Influence of the chemical and mineralogical composition on the reactivity of volcanic ashes during alkali activation

The influence of chemical and mineralogical composition of four natural volcanic ashes was investigated. NaOH was used as the sole alkaline activator. The reactivity of the systems was studied with Differential Scanning Calorimetry. The X-ray spectra of reacted materials showed a mixture of amorphous and crystalline materials. The SEM micrographs showed that the largest particles only acted as reactive filler while smaller particles dissolved in the activating solution forming the glassy aluminosilicate matrix. The infrared spectra showed a broad absorbance at 820 -1250 cm-1and 480-600 cm-1 assigned to internal vibration of Si-O-Si and Si-O-Al in both raw volcanic ash and resulting inorganic polymers. The reactivity of the volcanic ashes was found to increase correlate with their amorphous fractioncontent in amorphous phases. The dry compressive strengths of synthesized products from all the ashes were in the range 14-63 MPa, suggesting their possible utilization as building materials. However, the strength was found to decrease (1-28 MPa) after specimens' immersion overnight in water, but was partly or totally recovered after overnight drying at 90°C.