Porous tablets of crystalline calcium carbonate via sintering of amorphous nanoparticles

Porous tablets of crystalline calcium carbonate were formed upon sintering of a precursor powder of amorphous calcium carbonate (ACC) under compressive stress (20 MPa) at relatively low temperatures (120–400 uC), induced by pulsed direct currents. Infrared spectroscopy ascertained the amorphous nature of the precursor powders. At temperatures of 120–350 uC and rates of temperature increase of 20–100 uC min, the nanoparticles of ACC transformed into crystallites of mainly aragonite, which is generally difficult to achieve using wet-chemicals under kinetic control. The amorphous precursor particles (y10 nm) transformed into crystallites (y30–50 nm) during sintering. Consistently, the specific surface areas of 140–160 m g for the precursor particles were reduced to 10–20 m g for the porous tablets. The porous network within the tablets consisted of fused aragonite and vaterite particles in a ratio of y80 : 20. The fraction of aragonite to vaterite was invariant to the temperature and rate of temperature change used. The particle size increased only to a small amount on an increased rate of temperature change. At temperatures above 400 uC, porous tablets of calcite formed. The later transformation was under thermodynamic control, and led to a minor reduction of the specific surface area. The size of the crystallites remained small and the transformation to calcite appeared to be a solid-state transformation. Porous, templateand binder-free tablets of calcium carbonate could find applications in for example, biology or water treatment.