## 1 Introduction
In China, concrete pavements with fibers made from fly ash seemed to be more resistant to water and other stress factors (e.g., heat, frost), with material and energy savings compared to conventional production [20]. The use of fly ash mixed with other industrial byproducts in different proportions--such as 80% fly ash + 20% granulated blast furnace slag, or 70% copper slag + 30% fly ash--shows good results in laboratory tests combined with samples from road pavement [21]. A mixture of fly ash, an alkaline activator, and crushed aggregates was successfully tested in the laboratory for road base material purposes in Malaysia [22]. A mixture with 33% fly ash could prolong the use of the subbase material, with cost savings of around 23% compared to conventional materials [23]. Laboratory tests confirmed that fly ash could be used as a subbase material for road construction, replacing natural aggregates such as river sand [24]. Other research has focused on byproducts of the steel- and iron-making industries--such as electric arc furnace slag and blast furnace slag--as substitutive materials for sand in road construction [25]. Six foamed bitumen inverters were proposed and tested in a laboratory study using industrial byproducts (i.e., electric arc furnace slag, fly ash), bottom ash from municipal waste incineration plants, glass waste, or CDW materials such as reclaimed asphalt pavement, in a range of 10-20%, for potential use as road subbase layers, without toxicological problems [26]. Another approach is to use blast furnace slag activated with quicklime to bind dredge sediment for sublayer road applications [27]. Therefore, the literature review shows that different mixtures of industrial byproducts have been tested to check the strength and durability characteristics of road subbase layers [28], with benefits in the extension of lifetime, cost savings, lower surface roughness, and reducing GHG emissions [29]. Research regarding the mixture of industrial byproducts with cement and water has produced cement-stabilized pavement applications that fulfill the technical requirements for highways in China, in addition to reducing GHG emissions [30]. However, further research aims to increase industrial byproduct recycling performances and to reduce practical usage barriers for road subbases in highways with significant traffic loads [31]. In Vietnam, road transport accounts for more than 77.4% of the volume of transported goods and 94% of passengers [32; 33]. Decision No. 1454/QD-TTg [34], dated 1 September 2021, approves the master plan on road network development for the period 2021-2030, with a vision to 2050 in Vietnam, and sets the target for 2030 to essentially complete the inter-regional highways, connecting international gateway seaports, international airports, major international border gates with large import and export demand, and high-class cities, and in this framework to strive to build and complete about 5000 km of expressways. The contained vision to 2050 is to complete the road network throughout the country in a synchronous and modern manner, ensuring reasonable connection and development between modes of transport. Transport quality and services will be improved, ensuring convenience, safety, and reasonable costs. The road network planning comprises the expressway network, which is planned to be 41 routes, with a total length of about 9014 km, as well as the national highway network, which consists of 172 routes, with a total length of about 29,795 km. Moreover, the rural road system shall be completed. JICA and MOT [35] highlighted the need in Vietnam for the expansion of the national road networks by at least 10,000 km by 2030 for the network coverage to be considered to be well developed. Vietnam has six classes of roads: national motorways, provincial roads, district roads, rural roads, urban streets, and special roads for government officials. In Vietnam, common pavement types include asphalt concrete, cement concrete, bitumen-treated crushed stones, crushed stones, and soil.