Introduction

Asphalt binder is one of the essential components in the asphalt mixture to construct flexible pavements. Asphalt binder represents a small percentage of the total weight of the asphalt mixture (3–6%). However, it is the most significant contributor, with approximately 60%, of the total cost of the asphalt mixture [1]. Despite the great importance of asphalt in the manufacture of asphalt mixtures, it causes adverse environmental impacts due to the carbon emissions produced during the construction of asphalt roads [1, 2]. Utilizing improved asphalt reduces asphalt consumption during the infrastructure lifecycle, thus reducing fossil fuel consumption and global emissions. This fact coexists with the need to extend the pavement service life and preserve non-renewable asphalt sources for future generations by improving the virgin asphalt [3, 4]. This is especially important where the available asphalt is substandard.

Recently, implementing nanotechnology as a novel methodology to improve asphalt properties has received great attention [5]. The performance of the virgin asphalt is greatly enhanced by the introduction of nanomaterials as additives due to their unique huge surface area and good dispersion ability. Among the nanomaterials, nanosilica (NS) is one of the most widely studied materials [6,7,8,9]. However, NS is not applicable for improving asphalt mixtures from the economic point of view due to its higher cost than traditional paving materials [10, 11]. The price of the NS is highly dependent on the particle’s size range, specific surface area, and the preparation method, where it ranges from 100 to 7500 US $/kg [12,13,14]. Al-Taher et al. concluded that using NS in asphalt mixtures is not practical due to its high cost, whereas utilizing NS at a content of 7% by asphalt weight increased the asphalt mixture’s cost by about 5000% compared to the unmodified mix [10]. In another study, the authors compared the effects of nanosilica and hydrated lime (HL) on the properties of asphalt concrete. They concluded that the NS to HL cost ratio is much higher than the ratio of performance indices of these mixtures [11]. Filonzi et al. showed a significant increase (785%) in the cost of asphalt mixtures incorporating NS compared to conventional ones [15]. However, most researchers have employed chemically prepared NS particles, which are homogeneous and controlled-size particles but high cost and limited quantities. This forced some researchers to use lower contents of NS, which caused slight improvements in asphalt properties. For instance, Saltan et al. employed low contents of NS (0.1, 0.3, and 0.5% by asphalt weight) to improve the properties of virgin asphalt. Results showed limited improvements in the physical–rheological properties. Besides, there was no upgrading in the PG value of the base asphalt [16]. In another study, low contents of NS (0.2–0.9% by asphalt weight) were employed for improving the physical–rheological properties of the asphalt binder. The results also showed limited improvements in the binder properties, where there was no noticeable change in the penetration values at all used contents [17]. An additional drawback for the use of NS in asphalt modification is the tendency of its particles to agglomerate due to minimal dimensions, large surface area/particle-size ratio, and high surface free energy [18]. Several authors reported the agglomeration of NS particles during the asphalt modification [16, 19,20,21,22]. Despite those drawbacks, original asphalt performance is dramatically enhanced by adding NS [20, 23, 24]. Therefore, it is necessary to find a viable alternative that offers comparable performance to NS.

