Rheological studies and percolation modeling of microstructure development of fresh cement paste

This study combines rheology, scanning electron microscopy (SEM) and cement hydration studies as well as theoretical modeling to gain fundamental understanding about the microstructure, its control of flow behavior, its development due to hydration, and setting.By carefully examining the creep and recovery behavior of cement paste, two physical parameters are proposed to characterize quantitatively the microstructure of cement paste and effect of hydration, the yield stress ($\tau\sb{\rm y})$ and failure strain ($\gamma\sb{\rm y}$). It was found that $\tau\sb{\rm y}$ increased slowly in the first 1-2 hours hydration (Zone I), after which it increased much more rapidly (Zone II). The transition time between Zone I and Zone II corresponds to the initial setting time (t$\sb{\rm set})$ of cement. The failure strain, on the other hand, increases drastically by more than one order of magnitude at initial set. This transition time corresponds approximately to the end of the induction period as determined by calorimetry. Kinetic analysis based on the $\tau\sb{\rm y}$ measurements suggested that the rate limiting process in early cement hydration is diffusion.The oscillatory shear measurements, on the other hand, showed that the elastic modulus (G$\sp\prime$) increased continuously with hydration, forming an S-shaped curve, increasing slowly (Zone I), then more rapidly (Zone II) and finally slowly again (Zone III). The transition time between Zone II and III corresponds to the initial setting time as defined by the $\tau\sb{\rm y}$ measurements.The increase of both $\tau\sb{\rm y}$ and G$\sp\prime$ was attributed to the formation of hydration product (CSH) in the neck area between cement particles. A bond percolation model based on experimental results is proposed to describe this microstructure development. Cement particles are initially flocculated by Van der Waals bonds to form a three dimensional network structure. With hydration the weaker Van der Waals bonds are randomly replaced by the stronger chemical bonds due to cement hydration and formation of CSH. Cement paste sets when the chemical bonds percolate within the microstructure. Both a two dimensional triangular beam network and a two dimensional spring lattice were used to simulate microstructure development in an attempt to understand the different behaviors of G$\sp\prime$ and $\tau\sb{\rm y}$. Modulus and yield stress of the network were calculated as weak bonds are replaced by strong bonds. In combination with the cement hydration simulation model, the predicted evolution in modulus and yield stress agrees well with experimental results.The current study concludes that the rheological behavior of cement paste is similar to that of colloidal gels. However with the hydration, the nature of the bonding between particles changes, bringing about complex rheological changes.U of I OnlyETDs are only available to UIUC Users without author permissio