 illustrates the most common types of adsorption isotherms. Type I isotherm shown in Fig. 1 corresponds to monolayer adsorption typically seen in chemical adsorption. Adsorption Types II-V are multi-layer adsorption isotherms typically observed with physical adsorption of molecules on solid surfaces. 1.3. Motivation and objective In summary, there is significant amount of work done to understand the effect of fillers on the mechanical property of asphalt mastics as a composite. However, most of these studies have focused on the final outcome or overall behavior of the mastics without explicitly investigating the mechanisms of the fillerbinder interface that dictate the overall behavior. Other studies have investigated the detailed mechanisms of adsorption of organic molecules from crude oil or asphalt binder onto mineral aggregate surfaces. This study was motivated by the need to bridge the gap between these two streams of knowledge, i.e. not only to
Fig. 1. Types of adsorption isotherm representing the relationship between adsorbing capacity ðaÞ and relative pressure ðP=P 0 Þ. (Type I corresponds to monolayer adsorption, while types II-V correspond to multi-molecular layer adsorption; adapted from [10]).
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understand the nature of adsorption in proximity to the interface but also the localized influence of these mechanisms on binder rheology at a similar length scale in lieu of the bulk. As such, the objective of this study was to use selective binder adsorption and recovery techniques to simultaneously investigate the details of the physicochemical interactions that take place at the binderfiller interface as well as the quantitative influence of these interactions on the properties of the binder in proximity to the interface. 2. Materials and test methods 2.1. Materials An unmodified asphalt binder with Performance Grade (PG) 6422 was used in this study. Two kinds of mineral fillers were selected for this study: limestone and granite. The fillers used in this study were washed through a filter paper using distilled water prior to being used (procedure is discussed below). The specific surface area and particle size distribution were determined by laser particle size analyzer. 2.2. Laboratory tests The overall methodology used to evaluate the influence of binder-filler interaction was as follows. A stock solution of the binder in toluene was allowed to freely interact with the filler particles, after which the filler particles were filtered and the binder was recovered from the filtrate and used for further testing. Appropriate controls were used while making comparisons. These controls were designed to isolate the influence of solvent and filter paper from the properties of the binder and are discussed in more detail later. Pertinent details of the procedure are described below. 2.2.1. Adsorption test procedure The adsorption test procedure used was as follows: 1. A stock solution of 20% w/w of binder in toluene was first prepared using 10.0 g asphalt binder. Other concentrations of binder in the toluene solution were also used in the initial part of this study before arriving at this concentration as the optimum; this is discussed in the following section. 2. 30 g of the mineral filler was placed into a funnel lined with a filter paper that had pore size range of 5–10 micrometer. Distilled water was run through the fillers and filter paper several times until the filtrate water was clear. The objective of this step was to obtain the fillers without the particles that were smaller than the pore size of filter paper. This was to ensure that filler particles did not wash out with the binder-toluene solution in the subsequent steps of the experiment and influence the measurements of the binder recovered after running it through the fillers. The size composition of fillers before and after washing process are shown in Fig. 2. The results in this Figure show that the granite filler was very similar in size distribution before and after washing. On the other hand, the limestone filler was much finer in particle size distribution before washing. While washing was necessary to avoid artifacts and errors in the experimental measurement, these comparisons also show that the limestone filler in its original form have much finer particles and consequently much higher specific surface rendering the influence of fixed asphalt (discussed in more detail later) even more substantial. 3. The filler sample residue from the filter paper was then dried in an oven at 110 °C for 5 h, after which it was placed at room temperature for 24 h to reach thermal equilibrium. The sample was then stored in a sealed aluminum container.
4. 18 g dried filler was weighed and placed into a funnel lined with a similar filter paper as before (5–10 lm size). 50 g of the asphalt-toluene stock solution obtained in step (1) (i.e. 10 g of binder) was run through the fillers and filter paper and collected in a conical flask. Vacuum was applied to accelerate the process which took a total of 30 min. 5. A rotatory evaporator was used to separate the toluene from the original asphalt-toluene stock solution as well as from the three different residual asphalt-toluene filtrates: filtrate from a blank run through the filter paper and filtrates from runs through the limestone and granite fillers. 6. Two asphalt mastics were also prepared using the same mass ratio (filler: binder = 1.8: 1 by weight). Note that this is higher than the typical filler to binder ratio by mass used in mix designs (e.g. 0.6–1.2: 1 range recommended by the Superpave volumetric mix design methods). This ratio was decided after a few trials to improve sensitivity of the measured mastic properties to both free and fixed binder. A lower filler to binder ratio than that was used would increase the relative proportion and contribution of the free binder making the measurements less sensitive to determine the properties of the fixed binder. The following is a list and nomenclature of the different samples and controls that were used for further evaluation: 1. B-C1: A sample of the original asphalt binder. 2. B-C2: Residual asphalt binder after being dissolved in toluene and then recovered as a secondary control to assess impact of toluene, if any. 3. B-C3: Residual asphalt binder recovered from the filtrate after running the solution through a blank filter paper as tertiary control to evaluate whether any adsorption occurred in the filter paper. 4. B-LS-free and B-G-free: Residual asphalt binder recovered from the filtrate after running the solution through a filter paper and washed limestone and granite fillers, respectively. 5. B-LS-fix and B-G-fix: Asphalt binder adsorbed onto the surface of limestone and granite fillers, respectively. 6. M-LS and M-G: Mastic comprising of the original asphalt binder and washed limestone and granite fillers, respectively (1.8:1 ratio by weight). 2.2.2. Oscillatory shear test A Dynamic Shear Rheometer (DSR) was used to measure the rheological properties of the different binders. A frequency sweep test was conducted by applying torsional shear strain of 0.015% following a sinusoidal waveform with frequencies varying from 0.01 to 100 radians/s and at temperatures of 30, 40, 50, 60, and 70 °C. All measurements for the binder and mastic were carried out using a specimen diameter of 25 mm and thickness of 2 mm (note that 2 mm is significantly larger than the largest filler particle 75 lm size). 2.2.3. Asphaltene content The asphaltene contents of B-C1 (original binder), B-LS-free and B-G-free (binder recovered from filtrate after interacting with limestone and granite fillers) were measured according to ASTM D 4124-09. In summary, the asphalt binder was first dissolved in iso-octane and the undissolved asphaltenes were separated using a fritted glass funnel. The residue was then vacuum degassed to remove any traces of the solvent. The mass of the dry residue, i.e. the separated fractions, was used as an estimate for the asphaltene content. Two replicates were used for these measurements.
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Fig. 2. The size composition of fillers before and after washing.
