Effect of Surface Treatment of Polypropylene (PP) Fiber on the Sulfate Corrosion Resistance of Cement Mortar

Sulfate erosion is one of the most complex and harmful chemical corrosion actions. Following sulfate erosion, concrete expands, cracks, dissolves, peels off, and decreases in strength, which affects the durability of structures. Polypropylene fiber (PP) is widely used in various concrete structures because of its good mechanical properties and chemical corrosion resistance. However, PP fiber has a number of shortcomings, such as a smooth surface, poor hydrophilicity, lack of active groups in the molecular chain, and agglomeration and poor dispersion in cement-based materials. These issues limit its application in cement-based materials. Although the use of a silane coupling agent to modify the surface of PP fiber is effective, the influence of treated PP fiber on the sulfate resistance of cement-based materials is not significant. In this study, a PP fiber treated with a silane coupling agent was used to examine effects of different cement-to-sand ratios (C/S) and dosages of the treated PP fiber on the sulfate erosion resistance of cement mortar. Furthermore, the apparent morphology, mass loss rate, flexural strength, corrosion resistance coefficient, and microstructure of the concrete were investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results revealed that the PP fiber became rough after modification. Active groups were introduced on the fiber surface, which were well dispersed in the mortar and formed a good network distribution structure in the cement mortar, thereby slowing the erosion rate of the PP fiber mortar in a sodium sulfate solution. At a C/S ratio of 1:1 and a treated fiber dosage of 0.6%, the treated fiber mortar has exhibited good sulfate resistance. In addition, the monofilament fiber immersion test revealed that a layer of sodium sulfate crystals was deposited on the fiber surface, thereby increasing the roughness of the fiber surface and the pull-out force of the fiber from the cement matrix, this result indicated that the interfacial adhesion between the treated PP fiber and cement matrix was improved, which in turn led to the improvement in the sulfate erosion resistance of the treated PP fiber.