# Mechanical Properties and Freeze-thaw Durability of Basalt Fiber Reactive Powder Concrete
## 1 Introduction  
In the practical application of concrete structural engineering, the deterioration of concrete and the shortening of its service life are related to freeze-thaw damage, chloride ion penetration, sulfate attack and carbonization [1; 2]. Accordingly, it is important to improve the durability of concrete structures in practical applications, which is a crucial factor to reduce the cost of infrastructure construction and maintenance. In cold areas, most concrete structures appear in the natural environment, and the freeze-thaw cycle reduces the service life and becomes one of the main sources of damage to the concrete structure [3]. Especially in cold areas, where de-icing salt has been adopted, the concrete damage caused by the coupling action of chloride corrosion and freeze-thaw cycle is serious [4]. The destruction of concrete structures caused by freeze-thaw cycle has aroused widespread concern. The durability of concrete is closely related to the internal pore structure. Concrete structure with low-porosity could effectively reduce the pore solution inside the concrete, thus reducing the internal water pressure caused by the freezing of the pore solution and improving the freeze-thaw resistance of concrete [5]. Besides, the compact concrete structure could hinder the transmission of chloride and greatly reduce the damage caused by chloride ion penetration [6; 7]. Reactive powder concrete (RPC), proposed in the 1990s, is an ultra-high-performance concrete characterized by dense microstructure and uniform concrete matrix [8]. The traditional RPC is developed through microstructure enhancement technology and the main improvement is related to matrix uniformity, porosity and microstructure, including eliminating coarse aggregates and reducing the water-binder ratio to improve matrix uniformity [9], optimizing particle gradation to decrease matrix porosity [10] and increasing the silica component by adding silica fume to improve the microstructure of the matrix structure [11]. RPC is seen as a potential material for the field of special prestressed and precast concrete components due to its low permeability, superior mechanical properties and durability [12; 13]. Although the manufacturing cost of RPC is generally high, the thickness of concrete component is possible to be reduced due to its ultra-high mechanical properties, resulting in materials and costs saving. Therefore, RPC has certain economic advantages in practical applications [14]. However, RPC still has the characteristics of high brittleness of ordinary concrete and the brittleness of RPC increases as the strength increase. Therefore, the addition of fiber is usually adopted to increase the toughness of RPC [15]. Adding fiber into brittle concrete is a widely used technology. The bridging and pulling effects of fiber could improve the brittleness and impact resistance of the mixture [16; 17], and the three-dimensional randomly scattered fiber could effectively inhibit the generation and propagation of cracks [18]. Previous reports have found that polypropylene fiber [19], carbon fiber [20] and steel fiber [21] can be used as reinforcing materials for RPC, and the fiber commonly used to reinforce concrete is steel fiber [22]. To improve the flexural strength, compressive strength, elastic modulus and ductility of RPC, the recommended content of steel fiber is 2% volume fraction [23]. Moreover, adding steel fiber into concrete is judged as an effective method to prevent spalling. However, steel fiber with high content will converge into a spherical shape and the steel fiber is sensitive to chemical corrosion, which results in a decrease in properties of concrete. Besides, adding steel fiber into the concrete will reduce the workability and increase the cost [24]. Basalt fiber is an environmentally friendly and high-performance fiber made of natural volcanic basalt rock, which can be used to reinforce concrete.