Hydration of cement with retarder characterized via electrical resistivity measurements and computer simulation

Development of the electrical resistivity with time was measured for cement pastes with the addition of citric acid monohydrate (0%, 0.03%, 0.05% and 0.07%) using a non-contact electrical resistivity measurement device. As the retarder content increases, the electrical resistivity becomes larger in the dissolution and precipitation period but smaller after the setting period because of the protective layer formed around the clinker grains. The setting-time test confirmed that the time of the second peak in the curve of the electrical-resistivity development rate can reflect the setting behavior of the pastes. The time of the third peak in the curve of the electrical-resistivity development rate reflects the transition from a phase-boundary mechanism to a diffusion-controlled mechanism during cement hydration, and thus it was treated as the critical time in a computer simulation of the cement hydration. The computer simulation enabled the visualization of the microstructural evolution and the development of the hydration degree and porosity with time. The simulated results demonstrate the strong retarding effect of citric acid on cement hydration in the early stage. The hydration degree determined by the simulation is in consistent with the results of a non-evaporable water test. The microstructural observations obtained via scanning electron microscopy (SEM) show that the retarder delays cement hydration during the early stage, which confirms the findings of the computer simulation.