Exploring the post‐thermal exposure effects on mechanical and microstructural properties of alkali‐activated polypropylene fiber‐reinforced mortars

Abstract This study is one of the first to systematically investigate the effect of post‐fire air curing on the mechanical and microstructural properties of alkali‐activated mortars reinforced with polypropylene (PP) fibers. It evaluates the long‐term (90‐day) recovery behavior of mortars produced with both fiber reinforcement and varying slag/fly ash ratios after exposure to temperatures up to 800°C. Sodium metasilicate‐activated mortars with and without PP fiber were produced by replacing varying proportions of blast furnace slag and fly ash (0%, 25%, 50%, 75%, and 100%). The PP fiber was used on a mass basis of the binder at the ratios of 0.5%, 1%, and 1.5%. The flowability, unit weight, apparent porosity, flexural and compressive strength, and microstructural analysis were evaluated. Strength values were assessed at 1, 28, and 90 days before and after exposure to elevated temperatures. The experimental findings indicated that fiber reinforcement enhanced the flexural and compressive strength of the samples before exposure to elevated temperatures. Immediate loss of flexural and compressive strength occurred after exposure to elevated temperatures. For example, after exposure to 800°C, the flexural strength loss in reference mortars was 82%, while the compressive strength loss reached 88%. In fiber‐reinforced mortars, the flexural strength loss was 75%, whereas the compressive strength loss was as high as 94%. These results indicate that fiber addition is more effective in preserving flexural strength than compressive strength. Microstructural analysis revealed that PP fiber improves mortar strength at lower temperatures through a bridging effect, but its melting and burning create voids and cracks at elevated temperatures, ultimately reducing strength.