Evaluation of a Cross-Conductance Sensor for Cement Paste Hydration Monitoring and Setting Time Measurement

In this article, a sensor for monitoring hydration and detecting initial and final setting timings of freshly mixed cement paste is presented. It is a new application of a cross-conductance sensor and possesses the advantages of the cross-conductance approach, i.e., the output is sensitive only to the conductivity of the medium other than the length of the electrode. The new sensor overcomes several limitations of the conventional Vicat apparatus (VA) and other existing impedance-based schemes. The cross-conductance sensor cell is designed with the help of finite element analysis. The cell has four conductive electrodes separated by small gaps. It is fabricated using inexpensive but reliable materials. A simple measurement circuit is developed to obtain an output due to a change in the conductance of the sensor cell. The output of the sensor unit gives the conductance versus time response of the sample. From the rate of change of this output, the setting time of the cement paste samples is determined. The proposed sensor is simple to realize and it does not require calibration. The repeatability of the prototype sensor unit is found to be 1.5%. With the help of the fabricated sensor, the initial and the final setting timings of the cement paste samples are measured with an accuracy of ±2%.