Efflorescence is primarily composed of calcium carbonate, so there is no distinction. Calcium carbonate leachate is not a direct indicator of rebar corrosion. Carbonation within the cement paste increases the potential for rebar corrosion because it lowers the pH of the concrete matrix, thus reducing the passivation effect of high alkalinity.
Efflorescence occurs when water gets into the concrete, mortar or any cementitious material, takes calcium and potentially a few other elements in solution then comes back to the surface where it is exposed to carbon dioxide. The carbon dioxide combines with calcium oxide to produce calcium carbonate. There can be a variety of other compounds that develop from smaller quantity components of cement and concrete including aluminates and sulphates, however, calcium is the dominant element in portland cement so there is more of it available to leach.
Efflorescence can occur rather quickly, but carbonation can take years to reach the point of affecting rebar corrosion. The depth of carbonation can be checked by taking a core of the concrete, breaking the core in half along its length, then treating the freshly broken surface with phenolphthalein to see the depth of carbonation.
