Because they are even more basic, the alkali-loaded polymorphs are correspondingly more reactive. Appreciable amounts (>1%) in cement make set control difficult, and the cement becomes excessively hygroscopic. The cement powder flowability is reduced, and air-set lumps tend to form. They withdraw water from gypsum on storage of the cement, leading to false set. For this reason, their formation is avoided wherever possible. It is more energetically favorable for sodium and potassium to form sulfates and chlorides in the kiln, but if insufficient sulfate ion is present, any surplus alkalis congregate in the aluminate phase. The feed and fuel in the kiln system are preferably controlled chemically to keep the sulfate and alkalis in balance. However, this stoichiometry is only maintained if there is substantial surplus oxygen in the kiln atmosphere: if "reducing conditions" set in, then sulfur is lost as SO2, and reactive aluminates start to form. This is readily monitored by tracking the clinker sulfate level on an hour-to-hour basis.