Complex microstructures of biominerals such as seashell nacre, bone, and teeth are awe-inspiring. Nature has devised schemes to combine hard inorganic platelets of aragonite (CaCO3) and an organic matrix that produce tough biocomposites. The ability of the organic-inorganic components to “slide” internally leads to the toughening of the materials, though a recreation of this system at the nanoscale has yet to be shown. Here, we implement a poly(KAMPS)-based assembly, which is carried out entirely from dilute aqueous solutions of the materials to create a “brick and mortar”-type aragonite structure that mimics the platelet sliding and exhibits toughening. The negatively charged poly(KAMPS) chains are attracted to the positively charged divalent cations, by which addition of NaHCO3 to an aqueous mixture of Ca2+-poly(KAMPS), results in the growth of aragonite nanorods with a width of 120 nm. The reversible nature of the physical gel formation of poly(KAMPS) in solution results in adhesion of the nanorods into a microscale structure. The new nacre-like carbonate composite, has a modulus (44 GPa) and hardness (2.8 GPa) on a similar order as to that of nacre and other bio-composites, exhibits limited creep, and demonstrates a mechanism with nanoscale deformation.