## 1 Introduction  
Massive efforts and attempts are made to reduce the share of using ordinary Portland cement (OPC) as the main source of concrete production. It was reported that the second most used material after water is concrete [1]. However, a huge quantity of energy is required during the production of Portland cement (PC) concrete, which in turn contributes to the emission of large amounts of greenhouse gases [2]. A few decades back, a new concept of so-called geopolymers was first described by [PERSON] in 1978 [3]. Also known as alkali-activated materials, geopolymers have been widely investigated in many research works, due to their durability, physical and chemical properties, such as high strength, good corrosion resistance to aggressive environments, and ability to withstand high temperature [4; 5; 6]. Throughout the years, numerous studies have been conducted to expand wider the use of geopolymers, whose potential use in infrastructure rehabilitation was proposed by [PERSON] et al. [7]. However, limited studies can be found in the field of repair and rehabilitation geopolymer materials, in particular, in relation to the bonding mechanism when it is used as a repairing layer or as binding material to ensure its adhesion against OPC substrate.  
Much research has investigated the manufacturing of geopolymers using various precursors, such as fly ash, tungsten mine waste mud, granulated and blast furnace slag, where different alkaline solutions were used as chemical activators [8; 9; 10]. [PERSON] et al. [5] investigated the production of a geopolymer using tungsten mine waste, where calcium hydroxide was added as an additive to the geopolymeric mixture. Their results showed a high bonding strength of the geopolymeric binders formed when compared to that of commercial repair products. [PERSON] et al. [11] studied bond strengths of a geopolymer mix made from fly ash and of slag, and the results revealed that a high bonding strength (>10 MPa) can be obtained when sodium hydroxide with sodium silicate solution is used as an alkali activator, which was in contrast to those mixes where sodium hydroxide solution or sodium silicate solution were independently used. [PERSON] et al. [12] investigated the bond strength of a high calcium-based geopolymer repair material (GRM) using both slant shear and split tensile tests. Their findings showed a remarkable bonding strength can be achieved for their produced geopolymer material when compared to OPC-based repair material and ultra-high performance fibre concrete (UHPFC). Another study by [PERSON] et al. [13] reported that slag-based geopolymer can be a significant material to be used as concrete repairing binder. A similar study conducted by [PERSON] and [PERSON] [14] showed that in the production of slag-based geopolymer concrete, the replacement of 10% of slag by ultra-fine fly ash, ultra-fine slag and condensed silica fume enhanced its strength and workability. However, the same authors stated that the increase in fineness of the binders results in low workability and accelerates the setting time of geopolymer mortar or concrete when compared to binders with lower fineness [14].  
Based on the literature described above, there is a lack of studies that clearly describe the main bonding mechanism between geopolymer repair material and OPC concrete substrate. In this study, the authors attempted to investigate fresh and hardened properties of high calcium fly ash-based geopolymer as a repairing material, including setting time, morphological analysis, and a bonding strength mechanism against OPC substrate. The effect of variation of alkali activator concentration has also been investigated.