In situ powder X-ray diffraction was used to examine the hydration of API Class H cement slurries, with a water-to-cement ratio of 0.394, at.66, 93, 121, and 177 T under autogenous pressure in the presence of varying amounts of the additives tartaric acid, modified lignosulfonate, and AMPS (2-acrylamido-2-methylpropanesulfonic acid) copolymer. All of these retarding agents inhibited the hydration of crystalline C3S (Ca3SiO5), but other modes of action were also apparent. The formation of ettringite was suppressed when tartaric acid was used by itself or in combination with other additives. Changes in the hydration Of C3S vs time could not be correlated in a simple way with the observed pumping times for the cement slurries. The largest changes in pumping time as a function of temperature occurred in a temperature interval where ettringite/monosulfate decomposes and crystalline hydrogarnet starts to be formed.