Effect of a PCE-based superplasticizer on the viscoelastic stiffness of a cement paste: contrasting fresh-state and early-age behavior

The impact of a polycarboxylate ether-based superplasticizer (SP) on the viscoelastic stiffness of a cement paste at an early age is examined. Ordinary Portland cement, free of tricalcium aluminate is used, and pastes prepared with and without SP are characterized with isothermal calorimetry, small-amplitude oscillatory shear rheology, and hourly three-minute creep testing. Isothermal calorimetry enables an indirect estimation of the paste hydration degree as a function of its age. The stiffness moduli, quantified from rheology and creep tests on the pastes with and without SP, are compared as a function of the hydration degree. In the fresh state, at hydration degrees below 1 %, rheology shows that the paste containing SP is less stiff than the paste without. In the hardened state, the trend is reverse and the paste with SP exhibits higher elastic and creep moduli. A discussion follows, that proposes an interpretation based on a schematic representation of the microstructure derived from the Powers-Acker hydration model, and literature. We argue that the dispersing ability of the SP, well known to induce a more evenly distributed network of particles in the fresh cement state, persists in the hardened paste. The hypothetical finer microstructure, with a finer pore distribution, in the hardened paste containing SP, tentatively explains the higher moduli measured, and clarifies the apparently inconsistent mechanical behavior of the cement pastes prepared with and without a superplasticizer.