Estudo das propriedades das argamassas refratárias úmidas com o tempo de estocagem

Abstract

The evaluation of refractory mortars is commonly performed in the refractories industry under a macroscopic approach where the mortars workability is observed by means of the plasticity and its resistance after drying and further burning through strength bonding. The adherence of a mortar on the base from the substrate of a refractory brick occurs by mechanical or chemical mechanism. This adhesion is measured by the test of bonding strength and tends to show after drying, results slightly lower than after burning due to the formation of ceramic phases after burning. In order to demonstrate the changes in the rheological behavior and the mechanical properties with the storage time for the two different kinds of wet mortars based on sodium silicate binder, workability tests were conducted at predetermined intervals along twelve months, furthermore tests of bonding strength and microstructural analysis of interfaces mortar/refractory brick for each type of mortar were also performed. The selection and variation of the raw materials and constituent additives were made according to the authors own experimental and practical criteria, due to scarcity of articles or papers published about the subject. The basis of this experimental study was built up from two types of wet mortar linked to the same sodium silicate: one of them as mortar basis of fireclay chamote, clay and sodium silicate and the other aluminous mortar basis of fused alumina, clay and sodium silicate. The variations for preparing the mortars for this study were the amount of water defined by the workability desirable, the adding of additives modifiers of the interface solid / water, such as NaOH co-ion modifier superficial charges and CMC carboxymethylcellulose - a macromolecule that modifies the viscosity of system. The rheology of fireclay mortars studied was more affected than high alumina mortars due to the difference in porosity of its raw materials constituents. In general, it was observed that the loss of plasticity of the fireclay mortars was reduced when additives, modifiers of the interface solid/water, have been introduced in these experiments. In the first 15 days of storage wet mortars showed a big change in its rheology, from fluid to gelatinous state. The greatest evidence of change was observed for mortars rheological behavior for the fireclay classes without additives and the lowest one was for the high alumina classes with additives. The microstructural analyses of the interface refractory mortar/brick were conducted using scanning electron microscopy with EDS microanalysis (SEM-EDS) that led to the identification of sodium ion from the mortar penetrating into the micropores of the refractory brick. It is possible to conclude that the assumptions about the rheological changes truly are predominantly related to adsorption capillary mechanisms and when conditions are created for making this capillarity adsorption difficult, the mortars rheological characteristic is less affected during the storage. It was not confirmed that the loss of strength that occurred when modifiers, additives for the solid interface/water, were added in mortars during its mixing. In general, for both classes of mortars studied it was observed that the best strength bonding results have occurred after heat treatment at 1400°C during 5h.