Efeito da adição de Poliuretana (PU) nas propriedades termomecânicas de pastas de Cimento Portland sujeitas a altas temperaturas

Abstract

In order to adapt Portland cement to well at high temperature conditions and thermal gradients, the application of additives of polymeric nature can be a way to improve their thermomechanical behavior. Steam that is normally injected into the thermal cycling process is around 300 °C, which can damage the naturally fragile cement sheath, either during injection or cooling, as a result of incompatibility of thermal expansion between the cement sheath and Steel coating. The objective of this work is to evaluate the effect of the addition of different polyurethane (PU) concentrations on the thermo-mechanical properties of cement sluuries submitted to temperatures of up to 300 ° C and 3 kpsi (20.7 MPa) pressure. Additive formulations were prepared 1 gpc (0.052 kg), 2 gpc (0.102 kg) and 3 gpc (0.15 kg) of polyurethane, respectively named PU1, PU2 and PU3. A standard paste (without PU) was also prepared. The slurries were cured for 14 days, 28 days and 90 days, and the last three days of each cure were subjected to thermal cycling conditions at a temperature of up to 300 ° C and a pressure of 3 kpsi. The mechanical properties of the cured slurries were analyzed by compressive strength, tensile strength by diametrical compression, resonance frequency for the calculation of the Poisson's coefficient and ultrasonic test for the calculation of the modulus of elasticity. The thermal properties were investigated by the study of the behavior of the coefficient of linear thermal expansion (TSC) in argon atmosphere, with the thermal properties analyzer the study of thermal capacity, thermal conductivity and specific heat in the pastes at all curing times and also by the thermogravimetric (TG) analysis to monitor the mass variation under the same conditions temperature and heating rate of the dilatometry. The samples were also characterized by X - ray diffraction (XRD) and field emission scanning electron microscopy (SEM - FEG). The results of compressive strength and tensile strength showed a slight reduction of values with addition of polyurethane and with 90 days of cure the behavior pattern of the compositions was similar to those at 14 and 28 days. After 28 days of curing, the samples presented a good elastic behavior, evidenced by the modulus of elasticity and the Poisson coefficient, 16.5 GPa and 0.22 (reference sllury obtained 17.6 GPa of elastic modulus and Poisson of 0.20). After 28 and 90 days, the samples showed only high xonotlite peaks, which gives higher cement stability according to literature data. The presence of polyurethane also affected the coefficient of thermal expansion of the materials in the dilatometry test, since its presence decreases the hydration kinetics of the samples. Thus, the results showed that the thermomechanical properties of PU formulations were satisfactory compared to the standard paste, mainly with 28 and 90 days of curing, presenting better stability in the crystallographic and morphological characterization, in the study of the compressive and tensile strength, in the plastic (elastic) and dilatometric behavior, as they decreased the thermal incompatibility between the cement sheath and the steel coating. In addition, it reduces the brittle behavior of the cement, contributing in general to the thermomechanical behavior of the sheath material subjected to high temperatures and gradients.