3. Influence of binder concentration on the thickness of the film adsorbed It is important to briefly discuss the importance and rationale for the choice of the binder to toluene ratio used in the procedure described in the previous section. One of the goals of this study was to evaluate whether or not the binder at the interface of the mineral aggregate was different from the bulk in terms of its chemical composition and rheological properties. Strictly speaking there is no material at an interface. Therefore, in the context of this study, interfacial binder is defined as the binder that is uniformly adsorbed onto the surface of the filler particle, i.e. binder in the immediate vicinity of the surface of the filler. This will be referred to as fixed binder in the forthcoming discussion. In contrast, the binder in the bulk of a mastic and not directly interacting with the surface of the filler particle will be referred to as free binder in the forthcoming discussion. In this study, the adsorption of the binder onto the aggregate surface was carried out by allowing the solution of the binder to interact with mineral fillers as it drained through a filler sample in a funnel. In order to determine the extent of binder that was being adsorbed during this process and to delineate between what could be considered as fixed versus free binder, tests with toluene solutions containing different percentages of binders were conducted. Note that the toluene used in this research is a medium to accelerate the interaction between asphalt binder and mineral aggregate, and also to separate the free asphalt and fixed asphalt. In real mixes at typical mixing and compaction temperatures, a similar mobility of molecules and interaction can be expected on account of the high temperatures and liquid phase of the asphalt binder. Stock solutions with four different concentrations of asphalt binder in toluene were prepared: 0%, 10%, 20% and 40% (w/w). Washed limestone fillers in a funnel with filter paper (5– 10 lm) were used in this case. The stock solution of the binder was run through the limestone fillers; the volume of the stock solution used was such that the asphalt binder to filler was 1:1.8 by mass. Overall the adsorption test procedure was the same as described previously. After this process was completed, the residue filler was vacuum degassed to remove any solvent. The mass of asphalt binder adsorbed on the surface of fillers, madsorbed , was estimated as the increase in mass of the fillers before and after the adsorption test. The following procedure was used to estimate the average asphalt film thickness adsorbed on the surface of mineral fillers. The laser particle size analyzer was used to obtain an estimate for the specific surface area of mineral fillers (Table 1). The specific surface area calculated and shown in Table 1 is based on the assumption of a spherical particle shape. Researchers recognize
that even finer particles in the filler can have a variety of shapes; the use of the specific surface area in this case is to obtain an index for the film thickness. The asphalt film thickness was calculated according to Eq. 1. The trend of asphalt film thickness with concentration of stock solution is shown in Fig. 3.
d¼
madsorbed
ð1Þ
qbinder mfiller S
In Eq. (1), d is an estimate for the thickness of adsorbed asphalt film; madsorbed is the mass of the adsorbed asphalt binder on the surface of fillers; q is the density of adsorbed asphalt binder; mfillers is the total mass of mineral fillers; S is the specific surface area of mineral fillers. Generally, particles with more angular and tortuous shapes result in larger specific surface area. The main observation from Fig. 3 is that when the concentration was between 10% and 30%, the asphalt film thickness changed very little with concentration. This indicates that the asphalt film adsorbed on the surface of fillers was close to a point where a uniform film was being adsorbed while the residual binder was being washed away. This is somewhat analogous to the formation of a monolayer in a gas adsorption isotherm. When the concentration was more than 30%, additional layers of binder are adsorbed onto the filler particles already coated with the binder. It is possible to conceive that at 100% concentration (pure binder), all the binder would be adsorbed in multiple layers resulting in the mastic, while the properties could potentially vary from the inner to the outer most layer. Based on the results shown in Fig. 3 and a comparison of typical adsorption isotherms shown in Fig. 1, a concentration of 20% was considered as optimal and adequate to ensure complete coating of both types of filler particles for the subsequent adsorption tests. In other words, for the given set up, the use of 20% concentration of binder solution was considered to result in a binder film that uniformly coated the filler particles but was not excessively thick to become indistinguishable from the bulk, should there be any differences between the fixed binder and the bulk binder. Researchers note that while such a choice is somewhat arbitrary, future studies can use a similar approach to more
Table 1 The specific surface area and adsorbed asphalt film thickness for mineral fillers. Property
Limestone Fillers
Granite Fillers
Estimated specific surface area (m2/g) Mass of adsorbed or fixed asphalt per gram of filler (g) Average estimated film thickness (micrometer)
0.227 0.133
0.131 0.065
0.587
0.493
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demonstrated that even a slight increase of asphaltene can result in a substantial increase of both stiffness and tensile strength. For example, in their study they showed that an increase in the asphaltene content by 5 percentage points resulted in an increase in the binder stiffness by a factor of 1.25–3.25. This suggests that the complex modulus of the free binder in the bulk after being subjected to adsorption by the mineral filler would decrease. In fact, this is consistent with the rheological properties discussed in the following section. 4.3. Changes in rheological properties due to adsorption
Fig. 3. Asphalt film thickness on the surface of limestone fillers using different asphalt-toluene solution concentrations.
thoroughly investigate adsorption characteristics of binder onto mineral surfaces. Also while this choice was made using limestone fillers it should also result in at least a monolayer coating even with the granite fillers since the latter have smaller surface area compared to the former. 4. Results and discussion 4.1. Thickness of the film adsorbed As discussed in the earlier section, the laser particle analyzer was used to obtain the particle size distribution of the filler particles (Fig. 2), which was in turn used to estimate the specific surface area of the fillers. Table 1 summarizes the specific surface area of the two different types of fillers, mass and average film thickness of the fixed asphalt (at a 20% w/w binder concentration). Results shown in Table 1 suggest that limestone fillers can adsorb approximately twice as much asphalt binder as that of granite fillers. However, this can be mostly due to the fact that limestone fillers had a larger specific surface area than granite fillers. The estimated film thickness is intended to account for the differences in adsorbed mass and estimated surface area. In terms of the asphalt film thickness, limestone fillers also adsorbed thicker asphalt film than granite fillers, but the preponderance was not as obvious as the adsorbed asphalt amount. 4.2. Chemical nature of the film adsorbed 4.2.1. Results based on measurement of asphaltene content Based on previous research studies, it was hypothesized that polar species from the asphalt binder would be more prone to adsorption on to the filler surface. In other words, the adsorbed or fixed asphalt binder would have a relatively higher percentage of higher polar molecules. In order to evaluate the extent of polar species that were adsorbed, the asphaltene contents of B-C1, B-LS-free and B-G-free asphalt binder samples (original, filtrate from limestone, and filtrate from granite fines, respectively) were determined according to ASTM D 4124-09 [21] using two replicates. Based on these measurements, the binders B-C1, B-LS-free, and B-G-free had asphaltene contents of 27.5%, 24.5%, and 25.5%, respectively. Note that the aforementioned results are for the filtrate. Therefore a reduction in the asphaltene content suggests that the fixed asphalt binder has a relatively higher asphaltene content due to the adsorption by the filler particles. Although the change in asphaltene concentration was small (approximately 2–3 percentage points compared to the control), Sultana and Bhasin [22]
As discussed previously, frequency-temperature sweeps were conducted and the resulting data were used to construct master curves for the selected asphalt binders. A master curve provides the relationship between complex modulus and reduced frequency. The reduced frequency is in turn obtained by developing a relationship between the test temperature and the horizontal shift of data at different frequencies from different temperatures. In this case, data from the five different test temperatures were horizontally shifted to form a single smooth curve at a reference temperature of 40 °C. This temperature was selected because it was in between the range of temperatures in the temperaturefrequency sweep measurements. Recall that binders B-LS-free and B-G-free are referred to as the free asphalt binder that did not readily or immediately adsorb on to the filler surface. A simple rule of mixtures was used to compute the complex modulus of the fixed binder, i.e. the binder adsorbed on the mineral fillers (Eq. (2)). Researchers recognize that the properties of the fixed binder calculated as such would be contingent on the accuracy of the applicability of the rule of mixtures. While beyond the scope of this study, researchers intend to explore means of using stronger solvents to extract adsorbed binder and making such measurements directly in the future.
