## 1 Introduction  
Road construction is a key infrastructure development for the emerging economies of South-East Asia, and road transport plays an important role in Vietnam due to its high mobility, distribution, and door-to-door features. Road construction relies on the utilization of natural aggregates as building materials. However, growing pressures for sustainable roads highlight the importance of replacing primary materials with industrial byproducts, also known as substitutive building materials (SBMs). Aggregates with good geotechnical characteristics are becoming increasingly scarce. Road pavements comprise multiple layers of different thicknesses, generally including both unbound and bound materials. The essential components of the road structures are embankments, subgrade, subbase, base coarse, and surface coarse [1]. The subbase layer consists of well-graded granular or cement-bound material. However, the performance requirements are lower than those of the road base aggregate. This layer also provides drainage and frost protection. The usual subbase layer applications are composed of aggregates such as gravel, sand, and quarry stones, which are usually virgin minerals that provide the required strength and durability [2]. Sand and gravel are crucial materials used in construction industries, including civil infrastructure projects such as subbase utilization in road construction. However, the increasing sand demands for construction industries lead to the overexploitation of river or delta ecosystems around the globe. The Mekong Delta is particularly vulnerable to sand mining practices, in addition to other threats such as climate change or chemical pollution [3]. The real magnitude of sand mining impacts requires further investigations in countries such as Vietnam [4]. Unregulated sand mining operations pose serious risks to river ecosystems and nearby human settlements. Marine sand extraction is also an unsustainable option to be considered as an alternative to river sand mining [5]. Therefore, alternatives to river sand must be put in place to feed future road construction projects at the national and regional levels. One solution is to manufacture the sand (m-sand) instead of extracting it from rivers and deltas, avoiding damage to complex ecosystems [6]. Manufactured aggregate industries (m-sand, crushed stone) have emerged in China in the last decade due to the natural supply shortages reaching 78% of the total primary supply in 2018 [7]. In Asia, m-sand is already used as an SBM for roads [8]; however, this puts even more pressure on primary resources. Many types of secondary raw materials have been used as SBMs for road subbase applications in recent decades, replacing natural aggregates. Several types of research [9; 10; 11; 12] have indicated the successful feasibility of construction and demolition waste (CDW) as unbound granular materials in subbase applications. CDW has been commonly used in several countries in road subbase applications, after a recycling process involving the removal of other wastes such as wood, paper, steel, iron, etc., followed by sorting, crushing, and grading. Similar to construction waste, the other prominent waste products are slags, such as granulated blast furnace slag, electric are furnace slag, steel slags, etc. [13; 14; 15; 16]. Previous studies show different mixtures of byproducts and waste streams to be used in road construction [17]. Fly ash produced from thermal energy plants running on coal combustion ends up in industrial or municipal landfills. The mixture of fly ash with other materials or waste streams could be used in road construction. For example, glass waste and fly ash were incorporated in cold mixed asphalt used for road pavement construction [18]. The use of stabilized fly ash and sisal fiber in producing bituminous pavements showed positive benefits in terms of GHG emissions [19].