Cellulose Nanocrystals (CNCs) have previously been studied as a sustainable additive to cement, enhancing strength and workability of paste. In this work, rotational rheology measurements were conducted on Type V and I/II cement pastes with varying water to cement ratios (W/c). Different CNCs were added to the mixtures in a range of small dosages (<3.5 vol%). Rheology was approximated using the Bingham Plastic model and reductions in yield stress were observed up to 50% reduction at low CNC dosages. However, there were important differences between cement types under the same w/c and CNC dosages conditions, also minimum yield stress occurred at different CNC dosages for differentw/c, and some CNC work better than others. These results are the first step to develop an understanding of the role of cement chemistry on workability enhancement by CNCs, as well as identify the CNC characteristics that work the best for that purpose.