Study On Early Age Cement Hydration Process And Microstructure Using Resistivity Measurement

As we all know, the hydration of Portland cement is an exceedingly complexprocess involving numerous reactions that cause a variety of physical and chemicalchanges. The formation and development of the microstructure and properties aremainly controlled by hydration reaction. In particular, the early behaviors of hydrationplay an important role in determining the engineering properties such as mechanicalstrength, permeability and durability. Just as a weak newborn baby needs more care inhis early life, it is of special significance to clarify the characteristics of cement pasteat early-age.In this paper, a non-contact electrical resistivity measurement method was used todetect the hydration process and the structure evolution of cement at early age. Withthe results of the resistivity research, the meaning and stage division in earlyhydration of cement were conducted. And the effects of w/c ratio，the dosage ofmineral admixture on hydration process were also researched.Results show that the electrical properties of cement paste are very sensitive to thechanges of hydration process, including the physical (porosity, pore distribution, etc.)and chemical (ion concentration) changes. Thus, the non-contacting electricalresistivity device provides an effective way to continuously and accurately monitorthe early-age hydration process of cement paste with bright application prospects.Through measuring the electrical resistivity of cement pastes during hydration, it hasbeen found that the resistivity curve was well correlated with the hydration process.According to the critical points of curves, the hydration course of cement paste can bedivided into four stages: the dissolution period, the induction period, the accelerationperiod and the deceleration period. In the first two stages，the resistivity of pasties isdetermined by the conductivity of liquid phase, but in the later two stages, theresistivity is a sign of formation and development of microstructure in the paste.The researches on the different water-cement ratio indicate that the higher w/cratio leads to lower resistivity of paste and the delaying of hardening process. That isbecause the water-cement ratio increases the liquid phase, making the hydration of cement particle and ion migration more easily, and getting lower resistance finally.Fly ash has the dispersing function, packing effect and microfiller effect in earlyhydration process, which can prolong the hydration stages.New definition on setting times are given based on the resistivity-time curve, andit is also proved through experiments that it is possible to predict the28-day strengthusing1-day resistivity.