Influence of superplasticizer on activation energy and autogenous shrinkage of a cement paste

Special Publication of the American Concrete Institute, SP-217, Eds ACIThree superplasticizers (SP) have been studied in this research: the first is based on modified polycarboxylic ether and is used to improve the workability of concrete and to obtain high mechanical characteristics at early age; the second, which contains naphthalene sulphonate, is used to reduce drastically mixing water in concrete and improve mechanical strength at early age; the third SP investigated is melamine-based and is used to improve the workability of concrete creating electrostatic repulsion between cement grains. The intention of the present investigation was to provide more information about the role of these SP in concrete at early age. The apparent activation, initial and final set times by Vicat needle, chemical and autogenous shrinkage were measured for cement pastes having a water/cement ratio of 0.25. The apparent activation energy has been determined by the "setting times method" at different temperatures: 10, 20, 30 and 40 °C. The volumetric autogenous shrinkage was measured at the same temperatures immediately after setting. The experimental results show that the apparent energy activation is slightly modified by the presence of SP. Also, the evolution of chemical shrinkage shows clearly that the SP acts on the hydration kinetic of cement. The effect of a particular SP on autogenous shrinkage at different temperatures can be correctly predicted by means of the maturity concept