Aluminum reacts with water to form various oxides of aluminum plus hydrogen gas, with the release of a great deal of heat. The primary reactions are:

2Al + 6H2O ---> 2Al(OH)3 + 3H2
2Al + 4H2O ---> 2AlO(OH) + 3H2
2Al + 3H2O ---> Al2O3 + 3H2

In the ALICE propellant, aluminum nanopowder is mixed with ice, forming a thick paste which can be molded and cooled before flight so that the water remains frozen until launch. Once ignited, the aluminum and water react, the expanding hydrogen gas provides the thrust, and the heat generated allows the reactions to continue. Since there is no free oxygen in the combustion chamber, the hydrogen can't "burn" until it hits the outside air, at which point it is no longer supplying thrust to the rocket. Likewise, the presence of a vacuum outside would have no effect on the reactions taking place inside the rocket engine.
Source(s):
http://www1.eere.energy.gov/hydrogenandfuelcells/pdfs/aluminium_water_hydro...
http://www.physorg.com/news170080451.html

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