In addition to the fine filler (surface area) effect described above, a chemical reaction between calcium carbonate (limestone) filler and cement has been included in version 3.0 of CEMHYD3D. Based on experimental observations in the published literature [14-18], the formation of a monocarboaluminate phase [AFmc= (CaO)3(Al2O3)·CaCO3·11H2O] in preference to the formation of a monosulfoaluminate phase [AFm= (CaO)3(Al2O3)·CaSO4·12H2O] was added by modifying the CEMHYD3D computer codes to include the following reaction:
3 (CaO)3(Al2O3)·CaSO4·12H2O + 2 CaCO3 + 18 H2O
2 (CaO)3(Al2O3)·CaCO3·11H2O + (CaO)3(Al2O3)·3 CaSO4·32H2O
This reaction favors the production of AFmc (and ettringite) over that of the conventional AFm phase in the presence of calcium carbonate. In the CEMHYD3D model, this reaction becomes active only when the initial calcium sulfate is depleted and the previously formed ettringite begins to convert to the AFm phase by reaction with more of the cement clinker aluminate phases. This is in general agreement with experimental observations [15, 16]. While other reaction paths could be written for the formation of AFmc in a cement-based system, the above scheme was chosen for its simplicity in implementation in the CEMHYD3D codes and the fact that it does yield the desired effect: the formation of the AFmc phase at the expense of the AFm phase. Calcium carbonate generally has a low reactivity (because of its low solubility), and in typical simulations using the updated CEMHYD3D codes, for a 20 % by mass fraction substitution of ground limestone for cement, only about 5 % of the limestone present reacts during the first 180 d of simulated hydration.
To incorporate limestone and AFmc into the CEMHYD3D codes, they were simply assigned phase IDs, and their relevant properties (specific gravity, molar volume, etc.) included into the computer program disrealnew.c (see program listing in Appendix B). Then, the reaction scheme outlined above, with the CaCO3 dissolving to create DIFFCACO3 species that react at the surfaces of available AFm, was added to the disrealnew.c and hydrealnew.c modules (listings provided in Appendix B).