Gbinder ¼ /Gfixed þ ð1 /ÞGfree
ð2Þ
where, / is the volume fraction of fixed asphalt; Gbinder is the measured complex modulus of the original asphalt binder; Gfixed is the estimated complex modulus of the fixed asphalt binder that was adsorbed on the mineral fillers; Gfree is the measured complex modulus of the free asphalt binder that is the residual asphalt binder obtained from the filtrate after being adsorbed by mineral fillers. Figs. 4 and 5 present the actual and normalized master curves for the binders and mastics at the reference temperature of 40 °C. A close examination of Fig. 4 reveals that there are slight differences in the complex modulus of the three control binders (BC1, B-C2, B-C3). Recall that B-C2 is the same as B-C1, except that the binder in B-C2 was recovered after being dissolved in toluene. Similarly, B-C3 is the same as B-C1, except that the binder in B-C3 was recovered from the binder that was passed through a filter paper in a toluene solution. These results show that although the use of toluene itself had very little influence on the results (B-C1 vs. B-C2), the use of toluene combined with the process of running the binders through the filter paper can alter the rheological properties of the binder to some extent (B-C1 or B-C2 vs. B-C3). In other words, the process of running the binder solution through the filter paper resulted in a slight preferential adsorption of the binder to the filter paper. The contribution of these factors was normalized by using B-C3 as the basis in order to appropriately isolate and evaluate the influence of fillers on binder adsorption. Fig. 5 illustrates the normalized master curve for the binders with the complex modulus of B-C3 as the basis. By comparing asphalt binder and asphalt mastics (B-C1,2 or 3 versus M-LS, G) in Fig. 4, as expected, mastics have a higher
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Fig. 4. Master curve for all asphalt binders and mastic; note that the values for B-LS-fix and B-G-fix are estimated using Eq. ((2)), while other values are measured.
Fig. 5. Normalized master curve for all asphalt binders and mastic using B-C3 as the basis.
complex modulus compared to the binder. This is due to the contribution of the stiffness of the filler particles to the composite as well as the combined influence of the fixed and free binder. The following three conclusions can be drawn based on a comparison of the different binder properties in Fig. 4 and particularly in Fig. 5. First, adsorption of mineral fillers can change the composition of the asphalt binder. Specifically, the results for B-LS-free and B-G-free indicate that the free asphalt binder after being subjected to adsorption by the filler particles had relatively lower stiffness compared to the control binder. A corollary to this is that the estimated stiffness of the fixed asphalt adsorbed onto the mineral aggregate is much higher than the bulk (B-LS-fix and B-G-fix in Fig. 5). Note that the influence of adsorption on the increased stiffness of the adsorbed or fixed binder is significantly higher than the decrease in stiffness of the free binder. This is on account of the fact that the volume of binder adsorbed is significantly smaller than the volume of binder that passes through as the filtrate during the adsorption test. As such, even small changes in the properties of the free binder imply significant changes in the expected properties of the fixed binder. An important implication of the above is that the properties of asphalt binder change with the distance from the surface of the mineral aggregate particle. In other words, the properties of the binder farther away from the interface (as evidenced by measurements made on the binder that did not adsorb on the interface and recovered from the filtrate) are different from the properties of the binder in the immediate vicinity of the binder-aggregate interface (as evidenced by estimated differences between measurements of original binder and binder not adsorbed). Using Fig. 5 as an indicator, the change in these properties can be substantial particularly for fillers such as limestone. In fact, although these results are for
binder-filler interaction, they do not preclude the fact that similar mechanisms can also exist at the interface of binder and coarse and fine aggregate particles [23]. This has important implications on our understanding of cohesive failure in the binder and adhesive failure at the binder-aggregate interface [11,24,25]. Second, a comparison of B-LS-free to B-G-free reveals that the limestone filler had a greater softening effect than the granite filler. This information combined with the results related to the asphaltene content suggest that more polar fractions are adsorbed onto the surface of the limestone filler. Note that the difference in asphaltene content between B-LS-free and B-G-free was 1 percentage point, however, as discussed earlier a difference of 2–3 percentage points in asphaltene content can influence the stiffness by a factor of up to 3. The difference in the results between B-LSfree and B-G-free could be due to differences in polarity of the molecular species that are adsorbed or differences in available surface area for adsorption or both. In view of the differences in the estimated specific surface area of these two mineral fillers (Table 1), it appears that the specific surface area has a significant role in dictating the amount of polar fractions that can be adsorbed onto the surface of the mineral filler. This is consistent with the findings reported by Clopotel [26], who suggested that the specific surface area of the mineral fillers to be an important factor in the filler-binder interaction. It is emphasized that although the effect of specific surface area appears to be dominating in this case, the specific mineral composition of the filler surface will also have an influence on adsorption. Third, according to the previous hypothesis, the filler preferentially adsorbs more polar or ‘‘asphaltene” like fractions. In the other words, the free binder would have a relatively lower concentration of such polar fractions. Therefore, one would expect the stiffness of the free binder to be relatively lower than the stiffness of the control binder at lower frequencies (since stiffness contribution at low frequencies is attributed to polar and associated molecules) and vice versa. For the most part, the ratio of the complex modulus for the free binder gradually increased with an increase in the frequency. For example, the complex modulus ratio for B-LS-free varied from approximately 0.3–0.5 as the frequency changed from 0.1 to 100 Hz (recall that these data are from the master curve) and the complex modulus ration for B-G-free varied from approximately 0.7–0.85 for the same range of frequencies. A slight increase in this ratio is observed at frequencies lower than 0.001 Hz but it is unclear whether this was due to the material or an artifact due to excessive specimen deformation at very high temperatures. These results are generally consistent with the aforementioned hypothesis and also suggest that the magnitude of change for a given filler is not substantial across different frequencies.
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4.4. Estimating the contribution of binder-filler interaction The previous sections quantified the impact of binder adsorption on the rheology of the fixed and free binder and also compared these results with the changes in binder chemistry. Another approach to quantitatively assess the influence of the fillerbinder interaction is in terms of an interaction ratio discussed as follows. Previous studies report that there are three mechanisms for fillers to increase the modulus of asphalt: (i) volume contribution by the relatively stiffer filler particles; (ii) the physical interaction between filler particles, and (iii) the physicochemical interaction between asphalt and fillers [27–29]. When the volume fraction of fillers is less than a critical value (typically a percolation threshold that results in particle to particle contact), the contribution from the second mechanism is negligible. For the volume fractions used in this study and based on existing data [29], the particle to particle interaction is unlikely to be a contributor. Also, a qualitative comparison between the mastics with similar volumes of the two fillers can still be made. Considering the first and third mechanisms, the stiffening effect of the filler can be expressed according to Eqs. (3)–(5). In these equations it is assumed that the mastic is a composite of filler particles coated with a thin film of asphalt binder in a matrix of free asphalt binder. Gmastic Gmastic Gfree ¼ Gbinder Gfree Gbinder
GRphysical ¼
Gmastic Gfree
GRinteraction ¼
Gfree Gbinder
ð3Þ
ð4Þ
ð5Þ
In Eqs. (3)–(5), the subscripts mastic; binder; free denote the asphalt mastic, the original binder, and the free asphalt (i.e. the residual asphalt after allowing the original binder to interact with the filler particles). The parameter GRphysical in Eq. (4) denotes the stiffening ratio due to the physical presence of the filler particle replacement and GRinteraction is the modulus ratio that represents change in the bulk binder properties due to the physicochemical interaction between asphalt binder and filler particles. The magnitude of these two stiffening effects was computed across different frequencies and is presented in Fig. 6. As noted earlier, in order to account for the influence of experimental factors on these calculations, they were normalized based on the value of BC-3. In Fig. 6, values of GRinteraction that are less than 1 by definition indicate that the stiffness of the free binder is lower than the
stiffness of the original binder, which also implies that the mineral filler has adsorbed fractions of binder that contribute to its stiffness. The lower the value of this parameter, the higher the adsorption or interaction between the binder and the mineral filler (e.g. as in the case of the limestone filler vs granite). Based on the results shown in Fig. 6 and for the materials used in this study, it is clear that although the volume or physical contribution of limestone and granite filler was similar (GRphysical ), the interaction effect was substantially different (GRinteraction ). As discussed earlier, the latter could be due to (i) adsorption of more polar species (i.e. adsorption amount is same but more polar species are adsorbed by one compared to the other), or (ii) more adsorption of similar polar species (i.e. similar polar molecules are adsorbed by both fillers but one adsorbs more volume than the other) or (iii) more likely a combination of (i) and (ii). 5. Conclusions The objective of this study was to investigate the nature of binder adsorption that takes place on the surface of filler particles in tandem with the influence of such interactions on the rheological properties of the adsorbed or fixed binder and the residual or bulk free binder. The following is a summary of conclusions that can be drawn based on the aforementioned results and discussion: 1. Investigation of asphaltene content in the residual binder after subjecting it to adsorption with mineral fillers indicates that polar fractions are preferentially adsorbed on to the surface of fillers. It is noted that while the study investigated asphaltene content, it is just as likely that other polar fractions were adsorbed by the fillers as well. 2. Consistent with the previous observation, the stiffness of the free asphalt (binder that was not adsorbed on the filler surface) was approximately 0.3–0.85 times the stiffness of the original asphalt binder. Conversely, the stiffness of the adsorbed binder was estimated to be 5–8 times the stiffness of the original asphalt binder based on the rule of mixtures. These large differences have important implications in terms of understanding and modeling (cohesive and interfacial) failure in the micro structure of asphalt composites. 3. The magnitude of change in binder properties is related to the quality and quantity of the adsorbed binder, which is in turn related to the mineral character of the filler surface and the specific surface area of the mineral. For the minerals used in this study, it appears that the latter (specific surface area) plays a more significant role. Also, for a given filler, the magnitude of this change did not vary substantially with loading rate. 4. Binder – filler adsorption experiments must be carefully set up with appropriate controls as demonstrated in this study, since factors such as the use of a solvent and adsorption by a filter paper can influence the properties of the binder.
