Gel Composition and Strength Properties of Alkali‐Activated Oyster Shell‐Volcanic Ash: Effect of Synthesis Conditions

The gel composition and mechanical properties of alkali-activated oyster shell-volcanic ash were investigated at different NaOH concentrations (8, 12, and 15M) and curing temperatures (60°C and 80°C) in wet and dry conditions. XRD, FTIR, SEM-EDS, and TGA-DSC were used for microstructural characterization of the binder. The gel composition of the system was found to be influenced by NaOH concentration and was not affected when curing temperature was varied from 60°C to 80°C. The main phase was N,C–A–S–H for all alkali-activated oyster shell-volcanic ash, with C–S–H as secondary phase for some samples and contains high percentage of iron. The splitting at υ3 = 1400–1494 cm−1 on FTIR spectra corresponded to the elimination of the degeneracy due to the distortion of CO32− group. The high degree of splitting indicated that this carbonate group is linked to Ca2+. The compressive strength was influenced by curing temperature and the formation of a secondary phase. The compressive strength in dry condition increased roughly between 28 and 180 d for some samples, while in wet condition, the partial dissolution of Si–O–Si bonds of some silicate phases resulted in a reduction of strength.