To mix cement paste, the methodology adopted was that developed by PCA [4]. The system consists of a blender mixer with a capacity of 1 liter connected to a speed controller and to a temperature controlled water bath. The speed controller allows the speed of the blades in the mixer to be held constant regardless of the load. The cement paste resistance to mixing can vary with the viscosity or yield stress of the paste. The more viscous or higher the yield stress, the higher will be the load for the blades. The controlled temperature bath is set to 15 º C. This temperature allows the paste to be about 20 to 22 º C at the end of the mixing cycle, which is the temperature that is measured in concrete just after mixing. If the base of the mixer was not cooled, the temperature would be much higher and, therefore, the rheological parameters will correspond to a material at a different stage of the hydration process. In concrete the aggregates are a heat sink that replaces the role played by the cooled base of the mixer.
The mixing regime adopted was:
A precise volume of cement paste is transferred to the rheometer, using a disposable syringe (without the needle) having a maximum capacity of 1 ml. Depending on the gap the correct volume is selected. The amount of paste needed for the rheometer should uniformly fill the volume between the two plates with very little excess.
Once an amount of paste is placed on the lower plate of the rheometer, the two plates are automatically driven to the preset gap by the computer. The measurements are then initiated. To homogenize the paste between the plates, the top plate of the rheometer is rotated by predetermined steps up to a shear rate of 80 s−1 for 160 s. The torque measured is stable after this treatment. Then the shear rate is again swept between 0 and 24 s−1 (up curve) followed by the down curve with the same shear rate range as the up curve. Only the down curve is considered for the calculation of yield stresses and viscosities.
To avoid slippage during the measurements of the torque at varying shear rates, the plates of the rheometer are covered with an abrasive paper. The paper had a self adhesive backing and had a grit size of 240. This insured that the grit particles had the same average size as the cement particles.