Pre-treated corn straw fiber for fiber-reinforced concrete preparation with high resistance to chloride ions corrosion

Chloride ions erosion is a negative factor for concrete sustainable usage. Fiber addition is a potential solution, which promotes a tight concrete structure formation resulting in the resistance of chloride ions. Corn straw is a kind of crop waste widely sourced that can be used as sustainable concrete reinforcement material. The traditional recycling method of corn straw including burnt as straw ash is not environment friendly. This work aims to pre-treat corn straw fiber (CSF) by some convenient methods (untreated, water-treated, alkali-treated) and add treated CSFs directly into concrete to prepare fiber-reinforced concrete with excellent mechanical properties and high resistance ability to chloride ions. The component content of different treated CSFs was tested to study the fibers composition and microstructure change. The heat of hydration, the compressive strength and flexural strength were tested to study the hydration degree and the effect of different treated CSFs on the concrete mechanical performance. The rapid chloride permeability (RCPT) and chloride migration (RCM) tests were designed to study the reinforced concretes chloride ions resistance ability change. In addition, to further explore the mechanism of mechanical performance and chloride ions resistance ability change, water absorption rate (WAR) and mercury intrusion porosity (MIP) were tested to provide more supporting information from micro-level. It is concluded that CSF treated by Na2SiO3 has the least impurities and a rougher surface with a larger specific surface area bonding with cement substrate. Compared to the original concrete, the compressive strength of CSF concrete, WT-CSF concrete, and NT-CSF concrete increased by 15.7%, 17.8%, and 33.4% respectively. The results of RCM and RCPT show a decreasing trend with different treated CSFs addition, the NT-CSF concrete shows the greatest decrease with 20.1% and 17.5%, respectively. Except the rougher surface and less impurities of CSF, the improvement of mechanical performance and chloride ions resistance ability is mainly due to treated CSF, especially the NT-CSF contributes to a denser structure formation based on the MIP and WAR tests results. The viscosity of Na2SiO3 solution acts as a binder that helps to tighten the bonding between substrate and CSF. The water adsorbed by CSF and the alkaline environment provided by Na2SiO3 facilitates hydration decreasing the chloride ions migration. At the same time, the cellulose in CSF containing many hydroxyl and carboxyl groups further contributes to the concrete chloride ion resistance improvement due to its weak conductivity and the binding force of ions exchange.