Some powdery materials belonging to industrial waste are classified as nanomaterials, such as silica fume, which may be a suitable alternative to high-cost nanosilica, as their properties have remarkable similarities. Silica fume is a very fine material composed of amorphous silica produced by electric arc furnaces as a by-product of elemental silicon or ferro silicon alloys production [25, 26]. Silica fume is categorized as a nanomaterial due to its particles’ size range and shape [10, 19, 27,28,29]. Its high surface area and availability in large quantities may make it an appropriate economic additive to enhance the properties of the asphalt binder. The price of silica fume is relatively low compared to virgin asphalt and traditional modifiers as well. Where the average price of silica fume is 0.1 US $/kg [30, 31], while the price of virgin asphalt ranges from 0.2 to 0.6 US $/kg [32, 33]. As an industrial waste, using silica fume in road construction protects the environment from the risk of its improper disposal [30]. Moreover, silica fume can be directly added to the asphalt, unlike most modifiers that need to be prepared and treated before asphalt modification [34]. Silica fume has been widely utilized in concrete technology, but there is a limited research effort to explore using it in improving asphalt [30, 34, 35]. Some authors employed the silica fume as a binder modifier at percentages of 2–10% by asphalt weight. The results illustrated that adding silica fume in asphalt samples caused an enhancement in the rheological and physical properties compared with the virgin asphalt [36, 37]. Abutalib et al. studied the effect of using different contents of silica fume (2%, 4%, and 8% by asphalt weight) on the asphalt binder properties. Analysis of results showed that silica fume reduced the asphalt binder's aging index and temperature sensitivity [19]. Zghair et al. evaluated the effect of blending conditions on the properties of asphalt modified with silica fume. Three different speeds (2000, 4000, and 6000 rpm) of a high shear mixer were used to mix three percentages of silica fume (2%, 4%, and 6% by asphalt weight) with the control asphalt for 30 and 60 min. Their results showed that at high blending speed and time, slight improvements in the physical properties were gained [38, 39]. Zheng et al. studied the effect of adding silica fume on SBS-modified asphalt. Different contents of silica fume were used for this purpose (2%, 4%, 6%, and 8% by asphalt weight). They found that the asphalt incorporating silica fume has a high resistance against fatigue cracks and permanent deformation compared to the SBS-modified asphalt [34].

Based on the literature, silica fume was used to modify asphalt with contents of 2–10% by binder weight, and the results showed improvements in the properties of asphalt, but not to the extent achieved by nanosilica. The possibility of using high contents of silica fume for attaining the same performance as the nanosilica has not been studied. To address these issues, the current research attempts to investigate the possibility of modifying virgin asphalt with high contents of waste silica fume (low-cost nanomaterial) to obtain a similar performance of nanosilica (high-cost nanomaterial) as a prelude to the large-scale use of nanomaterials in the pavement. The current investigation also extends to study the attempt of using predictive models to estimate the rutting parameter of binders in the absence of the dynamic shear rheometer (DSR) device. Therefore, the physical–rheological properties of binders modified using NSF, at a wide percentage range, were evaluated by comparing them with binders modified using the high-cost nanosilica (NS).

Materials and methods

Materials

Base asphalt binder

A local asphalt obtained from the Alexandria Oil Company was used in this research. The basic properties of the base asphalt (as measured at the laboratory) are presented in Table 1.

Table 1 Properties of base asphalt
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Additives (nanomaterials)

In this research, two nanomaterials are used as asphalt binder modifiers: nanosilica fume (NSF) and chemically prepared nanosilica (NS). NSF is produced by EFACO (Egyptian Ferro-Alloys Company) in Egypt. NS is imported from Sigma-Aldrich Company. NS is a high-cost nanomaterial utilized in this study for comparison purposes. Table 2 presents the cost and technical properties of the used nanomaterials. Figure 1 shows the general texture of the employed nanomaterials.

Table 2 The prices and the technical properties of used nanomaterials
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Fig. 1
figure 1

General texture of the used nanomaterials

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Methods

The base asphalt was mixed with NSF at low contents (2%, 4%, 6%, and 8%) and high contents (20%, 30%, 40%, and 50%) by asphalt weight. The NS was employed as an asphalt modifier at the contents of 2, 4, and 6% by asphalt weight. Transmission electronic microscopy (TEM) was used to scan the nanostructure of the used nanomaterials. The penetration, softening point, and rotational viscosity tests were conducted to determine the properties of the modified asphalt. To achieve a homogeneous blending of modified binders, appropriate blending conditions were investigated. Scanning electron microscopy (SEM) was used to assess the homogeneity of modified binders. The changes in the chemical bonds of modified asphalts were investigated using the Fourier transform infrared spectroscopy (FTIR). The rheological properties of modified binders were investigated at the original and short-term aging conditions. To simulate the short-term aging condition of binders, the rolling thin film oven test (RTFO) was performed, according to AASHTO T240 [44]. Due to the unavailability of a DSR device, Bari and Witczak’s prediction model was applied to estimate the rutting parameter (G*/sin δ) of the modified binders [45]. The aging index and temperature sensitivity of modified binders were investigated at different levels of temperatures. Three replicates for each sample were conducted for all experimental tests. Then the average values were presented in the results. Figure 2 displays the outlines of the research work plan [46].

