The transport properties of an ionic model for liquid silica [1] at high temperatures and pressure are investigated using molecular dynamics simulations. With increasing pressure, a clear change from "strong" to "fragile" behaviour (according to Angell's classification of glass-forming liquids) is observed, albeit only an the small viscosity range that can be explored in MD simulations. This change is related to structural changes, from an almost perfect four-fold coordination to an imperfect five or six-fold coordination.