Avaliação da reatividade de adições minerais em pasta de cimento Portlan utilizando calorimetria de fluxo de calor (Heat Flow Calorimetry).

Abstract

The use of mineral additions in the production of cement compounds or in concrete elaboration generates some benefits, either from an economic point of view or when related to the environment preservation. Although limestone deposits are abundant all over the country, the introduction of residues decrease the production costs, besides the fact that the cement industry constitutes an alternative way for reciclation of some other industrial residues. The pozzolan and blast furnace slag are the main raw materials utilized as addition in the ordinary portland cement. The main advantage of their use is the low cost, the durability in aggressive environments and their low hydration heat. As for the hydration reaction, the ordinary portland cement produces hydrated compounds that directly influence its mechanical resistance. The main products are hydrated silicates, portlandite Ca(OH)2 and sulphoraluminates. On the other hand, the presence of addition minerals leads to the combination and reduction of the paste of the portlandite, with the formation of more stable and more resistant hydrated calcium silicates. The use of alternative agglomerate in ordinary portland cement has been the target of investigations in Brazil and abroad. Despite of great number of works on the subject published so far, such as Flores (1989), Marteli (1997), Hildebrando (1998), Barata & Dal Molin (2002) e Carneiro (2003), there are few data about the behavior of these residues when they are mixed to the cement, principally with respect to the study of the heat flux calorimetry of the hydration products of the cement. The isothermal or conduction calorimeter is extensively used in the research of reaction kinematics. Thermal ocouples are used to convert the thermal flow into voltage, which in turn can be monitored. The heat flow calorimeter can be used in the study of limestone carbonatation reactions, hydration reactions and liquid influence, as well as polymerization reactions and glasses, etc. The main objective of this work is the use of the heat flow calorimeter of the Applied Geochemistry and Mineralogy Group (AGMG) laboratory in the study of cement paste behavior with mineral additions, with the aim of comparing and verifying the influence of these materials in the reaction of hydration of the ordinary cement. For the calorimeter analysis, we have used an ordinary portland cement CP I, in which was added rice husk ash, fly ash, blast furnace slag (BFS), red mud, metakaolin, and silica fume in the preparation of the paste, with an water/cement ratio of 0.5, using concentrations of 10, 20, 30%, respectively, with a total duration of 50 hours. Only in the red mud we have added kaolin a calcinated at 900 ˚C, in order to neutralize the sodium hydroxide present in the sample, due to the Bayer process. X-ray diffraction analysis and thermal analysis (ATD/ATG) permitted the identification of the presence of the main products of the hydration reaction and of the portlandite. The results presented indicate that the substitution of part of the cement with mineral additions reduces the hydration heat formed during the reaction of the cement with water. However, the red mud and the metakaolin presented an antecipation of the hydration heat. The metakaolin is highly reactive, since it is composed of fine grains and a large amount of amorphous materials.