Fig. 2
figure 2

Outlines of the research work plan

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Nanomaterials characterization

Figure 3a shows the sizes and shapes of NSF particles. Images clearly show the distinct spherical shape of NSF particles. This spherical shape facilitates the process of spreading NSF in asphalt binder without agglomeration. Most NSF particles are in the range of 20–100 nm. On the other hand, NS has a limited range in particle size ranging from 10 to 20 nm, as presented in Fig. 3b.

Fig. 3
figure 3

TEM images of used nanomaterials

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Preparation of modified asphalt binders

The high-speed shear mixer outperforms the mechanical mixer in uniformly dispersing nanomaterials in asphalt [39, 47]. Therefore, a high-speed shear mixer (Silverson L5M-A) was used for blending samples at a temperature of 160 ± 10 °C. It is essential to choose the appropriate blending process to achieve a homogeneous modified binder. Most previous research used a wide range of blending conditions, where the blending speeds ranged from 1500 to 5000 rpm, and the duration ranged from 45 to 180 min [12, 48]. In the preparation process of the experimental samples in this research, it was noted that when using a blending speed higher than 4000 rpm, the temperature increased at an excessive rate which was challenging to control during the blending process. Therefore, the blending speed was kept lower than 4000 rpm. Samples were mixed at either 2000 or 4000 rpm for 60 or 120 min. The blending conditions were evaluated using two contents of NSF; 8% represents the low contents, and 40% represents the high contents. Penetration at 25 °C and rotational viscosity at 135 °C values were selected to evaluate the effect of blending conditions on the physical–rheological properties of modified asphalt [49]. The NS was mixed with base asphalt at the same blending conditions of NSF-modified asphalt. It should be noted that the base asphalt was mixed with no additives under the same blending conditions to avoid any varying degree of aging with the modified binders during the mixing process.

Prediction of rutting parameter for modified binders

The dynamic shear rheometer (DSR) test is one of the most critical superpave-specified tests for characterizing the viscoelastic behavior and determining the performance grade (PG) of the asphalt binder. The DSR test measures the asphalt complex modulus (G*) and phase angle (δ) at medium and high temperatures. There must be an alternative and applicable method for estimating approximate values of the G* and δ, in the event that the DSR device is not available. Bari and Witczak have developed prediction equations with a high level of correlation (R2 = 0.99) to compute the G* and δ at a given temperature and frequency based on the viscosity–temperature relationship [45]. Martinez et al. evaluated the model proposed by Bari and Witczak, and they confirmed the accuracy of this model with a high correlation coefficient (R2 = 0.987) [50]. If the asphalt viscosity is measured at different temperatures, then a relationship between the corresponding viscosity and investigated temperature can be formulated according to the ASTM D2493, as shown in Eq. (1). The viscosity–temperature relationship is modified to reflect the effect of loading frequency, formulated as follows in Eqs. (2) to (4). Equations (5) and (6) were used to calculate the G* and δ at different temperatures [45]. Loading frequency (fs) was used in these equations equal to 1.59 Hz (10 rad/s), which is the specified frequency in the DSR test simulating the shearing action for a traffic speed of about 90 km/h [51]. The G*/Sinδ parameter is a stiffness measure for rutting resistance of the binders at high pavement temperatures. To estimate the G*/Sinδ, Bari and Witczak’s model was employed with the assumption that it is applicable for modified asphalts.

