## 1 Introduction
In contrast to the conventional methods used for hot mix asphalt, the mix design of microsurfacing mixtures is complicated because it is a chemical system with several variables, including the type and content of asphalt emulsion, types and gradation of aggregates, water, and filler content, which affect the final properties of the mixtures [5; 19; 20]. Emulsion characteristics play a pivotal role in ensuring the workability and durability of microsurfacing, contributing to its final behavior. The emulsion's properties mainly depend on the design of the components selected for its production, like emulsifier type and dosage, acid content, binder grade and content, and polymer modifier type, and dosage [21]. In addition, the consistency and adhesion between aggregates and bitumen depend on the mineralogical composition of the aggregates. Consequently, each specific composition of aggregates and asphalt emulsion has unique chemical properties that affect the bond between the aggregate and bitumen emulsion [6; 12]. Another essential parameter affecting the microsurfacing mix design is the filler type and amount, which contribute significantly to the breaking and curing process due to the large surface area [21]. The mixtures typically contain 0-3% mineral filler, usually Portland cement. This component significantly influences the microsurfacing's performance, as the filler-mastic interaction critically affects the mixture cohesion [5; 22]. Regarding water, its addition improves the workability of the mixture, and its primary purpose is to wet, dissolve and adhere to other components, and moderate the chemical reaction [5; 12]. The optimum amount of water must be determined because a lack of water leads to the weak coating of aggregates and hardening of the mixture, and excessive water content can lead to the separation of aggregates or lengthen the mixture's curing time [6]. As each component of the microsurfacing mixture interacts with the other components to form a chemically complex compound, the amount and type of asphalt emulsion, cement, and water have a significant influence on the test results used to design the microsurfacing mixture [19; 21; 23]. Therefore, [PERSON] [18] stated that the primary mix design objectives of microsurfacing are ensuring good compatibility among the mixture ingredients and satisfying the strength and durability parameters. Thus, several mix design guidelines have been developed for an optimum job mix formula. The International Slurry Surfacing Association (ISSA), the Texas Transportation Institute (TTI), the American Society for Testing and Materials (ASTM), and the California Department of Transportation (Caltrans) developed microsurfacing mix design procedures, and the ISSA's guidelines are the most accepted and used worldwide. Similar standards have also been developed in the European Union and South Africa to support the design of slurry and microsurfacing materials [5; 19; 20; 23; 24]. However, various researchers have reported difficulties when using the existing mix design procedures, namely the repeatability and reproducibility of test results and poor correlation between field and laboratory results [5]. Some authors, like [PERSON] and [PERSON] [19] and [PERSON] and [PERSON] [25], have evaluated the ISSA method for microsurfacing mix design. They have concluded that modification to the current mix design procedure might be necessary to enhance reproducibility and provide successful mix designs on the basis of performance-related tests in the design method. Thus, the microsurfacing mix design methodologies described by the ASTM and ISSA should be used only as a guide [25]. Therefore, this work aimed to significantly contribute to developing an improved mix design method for microsurfacing mixtures that also included test methods from the European standards [26; 27; 28]. The method was also applied to study the influence of the type and amount of asphalt emulsion, filler (cement), and added water content on the composition and characteristics of a specific microsurfacing mixture.