THE INFLUENCE OF CaCO_3 ADDITIONS UPON HYDRATION AND HARDENING PROCESSES IN C_3A-(C S H_2)-H BINDING SYSTEM

摘要

The blended Portland cements containing limestone are interesting from the technical and economical point of view. For a long time, the ground limestone was considered as an inert material, like ground sand, having a filler effect in the binding systems. By several researches, a more complex effect of the ground limestone has been evidenced: by a filler effect, the fine ground limestone contributes to a densified hardening structure and by chemical reactions with cement compounds (tricalcium aluminate, tricalcium silicate) and water, it determines the formation of some calcium carbonate hydrates. The present paper brings information concerning the influence exerted by the presence of CaCO_3 additions (10-30%) upon the C_3A hydration and hardening, in the absence and in the presence of gypsum (C_3A/C S H_2~* =4/3). The formation of calcium carboaluminate hydrate was evidenced for the two types of binding systems, but the simultaneous presence of calcium sulphate had an unfavourable influence on this process. For an addition of 20% CaCO_3, their consumption degree after 28 days was of approximately 74-94%. By the hydration of the calcium aluminate system containing CaCO_3 without calcium sulphate, only the calcium carboaluminate hydrate was formed. In the simultaneous presence of calcium sulphate and calcium carbonate, both calcium carboaluminate hydrate and ettringite were formed. The mechanical strengths of the all hardened compositions were generally small, in correlation with the rapid hydration processes specific to such systems. However, the aluminate system containing both calcium sulphate and calcium carbonate (20%) showed a better binding behaviour. For this system, the hydration processes proceeded slower.