Mapping of Calorimetric Response for the Geopolymerisation of Mechanically Activated Fly ash

Abstract

The focus of this paper is on isothermal conduction calorimetric study of the geopolymerisation of mechanically activated fly ash. Mechanical activation was carried out in an eccentric vibratory mill due to its high efficiency. The samples used for calorimetry were characterised in terms of particle size distribution (by laser diffraction), morphology and chemical heterogeneity (by SEM–EDS) and structure (XRD and FTIR). The calorimetric response, rate of heat evolved ( q˙ ) with geopolymerisation time (t), was collected for 24 h. The 7 × 7 calorimetric maps were prepared using the data at seven reaction temperatures (TGP = 27, 32, 37, 42, 47, 53, 60 °C) for seven samples obtained after different duration of milling (tMA = 0, 5, 15, 30, 60, 90, 120 min). Comprehensive profiling of fly ash reactivity was done in terms of the maps for rate of heat evolved ( q˙ vs. time), total heat evolved (Q vs. time), fraction reacted (α vs. time) and iso-conversion time (tα). Each of the mechanically activated samples behaved uniquely. A model-free approach based on ‘iso-conversional methods’ was deployed to analyse the kinetics of geopolymerisation. The analysis revealed that activation energy changes with fraction reacted and displays three regimes of dependence. The merit of the model-free analysis over traditionally used ‘model-based analysis’ is emphasised. Further, in the context of geopolymerisation, empirical parameters based on fraction reacted are used to delineate efficacy of mechanical activation vis-à-vis reaction temperature.