Compressive behaviour and microstructure of geopolymer mortar subjected to cryogenic freeze-thaw cycle: Effects of cycles and polypropylene fiber

The in-depth investigation on mechanical and microstructural properties of cementitious materials subjected to cryogenic freeze-thaw cycles (CFTCs) is essential to efficiently improve their resistance to CFTCs. This study examines the effects of CFTCs ranging from 20 °C to −170 °C up to 9 cycles on geopolymer mortars (GPMs) to, including the role of polypropylene (PP) fiber on their performance. The compressive strength of GPMs initially improved with the CFTCs up to 3 cycles, followed by a decreasing trend with the CFTCs up to 9 cycles. The GPMs with PP fiber exhibited a monotonic and more significant decrease in the compressive strength after exposure to CFTCs. The permeability of GPMs increased with the increasing number of CFTCs due to the increasing cumulative pore volume and deteriorating microstructure, which are also found to be responsible for the decrease in compressive strength. Notably, pores with a diameter of 2500 nm are identified as critical in affecting GPMs' compressive strength. Higher proportions of pores with diameter smaller than 2500 nm enhance compressive strength, while pores exceeding the values have the opposite effect. Additionally, the compressive strength tends to decrease with the increase of microcrack attributes including total microcrack area, total microcrack length, and average microcrack width. GPMs with PP fiber exhibited weaker resistance to CFTCs due to the presence of more free water and the negative thermal expansion attribute of PP fiber. This exacerbates the adverse effects of frost-heave stress and thermal-stress on GPMs.