LAUSR

Abstract During placement cement-based materials are subject to different types of loadings, including high-rate shearing during pumping, hydrostatic pressure in formworks, etc. At the same time, the material undergoes microstructural evolution due to hydration that takes place at the cement-paste scale level. Superposition rheology, in which an oscillatory shear is superimposed to a steady shear (rate or stress), seems therefore to be the most representative of placement processes. In the present investigation the superposition rheology protocol is exploited for the first time in the case of cement pastes. Three different cement paste mixtures are considered: (i) a plain cement paste, (ii) a cement paste blended with a thixotropic agent (rod-like clay), and (iii) a cement paste blended with an organic thickening admixture (cellulose ether). First, the evolution of the linear viscoelastic properties at different superimposed stresses or shear rates are examined. Then, to probe mechanically the evolution of the microstructure under stress following a pre-shear, an oscillatory shear is superimposed to a constant stress. It is shown that low constant stresses lead to both reinforcement of the microstructure and the increase of the rebuilding kinetics. At higher superimposed steady stresses the microstructure breakage is enhanced, and the rebuilding is hindered. This is in contrast with shear-rate superposition in which the microstructure is always weakened.