Geopolymer, synthesized from ore-dressing tailing of bauxite (hereinafter referred to as tailing) and ground-granulated blast furnace slag (hereinafter referred to as slag), is exposed to severe conditions such as the low temperature during molding, setting, and hardening and the elevated temperature after hardening. The effect of low-temperature curing and high-temperature heating on strength, hydration products, and microstructure is investigated. The results show that the low-temperature curing hinders geopolymerization, and thus, lower early-age strength than that of ambient temperature curing samples is observed. However, this delayed effect does not prevent long-term strength from gradually increasing. The 90-day compressive strength can reach up to 60 MPa, and that is comparable value to 28-day strength of ambient temperature curing samples. The observation of retarded geopolymerization reaction and microstructure formation in TG-DSC and ESEM analysis is an essential reason for delayed strength development in low temperature. More large pores (>10 μm) observed in pore structure analysis also result in the lower strength. After firing at elevated temperature up to 1000 °C, strength of the hardened low-temperature curing geopolymer mortars severely decrease by more than 40 MPa. The reasons for this deteriorationare the chemical processes involving gels decomposition and crystalline-phase formation and physical processes such as cracking. After heating at 1200 °C, an increased strength is obtained, which is a result of sintering and densification.