Physico-chemical and mechanical properties of fly ash based-geopolymer pastes produced from pre-geopolymer powders obtained by mechanosynthesis

Abstract This work aims to study the feasibility of preparation of geopolymer pastes, from pre-geopolymer powders obtained by mechanosynthesis and water. These geopolymer pastes (MGP) were compared to the conventional geopolymer pastes (CGP) in terms of structural, microstructural and mechanical properties. CGP were manufactured by the mixture of fly ash as precursor with alkaline activator solution. Pre-geopolymer powders (PGP) were prepared by the high-energy milling of a dry mixture of fly ash and alkaline activators, with different Na2SiO3/NaOH mass ratios (2.5, 4 and 8) and milling times of 5, 10 and 15 min. CGP and MGP pastes were analyzed using X-ray diffraction (XRD), scanning electron microscopy (SEM), particle size distribution analysis, compressive strength tests and in situ attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), in order to highlight structural, microstructural and mechanical changes that could occur, and to follow geopolymerization kinetics respectively. The results pointed out that the MGP structures are mainly amorphous, with appearance of zeolitic crystals, depending of the chosen formula and the milling time. The in situ ATR-FTIR analysis indicated that the fly ash characteristic band (1027 cm−1) shifts towards lower wavenumbers, with a much larger shift for MGP than for CGP. The compressive strength results showed optimum values of 28 MPa for MGP and 15 MPa for CGP, after 7 curing days at 20 °C and 50% of relative humidity.