Acknowledgments The authors would like to acknowledge financial support from China Scholarship Council (Grant No.: 201306120187) and National Science Fund for Distinguished Young Scholars of China (Grant No.: 51225803). References
Fig. 6. Comparison of stiffening effect due to presence of filler particles versus interaction.
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M. Guo et al. / Construction and Building Materials 141 (2017) 152–159 [3] J.S. Miller, R.N. Traxler, Some of the fundamental physical characteristics of mineral filler intended for asphalt paving mixtures, Proc. Assoc. Asphalt Paving Technol. 3 (1932) 53–63. [4] P.J. Rigden, The use of fillers in bituminous road surfacing mixtures. A study of filler-binder systems in relation to filler characteristics, J. Soc. Chem. Ind. (1947) 299–309. [5] D.A. Anderson, W.H. Goetz, Mechanical behavior and reinforcement of mineral filler-asphalt mixtures, Proc. Assoc. Asphalt Paving Technol. 42 (1973) 37–66. [6] E.L. Dukatz, David A. Anderson, The effect of various fillers on the mechanical behavior of asphalt and asphaltic concrete, J. Assoc. Asphalt Paving Technol. 49 (1980) 530–549. [7] J Claine Petersen, Henry Plancher, P. Michael Harnsberger. Lime treatment of asphalt to reduce age hardening and improve flow properties. In: Association of Asphalt Paving Technologists Proceedings Technical Sessions, 1987, Reno, Nevada, USA, vol. 56, 1987. [8] Hao Wang, Imad Al-Qadi, Ahmed Faheem, Hussain Bahia, Shih-Hsien Yang, Gerald Reinke, Effect of mineral filler characteristics on asphalt mastic and mixture rutting potential, Transp. Res. Rec. J. Transp. Res. Board, 0361-1981 (2208) (2011) 33–39. [9] Ebrahim Hesami, Bjorn Birgisson, Niki Kringos, Numerical and experimental evaluation of the influence of the filler bitumen interface in mastics, Mater. Struct., 1359-5997 47 (8) (2014) 1325–1337. [10] Yiqiu Tan, Meng Guo, Micro-and nano-characteration of interaction between asphalt and filler, J. Test. Eval., 0090-3973 42 (5) (2014) 1089–1097. [11] Yiqiu Tan, Meng Guo, Interfacial thickness and interaction between asphalt and mineral fillers, Mater. Struct., 1359-5997 47 (4) (2014) 605–614. [12] Miomir Miljkovic, Martin Radenberg, Fracture behaviour of bitumen emulsion mortar mixtures, Constr. Build. Mater., 0950-0618 62 (2014) 126–134. [13] C.W. Curtis, K. Ensely, J. Epps. Fundamental Properties of asphalt-aggregate interactions including adhesion and absorption. Technical report, Strategic Highway Research Program, National Research Council, Washington, D.C., 1993. [14] J.A.N. Scott, Adhesion and disbonding mechanisms of asphalt used in highway construction and maintenance, Assoc. Asphalt Paving Technol. Proc. 47 (1978). [15] Gabrielle Fritschy, Eugène Papirer, Interactions between a bitumen, its components and model fillers, Fuel, 0016-2361 57 (11) (1978) 701–704. [16] Sócrates Acevedo, Maria A. Ranaudo, Gastón Escobar, Luis Gutiérrez, Pedro Ortega, Adsorption of asphaltenes and resins on organic and inorganic
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Effect of mineral fillers adsorption on rheological and chemical properties of asphalt binder Meng Guo a, Amit Bhasin b, Yiqiu Tan c,⇑ a
National Center for Materials Service Safety, University of Science and Technology Beijing, Beijing, China Department of Civil, Architectural, and Environmental Engineering, The University of Texas at Austin, Austin, TX, USA c School of Transportation Science and Engineering, Harbin Institute of Technology, Harbin, China b
h i g h l i g h t s Bitumen polar fractions preferentially adsorb on the surface of mineral fillers. Bitumen is 0.3–0.85 and 5–8 times stiffer close to and away from mineral interface. Magnitude of bitumen adsorption is dictated by specific surface area of fillers.
a r t i c l e
i n f o
Article history: Received 31 October 2016 Received in revised form 11 February 2017 Accepted 12 February 2017
Keywords: Bitumen-mineral interface Interfacial rheology Physicochemical interaction
a b s t r a c t An asphalt mastic imparts most of its characteristics to the asphalt concrete mixture and also dictates several forms of distresses in asphalt mixtures and pavements. Several studies have demonstrated that the interaction between asphalt binder and mineral fillers has a significant impact on the properties and performance of asphalt mastics and mixtures. The objective of this study was to investigate the nature of binder adsorption on mineral filler surface while simultaneously quantifying the influence of such adsorption on the properties of the binder in the immediate vicinity of the interface and bulk. An adsorption test using mineral fillers and binders was conducted to achieve this goal along with measurements of asphaltene content and rheology on the original binder and the residual binder from the adsorption tests. Results show that polar fractions preferentially adhered to the surface of the mineral filler. Such preferential adsorption resulted in a significant increase in the complex modulus of the adsorbed or fixed asphalt with a concomitant decrease in the complex modulus of the free or bulk asphalt binder. These changes in complex modulus varied only slightly as a function of the frequency. The magnitude of adsorption was dictated by the mineral nature of the surface and more importantly by the specific surface area of the particles. The findings from this study are useful to better understand and model the failure mechanisms in the micro structure of asphalt composites. Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction and Background Asphalt mastic is typically regarded as a matrix that binds aggregate particles together in an asphalt mixture. As such, the properties of the mastic have a strong influence on the properties of the asphalt mixture. In the context of this paper, asphalt mastic is defined as the mixture of asphalt binder and filler particles (particles finer than 75 lm or passing ASTM standard sieve number 200). In practice, the filler to binder ratio in a mastic is controlled based on weight or volumetric requirements. For example, the
original Superpave mix design method recommends the filler to binder ratio (by weight) to be anywhere from 0.6 to 1.2. European standards (EN 1097-4) call for measurement and control of the Rigden voids in the filler while selecting fillers and designing mixes. Interestingly, in contrast to practice, research studies have shown that in addition to volumetrics, the physicochemical interactions between the filler particles and the binder are equally important in dictating the performance of the mastic and consequently that of the mix [1–4]. Some of these studies are briefly described below. 1.1. Role of fillers in dictating the properties of mastic
⇑ Corresponding author. E-mail addresses: [email protected] (M. Guo), [email protected] (A. Bhasin), [email protected] (Y. Tan). http://dx.doi.org/10.1016/j.conbuildmat.2017.02.051 0950-0618/Ó 2017 Elsevier Ltd. All rights reserved.
