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The aim of this work is to valorize volcanic scoria by using them as starting material for geopolymers production. Nevertheless, volcanic scoria possesses low reactivity. Various amounts of metakaolin (5%, 10%, 15%, 20% and 25%) were added into two volcanic scoria (Z D and Z G) in order to improve their reactivity. Two alkaline solutions were used to activate the aluminosilicate materials. The starting materials were characterized by particle size distribution, specific surface area, chemical and mineralogical composition. The geopolymers were characterized by the setting time, XRD, FTIR, SEM and compressive strength. The results indicated that volcanic scoria have low specific surface area (2.3 m 2 /g for Z D , 15.7 m 2 /g for Z G), high average particle size (d 50 = 13.08 m and 10.68 m for Z D and for Z G respectively) and low glass phase contents. Metakaolin have a smaller average particle size (d 50 = 9.95 m) and high specific surface (20.5 m 2 /g). The compressive strength of geopolymers increased in the ranges of 23-68 MPa and 39-64 MPa for geopolymers from Z D-MK and Z G-MK respectively. This study shows that despite the low reactivity of volcanic scoria it can still be used to synthesize geopolymers with good physical and mechanical properties.

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.

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