Architecture Puzzler #103

The approximate setting time for concrete at 70 degrees Fahrenheit (21 degrees Centigrade) is six hours. How long would you guess the setting time is at 40 degrees F. (4 degrees C.)? At 20 degrees F. (-7 degrees C.)?

Acrylics and polyurethanes are two main types of sealant. Which is generally cheaper and can be painted sooner? Typically, a knowledgeable specifier would use the polyurethane on the exterior and the acrylic for the interior. Why?