Anderson et al. [5] found that different fillers resulted in different amount of stiffening when added to asphalt. The extent
M. Guo et al. / Construction and Building Materials 141 (2017) 152–159
of stiffening could not be explained solely on the basis of filler gradation. Other researchers also proposed that the effects of the physicochemical interactions must be studied carefully to better understand the influence of fillers on the performance of the mastic [6,7]. Petersen et al. [7] used steady state and dynamic test methods to quantify the stiffening effects of active fillers such as hydrated lime and showed that there were significant interactive effects between the asphalt binders and hydrated lime that can affect aging as well as rheological properties. More recently, Wang et al. [8] investigated the effect of mineral filler characteristics on the permanent deformation characteristics of asphalt mastics and mixtures. They found that asphalt mastic performance was significantly affected by the fractional voids in the filler and possibly by the CaO content and fineness modulus. This effect, however, varied with the type of binder indicating the possibility of binder-filler interactions. Hesami et al. [9] used a combination of numerical and experimental approaches to study the effects of interfacial interaction between asphalt binder and fillers on mastics. They arrived at a conclusion that was similar to the previous study, i.e. the shape and size of the filler particles, and the interfacial interaction between the filler and the binder had a significant influence on the properties of the composite. Several studies have tried to better understand and quantify the influence of such interfacial interaction on the performance of the mastic. For example, Tan and Guo [10,11] studied the mechanism of interfacial interaction between asphalt binder and mineral fillers by using physical and chemical test methods (e.g. dynamic mechanics analysis - DMA, differential scanning calorimetry – DSC, and Fourier transform infrared spectroscopy – FTIR) and micro/nano scale imaging techniques (e.g. atomic force microscope – AFM). They found that the impact of physicochemical interactions was significant and that such interactions were dictated by the specific surface area of fillers. In another similar study, Miljkovic and Radenberg demonstrated that the interfacial interaction between the binder and the sand in the localised contact regions influenced the fracture behavior of the bitumen emulsion mortar mixtures [12]. 1.2. Nature of interactions at the binder-filler interface While the aforementioned studies demonstrate the importance of binder-filler interaction on the overall or macroscopic properties of the mastic and consequently on the properties of the mixture, some studies have also investigated the nature of these interactions at the interface based on binder chemistry. For example, the adsorption of asphalt components on mineral fillers was investigated by many researchers. These studies showed that the polar components, such as asphaltenes, are preferentially adsorbed on the surface of the mineral filler [13–15]. Curtis et al. [13] studied the adsorption of asphalt model components (carboxylic acids, sulfoxides, phenol, pyridines, pyrrolics and ketones) representing the key asphaltic functional groups. They found that a competition between the functional groups existed towards mineral filler surface, and generally the polar functions adsorbed to a higher extent compared to nonpolar groups. Within the polar groups the sulfoxides, carboxylic acids, pyridines and phenols are the most adsorbed components. The nature of interactions at the interface of mineral aggregates (or particles) and organic molecules has also been closely studied by researchers working on flow of crude oil. For example, Acevedo et al. [16] reported that the asphaltenes dissolved in toluene were capable of creating a monolayer or a multilayer on the inorganic substrate. They also reported that the adsorption of resins dissolved in toluene on the silica was insignificant. Marczewski and Szymula [17] studied the adsorption of asphaltene dissolved in toluene on rock minerals, including natural Brazilian quartz,
153
dolomite, calcite and kaolin, as well as pure oxides: Fe2O3 and TiO2. Their adsorption measurements showed that although very often adsorption of asphaltenes on minerals was described as ‘‘Langmuir type”, it was in fact quite distant from such a simple model. Saraji et al. [18] measured the asphaltene adsorption in porous media under flow conditions using the ultraviolet visible spectrophotometry. They reported that the adsorption depended on the mineral type; Calcite showed higher adsorbance compared to quartz and dolomite. Similar adsorption studies have also been conducted using asphalt binders. For example, [19] found that different functional groups have different affinities towards mineral filler, and the Langmuir and Freundlich isotherm can be used to describe adsorption. The Langmuir isotherms are based on the assumption that a monolayer forms at the interface while Freundlich isotherms are not constrained by any such assumption. Also the Langmuir adsorption model explains adsorption by assuming an adsorbate behaves as an ideal gas at isothermal conditions. Freundlich isotherms are typically used for multisite adsorption on rough surfaces. [20] studied the effect of asphaltenes/maltenes on adsorption. They found that the addition of the asphaltenes increased the amount of binder adsorbed, while the addition of maltenes reduced the amount of bitumen adsorbed. [10] conducted a net adsorption test to study the adsorption type of asphalt binder on mineral fillers. They found that the type of adsorption isotherm that they observed corresponded to multimolecular layer adsorption. Fig. 1 illustrates the most common types of adsorption isotherms. Type I isotherm shown in Fig. 1 corresponds to monolayer adsorption typically seen in chemical adsorption. Adsorption Types II-V are multi-layer adsorption isotherms typically observed with physical adsorption of molecules on solid surfaces. 1.3. Motivation and objective In summary, there is significant amount of work done to understand the effect of fillers on the mechanical property of asphalt mastics as a composite. However, most of these studies have focused on the final outcome or overall behavior of the mastics without explicitly investigating the mechanisms of the fillerbinder interface that dictate the overall behavior. Other studies have investigated the detailed mechanisms of adsorption of organic molecules from crude oil or asphalt binder onto mineral aggregate surfaces. This study was motivated by the need to bridge the gap between these two streams of knowledge, i.e. not only to
Fig. 1. Types of adsorption isotherm representing the relationship between adsorbing capacity ðaÞ and relative pressure ðP=P 0 Þ. (Type I corresponds to monolayer adsorption, while types II-V correspond to multi-molecular layer adsorption; adapted from [10]).
