Rheological study of the interactions between MHEC and superplasticizers in aqueous solutions for applications in cement industry

A rheometer is a laboratory device used to measure the way in which a liquid, suspension or slurry flows in response to applied forces. It is used for those fluids which cannot be defined by a single value of viscosity and therefore require more parameters to be set and measured than is the case for a viscometer. It measures the rheology of the fluid.

The ideal parallel plate rheometer is practically impossible to use because of its kinematic conditions: in fact the plates translate one to each other and after time they completely separate. Moreover the stress applied to the sample continuously decreases because of the decreasing of the surface of application. The best way to reduce these undesired effects is to force the movement of the fluid along a close trajectory, repeatable in the time. This is the principle exploited in rotational rheometers: the instrument is made up of two plates, one is fixed and the other is in movement. The fluid is positioned between the plates. A motor provides the torque necessary to put the moving plate in rotation while a second device (transducer) is responsible to measure the torque necessary to move the plate. From the torque, with some calculations it is possible to obtain the value of the stress and also the value of the viscosity.

There are two kinds of commercial rotational rheometers: controlled-stress rheometers and controlled-strain rheometers. In the first ones the torque-transducer is the same motor used to put in rotation the plate while the shear rate is derived from the measure. They have recently been introduced on the market but, for the rapid development of the technology, they have competitive prices and they are presently the most employed rheometers.

The strain-controlled rheometers are older than the first ones; in this case the movement is produced by an independent motor to impose the shear rate while the torque is directly measured by a transducer positioned in correspondence of the fixed plate.
The strain-controlled rheometers are the less diffused but they are particularly convenient for some tests useful in the study of viscoelastic systems. Besides the motor of these rheometers is generally more powerful, allowing for the achievement of high torques, and so they are particular suitable for experiments on polymer melts.

Independently from the kind of rotational rheometer employed, some geometries are available; they change on the basis of the fluid tested and of the test performed. In the following, the main geometries for polymer melts are reported.