This tweet references an earlier article from The Space Review on using powdered dust found on objects throughout the solar system.

Electrostatic propulsion is the method used by many deep space probes currently in operation such as the Dawn spacecraft presently wending its way towards the asteroid Ceres. For that probe and several others, xenon gas is ionized and then electrical potential is used to accelerate the ions until they exit the engine at exhaust velocities of 15–50 kilometers per second, much higher than for chemical rocket engines, at which point the exhaust is electrically neutralized. This method produces very low thrust and is not suitable for takeoff from planets or moons.

However, in deep space and integrated over long periods of engine operation time, the gentle push of an ion engine can impart a very significant velocity change to a spacecraft, and do so extremely efficiently: for the Deep Space 1 spacecraft, the ion engine imparted 4.3 kilometers per second of velocity change (delta-v), using only 74 kilograms of propellant to do so. As of late September, Dawn’s ion thrusters have produced 10.2 kilometers per second of delta-v, using 367 kilograms of xenon...

Chemical rockets achieve their large thrust with high mass consumption rate (dm/dt) but low exhaust velocity; therefore, a large fraction of their total mass is fuel. Present day ion thrusters are characterized by high exhaust velocity, but low dm/dt; thus, they are inherently low thrust devices. However, their high exhaust velocity is poorly matched to typical mission requirements and therefore, wastes energy. A better match would be intermediate between the two forms of propulsion. This could be achieved by electrostatically accelerating solid powder grains.

Imagine a vehicle that is accelerated to escape velocity by a conventional rocket. It then uses some powder lifted from Earth for deep-space propulsion to make its way to a NEO, where it lands, collects a large amount of already-fractured regolith, and then takes off again. It is already known that larger NEOs such as Itokawa have extensive regolith blankets.

After leaving the NEO, onboard crushers and grinders convert small amounts of the regolith to very fine powder. (These processes would be perfected in low Earth orbit using regolith simulant long before the first asteroid mission.) Electrostatic grids accelerate and expel the powder at high exit velocities.

(Via Powering cislunar spaceflight with NEO powder)

Science fiction writers have long speculated on the use of what Jack Williamson called positive ray propulsion in his 1931 classic The Prince of Space:

These mammoth vacuum tubes, operated at enormous voltages from vitalium batteries, were little different in principle from the "canal ray" apparatus of some centuries before. Their "positive rays," or streams of atoms which had lost one or more electrons, served to drive the sunship by reaction—by the well-known principle of the rocket motor.

As Winchell Chung of Project Rho points out, Arthur C. Clarke described a finely divided dust propellant, that is, powdered lunar regolith:

The liners that plied from world to world obtained all their propellant mass here, filling their great tanks with the finely divided dust which the ionic rockets would spit out in electrified jets. By obtaining that dust from the Moon and not having to lift it through the enormous gravity field of Earth, it had been possible to reduce the cost of space-travel more than ten-fold.