## 1 Introduction  
Since Portland cement was developed in the 1820s, concrete has been used as the main construction material in construction fields due to its excellent economic feasibility, relatively high compressive strength, and durability [1; 2; 3]. However, it has inherently low tensile strength (about 10% of compressive strength), which results in susceptibility to cracking, and shows brittle behavior under tensile load, compared with compressive load.  
Recently, there is growing interest in highly ductile cement composites (HDCC) which is a class of cement mortar-based composite reinforced by synthetic fibers of less than 2% volume. This combination shows high tensile strain capacity, over 3%, which is approximately three hundred times greater than that of normal concrete [4; 5; 6; 7]. Unlike quasi-brittle concrete materials, HDCC shows tensile strain hardening under uniaxial tensile load. Such superior tensile behavior compared with normal concrete or fiber-reinforced concrete is achieved by proper selection of materials such as binder, aggregate, and fiber; and optimization of the mixture proportions based on micromechanics and steady-state cracking theory. The micromechanics and steady-state cracking theory are the theoretical backgrounds of the multiple cracking and strain-hardening behaviors of HDCC [8; 9]. Based on these theories, it is possible to select materials and determine mixture proportions to achieve a composite with high tensile-strain capacity and controlled crack width.  
In most types of HDCC, cement is used as the main binding material. However, a previous report indicated that the cement industry produces approximately 5% of the total carbon dioxide produced by human [10]. Therefore, many studies are being carried out to reduce the use of cement by partially replacing the cement with inorganic binders such as fly ash, blast furnace slag, and limestone powder, which are industrial by-products [11; 12; 13]. Limestone powder was assumed to be an inert filler before the 1980s, however, recent studies showed that calcium carbouluminate hydrates precipitate during the hydration of Portland limestone cement. Moreover, there is an interaction between calcium carbonate and tricalcium silicate [14]. Previous literature reported that the fluidity of the concrete was improved and cost efficiency improved by incorporating limestone powder [15; 16]. The amount of superplasticizer in concrete can be reduced by incorporating limestone powder while maintaining fluidity; although the performance depends on the particle size distribution and particle shape as well as surface characteristics of the limestone powder [17]. [PERSON] et al. (2010) also reported that the flexural deflection capacity and the tensile strain capacity of engineered cementitious composites (ECC) increased as the limestone powder content increased [18]. Although the feasibility of adapting limestone powder for HDCC has been demonstrated, study of the effects of limestone powder on the tensile behavior of HDCC, in terms of micromechanics, is still limited.  
The purposes of this study were to investigate the effects of limestone powder on the composite properties of HDCCs, in which the cement was partially replaced by limestone powder, and to investigate the underlying reasons for differences in the tensile behavior observed in the composites by performing matrix fracture and single fiber pullout tests and micromechanical analysis.