Identification Of Initial And Final Setting Time Of Cement Paste By Electrical And Ultrasonic Methods

The early setting behavior of cement pastes significantly affects the laterperformance of cementitious materials. Detecting of setting time of cement paste cannot only guide concrete construction, but also give a deeper understanding of the natureof cement hydration kinetics. Currently, setting time obtained by different methodsvaries, indicating that the nature of setting process hasn’t been fully understood. In thispaper, Electrical Impedance Tomography (EIT) and ultrasonic longitudinal wave havebeen used to monitor the early setting process of cement pastes.The setting process of cement slurries was studied by EIT within the first sevenhours after mixing.Sectional images of electrical impedance of cement pastes can reflectthe differences of hydration degree in different part at the very moment. But the EITimages couldn’t tell the accurate setting time. By the differential curve of electricalimpedance-time of the slurry on the boundary, early cement hydration process could bedivided into three stages, declining stage, steady stage and rising stage. Similarphenomenon has been found in cement pastes with accelerator or retarder. Combiningexperimental analysis and previous work, the electrical impedance of cement pastes isconsidered reflecting the electrical characteristics and connectivity of the liquid phase.Initial set of cement paste occurs when the electrical impedance begins to rise.Ultrasonic longitudinal waves of0.5~5MHz were used to monitor the cementhydration process within the first48hours. The ultrasonic experiments showed goodrepeatability.It was found that, according to the longitudinal wave velocity, earlyhydration of cement could be divided into four stages, steady stage, slowly rising stage,rapidly rising stage and laterplateau. The same trend has been found in longitudinalwaves with different frequencies. The initial velocity of0.5MHz wave is about1400m/s,while the velocity of5MHz wave is about1600m/s.Combiningthe test of evaporablewater in early stage, the final set is regarded to occur at the moment that ultrasonicvelocity starts to rise. After that turning point, the porosity of the cement pastes dropsrapidly. Therefore, the ultrasonic longitudinal wave velocity of early stage cementpastes is sensitive to solid phase evolution and the porosity of pastes.