Partial replacement of volcanic ash by bauxite and calcined oyster shell in the synthesis of volcanic ash-based geopolymers

• Bauxite and calcined oyster shell were used as partial replacements of volcanic ash.
• Replacements were weakly dissolved in alkaline medium.
• Low percentage of bauxite reduced efflorescence.
• Low percentage of calcined oyster shell lessened setting time.

Volcanic ash is an aluminosilicate material with generally variable chemical and mineralogical compositions from one deposit to another. When combined with suitable alkaline solution, its powder can be converted into geopolymer whose characteristics such as setting time and compressive strength are moderate. The present study investigates the effects of bauxite and calcined oyster shell powders as partial replacement for volcanic ash in order to enhance certain characteristics of resulted geopolymers. Depending on the nature of the concerned materials (volcanic ash, replacements and geopolymers), chemical and mineralogical analyses, Fourier transform infrared spectroscopy, X-ray diffractometry, visual aspect, durability, apparent porosity, initial setting time, linear shrinkage and 28 days compressive strength were performed. The replacements were weakly dissolved in alkaline medium due to their high crystalline phase contents. However, replacement of calcined oyster shell or up to 20% mass of bauxite lessened the setting time. For a given age of geopolymers, any quantity of replacement lowered linear shrinkage and about 10% mass of bauxite was enough to reduce efflorescence. Immersion of specimens in 3% solution of sulfuric acid for 28 days led to mass loss of less than 2.5% with surfaces that seemed to remain structurally intact. There was weak increase of 28 days compressive strength for geopolymers with small quantity of replacement. Conversely, great quantity of replacement produced the decrease of 28 days compressive strength. Anyhow, in geopolymer synthesis of volcanic ash, partial replacement by bauxite is a promising way of reducing efflorescence while calcined oyster shell is recommended for reducing of setting time.