Improvement on engineering properties of alkali activated ternary cementitious paste with calcined oyster shell ash and flue gas desulfurization gypsum

To reduce the environmental pollution and conserve the natural resources, this study utilized the calcined oyster shell ash (COSA), Class F fly ash (FFA), and flue gas desulphurization (FGD) gypsum to improve the engineering properties of alkali-activated ternary cementitious paste. The COSA was obtained by calcining the crushed oyster shell at 800°C for three hours and then ground to pass through sieved No. 100, which was used to substitute FFA with 2.5, 5, and 7.5 mass(%). The alkali solution was composed of 10M sodium hydroxide (NaOH) and sodium silicate (SS) with modulus 2.5. The hardened paste with 7.5% of COSA, 1.0% of FGD gypsum, and the mass of added water to dry binder (Waw/WB) of 0.09 at age of 56 days had the highest compressive strength of 62 MPa. The microstructural analyses of SEM and EDS showed that the resulting calcium alumino-silicates hydrates (C-A-S-H) gel was found to have compact microstructure and improve the compressive strength of specimens. This indicates that the COSA has successfully replaced the natural limestone as CaO activator to manufacture the alkali-activated ternary cementitious paste together with the FFA and FGD gypsum.