Improvement on engineering properties of alkali-activated paste with calcined oyster shell ash and flue gas desulfurisation gypsum

To reduce environmental pollution and conserve natural resources, calcined oyster shell ash (COSA), class F fly ash (FFA) and flue gas desulfurisation gypsum (FGDG) were used in an attempt to improve the engineering properties of alkali-activated ternary cementitious paste. The COSA – obtained by calcining crushed oyster shells at 800°C for 3 h and then grinding to pass through a no. 100 sieve – was used to substitute FFA at rates of 2.5, 5.0 and 7.5 mass%. The alkali solution used was 10M sodium hydroxide and sodium silicate with a modulus of 2.5. The hardened paste with 7.5% COSA, 1% FGDG and ratio of weight of added water to dry binder of 0.09 at 56 days had the highest compressive strength of 62 MPa. Microstructural analyses (scanning electron microscopy and energy-dispersive X-ray spectroscopy) showed that the resulting calcium aluminosilicate hydrate gel was found to have a compact microstructure, which improved the compressive strength of specimens. In summary, COSA successfully replaced natural limestone as a calcium oxide activator to manufacture an alkali-activated ternary cementitious paste containing FFA and FGDG.