Chemical Guy, it has been a long time since Chemistry 101 but drywall is made of gypsum. The rock (calcium carbonate??) is heated to drive off the waters of hydration and it is ground in a ball mill into powder. The powder is then mixed with water into a slurry and poured into the paper liners that make up the wall board. It is driedto drive off most of the water but some remains in the form of waters of hydration to bind the gypsum into one piece again. Therefore, the wallboard still retains a large amount of water but it is tied up with the gypsum. When you add additional plaster, the water from this plaster effloresces from the one surface since the gypsum fights it from going into the wallboard. With the increase in humidity in a closed room, the water vapor has no place to go and it condenses on other surfaces. Therefore, you have to have good ventilation to get the plaster to dry.