Using dynamic rheology to explore the microstructure and stiffening of cementitious mixtures

The microstructure and the stiffening process of cementitious mixtures were investigated using a dynamic rheology method. The rheology of fresh pastes of cementitious mixtures depends on its microstructure. The yield stress and the storage modulus increase with volume fraction, and are well described by a power-law dependence on volume fraction. The flocculated pastes have almost the same breaking strain though they may have different volume fractions. For dispersed pastes the breaking strain follows a power-law dependence on volume fraction. The yield stress shows a logarithmic relationship to superplasticizer dosage and the storage modulus follows an exponential relationship to superplasticizer dosage.; The stiffening process of cementitious mixtures is well described by empirical equations. Usually the yield stress and the storage modulus follow a power-law dependence on hydration time. Shear history has markedly influence on the stiffening process. The exponential parameter in the power-law function that describes the relationship between storage modulus and time increases as preshear intensity increases. The modulus of paste that experienced high shear is lower initially but increases more rapidly with time. The moduli converge after a few hours.; Cement composition and cement-admixture interactions affect the microstructure and stiffening process of cementitious mixtures. Usually for flocculated or partially dispersed pastes the storage modulus follows a power-law dependence on hydration time. For some well-dispersed pastes the storage modulus follows an exponential-law dependence on hydration time. The effectiveness of a dispersing admixture varies with cement. Stiffening is delayed for a more dispersed paste. Slag and silica fume have little effect on the kinetics of stiffening, whereas Class C fly ash have major effects on the kinetics of stiffening.; The cement paste and the wollastonite pastes made from pore solutions extracted from cement pastes both vary with time according to a power-law, and the exponents are approximately the same. Stiffening of cement paste in the first hour appears to be due to increase in ionic strength by hydration.