A non-contacting electrical resistivity measurement device has been adopted to measure the electrical resistivities, ρ(t), of Portland cement pastes with water cement (w/c) ratios of 0.35, 0.40, and 0.45. The electrical resistivity curves all exhibit the same type of s-shaped growth; however, the electrical resistivity curve of the cement paste with the lower w/c ratio is always above those of higher w/c ratios. The hydration process can be divided into the dissolution and precipitation period, the setting period, the acceleration period, and the deceleration period, according to the rate of electrical resistivity development. In order to gain insights on the evolution of the microstructure during hydration, this study includes a computer simulation of the hydration process, considering the influence of particle size, w/c ratio, temperature, and orientation on representative length scales in the microstructures. The visualization of the microstructure evolution in the paste can well interpret the hydration process, which is consistent with the theoretical analysis. The microstructure evolutions show that the lower the w/c ratio is, the more quickly the hydration products cluster; consequently, it is easier and takes less time to form clusters, making the porosity decrease more rapidly, offering reasonable explanation of the characteristics of the 24 h compressive strengths of the pastes with different w/c ratios. The porosity obtained can well explain the development of the electrical resistivities of cement pastes with different w/c ratios after the dissolution period.
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