**1. Introduction**

Natural rock asphalt refers to the asphalt that exists in rock crevices, which is often used as an additive and modifier for asphalt materials [1–5]. Buton Rock Asphalt (BRA) is a kind of rock asphalt that originates from Buton Island, Sulawesi province, Indonesia [6,7]. It is used as a modifier to prepare Buton Rock Asphalt-Modified Asphalt (BRA-MA) to improve high-temperature and moisture performances of pavement [6,7]. The preparation methods of BRA-modified asphalt mainly include wet processing and dry processing. Wet processing uses BRA as a modifier to prepare BRA-modified asphalt, whose performance is significantly improved compared to base asphalt [8]. Lv et al. [7] reported that the addition of BRA could help to improve the aging resistance of bio-asphalt. The use of BRA can reduce the consumption of petroleum asphalt and cost of materials. It is suggested that BRA is also helpful for the cleaner and cheaper production of asphalt pavement construction, signifying that the application of BRA is beneficial for environment-protection [9] and economic values [10–13]. Consequently, the application prospect of BRA-MA is very extensive.

However, the storage stability of BRA-MA requires further study since the segregation of BRA often occurs in the production plant and transportation of BRA-MA, especially in wet processing. Hence, the storage stability of BRA-MA can be divided into two parts:

**Citation:** Su, Y.; Hu, X.; Wan, J.; Wu, S.; Zhang, Y.; Huang, X.; Liu, Z. Physical Properties and Storage Stability of Buton Rock Asphalt Modified Asphalt. *Materials* **2022**, *15*, 3592. https://doi.org/10.3390/ ma15103592

Academic Editor: Francesco Canestrari

Received: 4 April 2022 Accepted: 12 May 2022 Published: 18 May 2022

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static storage [14] in the production plant and the transportation-storage process [15]. Segregation characteristic of the two parts is differential. Thus, the storage stability of BRA-MA during thermal static storage and transportation should be investigated and improved to address the segregation of BRA. Direct observation method, softening-point difference [16], Fourier infrared spectroscopy, and fluorescence microscope were used to evaluate the storage stability of modified asphalt [17]. Gu et al. [18] evaluated the storage stability of waterborne epoxy emulsified asphalt by fluorescence microscope. It was found that the content increase in waterborne-epoxy resin improves the compatibility of waterborne epoxy resin emulsified asphalt. Nevertheless, whether the storage stability evaluation method of polymer-modified asphalt is applicable to BRA-MA is not clear, since BRA is different with polymer modifier. For instance, it is quite difficult to be distinguished under a microscope, thus the micro-observation method is not applicable.

The factors [19] affecting the storage stability of modified asphalt include the content of modifier, storage temperature, storage time, and particle size. T J, Navarro et al. [20] and Liang et al. [21] studied the effects of the storage temperature and sample particle size on the storage stability of tire-rubber powder modified asphalt. It was found that the storage stability of tire-rubber-powder-modified asphalt gradually decreases with the increase in storage temperature and rubber powder particle size. Fang et al. [22] studied the effect of the preparation process on the storage stability of waste polyethylene-modified asphalt by using differential scanning calibration (DSC) and thermal gravimetric analysis (TGA). The preparation process was found to have little effect on the storage stability of modified asphalt. However, how particle size and BRA content determine the storage stability of BRA-MA has not been systematically studied. The effect of storage temperature, storage time, and cooling process on static storage and transportation storage is not yet explicit.

BRA powders of five different sizes were used as modifiers, and BRA-MA with 10%, 20%, 30%, and 40% BRA are introduced in this study. Penetration, softening point, ductility, viscosity, and viscosity-temperature susceptibility are characterized to understand the effect of particle size and BRA content on their physical properties. The softening-point difference test and viscosity difference test are separately conducted and discussed. Therefore, the appropriate evaluation method for the segregation of BRA-MA can be determined. Static storage stabilities by different temperature and time dependencies are characterized to illustrate the static storage process in the production plant before transportation. Storage stability during transportation is also tested in a simulation of the cooling process in a container in the vehicle. It provides a scientific basis for improving the physical properties and storage stability of BRA-MA.

#### **2. Materials and Experimental Methods**