$$\log \left( {\log \left( \eta \right)} \right) = A + {\text{VTS}}\,{\text{ Log}} \left( {T_{{\text{R}}} } \right)$$
(1)
$$\log \left( {\log \eta_{{\text{fs,t}}} } \right) = A^{\prime } + {\text{ VTS}}^{\prime } {\text{Log}} \left( {T_{{\text{R}}} } \right)$$
(2)
$$A^{\prime } = \left( {0.9699 \times f_{{\text{s}}}^{ - 0.0257} } \right){\text{A}}$$
(3)
$${\text{VTS}}^{\prime } = \left( {0.9668 \times f_{{\text{s}}}^{ - 0.0575} } \right){\text{VTS}}$$
(4)
$$\delta = 90 + \left( { - 7.3146 {-} 2.6162{\text{ VTS}}^{\prime } } \right) \log \left( {f_{{\text{s}}} \times \eta_{{\text{fs,t}}} } \right) + \left( {0.1124 + 0.2029 \cdot {\text{ VTS}}^{\prime } } \right) \times \left( {\log \left( {f_{{\text{s}}} \times \eta_{{\text{fs,t}}} } \right)} \right)^{2}$$
(5)
$$G^{*} = 0.0051f_{{\text{s}}} \times \eta_{{\text{fs,t}}} \times (\sin \delta )^{{7.1542 - 0.4929f_{{\text{s}}} + 0.0211f_{{\text{s}}}^{2} }}$$
(6)

where η is rotational viscosity, in cP; TR is testing temperature, in Rankine; A is regression intercept and VTS is regression slope of the log-linear model of viscosity–temperature relationship; ηfs,t is rotational viscosity as a function of fs, in cp; fs is a specified frequency in DSR test, in Hz; A′ is modified “A”; VTS′ is modified “VTS”; G* is the complex modulus of binder, in Pa; and δ is phase angle, in degrees.

Results and discussion

Optimization of blending speed and time

Figure 4a presents the effect of blending conditions on the penetration of modified asphalt with NSF. It is noted that both speed and time did not significantly affect the penetration values at 8% and 40% of NSF. The results also showed a slight difference in the viscosity values under different blending conditions, as shown in Fig. 4b. Increasing the blending time more than one hour or increasing the blending speed more than 2000 rpm did not significantly affect viscosity and penetration values. This indicates that one hour and 2000 rpm are sufficient to disperse the NSF through the binder. Therefore, in this study, all modified asphalt samples were mixed at 2000 rpm and for one hour.

Fig. 4
figure 4

Effect of blending conditions on physical–rheological properties of modified binders

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Fourier transform infrared spectroscopy (FTIR)

FTIR test was performed to evaluate the effectiveness of adding NSF and NS to the asphalt binder from a chemical point of view. The chemical change of NSF-modified binders was evaluated using two contents; 8% represents the low contents, and 40% represents the high contents of NSF. NS of 6% content was selected to represent the NS-modified binders. The bonds at 2923, 2852, 1628, 1458, and 1376 cm−1 are the distinctive bonds of asphalt, while the distinctive bonds of silica are 1116, 807, and 478 cm−1 [4, 52, 53]. As shown in Fig. 5, the positions of asphalt peaks did not change, and their intensities were almost the same in both the base asphalt and the modified binders with NSF and NS. The silica peaks appeared only in the spectrum of modified asphalt and did not appear in the base asphalt spectrum. The intensity of silica peaks was higher in the case of 40%NSF than 8%NSF and 6%NS, due to the high content of NSF in modified asphalt, thus providing evidence of the appropriate incorporation and dispersion of NSF into the binder [54]. According to the above observations, blending a high or low content of NSF with asphalt did not change the basic chemical bonds of base asphalt, but a simple physical blending occurred between them [4]. Likewise, this was observed when mixing 6% of NS with asphalt.

