what could it be when it throws white smoke through the tail pipes

i had a water leak 2 days ago where the water pump is loctated. i removed the whole base where the water pump is,i found out it was the 4 gaskets that bolt to the block i repaired it and put it back together, so when i put water in it i started up car and left the car running for about 15 mintues after the 15 minutes i saw the car trying to stall so i checked the gauge and saw that the car was over heating i saw white smoke coming from the tail pipes,first thing that came to my mind that it was the head gaskets so i checked the oil for any signs of water in the oil.i just bought the thermostat and will installed it,so i installed it and waited 25 minutes fans come on so i think that solve my problem but what would cause white smoke coming out of my tail pipes when it was over heating