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understand the nature of adsorption in proximity to the interface but also the localized influence of these mechanisms on binder rheology at a similar length scale in lieu of the bulk. As such, the objective of this study was to use selective binder adsorption and recovery techniques to simultaneously investigate the details of the physicochemical interactions that take place at the binderfiller interface as well as the quantitative influence of these interactions on the properties of the binder in proximity to the interface. 2. Materials and test methods 2.1. Materials An unmodified asphalt binder with Performance Grade (PG) 6422 was used in this study. Two kinds of mineral fillers were selected for this study: limestone and granite. The fillers used in this study were washed through a filter paper using distilled water prior to being used (procedure is discussed below). The specific surface area and particle size distribution were determined by laser particle size analyzer. 2.2. Laboratory tests The overall methodology used to evaluate the influence of binder-filler interaction was as follows. A stock solution of the binder in toluene was allowed to freely interact with the filler particles, after which the filler particles were filtered and the binder was recovered from the filtrate and used for further testing. Appropriate controls were used while making comparisons. These controls were designed to isolate the influence of solvent and filter paper from the properties of the binder and are discussed in more detail later. Pertinent details of the procedure are described below. 2.2.1. Adsorption test procedure The adsorption test procedure used was as follows: 1. A stock solution of 20% w/w of binder in toluene was first prepared using 10.0 g asphalt binder. Other concentrations of binder in the toluene solution were also used in the initial part of this study before arriving at this concentration as the optimum; this is discussed in the following section. 2. 30 g of the mineral filler was placed into a funnel lined with a filter paper that had pore size range of 5–10 micrometer. Distilled water was run through the fillers and filter paper several times until the filtrate water was clear. The objective of this step was to obtain the fillers without the particles that were smaller than the pore size of filter paper. This was to ensure that filler particles did not wash out with the binder-toluene solution in the subsequent steps of the experiment and influence the measurements of the binder recovered after running it through the fillers. The size composition of fillers before and after washing process are shown in Fig. 2. The results in this Figure show that the granite filler was very similar in size distribution before and after washing. On the other hand, the limestone filler was much finer in particle size distribution before washing. While washing was necessary to avoid artifacts and errors in the experimental measurement, these comparisons also show that the limestone filler in its original form have much finer particles and consequently much higher specific surface rendering the influence of fixed asphalt (discussed in more detail later) even more substantial. 3. The filler sample residue from the filter paper was then dried in an oven at 110 °C for 5 h, after which it was placed at room temperature for 24 h to reach thermal equilibrium. The sample was then stored in a sealed aluminum container.
4. 18 g dried filler was weighed and placed into a funnel lined with a similar filter paper as before (5–10 lm size). 50 g of the asphalt-toluene stock solution obtained in step (1) (i.e. 10 g of binder) was run through the fillers and filter paper and collected in a conical flask. Vacuum was applied to accelerate the process which took a total of 30 min. 5. A rotatory evaporator was used to separate the toluene from the original asphalt-toluene stock solution as well as from the three different residual asphalt-toluene filtrates: filtrate from a blank run through the filter paper and filtrates from runs through the limestone and granite fillers. 6. Two asphalt mastics were also prepared using the same mass ratio (filler: binder = 1.8: 1 by weight). Note that this is higher than the typical filler to binder ratio by mass used in mix designs (e.g. 0.6–1.2: 1 range recommended by the Superpave volumetric mix design methods). This ratio was decided after a few trials to improve sensitivity of the measured mastic properties to both free and fixed binder. A lower filler to binder ratio than that was used would increase the relative proportion and contribution of the free binder making the measurements less sensitive to determine the properties of the fixed binder. The following is a list and nomenclature of the different samples and controls that were used for further evaluation: 1. B-C1: A sample of the original asphalt binder. 2. B-C2: Residual asphalt binder after being dissolved in toluene and then recovered as a secondary control to assess impact of toluene, if any. 3. B-C3: Residual asphalt binder recovered from the filtrate after running the solution through a blank filter paper as tertiary control to evaluate whether any adsorption occurred in the filter paper. 4. B-LS-free and B-G-free: Residual asphalt binder recovered from the filtrate after running the solution through a filter paper and washed limestone and granite fillers, respectively. 5. B-LS-fix and B-G-fix: Asphalt binder adsorbed onto the surface of limestone and granite fillers, respectively. 6. M-LS and M-G: Mastic comprising of the original asphalt binder and washed limestone and granite fillers, respectively (1.8:1 ratio by weight). 2.2.2. Oscillatory shear test A Dynamic Shear Rheometer (DSR) was used to measure the rheological properties of the different binders. A frequency sweep test was conducted by applying torsional shear strain of 0.015% following a sinusoidal waveform with frequencies varying from 0.01 to 100 radians/s and at temperatures of 30, 40, 50, 60, and 70 °C. All measurements for the binder and mastic were carried out using a specimen diameter of 25 mm and thickness of 2 mm (note that 2 mm is significantly larger than the largest filler particle 75 lm size). 2.2.3. Asphaltene content The asphaltene contents of B-C1 (original binder), B-LS-free and B-G-free (binder recovered from filtrate after interacting with limestone and granite fillers) were measured according to ASTM D 4124-09. In summary, the asphalt binder was first dissolved in iso-octane and the undissolved asphaltenes were separated using a fritted glass funnel. The residue was then vacuum degassed to remove any traces of the solvent. The mass of the dry residue, i.e. the separated fractions, was used as an estimate for the asphaltene content. Two replicates were used for these measurements.
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Fig. 2. The size composition of fillers before and after washing.
3. Influence of binder concentration on the thickness of the film adsorbed It is important to briefly discuss the importance and rationale for the choice of the binder to toluene ratio used in the procedure described in the previous section. One of the goals of this study was to evaluate whether or not the binder at the interface of the mineral aggregate was different from the bulk in terms of its chemical composition and rheological properties. Strictly speaking there is no material at an interface. Therefore, in the context of this study, interfacial binder is defined as the binder that is uniformly adsorbed onto the surface of the filler particle, i.e. binder in the immediate vicinity of the surface of the filler. This will be referred to as fixed binder in the forthcoming discussion. In contrast, the binder in the bulk of a mastic and not directly interacting with the surface of the filler particle will be referred to as free binder in the forthcoming discussion. In this study, the adsorption of the binder onto the aggregate surface was carried out by allowing the solution of the binder to interact with mineral fillers as it drained through a filler sample in a funnel. In order to determine the extent of binder that was being adsorbed during this process and to delineate between what could be considered as fixed versus free binder, tests with toluene solutions containing different percentages of binders were conducted. Note that the toluene used in this research is a medium to accelerate the interaction between asphalt binder and mineral aggregate, and also to separate the free asphalt and fixed asphalt. In real mixes at typical mixing and compaction temperatures, a similar mobility of molecules and interaction can be expected on account of the high temperatures and liquid phase of the asphalt binder. Stock solutions with four different concentrations of asphalt binder in toluene were prepared: 0%, 10%, 20% and 40% (w/w). Washed limestone fillers in a funnel with filter paper (5– 10 lm) were used in this case. The stock solution of the binder was run through the limestone fillers; the volume of the stock solution used was such that the asphalt binder to filler was 1:1.8 by mass. Overall the adsorption test procedure was the same as described previously. After this process was completed, the residue filler was vacuum degassed to remove any solvent. The mass of asphalt binder adsorbed on the surface of fillers, madsorbed , was estimated as the increase in mass of the fillers before and after the adsorption test. The following procedure was used to estimate the average asphalt film thickness adsorbed on the surface of mineral fillers. The laser particle size analyzer was used to obtain an estimate for the specific surface area of mineral fillers (Table 1). The specific surface area calculated and shown in Table 1 is based on the assumption of a spherical particle shape. Researchers recognize
that even finer particles in the filler can have a variety of shapes; the use of the specific surface area in this case is to obtain an index for the film thickness. The asphalt film thickness was calculated according to Eq. 1. The trend of asphalt film thickness with concentration of stock solution is shown in Fig. 3.
