Effect of mix parameters on chloride content, sulfate ion concentration, and microstructure of geopolymer concrete

The present research work investigates the effect of mix parameters such as fly ash-ground granulated blast furnace slag (GGBS) blends (85 %/15 %, and 70 %/30 %), NaOH solution molarity (10 M and 14 M), different salts (i.e., 3.5 % NaCl and 3.5 % NaCl plus 3.5% MgSO4) on workability, compressive strength, chloride content, sulfate ion concentration, and microstructure of geopolymer concrete (Gc). Cube specimens were made for compressive strength test at the age of 7, and 28 days. Further, free chloride and total chloride contents of geopolymer concrete mixes were determined. The sulfate ion concentration and pH values of pore solutions made from different geopolymer concrete mixes were also determined. The effect of mix parameters on phase composition of geopolymer concrete mixes was evaluated by X-ray diffraction (XRD) analysis and the morphology of geopolymer concrete was studied by Field emission scanning electron microscope (FESEM) analysis. The results showed that the workability of geopolymer concrete decreased with increase in GGBS content and concentration of NaOH solution. With addition of salt, the slump value of geopolymer concrete increased. The compressive strength was increased with GGBS content and molarity of NaOH solution. The control geopolymer concrete mixes exhibited higher compressive strength than that added with salt. Free chloride and total chloride contents mostly reduced with increase in GGBS content in geopolymer concrete mixes. The increase in GGBS proportion and NaOH solution concentration reduced the concentration of sulfate ions in geopolymer concrete pore solutions contaminated with chloride-sulfate salt. From the XRD analysis, albite, nepheline, anorthoclase, and C-S-H gel peak intensities were mostly increased with age and GGBS replacement level. FESEM analysis showed formation of denser microstructure in geopolymer concrete made with 14 M NaOH solution than that made with 10 M NaOH solution. Overall, the geopolymer concrete made with higher GGBS content and higher concentration of NaOH solution exhibited improved performance through formation of denser microstructure leading to enhanced mechanical property and also reduced the concentration of aggressive ions i.e., chloride and sulfate ions in the pore solution of geopolymer concrete.