Effect of chloride and sulphate on ionic concentration of aqueous pore solution and microstructural properties of fly ash-GBFS geopolymer concrete

In present work, effect of admixed NaCl (NC), NaCl + Na2SO4 (NS), and NaCl + MgSO4 (MS) on ionic concentration (Na+, Ca2+, K+, sulphate, and chloride) of aqueous pore solution of geopolymer concrete (GepC) made with fly ash (FA), and FA plus ground granulated blast furnace slag (GBFS) was evaluated and correlated with microstructure. Results showed admixed salts led to reduction in compressive strength than control mix. These findings are in line with variations in concentrations of Na+ and Ca2+ ions of pore solution. Fly ash based GepC with 14 M NaOH solution and admixed with NaCl + MgSO4 salt showed higher strength loss of 61.58 %. Addition of salt led to increase in ionic concentration than control mix, where the concentration range of Na+ ion increased from 127.12-244.11 mmol/l to 133.73-541.3 mmol/l, and that of Ca2+ ion increased from 44.61-65.87 mmol/l to 46.71-143.72 mmol/l. GepC admixed with 3.5 % NC + 5 % MS showed lower sulphate concentration than 3.5 % NC + 5 % NS. The addition of GBFS resulted in decrease in free chloride content i.e., the range of free chloride content in fly ash based GepC was 0.71-0.89 % whereas that in case of fly ash-GBFS based GepC mix was 0.38-0.76 %. Further, the bound chloride content was higher in fly ash-GBFS based GepC (0.11-0.5 %) than fly ash based GepC (0.02-0.2 %). More formation of albite and CSH with GBFS content, and that of albite and nepheline with NaOH solution molarity as indicated by peak intensity from XRD analysis are in line with variations in compressive strength, and Na+ and Ca2+ concentrations of aqueous pore solution. Decalcification of CSH in magnesium sulphate environment as indicated by shifting of peak of Si-O-T bond to higher wavenumber is supported by variations of Ca2+ ion concentration, CSH peak intensity from XRD spectra, and compressive strength.