Investigation On The Early Hydration And Microstructure Of Cement-based Materials

With the development of cement and concrete industry, the researchers not only request the high strength, but also focus on the well durability of the cement and concrete. Therefore, the concrete with high strength and well durability could be found out. The hydration process (Especially early age hydration) affect the the development of strength and the durability of cement-based materials directly. So the cmenet chemistry research is foused on the reaction mechanism and the hydration products in the early age hydration.The objective of this paper is to study the effect of the hydration process of Portland cement and geopolymer cement pastes at early ages based on the ultrasound velocity and the electric resistivity in cement pastes and the dielectric constant of mortars. The results are compared to the outputs of simulation with the3-D computer model CEMHYD3D. The fraction of the percolated solid and pore phases are correlated to the UPV, resistivity and the dielectric constant. Besides, this study also established the heat of geopolymer cement hydration and the non-evaporable water content at early age. The main conclusions are as following:(1) The needle penetration depth is obviously increasing with curing age and decreasing with increasing w/c ratio. Ultrasound velocity method, the electric resistivity and dielectric constant method are the effective and reliable methods for monitoring hydration process of cement at early ages.(2) Based on the experimental and numerically simulated results, it can be concluded that during the early ages, the development UPV cannot directly be used for evaluating the percolation of solids, while it has clear correlation with the conventional Vicat methods. The development of UPV is not only affected by the percolation degree of solids, but the physical properties of the solids and the distribution of air phase play important roles as well.(3) Based on the experimental and numerically simulated results, it can be concluded that during the early ages, the development of the electric resistivity is closely affected by the volumetric fraction of pore spaces in the paste. During the early ages, the electric resistivity is mainly controlled by the ion concentrations in the pore solution, while later the pore fraction is the determinative parameter on the electric resistivity of the paste.(4)The additional admixture MK could delay the setting times of the water glass-slag geopolymer on the certain extent. Meanwhile, the1.5%H3PO4could delay them obviously.(5) Comparing the increment rate of ultrasound velocity to the heat evolution and non-evaporable water content in early hydrating geopolymer cement paste, the factors of hydration setting and hard process of geopolymeric cement and concrete are explained. The early hydration process of geopolymeric cement could obviously be divided to four stages, corresponding to the initial hydrolysis, dormant, acceleration, stabilization periods, respectively.