We study the rheology associated with gelation of hydrating cements used in oil well cementing and the use of accelerators to advance the rates of setting. Cement pastes made of API Grade H oil well cement with water/cement mass ratios of 0.25-0.40 are studied. Setting times are accelerated from 7 to 1 h by the addition of 5% wt/wt calcium chloride/cement. Complex viscosity follows an exponential time dependence and the shape of the gelation curve is essentially unaffected by the calcium addition levels. This leads to a "time-concentration" shifting law for the effect of the accelerator. A novel parallel plate geometry is introduced that eliminates slip and enables dynamic oscillatory measurements in the linear viscoelastic regime during setting. Another instrument based on the penetration of a sphere through the setting cement at rates of 1 mu m/s is presented. This instrument would enable high throughput evaluation of cement additives. The differences between small strain oscillatory measurements that measure a complex viscosity and continuous deformation measurements such as the penetrometer are discussed. (C) 2010 The Society of Rheology. [DOI: 10.1122/1.3494571]