The effect of polypropylene fiber with different length and dosage on the performance of alkali-activated slag mortar

Alkali-activated slag (AAS) cement is a cementitious material with high strength, high durability and environmental protection. AAS is considered as a potential substitute for Ordinary Portland Cement, although its large-scale application is limited by its large drying shrinkage. In this study, we demonstrate that the addition of polypropylene fiber (PPF) can reduce the drying shrinkage of AAS mortar. The influences of PPF length and dosage on the fluidity, mechanical properties, microstructure and impact resistance of AAS mortar are investigated. Three lengths (3 mm, 9 mm and 15 mm), and four dosages (volume fractions of 0.05%, 0.1%, 0.15% and 0.2%) of PPF are used in the AAS mixtures. Test results show that the addition of PPF can reduce the drying shrinkage of AAS mortar, as adding 0.2% of 15 mm PPF can reduce the drying shrinkage by 37.1%. The flexural strength of AAS mortar increases with PPF length and dosage, while introducing long PPF (9 mm and 15 mm) can slightly improve the compressive strength of AAS mortar. Mercury intrusion porosimetry (MIP) and Scanning electron microscope (SEM) results indicate that PPF with 9 mm and 15 mm can reduce the average pore size and improve the microstructure of AAS mortar. At the same time, effective bonding between the matrix and PPF is observed, which is consistent with the enhancement of mechanical properties. Furthermore, PPF addition significantly improves the ultimate toughness of the AAS mortar, which is manifested in the enhancement of impact resistance.