EFFECT OF HEMP FIBER ADDITION ON MECHANICAL PROPERTIES OF FLY ASH-BASED GEOPOLYMER MORTAR WITH PERLITE AGGREGATE

The construction industry’s strong interest in sustainable materials has driven efforts to develop environmentally friendly alternatives. Geopolymer concrete is a viable substitute for traditional Portland cement, capable of reducing CO₂ emissions by up to 80%. This study investigates the effect of adding hemp fiber in varying lengths (1 cm, 2 cm, and 3 cm) and dosages (0.5%, 0.75%, and 1%) on the performance of fly ash-based geopolymer mortar, using expanded perlite as a lightweight aggregate and a 10 M sodium hydroxide (NaOH) solution as the alkaline activator. The samples were heat-cured at 90°C and tested after 3 days for flexural and compressive strength. The flexural strength increased from 2.20 MPa (control) to 2.28 MPa (1 cm), 2.44 MPa (2 cm), and 2.43 MPa (3 cm). Compressive strength rose from 5.15 MPa (control) to 5.72 MPa (1 cm), 6.47 MPa (2 cm), and 6.54 MPa (3 cm). Additionally, fire resistance tests conducted at 300°C, 600°C, and 900°C on the 3 cm fiber-reinforced samples showed an increase in flexural strength compared to the control: 41.14%, 15.79%, and 14%, respectively. The unit weight test after fire exposure revealed greater mass loss in fibrous samples, particularly at 900 °C, where the 3 cm sample lost 19.8 g compared to 16.4 g for the control. However, the mechanical performance of the fiber-reinforced samples remained superior. SEM analysis showed improved integration between the fiber and the mortar matrix, resulting in better bonding, increased toughness, and crack-bridging effects. These findings suggest that the addition of hemp fiber, particularly at a dosage of 0.75% and a length of 3 cm, enhances both the mechanical properties and thermal resistance of fly ash-based geopolymer mortar.