d¼
madsorbed
ð1Þ
qbinder mfiller S
In Eq. (1), d is an estimate for the thickness of adsorbed asphalt film; madsorbed is the mass of the adsorbed asphalt binder on the surface of fillers; q is the density of adsorbed asphalt binder; mfillers is the total mass of mineral fillers; S is the specific surface area of mineral fillers. Generally, particles with more angular and tortuous shapes result in larger specific surface area. The main observation from Fig. 3 is that when the concentration was between 10% and 30%, the asphalt film thickness changed very little with concentration. This indicates that the asphalt film adsorbed on the surface of fillers was close to a point where a uniform film was being adsorbed while the residual binder was being washed away. This is somewhat analogous to the formation of a monolayer in a gas adsorption isotherm. When the concentration was more than 30%, additional layers of binder are adsorbed onto the filler particles already coated with the binder. It is possible to conceive that at 100% concentration (pure binder), all the binder would be adsorbed in multiple layers resulting in the mastic, while the properties could potentially vary from the inner to the outer most layer. Based on the results shown in Fig. 3 and a comparison of typical adsorption isotherms shown in Fig. 1, a concentration of 20% was considered as optimal and adequate to ensure complete coating of both types of filler particles for the subsequent adsorption tests. In other words, for the given set up, the use of 20% concentration of binder solution was considered to result in a binder film that uniformly coated the filler particles but was not excessively thick to become indistinguishable from the bulk, should there be any differences between the fixed binder and the bulk binder. Researchers note that while such a choice is somewhat arbitrary, future studies can use a similar approach to more
Table 1 The specific surface area and adsorbed asphalt film thickness for mineral fillers. Property
Limestone Fillers
Granite Fillers
Estimated specific surface area (m2/g) Mass of adsorbed or fixed asphalt per gram of filler (g) Average estimated film thickness (micrometer)
0.227 0.133
0.131 0.065
0.587
0.493
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demonstrated that even a slight increase of asphaltene can result in a substantial increase of both stiffness and tensile strength. For example, in their study they showed that an increase in the asphaltene content by 5 percentage points resulted in an increase in the binder stiffness by a factor of 1.25–3.25. This suggests that the complex modulus of the free binder in the bulk after being subjected to adsorption by the mineral filler would decrease. In fact, this is consistent with the rheological properties discussed in the following section. 4.3. Changes in rheological properties due to adsorption
Fig. 3. Asphalt film thickness on the surface of limestone fillers using different asphalt-toluene solution concentrations.
thoroughly investigate adsorption characteristics of binder onto mineral surfaces. Also while this choice was made using limestone fillers it should also result in at least a monolayer coating even with the granite fillers since the latter have smaller surface area compared to the former. 4. Results and discussion 4.1. Thickness of the film adsorbed As discussed in the earlier section, the laser particle analyzer was used to obtain the particle size distribution of the filler particles (Fig. 2), which was in turn used to estimate the specific surface area of the fillers. Table 1 summarizes the specific surface area of the two different types of fillers, mass and average film thickness of the fixed asphalt (at a 20% w/w binder concentration). Results shown in Table 1 suggest that limestone fillers can adsorb approximately twice as much asphalt binder as that of granite fillers. However, this can be mostly due to the fact that limestone fillers had a larger specific surface area than granite fillers. The estimated film thickness is intended to account for the differences in adsorbed mass and estimated surface area. In terms of the asphalt film thickness, limestone fillers also adsorbed thicker asphalt film than granite fillers, but the preponderance was not as obvious as the adsorbed asphalt amount. 4.2. Chemical nature of the film adsorbed 4.2.1. Results based on measurement of asphaltene content Based on previous research studies, it was hypothesized that polar species from the asphalt binder would be more prone to adsorption on to the filler surface. In other words, the adsorbed or fixed asphalt binder would have a relatively higher percentage of higher polar molecules. In order to evaluate the extent of polar species that were adsorbed, the asphaltene contents of B-C1, B-LS-free and B-G-free asphalt binder samples (original, filtrate from limestone, and filtrate from granite fines, respectively) were determined according to ASTM D 4124-09 [21] using two replicates. Based on these measurements, the binders B-C1, B-LS-free, and B-G-free had asphaltene contents of 27.5%, 24.5%, and 25.5%, respectively. Note that the aforementioned results are for the filtrate. Therefore a reduction in the asphaltene content suggests that the fixed asphalt binder has a relatively higher asphaltene content due to the adsorption by the filler particles. Although the change in asphaltene concentration was small (approximately 2–3 percentage points compared to the control), Sultana and Bhasin [22]
As discussed previously, frequency-temperature sweeps were conducted and the resulting data were used to construct master curves for the selected asphalt binders. A master curve provides the relationship between complex modulus and reduced frequency. The reduced frequency is in turn obtained by developing a relationship between the test temperature and the horizontal shift of data at different frequencies from different temperatures. In this case, data from the five different test temperatures were horizontally shifted to form a single smooth curve at a reference temperature of 40 °C. This temperature was selected because it was in between the range of temperatures in the temperaturefrequency sweep measurements. Recall that binders B-LS-free and B-G-free are referred to as the free asphalt binder that did not readily or immediately adsorb on to the filler surface. A simple rule of mixtures was used to compute the complex modulus of the fixed binder, i.e. the binder adsorbed on the mineral fillers (Eq. (2)). Researchers recognize that the properties of the fixed binder calculated as such would be contingent on the accuracy of the applicability of the rule of mixtures. While beyond the scope of this study, researchers intend to explore means of using stronger solvents to extract adsorbed binder and making such measurements directly in the future.
Gbinder ¼ /Gfixed þ ð1 /ÞGfree
ð2Þ
where, / is the volume fraction of fixed asphalt; Gbinder is the measured complex modulus of the original asphalt binder; Gfixed is the estimated complex modulus of the fixed asphalt binder that was adsorbed on the mineral fillers; Gfree is the measured complex modulus of the free asphalt binder that is the residual asphalt binder obtained from the filtrate after being adsorbed by mineral fillers. Figs. 4 and 5 present the actual and normalized master curves for the binders and mastics at the reference temperature of 40 °C. A close examination of Fig. 4 reveals that there are slight differences in the complex modulus of the three control binders (BC1, B-C2, B-C3). Recall that B-C2 is the same as B-C1, except that the binder in B-C2 was recovered after being dissolved in toluene. Similarly, B-C3 is the same as B-C1, except that the binder in B-C3 was recovered from the binder that was passed through a filter paper in a toluene solution. These results show that although the use of toluene itself had very little influence on the results (B-C1 vs. B-C2), the use of toluene combined with the process of running the binders through the filter paper can alter the rheological properties of the binder to some extent (B-C1 or B-C2 vs. B-C3). In other words, the process of running the binder solution through the filter paper resulted in a slight preferential adsorption of the binder to the filter paper. The contribution of these factors was normalized by using B-C3 as the basis in order to appropriately isolate and evaluate the influence of fillers on binder adsorption. Fig. 5 illustrates the normalized master curve for the binders with the complex modulus of B-C3 as the basis. By comparing asphalt binder and asphalt mastics (B-C1,2 or 3 versus M-LS, G) in Fig. 4, as expected, mastics have a higher
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157
Fig. 4. Master curve for all asphalt binders and mastic; note that the values for B-LS-fix and B-G-fix are estimated using Eq. ((2)), while other values are measured.
