The variation in rheological properties of normal portland cement type-10 and blended silica fume (SF) cement pastes was investigated as a function of temperature (0-40 degrees) in order to elucidate changes in concrete workability with ambient temperature. The rheological parameters measured included the Kantro mini-slump (spreading areas, S) and the dynamic viscosity (n) at various hear rates as a function of superplasticizer concentration (sodium polynaphthalene sulfonate, PNS). To interpret the changes in fluidity of the cement pastes, the concentration of the superplasticizer in the solution phase was monitored as a function of time (0-2 hours); calorimetric measurements of the early cement hydration rate (0-3 hours) in the pastes were also measured in some cases. The variations observed in paste fluidity (S, or 1/n) at a given PNS dosage exhibit significant non-linear variations with temperature; the rate of change of S and 1/n with time (i.e. slump loss rate) are also found to be non-linear, usually with a maximum value in the interval 5-20 degrees. The non-linear effects are more pronounced with the SF cement than with the type-10 cement. The observations are interpreted tentatively on the basis of coupled physico-chemical effects involving PNS adsorption on cement and on silica, and the influence of PNS on the early hydration rate.