For example, on the mortared brickwork of old forts and places of that sort, formations which look to the naked eye like stalactites and stalagmites sometimes form in less than one hundred years. However, those formations are composed of gypsum, which is a salt of calcium sulfate. Unlike calcium carbonate, gypsum is moderately soluble in water, which means that transport and recrystallization can take place much more rapidly (White, 1976, p.304). There is a whole class of cave deposits called evaporite minerals which consist of those minerals which dissolve readily in water. As might be expected, these formations are ephemeral when compared to the carbonates which form all the really large and impressive cave formations. The chemistry of all this is not particularly complex and is very well understood.
(Loftin, 1988, p.23)
Here's some more information.
Many people have found that stalactites forming on concrete or mortar outdoors may grow several centimeters each year. Stalactite growth in these environments, however, bears little relation to that in caves, because it does not proceed by the same chemical reaction. Although cement and mortar are made from limestone, the same rock in which the caves form, the carbon dioxide has been driven off by heating. When water is added to these materials, one product is calcium hydroxide, which is about 100 times as soluble in water as calcite is. A calcium hydroxide solution absorbs carbon dioxide rapidly from the atmosphere to reconstitute calcium carbonate, and produce stalactites. This is why stalactites formed by solution from cement and mortar grow much faster than those in caves. To illustrate, in 1925, a concrete bridge was constructed inside Postojna Cave, Yugoslavia, and adjacent to it an artificial tunnel was opened. By 1956, tubular stalactites 45 centimeters long were growing from the bridge, while stalactites of the same age in the tunnel were less than 1 centimeter long.