Fig. 5
figure 5

FTIR spectrum of base and modified asphalts

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Scanning electron microscopy (SEM)

The SEM images were employed to quantify the morphology of the modified binders. Figure 6 shows the SEM images of the binders modified with 8% NSF, 40% NSF, and 6% NS. As shown, nanoparticles have been well dispersed within the binder without clustering. However, some agglomerations of nanoparticles were observed in the 6% NS-modified asphalt due to its high surface area compared to NSF particles [16]. The sample surface of 8% NSF is smooth compared to 40% NSF indicating a difference in surface properties. It is expected that there will be a difference in the physical properties between them [55].

Fig. 6
figure 6

SEM images of modified binders at different contents of NSF and NS

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Physical properties

Figure 7 shows the effect of incorporating NSF and NS on asphalt binders' physical properties (penetration depth and softening point). As can be noted, all modified binders exhibited a higher softening point and lower penetration depth compared with the base asphalt. This indicates an improvement in binder temperature sensitivity and stiffness, leading to lower permanent deformation (decreased rutting) at higher temperatures [56]. It has also been observed that high contents of NSF achieve almost the same physical properties as low contents of NS.

Fig. 7
figure 7

Physical properties of NSF- and NS-modified binders

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Rheological properties

Brookfield viscosity results

Figure 8 shows the effect of adding NSF and NS on the rotational viscosity value at different temperatures before and after short aging. At all investigated temperatures, the viscosity increased with increasing NSF content. For unaged binders, the viscosity of base asphalt measured at 135 °C increased by 229, 357, and 569% due to incorporating 30, 40, and 50% NSF in binder blends, respectively. In contrast, it is increased by 78, 147, and 398% when incorporating 2, 4, and 6% NS, respectively. The large increase in viscosity values indicates a significant increase in the rutting parameter (G*/Sinδ), thereby increasing asphalt resistance to permanent deformation [51, 57, 58]. As can be noted, the rotational viscosity values of all modified binders before aging comply with the superpave limitations of 3000 cP at 135 °C [59]. The results showed a significant correlation between rotational viscosity and NSF contents. This prompted the inference of correlation equations between the NSF contents with rotational viscosity and temperature using the Minitab program for regression analysis, as shown in Eqs. (7) and (8) for unaged and aged binders, respectively.

$$\log \left( {\log \left( \eta \right)} \right)_{{{\text{Unaged}}}} = 0.00248*\left( {{\text{NSF}}\% } \right) {-} 1.1 {\text{Log}}\left( T \right) + 2.77\quad R^{2} = \, 0.99$$
(7)
$$\log \left( {\log \left( \eta \right)} \right)_{{\text{aged }}} = 0.00242*\left( {{\text{NSF}}\% } \right) {-} 1.07 {\text{Log}}\left( T \right) + 2.75\quad R^{2} = \, 0.99$$
(8)
Fig. 8
figure 8

Rotational viscosity of NSF- and NS-modified binders

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where η = rational viscosity in cP; and T = temperature in Celsius.

It is now possible to manufacture modified asphalt with preset (controlled) viscosity using NSF to suit the asphalt mix design for conditions of each region. This facilitates the use of NSF as an asphalt modifier on a large scale in road projects.

Rutting parameter of modified binders

As shown in Fig. 9, the relationship between log–log viscosity and log temperature was plotted for NSF- and NS-modified binders to compute the VTS and A values before and after short aging. It is stated by Rasmussen et al. that the log-linear model of viscosity–temperature relationship may be curvilinear or even discontinuous if a chemical change occurred in the binder [60]. As can be seen, the log-model of viscosity–temperature relationship is linear with a high level of correlation (R2 > 0.99) at all contents of NSF and NS. Accordingly, this indicates that no chemical change occurred in the modified binders. The FTIR analysis, in item 3.2, supports this finding. Table 3 summarizes the computed VTS and A values before and after short aging.

