Mechanical Performance, Phase Evolution and Microstructure of Natural Feldspathic Solid Solutions Consolidated Via Alkali Activation: Effect of NaOH Concentration

Five different alkali activators were prepared by mixing sodium hydroxide with molar concentrations of 8, 10, 12, 14 and 16 M and sodium silicate, Na2SiO3 in volume ratio 1:1. The obtained alkaline activators were used for producing metakaolin/feldspathic rock-based geopolymer composites. These geopolymer composites were obtained by mixing different solid precursors of natural solid solutions (trachyte, pegmatite and granite) in the range of 70 to 85 wt% with metakaolin and adding each alkaline solution at a liquid/solid mass ratio of 0.37. The pastes were cured at room temperature for 28 days before testing. The influence of NaOH concentration on structural modifications was investigated using Fourier Transform Infrared Spectroscopy (FTIR) and X-Ray Diffraction (XRD). Water absorption for open porosity, Environment Scanning Electron Microscope (ESEM) and compression tests were adopted to observe the morphology and mechanical properties. The results indicate that the NaOH concentration of 12 M was optimum for solid solution of trachyte with 98–106 MPa due to content of higher amorphous phases compared to solid solutions of pegmatite (94–105 MPa) or granite (101–108 MPa) where the better results were achieved at 10 M.