Modelling the hydrating behaviour of fly-ash in blended cements using thermodynamics

This paper presents a new method to thermodynamically model the hydration behaviour of fly-ash (FA) blended cements by deriving individual phase descriptions depending on the proportion of FA in the blended cement. The predicted hydrated phase assemblage, pore solution chemistries and pH over 1,000 days of hydration and with increasing FA proportions are presented. The thermodynamic data for the FA phases are derived using oxide proportions and mineral compositions are copied directly into the PHREEQC input file. The FA phases take account of all minerals to give a more accurate description of its behaviour during hydration. The calcium aluminosilicate hydrate (C-A-S-H) gel model consists of several Discrete Solid Phases (DSPs) derived from the quinary solid solution end-members in the cemdata18 database [1]. This method has been used previously by the authors to give reliable and computationally efficient results when modelling OPC hydration and extended here for C-A-S-H, accounting for its strongly incongruent dissolution. A number of blended cements with FA contents ranging from 0-35% (in 5% steps) were simulated. As the amount of FA in the blended cement increases, the results show a destabilization of calcium hydroxide at higher replacement levels, more hydrotalcite than OPC, the formation of strätlingite and AFm & AFt phases like monosulfate/monocarbonate and ettringite respectively. The dissolution of Portland cement is modelled using a well-known empirical approach. FA dissolution is modelled using an approach taken from the literature that gave good correlations with experimental data.