The performance of reological protocols usually employed for the rheological characterization of oil well cement slurries is investigated. To this end two cement slurries were employed, namely Paste A and Paste B. The API procedure is addressed and a modification to it is investigated. Flow curves are determined with the aid of a rotational rheometer using three different methods (ramp up/down, ramp down, and minimum-viscosity). The API procedure yielded flow curves that significantly overestimate (Paste A) or underestimate (Paste B) the viscosity throughout the whole range of shear rate. The deviations become particularly dramatic in the low end of this range and for Paste B, clearly because the steady state is not attained due to a thixotropic behavior. The investigated modification to the API procedure resulted in flow curves similar to the ones based on the minimum-viscosity method. A novel irrevesible phenomenological constitutive model is proposed to describe oil well cement slurry rheology under simple shear flow. To this end several rheological experiments were conducted, namely constant shear rate, construction and microstructural destruction experiment. The proposed model employ the fluidity itself, reciprocal of viscosity, to describe the paste s structure evolution. The macroscopical rheological effect of hydration phenomena is introduced in the model by means of a yield stress exponential function. All ten parameters needed in the model are experimentally obtainable. The present results have shown a good agreement between experimental data and modeling, albeit minor differences were observed for low shear rates below 7.1 s(−1).