engleski

STRENGTH AND MICROSTRUCTURE DEVELOPMENT OF ALKALI-ACTIVATED FLY ASH MORTARS

The enhancing of pozzolanic activity of the fly ash, which emerges as the coal combustion by-product, by alkali activation is the critical key to large-scale application. By an alkali activation of fly ash, alkaline aluminosilicate polymers, referred as „ geopolymers” , are formed. This new type of binder materials with empirical formula: Mn[-(Si-O2)z – Al-O2]n ∙ wH2O, where z is 1, 2 or 3 ; M is an alkali cation (Na or K) and n is the degree of polymerization, is characterized by a three-dimension amorphous network structures, and can exhibit superior chemical and mechanical properties compared to Portland cement-based materials. The present work reports preliminary results of the study on the products of reaction and strength development in an alkali-activated geopolymers material prepared using the Croatian Class F fly ash and sodium silicate solution (technical grade water glass, SiO2/Na2O = 3.1), and then thermally cured at 85 oC for different periods of time. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM) were utilized in this study. The results of the investigation show that the main reaction product formed in the alkali-activated fly ash-based geopolymers is an amorphous alkaline alumino-silicate that induce a shift in the T – O (T = Si or Al) asymmetric stretching band towards lower wave number (1029 cm -1) related to ones in the original fly ash. It was found that mechanical strength of alkali-activated fly ash mortars are affected substantially by the duration of thermal activation and by the curing time, when the mortars cured for 24 h at temperature of 85 oC had the highest compressive strength about 8 MPa at the age of 7 days, which then reached 15 MPa after 28 days of curing in the air at room temperature.

Class F fly ash ; alkali activation ; geopolymers mortars ; compressive strength ; XRD ; FTIR ; SEM.

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