A regular dust mask from Lowe's or Home Depot would do the trick, but if you want to spring for a 3M half face respirator you can usually find one that has a combination particulate and vapor filter cartridge. P95 or P100 filters are your friend for anything cementitious you are grinding. Very occasional grinding with a dust mask will not be too bad, but repeated exposure damages your lungs. Just make sure to ventilate the space well and maybe try to use a shop vac to capture the dust as you generate it or even consider using water as you grind to cut down on dust even getting into the atmosphere. A respirator is an easy safeguard though in case you don't want to mess around with all of the other dust control systems. Make sure it's snug on your face though