Good question. The over saturation of the saliva is what creates the deposition of "calculus" on the teeth. Too much and instead of being incorporated into the tooth, it accumulates on it. Remineralization is a process that happens on the nanometer scale in which calcium and phosphates are used to replace the lost ions (at least on the theoretical level). A great article that proposes a model is this one: Niu LN, Jee SE, Jiao K, et al. Collagen intrafibrillar mineralization as a result of the balance between osmotic equilibrium and electroneutrality. Nat Mater. 2017;16(3):370–378. doi:10.1038/nmat4789