Examination of Class G cement slurry retardation up to 250° F has indicated that commonly employed retarders usually give a threshold of unexpectedly long thickening times at ~160-190° F BHCT. Normally, thickening time is not linearly but exponentially dependent upon retarder concentration. The causes of the threshold of long thickening times have been investigated by carrying out appropriate hydration experiments on Class G cements at water/cement ratio 0.44 subjected to thickening under different API Schedule conditions. The cause of the threshold effect has been found to be surge in hydraulic reactivity of the ferrite (C4AF) phase from the Class G cement. The hydration products thus formed, mostly AFt phase or ettringite C3(A, F).3CaSO4.31-32H2O, are deposited on the hydrating clinker surfaces and in particular impede the hydration of the main cementitious alite (C3S) to calcium silicate hydrate (C-S-H), the initiation of which is the prime cause of cement thickening. As a result thickening time is extended and not diminished. However, as the temperature rises above ca 190°F, the increased hydraulic potential of the cement manifests itself. There is no longer an increased surge in ferrite phase hydration to obstruct C-S-H formation, so the C3S hydration rate rises again, culminating in lower thickening times once more. The threshold effect has important implications in cement slurry design.