Development of 3D printable fibre-reinforced cementitious composites: printability, mechanical properties, and fire performance

Fibre-reinforced cementitious composites (FRCC) are recognised for their superior mechanical performance, strain-hardening behaviour, and enhanced durability. Although 3D printing of cementitious composites has attracted considerable research interest in recent years, investigations on fibre reinforcement effect on 3D printing of cementitious composites (3DP-CC) are still required to advance the development and understanding of 3D printable fibre-reinforced cementitious composites (3DP-FRCC), particularly regarding the influence of fibre dosage and hybridisation on fresh properties, mechanical behaviour, shrinkage, and fire resistance. Building on a previously optimized low-carbon 3D printable mortar incorporating 60% ground granulated blast-furnace slag (GGBFS), this study developed high-performance 3D printable FRCC mixtures. The effects of polyethylene (PE) fibres at 0%, 0.5%, 0.75%, and 1% volume fractions, along with a hybrid system comprising 0.3% polypropylene (PP) and 0.7% PE, were systematically investigated. Printability was evaluated through flowability, extrudability, buildability, and pumpability tests, while rheological behaviour was characterised using static and dynamic yield stress and plastic viscosity. Mechanical performance was assessed via compressive and flexural testing of both 3D-printed and mould-cast specimens. Increasing fibre dosage slightly reduced flowability and pumpability due to elevated yield stress; however, all mixtures remained printable. The hybrid system exhibited the most balanced fresh performance. 3D-printed specimens demonstrated 10–20% higher flexural strength than mould-cast counterparts, attributed to preferential fibre alignment confirmed by microstructural analysis. Fibre incorporation significantly reduced shrinkage, achieving up to 43% reduction at 24 h and 36% at 28 d for the 1% PE mix. Under elevated temperatures up to 800 °C, fibre-reinforced systems showed improved spalling resistance and residual strength, with the hybrid mix performing best. Overall, fibre hybridisation offers an effective strategy for durable, low-carbon 3D printable FRCC.