Fig. 9
figure 9

Viscosity–temperature charts of the modified binders before and after short aging

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Table 3 VTS and A values of the modified binders at different contents of NSF and NS
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Figure 10 shows the effect of incorporating NSF and NS on the rutting parameter (G*/Sinδ) of the asphalt binder before and after short aging. As shown in Fig. 10a (left panel), the rutting parameter of the unaged binder significantly increased with increasing NSF contents, while it decreased with increasing temperature. A similar trend was observed for the short-aged binder in Fig. 10b (left panel). This indicates that the binders incorporating NSF resist the permanent deformation substantially higher when compared with the base asphalt, making the NSF-modified binder more suitable for use at high pavement temperatures. NS had a similar effect on the rutting parameter; the G*/Sinδ increased with increasing NS content. However, this effect of adding 6% NS to the base asphalt was more significant when compared with 2% and 4% NS, as shown in Fig. 10a, b (right panel).

Fig. 10
figure 10

Rutting parameter of NSF- and NS-modified binders

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The critical temperature is the maximum temperature attained when G*/Sinδ is 1.0 or 2.2 kPa for unaged or short aged binder, respectively [51, 59, 61]. The higher critical temperature indicates an enhancement in the high-temperature performance of binder, which causes upgrading in the performance grade (PG) of asphalt [34]. As shown in Fig. 10a, b (left panel), the critical temperature increased with increasing the NSF contents indicating a great improvement in the deformation resistance at high pavement temperatures. For both unaged and aged binders, the contents of 2% up to 8% NSF had a critical temperature lower than 64 °C, while the critical temperatures of 30%, 40%, and 50% NSF were 64 °C, 70 °C, and 76 °C, respectively. This indicates that high contents of NSF significantly improved the resistance to deformation compared to low contents. It is noticed that the addition of 6% NS upgrades the critical temperature of base asphalt from 52 to 70 °C, while 50% of NSF upgrades the critical temperature to 76 °C. This indicates the superiority of NSF (at high content) over the high-cost NS in this part. The maximum pavement temperatures for the Middle East and North Africa (MENA) regions are 70 °C and 76 °C for the hot summer season [61,62,63,64]. Based on the presented results, it is expected that the NSF-modified asphalts would fulfill the required high-performance temperatures for the MENA and similar states.

Prediction equations were inferred to correlate the NSF content with G*/Sinδ and temperature using the Minitab program for regression analysis, as shown in Eqs. (9) and (10) for unaged and aged binders, respectively.

$${\text{Log}}\left( {G^{*} {/}{\text{Sin}} \delta } \right)_{{{\text{Unaged}}}} = 2.34 + 0.0155 \times {\text{NSF}}\% - 0.041 \times T\quad R^{2} = \, 0.99$$
(9)
$${\text{Log}}\left( {G^{*} {/}{\text{Sin}} \delta } \right)_{{\text{aged }}} = 2.83 + 0.0155 \times {\text{NSF}}\% - 0.0429 \times T\quad R^{2} = \, 0.99$$
(10)

where G*/Sinδ is the rutting parameter in kPa; and T is the temperature in Celsius.

Aging index

The aging effect of asphalt can be investigated using the aging index (AI). AI can be computed according to Eq. (11) [65].

$${\text{AI}} = \frac{{{\text{The}}\,{\text{ measured}}\,{\text{ asphalt}}\,{\text{ property}}\,{\text{ after }}\,{\text{aging }} }}{{{\text{The }}\,{\text{measured}}\,{\text{ asphalt}}\,{\text{ property}}\,{\text{ before }}\,{\text{aging }}}}$$
(11)

Aging significantly affects the rheological properties of asphalt. Therefore, some researchers have used rotational viscosity and complex modulus (G*) to investigate the aging effect. AI based on the complex modulus values represents the effect of aging at in-service pavement temperatures (64–82 °C), while the AI based on the rotational viscosity values represents the effect of aging at the mixing and compaction temperatures (135–165 °C) [19, 57, 66, 67]. In this research, the AI values were calculated based on G* and rotational viscosity values to evaluate the short aging effect on the modified binders at different levels of temperatures. As shown in Fig. 11, all investigated binders became stiffer due to aging, where AI values are higher than one at different levels of temperatures. At all testing temperatures, the AI values calculated for G* (Fig. 11b) are greater than those computed using rotational viscosity (Fig. 11a) for both NSF and NS. This is due to the fact that the aged asphalt is stiffer at intermediate temperatures compared to high temperatures [57].

