Abstract Examination of Class G cement slurry retardation up to 250 degrees F has indicated that commonly employed retarders usually give a threshold of unexpectedly long thickening times at ~160–190 degrees 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 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 degrees 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. Introduction Cement slurries are designed in the laboratory using carefully controlled API and related procedures. From the viewpoint of cement slurry performance the key class of additives is retarders, because they are present to varying degrees in most cement slurries. Thickening time is a key factor in cement slurry design, because of its use in indicating the time available for pumping the slurry into position in the annulus before thickening and hardening take place. Thickening time has already been shown to be highly dependent upon cement and retarder types under given well conditions. In order to understand more fully the nature of thickening under retarded conditions, it was decided to investigate different common retarders under a variety of downhole conditions based mostly upon API temperature and pressure schedules with four different Class G cements. Liquid retarding additives were used here because of their greater overall utilisation worldwide. EXPERIMENTAL PART Four different API Class G cement (Cements 1–4) and four commonly employed retarders (A-D) were used in these investigations. Three of the retarders (A-C) were lignosulphonates and one (Retarder D) was a gluconate. P. 379