Utilization of volcanic ashes for the production of geopolymers cured at ambient temperature

Two types of volcanic ash were characterized (chemical and mineralogical compositions, amorphous phase composition, particle size distribution and specific surface area) and then used as raw materials for the synthesis of geopolymer cements cured at ambient temperature (24±3°C). The synthesized products were characterized by determination of setting time, 28-day compressive strength, X-ray diffraction and Fourier Transform Infrared Spectroscopy. The mineralogical composition, the amorphous phase composition, the particle size distribution, the specific surface area of the volcanic ashes as well as the molar ratios of Na2O/Al2O3 of the synthesized products and of SiO2/Na2O of the alkaline solutions were the main parameters which influenced the synthesis of geopolymers with attractive characteristics at ambient curing temperature. The volcanic ash sample whose mineralogical composition contained anhydrite, low content of free CaO, low specific surface area (2.3m2/g) and synthesized products with Na2O/Al2O3 molar ratios between 1.23 and 1.44 led to long setting time (test samples could be handled easily only after 14days at ambient atmosphere of the laboratory) and low 28-day compressive strength (9–19MPa) geopolymers. Moreover, its products swelled and presented cracks resulting from the formation of ettringite. The volcanic ash sample with large (Al2O3+SiO2)% wt of amorphous phase, high specific surface area (15.7m2/g) and synthesized products with Na2O/Al2O3 molar ratios between 1.04 and 1.31 led to more effective geopolymers: setting time was between 490 and 180min and 28-day compressive strength between 23 and 50MPa at ambient curing temperature (24±3°C).