Fig. 11
figure 11

Aging effect on modified binders at different levels of temperatures

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It is observed that the AI values show a linear variation with the content of modifiers (NSF and NS). Therefore, the slope of the line can be used as a parameter to evaluate the aging acceleration of the modified binders. Thus, the aging index slope (AIS) of the AI–modifier content relationship is computed at the investigated temperatures. The higher AIS indicates an acceleration of the aging process. Figure 11c depicts a comparison between the AIS values of NSF- and NS-modified binders. It clearly shows the big difference between the AIS values at all investigated temperatures, where its value for NS reaches almost six times compared to NSF. This promotes utilizing NSF-modified binders for sustainable pavement construction.

Temperature susceptibility

The binder’s sensitivity to temperature is defined as an indicator that expresses how the properties of the binder change with temperature [68]. The penetration index (PI) and the complex modulus temperature sensitivity (GTS) are usually used to evaluate the temperature sensitivity at in-service pavement temperatures [57], while viscosity–temperature sensitivity (VTS) is used to assess the temperature sensitivity of the binders at the mixing and compaction temperatures (135–165 °C) [60]. The temperature sensitivity of the modified binders with NSF and NS was investigated at three levels of temperatures.

The PI values of binders can be calculated according to Eq. (12) [69].

$${\text{PI}} = \frac{{\left( {1952 - 500 \times \log \left( {P25} \right) - 20 \times {\text{S.P}}} \right)}}{{\left( {50 \times \log \left( {P25} \right) - {\text{S.P}} - 120} \right)}}$$
(12)

where P25 is the penetration depth at 25 °C, in 0.1 mm; and S.P is the softening point, in °C.

Figure 12a depicts the penetration index values of the binders modified with NSF and NS. The results reveal that the PI values increased with increasing the NSF content, as its value changed from negative at the base asphalt to positive at all NSF contents. This indicates a significant improvement in the temperature sensitivity of NSF-modified asphalt at 25 °C was achieved. A similar trend is observed for NS-modified binders. All the PI values are within the acceptable range of − 2.0 to + 2.0, indicating that the modified asphalt can be used for road pavement [70].

Fig. 12
figure 12

Temperature sensitivity of modified binders at three levels of temperatures

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The log·log G* values versus the logarithm of temperature values for the base and modified asphalts are drowned to investigate the temperature sensitivity at in-service pavement temperatures (46–88 °C). The GTS values were then computed according to Eq. (13) [71, 72].

$${\text{Log}} \cdot {\text{Log}}\left( {G^{*} } \right) = {\text{GTS}} \times {\text{Log}}\left( {T_{{\text{K}}} } \right) + A_{{\text{G*}}}$$
(13)

where G* is the complex modulus, in Pa; TK is temperature, in Kelvin; AG* is regression intercept; and GTS is regression slope.

The high absolute value of GTS indicates the high-temperature susceptibility of the asphalt binders [57]. Figure 12b displays the regression lines of log·log (G*) and log (TK) relationship for the base asphalt and binders modified with NSF and NS. Table 4 lists the computed GTS values of base and modified binders. The GTS values decreased with increasing NSF and NS contents indicating an enhancement in the temperature sensitivity of modified binders. Figure 12c displays the VTS values of the base asphalt and binders modified with different contents of NSF and NS. Increased VTS means that the asphalt binder is more susceptible to change with temperatures [60]. As shown, the increase in NSF content can cause a significant decrease in VTS values, implying that binders with a higher content of NSF are less susceptible to temperature changes. The VTS value at 30% NSF is very close to its value at 4% NS, and the VTS value at 50% NSF is very close to its value at 6% NS. This indicates that binders modified with high contents of NSF have a comparable or better temperature susceptibility than NS-modified binders. Based on the values of PI, GTS, and VTS, the binders incorporating NSF are less sensitive to temperature.

