Mechanical activation of volcanic ash for geopolymer synthesis: effect on reaction kinetics, gel characteristics, physical and mechanical properties

Abstract

This paper looks at the possibility of using low reactive volcanic ash for making geopolymer cement. The research is directed towards (a) alteration of the reactivity of volcanic ash by mechanical activation, and (b) use of mechanically activated volcanic ash for the synthesis of a geopolymer. The effect of mechanical activation was quite visible on particle size distribution and the degree of crystallinity. The disappearance of some anorthite peaks and appearance of quartz peaks in volcanic ashes milled for 120 min demonstrate the change in mineralogy. The appearance of an intense carbonate band with milling time could be related to sorption of atmospheric CO₂ on the grains surface during mechanical activation. The manifestation of mechanical activation of volcanic ash was prominent on (a) the reaction kinetics, (b) microstructural development, and (c) physico-mechanical properties of the geopolymer product. The rate constant and extent of geopolymerization increased with milling time but decreased with curing temperature. This decrease is in non-conformity with other alumina-silicate materials used for geopolymerization such as metakaolin and fly ash. FEG-SEM and EDAX results revealed that the geopolymer gel obtained is mixture of poly(ferro-sialate-siloxo) and poly(ferro-sialate-disiloxo) binder type with a formula close to [Ca,Na,K,Mg]–[–Fe–O–]_x–[Si–O–Al–O–]_1−x–[–Si–O–]_y. The physico-mechanical properties changed significantly. Setting time reduced by >95% in samples milled for 60 min or more. The compressive strength which was negligible for 0–30 min milled volcanic ash reached 29–54 MPa after 60–120 min of milling time. Heat curing influenced the early age (7 and 28 days) compressive strength but the 90 day compressive strength of both ambient and heat cured samples were comparable.