Fig. 5. Normalized master curve for all asphalt binders and mastic using B-C3 as the basis.
complex modulus compared to the binder. This is due to the contribution of the stiffness of the filler particles to the composite as well as the combined influence of the fixed and free binder. The following three conclusions can be drawn based on a comparison of the different binder properties in Fig. 4 and particularly in Fig. 5. First, adsorption of mineral fillers can change the composition of the asphalt binder. Specifically, the results for B-LS-free and B-G-free indicate that the free asphalt binder after being subjected to adsorption by the filler particles had relatively lower stiffness compared to the control binder. A corollary to this is that the estimated stiffness of the fixed asphalt adsorbed onto the mineral aggregate is much higher than the bulk (B-LS-fix and B-G-fix in Fig. 5). Note that the influence of adsorption on the increased stiffness of the adsorbed or fixed binder is significantly higher than the decrease in stiffness of the free binder. This is on account of the fact that the volume of binder adsorbed is significantly smaller than the volume of binder that passes through as the filtrate during the adsorption test. As such, even small changes in the properties of the free binder imply significant changes in the expected properties of the fixed binder. An important implication of the above is that the properties of asphalt binder change with the distance from the surface of the mineral aggregate particle. In other words, the properties of the binder farther away from the interface (as evidenced by measurements made on the binder that did not adsorb on the interface and recovered from the filtrate) are different from the properties of the binder in the immediate vicinity of the binder-aggregate interface (as evidenced by estimated differences between measurements of original binder and binder not adsorbed). Using Fig. 5 as an indicator, the change in these properties can be substantial particularly for fillers such as limestone. In fact, although these results are for
binder-filler interaction, they do not preclude the fact that similar mechanisms can also exist at the interface of binder and coarse and fine aggregate particles [23]. This has important implications on our understanding of cohesive failure in the binder and adhesive failure at the binder-aggregate interface [11,24,25]. Second, a comparison of B-LS-free to B-G-free reveals that the limestone filler had a greater softening effect than the granite filler. This information combined with the results related to the asphaltene content suggest that more polar fractions are adsorbed onto the surface of the limestone filler. Note that the difference in asphaltene content between B-LS-free and B-G-free was 1 percentage point, however, as discussed earlier a difference of 2–3 percentage points in asphaltene content can influence the stiffness by a factor of up to 3. The difference in the results between B-LSfree and B-G-free could be due to differences in polarity of the molecular species that are adsorbed or differences in available surface area for adsorption or both. In view of the differences in the estimated specific surface area of these two mineral fillers (Table 1), it appears that the specific surface area has a significant role in dictating the amount of polar fractions that can be adsorbed onto the surface of the mineral filler. This is consistent with the findings reported by Clopotel [26], who suggested that the specific surface area of the mineral fillers to be an important factor in the filler-binder interaction. It is emphasized that although the effect of specific surface area appears to be dominating in this case, the specific mineral composition of the filler surface will also have an influence on adsorption. Third, according to the previous hypothesis, the filler preferentially adsorbs more polar or ‘‘asphaltene” like fractions. In the other words, the free binder would have a relatively lower concentration of such polar fractions. Therefore, one would expect the stiffness of the free binder to be relatively lower than the stiffness of the control binder at lower frequencies (since stiffness contribution at low frequencies is attributed to polar and associated molecules) and vice versa. For the most part, the ratio of the complex modulus for the free binder gradually increased with an increase in the frequency. For example, the complex modulus ratio for B-LS-free varied from approximately 0.3–0.5 as the frequency changed from 0.1 to 100 Hz (recall that these data are from the master curve) and the complex modulus ration for B-G-free varied from approximately 0.7–0.85 for the same range of frequencies. A slight increase in this ratio is observed at frequencies lower than 0.001 Hz but it is unclear whether this was due to the material or an artifact due to excessive specimen deformation at very high temperatures. These results are generally consistent with the aforementioned hypothesis and also suggest that the magnitude of change for a given filler is not substantial across different frequencies.
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4.4. Estimating the contribution of binder-filler interaction The previous sections quantified the impact of binder adsorption on the rheology of the fixed and free binder and also compared these results with the changes in binder chemistry. Another approach to quantitatively assess the influence of the fillerbinder interaction is in terms of an interaction ratio discussed as follows. Previous studies report that there are three mechanisms for fillers to increase the modulus of asphalt: (i) volume contribution by the relatively stiffer filler particles; (ii) the physical interaction between filler particles, and (iii) the physicochemical interaction between asphalt and fillers [27–29]. When the volume fraction of fillers is less than a critical value (typically a percolation threshold that results in particle to particle contact), the contribution from the second mechanism is negligible. For the volume fractions used in this study and based on existing data [29], the particle to particle interaction is unlikely to be a contributor. Also, a qualitative comparison between the mastics with similar volumes of the two fillers can still be made. Considering the first and third mechanisms, the stiffening effect of the filler can be expressed according to Eqs. (3)–(5). In these equations it is assumed that the mastic is a composite of filler particles coated with a thin film of asphalt binder in a matrix of free asphalt binder. Gmastic Gmastic Gfree ¼ Gbinder Gfree Gbinder
GRphysical ¼
Gmastic Gfree
GRinteraction ¼
Gfree Gbinder
ð3Þ
ð4Þ
ð5Þ
In Eqs. (3)–(5), the subscripts mastic; binder; free denote the asphalt mastic, the original binder, and the free asphalt (i.e. the residual asphalt after allowing the original binder to interact with the filler particles). The parameter GRphysical in Eq. (4) denotes the stiffening ratio due to the physical presence of the filler particle replacement and GRinteraction is the modulus ratio that represents change in the bulk binder properties due to the physicochemical interaction between asphalt binder and filler particles. The magnitude of these two stiffening effects was computed across different frequencies and is presented in Fig. 6. As noted earlier, in order to account for the influence of experimental factors on these calculations, they were normalized based on the value of BC-3. In Fig. 6, values of GRinteraction that are less than 1 by definition indicate that the stiffness of the free binder is lower than the
stiffness of the original binder, which also implies that the mineral filler has adsorbed fractions of binder that contribute to its stiffness. The lower the value of this parameter, the higher the adsorption or interaction between the binder and the mineral filler (e.g. as in the case of the limestone filler vs granite). Based on the results shown in Fig. 6 and for the materials used in this study, it is clear that although the volume or physical contribution of limestone and granite filler was similar (GRphysical ), the interaction effect was substantially different (GRinteraction ). As discussed earlier, the latter could be due to (i) adsorption of more polar species (i.e. adsorption amount is same but more polar species are adsorbed by one compared to the other), or (ii) more adsorption of similar polar species (i.e. similar polar molecules are adsorbed by both fillers but one adsorbs more volume than the other) or (iii) more likely a combination of (i) and (ii). 5. Conclusions The objective of this study was to investigate the nature of binder adsorption that takes place on the surface of filler particles in tandem with the influence of such interactions on the rheological properties of the adsorbed or fixed binder and the residual or bulk free binder. The following is a summary of conclusions that can be drawn based on the aforementioned results and discussion: 1. Investigation of asphaltene content in the residual binder after subjecting it to adsorption with mineral fillers indicates that polar fractions are preferentially adsorbed on to the surface of fillers. It is noted that while the study investigated asphaltene content, it is just as likely that other polar fractions were adsorbed by the fillers as well. 2. Consistent with the previous observation, the stiffness of the free asphalt (binder that was not adsorbed on the filler surface) was approximately 0.3–0.85 times the stiffness of the original asphalt binder. Conversely, the stiffness of the adsorbed binder was estimated to be 5–8 times the stiffness of the original asphalt binder based on the rule of mixtures. These large differences have important implications in terms of understanding and modeling (cohesive and interfacial) failure in the micro structure of asphalt composites. 3. The magnitude of change in binder properties is related to the quality and quantity of the adsorbed binder, which is in turn related to the mineral character of the filler surface and the specific surface area of the mineral. For the minerals used in this study, it appears that the latter (specific surface area) plays a more significant role. Also, for a given filler, the magnitude of this change did not vary substantially with loading rate. 4. Binder – filler adsorption experiments must be carefully set up with appropriate controls as demonstrated in this study, since factors such as the use of a solvent and adsorption by a filter paper can influence the properties of the binder.
Acknowledgments The authors would like to acknowledge financial support from China Scholarship Council (Grant No.: 201306120187) and National Science Fund for Distinguished Young Scholars of China (Grant No.: 51225803). References
Fig. 6. Comparison of stiffening effect due to presence of filler particles versus interaction.
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