Table 4 Complex modulus temperature sensitivity of NSF- and NS-modified binders
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Economic assessment

The economic assessment of using high NSF contents to improve the properties of base asphalt is compared with a 6% NS-modified binder. The price ratio (PR) and performance improvement ratio (PIR) can be computed using Eqs. (14) and (15) [73, 74].

$${\text{PR}} = \, P_{{\text{m}}} {/ }P_{{\text{b}}}$$
(14)
$${\text{PIR}} = {\text{PI}}_{{\text{m}}} {\text{/ PI}}_{{\text{b}}}$$
(15)

where Pm and PIm are the price and properties of the modified asphalt, respectively; Pb and PIb are the price and properties of the base asphalt.

NSF is available in large quantities at relatively low prices [30, 31]. In this study, the used prices in the economic assessment of both NSF and NS are 0.45 and 2370 US $/kg, respectively (as prevailing averages in the study area). The assumed average price of virgin asphalt is equal to 0.35 US $/kg, according to Egyptian General Petroleum Corporation (EGPC). The blending cost of modified asphalt is assumed to be the same in both cases, when using NSF or NS. Consequently, the price of NSF- and 6% NS-modified asphalts and the PR values are shown in Table 5. It can be inferred that using high contents of NSF improves the properties of the base asphalt at almost the same cost. This finding promotes utilizing NSF on a wide range of road projects.

Table 5 Economic benefit analysis of modified binders
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Finally, the results show that the use of NSF at high contents achieves a similar or better property of asphalt compared to NS. Also, modifying asphalt with high contents of NSF not only improved the physical–rheological properties of asphalt, but also industrial waste was reused, and more non-renewable resources would be saved. This finding implies that the NSF is a suitable asphalt modifier to be used in a wide range of road projects.

Conclusions

This paper investigates utilizing nanosilica fume (NSF) (low-cost nanomaterial) as a viable alternative to chemically prepared nanosilica (NS) (high-cost nanomaterial) as a prelude to the large-scale use of nanomaterials in the pavement. Therefore, the base asphalt was blended with NSF at different contents (2, 4, 6, 8, 20, 30, 40, and 50% by binder weight). Moreover, the base asphalt was blended with NS at the contents of 2, 4, and 6% by binder weight for comparison purposes. The physical–rheological properties, temperature susceptibility, aging effect, and economic benefit of modified binders were investigated. SEM and FTIR tests have been performed to help interpret the test results.

In view of the attained results, the following conclusions can be drawn:

  • Adding NS had a significant improvement on the physical–rheological properties of asphalt. However, the high price and limited available quantities of NS hinder its practical use in pavement construction.

  • The NSF-modified asphalts had a significant improvement on the physical–rheological properties of asphalt.

  • Predictive equations with high correlation have been inferred to correlate NSF content with both the rutting parameter and the rotational viscosity of modified asphalt. These equations can aid in selecting the percentage of NSF in modifying asphalt for different PG requirements to suit the asphalt mix design for conditions of each region.

  • The predicted rutting parameter (G*/Sinδ) increased significantly with the increase of NSF content from 20 to 50%, which can produce asphalt mixtures less susceptible to rutting.

  • The NSF additive significantly decreased the short aging acceleration compared to NS. Moreover, the asphalts incorporating NSF are less sensitive to temperature. This indicates that the NSF-modified asphalt is appropriate for utilization at a wide range of temperatures.

  • Economically, the price of NSF-modified asphalt is approximately equal to the price of unmodified asphalt, while the cost of 6%NS-modified asphalt increases about 384 times.

  • The NSF-modified binders were found to have comparable or better performance as compared to the NS-modified binders. This promotes the use of NSF as a viable alternative to NS, where NSF is available in abundance at low costs.