Simplistically; sand/dust results in abrasion (to the rotor blades as well as the compressor blades) but volcanic ash, with its much lower melting point, can result in glassification of the turbine blades as well. It is not the abrasion that results in engine failures (although it will affect their efficiency), it is melting ash in the hot end. That is how it can kill all engines within a short period of time.
Jim