Abstract: The calorimetric method was used as a main tool in the studies of the effect of superplasticizing and set retarding admixtures on cement hydration. The experiments were carried out using two types of cements (neat Portland cement CEM I 42.5R and blended Portland cement type CEM II/B-M (S-V) 42.5N), at varying w/c ratio 0.5 and 0.3 respectively. The hampering of cement hydration process at early age in the presence of these admixtures was generally observed. This is in practice equivalent to the set retardation. The dissolution, as well as the precipitation of hydration products were retarded too, as it has been proved by the DTA/TG, conductometric measurements and the evaluation of so-called chemical shrinkage. However in case of superplasticizers, after a prolonged “dormant” period the process is continued; the reactions occur at fairly good rate giving substantial heat effects – it can be concluded from the total heat evolved values after standard 41 h. At longer time a significant amounts of products are formed as well, but it seems that the crystallization of calcium oxide is disturbed; simultaneously the amount and composition of calcium silicate hydrates alters towards the “more amorphous” product. This could be observed under SEM. The set retarders give strong delaying effect, particularly at low amount of water and in the pastes produced from Portland cement. This can be due to the formation of insoluble layer of calcium and sugar or calcium and phosphorus containing compounds surrounding the hydrating grains; the formation of this “barrier” can be distinguished as a small peak on the calorimetric curve and it is poorly removable on further hydration. The shrinkage and conductivity measurements seem to prove not only the change of kinetics of heat evolution/hydration but also the modification of the process and resulting products.
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