Calcium carbonate has been found to precipitate into tablets which were self-aligned with respect to each other on limestone aggregates in Portland cement-based materials. A surface layer of water and reaction in 100% relative humidity air environments were critical for alignment of precipitates. The organization of the calcium carbonate precipitates resembled the cross-section of nacre, but without an organic phase. Possible implications for organized material synthesis, as well as biomineralization, are discussed.