The synergistic enhancement of bending and compression properties of glass fiber reinforced phosphate activated metakaolin geopolymer

Phosphoric acid-based geopolymer (PAG), an emerging, eco-friendly, and durable gel material, is characterized by a simple production process, excellent and stable performance, and expansive application potential. The addition of fiber will markedly enhance PAG's bending performance and toughness, however, it adversely impacts the compressive strength, complicating the achievement of a balance between bending and compressive properties. In this study, the effects of PA concentration (6-10 mol/L) and the liquid-solid ratio (L/S, 0.8-1.2) on the compressive strength and microstructure of phosphate-activated metakaolin geopolymer (PAMG), as well as the influences of glass fiber (GF) dosage (FD, 0.5%-2%) and length (FL, 3, 6, 9 mm) on the flowability, compressive strength, bending strength, and microstructure of GF-reinforced PAMG (GF/PAMG) were investigated. And the mechanisms of reinforcement and toughening were discussed. The compressive strength of PAMG increased firstly and then decreased with the increase of PA concentration and L/S. With the optimal conditions of 8 mol/L PA concentration and L/S of 1.0, PAMG's maximum compressive strength reached 29.7 MPa. Under these conditions, when the FL was 6 mm or 9 mm, GF/PAMG's flowability diminished with the increase of FD, while the compressive strength initially increased and then decreased. Meanwhile, the bending strength and toughness progressively improved. When the FD was 1% and FL was 6 mm, the negative effects of air interception were minimal, and the fiber bridging effects were predominant, leading to a maximum compressive strength of 43.5 MPa, an increase of 46.5%. Concurrently, the bending strength reached 2.7 MPa, marking an 80% enhancement, thereby achieving a synergistic improvement in both bending and compressive strength. Microscopic examination revealed the absence of chemical reactions between the fibers and the matrix, signifying a stable physical bond. • Phosphate activated geopolymer (PAMG) was prepared at the optimal ratio. • Appropriate amount of gel structure and AlPO 4 crystal formed a dense structure. • The compression and bending strength were synergistically enhanced. • Air interception effect and interaction mode between fibers and matrix. • The stable physical bond between fibers and matrix.