Calcium carbonate binding mechanisms in the setting of calcium and calcium-magnesium putty-limes

Calcium and calcium magnesium putty-limes (C-L and C-M-L) were characterized, through SEM, Porosimeter and Chemical Analysis, to obtain information on the microstructure of their solid-phases. Irregular agglomerates of portlandite grains and large acicular crystallites of brucite are differently interconnected to form a cellular solid matrix dispersed into the saturated aqueous solution of the hydroxides. The setting of these putty-limes was followed also in a thermobalance with a thermostatic chamber designed to keep the temperature in the range 15-30 °C. The tests were done at a constant humidity of 80%, under isothermal conditions, and a wet and carbonated N2 flux with 1% of CO2 was added. The microstructure of the final samples was characterized by the usual methods of investigation at micro-macro scale. Drying kinetics and the related shrinkage processes were discussed on the basis of a modified Kelvin equation, which predicts for the C-L lime putty a larger shrinkage (50.3%) than for C-M-L (35.5%). The binding mechanisms of the calcium carbonate in the strengthening of the putty-lime systems is due to the interconnected texture formed by the calcium carbonate fine crystallites formed during the precipitation process. The scientific reasons for this microstructure evolution have been discussed and explained on the basis of experimental data and theory, leading to a better understanding of the complex relationships between drying, shrinkage and the chemical processes occurring in the setting of putty-lime.