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Calcite particles (Source; CaCO3, ∼0.4 μm in size) were 3D-printed into cube honeycombs (honeycombs) with ∼0.7 mm thick struts and ∼0.6 mm cellular window opening. The sintered honeycombs, regardless of the presence of a sintering aid in the printing slurry, gave ∼3 MPa or more in the compressive stress, which exceeded that of trabecular bone. The conversion of calcite to hydroxyapatite (HAp) was assessed in 0.1 M K2HPO4 (pH: 7.0 or 7.4) at 20 °C–80 °C due to the X-ray diffraction intensity of the calcite and HAp peaks, surface microstructure, and P(V) accumulation on the honeycomb strut surface. Although a premature HAp layer yielded on all samples within 1 h soaking, further conversion depended on the samples. The conversion continued on Source until the volume fraction of the HAp shell layer reached ∼35% (37 °C). It was hardly detected on the grains of sintered honeycombs (from the slurry with the sintering aid) within the whole soaking period (≤24 h). In contrast, the conversion on the sintered honeycombs (from the slurry without the aid) became vigorous when soaked for 5 h, and, it reached at 24 h the highest level that the as-printed honeycombs achieved. The critical factors controlling the conversion were calcite dissolution, the equilibria among the carbonate and phosphate ions, and the rates of migration of Ca(II) and P(V) through the inter-granular channels of pores within the struts. A particle stacking model was proposed for the most plausible interpretation of the present results.