Q.: In several locations on the walls of an office building we have lighter colored concrete than elsewhere. We've determined that it has always been at locations where we began the day's work. The concrete was placed by pump. Did that have anything to do with it?
A.: Very likely. If the first half cubic yard or so of concrete through the pumpline wasn't wasted, it could account for the trouble. That first concrete loses some of its cement paste and mortar in the process of coating the pipeline and probably changes the water-cement ratio slightly as well. Both effects change the color. Either waste the first half cubic yard of concrete or prepare ahead of time to use it for some other and noncritical application.