To explain the incompatibility of some superplasticisers with class F fly ash, the effectiveness and mechanisms of superplasticisers based on lignosulfonate (LS), polynaphthalene (PNS) and polycarboxylates (PCE-1 and PCE-2) in dispersing the fly ash pastes were studied. The most effective superplasticisers, PCEs (PCE-1 and PCE-2), produced almost no additional electrostatic repulsion, are adsorbed in low amounts, and exhibited negligible adsorbed layer thickness. It is suggested that the excellent dispersing abilities of PCEs rely on the molecules remaining in the liquid phases of the pastes. For the inefficacy of LS and PNS, the absorption of molecules and bridging flocculation are the most likely causes. Superplasticiser molecules that entered the fly ash through the pores on the fly ash surface lost their dispersing abilities. The highly negatively charged LS and PNS can attract several fly ash particles to form the bridging flocculation, which reduced the dispersion of the fly ash pastes.