Influência das características físicas do MgO e do ADP na reação de formação e nas propriedades físico-mecânicas do cimento de fosfato de magnésio

Abstract

With the high consumption of non-renewable natural resources and the pollution associated with the manufacture of Portland cement, there is a growing interest in studies on alternative cement formulations, such as magnesium phosphate cement (MPC). Among the characteristics of this material are the high resistance to initial compression, high adhesion on different surfaces, resistance to aggressive environments, and low density. Its formation reactions take place at room temperature, without the need for treatments with high energy demand, such as conventional ceramics, or even a clinkerization step, such as Portland cement. However, the factors that influence the formation of MPC matrices are not completely understood, especially about the physical characteristics (surface area and granulometry) of the two main reagents: magnesium oxide (MgO) and ammonium dihydrogen phosphate (ADP). Thus, this work evaluated the influence of the surface area and granulometry of MgO and ADP on the formation reactions and mechanical properties of MPC. For this, MPC pastes were produced from the combination of MgO and ADP samples with different granulometry and surface areas (ASBET), two water contents, and three Mg/P ratios. Such parameters were shown to be relevant in the formation reactions and the hardening process of MPC matrices. The pastes were evaluated using XRD, isothermal calorimetry, compressive strength, porosity, and apparent density. MgO particles with higher ASBET led to an increase in the amount of accumulated heat. The water content proved to be important, as it affects the dissolution of the reagents, the supersaturation conditions, and the pH of the mixture, in the formation of struvite. The porosity of the matrix varies mainly as a function of Mg/P but is also affected by the ASBET of MgO. The pastes prepared with Mg/P higher than the stoichiometric showed higher mechanical strength, however, the presence of the more reactive MgO made it possible for the pastes prepared with the stoichiometric trait to increase the mechanical strength.