In this study, the pore structure of alkali-activated fly ash (AAFA) pastes characterized by different techniques, including mercury intrusion porosimetry (MIP), nitrogen adsorption and image analysis (based on backscattered electron images), was evaluated and compared critically. The degree of reaction of fly ash in AAFA pastes was derived from image analysis. It was found that due to a significant “ink-bottle” effect, the pore diameter of capillary pores derived from MIP was two orders of magnitude smaller than the size determined by image analysis. MIP and nitrogen adsorption results showed different peaks corresponding to the gel pores of AAFA pastes. Based on the experimental results, image analysis is regarded as a reliable technique for the characterization of large pores (> 1 μm) in AAFA pastes. Nitrogen adsorption is more suitable to characterize small pores (< 0.1 μm) in AAFA than MIP, and MIP data should be carefully interpreted, preferably in combination with other characterization techniques.