# Mechanical and Fiber-Bridging Behavior of Slag-Based Composite with High Tensile Ductility
## 1 Introduction  
It has been reported that approximately 1 ton of CO\({}_{2}\) is produced to produce 1 ton of cement and that annual CO\({}_{2}\) emission by the cement industry contributes approximately 7% of the world-yearly emissions of greenhouse gases [1]. Beside the environmental problems of cement, the number and the magnitude of natural and manmade disasters have increased due to environmental, social, and political problems. Therefore, high performance and material sustainability are becoming important in research on construction materials [2]. Highly-ductile fiber-reinforced cementless composite is a promising construction material due to its high tensile performance and material sustainability. Recently, several feasibility studies on highly ductile fiber-reinforced cementless composite were reported [3; 4; 5; 6; 7; 8; 9]. Although the reported materials had lower compressive and tensile strength values than those of cement composites, certain types of cementless composites showed higher tensile ductility, smaller crack width, and tighter crack spacing than those of cement composites. [PERSON] et al. demonstrated the strain-hardening behavior and high ductility of alkali-activated slag based cementless fiber-reinforced composite under tension load [3]. Polyvinyl alcohol (PVA) fiber of 2 vol.% was used as reinforcement. The compressive strength was 30.6 MPa and the tensile strength and tensile ductility were 4.7 MPa and 4.5%, respectively. [PERSON] and [PERSON] also established the feasibility of highly ductile fly-ash based cementless composite with high tensile ductility of up to 4.3% [4]. The compressive strength of composite was 27.6 MPa. After that, relatively high strength fly-ash based cementlesscomposites with a compressive strength of 63.7 MPa and tensile ductility of 4.3%, called engineered geopolymer composites, were developed [6; 7]. In [PERSON] et al. [8] it was experimentally proved that high ductility of up to 7.5% and high tensile strength of 13.1 MPa in a cementless composite can be achieved by using polyethylene (PE) fiber with high strength of up to 2 700 MPa. The effect of a defoamer on the tensile behavior of a cementless composite was also investigated [9]. Mechanical properties were investigated between cement and cementless composites with identical water to binder ratios and levels of fiber reinforcement (PE fiber) [10]. As described above, it has been demonstrated that, compared to cement-based composites, alkali-activated slag-based cementless composites reinforced with synthetic fibers have high tensile performance. However, to the best of the authors' knowledge, there is limited information available on the fundamental mechanisms that support the strain-hardening and highly ductile behavior of slag-based composite. The tensile behavior of fiber-reinforced composite depends on the characteristics of the matrix, fiber, and interface between matrix and fiber. It also depends on the structure and morphology. Therefore, to understand the fundamental mechanism on the strain-hardening and highly ductile behavior of the composite, it is necessary to investigate the micromechanical properties and evaluate the stress performance index (\(I_{SP}\)) and energy performance index (\(I_{EP}\)) [11; 12; 13]. Here, the two indices refer to the calculated values of the stress and energy conditions for the strain-hardening and steady-state multiple cracking behavior of composite. More specifically, for the stress condition, the maximum fiber-bridging stress must be higher than the cracking strength of the composite, while for the energy condition the complementary energy must be higher than the fracture energy of the matrix. The higher the two performance indexes are, the greater the likelihood is that the composite will exhibit strain-hardening and steady-state multiple cracking behavior.