Hybrid Effect of Nano-CaCO₃ and Polypropylene Fiber on Fresh and Hardened Properties of Alkali-Activated Material

Compared with traditional portland cementitious material, alkali-activated material (AAM) had the advantages of low carbon emission, energy saving, excellent durability, strength, and high temperature resistance. It has been proved that the addition of a fiber or nano-particle could improve the mechanical properties of AAM. The fresh and hardened properties of nano-CaCO3 (NCC) and polypropylene fiber (PPF) reinforced AAM were studied in this paper. The hybrid effects of PPF with various volume fractions (0%, 4%, and 8%), aspect ratios (0-464.52), and NCC with various content (0%, 1%, and 2%) on the slump spread, flow rate, flexural strength, compressive strength, and ultrasonic velocity of AAM were investigated. For flowability, the mixture with 0.4% 6 mm and 0.4% 12 mm PPF was the optimal, showing a positive hybrid effect. The hybrid use of PPF and NCC significantly improved the flexural strength, but the compressive strength was not significantly improved. The hybrid effect factors for compressive strength were lower than those for flexural strength. The positive hybrid effect of strength was most obvious when the nano-CaCO3 was 1%. The threshold for fiber factors was 200. Microstructure studies showed that the bridging effect of PPFs can limit the crack development and enhance the strength of AAM. Nano-CaCO3 promoted the denseness of AAM and bond between fiber and matrix. The hybrid of PPF in different lengths and NCC could decrease fiber consumption, reducing the cost and promoting the engineering application of fiber-reinforced AAMs.