SYNTHESIS OF METAKAOLIN-BASED GEOPOLYMERS AND THE EFFECT OF THE INCORPORATION OF GEOTHERMAL SILICA WASTE

摘要

Geopolymers based on metakaolin (MK) were synthesized using different replacement levels of geothermal silica waste (GSW) in dispersion. This material is a byproduct of the electric energy generation by means of geothermal brine and steam extracted from the underground. Following from that, the aim of this study was to analyze the effect of the GSW on the mechanical properties and the development of the microstructure of the geopolymers exposed at different conditions as sulfate attack and elevated temperatures up to 800°C. The mixtures were activated with a solution of sodium silicate and sodium hydroxide with molar ratios SiO2/Al2O3 = 2.8, 3.0 and 3.2, Na2O/SiO2 = 0.32 and H2O/Na2O = 10, utilizing 0, 5, 10, 15 and 20% of geothermal silica waste. The pre-curing treatment was carried out at 60°C during 2 hours. After the curing periods of 3, 7, 14 and 60 days the compressive strength was evaluated. The materials obtained were characterized by means of X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and scanning electronic microscopy (SEM). The tests to evaluate the sulfate attack and high temperatures involved an immersion for a period of 14 and 28 days into a solution of 5% sulfate magnesium and an exposure to 200, 400, 600 and 800°C during 2 hours, respectively. All the XRD patterns exhibited an amorphous peak displacement associated with geopolymeric gel formation and also a similar displacement in the characteristic band of aluminosilicates in FTIR spectrums was obtained indicating the geopolymerization. The results showed that geothermal silica waste affects negatively the mechanical performance mainly at early curing periods. The best mechanical performance was obtained with a ratio of SiO2/Al2O3 3.2 and 0% of geothermal silica waste. According to SEM, an increase in porosity was observed with a higher GSW load in the system. Also the GSW load affected negatively the chemical and thermal resistance of geopolymers. The sulfate attack promoted the re-formation of caolinite and the exposure to 800°C of 20% GSW systems promoted the highest loss of compressive strength.