Ceramics-Silikáty 57, (3) 232 - 242 (2013)

Most concrete produced today includes either chemical additions or chemical admixtures added to concrete, or both. These additives alter the properties of cementitious systems, including fluidity, hydration kinetics, microstructure and strength. Therefore, the development and use of the most suitable type, optimum dosage and delayed addition time (DAT) of superplasticiser in concrete technology have been of increasing importance. The present investigation aimed to evaluate the influence of DAT of sulfonated naphthalene formaldehyde (SNF) condensate on the hydration kinetics of sulfate resisting cement (SRC) pastes incorporating 10 mass % silica fume (SF). Superplasticized cement pastes were prepared using the required water of standard consistency with different dosages and DAT of SNF. The pastes were hydrated for 90 days under normal curing conditions. The results revealed that, the fluidity of cement pastes increases with SNF content and DAT up to 7.5 minutes, due to the electrostatic repulsion between cement particles and the decrease of anhydrous C₃A content that is minimized with hydration time. Also, the chemical and physico-mechanical characteristics improve with superplasticizer dosage up to 1.0 mass % and DAT up to 7.5 minutes, due to the improvement of superplasticizing effect, leading to increase the hydration of silicate phases (C₃S and β-C₂S) and formation of more CSH, CAH and CASH. These hydrates increase the gel/space ratio, modify the microstructure and consequently enhance the strength of cement paste. FTIR spectra are in a good agreement with those of chemical and physico–mechanical characteristics.