Enhanced Pozzolanic Reactions

Version 2.0 of CEMHYD3D also contains more accurate modelling of the reactions between silica (from silica fume or fly ash) and the cement hydration products. The silica typically reacts with the CH phase to form a pozzolanic C-S-H gel of a different specific gravity and molar stoichiometry than the C-S-H gel formed from conventional cement hydration. In fact, in the presence of excess silica, the conventional C-S-H may convert to the pozzolanic C-S-H form. Currently, the pozzolanic reactions included in the hydration model are:

1.1 CH + S +2.8 H -> C_1.1SH_3.9
C_1.7SH_4.0 + 0.5 H -> C_1.1SH_3.9 + 0.6 CH

with an assumed molar volume of 101.8 cm³/mol for the pozzolanic C_1.1SH_3.9 gel [24]. At this time, unlike the conventional C-S-H, the molar volume and stoichiometry of pozzolanic C-S-H are not temperature dependent. The current version of the model has been shown to provide good agreement with experimental data on the influence of silica fume additions on both the adiabatic temperature rise of concretes [24] and the chloride ion diffusivity of low w/c ratio cement-silica fume pastes [25].

The pozzolanic reactions are regulated by two parameters and one user input within the disrealnew.c program. The first parameter, PPOZZ (default value of 0.05), specifies the base probability for a reaction to occur when a diffusing CH species encounters a silica surface. The second parameter, PCSH2CSH (default value of 0.002), specifies the probability for primary or conventional C-S-H dissolution for conversion to the pozzolanic form of the C-S-H. If conversion to pozzolanic C-S-H is significantly increased due to a high temperature heat treatment, for example, as may be the case for reactive powder concrete [7], the parameter PCSH2CSH should be correspondingly increased to model this effect appropriately. The last user input value when initializing a run of CEMHYD3D, called csh2flag, indicates whether the conversion of conventional C-S-H to pozzolanic C-S-H is prohibited (value=0) or allowed (value=1).