Bioactive glass (BG) particles with micron-sized pores are important for the construction of BG scaffolds mimicking the hierarchical pore structure of natural cortical bone. In the present work, 45S5 BG was flame synthesised after alkali activation to prepare BG microspheres with micron-sized pores (mPBGMs), which can mimic the physiological pores of cortical bone appropriate for angiogenesis, cell settlement, proliferation and migration. The effects of three parameters of the alkali activation process, namely activation time, NaOH concentration and temperature, on the bioactivity, compositions and textural properties of mPBGMs were investigated. The results showed that all the mPBGMs exhibited good bioactivity. Compared to 45S5 BG as glass feedstock, mPBGMs lost a certain amount of sodium and phosphorus. The mPBGM treated with different alkali activation processes exhibited porosities ranging from 28 % to 54 %, with predominant pore sizes ranging from 1 to 10 µm. Inspired by alkali-silica reactions, the formation of pores in mPBGMs was considered to be the result of the combined action of foaming, decomposition and dehydration of sodium silicate hydrate, sodium carbonate hydrate and calcium silicate hydrate produced during alkali activation.